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  • View in gallery

    Overview of the study region in northern Australia. The boundaries of the three domains in the northern parts of Western Australia (western), Northern Territory (central), and Queensland (eastern) are plotted in red over surface elevation (shaded, m). Australian state names and boundaries are shown in black as are the names of major bodies of water, and the locations of major population centers are indicated in blue.

  • View in gallery

    The mean gridded total column water vapor (shaded, kg m−2) and 850-hPa winds (vectors) over northern Australia for (a)–(e) each month in the wet season and (f) the wet season as a whole, all over the period 1979–2013.

  • View in gallery

    Gridded (a)–(e) mean monthly accumulated precipitation (mm month−1) and (f)–(j) coefficient of variation (dimensionless) over northern Australia for each of the main months of the wet season during the period 1979–2010.

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    Boxplots of monthly accumulated mean areal precipitation (MAP, mm month−1) over the (a) western, (b) central, and (c) eastern domains during the period 1979–2010. Following the standard convention of Tukey boxplots, the bottom and top of the box represent the first and third quartiles, the thick line represents the median, and the ends of the “whiskers” extending from the boxes represent the lowest and highest datum still within 1.5 of the interquartile range.

  • View in gallery

    The mean number of PFs (number per month per square degree) gridded over northern Australia for (a)–(e) for each month in the wet season and (f) the wet season as a whole over the period 1998–2013.

  • View in gallery

    As in Fig. 4, but for monthly occurrence of PFs (number per month per square degree) during the period 1998–2013.

  • View in gallery

    Cumulative distribution functions of the population (solid lines) and rainfall contribution (dashed lines) of PFs as a function of (a) maximum echo top (km), (b) minimum IR temperature (K), and (c) total lightning flash count (number) in each of the three domains.

  • View in gallery

    Gridded proportion of (a)–(e) warm PFs (dimensionless) and (f)–(j) thunderstorm PFs (dimensionless) in the total number of PFs over northern Australia for each of the main months of the wet season during the period 1998–2013.

  • View in gallery

    (a) Proportion of warm PFs in the total population of PFs and (b) their contribution to the total rainfall during each month of the wet season, and also the wet season as a whole. Values are shown for the three domains during the period 1998–2013. Error bars represent the 95% confidence limits, as calculated by a bootstrap algorithm.

  • View in gallery

    As in Fig. 9, but for thunderstorm PFs.

  • View in gallery

    Cumulative distribution functions of the population (solid lines) and rainfall contribution (dashed lines) as a function of the area (km2) of (a) all PFs, (b) warm PFs, and (c) thunderstorm PFs in each of the three domains. The horizontal axes use a logarithmic scale.

  • View in gallery

    The daily mean moisture convergence (shaded, 10−4 g kg−1 s−1) and streamlines of moisture flux at the 850-hPa level for the (a) active phases and (b) suppressed phases of the MJO defined using the standard convention, and the (c) active phases and (d) suppressed phases of the MJO defined using the eastward-shifted convention. These quantities are calculated over the wet season as a whole during the period 1979–2013.

  • View in gallery

    The difference in daily mean gridded total column water vapor (shaded, kg m−2) between the active and suppressed phases of the MJO over northern Australia using the (a) standard and (b) eastward-shifted definition of the MJO over the wet season as a whole during the period 1979–2013. The values over the entire map exceed the 95% confidence level of statistical significance.

  • View in gallery

    Mean daily accumulated mean areal precipitation (MAP, mm day−1) during suppressed (red bars) and active (cyan bars) phases of the MJO and for all days (gray bars) over the (a) western, (b) central, and (c) eastern domains during the period 1979–2010. Error bars represent the 95% confidence limits, as calculated by a bootstrap algorithm. The active and suppressed phases of the MJO are calculated using the (a),(b) standard definition and (c) the eastward-shifted definition.

  • View in gallery

    The mean number of PFs (shaded, number per square degree per 30-day month) for the (a) active and (b) suppressed phases of the MJO defined using the standard definition, and the (c) active and (d) suppressed phases of the MJO defined using the eastward-shifted definition. These quantities are calculated over the wet season as a whole during the period 1998–2013.

  • View in gallery

    As in Fig. 14, but for daily occurrence of PF (number per day per square degree) during the period 1998–2013.

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    As in Fig. 15, but for the proportion of warm PFs.

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    As in Fig. 16, but for the proportion of warm PFs (%) during the period 1998–2013.

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    As in Fig. 15, but for the proportion of thunderstorm PFs.

  • View in gallery

    As in Fig. 16, but for the proportion of thunderstorm PFs (%) during the period 1998–2013.

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Regional Variation in the Wet Season of Northern Australia

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  • 1 School of Earth, Atmosphere, and Environment, Monash University, Clayton, Victoria, Australia
  • 2 Centre of Excellence for Climate System Science, Monash University, Clayton, Victoria, Australia
  • 3 School of Earth, Atmosphere, and Environment, Monash University, Clayton, Victoria, Australia
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Abstract

Variability in the wet season of tropical northern Australia is examined over its main months, November–March, with a focus on zonal differences between the western, central, and eastern domains, which encompass the northern parts of Western Australia, Northern Territory, and Queensland, respectively. The seasonal progression of the wet season is similar across the region, with steadily increasing atmospheric moisture and rainfall into the core months of the monsoon, January and February, decreasing into March. This seasonal progression differs in the eastern domain, where there is an extension of premonsoonal conditions into December, and a delay of the onset of the monsoon until January. An analysis of TRMM precipitation features (PFs) reveals more intense convection during the premonsoon, steadily decreasing in intensity to much shallower convection by March, with a steady increase in the overall number of PFs throughout the wet season. Regionally, the intensity of PFs steadily decreases eastward across northern Australia with significantly weaker, shallower PFs over the eastern domain. Intraseasonal variability associated with the Madden–Julian oscillation (MJO) has a consistent impact on the rainfall and the total number of TRMM PFs across northern Australia, with both increasing and decreasing during the active and suppressed phases, respectively. However, regional variations in the effect of the MJO lead to radically different characteristics of PFs during the suppressed phases; intense convection and thunderstorms become more frequent over the western and central domains, while shallow PFs associated with the warm rain precipitation process increase in number over the eastern domain.

Current affiliation: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California.

Corresponding author address: M. J. Murphy, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0225. E-mail: mjmurphy@ucsd.edu

Abstract

Variability in the wet season of tropical northern Australia is examined over its main months, November–March, with a focus on zonal differences between the western, central, and eastern domains, which encompass the northern parts of Western Australia, Northern Territory, and Queensland, respectively. The seasonal progression of the wet season is similar across the region, with steadily increasing atmospheric moisture and rainfall into the core months of the monsoon, January and February, decreasing into March. This seasonal progression differs in the eastern domain, where there is an extension of premonsoonal conditions into December, and a delay of the onset of the monsoon until January. An analysis of TRMM precipitation features (PFs) reveals more intense convection during the premonsoon, steadily decreasing in intensity to much shallower convection by March, with a steady increase in the overall number of PFs throughout the wet season. Regionally, the intensity of PFs steadily decreases eastward across northern Australia with significantly weaker, shallower PFs over the eastern domain. Intraseasonal variability associated with the Madden–Julian oscillation (MJO) has a consistent impact on the rainfall and the total number of TRMM PFs across northern Australia, with both increasing and decreasing during the active and suppressed phases, respectively. However, regional variations in the effect of the MJO lead to radically different characteristics of PFs during the suppressed phases; intense convection and thunderstorms become more frequent over the western and central domains, while shallow PFs associated with the warm rain precipitation process increase in number over the eastern domain.

Current affiliation: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California.

Corresponding author address: M. J. Murphy, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0225. E-mail: mjmurphy@ucsd.edu

1. Introduction

The tropical north of Australia is highly seasonal, with precipitation in this region commonly divided into wet and dry seasons, associated with the westerly monsoon and easterly trade winds, respectively. Recent research into moist convection and associated precipitation during the wet season has been concentrated in the center of this region, utilizing the observational infrastructure at Darwin (e.g., May and Ballinger 2007; Pope et al. 2009; Catto et al. 2012; Evans et al. 2014) (see Fig. 1 for location), with much less study in the western (e.g., Berry et al. 2011) and eastern (e.g., Robertson et al. 2006) parts of northern Australia. How the wet season in these regions might differ from that of Darwin remains unclear. Additionally, much of the previous research has examined the wet season as a whole or in three-month seasons, potentially masking important details of the evolution of the wet season and its variation across northern Australia.

Fig. 1.
Fig. 1.

Overview of the study region in northern Australia. The boundaries of the three domains in the northern parts of Western Australia (western), Northern Territory (central), and Queensland (eastern) are plotted in red over surface elevation (shaded, m). Australian state names and boundaries are shown in black as are the names of major bodies of water, and the locations of major population centers are indicated in blue.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

The wet season begins sometime between September and November, during the premonsoon (also known as the transition) period, when there is a gradual buildup of low-level moist static energy and convective available potential energy (CAPE) over northern Australia (Hendon et al. 1989). This buildup of CAPE is not fully released during the premonsoon due to the presence of considerable convective inhibition (CIN) (Hendon et al. 1989); however, this environment is conducive to isolated deep convection and lightning, as has been found around Darwin (Keenan and Carbone 1992). The onset of the monsoon typically occurs sometime in December, characterized by widespread convection of moderate to strong intensity associated with a decrease in both CAPE and CIN (Hendon et al. 1989), with the monsoon ending abruptly in March or April (e.g., Nicholls et al. 1982; Drosdowsky 1996). While the majority of the total rainfall over the wet season accumulates during the monsoon, a significant portion is still attributed to the premonsoon period, which at Darwin has been found to account for roughly 30% of the total rainfall (Nicholls et al. 1982).

The monsoon has strong variability on intraseasonal time scales, switching between active and break periods. Active periods of the monsoon (also known as “monsoon bursts”) are associated with westerly winds and a more oceanic nature of convection, with widespread rainfall and less intense convection; conversely, break periods are associated with easterly winds and a more continental nature of convection, with deeper convection, more frequent lightning and more isolated rainfall (Holland 1986; Keenan and Carbone 1992; Drosdowsky 1996; McBride and Frank 1999; May and Ballinger 2007; Xu and Zipser 2012). The time between active periods of the monsoon is roughly 40 days (Holland 1986), similar to the period of the Madden–Julian oscillation (MJO; Madden and Julian 1972), which is the dominant mode of intraseasonal variability in the tropics. Indeed active periods within the monsoon are most likely when the MJO’s convective anomaly arrives over the region; conversely, break periods are most likely when the MJO’s convective anomaly is far from northern Australia (Pope et al. 2009; Wheeler and McBride 2012; Evans et al. 2014; Berry and Reeder 2016). The tropical convective anomalies associated with the active phases of the MJO lead to strong increases in rainfall throughout northern Australia, particularly during austral summer (e.g., Wheeler et al. 2009; Evans et al. 2014). While active periods within the monsoon are especially wet, a significant contribution to the total rainfall during the wet season at Darwin has been associated with other weather regimes (Pope et al. 2009).

Recent studies have found an increasing trend in rainfall during the wet season over the past 50 years (Smith 2004; Taschetto and England 2009). This increase is concentrated in the northern and western parts of northern Australia with little change in rainfall found to the east over northern Queensland. Catto et al. (2012) linked the increasing trend in rainfall at Darwin to large-scale circulation changes, which are significantly lengthening the wet season, as well as increasing the frequency of weather regimes associated with the active monsoon during the austral summer months (December–February). Why these large-scale circulation changes would not lead to a similar trend in northern Queensland has not been addressed in the literature.

A useful tool for investigating the convection that generates rainfall over the tropics has emerged from the 16-yr record of remotely sensed observations collected by the now defunct Tropical Rainfall Measuring Mission (TRMM; Kummerow et al. 1998) satellite. These data have been organized into precipitation features (PFs), defined as contiguous raining pixels in the swath of TRMM’s precipitation radar, with characteristics of these PFs summarized using collocated observations from TRMM’s suite of sensors (Liu et al. 2008). The resulting TRMM PF database has been used in numerous studies, for example, to compare the properties of deep convection in monsoonal, continental, and oceanic rainfall regimes (Xu and Zipser 2012) and to investigate the characteristics of warm rain systems over the tropics (Liu and Zipser 2009). Some of the potential to use this TRMM PF database to investigate variability in the clouds and convection that produce rainfall over northern Australia is realized in this study.

The following study examines variability in the wet season of northern Australia during the recent 1979–2013 period. A central focus is on how the wet season varies regionally across northern Australia and on identifying key differences from the heavily studied region surrounding Darwin. The main months of the wet season are individually analyzed and the seasonal evolution of rainfall and characteristics of the convection producing it are described in the western, central, and eastern parts of northern Australia. Intraseasonal variability in the wet season is also examined through the MJO, and regional variations in its impacts over northern Australia are described.

2. Data and methods

a. Geography and regional definitions

This study is focused on the deep tropics, which is defined here as the region of Australia equatorward of 18°S. To quantify zonal differences across northern Australia, three domains are defined (see Fig. 1): 1) western (12°–18°S, 120.75°–128.75°E; a subset of the Kimberley region of Western Australia), 2) central (10.375°–16.5°S, 129.25°–137.25°E; the northern section of the Northern Territory, including Darwin), and 3) eastern (10°–18°S, 140.5°–146.5°E; the northern section of Queensland including Cairns) Only the land areas enclosed by these domains are examined in this study, and the only substantial orography is found in the “wet tropics” on the southeast coast of the eastern domain. Throughout this study the wet season is defined as the months of November–March, inclusive.

b. Gridded rain gauge analyses

A gridded precipitation analysis was obtained from the Australian Water Availability Project (AWAP; Jones et al. 2009). This dataset is based on daily gauge observations, collected at 0900 local time (LT), that have been spatially interpolated onto a 0.05° × 0.05° latitude–longitude grid that covers the land areas of Australia. The time offset from UTC time ranges from +8 to +10 h across northern Australia, resulting in the daily accumulations in the AWAP analysis roughly corresponding to 0000–2359 UTC per day. The period 1979–2010 was chosen for analysis as gauge density over tropical Australia had increased significantly by this time. For this study the resolution of the dataset was coarsened to 0.25° × 0.25° using local area averaging in order to make this large dataset easier to work with. Time series of both daily and monthly mean areal precipitation were calculated over the land areas in each of the three aforementioned domains.

c. TRMM precipitation features

Precipitation features over tropical Australia, as identified by instruments on board the TRMM (Kummerow et al. 1998) satellite, are taken from the University of Utah TRMM PF database (Liu et al. 2008) over the period 1998–2013. This database follows the concepts of PFs developed by Nesbitt et al. (2000). The radar precipitation feature definition groups contiguous pixels in the TRMM Precipitation Radar (PR) swath, determined by the TRMM PR 2A25 product (Iguchi et al. 2000) to have near-surface rain greater than zero. The TRMM PR operates at 13.8 GHz with horizontal polarization and has a minimum detectable signal of 17–18 dBZ, resulting in the exclusion of some light precipitation. The swath width of the PR is 247 km with a ground resolution of 5 km at nadir. Characteristics at the grouped pixels are summarized from collocated measurements and retrievals from the PR, TRMM Microwave Imager (TMI), Visible and Infrared Sensor (VIRS), and Lightning Imaging System (LIS) [for a detailed description see Liu et al. (2008)]. To eliminate noise, a minimum of two contiguous precipitating pixels were required to define a PF in this study.

Characteristics of PFs over the land areas in the three aforementioned domains are as follows: 1) size, as calculated from the number of PR pixels within the PF; 2) maximum detectable echo top, defined as the maximum storm height from TRMM 2A23 products; 3) cloud-top temperature, as indicated by the minimum VIRS infrared brightness temperature at 10.8-μ m wavelength (); 4) lightning flash count, defined as the number of lightning flashes detected in a PF; and 5) volumetric rain, calculated from 2A25 near-surface rain (Liu et al. 2008).

The PFs associated solely with the warm rain processes of droplet coalescence are identified in the TRMM PF database. Those PFs meeting both of the following criteria from Liu and Zipser (2009) are classified as warm PFs in this study: 1) minimum K, which excludes PFs with cloud-top temperatures colder than the freezing point; and 2) maximum PR echo top <4.5 km, the typical freezing level in the tropics, which ensures all large precipitation particles detectable by the PR are below the freezing level. The PFs associated with lightning are also identified in the TRMM PF database. Those PFs with at least one lightning flash detected during the roughly 80 s that a location is usually observed during a TRMM overpass (e.g., Cecil et al. 2005) are classified as thunderstorm PFs in this study.

d. ERAI reanalysis

The total column water vapor over northern Australia and its surrounding waters were obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERAI; see Dee et al. 2011) for the period 1979–2013. The ERAI has a spectral T255 horizontal resolution, which corresponds to approximately 79 km spacing on a reduced Gaussian grid, and 60 model layers from the surface up to the top of the atmosphere located at 0.1 hPa. The dataset used in this study has been interpolated to approximately a 0.7° horizontal resolution. These 6-hourly data were averaged over each day of the wet season (0000–2359 UTC) roughly corresponding to the time of the daily accumulations in the AWAP analysis.

Moisture convergence at 850 hPa was calculated from the ERAI fields by multiplying the specific humidity at 850 hPa with the u and υ components of the 850-hPa wind and taking the divergence of the resulting quantity (the sign of the result was reversed so that convergence is positive). The product of the specific humidity and u- and υ-wind components was also used to generate streamlines of moisture transport.

e. MJO index

The state of the Madden–Julian oscillation (MJO) is represented by the Real-time Multivariate MJO (RMM) index (Wheeler and Hendon 2004) throughout this study. The RMM index is the principal component time series of the two leading empirical orthogonal functions (EOFs) of combined daily mean outgoing longwave radiation (OLR) and 850- and 200-hPa zonal wind anomalies, which are averaged between 15°N and 15°S. The seasonal cycle is removed by subtracting the long-term mean and the first three harmonics of the annual cycle, based on the 1979–2001 period. Interannual variability is removed from the RMM index by regression with the first rotated EOF of Indo-Pacific SSTs (SST1), as described in Drosdowsky and Chambers (2001), and by subtracting out the 120-day mean of the previous 120 days. This index was obtained from the Australian Bureau of Meteorology (http://poama.bom.gov.au/project/maproom/RMM/index.htm) over the period 1979–2013.

In this study, phases 4, 5, and 6 of the MJO are grouped together and referred to as the active phases of the MJO, while phases 1, 2, and 8 are grouped together and referred to as the suppressed phases of the MJO over northern Australia. These active phases of the MJO are associated with generally enhanced precipitation over northern Australia and active periods during the monsoon; conversely, these suppressed phases of the MJO are associated with generally suppressed precipitation and the break periods during the monsoon (e.g., Wheeler et al. 2009; Evans et al. 2014). The sensitivity of the results of this study to a shifting of the phases used to define the active and suppressed phases of the MJO to the east and also to the west was examined. The only significant changes to the results from these sensitivity tests resulted from the eastward shifting of the phases so that phases 5, 6, and 7 define the eastward-shifted active phases of the MJO, and phases 1, 2, and 3 define the eastward-shifted suppressed phases of the MJO over northern Australia. These alternate definitions of the active and suppressed phases are hereafter referred to as the eastward-shifted active phases and eastward-shifted suppressed phases of the MJO. The sensitivity test using the westward-shifted active and suppressed phases of the MJO resulted in a much weaker modulation of the weather over northern Australia than the standard definition (not shown). An amplitude threshold of 1.0 was applied to all the active and suppressed phase groups to exclude times when the MJO was weak.

3. Seasonal evolution of the wet season

The seasonal evolution of the wet season is closely associated with a shifting of the low-level winds and subsequent enhancement of atmospheric moisture over northern Australia, as depicted in the monthly mean total column water vapor and 850-hPa winds (Fig. 2). The wet season begins with strong easterly trade winds dominant across most of the region and a relatively dry atmospheric state, characteristic of the premonsoon period (Hendon et al. 1989). As the trade winds weaken into December a progressive moistening of the atmosphere begins, culminating in very moist conditions during January and February when monsoonal westerly winds become dominant over the northernmost part of the region. By March the trade winds have reestablished themselves over northern Australia and moisture finally decreases. Overall, this seasonal moistening of the atmosphere is greatest in the northernmost parts of the Australian continent, where exposure to the monsoonal westerly winds is greatest, particularly the northern tip of Queensland and the northern coast of the Northern Territory (Fig. 2f). Much less moistening is found inland from the coast and in eastern Queensland near the town of Cairns (see Fig. 1 for all location names), places where the trade winds remain established throughout the wet season. Less moistening is also found along the northwest facing coastline of Western Australia, where winds remain light throughout most of the wet season.

Fig. 2.
Fig. 2.

The mean gridded total column water vapor (shaded, kg m−2) and 850-hPa winds (vectors) over northern Australia for (a)–(e) each month in the wet season and (f) the wet season as a whole, all over the period 1979–2013.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

a. Seasonal evolution of rainfall

The spatial pattern of rainfall across northern Australia is depicted in the long-term mean monthly accumulation for each month of the wet season (Figs. 3a–e). Overall the pattern of rainfall follows that of atmospheric moisture, with highest accumulated rainfall near the coast, decreasing inland. Rainfall is the least in November, when monthly accumulations greater than 100 mm are confined to the extreme northwest of the Northern Territory and the relatively mountainous wet tropics on the east coast of Queensland near Cairns. December is much wetter than November, with most of the study region receiving mean accumulations greater than 100 mm, with the exception of the extreme southwest of the region (poleward of 17°S), with maxima exceeding 200 mm in the northwestern part of the Northern Territory, the wet tropics, and the northwest tip of Queensland. The pattern and amount of accumulated rainfall during the core monsoon months of January and February are very similar, with accumulations exceeding 200 mm throughout most of tropical Queensland and the northwestern parts of Western Australia and the Northern Territory, and maxima exceeding 300 mm along the northwest coast of the Northern Territory and 400 mm along parts of the east and west coasts of Queensland. The pattern of precipitation in March, at the end of monsoon, is similar to that of December but with enhanced precipitation along the east coast and northwestern tip of Queensland.

Fig. 3.
Fig. 3.

Gridded (a)–(e) mean monthly accumulated precipitation (mm month−1) and (f)–(j) coefficient of variation (dimensionless) over northern Australia for each of the main months of the wet season during the period 1979–2010.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

The spatial pattern of variability in accumulated rainfall across northern Australia is depicted using the coefficient of variation (CV) for each month (Figs. 3f–j). Here the CV is defined as the ratio of the monthly standard deviation to the monthly mean; this statistic was chosen for its usefulness in comparing variability across grid points with widely different means. Variability is greatest during the premonsoonal month of November across much of the study region, with low CV values indicative of low monthly variability only found in the region between northern Western Australia and northwestern Northern Territory, with particularly high variability throughout northern Queensland. Progressing into December, the spatial pattern of variability is similar to November but with lower magnitude. The core months of the monsoon, January and February, have much lower CV values, particularly along the northwest-facing coastlines, where exposure to the moisture transported by the monsoon’s northwesterly winds is greatest. During March, when the monsoon is typically coming to an end, variability once again increases to similar levels to that found in November, with the notable exception of northern Queensland, where variability in monthly rainfall remains low.

A generally similar seasonal evolution of monthly accumulated mean areal precipitation (MAP) is found over each of the three domains (Fig. 4). The driest month is November with the median and 75th percentile of monthly rainfall below 100 mm across all domains. December and March, transitional months on either end of the monsoon, are considerably wetter with the 25th percentile of monthly rainfall exceeding the 75th percentile during November across all domains except March in the western domain. The wettest months in all domains are January and February, the core months of the monsoon, which have very similar statistical distributions.

Fig. 4.
Fig. 4.

Boxplots of monthly accumulated mean areal precipitation (MAP, mm month−1) over the (a) western, (b) central, and (c) eastern domains during the period 1979–2010. Following the standard convention of Tukey boxplots, the bottom and top of the box represent the first and third quartiles, the thick line represents the median, and the ends of the “whiskers” extending from the boxes represent the lowest and highest datum still within 1.5 of the interquartile range.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

Regional differences are also found in the seasonal evolution of monthly rainfall, particularly in the eastern domain. There is an abrupt increase in monthly rainfall from December into January in the eastern domain, while the remaining domains have a gradual increase in rainfall from November into the core months of the monsoon. This is illustrated by the interquartile ranges of monthly rainfall during December and January being distinct from each other in the eastern domain but overlapping elsewhere. The median values of rainfall in the eastern domain during the months before this abrupt increase into the core of the monsoon is well below that of the central domain and roughly the same as that of the western domain, but afterward meet or exceed the median values found in these domains. Rainfall in the eastern domain is also notable for very high extreme values during January, exceeding 600 mm month−1. The western and central domains have similar distributions of rainfall but with slightly higher intensity in the central domain, where higher median values of rainfall are found in each month of the wet season.

b. Seasonal evolution of the number of TRMM PFs

The spatial pattern of the mean number of TRMM PFs detected per month per square degree across northern Australia is shown for each month of the wet season (Fig. 5). At the start of the wet season PF counts are low, with similar values over the Australian continent and its surrounding waters. High PFs counts are found on the northwestern coasts of the Northern Territory and Western Australia with a maximum near Cairns on the coast of Queensland, and the lowest counts in the southwestern corner of the study region. This pattern persists into December, but with higher PF counts, particularly over the waters to Australia’s north and east. A different pattern is found from January to March, when a band of very high PF counts (roughly 20–30 PFs per month per square degree) extends from the waters north of the Northern Territory southeastward across the northernmost part of Queensland and covering much of the Coral Sea. This pattern leads to very high PF counts along northern and eastern Queensland and also the northern coasts of the Northern Territory and Western Australia. The magnitude of this pattern peaks in February and abates very little into March, actually increasing in the Coral Sea and Queensland, though decreasing somewhat in the interior of the continent.

Fig. 5.
Fig. 5.

The mean number of PFs (number per month per square degree) gridded over northern Australia for (a)–(e) for each month in the wet season and (f) the wet season as a whole over the period 1998–2013.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

Overall the three domains share a similar seasonal evolution of monthly PF counts, as shown by the distribution of the number of PFs detected per month per square degree of the land areas of each domain (Fig. 6). While the TRMM satellite’s sampling of PFs may have some regional and/or monthly variation, this would not significantly affect these statistics of the areal-normalized number of PFs per month in each region. The seasonal evolution of monthly PF counts is similar to that of monthly rainfall with an increase in the number of PFs from the premonsoon into the core months of the monsoon, though unlike with rainfall, PF counts decrease little into March. In the eastern domain there is a much more abrupt increase in PF counts from December to January than that found in the other domains, similar to what was found with monthly rainfall. Again, this is illustrated by the interquartile ranges during December and January being distinct from each other in the eastern domain but overlapping in the remaining domains. Before this abrupt increase in the eastern domain monthly PF counts are lower than the other two domains, but in January PF counts exceed that in the other domains. Also as was found with monthly rainfall, the western and central domains both have very similar seasonal evolutions of PF counts but with slightly higher amplitude in the central domain.

Fig. 6.
Fig. 6.

As in Fig. 4, but for monthly occurrence of PFs (number per month per square degree) during the period 1998–2013.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

c. Seasonal evolution of the intensity of TRMM PFs

Regional variations in the intensity of PFs during the wet season as a whole are depicted in cumulative distribution functions (CDFs) of the population and rainfall contribution of three metrics of PF intensity (Fig. 7). A tendency for less intense PFs over the eastern domain than the other domains is shown throughout the distributions of maximum echo top (Fig. 7a) and minimum IR temperature (Fig. 7b) of PFs. While PFs over the eastern domain have maximum echo tops roughly 1 km lower and minimum IR temperatures of 5–10 K warmer than the other domains, regional differences in the rainfall contribution of PFs by these metrics are small, with roughly half of all rainfall contributed by intense PFs with tops over 14 km and temperatures less than 185 K in all domains. A tendency for higher rainfall contributions from PFs associated with multiple lightning flashes is found over the western domain compared to the other domains (Fig. 7c), though the total proportion of PFs with lightning is under 7.5% in all domains (see Fig. 10a). The PFs with more than 25 detected lightning flashes contribute approximately 10% of the total rainfall in the eastern and central domains, but nearly 20% of the total in the western domain.

Fig. 7.
Fig. 7.

Cumulative distribution functions of the population (solid lines) and rainfall contribution (dashed lines) of PFs as a function of (a) maximum echo top (km), (b) minimum IR temperature (K), and (c) total lightning flash count (number) in each of the three domains.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

Using these three metrics of PF intensity discussed above, the spatial patterns of two important subclasses of PFs are identified (Fig. 8). The spatial pattern of the proportion of warm PFs (Figs. 8a–e), those PFs associated solely with the warm rain precipitation process (see section 2c), reveal a much higher proportion of these PFs over the ocean than over land, consistent with previous research (e.g., Liu and Zipser 2009). These warm PFs make up the highest proportion of PFs over the ocean during November, particularly over the Coral Sea where proportions exceed 0.6 over most of this region, and steadily decrease into March, with the exception of the southern Coral Sea where proportions remain relatively high even into March. The opposite pattern is found over land, where warm PFs make up the lowest proportions during November, and steadily increase into March. Over the land the proportion of warm PFs is consistently highest over Queensland (roughly 0.2–0.3 during March) and lowest over the center of the continent with scattered pockets of high proportions of warm PFs over the southwestern part of Western Australia. An exception to this stark contrast between the land and ocean is found along the east coast of Queensland, where the seasonal cycle in the proportion of warm PFs is much weaker than in the rest of Queensland, with proportions between 0.3–0.5 throughout the wet season. This weaker annual cycle is related to the high proportions of warm PFs over the Coral Sea extending into this coastline, with this effect most apparent during November.

Fig. 8.
Fig. 8.

Gridded proportion of (a)–(e) warm PFs (dimensionless) and (f)–(j) thunderstorm PFs (dimensionless) in the total number of PFs over northern Australia for each of the main months of the wet season during the period 1998–2013.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

The spatial pattern of the proportion of thunderstorm PFs (Figs. 8f–j), those PFs associated with lightning (see section 2c), shows that thunderstorms are much more common over land rather than ocean, consistent with previous research (e.g., Christian et al. 2003). Those thunderstorm PFs that do occur over the ocean are largely confined to the month of November and the waters just off the coast of Western Australia and the northwestern part of the Northern Territory, and also the southern part of the Gulf of Carpentaria. Over land thunderstorm PFs make up the highest proportions of PFs during November and in the western part of the continent, reaching proportions of roughly 0.2–0.4 in the west. The proportion of thunderstorm PFs steadily decreases into March when they make up proportions of 0.1 or less over most of the continent.

The regional differences and seasonal evolution of warm PFs are quantified through their proportion over the land areas of each of the three aforementioned domains (Fig. 9). Over the wet season as a whole the proportion of warm PFs and their rainfall contribution significantly increases eastward from the western domain to the eastern domain. The eastern domain has a much higher proportion of warm PFs (17%) than that found in the western domain (6.5%) or the central domain (9.1%), and though their contribution to the total rainfall over the eastern domain is small (1.2%) it is much higher than that found over the western and central domains (<0.5%). The seasonal evolution of warm PFs varies strongly by region, with a steady and significant increase in the proportion of warm PFs and their contribution to the total rainfall over the wet season in the western domain, while the remaining domains have similar values in all months except March, when warm PFs peak in all domains. The premonsoonal month of November has the most regional variation, with the least warm PFs in the western domain and the second highest in the eastern domain after March, while in the central domain all of the months November–February have similar values.

Fig. 9.
Fig. 9.

(a) Proportion of warm PFs in the total population of PFs and (b) their contribution to the total rainfall during each month of the wet season, and also the wet season as a whole. Values are shown for the three domains during the period 1998–2013. Error bars represent the 95% confidence limits, as calculated by a bootstrap algorithm.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

The regional differences and seasonal evolution of thunderstorm PFs are also quantified through their proportion over the land areas of each of the three aforementioned domains (Fig. 10). Over the wet season as a whole the proportion of thunderstorm PFs is significantly higher at the 95% confidence level in the western domain (nearly 7.5%) and lower in the eastern domain (4%) though their contributions to the total rainfall (52% and 47%, respectively) are not significantly different from one another. Throughout northern Australia thunderstorm PFs are most common and contribute the most rainfall in November, during the premonsoon, and steadily decrease in frequency and rainfall contribution throughout the remainder of the wet season, in agreement with previous studies around Darwin (e.g., Keenan and Carbone 1992). Once again the eastern domain shows an extension of premonsoonal conditions into December, when there are similar proportions of thunderstorm PFs as November, accompanied by an abrupt drop to significantly lower proportions in January. This is very different from the western and central domains, where there are significant drops in both the proportion of thunderstorm PFs and there rainfall contribution from November to December. The amplitude of the seasonal cycle in thunderstorm PFs decreases eastward from a peak in the western domain.

Fig. 10.
Fig. 10.

As in Fig. 9, but for thunderstorm PFs.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

Regional variations in the size of PFs during the wet season as a whole are revealed by the CDFs of their population and rainfall contribution as a function of PF area (Fig. 11). There is little regional variation in the area of the full population of PFs over each subregion (Fig. 11a), though PFs in the eastern domain are slightly smaller than the other subregions throughout their distribution. Overall, warm PFs tend to be much smaller in area than the full population of PFs, with more than 90% of warm PFs smaller than 200 km−2 in all domains (Fig. 11b). Conversely, thunderstorm PFs tend to be much larger in area than the full population with more than 95% of these PFs larger than 200 km−2 in all domains (Fig. 11c). Regionally, warm PFs have the largest areas in the eastern domain and the smallest in the central domain, though the differences are relatively small. Thunderstorm PFs increase in area eastward, though the western and central domains have relatively little difference in area compared to that of the eastern domain where thunderstorm PFs are 1.30–1.45 times larger than in the central domain throughout much of their distribution. Differences in the contribution to the total rainfall made by PFs is similar to that described for the population of PFs (Fig. 11, dashed lines). The differences in the size of PFs shown in Fig. 11 change little during the individual months of the wet season (not shown).

Fig. 11.
Fig. 11.

Cumulative distribution functions of the population (solid lines) and rainfall contribution (dashed lines) as a function of the area (km2) of (a) all PFs, (b) warm PFs, and (c) thunderstorm PFs in each of the three domains. The horizontal axes use a logarithmic scale.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

In summary, the intensity of PFs has a clear seasonal evolution marked up a steady decrease in intensity throughout the wet season, with thunderstorms most common at the beginning and warm PFs most common at the end of the wet season. Regionally the eastern domain has much less intense PFs than that found farther west, and warm PFs contribute much more to the total rainfall in this domain. There is also a tendency for more thunderstorms in the western domain, though this difference is less pronounced than that found for warm PFs in the eastern domain. Warm PFs tend to be much smaller in area than the full population of PFs whereas thunderstorm PFs tend to be much larger in area; regionally, both types of PFs tend to be larger in area in the eastern domain.

4. Intraseasonal variations in the wet season

Intraseasonal variability during the wet season of northern Australia is associated with pronounced changes in the low-level wind and transport of moisture over the region (Fig. 12; see section 2d for a detailed description of the calculation of these quantities). During the active phases of the MJO (see section 2e for the method used to define active and suppressed phases of the MJO) a westerly transport of moisture at 850 hPa is found in the waters to the north of northern Australia with easterly transport confined to the parts of the continent poleward of 14°S (Fig. 12a). The monsoon trough at 850 hPa rests between these two flow regimes, centered over the northern part of the central domain, directing maritime tropical moisture into the continent, with convergence of moisture found over much of northern Australia. During the suppressed phases of the MJO an easterly transport of moisture at 850 hPa is found across the whole of northern Australia (Fig. 12b) indicative of the easterly trade wind flow found during break periods in the monsoon (e.g., Holland 1986). This dominance of easterly flow during the suppressed phases is associated with much less transport of moisture into northern Australia, with divergence of moisture found over most of the region. A similar pattern is found with the eastward shifted active and suppressed phases of the MJO (Figs. 12c,d). The main differences are found during the shifted active phases, with an eastward shift in the position of the monsoon trough over the central domain toward the Gulf of Carpentaria and in a 1° poleward shift in the position of the monsoon trough.

Fig. 12.
Fig. 12.

The daily mean moisture convergence (shaded, 10−4 g kg−1 s−1) and streamlines of moisture flux at the 850-hPa level for the (a) active phases and (b) suppressed phases of the MJO defined using the standard convention, and the (c) active phases and (d) suppressed phases of the MJO defined using the eastward-shifted convention. These quantities are calculated over the wet season as a whole during the period 1979–2013.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

The difference in daily mean total column water vapor between the active and suppressed phases of the MJO was calculated for the wet season as a whole (Fig. 13). This difference is positive and statistically significant at the 95% confidence level over the entire region, indicating a significant increase in water vapor during the active versus suppressed phases, and is found for both the standard and eastward-shifted definition of the active and suppressed phases of the MJO. The spatial pattern of the moisture difference using the standard definition is relatively uniform across much of northern Australia with values of approximately 4–5 kg m−2 (Fig. 13a). This pattern breaks down over the eastern domain, where the difference in water vapor drops to approximately 1–4 kg m−2, with the lowest values along the east coast just south of Cairns. A very different pattern is found using the eastward-shifted definition of the active and suppressed phases of the MJO (Fig. 13b), with the highest values of the moisture difference centered over the Gulf of Carpentaria, and maxima of 5–7 kg m−2 over northern Queensland and the eastern coast of the central domain.

Fig. 13.
Fig. 13.

The difference in daily mean gridded total column water vapor (shaded, kg m−2) between the active and suppressed phases of the MJO over northern Australia using the (a) standard and (b) eastward-shifted definition of the MJO over the wet season as a whole during the period 1979–2013. The values over the entire map exceed the 95% confidence level of statistical significance.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

a. Intraseasonal variability in rainfall

Daily accumulated rainfall from the AWAP dataset was calculated over each domain for all days over the wet season and during both the active and suppressed phases of the MJO (Table 1). Compared to the mean daily rainfall over the entire wet season, the active and suppressed phases have higher and lower accumulations, respectively, in all domains. While this relationship is statistically significant at the 95% confidence limits in all three domains, the strongest modulation of daily rainfall is found in the western and central domains, with a much weaker modulation in the eastern domain.

Table 1.

The mean daily mean areal precipitation (MAP, mm day−1) in each region (in boldface) over the wet season as a whole during the standard active and suppressed phases of the MJO as well as during all days. The values for the eastward-shifted active and suppressed phases (in italics) of the MJO are also shown. The upper () and lower () 95% confidence limits (CL) were calculated by a bootstrap algorithm and observations are over the period 1979–2010.

Table 1.

Daily accumulated rainfall was also calculated for the eastward-shifted active and suppressed phases of the MJO (Table 1). Using this definition the western and central domains have slightly lower daily rainfall during both the active and suppressed phases (0.07–0.35 mm day−1) though they remain statistically significant from each other at the 95% confidence limits. Conversely, daily rainfall in the eastern domain has a much stronger response to the MJO using this definition, with much larger increases in rainfall (+1.36 mm day−1) during the active phases and much smaller decreases in rainfall (−0.84 mm day−1) during the suppressed phases. Because of this much stronger response to the eastward-shifted MJO phases in the eastern domain, this definition will be used for that domain while the standard definition of MJO phases will be used in both the western and central domains.

The seasonal evolution of the relationship between daily rainfall and the MJO was also examined in each domain (Fig. 14). In all domains, a similar modulation of rainfall by the MJO is found during the individual months of the wet season to that found seasonally, characterized by significant increases and decreases in daily rainfall during the active and suppressed phases, respectively, for each month. The weakest modulation of rainfall is found in November in the western and central domains. The modulation of rainfall in the eastern domain remains strong in all months, with the increases and decreases in rainfall during the active and suppressed phases statistically different at the 95% confidence limits from the daily rainfall during all phases of the MJO in each month.

Fig. 14.
Fig. 14.

Mean daily accumulated mean areal precipitation (MAP, mm day−1) during suppressed (red bars) and active (cyan bars) phases of the MJO and for all days (gray bars) over the (a) western, (b) central, and (c) eastern domains during the period 1979–2010. Error bars represent the 95% confidence limits, as calculated by a bootstrap algorithm. The active and suppressed phases of the MJO are calculated using the (a),(b) standard definition and (c) the eastward-shifted definition.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

b. Intraseasonal variability in the number of TRMM PFs

The spatial pattern of the mean number of TRMM PFs per square degree per 30-day month across northern Australia is depicted for the active and suppressed phases of the MJO during the wet season as a whole (Figs. 15a,b). Overall there is an increase in the number of PFs over both land and ocean during the active phases compared to the suppressed phases of the MJO. During the active phases, PF counts are high (>20 counts per square degree per 30-day month) across much of the waters surrounding northern Australia, particularly in a band extending from the ocean just north of the central domain, over the tip of northern Queensland, and into the Coral Sea, and also in a relatively small patch to the northwest of the coast the western domain. These high PF counts spill over into the northwest coasts of western and central domains and along the east coast of the eastern domain. During the suppressed phases, high PF counts (>20 counts per square degree per 30-day month) are largely confined to the Coral Sea and the east coast of the eastern domain, where PFs counts differ little from that found during the active phases. A similar pattern is found using the eastward-shifted definition of the active and suppressed phases of the MJO with the main difference being slightly less PFs over the western and central domains and slightly more PFs over the eastern domain during the active phases (Figs. 15c,d).

Fig. 15.
Fig. 15.

The mean number of PFs (shaded, number per square degree per 30-day month) for the (a) active and (b) suppressed phases of the MJO defined using the standard definition, and the (c) active and (d) suppressed phases of the MJO defined using the eastward-shifted definition. These quantities are calculated over the wet season as a whole during the period 1998–2013.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

The mean number of PFs detected per day per square degree of the land areas of each domain were calculated for all days over the wet season and during the active and suppressed phases of the MJO (Table 2). Compared to the mean number of PFs over the entire wet season, the active and suppressed phases have higher and lower counts, respectively, in all domains. While this relationship is strong and statistically significant at the 95% confidence limits in the western and central domains, it is weak and fails to reach statistically significant levels in the eastern domain. The mean number of PFs was also calculated for the eastward-shifted active and suppressed phases of the MJO (Table 2). Using this definition the western and central domains have slightly lower PF counts during the active phases and slightly higher PF counts during the suppressed phases, leading to a weaker modulation of PF counts by the MJO. The effect is the opposite in the eastern domain, where the modulation of PF counts is stronger than that found using the standard definition of MJO phases, and exceeds the 95% confidence limits of statistical significance.

Table 2.

As in Table 1, but for the mean daily mean occurrence of PFs over the period 1998–2013.

Table 2.

This general relationship of higher and lower PF counts during the active and suppressed phases of the MJO, respectively, is also found in the individual months of the wet season (Fig. 16). The eastward-shifted definition of the active and suppressed phases of the MJO was used for the eastern domain while the standard definition was used elsewhere. However, statistically significant differences in PFs are only present in the central domain during the mid–wet season months of January and February, and in the eastern domain during the first two months of the wet season (November and December), and are not found during any individual month in the western domain.

Fig. 16.
Fig. 16.

As in Fig. 14, but for daily occurrence of PF (number per day per square degree) during the period 1998–2013.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

c. Intraseasonal variability in the intensity of TRMM PFs

The spatial pattern of the proportion of warm PFs across northern Australia is depicted for the active and suppressed phases of the MJO during the wet season as a whole (Figs. 17a,b). Overall much higher proportions of warm PFs are found over the waters surrounding northern Australia during the suppressed phases compared to the active phases of the MJO, with the strongest modulation over the Coral Sea where proportions change from roughly 0.3–0.5 to 0.4–0.7. Little to no modulation of the proportion of warm PFs is found over much of the land areas of northern Australia, where warm PFs themselves are relatively rare (see Fig. 8); of the land areas, the strongest modulation is found over eastern Queensland. A similar pattern is found using the eastward-shifted definition of the active and suppressed phases of the MJO with the main difference being a slightly stronger modulation of the proportion of warm PFs over the Coral Sea (Figs. 17c,d).

Fig. 17.
Fig. 17.

As in Fig. 15, but for the proportion of warm PFs.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

The proportion of warm PFs and their contribution to the total rainfall was calculated over the wet season as a whole during all days and also the active and suppressed phases of the MJO (Table 3). The eastward-shifted definition of the active and suppressed phases of the MJO was used for the eastern domain while the standard definition was used elsewhere. The proportions of warm PFs are significantly higher (95% confidence limits) during the suppressed phases than the active phases in all domains except the western domain. This difference between the active and suppressed phases is greatest in the eastern domain, ranging from 13.7% to 20.3%, with a much smaller but still statistically significant difference in the central domain. The eastern domain is the only domain with a statistically significant difference in the rainfall contribution from warm PFs between the active and suppressed phases, though it only varies from 0.91% to 1.58%.

Table 3.

The proportion of and rainfall contribution (%) from both warm rain and thunderstorm (T-storm) PFs by region (in boldface) over the wet season as a whole during the active and suppressed phases of the MJO as well as during all days. The eastward-shifted active and suppressed phases of the MJO are used in the eastern domain, while the standard phases are used in the other domains. The upper () and lower () 95% confidence limits (CL) were calculated by a bootstrap algorithm and observations are over the period 1998–2013.

Table 3.

The seasonal evolution of this modulation of warm PFs by the MJO was also examined in each domain (Fig. 18). The strongest and most consistent seasonal evolution is found in the eastern domain, where the proportion of warm PFs is significantly higher during the suppressed phases in all months except February and March. In the central domain all months except March have significant differences between the active and suppressed phases, though the month of December has the opposite relationship to that found during the other months, with significantly higher proportions of warm PFs during the active phases of the MJO. This seasonal evolution is weakest in the western domain where only December has a significant modulation of warm PFs by the MJO, and similar to the central domain, this month has significantly higher proportions during the active phases.

Fig. 18.
Fig. 18.

As in Fig. 16, but for the proportion of warm PFs (%) during the period 1998–2013.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

The spatial pattern of the proportion of thunderstorm PFs across northern Australia is depicted for the active and suppressed phases of the MJO during the wet season as a whole (Figs. 19a,b). Overall higher proportions of thunderstorm PFs are found during the suppressed phases compared to the active phases of the MJO. The MJO’s modulation of thunderstorm PFs is concentrated over land, where thunderstorm PFs themselves are most common (see Fig. 8), with the only measurable impact over the waters surrounding northern Australia found just off the northwest coasts of the western and central domains and in the southern and eastern parts of the Gulf of Carpentaria. The modulation of thunderstorm PFs by the MJO is greatest in the westernmost part of the continent and becomes steadily weaker eastward into Queensland. A very similar pattern is found using the eastward-shifted definition of the active and suppressed phases of the MJO (Figs. 19c,d).

Fig. 19.
Fig. 19.

As in Fig. 15, but for the proportion of thunderstorm PFs.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

The proportion of thunderstorm PFs and their contribution to the total rainfall was calculated over the wet season as a whole during all days and also the active and suppressed phases of the MJO (Table 3). The eastward-shifted definition of the active and suppressed phases of the MJO was used for the eastern domain while the standard definition was used elsewhere. Over the wet season as a whole the proportion of thunderstorm PFs, as well as their contribution to the total rainfall, are significantly higher during the suppressed phases than the active phases in all domains except the eastern domain. The difference between the active and suppressed phases is slightly higher in the western domain than in the central domain, ranging from 6%–8.9% to 5.4%–7.1%, respectively. These two domains also have statistically significant differences in the contribution to the total rainfall made by thunderstorm PFs between the active and suppressed phases, ranging from 43%–63% to 42%–58% in the western and central domains, respectively.

The seasonal evolution of this modulation of thunderstorm PFs by the MJO was also examined in each domain (Fig. 20). The increase in the proportion of thunderstorm PFs during the suppressed phases is significant higher (95% confidence limits) than that of the active phases in December, February, and March in the western domain, and in December and February in the central domain, and only in February in the eastern domain. The opposite modulation of thunderstorm PFs by the MJO is found in November in both the central and eastern domains, which have significantly higher proportions of thunderstorm PFs during the active phases compared to the suppressed phases.

Fig. 20.
Fig. 20.

As in Fig. 16, but for the proportion of thunderstorm PFs (%) during the period 1998–2013.

Citation: Monthly Weather Review 144, 12; 10.1175/MWR-D-16-0133.1

In summary, strong regional differences are found in the MJO’s modulation of TRMM PFs during the suppressed phases. There is strong modulation of the number of PFs by the MJO in the western domain with many more PFs during the suppressed phases, and these PFs are also more intense, with higher proportions of thunderstorms. In the eastern domain there is little modulation of the number of PFs by the MJO, though PFs are much less intense during the suppressed phases, with higher proportions of warm PFs. In between these two extremes is the central domain, which has more PFs and higher proportions of thunderstorms during the suppressed phases but also has higher proportions of warm PFs, though the increases are less than that found in the western and eastern domains, respectively.

5. Discussion

The wet season of northern Australia has a pronounced seasonal evolution over its main months (November–March). The atmosphere over the region becomes increasingly more moist as the wet season progresses from the late premonsoon month of November into the core monsoon months of January and February, and both rainfall and the number of TRMM precipitation features (PFs) steadily increases, as the intensity of these PFs decreases. While atmospheric moisture and rainfall decrease into March, the final month of the wet season, the number of PFs decreases little as smaller PFs associated solely with the warm rain precipitation processes increase in number. Thunderstorms have the opposite seasonal evolution to these warm PFs, they make up the highest proportion of PFs during the early part of the wet season with their proportion steadily decreasing into March. The amplitude of this seasonal evolution in PF intensity is strongest in the western part of northern Australia and decreases eastward into northern Queensland.

These observations of the seasonal evolution of PFs are consistent with previous studies in northern Australia. The premonsoon has been characterized by elevated convective available potential energy and convective inhibition (Hendon et al. 1989), and more intense convection and lightning have been observed over the region surrounding Darwin during this period compared to the active monsoon (Keenan and Carbone 1992). The significant peak in the proportion of thunderstorms found during the early months of the wet season is in agreement with this previous research and extends this result to northern Australia as a whole. The seasonal peak in the proportion of warm PFs found during March is most likely related to the retreat of the monsoon trough from northern Australia, leading to a return to southeasterly trade wind flow with diminished CAPE but enhanced moisture related to the monsoon still present over the region (e.g., Manton and Bonell 1993).

Regional variability is also found in the seasonal evolution of the wet season, with clear differences distinguishing northern Queensland (eastern domain) from the rest of northern Australia. Northern Western Australia (western domain) and northern Northern Territory (central domain) have very similar seasonal evolutions but with a slightly stronger amplitude in the latter. In these regions December serves as a transition from the premonsoon into the core of the monsoon, with steadily increasing moisture and rainfall, as well as more, but less intense, PFs. However, in the eastern domain there is an extension of conditions associated with the premonsoon from November into December (i.e., lower rainfall, less PFs overall, and a high proportion of thunderstorms). While this lengthening of the premonsoon and accompanying abrupt change into the core of the monsoon found in the eastern domain could be related to examining months rather than individual weeks, it still suggests that the effects of the monsoon are not felt in the eastern domain until well after monsoon onset has occurred over the rest of northern Australia. This delay in the onset of the monsoon over the eastern domain is likely a consequence of the monsoonal westerlies being offset by the prevailing trade winds off the coast of northern Queensland, which persist later into the wet season due to a lack of the continental monsoon effect over the Coral Sea and Pacific Ocean. This leads to reduced exposure of the eastern domain to monsoonal moisture until January, when the monsoon westerlies become more firmly established over northern Australia.

The intensity of convective systems over northern Australia also has strong regional variations. Precipitation features are most intense in the western domain, where the proportion of thunderstorms and their rainfall contribution are highest, steadily decreasing in intensity eastward. These thunderstorms also tend to increase in area eastward, suggesting a more isolated nature in the west and a closer association with organized convective systems farther east. The higher proportion of thunderstorm PFs over the western domain is most likely related to this domain’s great distance from the low-level moisture sources for the easterly trade winds, the Coral Sea, and Gulf of Carpentaria, leading to a more continental nature of convection characterized by deeper convection and lightning (Xu and Zipser 2012) when the trade winds are blowing. This argument is supported by thunderstorms being most common over the western and central domains during the periods when the easterly trade winds dominate: the premonsoon and break periods during the monsoon, which are closely linked with the suppressed phases of the Madden–Julian oscillation (MJO; Keenan and Carbone 1992; May and Ballinger 2007; Xu and Zipser 2012).

Much less intense PFs are found over the eastern domain, characterized by lower echo tops, warmer cloud-top temperatures, and lower lightning flash rates than in the rest of northern Australia. In contrast to the western domain, the easterly trade winds advect copious low-level moisture from the Coral Sea into the eastern domain, leading to a more maritime nature of convection characterized by the shallow warm clouds typical of the oceanic trade winds. This results in the much higher proportion of warm PFs observed in the eastern domain over the wet season as a whole, as well as the much weaker amplitude of the seasonal cycle of the proportion of warm PFs compare to the rest of northern Australia. This regional difference in warm PFs is most pronounced during November, when warm PFs are extremely uncommon in the western domain but have the second highest proportion of any month in the eastern domain. Thunderstorms are still most common during the premonsoon period, November and December in the eastern domain, despite the more maritime nature of the trade winds, though they never reach the proportions or rainfall contributions found farther west.

The MJO was found to strongly modulate the low-level winds, moisture, and rainfall across northern Australia, in general agreement with previous studies in this region (e.g., Wheeler et al. 2009; Evans et al. 2014). The active phases of the MJO are generally associated with westerly low-level winds and increased moisture and rainfall, while the suppressed phases of the MJO are associated with easterly low-level winds and decreased moisture and rainfall over northern Australia. The importance of the propagation of the MJO over northern Australia was demonstrated by the eastward-shifted active and suppressed phases of the MJO having a much stronger impact on the eastern domain than the standard active and suppressed phases, which impacted the other domains the strongest.

The MJO’s effect on PFs provides insight into the mechanisms by which the MJO modulates rainfall during the wet season and its regional variation. Statistically significant increases in the number of PFs are found in the active phases compared to the suppressed phases in all of the three domains examined. However, the results for the three domains are averages over land areas; the increase in the number of PFs during the active phases of the MJO is much weaker in the waters surrounding northern Australia. In these maritime areas the modulation of PFs by the MJO results in increases in the number of warm PFs during the suppressed phases; in the most extreme case of the Coral Sea and the adjacent eastern coast of Queensland this leads to the overall number of PFs being nearly unchanged by the MJO. Over the continent the modulation of the intensity of PFs by the MJO is the opposite and results in increases in the number of thunderstorm PFs during the suppressed phases, with the strongest effect in the western domain.

The stark contrast in the MJO’s modulation of the intensity of PFs found over the study region is likely rooted in the MJO’s modulation of the low-level winds and the way they interact with the geography of northern Australia. The easterly low-level wind anomalies associated with the suppressed phases of the MJO would lead to typical maritime trade wind conditions over the waters surrounding northern Australia, particularly the Coral Sea and the adjacent eastern coast of Queensland, which are more exposed to easterly winds and any moisture they transport. In the more downstream and interior parts of the continent, these easterly winds would be much drier due to their distance from the moisture source of the Coral Sea, and lead to a continental air mass more conducive to thunderstorms than shallow trade wind cumulus clouds. This effect is illustrated by the differences between the western and eastern domains, where the suppressed phases of the MJO are associated with significant increases solely in the proportion of thunderstorm PFs and warm PFs, respectively. The central domain rests between these two extremes, both geographically and in terms of the MJO’s modulation of PFs, with significant increases in both warm and thunderstorm PFs during the suppressed phases of the MJO.

The lack of a strong increasing trend in summer rainfall in the eastern domain, like that found in the northwestern part of northern Australia (Smith 2004; Taschetto and England 2009), is potentially explained by the results presented in this study. The increased frequency of weather regimes associated with the active monsoon at Darwin found by Catto et al. (2012) could have less of an impact on rainfall in the eastern domain for two reasons: 1) the MJO’s different modulation of the intensity of convection in this domain, and 2) the eastern domain’s reduced exposure to the moisture transported in the monsoonal westerlies compared to the rest of northern Australia. The increased length of the wet season at Darwin found by Catto et al. (2012) was associated with a change in flow conditions from southeasterly to more moist easterly and northeasterly flow regimes during the early and late parts of the wet season. While relatively small changes to the direction of the trade wind flow could have a large impact on the advection of moisture in the central and western domains, this effect would likely be reduced in the eastern domain due to its much greater exposure to the moisture transported in the typical southeasterly trade wind flow.

6. Summary and conclusions

Based on the analyses presented the following conclusions are made on the wet season (November–March) in the western, central, and eastern domains of northern Australia over the period 1979–2013.

a. Seasonal evolution

  1. There is a steady increase in total column water vapor, rainfall, and the number of TRMM precipitation features (PFs) from the premonsoonal month of November into the core months of the monsoon, January and February.
  2. This seasonal evolution reverses in March, when the monsoon has ended, with the exception of PFs, whose overall numbers differ little but become dominated by small PFs associated solely with the warm rain precipitation process (warm PFs).
  3. Very similar seasonal evolutions are found in the western and central domains, with a slightly higher amplitude in the central domain.
  4. The seasonal evolution in the eastern domain is characterized by a delay in the onset of the monsoon compared to the rest of northern Australia. This is manifested by an extension of the premonsoon into December, with an abrupt change into the wetter conditions found in January.
  5. Thunderstorms make up the highest proportion of PFs and contribute the most rainfall during the beginning of the wet season, with steadily decreasing values into March.
  6. Warm PFs steadily increase in proportion and contribution to rainfall into March.
  7. Regionally, PFs are less intense over the eastern domain than the rest of northern Australia with lower echo tops, warmer cloud-top temperatures, and the lowest proportion of rainfall contributed by thunderstorms with multiple lightning flashes.
  8. Thunderstorms make up a significantly higher proportion of PFs in the western domain decreasing eastward. The area of these thunderstorms is smallest in the western domain, increasing eastward.
  9. Warm PFs make up a significantly higher proportion of PFs and contribute more rainfall over the eastern domain in every month of the wet season, decreasing westward.

b. Intraseasonal variability

  1. The active phases of the Madden–Julian oscillation (MJO) over northern Australia are associated with westerly low-level winds, increased atmospheric water vapor, and increased daily rainfall; while the suppressed phases of the MJO are associated with easterly low-level winds, decreased atmospheric water vapor, and decreased daily rainfall.
  2. The number of PFs during the wet season as a whole significantly increase during the active phases and decrease during the suppressed phases of the MJO across northern Australia.
  3. The proportion and contribution to the total rainfall from warm rain PFs significantly increases during the suppressed phases of the MJO in the eastern domain over the wet season. This relationship weakens westward into the western domain, where the increase becomes insignificant.
  4. The proportion and contribution to the total rainfall from thunderstorm PFs significantly increases during the suppressed phases of the MJO in the western domain over the wet season. This relationship weakens eastward into the eastern domain, where the increase becomes insignificant over the wet season as a whole.

The results of this study highlight the strong regional variability in rainfall and associated convection across northern Australia during the wet season. The strong regional variability of the impact of the MJO found over northern Australia has implications for the Maritime Continent, a region of particular importance as it is thought to have a large impact on global climate and global climate models are known to perform particularly poorly there (e.g., Ramage 1968; Neale and Slingo 2003). The Maritime Continent is also strongly affected by the MJO (e.g., Zhang 2005; Hidayat and Kizu 2010) and its geography, comprising an intricate arrangement of islands with relatively high mountain ranges, is much more complex than that of northern Australia and would likely lend itself to strong regional variability in the impact of the MJO.

Acknowledgments

This work was supported by a Monash Graduate Scholarship. The authors thank Matthew C. Wheeler, Gregory J. Connor, and Bhupendra A. Raut for many useful discussions and also for their reviews of early versions of this manuscript. We are also thankful to three anonymous reviewers whose input led to an improved final manuscript.

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