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Li Shi
,
Oscar Alves
,
Harry H. Hendon
,
Guomin Wang
, and
David Anderson

Abstract

The impact of stochastic intraseasonal variability on the onset of the 1997/98 El Niño was examined using a large ensemble of forecasts starting on 1 December 1996, produced using the Australian Bureau of Meteorology Predictive Ocean Atmosphere Model for Australia (POAMA) seasonal forecast coupled model. This coupled model has a reasonable simulation of El Niño and the Madden–Julian oscillation, so it provides an ideal framework for investigating the interaction between the MJO and El Niño. The experiment was designed so that the ensemble spread was simply a result of internal stochastic variability that is generated during the forecast. For the initial conditions used here, all forecasts led to warm El Niño–type conditions with the amplitude of the warming varying from 0.5° to 2.7°C in the Niño-3.4 region.

All forecasts developed an MJO event during the first 4 months, indicating that perhaps the background state favored MJO development. However, the details of the MJOs that developed during December 1996–March 1997 had a significant impact on the subsequent strength of the El Niño event. In particular, the forecasts with the initial MJOs that extended farther into the central Pacific, on average, led to a stronger El Niño, with the westerly winds in the western Pacific associated with the MJO leading the development of SST and thermocline anomalies in the central and eastern Pacific. These results imply a limit to the accuracy with which the strength of El Niño can be predicted because the details of individual MJO events matter. To represent realistic uncertainty, coupled models should be able to represent the MJO, including its propagation into the central Pacific so that forecasts produce sufficient ensemble spread.

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Wenju Cai
,
Peter van Rensch
,
Tim Cowan
, and
Harry H. Hendon

Abstract

Impacts of El Niño–Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD) on Australian rainfall are diagnosed from the perspective of tropical and extratropical teleconnections triggered by tropical sea surface temperature (SST) variations. The tropical teleconnection is understood as the equatorially trapped, deep baroclinic response to the diabatic (convective) heating anomalies induced by the tropical SST anomalies. These diabatic heating anomalies also excite equivalent barotropic Rossby wave trains that propagate into the extratropics. The main direct tropical teleconnection during ENSO is the Southern Oscillation (SO), whose impact on Australian rainfall is argued to be mainly confined to near-tropical portions of eastern Australia. Rainfall is suppressed during El Niño because near-tropical eastern Australia directly experiences subsidence and higher surface pressure associated with the western pole of the SO. Impacts on extratropical Australian rainfall during El Niño are argued to stem primarily from the Rossby wave trains forced by convective variations in the Indian Ocean, for which the IOD is a primary source of variability. These equivalent-barotropic Rossby wave trains emanating from the Indian Ocean induce changes to the midlatitude westerlies across southern Australia, thereby affecting rainfall through changes in mean-state baroclinicity, west–east steering, and possibly orographic effects. Although the IOD does not mature until austral spring, its impact on Australian rainfall during winter is also ascribed to this mechanism. Because ENSO is largely unrelated to the IOD during austral winter, there is limited impact of ENSO on rainfall across southern latitudes of Australia in winter. A strong impact of ENSO on southern Australia rainfall in spring is ascribed to the strong covariation of ENSO and the IOD in this season. Implications of this pathway from the tropical Indian Ocean for impacts of both the IOD and ENSO on southern Australian climate are discussed with regard to the ability to make seasonal climate predictions and with regard to the role of trends in tropical SST for driving trends in Australian climate.

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Harry H. Hendon
,
Eun-Pa Lim
, and
Hanh Nguyen

Abstract

Seasonal variations of subtropical precipitation anomalies associated with the southern annular mode (SAM) are explored for the period 1979–2011. In all seasons, high-polarity SAM, which refers to a poleward-shifted eddy-driven westerly jet, results in increased precipitation in high latitudes and decreased precipitation in midlatitudes as a result of the concomitant poleward shift of the midlatitude storm track. In addition, during spring–autumn, high SAM also results in increased rainfall in the subtropics. This subtropical precipitation anomaly is absent during winter. This seasonal variation of the response of subtropical precipitation to the SAM is shown to be consistent with the seasonal variation of the eddy-induced divergent meridional circulation in the subtropics (strong in summer and weak in winter). The lack of an induced divergent meridional circulation in the subtropics during winter is attributed to the presence of the wintertime subtropical jet, which causes a broad latitudinal span of eddy momentum flux divergence due primarily to higher phase speed eddies breaking poleward of the subtropical jet and lower speed eddies not breaking until they reach the equatorward flank of the subtropical jet. During the other seasons, when the subtropical jet is less distinctive, the critical line for both high and low speed eddies is on the equatorward flank of the single jet and so breaking in the subtropics occurs over a narrow range of latitudes. The implications of these findings for the seasonality of future subtropical climate change, in which a shift to high SAM in all seasons is expected to be promoted, are discussed.

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Eun-Pa Lim
,
Harry H. Hendon
, and
Harun Rashid

Abstract

Predictability of the southern annular mode (SAM) for lead times beyond 1–2 weeks has traditionally been considered to be low because the SAM is regarded as an internal mode of variability with a typical decorrelation time of about 10 days. However, the association of the SAM with El Niño–Southern Oscillation (ENSO) suggests the potential for making seasonal predictions of the SAM. In this study the authors explore seasonal predictability and the predictive skill of SAM using observations and retrospective forecasts (hindcasts) from the Australian Bureau of Meteorology dynamical seasonal forecast system [the Predictive Ocean and Atmosphere Model for Australia, version 2 (POAMA2)].

Based on the observed seasonal relationships of the SAM with tropical sea surface temperatures, two distinctive periods of high seasonal predictability are suggested: austral late autumn to winter and late spring to early summer. Predictability of the SAM in the austral cold seasons stems from the association of the SAM with warm-pool (or Modoki/central Pacific) ENSO, whereas predictability in the austral warm seasons stems from the association of the SAM with cold-tongue (or eastern Pacific) ENSO.

Using seasonal hindcasts for 1980–2010 from POAMA2, it is shown that the observed relationship between SAM and ENSO is faithfully depicted and SST variations associated with ENSO are skillfully predicted. Consequently, POAMA2 can skillfully predict the phase and amplitude of seasonal anomalies of the SAM in early summer and early winter for at least one season in advance. Zero-lead monthly forecasts of the SAM are furthermore shown to be highly skillful in almost all months, which is ascribed to predictability stemming from observed atmospheric initial conditions.

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Harry H. Hendon
,
Eun-Pa Lim
, and
Guo Liu

Abstract

Forecast skill for seasonal mean rainfall across northern Australia is lower during the summer monsoon than in the premonsoon transition season based on 25 years of hindcasts using the Predictive Ocean Atmosphere Model for Australia (POAMA) coupled model seasonal forecast system. The authors argue that this partly reflects an intrinsic property of the monsoonal system, whereby seasonally varying air–sea interaction in the seas around northern Australia promotes predictability in the premonsoon season and demotes predictability after monsoon onset. Trade easterlies during the premonsoon season support a positive feedback between surface winds, SST, and rainfall, which results in stronger and more persistent SST anomalies to the north of Australia that compliment the remote forcing of Australian rainfall from El Niño in the Pacific. After onset of the Australian summer monsoon, this local feedback is not supported in the monsoonal westerly regime, resulting in weaker SST anomalies to the north of Australia and with lower persistence than in the premonsoon season. Importantly, the seasonality of this air–sea interaction is captured in the POAMA forecast model. Furthermore, analysis of perfect model forecasts and forecasts generated by prescribing observed SST results in largely the same conclusion (i.e., significantly lower actual and potential forecast skill during the monsoon), thereby supporting the notion that air–sea interaction contributes to intrinsically lower predictability of rainfall during the monsoon.

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Murry L. Salby
,
Harry H. Hendon
,
Karen Woodberry
, and
Ken Tanaka

Synoptic images of the global cloud field have been created from infrared measurements taken aboard four geostationary and two polar-orbiting platforms simultaneously observing the earth. A series of spatial and temporal interpolations together with data reliability criteria are used to composite data from the individual satellites into synoptic images of the global cloud pattern. The composite Global Cloud Imagery (GCI) have a horizontal resolution of about half a degree and a temporal resolution of 3 h, providing an unprecedented view of the earth's cloud field. Each composite image represents a nearly instantaneous snapshot of the global cloud pattern. Collectively, the composite imagery resolve, on a global basis, most of the variability associated with organized convection, including several harmonics of the diurnal cycle.

The dense and 3-dimensional nature of the GCI make them a formidable volume of information to treat in a practical and efficient manner. To facilitate analysis of global cloud behavior, the GCI has been constructed with certain homogeneous properties. In addition to synoptic coverage of the globe, data are spaced uniformly in longitude, latitude, and time, and contain no data voids. An interactive Image Analysis System (IAS) has been developed to investigate the space-time behavior of global cloud activity. In the IAS, data, hardware, and software are integrated into a single system capable of providing a variety of space-time covariance analyses. Because of its customized architecture and the homogeneous properties of the GCI, the IAS can perform such analyses on the 3-dimensional data with interactive speed. Statistical properties of cloud variability are presented along with other preliminary results derived from the GCI.

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Toshiaki Shinoda
,
Michael A. Alexander
, and
Harry H. Hendon

Abstract

Remote forcing of sea surface temperature (SST) variations in the Indian Ocean during the course of El Niño–Southern Oscillation (ENSO) events is investigated using NCEP reanalysis and general circulation model (GCM) experiments. Three experiments are conducted to elucidate how SST variations in the equatorial Pacific influence surface flux variations, and hence SST variations, across the Indian Ocean. A control experiment is conducted by prescribing observed SSTs globally for the period 1950–99. In the second experiment, observed SSTs are prescribed only in the tropical eastern Pacific, while climatological SSTs are used elsewhere over the global oceans. In the third experiment, observed SSTs are prescribed in the tropical eastern Pacific, while a variable-depth ocean mixed layer model is used at all other ocean grid points to predict the SST.

Composites of surface fluxes and SST over the Indian Ocean are formed based on El Niño and La Niña events during 1950–99. The surface flux variations in the eastern Indian Ocean in all three experiments are similar and realistic, confirming that much of the surface flux variation during ENSO is remotely forced from the Pacific. Furthermore, the SST anomalies in the eastern tropical Indian Ocean are well simulated by the coupled model, which supports the notion of an “atmospheric bridge” from the Pacific. During boreal summer and fall, when climatological winds are southeasterly over the eastern Indian Ocean, remotely forced anomalous easterlies act to increase the local wind speed. SST cools in response to increased evaporative cooling, which is partially offset by increased solar radiation associated with reduced rainfall. During winter, the climatological winds become northwesterly and the anomalous easterlies then act to reduce the wind speed and evaporative cooling. Together with increased solar radiation and a shoaling mixed layer, the SST warms rapidly. The model is less successful at reproducing the ENSO-induced SST anomalies in the western Indian Ocean, suggesting that dynamical ocean processes contribute to the east–west SST dipole that is often observed in boreal fall during ENSO events.

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John W. Bergman
,
Harry H. Hendon
, and
Klaus M. Weickmann

Abstract

The role of the Madden–Julian Oscillation (MJO) for the onset of El Niño is examined. A preliminary analysis compares tropical Pacific variability during three boreal winters that initially had similar distributions of sea surface temperature (SST). During the winter of 1996–97, strong MJO activity led to west Pacific cooling and central Pacific warming. Subsequently, convective activity migrated from the west Pacific into the central Pacific and the accompanying westerly surface wind anomalies promoted further central Pacific warming. Strong MJO activity was also evident during winter 1989–90 and the early stages of El Niño development were evident that winter with similar evolution to that during 1996–97. However, the development of El Niño was aborted in May 1990. It is speculated that a full El Niño did not develop during 1990 because the subsurface ocean structure would not support that development. The MJO was relatively quiescent during the winter of 1981–82. A strong El Niño developed during 1982, but not as rapidly as it did during 1997. Thus, MJO might be relevant to the timing and initial growth of El Niño rather than responsible for the event itself.

A detailed analysis of dynamical interactions is performed for the winter of 1996–97, when two exceptionally strong MJOs accompanied substantial SST fluctuations in the Pacific. SST cooling in the west Pacific was, for the most part, forced by surface flux variations. Surface cooling was initiated by the reduction of short-wave surface fluxes due to enhanced cloud cover. Later, evaporative cooling during westerly wind anomalies reinforced that cooling. In February 1997, ocean dynamics were also important for the SST perturbation; off-equatorial upwelling, through an anomalously large vertical temperature gradient, contributed substantially to west Pacific cooling.

During late March and early April 1997, central Pacific SSTs warmed in response to a downwelling Kelvin wave that was forced during the February MJO. That warming was primarily due to zonal temperature advection, promoted by strong eastward currents acting on an east–west temperature gradient. After the passage of the Kelvin wave, zonal currents, surface winds, and SST gradients did not revert to their pre-Kelvin wave values. As a result, temperature advection was negligible after the Kelvin wave, and SST continued to warm due to the positive surface heat flux that is typical for the region. So, if the MJO did contribute to this important SST warming through nonlinear interactions, then those interactions involve the coupling of atmospheric and oceanic dynamics.

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Harry H. Hendon
,
David W. J. Thompson
, and
Matthew C. Wheeler

Abstract

Daily variations in Australian rainfall and surface temperature associated with the Southern Hemisphere annular mode (SAM) are documented using observations for the period 1979–2005. The high index polarity of the SAM is characterized by a poleward contraction of the midlatitude westerlies. During winter, the high index polarity of the SAM is associated with decreased daily rainfall over southeast and southwest Australia, but during summer it is associated with increased daily rainfall on the southern east coast of Australia and decreased rainfall in western Tasmania. Variations in the SAM explain up to ∼15% of the weekly rainfall variance in these regions, which is comparable to the variance accounted for by the El Niño–Southern Oscillation, especially during winter. The most widespread temperature anomalies associated with the SAM occur during the spring and summer seasons, when the high index polarity of the SAM is associated with anomalously low maximum temperature over most of central/eastern subtropical Australia. The regions of decreased maximum temperature are also associated with increased rainfall. Implications for recent trends in Australian rainfall and temperature are discussed.

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Mei Zhao
,
Harry H. Hendon
,
Oscar Alves
,
Yonghong Yin
, and
David Anderson

Abstract

The authors assess the sensitivity of the simulated mean state and coupled variability to systematic initial state salinity errors in seasonal forecasts using the Australian Bureau of Meteorology Predictive Ocean Atmosphere Model for Australia (POAMA) coupled model. This analysis is based on two sets of hindcasts that were initialized from old and new ocean initial conditions, respectively. The new ocean initial conditions are provided by an ensemble multivariate analysis system that assimilates subsurface temperatures and salinity and is a clear improvement over the previous system, which was based on univariate optimal interpolation, using static error covariances and assimilating only temperature without updating salinity.

Large systematic errors in the salinity field around the thermocline region of the tropical western and central Pacific produced by the old assimilation scheme are shown to have strong impacts on the predicted mean state and variability in the tropical Pacific for the entire 9 months of the forecast. Forecasts initialized from the old scheme undergo a rapid and systematic adjustment of density that causes large persistent changes in temperature both locally in the western and central Pacific thermocline, but also remotely in the eastern Pacific via excitation of equatorial waves. The initial subsurface salinity errors in the western and central Pacific ultimately result in an altered surface climate because of induced temperature changes in the thermocline that trigger a coupled feedback in the eastern Pacific. These results highlight the importance of accurately representing salinity in initial conditions for climate prediction on seasonal and potentially multiyear time scales.

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