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M. Issa Lélé
,
Lance M. Leslie
, and
Peter J. Lamb

Abstract

The major objective of this study is to re-evaluate the ocean–land transport of moisture for rainfall in West Africa using 1979–2008 NCEP–NCAR reanalysis data. The vertically integrated atmospheric water vapor flux for the surface–850 hPa is calculated to account for total low-level moisture flux contribution to rainfall over West Africa. Analysis of mean monthly total vapor fluxes shows a progressive penetration of the flux into West Africa from the south and west. During spring (April–June), the northward flux forms a “moisture river” transporting moisture current into the Gulf of Guinea coast. In the peak monsoon season (July–September), the southerly transport weakens, but westerly transport is enhanced and extends to 20°N owing to the strengthening West African jet off the west coast. Mean seasonal values of total water vapor flux components across boundaries indicate that the zonal component is the largest contributor to mean moisture transport into the Sahel, while the meridional transport contributes the most over the Guinea coast. For the wet years of the Sahel rainy season (July–September), active anomalies are displaced farther north compared to the long-term average. This includes the latitude of the intertropical front (ITF), the extent of moisture flux, and the zone of strong moisture flux convergence, with an enhanced westerly flow. For the dry Sahel years, the opposite patterns are observed. Statistically significant positive correlations between the zonal moisture fluxes and Sudan–Sahel rainfall totals are most pronounced when the zonal fluxes lead by 1–4 pentads. However, although weak, they still are statistically significant at lags 3 and 4 for meridional moisture fluxes.

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Irenea L. Corporal-Lodangco
,
Lance M. Leslie
, and
Peter J. Lamb

Abstract

This study investigates the El Niño–Southern Oscillation (ENSO) contribution to Philippine tropical cyclone (TC) variability, for a range of quarterly TC metrics. Philippine TC activity is found to depend on both ENSO quarter and phase. TC counts during El Niño phases differ significantly from neutral phases in all quarters, whereas neutral and La Niña phases differ only in January–March and July–September. Differences in landfalls between neutral and El Niño phases are significant in January–March and October–December and in January–March for neutral and La Niña phases. El Niño and La Niña landfalls are significantly different in April–June and October–December. Philippine neutral and El Niño TC genesis cover broader longitude–latitude ranges with similar long tracks, originating farther east in the western North Pacific. In El Niño phases, the mean eastward displacement of genesis locations and more recurving TCs reduce Philippine TC frequencies. Proximity of La Niña TC genesis to the Philippines and straight-moving tracks in April–June and October–December increase TC frequencies and landfalls. Neutral and El Niño accumulated cyclone energy (ACE) values are above average, except in April–June of El Niño phases. Above-average quarterly ACE in neutral years is due to increased TC frequencies, days, and intensities, whereas above-average El Niño ACE in July–September is due to increased TC days and intensities. Below-average La Niña ACE results from fewer TCs and shorter life cycles. Longer TC durations produce slightly above-average TC days in July–September El Niño phases. Fewer TCs than neutral years, as well as shorter TC durations, imply less TC days in La Niña phases. However, above-average TC days occur in October–December as a result of higher TC frequencies.

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Hamish A. Ramsay
,
Michael B. Richman
, and
Lance M. Leslie

Abstract

This study examines combining ENSO sea surface temperature (SST) regions for seasonal prediction of Coral Sea tropical cyclone (TC) frequency. The Coral Sea averages ~4 TCs per season, but is characterized by strong interannual variability, with 1–9 TCs per season, over the period 1977–2012. A wavelet analysis confirms that ENSO is a key contributor to Coral Sea TC count (TCC) variability. Motivated by the impact of El Niño Modoki on regional climate anomalies, a suite of 38 linear models is constructed and assessed on its ability to predict Coral Sea seasonal TCC. Seasonal predictions of TCC are generated by a leave-one-out cross validation (LOOCV). An important finding is that models made up of multiple tropical Pacific SST regions, such as those that comprise the El Niño Modoki Index (EMI) or the Trans-Niño Index (TNI), perform considerably better than models comprising only single regions, such as Niño-3.4 or Niño-4. Moreover, enhanced (suppressed) TC activity is expected in the Coral Sea when the central Pacific is anomalously cool (warm) and the eastern and western Pacific are anomalously warm (cool) during austral winter. The best cross-validated model has persistent and statistically significantly high correlations with TCC (r > 0.5) at lead times of ~6 months prior to the mean onset of the Coral Sea TC season, whereas correlations based heavily on the widely used Niño-3.4 region are not statistically significant or meaningful (r = 0.09) for the same lead times. Of the 38 models assessed, several optimized forms of the EMI and of the TNI perform best.

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Diandong Ren
,
Lance M. Leslie
, and
Mervyn J. Lynch

Abstract

Changes in storm-triggered landslide activity for Southern California in a future warming climate are estimated using an advanced, fully three-dimensional, process-based landslide model, the Scalable and Extensible Geofluid Modeling System for landslides (SEGMENT-Landslide). SEGMENT-Landslide is driven by extreme rainfall projections from the Geophysical Fluid Dynamics Laboratory High Resolution Atmospheric Model (GFDL-HIRAM). Landslide changes are derived from GFDL-HIRAM forcing for two periods: 1) the twentieth century (CNTRL) and 2) the twenty-first century under the moderate Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios A1B enhanced greenhouse gas emissions scenario (EGHG). Here, differences are calculated in landslide frequency and magnitude between the CNTRL and EGHG projections; kernel density estimation (KDE) is used to determine differences in projected landslide locations. This study also reveals that extreme precipitation events in Southern California are strongly correlated with several climate drivers and that GFDL-HIRAM simulates well the southern (relative to Aleutian synoptic systems) storm tracks in El Niño years and the rare (~27-yr recurrence period) hurricane-landfalling events. GFDL-HIRAM therefore can provide satisfactory projections of the geographical distribution, seasonal cycle, and interannual variability of future extreme precipitation events (>50 mm) that have possible landslide consequences for Southern California. Although relatively infrequent, extreme precipitation events contribute most of the annual total precipitation in Southern California. Two findings of this study have major implications for Southern California. First is a possible increase in landslide frequency and areal distribution during the twenty-first century. Second, the KDE reveals three clusters in both the CNTRL and EGHG model mean scarp positions, with a future eastward (inland) shift of ~0.5° and a northward shift of ~1°. These findings suggest that previously stable areas might become susceptible to storm-triggered landslides in the twenty-first century.

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Esther D. Mullens
,
Lance M. Leslie
, and
Peter J. Lamb

Abstract

Ice storms are an infrequent but significant hazard in the U.S southern Great Plains. Common synoptic profiles for freezing precipitation reveal advection of low-level warm moist air from the Gulf of Mexico (GOM), above a shallow Arctic air mass ahead of a midlevel trough. Because the GOM is the proximal basin and major moisture source, this study investigates impacts of varying GOM sea surface temperature (SST) on the thermodynamic evolution of a winter storm that occurred during 28–30 January 2010, with particular emphasis on the modulation of freezing precipitation. A high-resolution, nested ARW sensitivity study with a 3.3-km inner domain is performed, using six representations of GOM SST, including control, climatological mean, uniform ±2°C from control, and physically constrained upper- and lower-bound basin-average anomalies from a 30-yr dataset. The simulations reveal discernable impacts of SST on the warm-layer inversion, precipitation intensity, and low-level dynamics. Whereas total precipitation for the storm increased monotonically with SST, the freezing-precipitation response was more varied and nonlinear, with the greatest accumulation decreases occurring for the coolest SST perturbation, particularly at moderate precipitation rates. Enhanced precipitation and warm-layer intensity promoted by warmer SST were offset for the highest perturbations by deepening of the weak 850-hPa low circulation and faster eastward progression associated with enhanced baroclinicity and diabatic generation of potential vorticity. Air-parcel trajectories terminating within the freezing-precipitation region were examined to identify airmass sources and modification. These results suggest that GOM SST can affect the severity of concurrent ice-storm events in the southern Great Plains, with warmer basin SST potentially exacerbating the risk of damaging ice accumulations.

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Diandong Ren
,
Rong Fu
,
Lance M. Leslie
, and
Robert E. Dickinson

Abstract

An overview of storm-triggered landslides is presented. Then a recently developed and extensively verified landslide modeling system is used to illustrate the importance of two important but presently overlooked mechanisms involved in landslides. The model's adaptive design makes the incorporation of new physical mechanisms convenient. For example, by implementing a land surface scheme that simulates macropore features of fractured sliding material in the draining of surface ponding, it explains why precipitation intensity is critical in triggering catastrophic landslides.

Based on this model, the authors made projections of landslide occurrence in the upcoming 10 years over a region of Southern California, using atmospheric parameters provided by a highresolution climate model under a viable emission future scenario. Current global coupled ocean–atmosphere climate model (CGCM) simulations of precipitation, properly interpreted, provide valuable information to guide studies of storm-triggered landslides. For the area of interest, the authors examine changes in recurrence frequency and spatial distribution of storm-triggered landslides. For some locations, the occurrences of severe landslides (i.e., those with a sliding mass greater than 104 m3) are expected to increase by ~5% by the end of the twenty-first century.

The authors also provide a perspective on the ecosystem consequences of an increase in storm-triggered mudslides. For single plants, the morphological features required for defense against extreme events and those required to maximize growth and reproduction are at odds. Natural selection has resulted in existing plants allocating just enough resources to cope with natural hazards under a naturally varying climate. Consequently, many plant species are not prepared for the expected large changes in extremes caused by anthropogenic climate changes in the present and future centuries.

A supplement to this article is available online:

DOI: 10.1175/2010BAMS3017.2

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Bruce W. Buckley
,
Lance M. Leslie
, and
Milton S. Speer

Abstract

The recorded climatology of tropical cyclones that affect the Tasman Sea spans the period from 1911 to the present. This climatology is a subset of the much larger Australian Tropical Cyclone database, which is the official record of all tropical cyclones in the Australian area of responsibility. Such a long, detailed record should provide an excellent dataset for regional climate research. However, a detailed analysis of the database has revealed that it must be used with caution over the Tasman Sea, where statistically significant discontinuities are present, greatly reducing its quality and length for climate and climate change studies. Problems with the complete Australian Tropical Cyclone database have been identified and discussed earlier by a number of authors. This study is concerned with two statistically significant discontinuities that occurred in the Tasman Sea portion of the database in the mid-1950s and in 1977. The first discontinuity almost trebled the recorded frequency of tropical cyclones, whereas the second discontinuity exhibited an opposite trend, decreasing the recorded frequency of tropical cyclones by a factor of 8 from the previous period. Some possible explanations for the abrupt changes in this subset of one particular database are discussed. It is suggested here that the most likely explanation is the improved observing technology and the associated changes in interpretation of the new data. Finally, it is likely that other climate databases have been affected by similar problems and should be treated with the same degree of caution.

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Zewdu T. Segele
,
Peter J. Lamb
, and
Lance M. Leslie

Abstract

Horn of Africa rainfall varies on multiple time scales, but the underlying climate system controls on this variability have not been examined comprehensively. This study therefore investigates the linkages between June–September Horn of Africa (especially Ethiopian) rainfall and regional atmospheric circulation and global sea surface temperature (SST) variations on several key time scales. Wavelet analysis of 5-day average or monthly total rainfall for 1970–99 identifies the dominant coherent modes of rainfall variability. Several regional atmospheric variables and global SST are then identically wavelet filtered, based on the rainfall frequency bands. Regression, correlation, and composite analyses are subsequently used to identify the most important rainfall–climate system time-scale relationships.

The results show that Ethiopian monsoon rainfall variation is largely linked with annual time-scale atmospheric circulation patterns involving variability in the major components of the monsoon system. Although variability on the seasonal (75–210 days), quasi-biennial (QB; 1.42–3.04 yr), and El Niño–Southern Oscillation (ENSO; 3.04–4.60 yr) time scales accounts for much less variance than the annual mode (210 days–1.42 yr), they significantly affect Ethiopian rainfall by preferentially modulating the major regional monsoon components and remote teleconnection linkages. The seasonal time scale largely acts in phase with the annual mode, by enhancing or reducing the lower-tropospheric southwesterlies from the equatorial Atlantic during wet or dry periods. The wet QB phase strengthens the Azores and Saharan high and the tropical easterly jet (TEJ) over the Arabian Sea, while the wet ENSO phase enhances the Mascarene high, the TEJ, and the monsoon trough more locally. The effects of tropical SST on Ethiopian rainfall also are prominent on the QB and ENSO time scales. While rainfall–SST correlations for both the QB and ENSO modes are strongly positive (negative) over the equatorial western (eastern) Pacific, only ENSO exhibits widespread strong negative correlations over the Indian Ocean. Opposite QB and ENSO associations tend to characterize dry Ethiopian conditions. The relationships identified on individual time scales now are being used to develop and validate statistical prediction models for Ethiopia.

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Lance M. Leslie
,
Bruce W. Buckley
, and
Mark Leplastrier

Abstract

The preparation of accurate operational weather forecasts and the timely issuance of severe marine weather and ocean warnings and advisories for major oceanic weather systems impacting both coastal areas and the open ocean are major forecasting problems facing the Australian Bureau of Meteorology’s Regional Forecast Centre (RFC) and its collocated Tropical Cyclone Warning Centre (TCWC) in Perth, Western Australia. The region of responsibility for the Perth RFC is vast, covering a large portion of the southeast Indian and Southern Oceans, both of which are extremely data sparse, especially for near-surface marine wind data. Given that these coastline and open-ocean areas are subject to some of the world’s most intense tropical cyclones, rapidly intensifying midlatitude cyclones, and powerful cold fronts, there is now a heavy reliance upon NASA Quick Scatterometer (QuikSCAT) data for both routine and severe weather warning forecasts.

The focus of this note is on the role of QuikSCAT data in the Perth RFC for the accurate and early detection of maritime severe weather systems, both tropical and extratropical. First, the role of QuikSCAT data is described, and then three cases are presented in which the QuikSCAT data were pivotal in providing forecast guidance. The cases are a severe tropical cyclone in its development phase off the northwest coast of Australia, a strong southeast Indian Ocean cold front, and an explosively developing midlatitude Southern Ocean cyclone. In each case, the Perth RFC would have been unable to provide early and high-quality operational forecast and warning guidance without the timely availability of the QuikSCAT surface wind data.

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Esther D. Mullens
,
Lance M. Leslie
, and
Peter J. Lamb

Abstract

Winter storms in the southern United States can significantly impact infrastructure and the economy. In this study, National Centers for Environmental Information Storm Event Database and local climate summaries, are used to develop a spatial climatology of freezing precipitation (freezing rain and ice pellets) and snow over the southern Great Plains, 1993–2011. Principal component analysis is performed on the 500-hPa height field, at the approximate onset time of precipitation, for 33 freezing precipitation and 42 snow case studies, to differentiate common synoptic flow fields associated with precipitation type. The five leading patterns for each precipitation type are retained. Composites of temperature, moisture, pressure, and wind fields are constructed and extended 24 h before and after precipitation initiation to track the storm system evolution. Many 500-hPa flow fields are similar for both precipitation types. However, snow-dominant events have stronger and/or more frequent surface cyclone development. Freezing precipitation is associated with the southward propagation of an Arctic anticyclone well ahead of precipitation, weak or absent surface cyclone formation, and a more western trough axis. High-impact ice storms in the region often have slow-moving upper-level flow, persistent isentropic ascent over a surface quasi-stationary front with strongly positive moisture anomalies, and warm layer airmass trajectories originating over the Gulf of Mexico. The results here are based on a relatively small sample size. However, this work is intended to be useful for forecasters, in particular as a pattern recognition aid in predicting the evolution of precipitation within southern Great Plains winter storms.

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