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  • Author or Editor: Timothy M. Hall x
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Leonard M. Druyan
and
Timothy M. Hall

Abstract

Ensembles of three simulations each, forced by June–September 1987 and 1988 sea surface temperatures, respectively, were made with a new version of the general circulation model of the National Aeronautics and Space Administration/Goddard Institute for Space Studies. Time series of 6-h meridional winds at about 780 mb over West Africa were spectrally analyzed to detect African wave disturbances, whose properties for the two ensembles are compared and contrasted. The realistically simulated, stronger 1988 tropical easterly jet and the associated stronger upper-tropospheric divergence are components of interannual differences in the SST-forced planetary circulation, which correspond to higher amplitudes of African wave activity and concomitant excesses in 1988 Sahel rainfall rates. Results do not show, however, that most of the heavier precipitation was spatially organized by African wave structures. The excess rainfall is associated with stronger mean southerly circulation in the lower troposphere, which carried more moisture into the Sahel. Nevertheless, because waves modulate winds, convergence, humidity, and precipitation, the study suggests that they serve as a teleconnection mechanism, whereby extreme Pacific Ocean SST anomalies are able to influence climate variability in Africa's Sahel.

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Timothy M. Hall
and
Stephen Jewson

Abstract

Two statistical methods for predicting the number of tropical cyclones (TCs) making landfall on sections of the North American coastline are compared. The first method—the “local model”—is derived exclusively from historical landfalls on the particular coastline section. The second method—the “track model”—involves statistical modeling of TC tracks from genesis to lysis, and is based on historical observations of such tracks. Identical scoring schemes are used for each model, derived from the out-of-sample likelihood of a Bayesian analysis of the Poisson landfall number distribution. The track model makes better landfall rate predictions on most coastal regions, when coastline sections at a scale of several hundred kilometers or smaller are considered. The reduction in sampling error due to the use of the much larger dataset more than offsets any bias in the track model. When larger coast sections are considered, there are more historical landfalls, and the local model scores better. This is the first clear justification for the use of track models for the assessment of TC landfall risk on regional and smaller scales.

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Emmi Yonekura
and
Timothy M. Hall

Abstract

A new statistical model for western North Pacific Ocean tropical cyclone genesis and tracks is developed and applied to estimate regionally resolved tropical cyclone landfall rates along the coasts of the Asian mainland, Japan, and the Philippines. The model is constructed on International Best Track Archive for Climate Stewardship (IBTrACS) 1945–2007 historical data for the western North Pacific. The model is evaluated in several ways, including comparing the stochastic spread in simulated landfall rates with historic landfall rates. Although certain biases have been detected, overall the model performs well on the diagnostic tests, for example, reproducing well the geographic distribution of landfall rates. Western North Pacific cyclogenesis is influenced by El Niño–Southern Oscillation (ENSO). This dependence is incorporated in the model’s genesis component to project the ENSO-genesis dependence onto landfall rates. There is a pronounced shift southeastward in cyclogenesis and a small but significant reduction in basinwide annual counts with increasing ENSO index value. On almost all regions of coast, landfall rates are significantly higher in a negative ENSO state (La Niña).

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Emmi Yonekura
and
Timothy M. Hall

Abstract

Improvements on a statistical tropical cyclone (TC) track model in the western North Pacific Ocean are described. The goal of the model is to study the effect of El Niño–Southern Oscillation (ENSO) on East Asian TC landfall. The model is based on the International Best-Track Archive for Climate Stewardship (IBTrACS) database of TC observations for 1945–2007 and employs local regression of TC formation rates and track increments on the Niño-3.4 index and seasonally varying climate parameters. The main improvements are the inclusion of ENSO dependence in the track propagation and accounting for seasonality in both genesis and tracks. A comparison of simulations of the 1945–2007 period with observations concludes that the model updates improve the skill of this model in simulating TCs. Changes in TC genesis and tracks are analyzed separately and cumulatively in simulations of stationary extreme ENSO states. ENSO effects on regional (100-km scale) landfall are attributed to changes in genesis and tracks. The effect of ENSO on genesis is predominantly a shift in genesis location from the southeast in El Niño years to the northwest in La Niña years, resulting in higher landfall rates for the East Asian coast during La Niña. The effect of ENSO on track propagation varies seasonally and spatially. In the peak activity season (July–October), there are significant changes in mean tracks with ENSO. Landfall-rate changes from genesis– and track–ENSO effects in the Philippines cancel out, while coastal segments of Vietnam, China, the Korean Peninsula, and Japan show enhanced La Niña–year increases.

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