1.2
Leading Tropical Climate Modes Associated with Interannual and Multi-decadal Variations in Seasonal North Atlantic Hurricane Activity
Gerald D. Bell, NOAA/NWS, Camp Springs, MD; and M. Chelliah
The leading unrotated tropical interannual EOF (ENSO) and the leading unrotated tropical multi-decadal EOF in the NCEP-NCAR Reanalysis for the ASO season from 1949-2000 are described, and their relative contributions to atmospheric anomalies associated with extremes in seasonal Atlantic Basin activity are investigated. It is suggested that the combined modes can provide a unified framework for interpreting the tropical North Atlantic and associated global-scale atmospheric anomalies accompanying both seasonal extremes and multi-decadal variations in Atlantic Basin hurricane activity. Over the Main Development Region (MDR) of the tropical Atlantic these combined modes reproduce the essence of the atmospheric circulation anomalies known to accompany extremes in seasonal Atlantic Basin activity, including the subtropical ridges, the tropical easterly jet, the vertical wind shear, and the structural characteristics of the African Easterly Jet (AEJ). The tropical multi-decadal mode accounts for much of this combined signal over the eastern half of the MDR, suggesting that the circulation anomalies associated with individual seasonal extremes in Atlantic Basin activity will closely resemble those associated with multi-decadal variations in seasonal activity. The time series of the tropical multi-decadal mode is consistent with the observed multi-decadal variations in seasonal Atlantic hurricane activity, and with the temporal evolution of the Atlantic Multi-decadal mode. In contrast, the ASO atmospheric variability over the western Caribbean Sea and Gulf of Mexico is strongly linked to interannual variations associated with the ENSO mode, while the variability in the vicinity of the Caribbean Islands and surrounding regions is related equally to both modes. The vertical wind shear in this latter region is known to be particularly critical for hurricane development in the MDR. The distinct tropospheric behavior of the two modes suggests that the anomalies related to the tropical multi-decadal mode vary depending on the phase of ENSO, and vice versa. These behaviors are described for the 200-hPa streamfunction and vertical wind shear anomalies using various combinations of the two modes. The analysis suggests that successful dynamical predictions of seasonal Atlantic Basin activity will require an accurate representation of the atmospheric anomalies associated with both ENSO and the tropical multi-decadal mode.
Session 1, Seasonal to Interannual Prediction and Predictability I
Monday, 29 April 2002, 9:30 AM-10:50 AM
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