Leading Tropical Mode Associated with Increased Atlantic Hurricane Activity since 1995

Multi-decadal extremes in Atlantic hurricane activity result from a coherent and inter-related set of atmospheric and oceanic conditions associated with the leading multi-decadal modes of tropical climate variability. One of these leading modes captures the 1995 transition to an active Atlantic hurricane era, and to an inactive East Pacific hurricane era. This mode is related to fluctuations in the strength of the West African monsoon system and the Amazon Basin monsoon, as well as to Atlantic sea-surface temperatures. The mode captures an east-west seesaw in anomalous convection between the West African monsoon region and the Amazon Basin, which helps to account for the inter-hemispheric symmetry of the 200-hPa streamfunction anomalies across the Atlantic Ocean and Africa, the 200-hPa divergent wind anomalies, and both the structure and spatial scale of the low-level tropical wind anomalies, associated with multi-decadal extremes in Atlantic hurricane activity. By examining the leading tropical multi-decadal modes we can understand differences between the 1950-1969 and 1995-2004 periods of above-normal Atlantic hurricane activity. In particular, the period 1950-1969 featured a very strong West African monsoon circulation and near-average sea-surface temperatures across the central tropical Atlantic, while the period 1995-present features a modestly enhanced West African monsoon and exceptionally warm Atlantic sea-surface temperatures. Finally, it is shown that the ENSO teleconnections and impacts on Atlantic hurricane activity can be substantially masked or accentuated by the tropical multi-decadal modes. This leads to the important result that the multi-decadal modes provides a substantially more complete view of the climate control over Atlantic hurricane activity during individual seasons than is afforded by ENSO alone.