Climate Trends Associated with Multidecadal Variability of Atlantic Hurricane Activity

Abstract

Anomalous long-term variations of ocean heat transport offer an attractive and intuitively creditable explanation for many long-term climate trends. Multidecadal variations of intense Atlantic hurricane activity are but one manifestation of an extensive array of regional and global climate trends which appear to be linked to variations of heat transport by the Atlantic thermohaline circulation. In addition to influencing Atlantic tropical cyclones and the closely associated West African monsoon, Atlantic thermohaline variability appears to be linked to global SST variations and related trends occurring throughout the global climate system. Consequently, understanding decadal trends in hurricane activity may be critically dependent on understanding the somewhat broader issue of decadal variations of the major ocean circulations. The net transport of warm surface layer water to high latitudes by the so-called “Atlantic conveyor belt” (i.e., thermohaline) circulation is sensitive to surface-layer salinity anomalies in the “deep water” formation areas of the North Atlantic. A major decrease of surface-layer salinity appeared over portions of these areas during the late 1960s which reduced ocean water density and slowed the surface water sinking process associated with deep water formation. This trend, in turn, lead to diminished northward heat transport by the ocean hence, to cooling of ocean surface temperatures in much of the North Atlantic and warming of SSTs in much of the South Atlantic. These regional Atlantic SST anomalies initiated the atmospheric circulation anomalies associated with the long running Sahel drought and the associated decrease of intense Atlantic hurricane activity in recent decades. At approximately the same time, the ocean surface also cooled in much of the North Pacific while strong SST warming occurred in much of the Southern Hemisphere Atlantic, Indian, and Pacific Ocean areas. This global distribution of altered ocean surface temperatures has been directly linked to altered patterns of Atlantic and West African surface pressure and monsoon circulations. These global climate changes also include the energetics of ENSO and related variables in the tropical Pacific and Indian Oceans and numerous “teleconnected” interactions between the tropical Pacific, the North Pacific, North America and Europe. At present there are few long-term observational data for making reliable direct estimates of trends in the net Atlantic Ocean conveyor transport. Consequently, no such information is available for detecting and further anticipating forthcoming decadal trends in hurricane activity. Needed research is suggested which includes surveys and the synthesis of additional trend data for the specification of plausible and physically consistent global interactions linking the Atlantic conveyor circulation and other decadal trend associations in the global climate system. In this way, some of these global data may yield factors which are potentially useful for forecasting the onset and termination of new decadal trends of hurricane activity. This prospect is examined in data for the most recent 50-year period and in prior realizations of similar concurrent climate trends in earlier historical data.