Mapping High Sea Winds from Space: A Global Climatology

Abstract

High winds at sea are feared by sailors, but their distribution is poorly known because ships have avoided them as much as possible. The accumulation of spaceborne scatterometer measurements now allows a global mapping of high winds over the ocean. Seven years of Quick Scatterometer (QuikSCAT) data gathered since July 1999 show that high-wind events, defined as wind speeds greater than 20 m s?1 (?strong gale? and higher on the Beaufort scale), mostly happen in winter. Over coastal regions, land orography is the major cause of high winds, forcing wind jets of various types. Over the open ocean, high winds tend to be collocated with the extratropical storm tracks, along which migratory low and high pressure systems travel eastward. In comparison, tropical cyclones do not leave a strong signature in the climatology of high-wind occurrence except in the western Pacific east of Taiwan. In the extratropics, sea surface temperature (SST) fronts and their meanders significantly change the frequency of high-wind events. For example, high winds occur twice as often (or more) over the warmer than the colder flank of the Gulf Stream, and over the poleward than equa-torward meanders of the Antarctic Circumpolar Current. The collocation of frequent high winds and SST frontal zones is not a mere coincidence because SST gradients anchor storm tracks, which in turn sustain the surface westerlies against friction with lateral heat and momentum flux. Both the high mean speed and large variance of wind increase the probability of high winds. Implications for navigation safety and oceanographic and climate research are discussed.