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Shang-Ping Xie

Over most of the World Ocean, sea surface temperature (SST) is below 26°C and atmospheric deep convection rarely takes place. Cool ocean–atmosphere interaction is poorly understood and this lack of understanding is a stumbling block in the current effort to study non-ENSO climate variability. Using new satellite observations, the response of surface wind and low clouds to changes in SST is investigated over cool oceans, where the planetary boundary layer (PBL) is often capped by a temperature inversion. While one-way atmospheric forcing is a major mechanism for basinscale SST variability in the extratropics, clear wind response is detected in regions of strong ocean currents. In particular, SST modulation of vertical momentum mixing emerges as the dominant mechanism for SST-induced wind variability near oceanic fronts around the world, which is characterized by a positive SST–wind speed correlation. Several types of boundary layer cloud response are found, whose correlation with SST varies from positive to negative, depending on the role of surface moisture convergence. Noting that the surface moisture convergence is strongly scale dependent, it is proposed that horizontal scale is important for setting the sign of this SST–cloud correlation. Finally, the processes by which a shallow PBL response might lead to a deep, tropospheric-scale response and the implications for the study of extratropical basin-scale air–sea interaction are discussed.

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Takeaki Sampe
and
Shang-Ping Xie

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 equatorward 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.

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Hyein Jeong
,
Hyo-Seok Park
,
Jasti S. Chowdary
, and
Shang-Ping Xie

Abstract

In the summer of 2022, a long-lasting La Niña entered its third year. In Asia, southern China was in the grip of a historic drought while heavy rainfall ravaged Pakistan. Using a climate model forced by observed sea surface temperatures (SST) over the equatorial Pacific, we show that the back-to-back La Niña events from 2020 to 2022 are a key contributor to the global SST pattern in 2022, including the negative-phase Pacific decadal oscillation and exceptionally strong negative Indian Ocean dipole. The model reproduces the observed precipitation pattern over South and East Asia, including enhanced rainfall over Pakistan–northwest India and reduced rainfall over southern China. Additional model simulations indicate that the negative Indian Ocean dipole combined with La Niña reduces southern China rainfall by causing anomalous subsidence and anticyclonic flows. These results highlight the dominant role of long-lasting La Niña in modulating rainfall over heavily populated monsoon Asia.

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