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Predictability of Northwest Pacific climate during summer and the role of the tropical Indian Ocean |
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| Authors: | Jasti Sriranga Chowdary Shang-Ping Xie Jing-Jia Luo Jan Hafner Swadhin Behera Yukio Masumoto Toshio Yamagata |
| Institution: | 1. International Pacific Research Center, SOEST, University of Hawaii at Manoa, Honolulu, HI, 96822, USA 2. Department of Meteorology, University of Hawaii at Manoa, Honolulu, HI, USA 3. Frontier Research Center for Global Change, JAMSTEC, Yokohama, Japan 4. Department of Earth and Planetary Science, The University of Tokyo, Tokyo, Japan |
| Abstract: | A seasonal forecast system based on a global, fully coupled ocean?Catmosphere general circulation model is used to (1) evaluate the interannual predictability of the Northwest Pacific climate during June?CAugust following El Ni?o JJA(1)], and (2) examine the contribution from the tropical Indian Ocean (TIO) variability. The model retrospective forecast for 1983?C2006 captures major modes of atmospheric variability over the Northwest Pacific during JJA(1), including a rise in sea level pressure (SLP), an anomalous anticyclone at the surface, and a reduction in subtropical rainfall, and increased rainfall to the northeast over East Asia. The anomaly correlation coefficient (ACC) for the leading principal components (PCs) of SLP and rainfall stays above 0.5 for lead time up to 3?C4?months. The predictability for zonal wind is slightly better. An additional experiment is performed by prescribing the SST climatology over the TIO. In this run, designated as NoTIO, the Northwest Pacific anticyclone during JJA(1) weakens considerably and reduces its westward extension. Without an interactive TIO, the ACC for PC prediction drops significantly. To diagnose the TIO effect on the circulation, the differences between the two runs (Control minus NoTIO) are analyzed. The diagnosis shows that El Nino causes the TIO SST to rise and to remain high until JJA(1). In response to the higher than usual SST, precipitation increases over the TIO and excites a warm atmospheric Kelvin wave, which propagates into the western Pacific along the equator. The decrease in equatorial SLP drives northeasterly wind anomalies, induces surface wind divergence, and suppresses convection over the subtropical Northwest Pacific. An anomalous anticyclone forms in the Northwest Pacific, and the intensified moisture transport on its northwest flank causes rainfall to increase over East Asia. In the NoTIO experiment, the Northwest Pacific anticyclone weakens but does not disappear. Other mechanisms for maintaining this anomalous circulation are discussed. |
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