| 概要 |
Upper ocean responses to typhoons in the northwestern Pacific are studied using historical temperature data obtained from a buoy station (St. 21004, 29°N-135°E) of the Japan Meteorological Agency duri...ng 1982-2000 and a three-dimensional primitive equation model (the Princeton Ocean Model; POM). In the data period, 25 typhoons which passed through locations, approximately around ~200km to the right or left of the buoy station are analyzed. The typhoons cooled the sea surface by 1.6-4.3°C. In particular, several intense, slowly moving typhoons (≤4ms_<-1>) showed special-common features of temperature variations; they too much cooled the sea surface water by 3-4°C with the passage of the typhoons; even after the typhoons, SST cooling states continue long for about 20days, and the SST minimums reach 1-2days after the typhoon passages; temperatures in the subsurface layers (50m, 100m) rapidly increased against the SST decrease 2-3days before the passage of typhoons; near-inertial oscillations frequently occur after the typhoons, especially with amplitudes larger in the subsurface layers than that in the surface. Numerical simulation of an intense, slowly moving Typhoon Abby (August 1983) well reproduces these observed features and shows that the subsurface temperature response beneath a mixed layer to which vertical turbulent mixing is largely extended, crucially depends on a basin-scale stretching or shrinking of water column, directly concerned with isostatic effect (inverse barometric effect), i.e., a significant cooling in the central region of the typhoon due to the stretching (isostatic effect; upwelling), whereas a weak warming in the distant region out of the typhoon due to the shrinking (downwelling). The model shows that the subsurface time-variations of temperature may be roughly expressed by a linear thermal equation, describing the vertical movement of the water column. A striking feature in the model is an existence of a cyclonic eddy (CE) formed in the rear of the typhoon, even though it has not been observed yet. The model postulates that the CE plays largely important role in flow and temperature fields, tracking along the typhoon. A recent conceptual model with vertical mode expansions (Masuda and Hong, 2011) will give a clearly theoretical explanation of the CE as well as temperature response to a typhoon.続きを見る
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