16:00 〜 16:15
[AOS16-03] Interannual variability of the position of the Southern Gyre and associated surface wind conditions
キーワード：西部アラビア海、ソマリア海流、サザンジャイア、併合
Strong clockwise monsoonal winds over the western Arabian Sea (AS) excite the northeastward Somali Current (SC) during boreal summer, with two anticyclonic oceanic gyres embedded. These regional gyres, the Southern Gyre (SG) and the Great Whirl (GW), develop around 4°N and 10°N, respectively, in boreal spring and early summer. These gyres are accompanied by the strong upwelling areas, “Cold filaments”, at the northern flanks and transport the cold and nutrient-rich water offshore. Therefore, interannual variability of these gyres can significantly affect biological productivity, regional climates, and climate modes such as the Indian Ocean Dipole. While the SG moves northward and coalesces with the Great Whirl in some years (One-Gyre year), it remains near 4°N to keep a two-gyre structure in other years (Two-Gyre year). Although analytical and idealized model studies suggest that the SG stays further south when the SC and/or alongshore winds are strong, relations between the states of the Somali Current system and surface wind variability are still unclear in realistic conditions. This study, therefore, explores this question by using the outputs of the regional ocean model (ROMS).
Interannual-run is conducted from 1990 to 2017 with the conditions at the end of the 20 years spin-up run as the initial values, and it is confirmed that seasonal cycles of the sea surface temperature (SST), mixed layer depth (MLD) and transport of the SC in the western equatorial AS are well reproduced. Composite analysis is conducted by using a SST anomaly in the western equatorial AS and years of low (high) SST values are defined as the Cold (Warm) years, which correspond to Two-Gyre (One-Gyre) years well. Clockwise wind stress anomaly in the Southern Hemisphere during the monsoon onset period is associated with the Cold years and its northern branch is westerly wind anomaly near the equator. This anomalous wind stress curl is important to generate the stronger Somali Current, in addition to the effect of the alongshore winds suggested by the previous studies. The associated Ekman pumping velocity anomaly is upward and excites upwelling Rossby waves in both hemispheres. These Rossby waves can generate the positive (negative) western boundary current anomaly in the Southern (Northern) Hemisphere upon reaching the African coast. Therefore, the anomalous wind stress curl in the Cold years is another source of the stronger Somali Current associated with the Two-Gyre state. The western boundary current anomaly thus excited also result in the convergence of anomalous alongshore currents near the equator, which sets the favorable condition as well for the offshore flow associated with the cold filament of the SG.
Sensitivity experiments are conducted to separate the effect of the wind stress curl from the influences of the alongshore winds and to estimate contribution of the oceanic internal variability. In the experiments, referred to as the Ueq-run, the zonal wind anomaly confined near the equator is superposed on the original wind field in the Warm years of the interannual-run. Results on the position of the SG to the strength of the added zonal wind anomaly will be presented as well.
Interannual-run is conducted from 1990 to 2017 with the conditions at the end of the 20 years spin-up run as the initial values, and it is confirmed that seasonal cycles of the sea surface temperature (SST), mixed layer depth (MLD) and transport of the SC in the western equatorial AS are well reproduced. Composite analysis is conducted by using a SST anomaly in the western equatorial AS and years of low (high) SST values are defined as the Cold (Warm) years, which correspond to Two-Gyre (One-Gyre) years well. Clockwise wind stress anomaly in the Southern Hemisphere during the monsoon onset period is associated with the Cold years and its northern branch is westerly wind anomaly near the equator. This anomalous wind stress curl is important to generate the stronger Somali Current, in addition to the effect of the alongshore winds suggested by the previous studies. The associated Ekman pumping velocity anomaly is upward and excites upwelling Rossby waves in both hemispheres. These Rossby waves can generate the positive (negative) western boundary current anomaly in the Southern (Northern) Hemisphere upon reaching the African coast. Therefore, the anomalous wind stress curl in the Cold years is another source of the stronger Somali Current associated with the Two-Gyre state. The western boundary current anomaly thus excited also result in the convergence of anomalous alongshore currents near the equator, which sets the favorable condition as well for the offshore flow associated with the cold filament of the SG.
Sensitivity experiments are conducted to separate the effect of the wind stress curl from the influences of the alongshore winds and to estimate contribution of the oceanic internal variability. In the experiments, referred to as the Ueq-run, the zonal wind anomaly confined near the equator is superposed on the original wind field in the Warm years of the interannual-run. Results on the position of the SG to the strength of the added zonal wind anomaly will be presented as well.