[A-OS11] Physical, biogeochemical, and ecological processes and variability in the Indian Ocean

[AOS11-P05] Distinct wind stress pattern causing the two-gyre structure in the Somali Current during boreal summer

*Takahiro Kusumi¹, Yukio Masumoto¹ (1.the Unversity of Tokyo)

Keywords:Western Arabian Sea, Somali Current, Southern Gyre, Coalescence

Strong clockwise monsoonal winds over the western Arabian Sea (AS) excite the northeastward Somali Current during boreal summer, with two anticyclonic oceanic gyres embedded. These gyres, the Southern Gyre and the Great Whirl, develop around 4°N and 10°N, respectively, in boreal spring and early summer. While the Southern Gyre 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 it has been speculated that horizontal patterns of wind stress, instead of the wind stress magnitude, may be important for the above differences in the gyre movement, such an anomalous wind stress patterns have not been identified yet. This study explores the wind stress patterns causing the different behavior of the two-gyre system by analyzing the observational and reanalysis datasets.
There are 7 two-gyre years of the 29 years from 1993 to 2021. The composite analysis reveals that alongshore northeasterly wind stress anomalies off the coast of Somalia and anomalous positive (negative) wind stress curl in the northern (southern) AS with westerly wind stress anomalies near the equator appear during the monsoon onset periods of the two-gyre years. These wind stress anomalies tend to generate convergence of the western boundary current north of the equator by weakening (strengthening) the Somali Current in the northern (southern) region. This convergence of the western boundary current is tied to the offshore flows, which may generate a favorable condition for the Southern Gyre to stay near the equator and maintaining the two-gyre structure.