Annual Rossby waves in the Arabian Sea from TOPEX/POSEIDON altimeter and in situ data

Sea-surface height data acquired by the TOPEX/POSEIDON satellite over the Arabian Sea from October 1992 to October 1998 are analyzed. Strong seasonal fluctuations are found between 6° and 10°N, which are mainly associated with westward propagating annual Rossby waves radiated from the western side of the Indian subcontinent and that are continuously forced by the action of the wind-stress curl over the central Arabian Sea. An analysis of hydrographic data acquired during August 1993 and during January 1998 at 8°N in the Arabian Sea reveals the existence of first- and second-mode annual Rossby waves. These waves, which can be traced as perturbations in the density fields, have wavelengths of 12×103 and 4.4×103 km as well as phase velocities of 0.38 and 0.14 m/s, respectively. The waves are associated with a time-dependent meridional overturning cell that sloshes water northward and southward. Between 58° and 68°E in the central Arabian Sea, we found a Rossby-wave induced transport in the upper 500 m of about 10 Sv southward in August 1993 and northward in January 1998. Below 2000 m, there was still a northward transport of 3.2 Sv in August 1993 and a southward transport of 4.8 Sv in January 1998. A comparison of steric height differences between August 1993 and January 1998 calculated from the observed density fields as well as calculated from the reconstructed density fields using first- and second-mode annual Rossby waves agree quite well with the corresponding sea-surface height differences. Implications resulting from the reflection of annual Rossby waves, like fluctuations of the western boundary currents, are discussed.