Intraseasonal meridional current variability in the eastern equatorial Indian Ocean

Intraseasonal variability in meridional current in the eastern equatorial Indian Ocean is investigated by use of results from a high-resolution ocean general circulation model. Both the simulated and observed meridional current variability demonstrate energy peaks in two distinct time-scales; one in the biweekly (6-8 d) period above the thermocline and the other in lower frequency band (20-70 d) within the subsurface layer. In addition, the intraseasonal current variability can be seen even in the deep ocean at 2000 m and 4000 m depths. Composite analysis of the biweekly current field demonstrates that the horizontal structure is consistent with the one for the Mixed Rossby-gravity wave, with the phase speed and wavelength close to theoretical values of the Mixed Rossby-gravity wave at 15-d period. This meridional current variability shows large coherence with the local meridional wind stress, suggesting the upper-ocean responses to the local wind-forcing. A part of the energy of the biweekly variability penetrates into the deeper layer along the raypath of the 15-d Mixed Rossby-gravity wave, causing the intraseasonal variability observed in the current meter data at 4000 m depth. On the other hand, the lower frequency intraseasonal variability in the subsurface layer is also captured in the model simulation forced by climatological winds. Large barotropic energy conversion rate appears during boreal autumn and early winter in a region southeast of Sri Lanka, suggesting importance of instability due to the seasonally changing current systems to generate mesoscale eddy-like disturbances that result in the subsurface meridional current variability.