Observed variability in the upper layers at the Equator, 90°E in the Indian Ocean during 2001–2008, 2: meridional currents

In the Indian Ocean, the observed variability of meridional currents (MC) at the Equator, 90°E shows distinct 10–20 day (quasi-biweekly) north–south reversals in the near-surface 40–350 m water column. Unlike the zonal currents, the seasonal variability in the MC field is strikingly absent in the annual cycle over this water column. However, the strong amplitude oscillations penetrate deep during boreal winter, boreal spring and boreal fall seasons. There is also a suggestion that they are weak and shallow during summer monsoon season during the years when the intraseasonal zonal westerly wind bursts along the equator are weak or absent. These quasi-biweekly oscillations are attributed to the westward propagating equatorially trapped mixed Rossby-gravity (MRG) or Yanai waves triggered by the westward propagating meridional wind (MW) field along the equator suggesting a possibility of wind-forced response of the upper ocean. A careful examination reveals interannual modulation of this quasi-biweekly variability in the MC field. The salinity induced vertical stratification observed in the near-surface layer through barrier layer effects also shows a significant influence on the MC field on 10–20 day time scale. The upwelling and downwelling cycles caused in the near off-equatorial region as a result of the westward propagation of the MRG waves are noticed in the pycnocline inferred from the vertical temperature and salinity profiles recorded by a nearby TRITON CTD mooring deployed at 1.5°S, 90°E. The observed sharp differences in the MCs between years appear to be determined by the strengths of both MWs and barrier layer thickness. A suggestion of westward propagation of quasi-biweekly variability in SST along the equator with phase speed resembling that of the MRG waves is also episodically seen during most years