Dynamical response of equatorial Indian Ocean to intraseasonal winds: zonal flow

Current observations show a broad, secondary energy peak centered at 40-60 day period for the zonal current at 200 m in the western (47$^{\circ} $--62$^{\circ }$E) and at 25 m in the central ($80^{\circ }30^{\prime}$E) equatorial Indian Ocean. In the east (near 90$^{\circ}$E), a 50-day oscillation of zonal, near surface current was recently observed.

A nonlinear and linear $4\frac{1}{2}$-layer ocean models are used to investigate dynamics of the observed zonal flow. The former includes active thermodynamics and mixed layer physics, and the latter is purely dynamical and wind driven. Solutions are found in a basin resembles the actual Indian Ocean north of 29$^{\circ}$S, and they are forced by NCEP/NCAR 5-day-mean reanalysis for a period of 20 years (1980--1999).

Consistent with the observations, our nonlinear solution displays a 40-60 day, relative spectral peak of zonal currents near the observed locations. This energy peak vanishes in the nonlinear solution when the model is forced by the winds with periods lower than 90 days being filtered out, suggesting that the 40-60 day zonal flow is driven by the winds at the same frequency. In addition, the 40-60 day current peak appears and is much stronger in the wind-driven, linear solution, further indicating that the currents are forced by the 40-60 day peak in the wind, which results primarily from the atmospheric Madden-Julian Oscillation (MJO) that produces strong zonal winds in the central and eastern equatorial Ocean.

Interestingly, the strongest spectral peak for the zonal surface current is at 90-day period. This peak passes the 95$\%$ significance level over the 20-year record, and it dominates the 40-60 day current in the central-eastern ocean where the winds reach their maxima. Both the linear and nonlinear solutions suggest that that the 90-day current, the dominant current at intraseasonal time scales, results from the near resonance response of the equatorial ocean to the 90-day winds, which have a significant energy power that are not apparently associated with the MJO. Further observations are needed in the equatorial Indian Ocean to verify the 90-day current that appears in the models.