[1] The intraseasonal sea level variations along the southern coast of Sumatra and Java are investigated using in situ data and the TOPEX/Poseidon satellite altimetry data. The analysis shows that there are two intraseasonal variations of distinct timescale: 20-40 days during boreal summer (JJA) and 60-90 days during boreal winter (DJF). During boreal summer the shorter time variations of the sea level along the coast of Sumatra and Java are traced back to the eastern equatorial Indian Ocean (EIO); this indicates the importance of the remotely forced equatorial Kelvin waves. During boreal winter, on the other hand, both the remote winds over the EIO and the local alongshore winds are important in explaining the longer time variations of the sea level along the coast. Further analysis indicates that these intraseasonal variations are associated with the coastal Kelvin waves with phase speed ranging from 1.5 to 2.86 m/s. A simple analytical model forced by daily wind stress confirms the above intraseasonal variations along the southern coast of Sumatra and Java.

[1] This study examines the structure and dynamics of wind-forced intraseasonal zonal current variability in the equatorial Indian Ocean. We take advantage of a variety of satellite and in situ data sets, including unprecedented 4-8 year-long velocity time series records from the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) program. Spectral analysis reveals prominent intraseasonal zonal currents variations along the equator with periods of 30-70 days. These oscillations are vertically in phase above the thermocline and propagate eastward with the local zonal winds. In the thermocline, intraseasonal zonal velocity variations also propagate eastward across a broad range of phase speeds expected for low baroclinic equatorial Kelvin waves; amplitudes decrease with depth, with deeper levels leading those near surface. Collectively, these results suggest that the near-surface layer responds directly to intraseasonal zonal wind stress forcing and that subsequently energy radiates downward and eastward in the thermocline in the form of wind-forced equatorial Kelvin waves. In addition, intraseasonal zonal current variability on the equator is coherent with off-equatorial sea surface height fluctuations in the eastern and central of the basin. This coherence is primarily due to the fact that equatorial zonal wind variations are associated with off-equatorial wind stress curls that can generate local Ekman pumping and westward propagating Rossby waves.

[1] A high-resolution ocean general circulation model (OGCM) is used to explore dynamics of intraseasonal variability in surface and subsurface currents off Java. The results indicate that the surface current, the so-called South Java Coastal Current (SJCC), is dominated by variations with a period of 90 days. In the subsurface current, which is referred to as the South Java Coastal Undercurrent (SJCU), 60-day variations are the most prominent feature. A normal mode analysis demonstrates that the first baroclinic mode is the leading mode, which accounts for 70% of the total variance, whereas the second baroclinic mode explains 24% of the total variance. The 90-day variations in the SJCC captured mostly by the first baroclinic mode are found to be primarily driven by winds. Those are associated with propagation of the first baroclinic Kelvin waves generated in the central equatorial Indian Ocean. On the other hand, the 60-day variations in the SJCU enhanced by wind forcing over the eastern equatorial Indian Ocean off Sumatra are mostly captured by the second baroclinic mode.

[1] Variations of subsurface zonal current in the eastern equatorial Indian Ocean are investigated by examining 6-year data (December 2000-November 2006) from acoustic Doppler current profiler (ADCP) mooring at 0°S, 90°E. The analysis indicates the presence of an eastward equatorial subsurface current between 90 and 170 m depths during both boreal winter and summer. During boreal winter, the generation of eastward pressure gradient, which drives an eastward flow in the thermocline, is caused primarily by upwelling equatorial Kelvin waves excited by prevailing easterly winds. On the other hand, the downwelling Rossby waves generated by the reflection of the spring downwelling Kelvin waves in the eastern boundary, as well as the upwelling equatorial Kelvin waves triggered by easterlies, create an oceanic state that favors the generation of the eastward pressure gradient during boreal summer. The subsurface current reveals a distinct seasonal asymmetry. The maximum eastward speed of 63 cm s À1 is observed in April, and secondary maximum of 49 cm s À1 is seen in October. The zonal transport per unit width within depth of the subsurface current exhibits similar variations: reaching maximum eastward transport of 35 m 2 s À1 in April and secondary maximum of 29 m 2 s À1in October. Moreover, the subsurface current during boreal summer undergoes significant interannual variations; it was absent in 2003, but it was anomalously strong during 2006.