FY 2020

Indian Ocean Intraseasonal Kelvin waves: Equatorial structure and propagation into the Indonesian Seas

Pujiana, K., and M.J. McPhaden

J. Geophys. Res., 125(5), e2019JC015839, doi: 10.1029/2019JC015839, View online (2020)

Intraseasonal oceanic variations in the Indian Ocean are prominent mode of variability affecting evolution of the Indian Ocean Dipole, the semiannual Wyrtki jets, and the Indonesian throughflow. This study describes the structure and propagation of wind‐forced intraseasonal Kelvin waves along the equator in the Indian Ocean and their penetration into the Indonesian seas. For this purpose, we use an unprecedented multiyear (2004–2016) moored velocity time series data set from the equatorial central and eastern Indian Ocean, moored time series from Lombok, Makassar, and Ombai straits; satellite altimetry, and other oceanic and atmospheric data products. The waves are generated by zonal wind fluctuations with periods of approximately 20–90 days. They propagate eastward along the equator at speeds close to that of a second baroclinic mode (with estimates ranging between 1.3 and 1.7 ms−1 depending on method) and have an exponential latitudinal decay scale away from the equator of ∼250 km. Phase also propagates upward, implying downward energy propagation, consistent with wind energy source at the surface. As the waves encounter the coast of Sumatra, they partially reflect into Rossby waves that radiate energy westward back into the ocean interior. Some wave energy also propagates southward along the coasts of Sumatra and Java and can be traced into Lombok, Ombai, and Makassar straits at lags of about 2–3 weeks. The implications of these results for understanding variability in the Indian Ocean and its connectivity with the Indonesian seas are discussed.

Plain Language Summary: We investigate Indian Ocean equatorial Kelvin waves on weekly to monthly timescales and their propagation into the Indonesian seas. For this purpose, we use ocean current velocity data from an unprecedented number of moorings during 2004–2016, plus satellite data and other data sets. Consistent with theoretical expectations, the wave structures decay exponentially away from the equator to the north and south, with a decay scale of approximately 250 km. The phase of the zonal current variations propagates eastward and upward consistent with eastward and downward energy propagation expected for Kelvin waves forced by surface winds. When the waves encounter the west coast of Sumatra, they partially reflect into Rossby waves that propagate westward back into the interior of the ocean. Some energy also continues poleward as coastal waves along Sumatra and Java. These waves eventually make their way into Lombok, Ombai, and Makassar straits in the Indonesian seas, affecting the velocity there. We discuss the implications of these results for understanding broader regional circulation patterns in the Indian Ocean.