Australia: The Land Where Time Began

A biography of the Australian continent 

Wyrtki Jet Dynamics - Seasonal Variability

In this paper the authors¹ examine the dynamics of the Wyrtki jets, strong equatorial zonal flows typically occurring in the boreal spring and autumn in the Indian Ocean. A continuously stratified longwave ocean model that was driven by QuikSCAT zonal winds was primarily relied upon for their diagnosis. It is indicated by the results from the model, which were found to compare well with altimetry and in situ current observations, that zonal currents propagate to the west along the Equator at semiannual periods and their average velocity has been found to be -1.5 m s ^-1 [0.55 m s ^-1]². This speed of propagation is 3 times that of the dominant wave mode in zonal wave velocity, namely the first meridional, second baroclinic mode Rossby wave. The authors¹ interpreted the results in terms of a superposition of Rossby waves on a wind-forced jet, the jet being stronger by a factor of 2 than the wave. On the other hand the sea surface height (SSH) displays features of propagation varying in both speed and direction from region to region. Differing influences of Kelvin and Rossby wave dynamics on the variability result in this contrasting behaviour between SSH and zonal velocity. In many respects these results are analogous to the distinction between SSH zonal current behaviour that was found in previous studies, on seasonal timescales, of the Pacific and Atlantic oceans. 

Sources & Further reading

1. Nagura, Motoki, and Michael J. McPhaden. "Wyrtki Jet Dynamics: Seasonal Variability." Journal of Geophysical Research: Oceans 115, no. C7 (2010): C07009.
2. Sengupta, Debasis, Retish Senan, B. N. Goswami, and Jérôme Vialard. "Intraseasonal Variability of Equatorial Indian Ocean Zonal Currents." Journal of Climate 20, no. 13 (2007/07/01 2007): 3036-55.