Wyrtki Jet dynamics: Seasonal variability

Abstract

[1] This study examines the dynamics of the Wyrtki jets, which are strong equatorial zonal flows that occur typically during boreal spring and fall in the Indian Ocean. Our diagnosis relies primarily on a continuously stratified linear longwave ocean model driven by QuikSCAT zonal winds. Model results, which compare well with satellite altimetry and in situ current observations, indicate that the zonal currents propagate westward along the equator at semiannual periods with an average speed of −1.5 m s −1 . This propagation speed is three times faster than the propagation speed of the dominant wave mode in model zonal velocity, namely the first meridional, second baroclinic mode Rossby wave. We interpret this result in terms of a superposition of Rossby waves on a wind-forced jet, with the jet stronger than the waves by a factor of 2. Sea surface height (SSH), on the other hand, shows propagating features that vary in both speed and direction from region to region. This contrasting behavior between SSH and zonal velocity results from differing influences of Kelvin and Rossby wave dynamics on the variability. These results are in many respects analogous to the distinction between SSH and zonal current behavior found in previous studies of the equatorial Pacific and Atlantic oceans on seasonal time scales.

Ocean Response to CINDY/DYNAMO MJOs in Air-Sea-Coupled COAMPS

“…They were explained as a wind-driven response to zonal winds by O'Brien and Hurlburt (1974), who applied the theory of Yoshida (1959). The WJs have been observed to appear in the spring and the fall of every year on a regular basis (e.g., Nagura and McPhaden 2010;Nyadjro and McPhaden 2014), although the spring jets have been found to be weak in 2006 through 2009 (Joseph et al 2012). The strong response of the WJ to semi-annual forcing has been explained by resonant basin modes (Jensen 1993;Han et al 1999) based on the theoretical work of Cane and Moore (1981).…”

Impact of Indian Ocean Dipole and El Niño/Southern Oscillation wind‐forcing on the Wyrtki jets

“…Qiu et al [2009] reported the westward phase propagation in the equatorial currents, which they attributed to the westward phase propagation of zonal winds in the western equatorial Indian Ocean. Nagura and McPhaden [2010a] studied this westward propagation in detail using a linear continuous stratified model, and interpreted the westward propagation as a result of a superposition of Rossby waves on a wind forced jet.…”

Climatology and seasonality of upper ocean salinity: a three-dimensional view from argo floats

“…8a-c). In particular, a prominent signature of the Wyrtki Jet is evident in the subsurface Indian Ocean, which corresponds to strong equatorial zonal flows that occur typically during boreal spring and fall at semiannual periods with an average speed of −1.5 m/s (Nagura and McPhaden 2010). Further downward, the phase pattern basically exhibits a random nature, except for the SPCZ where a coherent feature can be marginally detected between ~100-300 m (Fig.…”