Nonlinear Tropical Instability Waves

Tropical Instability Waves (TIWs) are westward-propagating oscillations observed in both the equatorial Pacific and Atlantic. Two distinct types of TIWs are commonly seen. The first species appears very near the equator, has a period of about 17 days, and is most prominent in meridional velocity. The second type is centered about 5°N, and has a period around one month, and has been observed in sea surface height (SSH), sea surface temperature (SST), and velocity. Barotropic instability of the shear between the South Equatorial Current (SEC) and the Equatorial Undercurrent/North Equatorial Countercurrent (EUC/NECC) has long been regarded as the most important energy source of these waves. In our work, the nonlinear shallow water equations (equivalent to a one and half layer reduced gravity model) are used to study the nonlinear development of TIWs with two typical zonal mean profiles from the Pacific and Atlantic Oceans as the initial mean states. During the early, linear development, the most unstable wave has a period of about one month, a wavelength about 1100km (consistent with the linear instability theory), and dominates the whole domain. After the TIWs centered near 5°N have developed into mature vortices, fast neutral Yanai-wave type waves with a period of about half month appear and dominate from 3°S to 2°N. In the Pacific Ocean case, merger of two vortices and generation of new vortices are occasionally observed for very strong mean currents. Also, secondary instability in the outer region of these vortices may appear when the magnitude of the initial mean flow is large. One implication of this study is that linear instability theory fails to explain the cross equatorial structure of TIWs and the emergence of the Yanai-like waves at the equator.