11:00 AM - 11:15 AM
[AOS19-08] Generation Mechanism of Tropical Instability Waves in the Equatorial Pacific Ocean
Keywords:Tropical Instability Wave, Equatorial Pacific Ocean, Potential Vorticity Gradient, Rossby Wave, 1.5-Layer Shallow Water Model, Linear Stability Analysis
In this study, we show that a linear stability analysis using a 1.5-layer shallow water model with a zonal mean flow on the equatorial beta plane can successfully predict an unstable mode whose phase speed and meridional structure are both very similar to those of the TIWs simulated in an eddy-resolving ocean general circulation model (OGCM), although the wavelength and the growth rate are somewhat larger and smaller, respectively. This unstable mode can be interpreted as resulting from a coupling between a Rossby wave propagating eastward in the negative potential vorticity (PV) gradient just north of the equator (~1o–3.5oN) while advected westward by the SEC, and a Rossby wave propagating westward in the positive PV gradient further north (~3.5o–8oN) while weakly advected eastward by the NECC. The unstable mode tends to have smaller growth rate as the negative PV gradient becomes weaker, and even disappears when it is completely absent.
The negative PV gradient just north of the equator thus plays an essential role in forming the unstable mode. Actually, the seasonal and interannual variability of the TIWs simulated in the OGCM is shown to be significantly controlled by the strength of the negative PV gradient, suggesting the possibility that it can serve as a key parameter for a dynamically based parameterization of the heat and momentum transfer associated with TIWs in coarse-resolution OGCMs.