Estimating the Surface Current of 17-day and 33-day Tropical Instability Waves in the Equatorial Pacific with Altimeter Observations

Minyang Wang, South China Sea Institute Of Oceanology, Guangzhou, China, Xie shang-Ping, Scripps Institution of Oceanography, La Jolla, CA, United States, Yan DU, South China Sea Institute of Oceanology, Chinese Academy of Sciences, State Key Laboratory of Tropical Oceanography, Guangzhou, China and Samuel Shanpu Shen, San Diego State University, Department of Mathematics and Statistics, San Diego, CA, United States
Abstract:
There are two kinds of tropical instability waves (TIWs) in the equatorial Pacific Ocean. One is the classical 33-day TIWs north of the equator, and another is the 17-day TIWs on the equator (Lyman et al. 2007). Both of them play an important role on the momentum and heat budget of the equatorial Pacific. However, the 17-day TIWs gain little attention due to the lack of gridded current product resolving them. Our study aims to capture the different TIWs patterns and estimate the associated current perturbations in the equatorial Pacific with satellite observations, resolving the 17-day TIWs as well as the 33-day ones.

Satellite observing SSH and SST perturbations (60-day time and 20-deg zonally high-passed) from 1993-2018 are used. A joint EOF analysis of SSH and SST in a box centered at 140W is applied to capture the two TIWs patterns. The first and second EOFs (Figure 1 top two) corresponds to 33-day TIWs, called as Rossby-mode TIWs as their resemblance to equatorially-trapped Rossby waves. The PC1 and PC2 are highly correlated with the 33-day zonal current perturbations from ADCP at 140W, 0N with phase shift (rmax=0.4). The third and fourth EOFs (Figure 1 bottom two) corresponds to 17-day TIWs, called as Yanai-mode TIWs as their resemblance to Yanai waves. The PC3 and PC4 are highly correlated with the 17-day meridional current perturbations from ADCP at 140W, 0N with phase shift (rmax=0.5).

To estimate the Rossby-mode and Yanai-mode TIWs currents, we develop a diagnostic model with acceleration terms, Coriolis force, pressure gradient force and background zonal current advection retained in the momentum equations. The only input is SSH perturbations from an altimeter product (AVISO). The output of meridional currents shows high correlation with ADCP (r=0.56). However, the zonal one shows relatively low correlation with ADCP (r=0.31). The origin of zonal current bias is probably the mapping errors of AVISO amplified in the zonal pressure gradient force. We need to avoid amplifying mapping errors by changing our model in the work later on.

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