Effective generation mechanisms of tropical instability waves as represented by high-resolution coupled atmosphere–ocean prediction experiments

Abstract

Cusp-shaped fluctuations of the sea surface temperature (SST) front in the tropical Pacific, now known as tropical instability waves (TIWs), were discovered by remote sensing in the 1970s. Their discovery was followed by both theoretical and analytical studies, which, along with in situ observations, identified several possible generation mechanisms. Although modeling studies have shown that TIWs strongly influence the heat budget, their influence on local variations of realistically initialized predictions is not yet understood. We here evaluate a series of medium-range (up to ~ 10 days) coupled atmosphere–ocean predictions by a coupled model with different horizontal resolutions. Observational SST, surface wind stress, heat flux, and pressure data showed that representation of temporally and spatially local variations was improved by resolving fine-scale SST variations around the initialized coarse-scale SST front fluctuations of TIWs. Our study thus demonstrates the advantage of using high-resolution coupled models for medium-range predictions. In addition, analysis of TIW energetics showed two dominant sources of energy to anticyclonic eddies: barotropic instability between equatorial zonal currents and baroclinic instability due to intense density fronts. In turn, the eddy circulation strengthened both instabilities in the resolved simulations. This revealed feedback process refines our understanding of the generation mechanisms of TIWs.

Wind stress curl as a driving force of annual waves in the upper ocean for interpreting energetics at all latitudes

“…Goddard and Philander (2000) pointed out that sea surface temperature (SST) and available potential energy are closely correlated, implying a connection between the wind input of wave energy and El Niño events. Toyoda et al (2021Toyoda et al ( , 2023 diagnosed an ocean reanalysis dataset in terms of the temporal evolution of wave energy associated with ENSO. They provided an energetic view for investigating ENSO which had often been understood in terms of raw quantities including SST, SSH, and thermocline depth in previous studies.…”

“…In the Pacific Ocean, equatorial KWs traverse toward the eastern boundary, undergo reflection, and subsequently diverge into two packets of westward RWs. These wave packets then coalesce into a single packet of westward propagating waves at 160° W as indicated by Toyoda et al (2021Toyoda et al ( , 2023. We need to modify the AGC17 energy flux in such a way as to be compatible with the modified wind input in the wave energy budget Eq.…”

“…8a. The AGC17 scheme was used for investigating waves in the Indian Ocean (Ogata and (16b) Aiki 2019;Li and Aiki 2020, 2022Li et al 2021), the Atlantic Ocean (Song and Aiki 2020Song et al 2023a, b), the Pacific Ocean (Toyoda et al 2021(Toyoda et al , 2023, and the atmosphere (Aiki et al 2021). In Toyoda et al (2021Toyoda et al ( , 2023 and Song and Aiki (2023), the horizontal variation of gravity wave speed c was considered in their calculation of the pseudo-streamfunction.…”