Energy Flow Diagnosis of ENSO from an Ocean Reanalysis

Abstract

A method is introduced for diagnosing the time evolution of wave energy associated with ENSO from an ocean reanalysis. In the diagnosis, time changes of kinetic and available potential energy are mainly represented by energy inputs caused by surface wind stress and horizontal energy fluxes for each vertically decomposed normal mode. The resulting time evolutions of the wave energy and vertical thermocline displacements in the 1997–1998 and 2014–2016 El Niño events are consistent with our previous knowledge of these events. Further, our result indicated that representation of several vertical modes is necessary to reproduce the broadly distributed downward thermocline displacements in the central to eastern equatorial Pacific, generated by a westerly wind event in the western equatorial Pacific (e.g., in March 1997), that are preconditioning for El Niño development. In addition, we investigated the wave energy budget, including the influence of data assimilation, on the complicated time evolution of equatorial thermocline displacements caused by repeated westerly and easterly wind events during the 2014–2016 El Niño event. Our result suggests that noise from a momentum imbalance near the equator associated with data assimilation, which possibly affected the El Niño prediction failure in 2014, was much reduced by our developed ocean data assimilation system and reanalysis. This study, which provides a new connection between the theoretical works and reanalysis products that use sophisticated systems for synthesizing OGCMs and observations, should be useful for climate research and operational communities interested in ENSO.

The vertical structure of annual wave energy flux in the tropical Indian Ocean

“…The first six modes explain about 80% of baroclinic kinetic energy fraction in the equatorial regions of a global ocean reanalysis, as shown by Fig. 1 in Toyoda et al (2021). The Sturm-Liouville equation for the baroclinic normalmode decomposition associated with a stratified ocean may be written as…”

“…The scheme is diagnostic and provides a computationally cheaper way compared with sensitivity experiments using an elaborated numerical model. The AGC17 scheme allowed Ogata and Aiki (2019), LA20, Song andToyoda et al (2021) to obtain wholly new perspectives on the horizontal transfer routes of waves in the tropical oceans. We extend their work to a three-dimensional system and describe vertical wave propagation.…”

Interactions Between Ocean and Successive Typhoons in the Kuroshio Region in 2018 in Atmosphere–Ocean Coupled Model Simulations

“…The atmospheric initial conditions were based on JMA 6‐hourly global objective analysis data. The oceanic initial conditions were derived from an ocean data assimilation system (Toyoda et al., 2021): for both high‐ and low‐resolution experiments, long‐term data assimilative simulations were conducted with assimilated data of in situ temperature and salinity profiles and satellite‐derived SST, sea surface salinity, sea surface height (SSH), and sea‐ice concentration data and with the 3‐hourly JRA55‐do (Tsujino et al., 2018) surface boundary conditions. These long‐term data assimilative simulations used 10‐day assimilation window cycles with 3‐dimensional variational method.…”

The Role of Equatorially Forced Waves in Triggering Benguela Niño/Niña as Investigated by an Energy Flux Diagnosis

“…Recently, unified wave energy flux schemes for equatorial waves by Aiki et al. (2017), which are named as AGC (Aiki, Greatbatch, and Claus) flux, have been applied in the Atlantic, Indian, and Pacific Ocean (Li & Aiki, 2020; Song & Aiki, 2020; Toyoda et al., 2021), showing its ability in explaining the wave‐related climate variability from the view of wave energy. Song and Aiki (2021) utilized the named AGC level0 (hereafter referred to as AGC‐L0) scheme and successfully revealed the wave energy flux of intraseasonal CTWs along the African coast, which has certainly provided a new view to address the remote forcing mechanism for the Benguela Niño/Niña.…”