The Climatological Horizontal Pattern of Energy Flux in the Tropical Atlantic as Identified by a Unified Diagnosis for Rossby and Kelvin Waves

Abstract

This study investigates the horizontal distribution of wave energy flux in the tropical Atlantic Ocean using shallow‐water model experiments for three gravest baroclinic modes forced by climatological winds. This is the first attempt to apply a unified diagnostic scheme to the analysis of energy fluxes associated with both Rossby waves (RWs) and Kelvin waves (KWs) in the tropical Atlantic Ocean. Those analyses were difficult in previous studies owing to the difference of equatorial and quasigeostrophic dynamics. The scheme yields the transfer route (i.e., trajectories by group velocity vector) of wave energy originating from both local wind forcing and boundary reflection. The meridional flux of wave energy crossing two zonal sections (at 3°N and at 2°S) is examined for understanding where the equatorial and off‐equatorial regions interconnect. Equatorward energy fluxes into the basin interior are found mainly on the zonal section in the Northern Hemisphere (at 3°N), indicating the radiation of RWs from the Guinea coast. At the equator, seasonal transition in May from eastward wind anomaly to westward anomaly yields a significant energy input at the central basin that radiates both RWs and KWs. The energy flux of these RWs arrives at the western boundary in September, followed by reflection of KWs to yield eastward transbasin energy fluxes in October‐December. The analysis also identifies an unexpected wind input in May at 4°S which induces both RWs and inertial gravity waves.

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

“…The mesh grid and model setup in our experiments has already been applied by Song and Aiki (2020, 2021) to cover the Atlantic basin. The model involves the 100‐m isobath in the bathymetry data provided by the GEBCO (The General Bathymetric Chart of the Oceans) data set for realistic coastline with the latitude range from 44°S to 72°N.…”

“…The yielded c ( n ) , H ( n ) , and Ψ ( n ) for the first five modes have been presented by Table 1 and Figure 1. Readers can refer to Song and Aiki (2020, 2021) for detailed information about the solution of the classical eigenvalue problem for c ( n ) and H ( n ) . It should be pointed out here is that, without considering the topography of the continental shelf, in the coastal region off the equator, c ( n ) still represent the gravity wave speed of the n th equatorial baroclinic mode hence might be different from the CTW mode described by Illig et al.…”

“…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.…”

“…Previous attempts using the SLA or SST as a proxy by means of decomposition algorithm designed for either equatorial waves or CTW by Illig et al (2018) to diagnose the wave energy failed to reveal the energy transfer between waves in all involved baroclinic modes along their routes from the equator to the ABU region. 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.…”

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

“…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.…”