Horizontal energy flux of wind-driven intraseasonal waves in the tropical Atlantic by a unified diagnosis

Abstract

Intraseasonal waves in the tropical Atlantic Ocean have been found to carry prominent energy that affects interannual variability of zonal currents. This study investigates energy transfer and interaction of wind-driven intraseasonal waves using single-layer model experiments. Three sets of wind stress forcing at intraseasonal periods of around 30 days, 50 days and 80 days with a realistic horizontal distribution are employed separately to excite the second baroclinic mode in the tropical Atlantic. A unified scheme for calculating the energy flux, previously approximated and used for the diagnosis of annual Kelvin and Rossby waves, is utilized in the present study in its original form for intraseasonal waves. Zonal velocity anomalies by Kelvin waves dominate the 80-day scenario. Meridional velocity anomalies by Yanai waves dominate the 30-day scenario. In the 50-day scenario, the two waves have comparable magnitudes. The horizontal distribution of wave energy flux is revealed. In the 30-day and 50-day scenarios, a zonally alternating distribution of cross-equatorial wave energy flux is found. By checking an analytical solution excluding Kelvin waves, we confirm that the cross-equatorial flux is caused by the meridional transport of geopotential at the equator. This is attributed to the combination of Kelvin and Yanai waves and leads to the asymmetric distribution of wave energy in the central basin. Coastally-trapped Kelvin waves along the African coast are identified by along-shore energy flux. In the north, the bend of the Guinea coast leads the flux back to the equatorial basin. In the south, the Kelvin waves strengthened by local wind transfer the energy from the equatorial to Angolan regions.

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

“…However, it is inappropriate to this study. Song and Aiki (2021) has revealed the deficiency of AGC-L2 scheme in estimating the wave energy flux in the wave event of which planetary vorticity changes rapidly. Specifically, in this study, when CTWs travel off the equator, AGC-L2 scheme underestimates the wave energy flux in the coastal region (see Figure S1 in Supporting Information S1).…”

“…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. This study hence attempts performing numerical experiments with ocean linear models to reproduce anomalous SST events along the southwest coast of Africa purely by linear wave dynamics, in which the development of waves is diagnosed through the AGC-L0 scheme so as to figure out the role of Atlantic equatorial waves in triggering the events.…”

Dual Wave Energy Sources for the Atlantic Niño Events Identified by Wave Energy Flux in Case Studies

“…We perform numerical experiments with the linear ocean model (hereafter referred to as LOM) by manipulating the wind forcing. We apply the diagnosis scheme for wave energy flux following Aiki et al (2017), Song and Aiki (2020), and Song and Aiki (2021) (hereafter referred to as AGC scheme short for the name of its authors, Aiki, Greatbatch and Claus) to identify equatorial waves in the development of the events, attempting to figure out the spatial sensitivity of forcing and the mechanism that leads to the diversity of the Atlantic Niño in terms of wave dynamics.…”