Abstract
A recently developed energy flux diagnosis scheme, which incorporates a smooth connection between the tropical and subtropical zones, is used in the present study to investigate vertically propagating waves in the tropical Indian Ocean (IO) based on the result of a linear, continuously stratified ocean model driven by climatological wind forcing. This extended diagnosis reveals deep-reaching eastward energy fluxes at the equator which develop four times per year and are associated with equatorial Kelvin waves (KWs) generated by semiannual winds. The authors find that the downward transfer of wave energy is particularly deep in the southern Bay of Bengal (SBoB). This downward flux is attributed to off-equatorial Rossby waves and appears four times per year, maximizing its amplitude during November–December. Southwesterly winds in the Arabian Sea intensify eastward energy flux of KWs at mid-depth, which maximizes in amplitude in August. This is contrastive to KW energy flux at the surface which peaks in May. These mid-depth equatorial KW packets subsequently arrive at the eastern boundary of the IO and are diffracted poleward to produce downward energy flux in November and December detected in the SBoB.
Highlights
In the tropical regions, the dynamics of the atmosphere– ocean interactions dictating the growth and morphology of large-scale surface currents strongly depend on the subsurface oceanic structure
In terms of model performance at the key depths of 550 m and 1100 m, the Pressure flux (PF) scheme fails to reveal important energy fluxes with distinct eastward Kelvin waves (KWs) signatures and misses a series of westward signals associated with midlatitude Rossby waves (RWs) (Fig. 4c, e)
The Aiki et al 2017 (AGC17) scheme is able to show the eastward transfer of wave energy along the equator associated with equatorial KWs and the westward transfer of wave energy in offequatorial regions associated with midlatitude RWs (Fig. 4d, f)
Summary
The dynamics of the atmosphere– ocean interactions dictating the growth and morphology of large-scale surface currents strongly depend on the subsurface oceanic structure. Further to the west and in the regions off the equator, the monsoonal phase transition produces a seasonal cycle in wind forcing (Reppin et al 1999; Schott and McCreary 2001) The former semiannual wind forcing induces equatorial Kelvin waves (KWs) with a downwelling. Owing to the dominance of semiannual wind forcing, variability in mid-depth of the equatorial IO below the main pycnocline is characterized by semiannual variability (Luyten and Roemmich 1982; Nagura and McPhaden 2016; Huang et al 2018a, b; Zanowski and Johnson 2019; Chen et al 2020) They consist of equatorial KWs and long RWs, which carry energy to the east and to
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