Evidence for the Maintenance of Slowly Varying Equatorial Currents by Intraseasonal Variability

Abstract

Recent evidence from mooring data in the equatorial Atlantic reveals that semiannual and longer time scale ocean current variability is close to being resonant with equatorial basin modes. Here we show that intraseasonal variability, with time scales of tens of days, provides the energy to maintain these resonant basin modes against dissipation. The mechanism is analogous to that by which storm systems in the atmosphere act to maintain the atmospheric jet stream. We demonstrate the mechanism using an idealized model setup that exhibits equatorial deep jets. The results are supported by direct analysis of available mooring data from the equatorial Atlantic Ocean covering a depth range of several thousand meters. The analysis of the mooring data suggests that the same mechanism also helps maintain the seasonal variability.

From Mixing to the Large Scale Circulation: How the Inverse Cascade Is Involved in the Formation of the Subsurface Currents in the Gulf of Guinea

“…Rossby waves, instead of nonlinear eddies, have been shown to play a significant role in the formation of mean jet-like currents in some studies (see [109] and references therein). Rossby waves can be important to explain other jet-like structures observed in the present simulation, in particular, the western region or deeper low latitude intermediate currents [23,44,45,124].…”

“…Huttl-Kabus et al [21] showed that the cores of NEUC and SEUC mostly originate from the southeastern tropical Atlantic. Jochum and Malanotte-Rizzoli [22] used idealized simulations to show that tropical instability waves (TIW) helped to maintain the SEUC against dissipation west of 10 • W. Greatbatch et al [23] showed that intraseasonal variability ( 10 s of days) provides the energy to maintain semiannual and longer time scale ocean currents against dissipation, but they focused on the Equatorial Deep Jets (EDJ). In the Gulf of Guinea, there is also a strong seasonal cycle associated with the annual equatorial upwelling forced by the trade winds and the formation of the so called cold-tongue from June to August and a second shorter season from November to December.…”

PIRATA: A Sustained Observing System for Tropical Atlantic Climate Research and Forecasting

“…Idealized simulations (Ascani et al, ) and the reconstruction of EDJ forcing derived from the 23°W mooring data (Claus et al, ) suggest an energy transfer from downward‐propagating intraseasonal Yanai waves to the EDJ at intermediate depths. Using idealized simulations and velocity data from three moorings along 23°W between 0.75°N and 0.75°S, Greatbatch et al () showed that the meridional flux of zonal momentum associated with intraseasonal variability acts to maintain the EDJ against dissipation. The inclusion of EDJ or their effects in coupled climate simulations remains an open issue to be solved in the future.…”

“…Using idealized simulations and velocity data from three moorings along 23°W between 0.75°N and 0.75°S, Greatbatch et al (2018) showed that the meridional flux of zonal momentum associated with intraseasonal variability acts to maintain the EDJ against dissipation. The inclusion of EDJ or their effects in coupled climate simulations remains an open issue to be solved in the future.…”

“…Using satellite data, Athie and Marin () could show that two types of intraseasonal waves dominate in the equatorial Atlantic: westward propagating TIWs dominate west of 10°W, and shorter period, wind‐driven Yanai waves dominate in the Gulf of Guinea east of 10°W. Generally, intraseasonal variability has a great importance for the mixed layer heat budget (Hummels et al, ) due to the associated divergence in meridional and vertical heat fluxes, but at the same time intraseasonal waves provide energy to the deep ocean to maintain the equatorial Atlantic circulation (Ascani et al, ; Greatbatch et al, ), as presented in more detail in section . High‐resolution model simulations that were validated against moored observation taken at the equator, 23°W were used to calculate eddy production rates and show that barotropic and baroclinic instabilities contribute to generate TIWs in the central equatorial Atlantic (Jochum et al, ; Von Schuckmann et al, ), thereby confirming early results obtained from analyzing observational data (Grodsky et al, ).…”