Sources and Pathways of Intraseasonal Meridional Kinetic Energy in the Equatorial Atlantic Ocean

Abstract

In the equatorial Atlantic Ocean, meridional velocity variability exhibits a pronounced peak on intraseasonal timescales whereas zonal velocity dominantly varies on seasonal to interannual timescales. We focus on the intraseasonal meridional velocity variability away from the near-surface layer, its source regions and its pathways into the deep ocean. This deep intraseasonal velocity variability plays a key role in equatorial dynamics as it is an important energy source for the deep equatorial circulation. The results are based on the output of a high-resolution ocean model revealing intraseasonal energy levels along the equator at all depths that are in good agreement with shipboard and moored velocity data. Spectral analyses reveal a pronounced signal of intraseasonal Yanai waves with westward phase velocities and zonal wavelengths longer than 450 km. Different sources and characteristics of these Yanai waves are identified: near the surface between 40°W and 10°W low-baroclinic-mode Yanai waves with periods of around 30 days are exited. These waves have a strong seasonal cycle with a maximum in August. High-frequency Yanai waves (10–20-day period) are excited at the surface east of 10°W. In the region between the North Brazil Current and the Equatorial Undercurrent high-baroclinic-mode Yanai waves with periods between 30 and 40 days are generated. Yanai waves with longer periods (40-80 days) are shed from the Deep Western Boundary Current. The Yanai wave energy is carried along beams east- and downward thus explaining differences in strength, structure and periodicity of the meridional intraseasonal variability in the equatorial Atlantic Ocean.

Tropical Instability Waves and Wind‐Forced Cross‐Equatorial Flow in the Central Atlantic Ocean

“…First, we will focus on meridional velocity fluctuations averaged between the uppermost available velocity records and 75 m. The derived fluctuations are a manifestation of TIWs, a continuous band of wave‐like structures passing the mooring site from east to west, thereby changing the flow direction from northward in the leading phase of a TIW to southward in the subsequent phase. The moored velocity records show that the amplitude of the meridional velocity fluctuations undergoes a distinct seasonal cycle (Körner et al., 2022; Tuchen et al., 2018) with increasing amplitudes during boreal spring and early summer, maximum velocities occasionally exceeding 50 cm/s in boreal summer and early fall before decreasing again in late boreal fall and winter. Despite the seasonality, the record also reveals strong year‐to‐year variability in the strength and occurrence of TIWs.…”

Multidecadal Intensification of Atlantic Tropical Instability Waves

“…Through their impact on SST, TIWs also feed back onto the large‐scale atmospheric circulation (Caltabiano et al., 2005; Seo & Xie, 2011; Seo et al., 2007; Wu & Bowman, 2007b). More recently, TIWs and intraseasonal waves excited by TIWs have been identified as an important component and maintenance mechanism of the deep equatorial Atlantic circulation (Ascani et al., 2010, 2015; Bastin et al., 2020; Greatbatch et al., 2018; Körner et al., 2022; Tuchen et al., 2018).…”

“…. More recently, TIWs and intraseasonal waves excited by TIWs have been identified as an important component and maintenance mechanism of the deep equatorial Atlantic circulation (Ascani et al, 2010(Ascani et al, , 2015Bastin et al, 2020;Greatbatch et al, 2018;Körner et al, 2022;Tuchen et al, 2018).…”

Factors influencing the meridional width of the equatorial deep jets

“…Although the EDJs' generation mechanisms are still not entirely clear, it is thought that they are excited by intraseasonal Yanai waves that originate from instabilities in the western boundary currents and/or between the near-surface ocean currents, for example in the form of Tropical Instability Waves (d 'Orgeville et al, 2007;Hua et al, 2008;Ascani et al, 2015;Ménesguen et al, 2019). Although Tropical Instability Waves are generated near the surface, they sometimes excite intraseasonal variability that can propagate to depths of a few thousand metres (Tuchen et al, 2018;Körner et al, 2022), where they can provide energy to the EDJ.…”