Variations of Equatorial Shear, Stratification, and Turbulence Within a Tropical Instability Wave Cycle

Abstract

Equatorial Internal Wave Experiment observations at 0°, 140°W from October 2008 to February 2009 captured modulations of shear, stratification, and turbulence above the Equatorial Undercurrent by a series of tropical instability waves (TIWs). Analyzing these observations in terms of a four‐phase TIW cycle, we found that shear and stratification within the deep‐cycle layer being weakest in the middle of the N‐S phase (transition from northward to southward flow) and strongest in the late S phase (southward flow) and the early S‐N phase (transition from southward to northward flow). Turbulence was modulated but showed less dependence on the TIW cycle. The vertical diffusivity (KT) was largest during the N (northward flow) and N‐S phases, when stratification was weak, despite weak shear, and was smallest from the late S phase to the S‐N phase, when stratification was strong, despite strong shear. This tendency was less clear in turbulent heat flux because vertical temperature gradients were small at times when KT was large, and large when KT was small. We investigated the dynamics of shear and stratification variations within the TIW cycle by using an ocean general circulation model forced with observed winds. The model successfully reproduced the observed strong shear and stratification in the S phase, except for a small phase difference. The strong shear is explained by vortex stretching by TIWs. The strong stratification is explained by meridional and vertical advection.

Direct Measurements of Upper Ocean Horizontal Velocity and Vertical Shear in the Tropical North Atlantic at 4°Ν, 23°W

“…The composite TIW SST response is also location specific. Some studies find surface cooling (warming) during or just after the northward (southward) flow phase (Cronin & Kessler, ; Kennan & Flament, ; Menkes et al, ; Wenegrat & McPhaden, ), consistent with our study, whereas others find a less well‐defined relationship, as other processes like surface heating can modulate temperatures within a TIW cycle (e.g., Inoue et al, , ).…”

“…oscillations themselves, the MSSD remains almost constant near 45 m. For most of the TIWs, a shallower (deeper) MSSD is seen during maximum northward (southward) flow. This is because southward flow typically penetrates deeper than the northward flow during a TIW, consistent with equatorial TIWs (Grodsky et al, ; Inoue et al, , ; Wenegrat & McPhaden, ).…”

“…Both du/dz and dv/dz contribute substantially to total vertical shear at all depths (Figures d and e), and often the contribution of dv/dz to the total shear squared exceeds that of du/dz (red and blue lines in Figure a, respectively). This differs from the equator where du/dz associated with the westward SECN, and the underlying eastward flowing Equatorial Undercurrent dominates the total shear as seen in the Pacific during a TIW season (e.g., Inoue et al, , ), and du/dz is often the only component documented in the equatorial Atlantic (e.g., Wenegrat & McPhaden, ).…”

“…Until now, TIW composites have not been generated for the tropical Atlantic. However, TIW composite analysis has been similarly applied to several months of moored data in the tropical Pacific at 2°N, 140°W influenced by the SECN and tropical cells (Cronin & Kessler, 2009) and at 0°N, 140°W influenced by the SECN and the Equatorial Undercurrent (Inoue et al, 2019). In addition, the velocity structure of a single equatorial TIW was studied in the Atlantic by Wenegrat and McPhaden (2015) and in the Pacific by Inoue et al (2012), as well as the flow field of tropical instability vortices (TIVs) located to the north of the cold tongue front between TIW crests (Kennan & Flament, 2000;Menkes et al, 2002).…”

“…Whether the composite TIW flow is symmetric, and whether there is eastward or westward flow during the northward phase, depends on the latitude of the cold tongue front and the position and orientation of TIVs that form north of the cold tongue between successive TIWs (Cronin & Kessler, 2009;Hormann et al, 2013;Inoue et al, 2012Inoue et al, , 2019Kennan & Flament, 2000;Menkes et al, 2002;Wenegrat & McPhaden, 2015). For example, more symmetry would also be expected if the vortices crossed the mooring at their latitudinal midpoint and were longitudinally centered between successive TIWs.…”