Deep cycle turbulence in Atlantic and Pacific cold tongues

Abstract

Typically, open ocean surface mixed layers away from the equator are driven by a combination of wind stress and convection from nighttime cooling or other forms of cold air outbreaks (Shay & Gregg, 1986) and these mixed layers largely contain the turbulence within (Anis & Moum, 1994). A unique aspect of equatorial smallscale fluid dynamics that has been observed in the Pacific's cold tongue (PCT) at 0°140°W is the existence of diurnally-varying turbulence beneath a nighttime surface mixed layer (Lien et al., 1995;Moum et al., 1989). This sub-mixed layer turbulence at the equator has been termed deep cycle (DC) turbulence.The existence of DC turbulence is linked to the strongly sheared current system above the core of the Equatorial Undercurrent (EUC) where the gradient Richardson number (Ri) persistently maintains a near-critical state with values fluctuating around 0.25 , a state of marginal instability (MI). This persistent MI state is nudged beyond critical toward the end of the solar day, associated with the deepening of the sheared base of the diurnal warm layer that forms during the period of net daytime heating and deepens by shear-induced mixing (K. G. Hughes et al., 2021). A diurnal composite derived from 8 days of microstructure profiling revealed that the shear layer descends from near the surface to about 60 m at a rate close to 6 m per hour . The arrival of the shear layer was found to trigger shear instability and also to immediately precede enhanced turbulence, suggesting causality (also, see model results by Pham et al. ( 2013)).To summarize, DC turbulence has two defining properties:1. it occurs below the ML base, and 2. it cycles diurnally.

Multidecadal Intensification of Atlantic Tropical Instability Waves

“…Aside from the observed increase in horizontal ETA (Figure 4), intensified TIWs are expected to cause stronger subsurface mixing and intensified sea surface cooling (Moum et al., 2009). However, the net effect of TIWs on the SST budget, and specifically the interplay of advection and vertical mixing due to TIWs, is still an open question (Moum et al., 2022).…”

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

“…Moreover, we make use of daily mean wind speed recorded at the PIRATA buoy between 2001 and 2021 as well as a wind stress curl climatology from Risien and Chelton (2008). Further, daily mean χ‐pod turbulence data (Moum et al., 2022, Text S1 in Supporting Information ) from the mooring site are investigated.…”