An Annual Cycle of Submesoscale Vertical Flow and Restratification in the Upper Ocean

Abstract

Numerical simulations suggest that submesoscale turbulence may transform lateral buoyancy gradients into vertical stratification and thus restratify the upper ocean via vertical flow. However, the observational evidence for this restratifying process has been lacking due to the difficulty in measuring such ephemeral phenomena, particularly over periods of months to years. This study presents an annual cycle of the vertical velocity and associated restratification estimated from two nested clusters of meso- and submesoscale-resolving moorings, deployed in a typical midocean area of the northeast Atlantic. Vertical velocities inferred using the nondiffusive density equation are substantially stronger at submesoscales (horizontal scales of 1–10 km) than at mesoscales (horizontal scales of 10–100 km), with respective root-mean-square values of 38.0 ± 6.9 and 22.5 ± 3.3 m day−1. The largest submesoscale vertical velocities and rates of restratification occur in events of a few days’ duration in winter and spring, and extend down to at least 200 m below the mixed layer base. These events commonly coincide with the enhancement of submesoscale lateral buoyancy gradients, which is itself associated with persistent mesoscale frontogenesis. This suggests that mesoscale frontogenesis is a regular precursor of the submesoscale turbulence that restratifies the upper ocean. The upper-ocean restratification induced by submesoscale motions integrated over the annual cycle is comparable in magnitude to the net destratification driven by local atmospheric cooling, indicating that submesoscale flows play a significant role in determining the climatological upper-ocean stratification in the study area.

Kinetic Energy of Eddy‐Like Features From Sea Surface Altimetry

“…Figure a shows daily variability, where the observed seasonal cycle peaked during October is attributed to a lagged response between the eddy field and the seasonality of the mixed layer (Nardelli et al., ) and is consistent with the submesoscale observations presented by Yu et al. (). Additionally, the running annual mean shows a significant decrease of −35.14 eddies per decade.…”

Dynamics of Mesoscale Eddies Interacting With a Western Boundary Current Flowing by a Gap

“…In addition, the seasonally variable WBC and the South China Sea throughflow are shown to alter the WBC hysteresis strongly (Song et al, ), which will impact the WBC‐eddy interactions. The seasonal variability of the mesoscale eddies (Yuan et al, ) and its effects on the seasonal variability of the WBC, eddy kinetic energy, and ocean stratification (Chang & Oey, ; Yu et al, ; Zhai et al, ) will also be considered in the future studies of WBC‐eddy interactions.…”

Wind‐Forced Symmetric Instability at a Transient Mid‐Ocean Front

“… Mooring observations . As part of the OSMOSIS project, nine subsurface moorings were deployed over the Porcupine Abyssal Plain (48.63–48.75° N, 16.09–16.27° W; Figure a) in the Northeast Atlantic Ocean for the period September 2012 to September 2013 (Buckingham et al., ; Yu et al., ). The mooring array was arranged in two concentric quadrilaterals with side lengths of ∼13 km (outer cluster) and ∼2 km (inner cluster) around a centrally located single mooring (Figure S1a in the supporting information).…”

“…More details on the OSMOSIS moorings, including observational uncertainties, are provided in Yu et al. (). Seaglider observations . In addition to the mooring observations, the OSMOSIS region (approximately 20 × 20 km) was continuously sampled by at least two (five in total) autonomous underwater gliders for the entire year (Damerell et al., ; Erickson & Thompson, ; Evans et al., ; Thompson et al., ).…”