Abstract
AbstractThe ocean surface boundary layer is a critical interface across which momentum, heat, and trace gases are exchanged between the oceans and atmosphere. Surface processes (winds, waves, and buoyancy forcing) are known to contribute significantly to fluxes within this layer. Recently, studies have suggested that submesoscale processes, which occur at small scales (0.1–10 km, hours to days) and therefore are not yet represented in most ocean models, may play critical roles in these turbulent exchanges. While observational support for such phenomena has been demonstrated in the vicinity of strong current systems and littoral regions, relatively few observations exist in the open‐ocean environment to warrant representation in Earth system models. We use novel observations and simulations to quantify the contributions of surface and submesoscale processes to turbulent kinetic energy (TKE) dissipation in the open‐ocean surface boundary layer. Our observations are derived from moorings in the North Atlantic, December 2012 to April 2013, and are complemented by atmospheric reanalysis. We develop a conceptual framework for dissipation rates due to surface and submesoscale processes. Using this framework and comparing with observed dissipation rates, we find that surface processes dominate TKE dissipation. A parameterization for symmetric instability is consistent with this result. We next employ simulations from an ocean front‐resolving model to reestablish that dissipation due to surface processes exceeds that of submesoscale processes by 1–2 orders of magnitude. Together, these results suggest submesoscale processes do not dramatically modify vertical TKE budgets, though such dynamics may be climatically important owing to their ability to remove energy from the ocean.
Highlights
Mechanisms that control the exchange of momentum, heat, and trace gases between the oceans and atmosphere are critical to Earth's climate
In all of our analysis, we approximated the ocean surface boundary layer (OSBL) thickness, H, by the mixed layer (ML) depth, defined as the depth at which the potential density exceeds its value at 10 m by 0.03 kg/m3. This has the distinct advantage of being consistent with our representation of surface- and submesoscale-forced turbulence in the models below. (We considered a definition of OSBL thickness based on the gradient in potential vorticity [PV] but found it made little difference in our results—at least in winter.) The surface buoyancy flux from the ocean to the atmosphere Bo was estimated in the usual manner, namely, Bo
We are able to exclude baroclinic instability (BCI) as a mechanism for dissipation in our environment since BCI generally fluxes energy to the large scale rather than to smaller scales where dissipation takes place. (Note, there is evidence of a forward cascade of energy—i.e., to dissipative scales—occurring during submesoscale BCI in high-resolution numerical simulations; Molemaker et al, 2010; Skyllingstad & Samelson, 2012.) We argue that centrifugal instability (CI) can be neglected since vorticity values at our moored site are much smaller than f in our domain
Summary
Mechanisms that control the exchange of momentum, heat, and trace gases between the oceans and atmosphere are critical to Earth's climate. Processes taking place at horizontal and temporal scales of 0.1–10 km and hours to days scales and not yet represented in climate models have been identified as influencing turbulence and buoyancy budgets within the ocean surface boundary layer (OSBL) (Boccaletti et al, 2007; Callies et al, 2016; Fox-Kemper et al, 2008, 2011; Grisouard, 2018; Haine & Marshall, 1998; Thomas, 2005; Thomas & Lee, 2005; Taylor & Ferrari, 2009, 2010; Thomas & Taylor, 2010). These have collectively been referred to as submesoscale processes (McWilliams, 2016; Thomas et al, 2008). The chief reason is that present-day observing strategies do not sufficiently resolve submesoscale motions owing to their quickly
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