Midlatitude Wind Stress–Sea Surface Temperature Coupling in the Vicinity of Oceanic Fronts

Abstract

The influences of strong gradients in sea surface temperature on near-surface cross-front winds are explored in a series of idealized numerical modeling experiments. The atmospheric model is the Naval Research Laboratory Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) model, which is fully coupled to the Regional Ocean Modeling System (ROMS) ocean model. A series of idealized, two-dimensional model calculations is carried out in which the wind blows from the warm-to-cold side or the cold-to-warm side of an initially prescribed ocean front. The evolution of the near-surface winds, boundary layer, and thermal structure is described, and the balances in the momentum equation are diagnosed. The changes in surface winds across the front are consistent with previous models and observations, showing a strong positive correlation with the sea surface temperature and boundary layer thickness. The coupling arises mainly as a result of changes in the flux Richardson number across the front, and the strength of the coupling coefficient grows quadratically with the strength of the cross-front geostrophic wind. The acceleration of the winds over warm water results primarily from the rapid change in turbulent mixing and the resulting unbalanced Coriolis force in the vicinity of the front. Much of the loss/gain of momentum perpendicular to the front in the upper and lower boundary layer results from acceleration/ deceleration of the flow parallel to the front via the Coriolis term. This mechanism is different from the previously suggested processes of downward mixing of momentum and adjustment to the horizontal pressure gradient, and is active for flows off the equator with sufficiently strong winds. Although the main focus of this work is on the midlatitude, strong wind regime, calculations at low latitudes and with weak winds show that the pressure gradient and turbulent mixing terms dominate the cross-front momentum budget, consistent with previous work.

Wind changes above warm Agulhas Current eddies

“…They found, using a coupled model, that vertical mixing in the atmospheric layer was the main driver of such phenomenon rather than pressure gradient. This is quite similar to results of Spall (2007), who added that background wind speed was an important parameter to determine which of the numerous proposed mechanisms was responsible of the wind increase effect. Figure 1 shows the mean SST in August 2003 south of Africa done using AMSR-E SST.…”

“…Modifications to the marine atmospheric boundary layer and ocean-atmosphere interaction across frontal regions were documented during experiments at sea (Sweet et al, 1981;Davidson et al, 1991;Friehe et al, 1991;, Bourras et al, 2004 and also modelled (Samelson et al, 2006;Bourras et al, 2004;Spall, 2007;Byrne et al, 2015). Satellite remote sensing confirms and extends in space and time the relationship demonstrated during those experiments.…”

“…In our study, we note that because we have quite a strong wind speed here (mean of 11 m s −1 ) the conditions are generally near neutral, so this argument (QuikScat wind speed was not calibrated for unstable condition) does not hold to explain the difference between surface wind speed assuming neutral condition only and surface wind speed taking into account unstable condition due to buoyancy. This also probably means that processes involving the whole marine boundary layer as modelled and proposed by Spall (2007) or Byrne et al (2015) are more likely to be responsible for the increase rather than processes involving the downward transport of momentum in the constant flux layer only at the air-sea interface.…”

Spatial Patterns and Intensity of the Surface Storm Tracks in CMIP5 Models

“…In the Gulf Stream region, both mechanisms have been shown to play some role in at least one general circulation model (GCM) (Brachet et al 2012). However, other work suggests that the pressure gradient mechanism, which was created for the tropics, may not be very strong in high-wind regimes of the storm tracks (Spall 2007;Small et al 2008;Schneider and Qiu 2015). Additionally, recent analysis by Liu et al (2013) shows that the momentummixing mechanism tends to dominate on shorter time scales, such as those captured by the storm tracks.…”

Diagnostics for Near-Surface Wind Response to the Gulf Stream in a Regional Atmospheric Model*

“…When background wind blows parallel to an SST front, this mechanism yields horizontal curls of near-surface winds. On the other hand, when background wind blows over a warmto-cold SST gradient across a front, the mechanism yields horizontal convergence of near-surface winds (Spall 2007). Hereinafter, we use the term ''downward momentum mixing (DMM) mechanism'' to refer to downward momentum transport from aloft that should be externally given in the surface-layer system.…”