Dynamical analyses of marine atmospheric boundary layer structure near the Gulf Stream oceanic front

Abstract

The effects of the sea surface temperature (s.s.t.) front at the edge of the Gulf Stream on the marine atmospheric boundary layer (MABL) are investigated using a numerical model to study the modification effects of an oceanic front on the MABL structure. The situation simulated is flow from over cold shelf water to over the warm water of the Gulf Stream. The initial temperature and humidity profiles of the air are specified to be near neutral over the cold water and are therefore typical of undisturbed conditions. The differential in s.s.t. across the oceanic front creates a horizontal variation in the surface perturbation pressure and the stability. The surface perturbation pressure and turbulent fluxes modulate the flow and produce horizontal variations in horizontal wind components with associated vertical motions. A thermally direct cell is produced as a result of the s.s.t. difference across the front. The isotherms slope upward towards the warm water. Entrainment of inversion layer air and upward vertical motion over the warm water cause the MABL to be deeper there. A layer of cloud forms over warm water and is associated with mixed layer deepening rather than lowering of the condensation level. Turbulent fluxes in the MABL show considerable spatial variation. Surface stress is much larger over the front and over the warm water than over the cold water. This is mostly caused by wind speed changes associated with the front. Changes in the drag coefficient due to changes in surface roughness and stability are much less important.Mean budgets for temperature and total water indicate that there is a balance between horizontal advection and turbulent flux divergence. The U momentum budget shows that once the geostrophic balance terms are subtracted, the balance is mainly between the pressure gradient force associated with the induced temperature field and turbulent friction, with horizontal advection and the Coriolis force acting on the geostrophic departure playing minor roles. The V momentum budget shows a balance between horizontal advection, Coriolis force and friction.Although there are few data for comparison, the results are in qualitative agreement with observations in the area. This study shows that the s.s.t. front at the Gulf Stream edge produces marked local changes in the nearby atmospheric surface layer. 29

On the Coupling of Wind Stress and Sea Surface Temperature

“…The main goal is to clarify the effect of variations in the intensity of turbulent mixing within the boundary layer on near-surface winds under quasiequilibrium conditions by considering a model that retains a basic representation of the essential physics, but is simple enough to yield explicit analytical solutions whose dependence on parameters can be readily identified and interpreted. The discussion of numerical model results by Wai and Stage (1989) anticipates some aspects of the present hypothesis, but focuses on the pressure gradient mechanism and on variability with smaller horizontal scales. The present study does not address the pressure gradient mechanism or the force balances in the complex transition region in the immediate vicinity of SST fronts.…”

“…Other potentially important mechanisms not explored here include thermodynamically induced pressure gradients (e.g., Lindzen and Nigam 1987;Wai and Stage 1989;Small et al 2003). However, the present model can account for the observed influence of SST on low-level winds in the far field, where quasi-equilibrium conditions are more likely to pertain; the model predicts that, for a given fixed pressure gradient, the low-level winds will be stronger over warmer water than over cooler water, because of the greater depth of the stress boundary layer over the warmer water.…”

“…In this scenario, it is argued that the enhanced winds over warm water are maintained by enhanced downward fluxes, induced by convective instability, of horizontal momentum from the faster flow above to the slower near-surface flow. Other possible mechanisms include flows driven by thermodynamically induced pressure gradients (e.g., Lindzen and Nigam 1987;Wai and Stage 1989;Small et al 2003).…”

On the air‐sea interaction in areas of thermal marine fronts in the Greenland Sea

“…Hsu (1984) has devised an analytic model for the phenomenon regarding it to be similar to the classical sea-breeze circulation. Later, Huang and Raman (1988) and Wai and Stage (1989) have shown in their simulations with two-dimensional models that this kind of SST distribution with respect to the wind direction results in a thermally driven direct cell with accelerated wind speeds over the warm water side. According to the model simulations of Warner et al (1990) the SST gradient associated with the north wall of the Gulf Stream can produce horizontal velocities in excess of 7 m s" 1 when simulated from calm initial conditions.…”

Acknowledging contributions

“…iii) As the air temperature and moisture starts to respond to the surface fluxes, the atmospheric pressure also changes (Lindzen and Nigam 1987). This leads to a spatial pressure gradient which can drive secondary circulations (Wai and Stage 1989). iv) The surface currents of ocean fronts or eddies will either increase or reduce the relative motion of the air and ocean, acting to change the surface stress, thus affecting the atmosphere as well as feeding back onto the ocean (Kelly et al 2001, Cornillon andPark 2001).…”