Air-sea heat exchange along the northern sea surface temperature front in the eastern tropical Pacific

The atmospheric response to the oceanic forcing in the eastern Pacific along the northern equatorial sea surface temperature (SST) front is investigated in terms of sensible and latent heat flux during the 6-month period 28 July 1999 through 27 January 2000. Of particular interest is the atmospheric boundary layer (ABL) response to oceanic Tropical Instability Waves (TIWs) that distort the SST front during May through January in normal years. In previous studies, time series of boundary layer properties clearly show the influence of TIWs but the relationship to spatial patterns of SST and wind stress has been inferred only from sparse in situ data. In this study, satellite observations are used to composite in situ data from moorings to compensate for the lack of a spatially dense mooring array. The variability in the position of the SST front caused by propagating TIWs enables fixed mooring locations to measure the atmospheric boundary layer (ABL) response from a large range of locations relative to the front. The satellite data enable determination of the precise location of the mooring relative to the front. The advantage of this strategy is the recurring measurement of the ABL response to the SST front over the six month period considered here. The results indicate that the TIW-induced perturbations of sensible and latent heat flux are spatially shifted in phase towards the east relative to the perturbations of SST. The maximum fluxes are not centered directly over the warmest water, but are shifted towards the portion of the frontal region where a disequilibrium boundary layer is expected due to the advection of colder air from the equatorial region. The changes of sensible and latent heat fluxes across the SST front have magnitudes of about 11 Wm⁻² and 126 Wm⁻², respectively. The sensible and latent heat flux patterns are interpreted in two complementary ways: (1) as an atmospheric response to the change of oceanic forcing as air flows across the SST front; and (2) as the atmospheric response to westward propagating TIWs along the SST front

Atmospheric boundary layer coupling to midlatitude mesoscale sea surface temperature anomalies

This thesis examines the mechanisms that couple the monthly-averaged atmospheric boundary layer (ABL) to open-ocean sea surface temperature (SST) perturbations on scales of 50-500 km. The observed positive correlation between surface wind speed anomalies and SST anomalies is successfully simulated using the Weather Research and Forecasting (WRF) model. In numerical experiments with idealized SST fronts, the cross-frontal surface wind acceleration in the cold-to-warm case and deceleration in the warm-to- cold case are found over narrow transition zones co-located with the narrow regions of large frontal SST changes. In the transition zone, horizontal momentum is redistributed vertically in the ABL by turbulence and convection. The largest pressure adjustments, on the other hand, take place over a much broader region downstream from the SST front. In the cold-to-warm transition zone the model simulates an unstable thermal internal boundary layer (TIBL) in the lower part of the ABL. As the TIBL grows, higher velocity air aloft is incorporated into the TIBL, accelerating the flow. Over the warm-to-cold transition zone, the momentum boundary layer collapses, and vertical mixing of momentum by turbulence and convection ceases in the upper part of the ABL. The WRF model is also applied to open-ocean ABL flow across idealized sinusoidal SST anomalies having scales similar to those observed in the Agulhas return current region. The simulated horizontal pressure gradient force anomalies are crucial to the response over the entire domain, and the vertically integrated momentum budget is found to be approximately linear. A linear diagnostic model is therefore developed which successfully predicts the observed phase and amplitude of the ABL wind, pressure and temperature response to the SST anomalies, with largest quantitative discrepancies found in the perturbation wind component perpendicular to the mean wind direction. By using the divergence and vorticity budgets, the diagnostic model shows that differences in the vertical structure of the perturbation wind components down and across the mean wind can explain the differences in the coupling coefficients for the divergence and curl as functions of downwind and crosswind SST gradients, and as functions of the angle between the SST gradient and the mean wind

Coupling between Sea Surface Temperature and Low-Level Winds in Mesoscale Numerical Models

This study evaluates the impacts of sea surface temperature (SST) specification and grid resolution on numerical simulations of air–sea coupling near oceanic fronts through analyses of surface winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) model. The 9 May 2001 change of the boundary condition from the Reynolds SST analyses to the NOAA Real-Time Global (RTG) SST in the ECMWF model resulted in an abrupt increase in mesoscale variance of the model surface winds over the ocean. In contrast, the 21 November 2000 change of the grid resolution resulted in an abrupt increase in mesoscale variability of surface winds over mountainous regions on land but had no significant effect on winds over the ocean. To further investigate model sensitivity to the SST boundary condition and grid resolution, a series of simulations were made with the Weather Research and Forecasting (WRF) model over a domain encompassing the Agulhas return current (ARC: also called “retroflection”) region in the south Indian Ocean. Results from three WRF simulations with SST measured by the Advanced Microwave Scanning Radiometer on the Earth Observing System Aqua satellite (AMSR-E) and the Reynolds and RTG SST analyses indicate the vital importance of the resolution of the SST boundary condition for accurate simulation of the air–sea coupling between SST and surface wind speed. WRF simulations with grid spacings of 40 and 25 km show that the latter increased energy only on scales shorter than 250 km. In contrast, improved resolution of SST significantly increased the mesoscale variability for scales up to 1000 km. Further sensitivity studies with the WRF model conclude that the weak coupling of surface wind speeds from the ECMWF model to SST is likely attributable primarily to the weak response of vertical turbulent mixing to SST-induced stability in the parameterization of boundary layer turbulence, with an overestimation of vertical diffusion by about 60% on average in stable conditions and an underestimation by about 40% in unstable conditionsKeywords: Wind, Mesoscale models, Air–sea interaction, Fronts, Sea surface temperatur