scholarly journals U.S. West Coast Surface Heat Fluxes, Wind Stress, and Wind Stress Curl from a Mesoscale Model

2005 ◽  
Vol 133 (11) ◽  
pp. 3202-3216 ◽  
Author(s):  
T. Haack ◽  
S. D. Burk ◽  
R. M. Hodur
Keyword(s):  
Relative Humidity ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Wind Stress ◽  
Mesoscale Model ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Wind Stress Curl ◽  
Low Level

Abstract Monthly averages of numerical model fields are beneficial for depicting patterns in surface forcing such as sensible and latent heat fluxes, wind stress, and wind stress curl over data-sparse ocean regions. Grid resolutions less than 10 km provide the necessary mesoscale detail to characterize the impact of a complex coastline and coastal topography. In the present study a high-resolution mesoscale model is employed to reveal patterns in low-level winds, temperature, relative humidity, sea surface temperature as well as surface fluxes, over the eastern Pacific and along the U.S. west coast. Hourly output from successive 12-h forecasts are averaged to obtain monthly mean patterns from each season of 1999. The averages yield information on interactions between the ocean and the overlying atmosphere and on the influence of coastal terrain forcing in addition to their month-to-month variability. The spring to summer transition is characterized by a dramatic shift in near-surface winds, temperature, and relative humidity as offshore regions of large upward surface fluxes diminish and an alongshore coastal flux gradient forms. Embedded within this gradient, and the imprint of strong summertime topographic forcing, are small-scale fluctuations that vary in concert with local changes in sea surface temperature. Potential feedbacks between the low-level wind, sea surface temperature, and the wind stress curl are explored in the coastal regime and offshore waters. In all seasons, offshore extensions of colder coastal waters impose a marked influence on low-level conditions by locally enhancing stability and reducing the wind speed, while buoy measurements along the coast indicate that sea surface temperatures and wind speeds tend to be negatively correlated.

scholarly journals Forward and Reverse Shear Environments during Polar Low Genesis over the Northeast Atlantic

2016 ◽  
Vol 144 (4) ◽  
pp. 1341-1354 ◽  
Author(s):  
Annick Terpstra ◽  
Clio Michel ◽  
Thomas Spengler
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Synoptic Scale ◽  
Low Level ◽  
Wind Speed Maximum ◽  
Polar Low ◽  
Baroclinic Zone

Abstract The synoptic and subsynoptic environments associated with polar low genesis are examined. Ambient pre–polar low environments are classified as forward or reverse shear conditions based on the angle between the thermal and mean wind. Forward shear environments are associated with a synoptic-scale ridge over Scandinavia, featuring a zonally oriented baroclinic zone extending throughout the troposphere with a wind speed maximum at the tropopause. Similar to typical midlatitude cyclogenesis, concurrent wavelike development occurs both in the lower and upper troposphere along the baroclinic zone and the mean propagation direction is eastward, parallel to isolines of sea surface temperature. Reverse shear environments exhibit a distinctly different structure and are characterized by a trough over Scandinavia, associated with a synoptic-scale, occluded cyclone. The genesis area exhibits strong cold air advection on its right-hand side and polar low development occurs on the warm side of an intense low-level jet. The environment resembles the characteristics conducive to secondary development associated with frontal instability. Polar lows developing in this configuration propagate mainly southward, perpendicular to isolines of sea surface temperature. The two genesis environments exhibit similar temperature differences between the sea surface and atmosphere near the surface, yet the magnitude of the surface fluxes is approximately double during reverse shear conditions due to stronger low-level winds. The ratio between surface sensible and latent heat fluxes is close to unity for both shear environments.

scholarly journals Sea Surface Temperature and Wind Stress Curl Variability near a Cape

2012 ◽  
Vol 42 (11) ◽  
pp. 2073-2087 ◽  
Author(s):  
Renato M. Castelao
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Wind Stress ◽  
Current System ◽  
Empirical Orthogonal Functions ◽  
Sea Surface ◽  
Western Boundary ◽  
Surface Cooling ◽  
Wind Stress Curl ◽  
California Current System

Abstract The coupling between sea surface temperature (SST), SST gradients, and wind stress curl variability near a cape off Brazil is investigated using satellite observations and several different SST high-resolution analyses. The cape is characterized by strong SST fronts year-round, associated with upwelling and advection of warm water offshore in a western boundary current. Observations reveal a strong coupling between crosswind SST gradients and wind stress curl variability, with the predominantly negative crosswind gradients leading to negative, upwelling favorable wind stress curl anomalies. The spatial correlation between empirical orthogonal functions (EOF) of those variables is ~0.6, while the correlation between the EOF amplitude time series of the wind stress curl and crosswind SST gradients is larger than 0.7. The coupling occurs during summer and winter and is strongly modulated by variations in the wind stress directional steadiness. The intensity of the coupling is weaker than around capes on the California Current system, presumably because of higher variability in wind direction off Brazil. During periods of high wind stress directional steadiness off Cape Frio, the coupling is intensified by up to 40%–75%. Wind stress curl is also correlated with SST itself, especially in the vicinity of the cape, although not as strongly as with crosswind SST gradients. The analyses suggest that the observed wind stress curl anomalies can lead to surface cooling of as much as 1°C. If the enhanced upwelling leads to further strengthening of the upwelling front, negative wind stress curl anomalies may be intensified in a positive feedback mechanism.

scholarly journals The Atmospheric Response to Weak Sea Surface Temperature Fronts*

2015 ◽  
Vol 72 (9) ◽  
pp. 3356-3377 ◽  
Author(s):  
Niklas Schneider ◽  
Bo Qiu
Keyword(s):  
Boundary Layer ◽  
Surface Temperature ◽  
Atmospheric Boundary Layer ◽  
Sea Surface Temperature ◽  
Wind Stress ◽  
Large Scale ◽  
Sea Surface ◽  
Coupling Coefficients ◽  
Wind Stress Curl ◽  
Sst Gradient

Abstract The response of the atmospheric boundary layer to fronts of sea surface temperature (SST) is characterized by correlations between wind stress divergence and the downwind component of the SST gradient and between the wind stress curl and the crosswind component of the SST gradient. The associated regression (or coupling) coefficients for the wind stress divergence are consistently larger than those for the wind stress curl. To explore the underlying physics, the authors introduce a linearized model of the atmospheric boundary layer response to SST-induced modulations of boundary layer hydrostatic pressure and vertical mixing in the presence of advection by a background Ekman spiral. Model solutions are a strong function of the SST scale and background advection and recover observed characteristics. The coupling coefficients for wind stress divergence and curl are governed by distinct physics. Wind stress divergence results from either large-scale winds crossing the front or from a thermally direct, cross-frontal circulation. Wind stress curl, expected to be largest when winds are parallel to SST fronts, is reduced through geostrophic spindown and thereby yields weaker coupling coefficients.

scholarly journals Influences of Sea Surface Temperature Gradients and Surface Roughness Changes on the Motion of Surface Oil: A Simple Idealized Study

2013 ◽  
Vol 52 (7) ◽  
pp. 1561-1575 ◽  
Author(s):  
Yangxing Zheng ◽  
Mark A. Bourassa ◽  
Paul Hughes
Keyword(s):  
Surface Roughness ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Wind Stress ◽  
Surface Wind ◽  
Free Water ◽  
Sea Surface ◽  
Ekman Transport ◽  
Wind Stress Curl ◽  
Transition Zones

AbstractThe authors' modeling shows that changes in sea surface temperature (SST) gradients and surface roughness between oil-free water and oil slicks influence the motion of the slick. Physically significant changes occur in surface wind speed, surface wind divergence, wind stress curl, and Ekman transport mostly because of SST gradients and changes in surface roughness between the water and the slick. These remarkable changes might affect the speed and direction of surface oil. For example, the strongest surface wind divergence (convergence) occurring in the transition zones owing to the presence of an oil slick will induce an atmospheric secondary circulation over the oil region, which in turn might affect the surface oil movement. SST-related changes to wind stress curl and Ekman transport in the transition zones appear to increase approximately linearly with the magnitude of SST gradients. Both surface roughness difference and SST gradients give rise to a net convergence of Ekman transport for oil cover. The SST gradient could play a more important role than surface roughness in changes of Ekman transport when SST gradients are large enough (e.g., several degrees per 10 km). The resulting changes in Ekman transport also induce the changes of surface oil movement. Sensitivity experiments show that appropriate selections of modeled parameters and geostrophic winds do not change the conclusions. The results from this idealized study indicate that the feedbacks from the surface oil presence to the oil motion itself are not trivial and should be further investigated for consideration in future oil-tracking modeling systems.

scholarly journals Intercomparison of oceanic and atmospheric forced and coupled mesoscale simulations Part I: Surface fluxes

1999 ◽  
Vol 17 (4) ◽  
pp. 566-576 ◽  
Author(s):  
P. Josse ◽  
G. Caniaux ◽  
H. Giordani ◽  
S. Planton
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Heat Budget ◽  
Weather Forecast ◽  
Atmospheric Model ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Storm Event ◽  
Fine Mesh

Abstract. A mesoscale non-hydrostatic atmospheric model has been coupled with a mesoscale oceanic model. The case study is a four-day simulation of a strong storm event observed during the SEMAPHORE experiment over a 500 × 500 km2 domain. This domain encompasses a thermohaline front associated with the Azores current. In order to analyze the effect of mesoscale coupling, three simulations are compared: the first one with the atmospheric model forced by realistic sea surface temperature analyses; the second one with the ocean model forced by atmospheric fields, derived from weather forecast re-analyses; the third one with the models being coupled. For these three simulations the surface fluxes were computed with the same bulk parametrization. All three simulations succeed well in representing the main oceanic or atmospheric features observed during the storm. Comparison of surface fields with in situ observations reveals that the winds of the fine mesh atmospheric model are more realistic than those of the weather forecast re-analyses. The low-level winds simulated with the atmospheric model in the forced and coupled simulations are appreciably stronger than the re-analyzed winds. They also generate stronger fluxes. The coupled simulation has the strongest surface heat fluxes: the difference in the net heat budget with the oceanic forced simulation reaches on average 50 Wm-2 over the simulation period. Sea surface-temperature cooling is too weak in both simulations, but is improved in the coupled run and matches better the cooling observed with drifters. The spatial distributions of sea surface-temperature cooling and surface fluxes are strongly inhomogeneous over the simulation domain. The amplitude of the flux variation is maximum in the coupled run. Moreover the weak correlation between the cooling and heat flux patterns indicates that the surface fluxes are not responsible for the whole cooling and suggests that the response of the ocean mixed layer to the atmosphere is highly non-local and enhanced in the coupled simulation.Key words. Oceanography: physical (air · sea interac- tion; eddies and mesoscale processes). Meteorology and atmospheric dynamics (ocean · atmosphere interactions)

scholarly journals On the Influence of Sea Surface Temperature distributions on the Development of Extratropical Cyclones

2021 ◽  
Author(s):  
Hai BUI ◽  
Thomas Spengler
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Latent Heat ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Minor Role ◽  
Extratropical Cyclones ◽  
The Absolute ◽  
Sst Gradient

AbstractThe sea surface temperature (SST) distribution can modulate the development of extratropical cyclones through sensible and latent heat fluxes. However, the direct and indirect effects of these surface fluxes, and thus the SST, are still not well understood. This study tackles this problem using idealized channel simulations of moist baroclinic development under the influence of surface fluxes. The model is initialized with a zonal wind field resembling the midlatitude jet and a different SST distribution for each experiment, where the absolute SST, the SST gradient, and the meridional position of the SST front are varied.The surface latent heat flux associated with the absolute SST plays a key role in enhancing the moist baroclinic development, while the sensible heat fluxes associated with the SST gradient play a minor role that can be detrimental for the development of the cyclone. The additional moisture provided by the latent heat fluxes originates from about 1000 km ahead of the cyclone a day prior to the time of the most rapid deepening. When the SST in this region is higher than 16°C, the additional latent heat is conducive for explosive cyclone development. For SSTs above 20°C, the cyclones feature characteristics of hybrid cyclones with latent heat release close to their core, maintaining their intensity for a longer period due to continuous and extensive moisture supply from the surface. A high absolute SST with a weak SST gradient, however, can lead to a delay of the deepening stage, because of unorganized convection at early stages reducing environmental baroclinicity.

scholarly journals Distinct Influence of Air–Sea Interactions Mediated by Mesoscale Sea Surface Temperature and Surface Current in the Arabian Sea

2017 ◽  
Vol 30 (20) ◽  
pp. 8061-8080 ◽  
Author(s):  
Hyodae Seo
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Wind Stress ◽  
Arabian Sea ◽  
Spatial Scales ◽  
Surface Current ◽  
Sea Surface ◽  
Surface Currents ◽  
Ekman Pumping ◽  
Wind Stress Curl

Abstract During the southwest monsoons, the Arabian Sea (AS) develops highly energetic mesoscale variability associated with the Somali Current (SC), Great Whirl (GW), and cold filaments (CF). The resultant high-amplitude anomalies and gradients of sea surface temperature (SST) and surface currents modify the wind stress, triggering the so-called mesoscale coupled feedbacks. This study uses a high-resolution regional coupled model with a novel coupling procedure that separates spatial scales of the air–sea coupling to show that SST and surface currents are coupled to the atmosphere at distinct spatial scales, exerting distinct dynamic influences. The effect of mesoscale SST–wind interaction is manifested most strongly in wind work and Ekman pumping over the GW, primarily affecting the position of GW and the separation latitude of the SC. If this effect is suppressed, enhanced wind work and a weakened Ekman pumping dipole cause the GW to extend northeastward, delaying the SC separation by 1°. Current–wind interaction, in contrast, is related to the amount of wind energy input. When it is suppressed, especially as a result of background-scale currents, depth-integrated kinetic energy, both the mean and eddy, is significantly enhanced. Ekman pumping velocity over the GW is overly negative because of a lack of vorticity that offsets the wind stress curl, further invigorating the GW. Moreover, significant changes in time-mean SST and evaporation are generated in response to the current–wind interaction, accompanied by a noticeable southward shift in the Findlater Jet. The significant increase in moisture transport in the central AS implies that air–sea interaction mediated by the surface current is a potentially important process for simulation and prediction of the monsoon rainfall.

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