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.

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 Improving the Altimeter-Derived Surface Currents Using High-Resolution Sea Surface Temperature Data: A Feasability Study Based on Model Outputs

2016 ◽  
Vol 33 (12) ◽  
pp. 2769-2784 ◽  
Author(s):  
M.-H. Rio ◽  
R. Santoleri ◽  
R. Bourdalle-Badie ◽  
A. Griffa ◽  
L. Piterbarg ◽  
...  
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Ocean Circulation ◽  
Large Scale ◽  
Spatial Scales ◽  
Ocean Surface ◽  
Sea Surface ◽  
High Temporal Resolution ◽  
Surface Currents ◽  
Ocean Surface Currents

AbstractAccurate knowledge of ocean surface currents at high spatial and temporal resolutions is crucial for a gamut of applications. The altimeter observing system, by providing repeated global measurements of the sea surface height, has been by far the most exploited system to estimate ocean surface currents over the past 20 years. However, it neither permits the observation of currents moving away from the geostrophic balance nor is it capable of resolving the shortest spatial and temporal scales of the currents. Therefore, to overcome these limitations, in this study the ways in which the high-spatial-resolution and high-temporal-resolution information from sea surface temperature (SST) images can improve the altimeter current estimates are investigated. The method involves inverting the SST evolution equation for the velocity by prescribing the source and sink terms and employing the altimeter currents as the large-scale background flow. The method feasibility is tested using modeled data from the Mercator Ocean system. This study shows that the methodology may improve the altimeter velocities at spatial scales not resolved by the altimeter system (i.e., below 150 km) but also at larger scales, where the geostrophic equilibrium might not be the unique or dominant process of the ocean circulation. In particular, the major improvements (more than 30% on the meridional component) are obtained in the equatorial band, where the geostrophic assumption is not valid. Finally, the main issues anticipated when this method is applied using real datasets are investigated and discussed.

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 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 Improving the Altimeter-Derived Surface Currents Using Sea Surface Temperature (SST) Data: A Sensitivity Study to SST Products

2020 ◽  
Vol 12 (10) ◽  
pp. 1601 ◽  
Author(s):  
Daniele Ciani ◽  
Marie-Hélène Rio ◽  
Bruno Buongiorno Nardelli ◽  
Hélène Etienne ◽  
Rosalia Santoleri
Keyword(s):  
High Resolution ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Surface Current ◽  
Zonal Flow ◽  
Sensitivity Study ◽  
Sea Surface ◽  
Global Network ◽  
Altimeter Data ◽  
Surface Currents

Measurements of ocean surface topography collected by satellite altimeters provide geostrophic estimates of the sea surface currents at relatively low resolution. The effective spatial and temporal resolution of these velocity estimates can be improved by optimally combining altimeter data with sequences of high resolution interpolated (Level 4) Sea Surface Temperature (SST) data, improving upon present-day values of approximately 100 km and 15 days at mid-latitudes. However, the combined altimeter/SST currents accuracy depends on the area and input SST data considered. Here, we present a comparative study based on three satellite-derived daily SST products: the Remote Sensing Systems (REMSS, 1/10 ∘ resolution), the UK Met Office OSTIA (1/20 ∘ resolution), and the Multiscale Ultra-High resolution SST (1/100 ∘ resolution). The accuracy of the marine currents computed with our synergistic approach is assessed by comparisons with in-situ estimated currents derived from a global network of drifting buoys. Using REMSS SST, the meridional currents improve up to more than 20% compared to simple altimeter estimates. The maximum global improvements for the zonal currents are obtained using OSTIA SST, and reach 6%. Using the OSTIA SST also results in slight improvements (≃1.3%) in the zonal flow estimated in the Southern Ocean (45 ∘ S to 70 ∘ S). The homogeneity of the input SST effective spatial resolution is identified as a crucial requirement for an accurate surface current reconstruction. In our analyses, this condition was best satisfied by the lower resolution SST products considered.

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 Wind Stress Curl and Coastal Upwelling in the Area of Monterey Bay Observed during AOSN-II

2011 ◽  
Vol 41 (5) ◽  
pp. 857-877 ◽  
Author(s):  
Q. Wang ◽  
J. A. Kalogiros ◽  
S. R. Ramp ◽  
J. D. Paduan ◽  
G. Buzorius ◽  
...  
Keyword(s):  
Wind Speed ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Wind Stress ◽  
Coastal Upwelling ◽  
Surface Wind ◽  
Sea Surface ◽  
Monterey Bay ◽  
Surface Currents ◽  
Surface Wind Stress

Abstract Aircraft measurements obtained during the 2003–04 Autonomous Ocean Sampling Network (AOSN-II) project were used to study the effect of small-scale variations of near-surface wind stress on coastal upwelling in the area of Monterey Bay. Using 5-km-long measurement segments at 35 m above the sea surface, wind stress and its curl were calculated with estimated accuracy of 0.02–0.03 N m−2 and 0.1–0.2 N m−2 per 100 kilometers, respectively. The spatial distribution of wind speed, wind stress, stress curl, and sea surface temperature were analyzed for four general wind conditions: northerly or southerly wind along the coastline, onshore flow, and offshore flow. Wind stress and speed maxima frequently were found to be noncollocated as bulk parameterizations imply owing to significant stability and nonhomogeneity effects at cold SST pools. The analyses revealed that complicated processes with different time scales (wind stress field variation, ocean response and upwelling, sea surface currents, and heating by solar radiation) affect the coastal sea surface temperature. It was found that the stress-curl-induced coastal upwelling only dominates in events during which positive curl extended systematically over a significant area (scales larger than 20 km). These events included cases with a northerly wind, which resulted in an expansion fan downstream from Point Año Nuevo (wind speed peaks greater than about 8–10 m s−1), and cases with an offshore/onshore flow, which are characterized by weak background upwelling due to Ekman transport. However, in general, observations show that cold pools of sea surface temperature in the central area of Monterey Bay were advected by ocean surface currents from strong upwelling regions. Aircraft vertical soundings taken in the bay area showed that dominant effects of the lee wave sheltering of coastal mountains resulted in weak atmospheric turbulence and affected the development of the atmospheric boundary layer. This effect causes low wind stress that limits upwelling, especially at the northern part of Monterey Bay. The sea surface temperature is generally warm in this part of the bay because of the shallow oceanic surface layer and solar heating of the upper ocean.

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