Mesoscale Sea Surface Temperature Research Articles

Mesoscale structures of two types of cold-air outbreaks over the East China Sea and the effect of coastal sea surface temperature

Two types of cold-air outbreaks over the Yellow and East China Seas are investigated using a regional mesoscale model. Distinct patterns of surface turbulent heat fluxes and precipitation are identified between the two cases. The sea surface heat flux and frictional velocity are strongly influenced by mesoscale differences between high- and low-resolution datasets of sea surface temperature (SST). The influence of the SST difference on atmospheric water is spread to the downstream area of the outbreak with the phase transition of water. The air mass transformation around 800 hPa over the Kuroshio is partly influenced by the upstream SST difference. In particular, the SST difference near the mouth of the Yanzi River strongly modifies the air mass around Taiwan. Thus, in addition to the Kuroshio front, the mesoscale Chinese coastal SST structure is also important in the air mass transformation over the East China Sea.

Cloud-resolving simulation of heavy snowfalls in Japan for late December 2005: application of ocean data assimilation to a snow disaster

Abstract. We applied eddy-resolving ocean data assimilation to a cloud-resolving atmospheric simulation of a snow disaster and investigated the effects of mesoscale eddies on a heavy snowfall event in late December 2005. Ocean circulation model (OCM) data assimilation reproduces mesoscale sea surface temperature (SST) structures, which are smoothed out by optimum interpolation. This difference between OCM-assimilation and optimum-interpolation SSTs affects the atmospheric boundary layers over oceanic mesoscale eddies. The atmospheric response to the SST difference is complex at the cold tongue in the central Sea of Japan. Although the horizontal wind and turbulent mixing are quickly and locally affected by the low SST, the atmospheric temperature and water amounts are greatly affected by the upstream high SST via the northwesterly advection. In the heavy snowfall areas, the OCM assimilation greatly affects 10-day accumulated precipitation, though it does not largely influence 10-day mean vertical structures of wind, temperature and water vapor. Thus, we should recognize the significance of oceanic mesoscale eddies for heavy snowfall.

Frontal Scale Air–Sea Interaction in High-Resolution Coupled Climate Models

Abstract The emerging picture of frontal scale air–sea interaction derived from high-resolution satellite observations of surface winds and sea surface temperature (SST) provides a unique opportunity to test the fidelity of high-resolution coupled climate simulations. Initial analysis of the output of a suite of Community Climate System Model (CCSM) experiments indicates that characteristics of frontal scale ocean–atmosphere interaction, such as the positive correlation between SST and surface wind stress, are realistically captured only when the ocean component is eddy resolving. The strength of the coupling between SST and surface stress is weaker than observed, however, as has been found previously for numerical weather prediction models and other coupled climate models. The results are similar when the atmospheric component model grid resolution is doubled from 0.5° to 0.25°, an indication that shortcomings in the representation of subgrid scale atmospheric planetary boundary layer processes, rather than resolved scale processes, are responsible for the weakness of the coupling. In the coupled model solutions the response to mesoscale SST features is strongest in the atmospheric boundary layer, but there is a deeper reaching response of the atmospheric circulation apparent in free tropospheric clouds. This simulated response is shown to be consistent with satellite estimates of the relationship between mesoscale SST and all-sky albedo.

Atmospheric Boundary Layer Response to Mesoscale SST Anomalies in the Kuroshio Extension

Abstract The atmospheric boundary layer (ABL) response to mesoscale eddies in sea surface temperature (SST) in the Kuroshio Extension was investigated using a high-resolution (T213L30) atmospheric general circulation model. A control run was performed first by integrating the model for 40 days, driven by the satellite-derived, eddy-resolving SST during January 2006. The spatial pattern of surface wind anomalies—that is, a deviation from large-scale winds—reveals a positive correlation with the spatial pattern of mesoscale SST anomalies. The momentum budget analysis of the anomalous zonal wind was performed to investigate the formation of the ABL response. The most dominant term was the pressure gradient force; the advection term was comparable but in the opposite sense. Vertical mixing acts to weaken the anomalous zonal wind near the surface; however, the downward (upward) vertical turbulent flux anomalies were dominant near the ABL top over the warm (cold) SST anomalies, suggesting that the vertical mixing mechanism is effective. The role of the vertical mixing was further examined by a sensitivity experiment in which the turbulent diffusion coefficient for momentum was spatially smoothed. While the pressure gradient force and the advection terms were almost unchanged in the momentum budgets, the deceleration due to turbulence was enhanced because of the absence of the momentum input from the free atmosphere. The result is a reduction in the amplitude of the surface zonal wind anomalies to approximately half in the sensitivity experiment.

Dynamical Analysis of the Boundary Layer and Surface Wind Responses to Mesoscale SST Perturbations

Abstract The dynamical response of the marine atmospheric boundary layer (MABL) to mesoscale sea surface temperature (SST) perturbations is investigated over the Agulhas Return Current during winter from a 1-month, high-resolution, three-dimensional simulation using the Weather Research and Forecasting (WRF) mesoscale model. A steady lower boundary condition for July 2002 is obtained using SST measurements from the Advanced Microwave Scanning Radiometer on the Earth Observing System (EOS)–Aqua satellite (AMSR-E). The WRF models’ ability to accurately simulate the SST-induced surface wind response is demonstrated from a comparison with satellite surface wind observations from the SeaWinds scatterometer on the Quick Scatterometer (QuikSCAT) satellite. Relevant features of this simulation include a quasi-periodic distribution of mesoscale SST perturbations with spatial scales ∼200 km and strong winds that lead to a large surface sensible heat flux response, whose broad range of 80–100 W m−2 between warm and cool SST perturbations is much larger than seen in most previous simulations of mesoscale wind–SST coupling. This simulation provides the first realistic example of vertical turbulent redistribution of momentum driven by the SST-induced surface heating perturbations acting in concert with the SST-induced pressure gradients to accelerate near-surface flow toward warm water and decelerate near-surface flow toward cool water. This simulation is also the first example of a near-surface wind speed response to mesoscale SST perturbations that differs qualitatively and substantially from the vertically averaged MABL wind response. In the vertically averaged MABL momentum budget, the surface wind stress acts as a drag on the SST-induced perturbation flow as it is being accelerated by SST-induced pressure gradients. However, only in the middle and upper reaches of the MABL does the turbulent stress divergence act as a drag on the SST-induced winds perturbations in this simulation. These mesoscale SST perturbations are also shown to modify the wind direction within the MABL. Dynamically, this is accomplished through SST-induced perturbations to the crosswind components of the pressure gradient, turbulent stress divergence, and the Coriolis force.

The Effects of SST-Induced Surface Wind Speed and Direction Gradients on Midlatitude Surface Vorticity and Divergence

Abstract The effects of surface wind speed and direction gradients on midlatitude surface vorticity and divergence fields associated with mesoscale sea surface temperature (SST) variability having spatial scales of 100–1000 km are investigated using vector wind observations from the SeaWinds scatterometer on the Quick Scatterometer (QuikSCAT) satellite and SST from the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) Aqua satellite. The wind–SST coupling is analyzed over the period June 2002–August 2008, corresponding to the first 6+ years of the AMSR-E mission. Previous studies have shown that strong wind speed gradients develop in response to persistent mesoscale SST features associated with the Kuroshio Extension, Gulf Stream, South Atlantic, and Agulhas Return Current regions. Midlatitude SST fronts also significantly modify surface wind direction; the surface wind speed and direction responses to typical SST differences of about 2°–4°C are, on average, about 1–2 m s−1 and 4°–8°, respectively, over all four regions. Wind speed perturbations are positively correlated and very nearly collocated spatially with the SST perturbations. Wind direction perturbations, however, are displaced meridionally from the SST perturbations, with cyclonic flow poleward of warm SST and anticyclonic flow poleward of cool SST. Previous observational analyses have shown that small-scale perturbations in the surface vorticity and divergence fields are related linearly to the crosswind and downwind components of the SST gradient, respectively. When the vorticity and divergence fields are analyzed in curvilinear natural coordinates, the wind speed contributions to the SST-induced vorticity and divergence depend equally on the crosswind and downwind SST gradients, respectively. SST-induced wind direction gradients also significantly modify the vorticity and divergence fields, weakening the vorticity response to crosswind SST gradients while enhancing the divergence response to downwind SST gradients.

Atmospheric Simulations Using OGCM-Assimilation SST: Influence of the Wintertime Japan Sea on Monthly Precipitation

Temperature data for the Japan Sea obtained from ocean data assimilation modeling is applied to atmospheric simulations of monthly precipitation for January 2005. Because the volume of flow of the Tsushima Warm Current was large during the winter season, the sea surface temperature (SST) and coastal precipitation were higher in comparison with those in 2003. In order to evaluate influence of SST on monthly precipitation, we use surface temperatures of the Japan Sea in 2003 and 2005 for comparative simulations of precipitation for January 2005. The precipitation in experiment C (using cool SST data in 2003) is smaller than that in experiment W (using warm SST data in 2005) in a large part of the sea area, since the small evaporation results from the low SST over the upstream area of northwesterly winter monsoon. In the domain of 33.67-45.82°N and 125.89-142.9°E, the averaged evaporation and precipitation in experiment C are 10% and 13% smaller than those in experiment W, respectively. About half of the difference between the precipitations observed for January 2003 and 2005 in a heavy snow area is equal to the difference between the two simulations. Our results show that the mesoscale SST difference between 2003 and 2005 is related to the local difference of monthly precipitation.

Ocean–Atmosphere Interaction over Agulhas Extension Meanders

AbstractMany years of high-resolution measurements by a number of space-based sensors and from Lagrangian drifters became available recently and are used to examine the persistent atmospheric imprints of the semipermanent meanders of the Agulhas Extension Current (AEC), where strong surface current and temperature gradients are found. The sea surface temperature (SST) measured by the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) and the chlorophyll concentration measured by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) support the identification of the meanders and related ocean circulation by the drifters. The collocation of high and low magnitudes of equivalent neutral wind (ENW) measured by Quick Scatterometer (QuikSCAT), which is uniquely related to surface stress by definition, illustrates not only the stability dependence of turbulent mixing but also the unique stress measuring capability of the scatterometer. The observed rotation of ENW in opposition to the rotation of the surface current clearly demonstrates that the scatterometer measures stress rather than winds. The clear differences between the distributions of wind and stress and the possible inadequacy of turbulent parameterization affirm the need of surface stress vector measurements, which were not available before the scatterometers. The opposite sign of the stress vorticity to current vorticity implies that the atmosphere spins down the current rotation through momentum transport. Coincident high SST and ENW over the southern extension of the meander enhance evaporation and latent heat flux, which cools the ocean. The atmosphere is found to provide negative feedback to ocean current and temperature gradients. Distribution of ENW convergence implies ascending motion on the downwind side of local SST maxima and descending air on the upwind side and acceleration of surface wind stress over warm water (deceleration over cool water); the convection may escalate the contrast of ENW over warm and cool water set up by the dependence of turbulent mixing on stability; this relation exerts a positive feedback to the ENW–SST relation. The temperature sounding measured by the Atmospheric Infrared Sounder (AIRS) is consistent with the spatial coherence between the cloud-top temperature provided by the International Satellite Cloud Climatology Project (ISCCP) and SST. Thus ocean mesoscale SST anomalies associated with the persistent meanders may have a long-term effect well above the midlatitude atmospheric boundary layer, an observation not addressed in the past.

Modes of mesoscale sea surface height and temperature variability in the East Australian Current

Mesoscale variability where the East Australian Current (EAC) separates from the coast is studied using sea surface temperature and surface velocity streamfunction observed by satellite and a regional numerical model. The mean circulation simulated by the model (the Regional Ocean Modeling System (ROMS)) is compared to a high‐resolution regional climatology, and the realism of the simulated mesoscale variability is tested by comparison to statistical analyses of the satellite data. Both ROMS and data show spectral peaks in the mesoscale energy band at periods between 90 and 180 days. Complex Empirical Orthogonal Function (EOF) analysis identifies two significant modes of mesoscale variability in the data; an Eddy Mode, for which the variability propagates southwestward along the coast, and a Wave Mode, for which phase propagation is predominantly onshore. The regional model open boundary conditions include only annual and semiannual harmonics of variability so remote mesoscale forcing is absent. The Eddy Mode is represented well in the model indicating this aspect of the circulation results from local instabilities of the flow and that its underlying dynamical process is simulated well. While the observed and modeled Wave Modes have some similarities, their differences suggest the model is deficient in representing westward propagation of mesoscale period variability in the region. Whatever the source of this energy, the orthogonality property of the EOF analysis indicates the Wave Mode does not interact significantly with eddy processes in the EAC separation.

Effects of mesoscale sea-surface temperature fronts on the marine atmospheric boundary layer

A numerical modelling study is presented focusing on the effects of mesoscale sea-surface temperature (SST) variability on surface fluxes and the marine atmospheric boundary-layer structure. A basic scenario is examined having two regions of SST anomaly with alternating warm/cold or cold/warm water regions. Conditions upstream from the anomaly region have SST values equal to the ambient atmosphere temperature, creating an upstream neutrally stratified boundary layer. Downstream from the anomaly region the SST is also set to the ambient atmosphere value. When the warm anomaly is upstream from the cold anomaly, the downstream boundary layer exhibits a more complex structure because of convective forcing and mixed layer deepening upstream from the cold anomaly. An internal boundary layer forms over the cold anomaly in this case, generating two distinct layers over the downstream region. When the cold anomaly is upstream from the warm anomaly, mixing over the warm anomaly quickly destroys the shallow cold layer, yielding a more uniform downstream boundary-layer vertical structure compared with the warm-to- cold case. Analysis of the momentum budget indicates that turbulent momentum flux divergence dominates the velocity field tendency, with pressure forcing accounting for only about 20% of the changes in momentum. Parameterization of surface fluxes and boundary-layer structure at these scales would be very difficult because of their dependence on subgrid-scale SST spatial order. Simulations of similar flow over smaller scale fronts (<5 km) suggest that small-scale SST variability might be parameterized in mesoscale models by relating the effective heat flux to the strength of the SST variance.

Ocean–Atmosphere Covariability in the Western Arabian Sea*

The western Arabian Sea exhibits strong spatial variability in sea surface temperature (SST) during the southwest monsoon, with changes in SST that can exceed 5°C over 200 km. Exploration of satellite-based and in situ data shows a strong connection between mesoscale SST features and changes in the atmospheric boundary layer. The fundamental relationship is that of weak (strong) wind velocities overlying cold (warm) SST features. There are also coherent changes in other near-surface meteorological parameters, such as the air–sea temperature difference and relative humidity—indicating changes in the stability of the planetary boundary layer over the mesoscale SST features. These relationships are similar to those recently reported over the equatorial Pacific tropical instability wave region. This observed covariability of atmospheric boundary layer structure and SST results in variations of the surface heat and moisture fluxes; latent heat flux is modified by changes in relative humidity (principally through the temperature dependence of saturation specific humidity), wind speed, and boundary layer stability over the cold filaments. The nonlinear dependence of latent heat flux on the three parameters leads to a net enhancement of latent heat flux from the mesoscale features, as compared to that computed using spatially averaged parameters. Additionally, the spatial structure of the heat-flux variability will tend to dampen the mesoscale SST features. The mesoscale wind variability results in strong wind stress curl patterns on the same spatial scales as the oceanic features. The resulting Ekman pumping variations may play an important role in the evolution of the ocean eddy fields in this region. Further examination of the processes controlling the observed covariability, and the oceanic and atmospheric response to the coupling should therefore be undertaken.

Optimal averaging of NOAA/NASA Pathfinder satellite sea surface temperature data

Statistical interpolation is applied to 8 years of cloud‐interrupted satellite radiometer data to produce estimates of sea surface temperature (SST) at 10‐day intervals in the Indo‐Australian region. The data are 9‐km resolution daily “best SST” values calculated globally by the NOAA/NASA Pathfinder project reanalysis of advanced very high resolution radiometer (AVHRR) data. The optimal averaging technique determines an unbiased estimate of the signal that has the minimum mean square variance from the data, within the limits of the expected measurement error. Previous studies have shown that the method is superior to other linear averaging techniques, especially that of simple composite averaging. The method is applied in the time domain only, preserving the 9‐km spatial resolution of the data. The signal and noise covariances were evaluated from the data. This was done with care so that accurate estimates of the error bounds that bracket the optimally averaged values might be obtained. These error bounds were then verified against in situ data. A Markov function, (1 + τ/a) exp (−τ/a), where τ is the time lag and a is a characteristic timescale, was fitted to the data and used for the signal correlation function. This was selected after evaluation of functional forms proposed in other studies. The effect on the analysis of geographical variation in the correlation function was considered. The computational demand of the repeated matrix operations in optimal interpolation was reduced by using a limited duration data window. The complete analysis procedure for the 8‐year data set, comprising over 106 time series, was tractable on a modern workstation. The result is a set of SST maps for 1987–1994 at an interval of 10 days and a spatial resolution of 9 km. The analyses are suitable for applications such as high‐resolution ocean and atmosphere modeling where the timescales and space scales of interest are comparable to the analysis (i.e., of the order of 10 days, 9 km) and for which the presence of gaps due to clouds is problematic. Some features of Indo‐Australian regional mesoscale circulation that the analysis highlights are examined, including examples of detailed mesoscale SST evolution and interannual variability.

Geographic and Temporal EddY Variability in the Western North Atlantic as Sensed by Satellite: An Eddy Generation Mechanism

Abstract This paper has two objectives: to establish the satellite-derived mesoscale sea surface temperature anomaly as a useful oceanographic measure and to use this measure to explore an eddy generation mechanism. The use of satellite-derived sea surface temperature anomalies and their movement as a means of developing ocean eddy statistics is made plausible through a comparison with POLYMODE and other studies. This, plus the marked improvement in satellite resolution since POLYMODE and the use of anomaly thresholds, provides confidence in the statistical results from satellite SST. The distribution in space and time of 65 coherent eddies as sensed by satellites in the western North Atlantic is presented. The region considered is west of 50°W and the time period is March 1981 to December 1984. A region and season of enhanced warm core eddy formation that becomes apparent from a consideration of the data is discussed, a connection is made with the seasonal interior wind stress, and an instability and gen...

Sea surface temperatures of the coastal zones of France observed by the HCMM satellite

The Heat Capacity Mapping Mission experimental satellite was launched in April 1978 and provided data until July 1980. Although the basic mission objective was the measurement of diurnal temperature variations of the earth's surface for soil and geology applications, the characteristics of the heat capacity mapping radiometer onboard the satellite (temperature sensitivity of 0.3°C, instantaneous field of view of 0.5 km) also recommended it for use in oceanographic studies. The data acquired during the period from May 1978 to May 1979 were systematically utilized for evaluation of sea surface temperature in French oceanic regions (North Sea, English Channel, Celtic Sea, Bay of Biscay, and western Mediterranean Sea) and for study of sea surface temperature‐related dynamic phenomena. Comparisons were made of the radiometric performance of the heat capacity mapping radiometer to that of the very high resolution radiometer and the advanced very high resolution radiometer onboard meteorological satellites. They demonstrate the decisive gain in quality of the heat capacity mapping radiometer over the very high resolution radiometer and the similarly improved quality of the heat capacity mapping radiometer and advanced very high resolution radiometer for the observation of mesoscale sea surface temperature features. The utilization of available photographic products proved very suitable, since they had been geometrically corrected and enhanced in the sea surface temperature range, consequently avoiding many of those cases requiring involved computer treatment. On the basis of the interpretation of photographic products, conclusions have been drawn regarding several oceanic phenomena: (1) The thermal effluent of the Rhine‐Meuse system is affected by the residual tidal current of the North Sea. The extent of the offshore diffusion of the effluent is influenced by winds from the northeast and west which, respectively, retard or accelerate the residual current; (2) images exhibiting cold water along the edge of the continental shelf strongly support the hypothesis of a mixing process due to internal waves generated by the action of tidal currents at the edge of the shelf; (3) large‐scale eddy structures detected during the summer in the region of the Mediterranean Sea around 6°E and 38°N may be linked to a phenomenon of barotropic‐baroclinic instability; (4) significant diurnal heating of the surface layer (several degrees Celsius) observed in the Mediterranean Sea is related to the presence of weak winds, leading one to interpret with caution the daytime sea surface temperature satellite observations made during the summer period.

Mesoscale features along the first Oyashio Intrusion

Satellite and hydrographic observations obtained during May 1981 are used to examine the sea surface and subsurface temperature structures within a portion of the Tohoku Area (38°–43°N) extending east of Japan to 149°E longitude. These data delineate the Oyashio Front and the evolution of mesoscale features dominating the surface layer circulation east of northern Honshu and Hokkaido. A perturbation on the first Oyashio Intrusion generated eddies adjacent to the Oyashio Front and contributed to a jetlike feature with associated, paired vortices that intruded into what has been defined as the “perturbed area” (39°–42°N) southeastward from the vicinity of the Tsugara Strait. NOAA6 satellite Advanced Very High Resolution Radiometer infrared images and sea surface temperature distributions identified specific eddies and fronts, that also appeared as banded warm and cold regions within XBT (expendable bathythermograph) sections. The satellite data helped define the lateral extent of the features, while the sections provided estimates of their vertical extent. A new procedure for the determination of satellite sea surface temperatures was used to produce a composite temperature distribution over a portion of the Tohoku Area. The satellite‐derived temperature estimates exhibited a 0.8°C rms scatter about a 0.6°C warm bias relative to the XBT data. This map displays the intense mesoscale sea surface temperature variability in the region along the Oyashio Front and within the subarctic regime.

Mesoscale Variability of Sea Surface Temperatures

Abstract Results of an investigation of the nature of the mesoscale variability of sea surface temperature (SST) inthe upwelling zone off the central coast of Oregon are presented. A knowledge of SST mesoscale variability isimportant toward understanding the mesoscale air-sea interaction process in an upwelling zone. Almost-daily sea surface temperature data gathered by remote sensing techniques provided the basis for this investi-gation. These data were gathered over a period of 60 days during the COHO Project in summer 1973. In order to study the influence of wind forcing on the mesoscale SST field, wind data were gathered from ananemometer located at Newport, Ore. Some important results of this investigation are: 1) the daily SST fields respond rapidly to wind forcing;2) the three-week mean SST's tend to follow the large-scale bathymetry; and 3) identification of SST eddiesfrom day to day on daily perturbation (from the three-week means) maps is made difficult because of theexistence of strong horizont...