Numerical study on the summer upwelling system in the northern continental shelf of the South China Sea

A three-dimensional baroclinic nonlinear numerical model is employed to investigate the summer upwelling in the northern continental shelf of the South China Sea (NCSCS) and the mechanisms of the local winds inducing the coastal upwelling, associated with the QuikSCAT wind data. First, the persistent signals of the summer upwelling are illustrated by the climatological the Advanced Very High-Resolution Radiometer (AVHRR) Sea Surface Temperature (SST) image over 1985–2006 and field observations in 2006 summer. Then, after the successful simulation of the summer upwelling in the NCSCS, four numerical experiments are conducted to explore the different effects of local winds, including the wind stress and wind stress curl, on the coastal upwelling in two typical strong summer upwelling regions of the NCSCS. The modeled results indicate that the summer upwelling is a seasonal common phenomenon during June–September in the NCSCS with the spatial extent of a basin-scale. Typical continental shelf upwelling characteristics are clearly shown in the coastal surface and subsurface water, such as low temperature, high salinity and high potential density in the east of the Hainan Island, the east of the Leizhou Peninsula and the southeast of the Zhanjiang Bay (noted as the Qiongdong-QD), and the inshore areas from the Shantou Coast to the Nanri Islands of the Fujian Coast (noted as the Yuedong-YD). The analysis of the QuikSCAT wind data and modeled upwelling index suggests that the local winds play significant roles in causing the coastal upwelling, but the alongshore wind stress and wind stress curl have different contributions to the upwelling in the Qiongdong (QDU) and the coastal upwelling in the Yuedong (YDU), respectively. Furthermore, model results from the numerical experiments show that in the YD the stable alongshore wind stress is a very important dynamic factor to induce the coastal upwelling but the wind stress curl has little contribution and even unfavorable to the YDU. However, in the QD the coastal upwelling is strongly linked to the local wind stress curl. It is also found that not only the offshore Ekman transport driven by the alongshore wind stress, the wind stress curl-induced Ekman pumping also plays a crucial effect on the QDU. Generally, the wind stress curl even has more contributions to the QDU than the alongshore wind stress.     

Ocean wind fields retrieved from the advanced synthetic aperture radar aboard ENVISAT

In this paper an algorithm is presented which enables high-resolution ocean surface wind fields to be retrieved from the advanced synthetic aperture radar (ASAR) data acquired by the European remote sensing satellite ENVISAT. Wind directions are extracted from wind-induced streaks that are visible in ASAR images at scales above 200 m and that are approximately in line with the mean surface wind direction. Wind speeds are derived from the normalized radar cross section (NRCS) and image geometry of the calibrated ASAR images, together with the local ASAR-retrieved wind direction. Therefore the empirical C-band model CMOD4, which describes the dependency of the NRCS on wind and image geometry, is used. CMOD4 is a semi-empirical model, which was originally developed for the scatterometer of the European remote sensing satellites ERS-1 and 2 operating at C-band with vertical polarization. Consequently, CMOD4 requires modification when applied to ASAR images that were acquired with horizontal polarization in transmitting and receiving. This is performed by considering the polarization ratio of the NRCS. To demonstrate the applicability of the algorithm, wind fields were computed from several ENVISAT ASAR images of the North Sea and compared to atmospheric model results of the German weather service.Acknowledgements The authors were supported by the German Bundesministerium für Bildung und Forschung (BMBF) in the framework of the project. A new perspective of the Ocean ENVISAT Oceanography (ENVOC). The ENVISAT ASAR data were kindly made available by the European Space Agency in the framework of the ENVISAT Project AO-ID 220, Biological and geophysical parameters from synthetic aperture radar over the ocean (BIGPASO).     

Seasonal and interannual patterns of sea surface temperature in Banda Sea as revealed by self-organizing map

Seasonal and interannual variations of sea surface temperature (SST) in the Banda Sea are studied for the period of January 1985 through December 2007. A neural network pattern recognition approach based on self-organizing map (SOM) has been applied to monthly SST from the Advanced Very High Resolution Radiometer (AVHRR) Oceans Pathfinder. The principal conclusions of this paper are outlined as follows. There are three different patterns associated with the variations in the monsoonal winds: the southeast and northwest monsoon patterns, and the monsoon-break patterns. The southeast monsoon pattern is characterized by low SST due to the prevailing southeasterly winds that drive Ekman upwelling. The northwest monsoon pattern, on the other hand, is one of high SST distributed uniformly in space. The monsoon-break pattern is a transitional pattern between the northwest and southeast monsoon patterns, which is characterized by moderate SST patterns. On interannual time-scale, the SST variations are significantly influenced by the El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) phenomena. Low SST is observed during El Niño and/or positive IOD events, while high SST appears during La Niña event. Low SST in the Banda Sea during positive IOD event is induced by upwelling Kelvin waves generated in the equatorial Indian Ocean which propagate along the southern coast of Sumatra and Java before entering the Banda Sea through the Lombok and Ombai Straits as well as through the Timor Passage. On the other hand, during El Niño (La Niña) events, upwelling (downwelling) Rossby waves associated with off-equatorial divergence (convergence) in response to the equatorial westerly (easterly) winds in the Pacific, partly scattered into the Indonesian archipelago which in turn induce cool (warm) SST in the Banda Sea.     

SAR observation and model tracking of an oil spill event in coastal waters

Oil spills are a major contributor to marine pollution. The objective of this work is to simulate the oil spill trajectory of oil released from a pipeline leaking in the Gulf of Mexico with the GNOME (General NOAA Operational Modeling Environment) model. The model was developed by NOAA (National Oceanic and Atmospheric Administration) to investigate the effects of different pollutants and environmental conditions on trajectory results. Also, a Texture-Classifying Neural Network Algorithm (TCNNA) was used to delineate ocean oil slicks from synthetic aperture radar (SAR) observations. During the simulation, ocean currents from NCOM (Navy Coastal Ocean Model) outputs and surface wind data measured by an NDBC (National Data Buoy Center) buoy are used to drive the GNOME model. The results show good agreement between the simulated trajectory of the oil spill and synchronous observations from the European ENVISAT ASAR (Advanced Synthetic Aperture Radar) and the Japanese ALOS (Advanced Land Observing Satellite) PALSAR (Phased Array L-band Synthetic Aperture Radar) images. Based on experience with past marine oil spills, about 63.0% of the oil will float and 18.5% of the oil will evaporate and disperse. In addition, the effects from uncertainty of ocean currents and the diffusion coefficient on the trajectory results are also studied.     

Satellite-derived variability of the Aegean Sea ecohydrodynamics

Eight years of AVHRR-derived sea surface temperature (SST) and SeaWiFS-derived surface chlorophyll (Chl) data (1998–2005) are used to investigate key processes affecting the spatial and temporal variability of the two parameters in the Aegean Sea. Seasonal mean SST and Chl maps are constructed using daily data to study seasonal dynamics whereas empirical orthogonal function (EOF) and correlational analysis is applied to the 8-day composite SST and Chl anomaly time-series in order to study the variability and co-variability of the two parameters from subseasonal to interannual time-scales. The seasonal mean fields show that Black Sea cold and chlorophyll-rich waters enter through the Dardanelles Strait and they are accumulated in the north-eastern part of the Aegean Sea, steered by the Samothraki anticyclone. Large chlorophyll concentrations are encountered in the hydrological front off the Dardanelles Strait as well as in coastal areas affected by large riverine/anthropogenic nutrient loads. The SST seasonal mean patterns reveal strong cooling that is associated with upwelling along the eastern boundary of the basin during summer due to strong northerly winds, a process which is not present in the surface chlorophyll climatology. The Chl dataset presents much stronger sub-seasonal variability than SST, with large variations in the phase and strength of the phytoplankton seasonal cycles. EOF analysis of the anomaly time-series shows that SST non-seasonal variability is controlled by synoptic weather variations and anomalies in the north–south wind-stress component regulating the summer coastal upwelling regime. Mean SST and Chl patterns, and their associated variations, are not closely linked implying that Black Sea and riverine inputs mainly control the intra-annual and interannual variability of the surface chlorophyll in the Aegean Sea rather than mixing and/or upwelling processes.     

Summertime sea surface temperature fronts associated with upwelling around the Taiwan Bank

It is well known that upwelling of subsurface water is dominant around the Taiwan Bank (TB) and the Penghu (PH) Islands in the southern Taiwan Strait in summertime. Sea surface temperature (SST) frontal features and related phenomena around the TB upwelling and the PH upwelling were investigated using long-term AVHRR (1996–2005) and SeaWiFS (1998–2005) data received at the station of National Taiwan Ocean University. SST and chlorophyll-a (Chl-a) images with a spatial resolution of 0.01° were generated and used for the monthly SST and Chl-a maps. SST fronts were extracted from each SST images and gradient magnitudes (GMs); the orientations were derived for the SST fronts. Monthly maps of cold fronts where the cooler SSTs were over a shallower bottom were produced from the orientation.     

Validation of AVHRR and TMI-derived sea surface temperature in the northern South China Sea

Satellite-derived SSTs are validated in the northern South China Sea (NSCS) using in situ SSTs from the drifting buoys and well-calibrated sensors installed on Research/Vessel(R/V) Shiyan 3. The satellite SSTs are Advanced Very High Resolution Radiometer (AVHRR) daytime SST, AVHRR nighttime SST, Tropical rainfall Measuring Mission Microwave Imager (TMI) daytime SST and TMI nighttime SST. Availability of satellite SST, which is the ratio that the number of available satellite SST to the total ocean pixels in NSCS is calculated; annual average SST availabilities of AVHRR daytime SST, AVHRR nighttime SST, TMI daytime SST and TMI nighttime SST are 68.42%, 69.99%, 56.57% and 52.80%, respectively. Though the TMI SST availability is nearly constant throughout the year, the variations of the AVHRR SST availability are much larger because of seasonal variations of cloud cover in NSCS. Validation of the satellite-derived SSTs shows that bias±standard deviation (STD) of AVHRR SST is −0.43±0.76 and −0.33±0.79 °C for daytime and nighttime, respectively, and bias±STD of TMI SSTs is 0.07±1.11 and 0.00±0.97 °C for daytime and nighttime, respectively. It is clear that AVHRR SSTs have significant regional biases of about −0.4 °C against the drifting buoy SSTs. Differences between satellite-derived−in situ SSTs are investigated in terms of the diurnal SST cycle. When satellite-derived wind speeds decrease down below 6 m/s, the satellite SSTs become higher than the corresponding in situ SSTs, which means that the SST difference (satellite SST−Buoy SST) is positive. This wind-speed dependence of the SST difference is consistent with the previous results, which have mentioned that low wind speed coupled with clear sky conditions (high surface solar radiation) enhance the diurnal SST amplitude and the bulk-skin temperature difference.     

SST observations of upwelling induced by tidal straining in the Rhine ROFI

In this paper SST imagery and a three-dimensional numerical model of a river plume were employed to detect upwelling induced by tidal straining in the Rhine ROFI (region of fresh water influence). Previous studies have shown that the Rhine ROFI in the North Sea exhibits strong cross-shore density gradients that compete with tidal and wind mixing to establish stratification. During neap periods with low mixing energy an area measuring 30 km offshore by 100 km alongshore becomes stratified. When the ROFI is stratified strong cross-shore currents are observed, with surface currents rotating anti-cyclonically and bottom currents rotating cyclonically. The cross-shore currents interact with the cross-shore density gradients to produce a semi-diurnal cycle of stratification. Due to continuity requirements imposed by the proximity of the coast, the offshore-directed surface currents and onshore-directed bottom currents should lead to coastal upwelling.     

Seasonal upwelling on the Western and Southern Shelves of the Gulf of Mexico

An 8-year database of sea surface temperature (SST), 7 years of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color images, wind fields, and numerical model results are analyzed to identify regions and periods of coastal upwelling on the western and southern shelves of the Gulf of Mexico. On the seasonal scale, it is found that on the Tamaulipas, Veracruz, and southwestern Texas–Louisiana shelves there are upwelling favorable winds from April to August, when southeasterly winds are dominant and cold SST anomalies associated with upwelling are observed along their coasts. However, during summer, values of chlorophyll-a concentration are lower than those in autumn and winter, which are high due to advection of old bloom biological material from upstream. During winter, there is a cold front on the Tamaulipas shelf produced by advection of cold water from the Texas–Louisiana shelf and not due to upwelling. On the eastern Campeche Bank, persistent upwelling is observed due to favorable winds throughout the year with cold SST and large chlorophyll-a content along the inner shelf from May to September. On the Tamaulipas shelf, the summer upwelling delays the annual SST peak until September, while in most of the Gulf SST peaks in August. This difference is due to the end of the upwelling favorable wind conditions and the September seasonal current reversal.     

Interannual variability of summer coastal upwelling in the Taiwan Strait

This study dealt with the interannual variability of summer coastal upwelling in the Taiwan Strait, based on empirical orthogonal function (EOF) analysis. Three datasets were used for the analysis: the National Oceanic and Atmospheric Administration (NOAA), Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature dataset from 1985 to 2005; hydrographic records at two coastal stations from 1970 to 2001; and cruise measurements in 1988 and 2004. The results indicated that the first mode (85.3%) of the spatial variance showed a persistent front, which was generally aligned northeast–southwestward in the western Taiwan Strait. This front separated colder water on the west side from warmer water on the east side. The eigenvector time series showed that the variability of this front with time was closely correlated with the change in the wind stress anomaly of the alongshore wind component, derived from 17 years of the European remote sensing (ERS) satellite and QuickScat wind dataset from 1992 to 2005. Records of water temperature and salinity anomaly at Pingtan Island (Is.) located in the northwestern Taiwan Strait, and Dongshan Is. located in the southwestern Taiwan Strait, showed that a negative temperature anomaly appeared along with a positive salinity anomaly in some years. This suggested a dominant influence of cold and saline upwelling water at the surface. The years of notable cooling events derived from the station measurements were generally consistent with the time series of the EOF Mode 1. The change in upwelling derived from cruise measurements further confirmed the change shown by the EOF Mode 1 time series. These 1985–2005 results indicated that for the entire western Taiwan Strait summer coastal upwelling was strong in 1987, 1993, and 1998, and that upwelling in the northwest and the southwest Taiwan Strait showed different behavior. A delayed ENSO (El Niño Southern Oscillation) effect was suggested as a major mechanism for the interannual variability of Taiwan Strait coastal upwelling.     

Surface current patterns observed by HF radar: methodology and analysis of currents to the north of the Yaeyama Islands,East China Sea

A new method was developed to compare the classifications of different kind of data maps based on the self-organizing map (SOM) analysis. The surface current maps of the northern coast of Ishigaki and Iriomote Islands, East China Sea, observed by high-frequency (HF) radar were classified by SOM. Winds, sea surface temperatures (SST), and reanalysis data were also classified by SOM. The optimum area for classification was determined objectively by the new method, which relates these patterns to HF radar current patterns. We found two typical surface patterns: the first was that northeastward flows are dominant in the observation area and the second was that a clockwise eddy was also dominant in this area. The southwestward wind pattern was strongly related to the dominant clockwise eddy pattern of the HF radar current field.     

Regional coupled ocean–atmosphere downscaling in the Southeast Pacific: impacts on upwelling, mesoscale air–sea fluxes, and ocean eddies

Ocean–atmosphere coupling in the Humboldt Current System (HCS) of the Southeast Pacific is studied using the Scripps Coupled Ocean–atmosphere Regional (SCOAR) model, which is used to downscale the National Center for Environmental Prediction (NCEP) Reanalysis-2 (RA2) product for the period 2000–2007 at 20-km resolution. An interactive 2-D spatial smoother within the sea-surface temperature (SST)–flux coupler is invoked in a separate run to isolate the impact of the mesoscale (～50–200 km, in the oceanic sense) SST field felt by the atmosphere in the fully coupled run. For the HCS, SCOAR produces seasonal wind stress and wind stress curl patterns that agree better with QuikSCAT winds than those from RA2. The SCOAR downscaled wind stress distribution has substantially different impacts on the magnitude and structure of wind-driven upwelling processes along the coast compared to RA2. Along coastal locations such as Arica and Taltal, SCOAR and RA2 produce seasonally opposite signs in the total wind-driven upwelling transport. At San Juan, SCOAR shows that upwelling is mainly due to coastal Ekman upwelling transport, while in RA2 upwelling is mostly attributed to Ekman pumping. Fully coupled SCOAR shows significant SST–wind stress coupling during fall and winter, while smoothed SCOAR shows insignificant coupling throughout, indicating the important role of ocean mesoscale eddies on air–sea coupling in HCS. Coupling between SST, wind speed, and latent heat flux is incoherent in large-scale coupling and full coupling mode. In contrast, coupling between these three variables is clearly identified for oceanic mesoscales, which suggests that mesoscale SST affects latent heat directly through the bulk formulation, as well as indirectly through stability changes on the overlying atmosphere, which affects surface wind speeds. The SST–wind stress and SST–heat-flux couplings, however, fail to produce a strong change in the ocean eddy statistics. No rectified effects of ocean–atmosphere coupling were identified for either the atmospheric or oceanic mean conditions, suggesting that mesoscale coupling is too weak in this region to strongly alter the basic climate state.     

A simulation of the Chilean Coastal Current and associated topographic upwelling near Valparaíso, Chile

A 4-year simulation of the surface circulation driven by the local wind on a section of the central Chilean coast is presented. The model is shown to reproduce the major observed features of the circulation. Comparison to observations of sea-surface temperature (SST) taken within the study area suggests that the model captures well coastal upwelling processes in the region. The circulation is shown to have two distinct modes corresponding to spring/summer and autumn/winter. During spring/summer sustained strong south-westerly wind forcing drives an equatorward coastal jet consistent with the Chile Coastal Current (CCC) and coastal upwelling at previously identified locations of intense upwelling at Topocalma Point and Curaumilla Point. Weaker winds during autumn/winter produce a slower CCC and a more homogenous SST field. Upwelling/relaxation and topographic eddies provide the main sources of variability on sub-seasonal time-scales in the model. The mechanisms responsible for each of these are discussed. Upwelling at Topocalma and Curaumilla Points is shown to be produced through generation of an upwelling Ekman bottom boundary layer following acceleration of the CCC close to the coast, reinforced by secondary circulation due to flow curvature around the headlands. Additional upwelling occurs north of Curaumilla Point due to development of shallow wind-driven overturning flow. Wind-sheltering is shown to be an important factor for explaining the fact that Valparaíso Bay is typically an upwelling shadow. Flow separation and eddy formation within Valparaíso Bay is seen to occur on the order of 10 times per year during relaxation after strong wind events and may persist for a number of weeks. Shorter lived topographic eddies are also seen to occur commonly at Topocalma and Toro Points. These eddies are shown to form in response to the surface elevation minima produced at each of these locations during upwelling.     

The influence of rainfall on PR radar measurement of ocean surface wind speed and its calibration

Rain can significantly degrade the wind vector retrieval from Precipitation Radar(PR) by three mechanisms, namely, two-way rain attenuation, rain volume-backscattering, and ocean surface roughening from the rain splash effect. Here we first derive the radar equation for PR in rainy conditions. Then we use the rain attenuation model for Ku band, volume backscatter model for spherical raindrops and PR–TMI(TRMM Microwave Imager, TMI) matchup datasets from June to August in 2010 to solve the radar equation, and quantitatively analyze the influence of rainfall on PR radar measurement of ocean surface wind speed. Our results show that the significant effect of rain on radar signal is dominated by two-way rain attenuation and rain splash effect, and the effect of rain volume-backscattering is relatively the weakest, which can even be neglected in rain-weak conditions. Moreover, both the two-way rain attenuation and rain splash effect increase with the increasing of integration rain rate and incident angle. Last, we combine volume-backscattering effect and splash effect into a simple phenomenological model for rain calibration and select three typhoon cases from June to August in 2012 to verify the accuracy of this model. Before calibration, the mean difference and mean square error(MSE) between PR-observed ? 0 and wind-induced ? 0 are about 2.95 dB and 3.10 dB respectively. However, after calibration, the mean difference and MSE are reduced to 0.64 dB and 1.61 dB respectively. The model yields an accurate calibration for PR near-nadir normalized radar cross section(NRCS) in rainy conditions.     

The Guajira upwelling system

The coastal upwelled waters of the Guajira coast, the most northerly peninsula of South America, were studied on the basis of historical data bases, remotely sensed data, and three oceanographic cruises. The Guajira Peninsula is the locus of particularly strong upwelling because it protrudes into the Caribbean Low-Level Wind Jet and its west coast parallels the direction of the strongest winds. The year-round upwelling varies with the wind forcing: strongest in December–March and July, and weakest in the October–November rainy season. The east–west temperature, salinity and density front that delimits the upwelling lies over the shelf edge in the east of the peninsula but separates from the south-westward trending topography to the west. A coastal westward surface jet geostrophically adjusted to the upwelling flows along the front, and an eastward sub-surface counterflow is trapped against the Guajira continental slope. The undercurrent shoals toward the western limit of the upwelling, Santa Marta, beyond which point it extends to the surface. Some of the westward jet re-circulates inshore with the counterflow but part continues directly west to form an upwelling filament. Much of the mesoscale variation is associated with upwelling filaments, which expel cooler, chlorophyll-rich coastal upwelling waters westward and northward into the Caribbean Sea. Freshwater plumes from the Magdalena and Orinoco rivers influence the area strongly, and outflow from Lake Maracaibo interacts directly with upwelled waters off Guajira. Another important factor is the Aeolian input of dust from the Guajira desert by episodes of offshore winds.     

Coastal upwelling along the north coast of Papua New Guinea and El Niño events during 1981–2005

We investigate the relationship between sea surface temperature (SST) cooling and upwelling along Papua New Guinea’s (PNG) north coast before the onset of El Niño events using a hindcast experiment with a high-resolution ocean general circulation model. Coastal upwelling and related SST cooling appear along PNG north coast during the boreal winter before the onsets of six El Niño events occurring during 1981–2005. Relatively cool SSTs appear along PNG north coast during that time, when anomalous northwesterly surface wind stress, which can cause coastal upwelling by offshore Ekman transport appearing over the region. In addition, anomalous cooling tendencies of SST are observed, accompanying anomalous upward velocities at the base of the mixed layer and shallow anomalies of 27°C isotherm depth. It is also shown that entrainment cooling plays an important role in the cooling of the mixed layer temperature in this region.     

Processes of India’s offshore summer intraseasonal sea surface temperature variability

Active and break phases of the Indian summer monsoon are associated with sea surface temperature (SST) fluctuations at 30–90 days timescale in the Arabian Sea and Bay of Bengal. Mechanisms responsible for basin-scale intraseasonal SST variations have previously been discussed, but the maxima of SST variability are actually located in three specific offshore regions: the South-Eastern Arabian Sea (SEAS), the Southern Tip of India (STI) and the North-Western Bay of Bengal (NWBoB). In the present study, we use an eddy-permitting 0.25° regional ocean model to investigate mechanisms of this offshore intraseasonal SST variability. Modelled climatological mixed layer and upper thermocline depth are in very good agreement with estimates from three repeated expendable bathythermograph transects perpendicular to the Indian Coast. The model intraseasonal forcing and SST variability agree well with observed estimates, although modelled intraseasonal offshore SST amplitude is undere-stimated by 20–30 %. Our analysis reveals that surface heat flux variations drive a large part of the intraseasonal SST variations along the Indian coastline while oceanic processes have contrasted contributions depending of the region considered. In the SEAS, this contribution is very small because intraseasonal wind variations are essentially cross-shore, and thus not associated with significant upwelling intraseasonal fluctuations. In the STI, vertical advection associated with Ekman pumping contributes to ～30 % of the SST fluctuations. In the NWBoB, vertical mixing diminishes the SST variations driven by the atmospheric heat flux perturbations by 40 %. Simple slab ocean model integrations show that the amplitude of these intraseasonal SST signals is not very sensitive to the heat flux dataset used, but more sensitive to mixed layer depth.     

Upwelling or differential cooling? Analysis of satellite SST images of the Southeastern Baltic Sea

The results of comparative analysis of sea surface temperature variations along horizontal sections in the coastal zone are given. The data used had been taken by MODIS spectroradiometers (Aqua, Terra) in the Southeastern Baltic, in periods of coastal upwelling—in the periods of autumn differential cooling over coastal continental slopes (facilitating water subsidence along these slopes). Studying 135 SST images of coastal upwelling events in May–October 2000–2014 and four cooling events in October–November 2002, 2004, 2005, and 2009 revealed the specific features of the shape of horizontal temperature profiles on sea surface along sections over coastal continental slopes. In addition to the higher differences between water surface temperatures in the deep and coastal parts of the sea (up to 14°C), upwelling features an appreciable distance from the cold-water core to the coast (up to 3–15 km) and a variable shape of horizontal profiles of water temperature on the sea surface along the sections. Conversely, during autumn differential cooling, water temperature difference on the surface is relatively small, the shape of the dependence of surface water temperature on the distance to the shore does not change over time, varies only slightly with the alongshore displacement of the section, and shows low sensitivity to bathymetry and even to wind effect. Thus, the analysis of the shape of the temperature on the sea surface along horizontal sections over coastal continental slopes enables the diagnostics of the regime of vertical water exchange in the coastal zone.     

Correcting surface winds by assimilating high-frequency radar surface currents in the German Bight

Surface winds are crucial for accurately modeling the surface circulation in the coastal ocean. In the present work, high-frequency radar surface currents are assimilated using an ensemble scheme which aims to obtain improved surface winds taking into account European Centre for Medium-Range Weather Forecasts winds as a first guess and surface current measurements. The objective of this study is to show that wind forcing can be improved using an approach similar to parameter estimation in ensemble data assimilation. Like variational assimilation schemes, the method provides an improved wind field based on surface current measurements. However, the technique does not require an adjoint, and it is thus easier to implement. In addition, it does not rely on a linearization of the model dynamics. The method is validated directly by comparing the analyzed wind speed to independent in situ measurements and indirectly by assessing the impact of the corrected winds on model sea surface temperature (SST) relative to satellite SST.     

Upwelling and surface cold patches in the Yellow Sea in summer: Effects of tidal mixing on the vertical circulation

A three-dimensional, prognostic, wave–tide–circulation coupled numerical model is developed to study the effects of tidal mixing on the summertime vertical circulation in the Yellow Sea (YS). The distribution and mechanisms of upwelling are investigated by numerical means. Validated by historical tide gauge data, satellite sea surface temperature (SST) data, and cruise observation data, the model shows satisfactory performances in reproducing the dominant tidal system and three-dimensional sea temperature structure. Model results suggest that strong tidal mixing plays an important role in the formation of the vertical circulation in the YS. The Yellow Sea Cold Water Mass (YSCWM) is fringed by typical tidal mixing fronts (TMFs), which separate the cold, stratified water at the offshore side from the warm, well-mixed, shallow water at the other side. Considerable baroclinic gradient across the TMF makes the frontal zone the spot where the most active vertical circulation occurs; a secondary circulation is triggered with a distinct upwelling branch occurring mainly on the mixed side of the front. The numerical model produces systematic upwelling belts surrounding the YSCWM, and the upwelling is essentially induced by the TMF over sloping topography. The relative importance of tidal mixing and wind forcing for upwelling is further examined in numerical experiments. The southerly wind enhances the upwelling off the western coasts, but its overall influences in the whole YS are less important than tidal mixing. As shown by both satellite data and numerical modeling, the summertime SST field in the YS is featured by the stable existence of several site-selective surface cold patches (SCPs), most of which scatter in the waters off convex coastlines. One of the SCPs is found off Subei Bank, and the others are located off the eastern tip of Shandong Peninsula and off the three tips of Korean Peninsula. Two processes give rise to the SCP: on the one hand, TMF-induced upwelling supplies cold water from the deep layer; on the other hand, tidal mixing itself can stir the bottom water upward and homogenize the water column vertically. In the waters around the tips of peninsula in the YS, the tidal currents are extraordinarily strong, which provides a possible explanation for the site-selectivity of the SCPs.