Sea Surface Temperature Images Research Articles

Application of an edge detection method to satellite images for distinguishing sea surface temperature fronts near the Japanese coast

This study applies an edge detection method to satellite sea surface temperature (SST) images in order to distinguish SST fronts frequently occurring near the Japanese coast. First, we develop a new set of edge detection algorithms. The method is based on an entropic approach using Jensen–Shannon divergence, which is certified as a robust technique for detecting edge pixel candidates. Additionally, we incorporate ridge-line extraction morphological filter to obtain edge pixels, which is suited to delineate intricate frontal features. The methodology allows us to detect reasonable amount of possible fronts to the fullest extent according to the original image resolution. Therefore, it enables us to analyze finer-scale fronts near the coast as well as larger-scale fronts associated with major currents. Then, we present four case study examples of SST fronts near the Japanese coast by applying the method to Advanced Very High Resolution Radiometer (AVHRR) SST images with a grid size of 0.01°. We describe the configuration and seasonal variability of the SST frontal systems from the viewpoint of occurrence probability, SST gradient magnitude, and orientation to the local bathymetry as compared with the previous observational studies. Consequently, it is demonstrated that application of the method can derive new views of finer-scale and highly variable SST fronts. Moreover, this study first verifies the capability of comprehensive and statistical description of SST front configurations and variations near the Japanese coast by applying edge detection method to sequences of satellite SST images.

The winter St. Helena climate index and extreme Benguela upwelling

Climate changes in the subtropical South-east Atlantic turn out to be well described by the St. Helena Island Climate Index (HIX) and observed fluctuations are in good agreement with inter-decadal variability of the entire South Atlantic Ocean. Year-to-year variations of the averaged austral winter HIX (July–September), representative of the main upwelling season, were compared with (i) corresponding averages of the geostrophic alongshore component of the south-east trade wind (SET) between St. Helena Island in the south-west and Luanda/Angola in the north-east, (ii) the meridional distribution of surface waters colder than 13 °C to characterise intense Benguela upwelling (IBU), and (iii) the meridional position of the Angola-Benguela Frontal Zone (ABFZ) determined by means of sea surface temperature images for offshore distances between 50 and 400 km. Temporal changes of these parameters were investigated and showed that the frequency of consecutive years of strong and relaxed Benguela upwelling is characterised by a quasi-cycle of about 11–14 years. It is proposed that the index of the winter HIX may be used as a ‘surveyor’s rod' to describe interannual changes in the Benguela upwelling regime as well as those of the embedded marine ecosystem.

A field study of the behaviour of an anticyclonic eddy on the Catalan continental shelf (NW Mediterranean)

Through the analysis of satellite thermal images, mesoscale anticyclonic eddies have been observed to recurrently drift along the NW Mediterranean coasts. The presence of these eddies is known to cause transient “inversions” of the shelf–slope current. By means of an exhaustive analysis of sea surface temperature images a group of anticyclonic structures were monitored in September 2001. One of these eddy-like features was tracked and intensely surveyed by means of conductivity–temperature–depth (CTD) casts and repeated fast surveys with N ν-Shuttle (oscillating CTD) and Acoustic Doppler Current Profiler (ADCP) measurements during the ET2001 oceanographic cruise. A well developed anticyclonic eddy was surveyed over the Catalan continental shelf, 60 km north of Barcelona. In situ velocity data revealed that the anticyclonic motion was characterized by surface velocities of 50 cm/s and a diameter of about 45 km. The signal of the eddy was found down to 100 m depth, well below the seasonal thermocline. The passage of the eddy modified the local flow, involving advection and subduction of surrounding waters.

Significance of High-Frequency Wind Forcing in Modelling the Kuroshio

Motivated by an analysis of a satellite sea surface temperature image suggesting that a train of extra-tropical cyclones induces amplification of the Kuroshio meander, a regional Kuroshio/Oyashio general circulation model was used to investigate the impact of high-frequency wind on the Kuroshio path variations. Near Japan, the standard deviation of the wind stress curl can be 10 times larger than the monthly mean, so the synoptic variations of the wind stress curl cannot be neglected. With the bimodal Kuroshio case realized in the model, sensitivity tests were conducted using monthly and daily mean QuikSCAT-derived wind stress forcings. The comparison showed that the high-frequency local wind perturbed the Shikoku recirculation gyre (SRG) and caused a transition of the path from straight to meander. The strong anticyclonic eddy within the SRG triggered the meander in the latter case. The high-frequency wind perturbed the motion of the eddy that would have otherwise detached from the Kuroshio, migrated south and terminated the meandering state. The result reinforces the suggestion from previous studies that the anticyclonic eddy within the SRG plays an active role in controlling the Kuroshio path variations.

Differences in foraging strategy and maternal behaviour between two sympatric fur seal species at the Crozet Islands

Marine top-predators such as marine mammals forage in a heterogeneous environment according to their energetic requirements and to the variation in environmental characteristics. In this study, the behaviour of breeding females in 2 sympatric fur seal species, Antarctic fur seal Arctocephalus gazella and Subantarctic fur seal A. tropicalis, was investigated in relation to foraging effort. Foraging effort was hypothesised to be greater in Antarctic fur seal than in Subantarctic fur seal due to their shorter lactation period. Using satellite telemetry, time-depth recorders and satellite images of sea-surface temperature and chlorophyll a concentration, the foraging grounds, the at-sea activity budgets and the environmental features were determined for both species breeding on the Crozet Archipelago. Foraging cycle duration was similar for the 2 species, and the seals exhibited similar at-sea activity budgets. Only the proportion of time spent at sea was higher in Antarctic fur seals. Separate foraging areas were identified for the 2 species. Antarctic fur seal distribution was related to bathymetric features, while we did not find any direct relationship between chlorophyll a concentration and seal foraging areas. Our results suggest that Antarctic fur seals tend to respond to the higher needs of their pups by having a higher foraging efficiency and concentrating their foraging activity in the most productive areas.

Seasonal variation in phytoplankton in the southern Benguela: pigment indices and ocean colour

Pigment indices and satellite imagery were used to investigate the seasonal variation in phytoplankton in the southern Benguela, with the aid of monthly monitoring on two transect lines. Sea surface temperature images revealed the similarity in thermal conditions during autumn, winter and spring, with a broad zone of 11–14°C water extending northwards from Cape Columbine and a narrow zone extending south along the coast to the Cape Peninsula. In summer, warm water (18–20°C) intruded from the South Coast into the West Coast offshore region and cold water (10–12°C) prevailed inshore at particular upwelling centres. The phytoplankton response to these thermal conditions resulted in elevated chlorophyll a (2–15mg m –3) in the 11–14°C waters during all four seasons. The zones of elevated chlorophyll were generally broader from Cape Columbine northwards and narrower south of Cape Columbine to the Cape Peninsula. Pigment indices indicated that phytoplankton biomass was high in summer, but also quite extensive in the autumn and spring, with diatoms generally dominating inshore and small flagellates offshore. Diagnostic indices indicated that flagellates were prominent at inshore localities during all seasons, however, suggesting that this group contributed significantly to phytoplankton communities on the shelf.

Assessing the variability of hydrographic processes influencing the life cycle of the Sicilian Channel anchovy, Engraulis encrasicolus, by satellite imagery

AbstractThree oceanographic surveys carried out in the Sicilian Channel during the spawning season (June to July) of anchovy (Engraulis encrasicolus) showed a close relationship between anchovy reproductive strategy and important hydrographic structures. A time series of satellite‐derived sea surface temperature images of the Sicilian Channel were analysed by means of empirical orthogonal functions and the dominant empirical modes were studied in detail. The first empirical mode captured much of the original variance and reproduced the trajectory of the Atlantic Ionian Stream (AIS), the principal hydrodynamic feature of the area. The time coefficients of modes 1 and 2 had seasonal signals which, when combined, accounted for the enhancement of the thermal front, clearly visible off Cape Passero (southernmost coast of Sicily) during summer. As the area constituted the principal nursery ground of the Sicilian Channel anchovy, the combination of the time coefficients of these modes was considered a potential indicator of the food particle concentration usually associated with oceanic fronts, which provided the energy requirements for larval growth. Mode 3 described the north/south displacements of the mean AIS trajectory, which modified the surface temperature regime of the anchovy spawning habitat. Therefore, the time coefficients of this mode were used as a potential indicator of anchovy spawning habitat variability. The capability of time coefficients of modes 2 and 3 to modify the main pattern depicted by mode 1 were tested successfully against in situ oceanographic observations.

Sea surface temperature fronts in the East (Japan) Sea and temporal variations

The spatial pattern of sea surface temperature(SST) fronts and its seasonality are presented for the entire East(Japan) Sea. They are based on semi‐monthly sea surface temperature(SST) gradient maps constructed from declouded SST images over the period 1990–1995. Fronts in the East Sea can be assigned to the Subpolar Front (SPF) in its central sea areas and to other fronts along the continental shelves. It is observed for the first time that the SPF, located mostly along about 40°N between 132.3°E and 138°E, bifurcates at both its eastern and western ends. The spatial structure of the SPF shows strong seasonality associated with: temporal variations of a warm eddy off the east coast of Korea; surface cooling due to Siberian cold air south of Vladivostok; and bifurcation of the Tsushima Warm Current west of the Tsugaru Strait. We have also observed an isolated frontal zone trapped by a seamount in the northeastern part of the Japan Basin.

Clean seas: a North Sea test site

The Clean Seas project focused on the role that existing Earth observing satellites might play in monitoring marine pollution. Results are presented here from August 1997, for the North Sea test site, using sea surface temperature (SST), colour and synthetic aperture radar (SAR) images in conjunction with a hydrodynamic model. There was good correlation between data sources, e.g. between SST and ERS-2 SAR images. Both datasets showed the development of fine plume structures close to the Rhine outflow, apparently associated with the outflow, and possibly caused by tidal pulsing of the Rhine Plume. The model reproduced general temperature and sediment distributions well, but fine structures were not reproduced. Model sediment distribution patterns were verified using ‘chlorophyll concentration’ data from colour sensors, representative of sediment concentration in turbid water. In conjunction with the visible channels of the Advanced Very High Resolution Radiometer and Along-Track Scanning Radiometer, they give an uncalibrated measure of the sediment load. The model gives a more complete picture of the temporal dispersion of the Rhine Plume over time than is evident from the remotely sensed data alone.

Temporal and spatial variability of satellite sea surface temperature and ocean colour in the Japan/East Sea

Temporal and spatial variability of Sea Surface Temperature (SST) and ocean colour in the Japan/East Sea (JES) are examined during winter and spring using satellite data from Advanced Very High Resolution Radiometer (AVHRR) and Sea-viewing Wide Field of view Sensor (SeaWiFS). The timing of the spring phytoplankton bloom and the locations of the chlorophyll fronts are related to changes in the thermal fields and the locations of the temperature fronts. Daily images of SST and chlorophyll concentration show both differences and similarities of bio-optical and thermal front location, depending on region and season. Four sub-regions in the JES were defined and SST and chlorophyll values were extracted from weekly and monthly composite images to derive summary statistics. SST at the Subpolar Front increased about 7°C over a 1.5-month period from late April to early June in 1999. During this same period, elevated chlorophyll values near the Korean coast and in the southern basin decreased sharply as the phytoplankton bloom that first developed in the southern basin progressed to the front and northward. The SST/chlorophyll relationship is complex and seasonal. Near the Subpolar Front, SST and chlorophyll were positively related in April. In May, highest chlorophyll values corresponded to mixing regimes (such as areas of convergence and divergence at the edges of meanders) and, by June, SST and chlorophyll near the front were inversely related.

Space–time behaviour of the Angola–Benguela Frontal Zone during the Benguela Niño of April 1999

The hydrographic conditions in the south-east Atlantic were investigated during the cruise of R/V Poseidon in April 1999. The area of investigation covers the two major eastern boundary currents of the Southern Atlantic and its convergence in the Angola–Benguela Frontal Zone (ABFZ). Hydrographic measurements have been carried out in combination with current measurements and are supplemented with remote sensing data of sea surface temperature (SST) and wind stress. The Angola Current transported in its surface part less saline water from the great rivers in the north towards the ABFZ. At the same time, south of the front, strong coastal upwelling in the Benguela was observed. At the beginning of April 1999 the ABFZ was found at an unusual southern position. The rapid dynamic response of the ABFZ to wind forcing is discussed on the basis of satellite SST images combined with hydrographic measurements.

Unusual upwelling event and current reversal off the Italian Adriatic coast in summer 2003

An exceptional upwelling event occurred near the Conero Promontory along the Italian Adriatic coast between 11 and 23 July 2003, as a result of dominant southeasterly sirocco winds. Contemporarily, surface currents off central and southern Italy reversed to the northwest direction. This peculiar dynamic situation happened when the Adriatic river discharge rates were extremely low due to a prolonged dry season. Satellite images of sea surface temperature and chlorophyll concentration and satellite‐tracked drifters revealed that the upwelling event first appeared northwest of the promontory and then progressed to the southeast. Its offshore extension (20–30 km) includes convoluted filament structures. Surface temperature and ocean color signatures are correlated, indicating that the cold‐upwelled water is rich in nutrients. Coldest temperatures are as low as 18–19°C (8°C lower than the open sea). The northwestward currents reached maximum speeds larger than 40 cm/s inshore of the 20 m‐isobath (<10 km from shore).

Internal structure of SST features in the central Gulf of California

Abstract Closely spaced XBT and CTD data were used to study the sub-surface structure of features observed in satellite AVHRR sea-surface temperature (SST) images of the Gulf of California (GC), such as fronts, jets and gyres. The lowest SSTs in the GC are found around the midriff islands, and especially over the sills, because of strong mixing from tidal currents and breaking internal waves. This minimum SST area is limited to the south and north by sharp SST fronts, which frequently show convolutions, gyres, and filaments. A vertical section of temperature along the Gulf length shows that about 300 km south of the sills, the isotherms begin to spread out in the vertical, reducing stratification. The thermocline isotherms begin surfacing about 100 km south of the sill, forming the SST minimum area, which has a maximum temperature difference of ∼6 °C with waters to the south and a horizontal SST gradient of ∼0.05 °C km −1 . The evolution and subsurface structure of a cool filament, apparently originated as an instability of the SST front, was intensely sampled. Satellite images show that its head advanced due south at about 0.5 ms −1 , along the edge of a pool of warm surface water about 110 km in diameter. The thermocline below the warm pool was raised at its edge, and some isotherms reached the surface, producing the cool filament. The tilt of the temperature, salinity and density isolines at the edge of the warm pool produced a geostrophic jet, adjacent to the cool filament and just inside the warm pool, with maximum speed ∼0.5 ms −1 and extending down to ∼80 m. The filament lasted about 10 days, its width was ∼30 km and it reached a maximum length of ∼120 km.The volume transport of the jet in the top 100 m was around 0.7 Sv.

PICO-PHYTOPLANKTON DISTRIBUTION IN WESTERN PACIFIC OCEAN ESTIMATED FROM OCEAN COLOR AND SST

To investigate the spatial distribution of pico-phytoplankton in surface water, photosynthetic pigments were analyzed by a high-performance liquid chromatography (HPLC) for three cruises in the western Pacific Ocean carried out on 2000, 2001 and 2002. Analyzed chlorophyll-a showed peaks at the equatorial region and near Japan. In contrast, zeaxanthin, a specific pigment of pico-phytoplankton such as Synechococcus and Prochlorococcus showed a broad peak around equatorial region. From these results, we found inverse relationship between the ratio of zeaxanthin to chlorophyll-a concentration (ZCR) and chlorophyll-a concentration, and also positive relationship between ZCR and sea surface temperature (SST). And we derived a multiple regression equation to estimate ZCR from chlorophyll-a concentration and SST. The equation was applied to estimate spatial distribution of ZCR in the Pacific Ocean using satellite chlorophyll-a images and SST images

Lagrangian and satellite observations of the Brazilian Coastal Current

The waters dominating the Brazilian Continental Shelf to the south of Santa Marta Cape (28°40′S) are marked by their strong interannual variability. Both the seasonal oscillation of the Brazil–Malvinas (Falkland) Confluence (BMC) region and the seasonal variations of the La Plata River and Patos Lagoon outflows are reflected in the seasonal changes of the vertical and horizontal water mass structure in the Southern Brazilian Shelf. In the region to the north of Santa Marta Cape, the shelf is mainly described in the literature as dominated by Tropical Waters (TW) transported southwards by the Brazil Current (BC). However, the first Lagrangian (buoy) measurements made on the inner Brazilian shelf have shown that a coastal current flowing in the opposite direction in relation to the BC occurred on the shelf as far north as 24°S during the 1993 austral autumn and winter. Recent papers have suggested that the arrival at low latitudes of cold waters originating in the BMC region is an anomalous phenomenon and that it can be either forced by local winds during wintertime or related to the ENSO. High-resolution sea surface temperature (SST) imagery and the Lagrangian measurements taken in 1993 and 1994 are used in this paper to describe the temperatures, velocity, energy and oscillations present in this coastal current. These two data sets show that the current is not only fed by waters of Subantarctic or coastal origin but also receives a contribution of TW at the surface by lateral mixing. By analysing a set of monthly averaged SST images from 1982 to 1995, this work suggests that the intrusion of cold waters transported by the coastal current can be a regular winter phenomenon occurring on the Brazilian shelf at latitudes up to the vicinity of 25°S. Given its consistency, this current is named here the Brazilian Coastal Current.

Kuroshio path meander between the continental slope and the Tokara Strait in the East China Sea

Horizontal patterns and meander motions of the Kuroshio paths in the northern Okinawa Trough between the continental slope and the Tokara Strait are investigated using surface drifter buoy trajectory data, NOAA sea surface temperature (SST) measurements, and shipboard acoustic Doppler current profiler (ADCP) current observations. Temporal variations are also examined by spectral analyses of 1‐year moored velocity/temperature records along the continental slope near 28.8°–30.5°N and of the Kuroshio position time series in the Tokara Strait. Drifter buoy trajectories show that the Kuroshio paths in the northern Okinawa Trough are quasibimodal in character consisting of the northern paths and southern ones, which are associated with anticyclonic and cyclonic Kuroshio circulations, respectively. The Kuroshio position time series show that the northern paths tend to be persistent and intermittently undergo transition to southern paths at periods of 1–3 months. Moored current variations in the slope area and the Kuroshio path variations in the Tokara Strait are highly coherent near a period of 34 days due to the meander motions resulting from the transitions between the northern and southern paths. Successive NOAA SST images and shipboard ADCP current fields show that the transition from the northern path to the southern one is associated with a spatially growing cyclonic eddy, which is initially generated from a downstream‐propagating frontal meander with wavelength of about 200 km. When the cyclonic eddy grows into the scale of the northern Okinawa Trough (about 200‐km E–W, 250‐km N–S), the Kuroshio path changes from the northern path to the southern one.

In situ and satellite observations of a harmful algal bloom and water condition at the Pearl River estuary in late autumn 1998

Harmful algal blooms (HABs) have posed a serious threat to the aquaculture and fisheries industries in recent years, especially in Asia. During 1998 there were several particularly serious blooms in the coastal waters of south China, which caused a serious damage to aquaculture. We report a massive dinoflagellate bloom near the mouth of Pearl River in November 1998 with analyses of data from both in situ sea water measurements and satellites. A multi-parameter environmental mapping system was used to obtain real-time measurements of water quality properties and wind data through the algal bloom area, which allow us to compare water measurements from inside and outside of the bloom areas. This bloom with high concentrations of algal cells was evident as a series of red colored parallel bands of surface water that were 100–300 m long and 10–30 m wide with a total area of about 20–30 km 2 by visual. The algal density reached 3.8×10 7 cells l −1 and the surface chlorophyll-a (Chl-a) concentration was high. The algal species has been identified as Gymnodinium cf. catenatum Graham. The water column in the bloom area was stratified, where the surface temperature was 24–25 °C, the salinity was 18–20%, and the northern wind was about 3–4 m s −1 in the bloom area. The SeaWiFS image has shown high Chl-a area coinciding with the bloom area. The sea surface temperature (SST) image of the Pearl River estuary combined with the in situ measurements indicated that the bloom occurred along a mixing front between cooler lower salinity river water and warmer higher saline South China Sea (SCS) water.

Spatial patterns of wind and sea surface temperature in the Galician upwelling region

Cape Finisterre is the most northwest point in the Galician region and separates the meridional west and zonal north coasts of Galicia. The wind field there has high spatial and temporal variability throughout the year. No clear seasonal signal is evident; upwelling and nonupwelling patterns alternate in all seasons. Two outstanding features of the Galician region are persistent upwelling near Cape Finisterre even when not present farther south and alternation of strong upwelling on north and west coasts. Up to now, explanations have relied upon particular dynamics of oceanic flow past Finisterre. We find that major features of upwelling around Finisterre are related to strong spatial structure in the wind field. Analysis of QuikScat wind data for July 1999 to May 2001 shows strong repeatable patterns in the synoptic wind field. These wind patterns emerge as the combination of the two dominant modes in a complex empirical orthogonal function (CEOF) analysis representing over 85% of the variance. Summer wind patterns give rise to characteristic distributions of upwelling along the coast and favor development of filaments in particular locations. The wind measured at Finisterre itself, often used as a general indicator of upwelling conditions around the Galician coast, is not always representative of the overall wind field. The relevant wind fields allowed a qualitative explanation of temperature structure seen in sea surface temperature images, and of differences in both the upwelling and downwelling regimes between the two years.

Cyclonic Eddies Northeast of the Campeche Bank from Altimetry Data

Eight cyclonic eddies were identified near the western edge of the Loop Current, at the northeast shelf break of the Campeche Bank, through TOPEX/Poseidon sea surface height anomaly data from January 1993 through March 2000. The eddies' migration and their lifecycle are described. The formation of the eddies appears to be related to the dynamics of the Loop Current because the timing of their generation corresponds to the last stage of the anticyclone shedding from the Loop Current. The cyclones intensify while moving slightly to the northwest along the shelf break of the Campeche Bank; later, some cyclones are observed to move northward toward the Mississippi shelf break. The cyclones remain next to the Campeche Bank, south of 26°N, from 1.3 to 9.6 months; later, some of them move northward and strengthen by merging with other eddies, extending their life span. The eddies' vertical structure is analyzed with hydrographic data, and the connection between the formation of the cyclones and the evolution of the Loop Current is further described using sea surface temperature images.

Evolución de un evento de surgencia frente a punta Curaumilla, Valparaíso

The beginning, maturity and relaxation phases of a coastal upwelling event (16 to 20 January 1986), off punta Curaumilla (33°06'S), were characterized. This characterization was based on the spatial and time distribution of temperature, salinity, dissolved oxygen, nutrients (nitrate, phosphate and silicate), wind and Ekman transport. Satellite images of sea surface temperature were used as complementary information. The results showed that the winds were fluctuating and mainly blowing from the south and southwest, so it was possible to observe the initial and mature phases and the beginning of the relaxation phase. The initial phase was characterized by an increase of the south and southwest wind and the Ekman transport, a rise of the seasonal thermocline at the coast and the presence of a cold surface water plume 45-55o towards the left of the coastline. The mature phase was characterized by the presence of strong winds (>10 m·s -1 ), high mean Ekman transport (>1,000 m 3 ·s -1 per 1,000 m of coast), a lowering of the temperature ( 34.4 psu) and nutrients (phosphate >2.0 µM; nitrate >15 µM; silicate >10 µM ) at the coastal margin. The declination phase was partially observed, because the wind did not relax enough during the sampling period. However, the surface temperature satellite images on 23 and 24 January, clearly showed a decrease of the area covered with cold upwelled water. The water mass displaced towards the ocean had Subantarctic characteristics and the water mass upwelled to the surface, had Equatorial Subsurface characteristics.