Sea Surface Temperature Satellite Images Research Articles

Some Aspects of the Flow-Topography Interactions in the Taiwan Strait

The Taiwan Strait is a shallow channel with complicated topographic variations that connects the South China Sea with the East China Sea. Previous hydrographic and numerical studies have suggested that a distinct feature of the flow pattern in the strait is the transfer of the vertical stratification of the incoming flow from the south into the horizontal density gradient in the middle of the northern portion of the strait. This work applies a numerical model with a free surface to examine the influence of a change in the volume transport on the density distribution in the strait. According to the model results, a reduction in the incoming flow rate will cause a weakening of the original density gradient. The release of the available potential energy associated with the original density gradient causes the current to meander. The model results are consistent with the satellite sea surface temperature images on 12 and 17 August, 1998.

Tidal mixing near the Kimberley coast of NW Australia

A complex of reefs and islands atop raised topography that is 140 km long and up to 35 km wide extends roughly westward from the Kimberley coast of NW Australia. The surrounding seafloor has depths of 60–80 m. Satellite images of sea surface temperature in spring, summer and autumn suggest that tides commonly mix sub-thermocline waters to the surface along this ridge, particularly along its western half, which includes Adele Island and Churchill, Albert, Beagle and Mavis reefs. The cool surface waters (~3˚C cooler than surroundings) are seen where the depths are less than ~50 m, such that they closely reflect the bottom topography. The semi-diurnal tides have a maximum daily range of ~10 m and it is when the range exceeds ~3 m that mixing to the surface occurs. Thus, the mixing can cease during some neap periods. Historical data from the shelf show the sub-thermocline water to be nutrient-rich (nitrate concentrations of 6.0 µM), so the mixing is likely to be important for the marine ecosystem. As winter approaches, the distinctive cool mixed region around the Adele–Mavis banks is overwhelmed by surface cooling, and historical ship data reveal that the stratification on the surrounding shelf disappears.

Red tides during spring 1998 in Hong Kong:is El Niño responsible?

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 187:289-294 (1999) - doi:10.3354/meps187289 Red tides during spring 1998 in Hong Kong: is El Niño responsible? Kedong Yin1,2,*, Paul J. Harrison3, Jay Chen2, Wei Huang2, Pei-Yuan Qian1,2 1Department of Biology and 2The Centre for Coastal and Atmospheric Research, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR 3Oceanography, Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada *E-mail: kyin@ust.hk ABSTRACT: The magnitude of El Niño in 1997-98 was one of the strongest of the century. A series of red tides occurred in Hong Kong territorial waters between mid-March and mid-April 1998, resulting in the loss of HK$250 (US$32) million in fish kill damage. The causative species was Gyrodinium aureolum. We used a time series of satellite images to track the development of the harmful algal bloom and relate its movement to physical oceanographic conditions. Prior to the red tide event in Hong Kong, harmful algal blooms had occurred earlier (November 1997-February 1998) along the south China coast to the east of Hong Kong, although the species were different. The progression of harmful algal blooms from northeast to southwest coincided with the southwesterly movement of the south China Coastal Current as indicated by the satellite images of SST (sea surface temperature) from the satellite AVHRR. Remote sensing images of chlorophyll a (chl a) from SeaWiFS (sea-viewing wide field of view sensor) confirmed high near-shore chl a for the same region. The entire event coincided with the dramatic change in the oceanographic conditions of the northern portion of the South China Sea between 1997 and 1998 for the period of March to mid-April, as revealed in the weekly composite SST. The SST images showed a warm tongue pointing north to the south China coast in 1998 versus a cold tongue pointing south in 1997 in the middle of the South China Sea. The differences are believed to be due to El Niño and responsible for setting up the physical oceanographic conditions which were favorable for the formation of harmful algal blooms along the south China coast. The warm tongue in SST suggested that the warm water from the South China Sea might have been piling up towards the south China coast. On the other hand, downwelling of the south China Coastal Current along the coast due to the Northeast Monsoon during March might have been moving against the South China Sea warm water at the bottom. As a result, the coastal waters of the south China coast including Hong Kong became trapped along the coast. Given local eutrophied conditions of the China coast, the outbreak of harmful algal blooms occurred over a coast-wide scale (~400 km) in winter 1997 and spring 1998. It appears that the use of satellite SST images is helpful in detecting large-scale changes in oceanographic conditions and relating the changes to possible outbreak of harmful algal blooms. KEY WORDS: Red tides · El Niño · South China Sea · Satellite images · SeaWiFS Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 187. Publication date: October 14, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

Temporal and spatial patterning of sea surface temperature in the northern Benguela upwelling system: possible environmental indicators of clupeoid production

The physical dynamics of the northern Benguela upwelling system between July 1981 and August 1987 were investigated by applying standardized Principal Components Analysis to a time-series of 235 mean, weekly sea surface temperature satellite images of the region. The first three principal components accounted for 87% of the total variance in the standardized input images and were the only components retained for interpretation. Principal Component I (PC I) represents the mean spatial structure of the system in terms of relative SST gradients, PC II represents the balance in dominance between inshore-offshore v. longshore SST gradients and PC III highlights patterns associated with the warming of the central Namibian region (19–23°S) in conjunction with the maintenance of cooler conditions to the north and/or south. These results are interpreted within the context of previous findings on the behaviour of the system, and their relevance to clupeoid recruitment is discussed. Strong negative loadings on PC II indicated conditions which would promote the longshore “retention” of clupeoid eggs and larvae and “concentration” of planktonic food across thermoclines and thermal fronts, whereas strong positive loadings on PC III indicated conditions which would promote onshore retention and concentration. Conditions promoting onshore retention would, on occasion, have also been reflected by positive loadings on PC II whenever there was a uniform contraction of the offshore upwelling front, as a result of reduced, but similar, levels of upwelling along the entire coast.

Sea surface cooling caused by typhoons in the Tohoku area in August 1989

A remarkable sea surface cooling event was observed in the north western Pacific off Japan coast, which is known as the Tohoku Area, by using well‐calibrated satellite sea surface temperature images, when typhoons T8914 and T8915 passed over the Tohoku Area in August 1989. The sea surface cooling maximum reached up to 9°C, which is much larger than that reported in the previous studies. The corresponding hydrographic data show that the sea surface cooling state is closely related with the ocean structure in the Tohoku Area; the larger sea surface cooling occurs in the Oyashio area. The numerical experiments taking account of the ocean structure show that the surface cooling maximum is 3–4 times larger in case of the Oyashio profile than that of the Kuroshio profile and clarify the mechanism of the sea surface cooling.

Variability of sea-surface temperature in the South Atlantic bight as observed from satellite: Implications for offshore-spawning fish

We examined full-resolution (1 × 1 km) satellite images of sea-surface temperature (SST) over five consecutive years (1981–1986) covering the Atlantic menhaden ( Brevoortia tyrannus) recruitment period (November–April) in the SABRE (South Atlantic Bight recruitment experiment) study site. The results of our image time series indicated two processes which could be possible mechanisms for the onshore transport of fish larvae into coastal regions. One is the influx of warm Gulf Stream water that oscillates in and out of the Carolina Bays. These oscillations occurred throughout the study period over distances of 20–40 km and on time-scales as short as two days. The other is a tongue of relatively cold water located adjacent to the Virginia coast that moved southward and penetrated into Onslow Bay between January and March. Previous studies showed that Atlantic menhaden preferentially spawn in 18–22°C waters on the outer shelf. On the assumption that the 18°C isotherm (18DI) indicates where high larval abundance may occur, we used AVHRR-SST imagery to track the onshore-offshore movement of the 18DI along a transect extending onshore-offshore in Onslow Bay. Owing to seasonal warming and cooling, this isotherm was always found closest to the coast in early November, reached maximum offshore displacement by January/March, and then moved onshore again in April/May. Our results also showed that the position of this isotherm can move offshore or onshore in a matter of a few days. An important influence and possibly the major cause of the higher frequency displacements of the 18DI are Gulf Stream meanders or filaments moving through Onslow Bay. Our estimates of onshore isotherm speeds as determined from satellite SST ranged from 2 to 25 cm s −1 and are within the same order as those calculated by physical models or larval age determinations. If the onshore pulses of warm Gulf Stream water are indeed a mode by which menhaden larvae are transported cross-shelf, then the use of satellite-based observations to determine their frequency and onshore extent, as done in the present study, is a useful tool to study variations in fish recruitment.

A relaxation labeling technique for computing sea surface velocities from sea surface temperature

The paper applies a relaxation labeling technique to computing sea surface advective velocities from sequential satellite sea surface temperature (SST) images. The technique first identifies prominent feature points in a pair of time-lapsed SST images, and then estimates sea surface current velocities by establishing feature point correspondences between the two images. In the context of relaxation labelling operation the authors have constructed a compatibility coefficient expressing flow field spatial smoothness, or coherency, suitable for fluid motion. New results are reported and are compared to those obtained using two other methods: the geostrophic currents from in situ measurements and the maximum cross-correlation (MCC) method. The three kinds of velocities are shown to be consistent to one another. >

Variability of water properties in late spring in the northern Great South Channel

Regional CTDIADCP surveys made in the northern Great South Channel (GSC) in late spring of 1988 and 1989 show different patterns of surface salinity in the extent of the freshwater plume east of Cape Cod. In April 1988, the surface plume was just beginning to form along the outer coast of Cape Cod, while 6 weeks later in the season in 1989, the minimum salinity was about 1.5 less, and a large pool of water fresher than 31.6 had pushed eastward over much of the northern GSC region. The difference in the amount of freshening between these two years is due primarily to the 6-week difference in the seasonal cycle and increased river discharge in 1989. The offshore spreading of the low-salinity plume was driven by the deeper circulation and upwelling-favorable winds. The distribution of Maine Intermediate Water (MIW) also significantly differed between April 1988 and June 1989. In April 1988, the seasonal thermocline was just beginning to form, and the spatial structure of MIW was relatively uniform. In June 1989, a narrow core of temperature minimum water (with T min in a range of 3.2–4.4°C) was found along the western flank of the northern GSC between 40 m and 120 m. This colder and fresher water spread to mix with the interior MIW as the core flowed southward into the central GSC. Hydrographic data plus satellite sea-surface temperature images showed a relatively permanent continuous thermal front (with a 10-km cross-isobath variation) along the eastern flank of Nantucket Shoals, across the northern shallow region of the GSC and along the northwestern flank of Georges Bank, which separated the well-mixed water over the shallow region of the GSC from stratified water in the center of the northern GSC. Comparison of the location of this front with theoretical predictions by LODER and GREENBERG [(1986) Continental Shelf Research, 6, 397–414] suggests that enhanced tidal mixing due to the spring-neap cycle is important in determining the relative balance between buoyancy import and tidal mixing in the GSC region.

The Point Arena eddy: A recurring summer anticyclone in the California Current

The propensity for an anticyclonic eddy (∼100‐ to 200‐km diameter) to develop west of Point Arena during the summer is documented with an analysis of satellite derived sea surface temperature (SST) data and a review of published historical hydrographic data. The satellite data span the period from 1981 through 1989 and the hydrographic data include CalCOFI surveys from 1949–1959 and a few more recent studies. A closed anticyclone was apparent in many of the historical dynamic height maps, while a meander pattern, lacking closed contours, characterized the coincident hydrographic 10‐m temperature maps. A conceptual model of an eddylike perturbation imbedded in a broader equatorward flow is adopted as a working hypothesis. In this simplistic model, the only distinction between an eddy (with closed contours) and a meander (without closed contours) is the relative strength of the eddylike perturbation. This led to a method to detect the eddies in the satellite SST data, which were dominated by meanderlike patterns: A linear trend was subtracted from each image to remove large‐scale SST gradients, leaving an expression of the residual eddylike perturbation. Composite images over select time periods were produced for analysis by computing spatial variance EOFs of the detrended data. With this approach, a warm‐core eddy was evident at Point Arena in summertime satellite SST data from 1981–1985 and 1987–1989 (formation of an eddy in 1986 remains unresolved because the readily available SST data were incomplete). The eddy was bounded on the inshore side by the jet and filament system studied during the 1987 and 1988 Coastal Transition Zone (CTZ) program. Recurrence of this eddy was apparently coupled with the recurring cold filament commonly observed at Point Arena during the summer. The coevolution of the eddy and meandering jet during 1988 was studied with sequential composite empirical orthogonal function (EOF) satellite SST images. These show that a deep anticyclonic meander was present near Cape Mendocino and directed the jet toward shore. In late July 1988 this meander pinched off a new eddy, and the subsequent jet flow was redirected southward and displaced farther offshore. These events account for the transition in the jet observed in the July CTZ hydrographic surveys. The length scale and position of the Point Arena eddy is consistent with a standing wave, phase locked to a prominent cape (in this case, Mendocino). The significance of the apparent splitting off of an eddy in 1988, as well as the apparent preponderance of anticyclones evident in these EOFs, is discussed relative to recent theoretical and numerical model developments.

Observations of Frontal Instabilities on an Upwelling Filament

Frontal instabilities were observed along a density front on the cyclonic boundary of an upwelling filament that formed north of Pt. Arguello, California in October 1983. Observations of the instabilities were conducted using satellite sea surface temperature images and in situ sampling. The instabilities formed on the southern (cyclonic) boundary of the filament at a wavelength of about 20 km and consisted of two lobes, one warm and one cool, each with a width of about 4 km. The time scale for formation of the instabilities is about 1 day. Near-surface distributions of temperature, salinity, and density within the cool lobes of the instabilities are consistent with local upwelling at the rate of about 30 m d−1. A simple model based on conservation of potential vorticity is presented, which accounts for the observed upwelling. Based on isopycnal displacements and the distribution of salinity, the signature of the instabilities appears to be confined to the upper 50 m of the water column.

The Coastal Transition Zone Program

The Coastal Transition Zone (CTZ) Program, sponsored by the Office of Naval Research (Coastal Sciences and Oceanic Biology programs), is designed to investigate the cold tongues (“filaments”) often observed in satellite sea surface temperature images of the waters off the west coast of North America. The cold filaments are not unique to this region, since similar features have also been observed along other coasts around the world, including those near Portugal and southwestern Africa. The discovery of these features is an excellent example of the power of satellite observations, because although the filaments are quite prominent in the satellite images, years of regular shipboard observations did not reveal them. On the other hand, the study of cold tongues also illustrates the necessity of on‐site observations, because the nature, structure, causes, and effects of filaments cannot be determined from the satellite observations alone.