Sea Surface Temperature Imagery Research Articles

Estimating Coastal Lagoon Tidal Flooding and Repletion with Multidate ASTER Thermal Imagery

Coastal lagoons mix inflowing freshwater and tidal marine waters in complex spatial patterns. This project sought to detect and measure temperature and spatial variability of flood tides for a constricted coastal lagoon using multitemporal remote sensing. Advanced Spaceborne Thermal Emission Radiometer (ASTER) thermal infrared data provided estimates of surface temperature for delineation of repletion zones in portions of Chincoteague Bay, Virginia. ASTER high spatial resolution sea-surface temperature imagery in conjunction with in situ observations and tidal predictions helped determine the optimal seasonal data for analyses. The selected time series ASTER satellite data sets were analyzed at different tidal phases and seasons in 2004–2006. Skin surface temperatures of ocean and estuarine waters were differentiated by flood tidal penetration and ebb flows. Spatially variable tidal flood penetration was evaluated using discrete seed-pixel area analysis and time series Principal Components Analysis. Results from these techniques provide spatial extent and variability dynamics of tidal repletion, flushing, and mixing, important factors in eutrophication assessment, water quality and resource monitoring, and application of hydrodynamic modeling for coastal estuary science and management.

Direct interaction between the Gulf Stream and the shelfbreak south of New England

Sea surface temperature imagery, satellite altimetry, and a surface drifter track reveal an unusual tilt in the Gulf Stream path that brought the Gulf Stream to 39.9°N near the Middle Atlantic Bight shelfbreak—200 km north of its mean position—in October 2011, while a large meander brought Gulf Stream water within 12 km of the shelfbreak in December 2011. Near-bottom temperature measurements from lobster traps on the outer continental shelf south of New England show distinct warming events (temperature increases exceeding 6°C) in November and December 2011. Moored profiler measurements over the continental slope show high salinities and temperatures, suggesting that the warm water on the continental shelf originated in the Gulf Stream. The combination of unusual water properties over the shelf and slope in late fall and the subsequent mild winter may affect seasonal stratification and habitat selection for marine life over the continental shelf in 2012.

Thermal fronts of the southern South China Sea from satellite and in situ data

The major goal of this study was to find match-ups between thermal fronts mapped from satellite sea surface temperature (SST) imagery and from in situ data in the southern South China Sea (SSCS), using 11 ship surveys conducted by the South China Sea Institute of Oceanology (SCSIO) between 1987 and 1999. Fronts were automatically detected by the Cayula–Cornillon multi-image edge detection algorithm (CCA) applied to satellite-derived maps of the Advanced Very High Resolution Radiometer (AVHRR) SST obtained from the Pathfinder project (8364 twice-daily global fields with 9 km resolution between 1985 and 1996). Twice-daily near-instant frontal maps were composited without any averaging or smoothing to produce individual monthly frontal maps covering the period from January 1985 through December 1996 (144 maps in total). Although the SSCS is a tropical sea with little SST difference between water masses, the CCA turned out to be an effective tool for front mapping in the SSCS. Out of the 11 ship surveys analysed in this study, four surveys produced satisfactory match-ups. The percentage of match-ups is considered reasonably high given that (1) ship surveys were not optimized to cross fronts, therefore most in situ sections missed fronts; (2) satellite measurements of SST with AVHRR are hampered by cloudiness, therefore satellite-derived frontal maps might miss some fronts masked by persistent cloudiness. Fronts are more distinct in winter, when cross-frontal SST gradients are enhanced. From oceanographic vertical sections and horizontal maps, fronts are much sharper in the subsurface layer (represented here by 50 m level). Nonetheless, the CCA successfully detected SST fronts with a cross-frontal step as small as 1°C.

Current separation and upwelling over the southeast shelf of Vietnam in the South China Sea

The high‐resolution, unstructured grid Finite‐Volume Community Ocean Model (FVCOM) was used to examine the physical mechanisms that cause current separation and upwelling over the southeast shelf of Vietnam in the South China Sea (SCS). Process‐oriented experiments suggest that the southwesterly monsoon wind is a key physical mechanism for upwelling in that area but not a prerequisite to cause current separation. With no wind forcing, current separation in summer can occur as a result of the encounter of a southward along‐shelf coastal current from the north and northeastward buoyancy‐driven and stratified tidal‐rectified currents from the southwest. The southward current can be traced upstream to the Hong River in the Gulf of Tonkin. This current is dominated by semigeostrophic dynamics and is mostly confined to the narrow shelf along the northern Vietnamese coast. The northeastward currents are generated by tidal rectification and are intensified by the Mekong River discharge and southwesterly monsoon wind forcing. The dynamics controlling this current are fully nonlinear, with significant contributions from advection and vertical turbulent mixing. Upwelling in the current separation zone can be produced by a spatially uniform constant wind field and can be explained using simple wind‐induced Ekman transport theory. This finding differs from previous theory in which the regional dipole wind stress curl is claimed as a key mechanism for current separation and upwelling in this coastal region. Our SCS FVCOM, driven by the wind stress, river discharge, and tides, is capable of reproducing the location and tongue‐like offshore distribution of temperature as those seen in satellite‐derived sea surface temperature imagery.

New perspectives on Natal Pulses from satellite observations

More than 6 years of high‐frequency sea surface temperature (SST) imagery are used to follow perturbations at the inshore front of the Agulhas Current, between the northern Natal Bight and Port Elizabeth. Selected case studies as well as a front detection algorithm applied to the SST data provide some new insights into the dynamics of Natal Pulses and the variability of the northern Agulhas Current. Over the measurement period, the path of the Agulhas Current's inshore front does not exhibit any seasonality. Instead, large excursions in the Agulhas Current's inshore front are dominated by the intermittent passage of offshore cyclonic meanders from the Natal Bight and Port Edward regions to Port Elizabeth. SST imagery reveals that as they progress downstream, Natal Pulses interact with the topography to form secondary offshore meanders which propagate in the lee of the original perturbation. Over time, these secondary meanders dissipate, remerge with the original Natal Pulse, or detach from the Agulhas Current. Fluctuations in the Agulhas Current inshore path do not display the same frequency characteristics at all transects. From the northern Natal Bight to Port Edward both the size and frequency of Natal Pulses increases, while south of Port Edward the number of Natal Pulses decreases. At Port Elizabeth, the number of Natal Pulses observed is lower in comparison to previous estimates, with an average of 1.6 Natal Pulses per year.

Effects of Hurricane Ike on the Louisiana–Texas coast from satellite and model data

The river-influenced Mississippi–Louisiana–Texas (MsLaTex) coast in the northern Gulf of Mexico was in 2008 impacted by Hurricane Ike that made landfall along the Texas coast on 13 September 2008, causing widespread damage due to storm surge and coastal inundation. We examine the effects of the hurricane on the MsLaTex coast using remote sensing, model simulation and field observations. Moderate resolution imaging spectroradiometer sea surface temperature imagery indicated an approximately 2–4°C decrease in temperature to the right of the hurricane track. The high-resolution (∼2 km) three-dimensional Navy Coastal Ocean Model simulation and tide data suggested the generation of a coastally trapped ‘barotropic Kelvin’ wave during the passage of Ike, which may have contributed to an earlier than expected surge in sea level. A distinct elevated band of suspended particulate matter (SPM) concentration extending from Louisiana to Texas (∼90.5–94.5°W) coast was observed offshore in sea-viewing wide field-of-view sensor-derived imagery of 17 and 25 September that appeared to be associated with the coastally trapped Kelvin wave that was enhanced west of the Mississippi Canyon. Two intermittent wind events on 15 and 21 September likely kept the SPM in suspension for an extended period. Subsequently, on 27 and 30 September, SPM decreased along the elevated band and near the landfall site but increased near the bays and passes due to receding flood waters. Off the Atchafalaya and Mississippi deltas a large pulse of river discharge containing elevated SPM levels caused a plume around the two river deltas and its offshore dispersion into the open Gulf of Mexico.

An empirical approach to derive MODIS ocean color patterns under severe sun glint

[1] Oil tracking in the Gulf of Mexico in response to the Deepwater Horizon accident requires timely and accurate observations of major circulation patterns such as the Loop Current and LC eddies. When the eastern GOM becomes nearly isothermal at the surface and the use of sea surface temperature imagery is limited, MODIS ocean color data can be used instead. However, frequent and extensive sun glint prevents such an application when glint reflectance, Lg, is >0.01 sr−1. Here, an empirical approach is developed to remove sun glint and clouds based on band ratios between the Rayleigh-corrected reflectance (Rrc) at 469, 555, 645, 859, and 1240-nm. To minimize the effect of residual errors due to variable aerosols and imperfect glint correction, a color index (CI) is derived to represent the color patterns. Comparison between results from adjacent days with different glint and aerosol patterns suggests that the approach is able to derive consistent color patterns under severe sun glint (Lg < 0.15 sr−1). Tests of the approach over the Tropical Atlantic, East China Sea, and ocean waters off South Africa further validate the approach's general applicability. The color index (CI) also shows significant correlation with MODIS band-ratio Chl (<1 mg m−3) for each case examined. The simple design of the approach makes it straightforward to implement for other subtropical and tropical regions when a qualitative MODIS CI is desired to infer circulation patterns and to trace eddies under severe sun glint.

Evaporative dense water formation and cross‐shelf exchange over the northwest Australian inner shelf

High‐resolution surveys of oceanographic and atmospheric conditions made during the winter over the inner shelf off northwest Australia are used to examine the coastal ocean response to large outgoing heat and freshwater fluxes. Relatively cool, low‐humidity air blows off the Australian continent out over the tropical continental shelf, resulting in a large mean latent heat flux (−177 W m−2) that overwhelms insolation and, along with the outgoing long‐wave radiation, results in substantial net cooling (−105 W m−2) and evaporative freshwater flux (0.6 cm d−1). The inner shelf is characterized by increasingly cool, salty, and dense waters onshore, with a strong front near the 25 m isobath. The front is evident in satellite sea surface temperature (SST) imagery along the majority of the northwest Australian shelf, exhibiting a complex filamentary and eddy structure. Cross‐shelf buoyancy fluxes estimated from the mean, two‐dimensional heat and salt budgets are comparable to parameterizations of cross‐shelf eddy driven fluxes; however, the same fluxes can be achieved by cross‐shelf transports in the bottom boundary layer of about 0.5 m2 s−1 (and an overlying return flow).

Origin and dynamics of mesoscale eddies in the Catalan Sea (NW Mediterranean): Insight from a numerical model study

Past observations and satellite sea surface temperature imagery indicate the presence of mesoscale anticyclonic eddies drifting along the Catalan coast. In September 2001 one of these anticyclonic eddies was surveyed over the shelf break during an oceanographic cruise which permitted the 3‐D description of its structure. In this work we investigate the origin and dynamics of such “Catalan eddies” using a numerical circulation model of the northwest Mediterranean at 3 km resolution driven by high‐resolution atmospheric analyses and compare model eddies with the observations in the Catalan Sea. We identify two zones of eddy formation in the Gulf of Lions, in front of the city of Marseille and at the southeast of coast of Roussillon, from which anticyclonic eddies are observed to drift toward the Catalan Sea. The hydrology and dynamics of the structures observed in the simulations are characterized. Sensitivity experiments and energy analysis are performed which allow us to identify the mechanisms associated with their generation. Properties of the eddy observed during the 2001 cruise at the Catalan shelf break are found to compare well with model eddies generated at the southeast of the Roussillon coast. The model relates the origin of these eddies to the separation of the coastal current downstream from Creus Cape: flow separation is linked to intense downwelling taking place in front of the Roussillon coast when strong northwesterly winds events occur.

An algorithm for oceanic front detection in chlorophyll and SST satellite imagery

An algorithm is described for oceanic front detection in chlorophyll (Chl) and sea surface temperature (SST) satellite imagery. The algorithm is based on a gradient approach: the main novelty is a shape-preserving, scale-sensitive, contextual median filter applied selectively and iteratively until convergence. This filter has been developed specifically for Chl since these fields have spatial patterns such as chlorophyll enhancement at thermohaline fronts and small- and meso-scale chlorophyll blooms that are not present in SST fields. Linear Chl enhancements and localized (point-wise) blooms are modeled as ridges and peaks respectively, whereas conventional fronts in Chl and SST fields are modeled as steps or ramps. Examples are presented of the algorithm performance using modeled (synthetic) images as well as synoptic Chl and SST imagery. After testing, the algorithm was used on > 6000 synoptic images, 1999–2007, to produce climatologies of Chl and SST fronts off the U.S. Northeast.

Intrusion of warm Bering/Chukchi waters onto the shelf in the western Beaufort Sea

Wind‐driven changes in the path of warm Bering/Chukchi waters carried by the Alaska Coastal Current (ACC) through Barrow Canyon during late summer are described from high‐resolution hydrography, acoustic Doppler current profiler–measured currents, and satellite‐measured sea surface temperature imagery acquired from mid‐August to mid‐September 2005–2007 near Barrow, Alaska. Numerical simulations are used to provide a multidecadal context for these observational data. Four generalized wind regimes and associated circulation states are identified. When winds are from the east or east‐southeast, the ACC jet tends to be relatively strong and flows adjacent to the shelf break along the southern flank of Barrow Canyon. These easterly winds drive inner shelf currents northwestward along the Alaskan Beaufort coast where they oppose significant eastward intrusions of warm water from Barrow Canyon onto the shelf. Because these easterly winds promote sea level set down over the Beaufort shelf and upwelling along the Beaufort slope, the ACC jet necessarily becomes weaker, broader, and displaced seaward from the Beaufort shelf break upon exiting Barrow Canyon. Winds from the northeast promote separation of the ACC from the southern flank of Barrow Canyon and establish an up‐canyon current along the southern flank that is fed in part by waters from the western Beaufort shelf. When winds are weak or from the southwest, warm Bering/Chukchi waters from Barrow Canyon intrude onto the western Beaufort shelf.

The summer distribution of coastal minke whales ( Balaenoptera acutorostrata) in the southern outer Moray Firth, NE Scotland, in relation to co-occurring mesoscale oceanographic features

Data on the distribution of northern minke whales ( Balaenoptera acutorostrata) in the coastal waters of the southern outer Moray Firth in northeast Scotland were collected from May to October, 2000 to 2004 inclusive. During this period, 127 encounters with B. acutorostrata were recorded during systematic boat-based surveys. The encounters were subsequently converted to number of animals sighted per unit of survey effort (SPUE) for comparison with mesoscale oceanographic features. Using Advanced Very High Resolution Radiometer (AVHRR) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) imagery, monthly composite images of sea surface temperature (SST) and chlorophyll- a concentration (Chl-a) were obtained. In the SST imagery two oceanographic features appeared to dominate the Moray Firth region: a cold water current and a warm water plume. The results show that the SPUE of B. acutorostrata was significantly higher during warm plume events than when the cold current was dominant (Mann–Whitney U, P = 0.0167). Levels of phytoplankton biomass also appeared to be substantially greater during warm water plume events. In conclusion, it is hypothesised that those areas of highest minke SPUE are directly related to the presence of targeted prey attracted by high densities of phytoplankton. Although these whales were found to occur across the survey area, throughout the study period, their distribution is concluded to be highly dependent upon the presence of co-occurring mesoscale features. These, as well as other physiographic environmental determinants, are all thought to be important in forming this heterogeneous coastal ecosystem.

Physical dynamics and biological implications of a mesoscale eddy in the lee of Hawai’i: Cyclone Opal observations during E-Flux III

Abstract E-Flux III (March 10–28, 2005) was the third and last field experiment of the E-Flux project. The main goal of the project was to investigate the physical, biological and chemical characteristics of mesoscale eddies that form in the lee of Maui and the Island of Hawai’i, focusing on the physical–biogeochemical interactions. The primary focus of E-Flux III was the cyclonic cold-core eddy Opal, which first appeared in the NOAA GOES sea-surface temperature (SST) imagery during the second half of February 2005. During the experiment, Cyclone Opal moved over 160 km, generally southward. Thus, the sampling design had to be constantly adjusted in order to obtain quasi-synoptic observations of the eddy. Analyses of ship transect-depth profiles of CTD, optical and acoustic Doppler current profiler (ADCP) data revealed a well-developed feature characterized by a fairly symmetric circular shape with a radius of about 80 km. Depth profiles of temperature, salinity and density were characterized by an intense doming of isothermal, isohaline and isopycnal surfaces. Isopleths of nutrient concentrations were roughly parallel to isopycnals, indicating the upwelling of deep nutrient-rich water. The deep chlorophyll maximum layer (DCML) shoaled from a depth of about 130 m in the outer regions of the eddy to about 60 m in the center. Chlorophyll concentrations reached their maximum values in Opal's core region (about 40 km in diameter), where nutrients were upwelled into the euphotic layer. ADCP velocity data clearly showed the cyclonic circulation associated with Opal. Vertical sections of tangential velocities were characterized by values that increased linearly with radial distance from near zero close to the center to a maximum of about 60 cm s - 1 at roughly 25 km from the center, and then slowly decayed. The vertical extent of the cyclonic circulation was primarily limited to the upper mixed layer, as tangential velocities decayed quite rapidly within a depth range of 90–130 m. Potential vorticity analysis suggests that only a relatively small (about 50 km in diameter) and shallow (to a depth of approximately 70 m) portion of the eddy is isolated from the surrounding waters. Radial movements of water can occur between the center of the eddy and the outer regions along density surfaces within an isopycnal range of σ – t 23.6 ( σ t = 23.6 kg m - 3 ) and σ – t 24.4 ( σ t = 24.4 kg m - 3 ). Thus the biogeochemistry of the system might have been greatly influenced by these lateral exchanges of water at depth, especially during Opal's southward migration. While the eddy was translating, deep water in front of the eddy might have been upwelled into the core region, leading to an additional injection of nutrients into the euphotic zone. At the same time, part of the chlorophyll-rich waters in the core region might have remained behind the translating eddy and, thus contributed to the formation of an eddy wake characterized by relatively high chlorophyll concentrations.

Physical and bio-optical observations of oceanic cyclones west of the island of Hawai’i

Abstract Interdisciplinary observations of mesoscale eddies were made to the west of the island of Hawai’i. A central goal of the studies is to improve our understanding of the coupling of physical, biological, and biogeochemical processes that occur within these eddies. A specific objective was to test the hypothesis that the physical mechanisms of mesoscale eddies result in increases in nutrient availability to the euphotic layer, increases in primary production, changes in biological community compositions and size distributions, and increases in carbon flux to the deep sea. Data were obtained from ships, surface drifters, and satellite sensors during three separate field experiments. Variability was associated with two well-developed cyclonic, cold-core mesoscale eddies, Cyclone Noah and Cyclone Opal, which were observed during the E-Flux I (survey November 6–20, 2004) and E-Flux III (survey March 10–27, 2005) field campaigns, respectively. No mesoscale eddies were found during E-Flux II (survey January 10–19) when winds were erratic in magnitude and direction, supporting the hypothesis that persistent trade winds drive the production of cold-core mesoscale eddies in the lee of the Hawaiian Islands. Cold-core eddies were present in the E-Flux study area for about 2/3 of a year beginning May 1, 2004 and trade winds prevailed for about 3/4 of the same year. Both Cyclone Noah and Cyclone Opal were generated during strong, persistent northeasterly trade wind conditions and appeared downwind of the ’Alenuihaha Channel separating the islands of Maui and Hawai’i. The likely production mechanism for both mesoscale cold-core eddies involves localized wind-stress-curl-induced upwelling produced by trade wind forcing. Cyclone Noah was likely spun up by strong trade winds just to the southwest of the ’Alenuihaha Channel ( ∼ 20 . 10 ∘ N , 156 . 40 ∘ W ) between August 13 and 21, 2004 based on MODIS satellite sea-surface temperature (SST) imagery and QuikScat satellite wind data, and apparently began to dissipate by mid-December 2004. Cyclone Opal was likely spun up by strong trade winds between February 2 and 18, 2005 southwest of the ’Alenuihaha Channel ( ∼ 20 . 30 ∘ N , 156 . 30 ∘ W ), but was no longer evident in April 2005. Both Cyclone Noah and Cyclone Opal had strong physical, chemical, and biological expressions and displayed similar maximum tangential current speeds of ∼ 60 cm s - 1 . However, Cyclone Opal was more symmetric and larger in scale (roughly 180–200 km in diameter compared to ∼ 160 km in horizontal scale for Cyclone Noah). Both mesoscale eddies displayed significant doming in their centers and in some cases outcroppings of isothermal, isopycnal, nutrient, and chlorophyll a isopleths. After formation and a slow drift southward, Cyclone Noah remained in nearly the same location (roughly 19 . 60 ∘ N , 156 . 50 ∘ W ) during the 3-week in situ sampling period, whereas Cyclone Opal drifted southward by ∼ 165 km over a similar time span of sampling. Interestingly, the physical manifestations of both features were relatively unchanged during the ship-based surveys; however, the biology appears to have evolved within Cyclone Opal. The present report sets the context for several other E-Flux studies.

Feasibility of using sea surface temperature imagery to mitigate cheloniid sea turtle-fishery interactions off the coast of northeastern USA

As sea turtles migrate along the Atlantic coast of the USA, their incidental capture in fisheries is a significant source of mortality. Because distribution of marine cheloniid turtles appears to be related, in part, to sea surface temperature (SST), the ability to predict water temperature over the continental shelf could be useful in minimizing turtle-fishery interactions. We analyzed 10 yr of advanced very high resolution radiometer (AVHRR) SST imagery to estimate the proportion of 18 spatial zones, nearshore and offshore of Hatteras, North Carolina, USA (35° N), to north of Cape Sable, Nova Scotia (44° N), at temperatures >10 to 15°C, by week. Detailed examples for 11°C, the temperature employed by some management actions in the study area, and for 14°C, the lowest tem- perature at which turtles were sighted by some studies in the area, demonstrate a predictable pattern of rapid warming in March and April, followed by rapid cooling in October and November, with nearshore waters warming more rapidly than those offshore. Of those loggerhead turtles Caretta caretta that stranded, were sighted, or were incidentally captured between Cape Hatteras, North Carolina, and Cape Cod, Massachusetts, those at lower latitudes occurred when 25% or more of the area reached a water temperature of 11°C, while those in the northern zones did not occur until 50% or more of the area had reached a water temperature of 14°C. This analysis provides a means of pre- dicting marine cheloniid turtle presence, which can be helpful in regulating fisheries that seasonally interact with turtles.

Onset and development of the Iberian poleward flow along the Galician coast

Satellite, cruise and mooring observations in October and November 1999 have characterised the onset and development of the poleward flow along the Galician continental shelf and slope. Downwelling favourable winds during the cruise triggered the onset of the poleward flow. An offshore zonal temperature gradient, identifiable near 42 ∘ in Sea Surface Temperature imagery, extended shorewards as coastal upwelling was suppressed by downwelling winds. Over the continental slope, the front turned more parallel to shore, as surface warm water was advected northward in the poleward flow. Downwelling winds increased mixing nearshore causing the pycnocline to intersect the upper slope to form a bottom front enhancing poleward geostrophic flow over the shelf. On the slope, a sub-surface salinity maximum was associated with the poleward flow, becoming better defined as the poleward flow persisted. The flow was surface intensified and continuous to 650 m with maximum speeds of 0.2 m s - 1 . The associated mean poleward transport was estimated at 2 ± 0.5 Sv . Direct transfer of momentum by the wind, elevation of sea level at the coast resulting from onshore Ekman transport, and the JEBAR mechanism all contribute towards the development of the poleward flow over the Galician shelf.

Ocean color observations of a surface water transport event: Implications for Pseudo-nitzschia on the Washington coast

A persistent patch of high biomass water, associated with the Juan de Fuca Eddy, is often observed in surface chlorophyll a images off the southwest coast of Vancouver Island, Canada. Outbreaks of toxic Pseudo-nitzschia spp. along the Washington, USA, coast are believed to correlate with the transport of waters from Juan de Fuca Eddy southward to Washington beaches. A time series of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite ocean color images from late May 1999 of coastal waters off Washington and Vancouver Island, processed for surface chlorophyll a concentration and spectral remote sensing reflectance, captured a transport event where water from the Juan de Fuca Eddy was transported onto the Washington shelf. Strong upwelling-favorable winds appeared to deform the patch over an 8-day period and move it southward into Washington coastal waters with surface velocities of approximately 8–16 km d −1. SeaWiFS and sea surface temperature imagery showed the local phytoplankton response to wind-driven coastal upwelling restricted to a narrow (10–15 km) region along the Washington coast. Although we did not observe transport of high biomass water originating in the Juan de Fuca Eddy to Washington beaches in May 1999, transport of Pseudo-nitzschia cells could occur following a rapid shift to downwelling-favorable conditions. Tracking the trajectory of surface waters from the Juan de Fuca Eddy by remote sensing could be used to trigger conditional sampling for domoic acid along the Washington coast.

Marine GIS: Identification of mesoscale oceanic thermal fronts

A new method (the ‘sink’ method) is proposed for the mapping of productive mesoscale oceanic thermal fronts based on the combined analysis of satellite imagery for sea surface temperature (SST) distribution and chlorophyll (CHL) concentration under a Geographic Information System (GIS). In an SST lattice data array, data value sinks describe heterogeneous drops in SST distribution as this is compared to more homogeneously distributed SST in the surrounding area. Using Arc/Info GIS, these thermal discontinuities are flagged and applied on SST and CHL imagery for the calculation of differences in SST and CHL patterns (DSST and DCHL) inside and outside of these flagged areas. Spatially connected sinks that are characterized by simultaneous negative DSST and positive DCHL pattern are mapped as thermal fronts. Results include a time series of monthly front occurrence GIS maps in Eastern Mediterranean waters along with three calculated front characteristics (DSST, DCHL and bathymetry). Comparisons between the mapped fronts and isobath distributions present clearly visible front‐to‐isobath spatial and shape associations while spatial analysis between front and wind data explains 66% of the number of mapped fronts.

Property fields in a Tortugas Eddy in the southern straits of Florida

Sea surface temperature imagery, ship-based surveys, and moored current meters described the passage of a Tortugas Eddy as it moved east at ca. 6 km day −1 through the southern Straits of Florida (SSF). In mid-April 1999 the eddy SST signature extended across half the width of the Straits. While in the western SSF, the eddy center was ca. 30 km seaward of the outer reef. The upper pycnocline, the subsurface chlorophyll a maximum (SCM), and nutricline shoaled from ca. 80 m at the eddy edge to <60 m at the center. Maximum chlorophyll a concentrations in the SCM were highest near the eddy center, at 1 mg m −3, although the depth-integrated concentrations (mg m −2) were similar across the feature. Nutrient-density relationships show nitrate+nitrite, phosphate, and silicate decreased to detection limits at σ t < 25.0 ; the SCM was centered near this isopycnal surface. As the Eddy passed Looe Key (81.5°W) the alongshore currents reversed to the west. During this period high-nutrient, cool waters shoaled near the bottom on the outer reef. By early May, the eddy SST signature was compressed into a thin band of cool surface waters off the Middle to Upper Keys. As the feature moved towards shore in the Middle to Upper Florida Keys, the nitrate+nitrite and chlorophyll concentrations increased in bottom waters along the outer reef. Processes such as internal tidal bores and breaking internal waves are likely responsible for delivering nutrients from Tortugas Eddies to the outer reef in the Middle to Upper Keys.

Structure and variability of the Western Maine Coastal Current

Abstract Analyses of CTD and moored current meter data from 1998 and 2000 reveal a number of mechanisms influencing the flow along the western coast of Maine. On occasions, the Eastern Maine Coastal Current extends into the western Gulf of Maine where it takes the form of a deep (order 100 m deep) and broad (order 20 km wide) southwestward flow with geostrophic velocities exceeding 20 cm s−1. This is not a coastally trapped flow, however. In fields of geostrophic velocity, computed from shipboard-CTD data, the core of this current is roughly centered at the 100 m isobath and its onshore edge is no closer than 10 km from the coast. Geostrophic velocity fields also reveal a relatively shallow (order 10 m deep) baroclinic flow adjacent to the coast. This flow is also directed to the southwest and appears to be principally comprised of local river discharge. Analyses of moored current meter data reveal wind-driven modulations of the coastal flow that are consistent with expectations from simple theoretical models. However, a large fraction of the near-shore current variance does not appear to be directly related to wind forcing. Sea-surface temperature imagery, combined with analysis of the moored current meter data, suggests that eddies and meanders within the coastal flow may at times dominate the near-shore current variance.