Shelf Water Masses Research Articles

Recent oceanographic variability and warming of the continental shelf south of New England and implications for acoustic propagation and sub-bottom characterization

The continental shelf south of New England is a complex oceanographic environment for underwater acoustics. The combination of large seasonal changes in stratification, complicated frontal phenomena associated with the shelfbreak front, and small spatial scales of variability for soundspeed results in important effects on acoustic propagation and sub-bottom characterization. Two recent trends have also become important. Longer-term warming, on the decadal scale, is affecting shelf water temperatures and soundspeed profiles. In addition, from April 2014 to December 2015, there was a substantial influence on shelf water mass properties from Gulf Stream warm core rings impinging on the continental shelf. The rings also diverted the shelfbreak front onshore at times as well. Some implications for propagation and sub-bottom characterization will be discussed.

Living benthic foraminiferal species as indicators of cold-warm water masses interaction and upwelling areas

The western South Atlantic continental margin, between 27° and 37°S, is dominated by four main water masses: cold-fresh Subantarctic Shelf Water (SASW), warm-salty Subtropical Shelf Water (STSW), cold upwelled South Atlantic Central Water (SACW), and fresh Plata Plume Water (PPW). Despite the large seasonal variability of PPW extension along the shelf, an intense and relatively stable temperature-salinity gradient separates the SASW and the STSW forming the Subtropical Shelf Front (STSF) around 32°S. The two dominant shelf water masses (SASW and STSW) arise from the process of mixing of oceanic waters. The SASW originates from the dilution of Subantarctic Water due to excess precipitation and continental runoff, and the STSW consists of modified warm tropical waters and South Atlantic Central Water (SACW) diluted below PPW. A previous article demonstrates distribution of Bulimina marginata, a shelf environment and deep-sea species of benthic foraminifera, is influenced by the front location and it can be used as a proxy of the STSF in sediment core analysis. Here we show three other infaunal living species inhabiting at the Continental margin: Buccella peruviana, Globocassidulina subglobosa and Uvigerina peregrina and their distribution limits show the interaction of Subantartic Shelf Water, Subtropical Shelf Water, and upwelling of SACW, in the bottom sediment of coastal studied areas.

The seasonal evolution of shelf water masses around Bouvetøya, a sub-Antarctic island in the mid-Atlantic sector of the Southern Ocean, determined from an instrumented southern elephant seal

Our study makes use of a fortuitous oceanographic data set collected around the remote sub-Antarctic island of Bouvetoya by a conductivity–temperature–depth recorder (CTD) integrated with a satellite-relayed data logger deployed on an adult female southern elephant seal ( Mirounga leonina ) to describe the seasonal evolution of the western shelf waters. The instrumented seal remained in waters over the shelf for 259 days, collecting an average of 2.6 (±0.06) CTD profiles per day, providing hydrographic data encompassing the late austral summer and the entire winter. These data document the thermal stratification of the upper water layer due to summer surface heating of the previous year’s Antarctic Surface Water, giving way to a cold subsurface layer at about 100 m as the austral winter progressed, with a concomitant increase in salinity of the upper layer. Upper Circumpolar Deep Water was detected at a depth of approximately 200 m along the western shelf of Bouvetoya throughout the year. These oceanographic data represent the only seasonal time series for this region and the second such animal–instrument oceanographic time series in the sub-Antarctic domain of the Southern Ocean. Keywords: Biotelemetry; diving; elephant seal; oceanography; marine predators; CTD-SRDL. (Published: 27 October 2016) To access the supplementary material for this article, please see supplementary files in the column to the right (under Article Tools). Citation: Polar Research 2016, 35 , 28278, http://dx.doi.org/10.3402/polar.v35.28278

Influence of oceanographic features on the spatial and seasonal patterns of mesozooplankton in the southern Patagonian shelf (Argentina, SW Atlantic)

Surveys conducted during spring, summer and late winter in 2005–2006 over the southern Patagonian shelf have allowed the seasonal distribution of mesozooplankton communities in relation to water masses and circulation to be investigated. In this system, most of the shelf is dominated by a distinct low salinity plume that is related to the runoff from the Magellan Strait (MSW), while the outer shelf is highly influenced by the cold and salty Subantarctic water (SAW) of the boundary Malvinas Current. Separating these two, the Subantarctic Shelf water mass (SASW) extends over the middle shelf. Correspondingly, the structure of the MSW and SAW mesozooplankton communities was found to be clearly different, while the former and the SASW assemblages were barely separable. This relatively fresh water mass is actually a variant of Subantarctic water that enters into the region from the south and the shelf-break, and hence its mesozooplankton community was not significantly different from that of the SAW water mass. Dissimilar species abundance, in turn associated with different life histories and population development, was more important than species composition in defining the assemblages. Total mesozooplankton abundance increased about 2.5-fold from the beginning of spring to late summer, and then decreased at least two orders of magnitude in winter. Across all seasons copepods represented >70–80% of total mesozooplankton over most of the shelf. Copepod species best represented through all seasons, in terms of both relative abundance and occurrence, were Drepanopus forcipatus and Oithona helgolandica. Although seasonal differences in abundance were striking, the spatial distribution of mesozooplankton was largely similar across seasons, with relatively higher concentrations occurring mainly in Grande Bay and surroundings. The well defined spatial patterns of mesozooplankton that appear from our results in conjunction with the southward wide extension of the shelf and the predicted current path and speed suggest that plankton production is locally enhanced in the Grande Bay area and has the potential to be exported downstream.

Hudson submarine canyon head offshore New York and New Jersey: A physical and geochemical investigation

Hudson Canyon is the largest shelf-sourced canyon system off the east coast of the United States, and hosts a productive ecosystem that supports key fisheries. Here we report the results of a multi-year interdisciplinary study of the geological, geochemical, and physical oceanographic features and processes in the canyon that underpin that ecosystem. High-resolution multi-beam bathymetric and backscatter data show that the contrasting morphology of the two perpendicularly oriented branches at the head of the Hudson Canyon is indicative of different states of geomorphological activity and sediment transport. Tightly spaced ridges and gullies extend perpendicularly towards the canyon axis from the canyon walls. Numerous depressions are found at the base of the canyon walls or along the canyon axis at depths from 300m to 600m. Elevated concentrations of dissolved methane in the water column, where the highest density of depressions occur, suggests that methane is actively venting there. The topography and reflective floors of circular depressions in canyon walls and their association with methane maxima suggest that these represent active methane gas release-collapse pockmarks with carbonate floors. Patterns of irregular, low-relief, reflective depressions on the canyon floor may also represent methane release points, either as gas release or cold-seep features. The presence of methane maxima in a region of strong advective currents suggests continuous and substantial methane supply. Hydrographic observations in the canyon show that multiple layers of distinct inter-leaved shelf (cold, fresh) and slope (warm, salty) water masses occupy the head of the canyon during the summer. Their interactions with the canyon and with each other produce shifting fronts, internal waves, and strong currents that are influenced by canyon topography. Strong tidal currents with along-canyon-axis flow shear help to drive the advection, dispersion and mixing of dissolved materials in the water column that likely help support the rich canyon ecosystem.

Winter sea ice melting in the Atlantic Water subduction area, Svalbard Norway

AbstractHerein, we study a small area along the shelf west of Spitsbergen, near Prins Karls Forland, where warm, saline Atlantic Water of the West Spitsbergen Current currently first encounters sea ice. This sea ice is drifting in a coastal current that carries Arctic Water originating from the Barents Sea northward over the shelf. Our aim was to investigate whether melting of sea ice by Atlantic Water in this area might be a significant factor that could contribute to the formation of a cold halocline layer that isolates the sea ice from further melting from below. Observations of temperature and salinity profiles were collected during two winters, via CTD‐SRDL instruments deployed on harbor seals (Phoca vitulina), and fed into a heat and freshwater budget box model in order to quantify the importance of melting relative to other processes that could transform the shelf water mass during winter. Cross‐frontal exchange of Atlantic Water from the West Spitsbergen Current, driven by buoyancy forcing rather than Ekman upwelling, was determined to be the source of the heat that melted drift ice on the shelf. Some local sea ice formation did take place, but its importance in the total heat and freshwater budgets appeared to be minor. The data suggest that the production of a cold halocline layer was preceded by southerly winds and rapid drift ice melting.

The Faroe shelf circulation and its potential impact on the primary production

The ecosystem on the Faroe shelf has been shown to be tightly controlled by the primary production. It has been suggested that the primary production is governed by the physical processes controlling this water mass. The objective of this study is to identify the physical control mechanisms that control this water mass, link these to the interannual variability of the chlorophyll content on the Faroe shelf and through this discuss the influence on the primary production. In order to achieve this, a 10year hindcast (2000–2009) with a regional ocean circulation model has been set up for the focus area. Results are compared with measurements on the Faroe shelf.The model reproduces the clockwise residual circulation around the Faroe Islands. The vertical velocity profile is validated using observations at a location west of the Islands. Observations show a logarithmic profile in the entire water column indicating a fully developed boundary layer. The modeled profile matches the observations in the bottom part of the water column, however the thickness of the bottom boundary layer is underestimated, which results in a constant profile in the upper part of the water column. As a consequence, the modeled velocity in the upper part of the water column is up to 20% lower than the observed velocity. The direction of the modeled velocity profile compares well with observations. The model realistically forms the partly isolated unique shelf water mass. Years with anomalously early and persistent modeled spring stratification correspond with years with a high on-shelf chlorophyll concentration.An integration of the exchange across the 120m isobath shows intense water mass exchange across this depth contour. The major part of this includes tidal shifting of the front between on-shelf and off-shelf waters and is associated with little effective water mass exchange. The result is a shelf water mass that is relatively isolated. The modeled net exchange is constituted by an on-shelf flow near the surface and an off-shelf flow near the bottom associated with the frictional boundary layer. This is confirmed by the tracer experiment. Both the tracer experiment and the exchange across the 120m isobath indicate that there is spatial variability in the exchange of the shelf water.The renewal rate of the shelf water mass during the spring bloom period is assessed by a set of passive tracer simulations. Using passive tracers, the time scale for the half-life is found to be between 30 and 40days. This compares favorably with the time-scale based on the net, advected water mass exchange that yields 60–70days, considering that this estimate neglects turbulent and diffusive exchange.

Shelf water and chlorophyll export from the Hatteras slope and outer shelf

AbstractUsing high‐resolution data acquired from a shipboard ADCP and a towed Scanfish equipped with a CTD and fluorometer, we examine the properties and transport of Middle Atlantic Bight (MAB) shelf water over a region of the Hatteras outer shelf and slope where MAB shelf water is commonly deflected offshore and entrained into the Gulf Stream. The data are from a period in early August 2004 when the seasonal pycnocline of the MAB is well developed and situated over a weakly stratified, near‐bottom shelf water mass commonly referred to as the cold pool. Our data show chlorophyll‐rich cold pool water carried rapidly southward over the slope and outer shelf, at a rate of up to 60 cm s−1, as part of the shelf‐edge frontal jet. This southward transport of chlorophyll‐rich cold pool water is shunted eastward and entrained into the Gulf Stream. However, the latitude band over which this export occurs varies significantly over the 7 day course of our study, a variation which is linked to an order 50 km shift in the latitude at which the Gulf Stream separates from the continental margin. The coupled rapid translation of the Gulf Stream frontal separation and the cold pool export zone is likely to have a significant impact on the movement and accumulation of biogenic material over the Hatteras slope and rise.

Continental shelf water masses off the Jaguaribe River (4S), northeastern Brazil

This work analyzed temperature and salinity data obtained from two oceanographic cruises carried out over the western portion of the continental shelf adjacent to the Jaguaribe River in NE Brazil (May and October 2009) and included the rainy and dry seasons, which correspond to different levels of continental contribution to shelf waters. The results of the analysis identified three different water masses: River Water (RW), Coastal Water (CW) and Tropical Water (TW). During the period of maximum freshwater discharge, an estuarine plume that extended for 6km along the coastline was observed, and its influence could be felt over the inner shelf. The plume was associated with the presence of CW separated by a strong halocline. However, in the middle and outer shelf regions, the CW and the TW were observed below the 50m isobath, while the RW was not observed, most likely due to the presence of northeast trade winds that press the plume along the coast, reducing its seaward expansion. During the dry season, a higher influence of the TW was observed, most likely due to the space-time variability of the North Brazil Current (NBC) and the role of anticyclonic eddies that meander from the adjacent ocean towards the continental shelf. The results obtained suggest a strong seasonal influence upon the water mixing process along the continental shelf with the CW being affected by the RW and TW depending upon the season.

Reprint of: Benthic and planktic community changes at the North Siberian margin in response to Atlantic water mass variability since last deglacial times

Abstract The eastward penetration of Atlantic-derived water (ADW) into the Eurasian Basin of the Arctic Ocean was investigated at the western Laptev Sea continental margin for the time since c. 17.6 ka. Using a high-resolution investigation of the lithology, geochemistry, planktic and benthic foraminifers, and ostracods on a sediment core from 270 m water depth major steps in the environmental evolution of the region are recognized. In general, ADW was continuously present in the study area. Between 17.6 and 15.4 ka ADW manifested itself through open-water polynyas and associated upwelling events. Comparison between the Laptev Sea and northern Svalbard shelf using Cassidulina neoteretis allows assuming an unmodified subsurface inflow of ADW within its northern branch between 15.4 and 13.2, which was strongest after 14.7-ka and in line with the overall climate amelioration. A local freshwater event at 13 ka followed by shelf flooding and the establishment of a freshened shelf water mass resulted in an off-shelf displacement of ADW from the studied site as suggested by the disappearance of C. neoteretis between 12 and 7 ka. As evidenced by an abundance peak in Nonion labradoricum , the sea-ice marginal zone was located at the site around 12–11 ka but then shifted northward during the early Holocene warming. Enhanced ADW inflow since 7 ka correlated with climate cooling and southward retreat of the seasonal drift-ice margin. The inflow of ADW during mid–late Holocene differed from deglacial times because of the combined influence of northern and eastern ADW branches.

Horizontal Scales of Variability over the Middle Atlantic Bight Shelf Break and Continental Rise from Finescale Observations

Abstract Observations with fine horizontal resolution are used to identify the horizontal scales of variability over the Middle Atlantic Bight (MAB) shelf break and continental rise. Spray gliders collected observations along two alongshelf transects over the continental rise in March–April 2006 and along 16 cross-shelf transects over the shelf break and continental rise during July–October 2007. Horizontal resolution varied from 1 km or finer over the shelf to 6 km in deep water. These observations allow horizontal thermohaline variability offshore of the MAB shelf break to be examined for the first time. Structure functions of temperature and salinity, the mean square difference between observations separated by specified distances, reveal the horizontal spatial scales in the region. Exponential (e-folding) scales of temperature and salinity increase from 8–13 km near the shelf break to about 30 km over the continental rise. Just offshore of the shelf break, alongshelf structure functions exhibit periodicity with a 40–50-km wavelength that matches the wavelength of shelfbreak frontal meanders. Farther offshore, alongshelf structure functions suggest a dominant wavelength of 175–250 km, but these scales are only marginally resolved by the available observations. Examination of structure functions of along-isopycnal salinity (i.e., spice) suggests that interleaving of shelf and slope water masses contributes most of the horizontal variability near the MAB shelf break, but heaving of isopycnals is the primary source of horizontal variability over the continental rise.

Benthic and planktic community changes at the North Siberian margin in response to Atlantic water mass variability since last deglacial times

The eastward penetration of Atlantic-derived water (ADW) into the Eurasian Basin of the Arctic Ocean was investigated at the western Laptev Sea continental margin for the time since c. 17.6 ka. Using a high-resolution investigation of the lithology, geochemistry, planktic and benthic foraminifers, and ostracods on a sediment core from 270 m water depth major steps in the environmental evolution of the region are recognized. In general, ADW was continuously present in the study area. Between 17.6 and 15.4 ka ADW manifested itself through open-water polynyas and associated upwelling events. Comparison between the Laptev Sea and northern Svalbard shelf using Cassidulina neoteretis allows assuming an unmodified subsurface inflow of ADW within its northern branch between 15.4 and 13.2, which was strongest after 14.7-ka and in line with the overall climate amelioration. A local freshwater event at 13 ka followed by shelf flooding and the establishment of a freshened shelf water mass resulted in an off-shelf displacement of ADW from the studied site as suggested by the disappearance of C. neoteretis between 12 and 7 ka. As evidenced by an abundance peak in Nonion labradoricum, the sea-ice marginal zone was located at the site around 12–11 ka but then shifted northward during the early Holocene warming. Enhanced ADW inflow since 7 ka correlated with climate cooling and southward retreat of the seasonal drift-ice margin. The inflow of ADW during mid–late Holocene differed from deglacial times because of the combined influence of northern and eastern ADW branches.

On carbon transport and fate in the East Siberian Arctic land–shelf–atmosphere system

This review paper summarizes current understanding of the transport of organic carbon to, and the fate of organic carbon within, the East Siberian Arctic Shelf (ESAS), and of processes determining carbon dioxide (CO2) and methane (CH4) fluxes from the ESAS to the atmosphere achieved from analyzing the data sets obtained on 20 expeditions performed from 1999 to 2011. This study of the ESAS was aimed at investigating how redistribution of old carbon from degrading terrestrial and sub-sea permafrost and from coastal erosion contributes to the carbon pool of the ESAS, how changes in the hydrological cycle of the surrounding land and alteration of terrestrial carbon cycles affect the hydrological and biogeochemical parameters of shelf water masses, and which factors control CH4 and CO2 emissions from the ESAS. This report describes selected results achieved by a developing international scientific partnership that has been crucial at every stage of the study and will be even more important in the future.

Seasonal and inter-annual temperature variability in the bottom waters over the western Black Sea shelf

Abstract. Long-term changes in the state of the Bottom Shelf Water (BSW) on the Western shelf of the Black Sea are assessed using analysis of intra-seasonal and inter-annual temperature variations. For the purpose of this study the BSW is defined as such shelf water mass between the seabed and the upper mixed layer (bounded by the σθ = 14.2 isopycnal) which has limited ability to mix vertically with oxygen-rich surface waters during the warm season due to formation of a seasonal pycnocline. A long-term time series of temperature anomalies in the BSW is constructed from in-situ observations taken over the 2nd half of the 20th century. The BSW is shown to occupy nearly half of the shelf area during the summer stratification period (May–November).The results reveal a warm phase in the 1960s/70s, followed by a cold phase between 1985 and 1995 and a further warming after 1995. The transition between the warm and cold periods coincides with a regime shift in the Black Sea ecosystem. While it was confirmed that the memory of winter convection is well preserved over the following months in the deep sea, the signal of winter cooling in the BSW significantly reduces during the warm season. The potential of the BSW to ventilate horizontally during the warm season with the deep-sea waters is assessed using isopycnic analysis of temperature variations. It is shown that temperature in the BSW is stronger correlated with the temperature of Cold Intermediate Waters (CIW) in the deep sea than with the severity of the previous winters, thus indicating that the isopycnal exchanges with the deep sea are more important for inter-annual/inter-decadal variability of the BSW on the western Black Sea shelf than effects of winter convection on the shelf itself.

An asymmetric upwind flow, Yellow Sea Warm Current: 1. New observations in the western Yellow Sea

[1] The winter water mass along the Yellow Sea Trough (YST), especially on the western side of the trough, is considerably warmer and saltier than the ambient shelf water mass. This observed tongue-shape hydrographic feature implies the existence of a winter along-trough and onshore current, often referred to as the Yellow Sea Warm Current (YSWC). However, the YSWC has not been confirmed by direct current measurements and therefore skepticism remains regarding its existence. Some studies suggest that the presence of the warm water could be due to frontal instability, eddies, or synoptic scale wind bursts. It is noted that in situ observations used in most previous studies were from the central and eastern sides of the YST even though it is known that the warm water core is more pronounced along the western side. Data from the western side have been scarce. Here we present a set of newly available Chinese observations, including some from a coordinated effort involving three Chinese vessels in the western YST during the 2006–2007 winter. The data show unambiguously the existence of the warm current on the western side of YST. Both the current and hydrography observations indicate a dominant barotropic structure of YSWC. The westward deviation of YSWC axis is particularly obvious to the south of 35°N and is clearly associated with an onshore movement of warm water. To the north of 35°N, the YSWC flows along the bathymetry with slightly downslope movement. We conclude that the barotropic current is mainly responsible for the warm water intrusion, while the Ekman and baroclinic currents play an important but secondary role. These observations help fill an observational gap and establish a more complete view of the YSWC.

Phytoplankton phenology on the Scotian Shelf

AbstractZhai, L., Platt, T., Tang, C., Sathyendranath, S., and Hernández Walls, R. 2011. Phytoplankton phenology on the Scotian Shelf. – ICES Journal of Marine Science, 68: . The impact of physical forcing on seasonal and interannual phytoplankton dynamics was examined using SeaWiFS chlorophyll, AVHRR sea surface temperature (SST), nitrate, and other hydrographic measurements for the Scotian Shelf and Scotian Slope. The spring bloom was characterized by a shifted Gaussian function fitted to seasonal chlorophyll time-series. The background chlorophyll (a constant term in the Gaussian function) is a joint property of the stratification and bio-optics of the mixed layer. Rapid shoaling of the mixed-layer depth in spring promoted the early spring bloom on the middle Scotian Shelf and Slope, triggered when averaged light in the mixed layer reached 15 W m−2. The duration of the spring bloom was prolonged in slope water, resulting in a discontinuity in duration between the shelf and slope water masses. The position of the latitudinal discontinuity in duration was correlated with that of the shelf–slope front in SST. The amplitude of the spring bloom was correlated with the nitrate inventory in the surface layer at the end of winter. The rate of decrease (increase) in chlorophyll after the spring bloom was related to the depletion (resupply) of nitrate in summer. The position of the shelf–slope front influenced the interannual variability of bloom characteristics.

History of ice-rafting and water mass evolution at the northern Siberian continental margin (Laptev Sea) during Late Glacial and Holocene times

The Late Glacial and Holocene history of environmental changes in the western Laptev Sea is reconstructed on the basis of continuous records of ice-rafted debris (IRD) and Atlantic water indicative subpolar planktic foraminifers and benthic foraminifer Cassidulina neoteretis from the two AMS 14C dated sediment cores, one from the upper continental slope and another from the Khatanga paleoriver valley on the outer shelf. Since seasonal sea ice cover is a permanent feature in this high Arctic region the prominent IRD-rich intervals in the oldest (>15.4 ka) and youngest (<7.2 ka) sediment units are attributed to iceberg-rafting. Rock types like phyllites are used as indicators of sediment material supplied by icebergs from local ice caps of the Severnaya Zemlya Archipelago. Authigenic concretions of vivianite and rhodochrosite found in the basal, IRD-rich core section on the slope, point to strong water stratification likely caused by meltwater inputs. The oldest Late Glacial IRD peaks provide evidence for iceberg production by the local ice caps on Severnaya Zemlya. Subsequent meltwater influence is tentatively related to the freshwater release from a retreating Barents–Kara ice sheet in the northeastern Kara Sea. Already prior to 15.4 ka Atlantic-derived waters intermittently reached the western Laptev Sea continental slope probably facilitated by upwelling in coastal polynyas. While subsurface Atlantic-derived waters were constantly present at the continental slope between 15.4 and 12 ka, the disappearance of C. neoteretis together with reduced abundance of subpolar planktic foraminifers after 12 ka suggests establishment of a freshened shelf water mass on the flooded outer shelf which affected the water mass structure on the continental slope. After 7 ka climate cooling and enhanced Atlantic-derived water inflow caused re-growth of ice caps on Severnaya Zemlya leading to a recurrence of IRD. Peaks of this renewed IRD input are centered at 7.2, 6.4, 5.4, 3, and 2 ka, implying that this millennial-scale variability in the IRD input might be related to an overregional changes in atmospheric circulation patterns.

Ocean‐atmosphere heat fluxes at the Ronne Polynya, Antarctica

The Ronne Polynya is a coastal polynya, a region of thin ice or open water in sea ice, caused by the offshore transport of the ice by strong winds from the land (Figure 1). As soon as the ice is transported offshore, new ice forms on the exposed ocean surface and is also advected offshore in a continual process, earning this type of polynya the nickname ‘ice factory’. These polynyas have an important impact on the regional meteorology and oceanography of the high latitudes as well as on the global ocean circulation. The exposed ocean surface is relatively warm compared to the cold polar atmosphere, and the large temperature and humidity differences result in large sensible and latent heat fluxes from the ocean to the atmosphere. This leads to a warming and moistening of the atmospheric boundary layer above and downwind of the polynya and, through vigorous convective mixing, the formation of a CIBL (convective internal boundary layer) (Figure 1). A decrease in the ocean-atmosphere temperature and humidity gradients is caused by this warming and moistening, which results in a decrease in the surface heat fluxes with fetch from the shore, or ice shelf front (Renfrew and King, 2000). The depth of the CIBL increases with fetch due to the warming and also the entrainment of warm air from above the CIBL (Garratt, 1992). As well as this turbulent heat transfer, polynyas can also influence the balance of radiative heat transfer through the generation of ice fog (Smith et al., 1990) and convective clouds or plumes (Pinto and Curry, 1995). Polynyas, therefore, have the potential to modify and induce mesoscale atmospheric motion, impacting on regional climate (Pinto et al., 1995). High rates of ice production due to the large ocean-atmosphere heat fluxes and the continual removal of the newly formed ice by the wind result in extensive brine rejection, whereby sea water rejects salt on freezing, leaving the sea ice relatively fresh and the modified water column relatively salty and therefore dense. This dense water sinks, as shown in Figure 1, accumulating on the continental shelf and forming a water mass, which eventually contributes to the temperature- and salinity-driven global ocean circulation, known as the thermohaline circulation (THC). Therefore the ice-formation mechanism within polynyas is important for the ventilation of deep and bottom water in both the Southern and Arctic Oceans (Morales Maqueda et al., 2004). It follows that, in order to accurately model the response of both the high latitudes and global THC to a changing climate, processes occurring within polynyas must be investigated. I was lucky enough to be given the opportunity to participate in a British Antarctic Survey (BAS) fieldwork campaign at the end of the Antarctic summer field season in February 2007. Using an instrumented Twin Otter aircraft belonging to the BAS, three flights were conducted over the Ronne Polynya between 25 and 28 February 2007 to investigate ocean-atmosphere heat fluxes. Quantification of these heat fluxes is a step towards quantifying the surface heat budget, which, together with the surface salinity budget, will aid understanding of the key processes governing deep-water formation within polynyas. Aircraft observations of the

Satellite Assessment of Bio-Optical Properties of Northern Gulf of Mexico Coastal Waters Following Hurricanes Katrina and Rita.

The impacts of major tropical storms events on coastal waters include sediment resuspension, intense water column mixing, and increased delivery of terrestrial materials into coastal waters. We examined satellite imagery acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) ocean color sensor aboard the Aqua spacecraft following two major hurricane events: Hurricane Katrina, which made landfall on 29 August 2005, and Hurricane Rita, which made landfall on 24 September. MODIS Aqua true color imagery revealed high turbidity levels in shelf waters immediately following the storms indicative of intense resuspension. However, imagery following the landfall of Katrina showed relatively rapid return of shelf water mass properties to pre-storm conditions. Indeed, MODIS Aqua-derived estimates of diffuse attenuation at 490 nm (K_490) and chlorophyll (chlor_a) from mid-August prior to the landfall of Hurricane Katrina were comparable to those observed in mid-September following the storm. Regions of elevated K_490 and chlor_a were evident in offshore waters and appeared to be associated with cyclonic circulation (cold-core eddies) identified on the basis of sea surface height anomaly (SSHA). Imagery acquired shortly after Hurricane Rita made landfall showed increased water column turbidity extending over a large area of the shelf off Louisiana and Texas, consistent with intense resuspension and sediment disturbance. An interannual comparison of satellite-derived estimates of K_490 for late September and early October revealed relatively lower levels in 2005, compared to the mean for the prior three years, in the vicinity of the Mississippi River birdfoot delta. In contrast, levels above the previous three year mean were observed off Texas and Louisiana 7-10 d after the passage of Rita. The lower values of K_490 near the delta could be attributed to relatively low river discharge during the preceding months of the 2005 season. The elevated levels off Texas and Louisiana were speculated to be due to the presence of fine grain sediment or dissolved materials that remained in the water column following the storm, and may also have been associated with enhanced phytoplankton biomass stimulated by the intense vertical mixing and offshore delivery of shelf water and associated nutrients. This latter view was supported by observations of high chlor_a in association with regions of cyclonic circulation.

Benthic foraminiferal assemblages of the South Brazil: Relationship to water masses and nutrient distributions

Using distributions of benthic Foraminifera and bottom-water variables (depth, salinity, temperature, oxygen, suspended matter, organic matter, phosphate, silicate, nitrite, and nitrate), we investigated movements of water masses on the South Brazilian Shelf (27–30°S) and assessed the seasonality of continental runoff on the distribution of shelf water masses. The data were obtained from water and sediment samples collected in the austral winter of 2003 and austral summer of 2004 in three transects. The terrestrial nutrient input was significantly reduced at stations away from the coast, but high values of nutrients were maintained in subsurface waters due the presence of South Atlantic Central Water (SACW) at greater depths. At shallow sampling stations the influence of freshwater runoff was related to (1) the dominance of calcareous benthic Foraminifera, such as lagoon-related Pseudononion atlanticum, Hanzawaia boueana, Bulimina marginata, Bolivina striatula, Elphidium poeyanum, together with several agglutinated species, including Arenoparrella mexicana, Gaudryina exilis, and Trochammina spp., common in coastal environments subject to wide salinity fluctuations. In contrast, smaller forms and higher species diversity characterized the assemblage at offshore stations. In winter, the presence of Buccella peruviana and Uvigerina peregrina at Santa Marta Cape suggest the possible transport of those species of Subantarctic Shelf Waters (SASW) origin. Foraminifera associated to Subtropical Shelf Water (STSW) were dominated by Globocassidulina subglobosa in both seasons. In summer, the occurrence of U. peregrina in the shallower stations suggested the influence of SACW nutrients brought up by upwelling of deeper waters.