1,721,034 research outputs found
Circulation and fluxes in the sub-polar North Atlantic
No full textThis thesis is primarily concerned with inverse techniques which are developed and applied to hydrographic data. A standard linear inverse method with an established history is chosen, the application of which is intended to calculate geostrophic reference currents. The first development of the method is a means to estimate skill in the inversion solution; this is also used to assist in solution selection. The second development of the method concerns the finding of an optimal hydrographic configuration to input to the inversion.The inverse method, with the above modifications, is applied to a data set which consists of three approximately 500-km-sided boxes of CTD stations, with vessel-mounted acoustic Doppler current profiles, collected as the U.K. Control Volume Experiment (CONVEX-91) during summer 1991 on the RRS Charles Darwin in the North Atlantic between Cape Farewell at the southern tip of Greenland and the European continental shelf west of Ireland. The East Greenland Current is represented by selected stations from the International Geophysical Year surveys of the R/V Anton Dohrn, because foul weather prevented CONVEX-91 from sampling there. Climatological wind stress data are used to estimate Ekman fluxes. We derive from the results an estimate of net (poleward) heat flux across the CONVEX-91 region (0.28±0.08 PW), and an estimate for the rate of freshwater gain by the Arctic Basin (0.17±0.04x106m3s-1). The results are compared, where possible, with work by previous authors. A quantified circulation scheme for the part of the Sub-Polar Gyre covered by the survey is presented, in which the most significant difference over previous schemes is that the Denmark Strait Overflow appears very weak - 5-6x106m3s-1, compared with ca. 13x106m3s-1.</p
The spatial and temporal variability of the East Greenland Coastal Current from historic data
No full textAnalysis of historic hydrographic data collected between 1932 and 1997 show a freshwater jet flowing southward along the East Greenland Shelf between Denmark Strait and Cape Farewell with a typical baroclinic transport of between 0.5 and 2 Sv. This jet has been labelled the East Greenland Coastal Current (EGCC). The depth of the jet is a function of transport but the lateral extent of the jet is mainly defined by the position of the shelf break. Comparing section transports from different times and locations it is seen that significant short-term variability masks both longer-term variability and latitudinal variations. The data suggest an increase in EGCC transport during the early 1930s Greenland warm period followed by reduced average fluxes thereafter but there is insufficient data for this result to be considered statistically significant. In addition there is no evidence of significant growth in the transport as the jet progresses south as might be expected with the addition of run-off from the SE Greenland fjords
The Deep Western Boundary Current at Cape Farewell: results from a moored current meter array
No full textAn analysis ismade of data from 30 Aanderaa recording currentmeters (RCMs) set on ninemoorings locatedeast of Cape Farewell, the southern tip of Greenland. The purpose of the measurements was to allow for theestimation of transport in the deep western boundary current (DWBC) below a depth of about 1500 m. Therecords commenced in September 2005 and lasted from9.5 to 11.5months.After calibration of the rawdata, 12-haverages of temperature and current were derived and the latter employed to estimate the flow across and alongthe array direction. The 9.5-month average transport of water colder than 38Cwas found to be 7.8 Sv (1 Sv[13106 m3 s21) with a standard error of 0.8 Sv. For water denser than su527.85 kg m23, the transport is calculatedas 4.5 Sv. Whether either of these values is significantly different fromcomparablemeasurementsmade 500 kmupstream cannot be determined. In marked contrast, for su . 27.8 kg m23, the transport is estimated as only9.0 Sv, smaller than the widely accepted value of 13 Sv for nearby measurements made in 1978. A reevaluationof the calculations and assumptions made then allows one to determine the uncertainty of the earlier estimateand thereby conclude that the difference between the previous and present measurements is significant, that is,that the transport has decreased between 1978 and 2005–06. Aweakening of the transport during the 9.5-monthperiod is also observed, along with a warming and an increase in salinity in the core of the DWBC. These latterchanges are shown to be consistent with interannual variability rather than a long-term trend
An isopycnal view of the Nordic Seas hydrography with focus on properties of the Lofoten Basin
No full textFew basins in the world exhibit such a wide range of water properties as those of the Nordic Seas with cold freshwaters from the Arctic in the western basins and warm saline waters from the Atlantic in the eastern basins. In this study we present a 50-year hydrographic climatology of the Nordic Seas in terms of depth and temperature patterns on four upper ocean specific volume anomaly surfaces. This approach allows us to better distinguish between change due to variations along such surfaces and change due to depth variations of the stratified water column. Depth variations indicate changes in the mass field while property variations along isopycnals give insight into isopycnal advection and mixing, as well as diapycnal processes. We find that the warmest waters on each surface are found in the north, close to where the isopycnal outcrops, a clear indication of downward mixing of the warmer, more saline waters on shallower isopycnals due to convective cooling at the surface. These saline waters come from the Norwegian Atlantic Slope Current by means of a very high level of eddy activity in the Lofoten Basin.The isopycnal analyses further show that the principal water mass boundary between the waters of Arctic origin in the west and Atlantic waters in the east aligns quite tightly with the Jan Mayen, Mohn, Knipovich Ridge system suggesting little cross-ridge exchange. Instead, the main routes of exchange between the eastern and western basins appear to be limited to the northern and southern ends of ridge system: Atlantic waters into the Greenland Sea in the Fram St and Arctic waters into the southern Norwegian Sea just north of the Iceland-Faroe Ridge.Analysis of a representative isopycnal in the main pycnocline shows it to be stable over time with only small variations with season (except where it outcrops in winter in the Greenland and Iceland Seas). However, two very cold winters, 1968–1969, led to greater than average heat losses across the entire Lofoten Basin that eroded away much of the Lofoten eddy and induced the greatest temperature anomaly in the entire 50-year record. Interannual variations in isopycnal layer temperature correlate with the NAO index such that waters in the Iceland Sea become warmer than average with warming air temperatures and conversely in the Lofoten Basin
The North Atlantic inflow to the Arctic Ocean: high-resolution model study
No full textNorth Atlantic Water (NAW) plays a central role in the ocean climate of the Nordic Seas and Arctic Ocean. Whereas the pathways of the NAW in the Nordic Seas are mostly known, those into the Arctic Ocean are yet to be fully understood. To elucidate these routes the results of a high-resolution global coupled ice–ocean model are used. We demonstrate that in 1989–2004 the NAW inflow was equally divided between the Fram Strait and Barents Sea. We find that salt influx within the branches is comparable but that most of the heat entered the Arctic Ocean through Fram Strait. The model shows complex NAW circulation patterns in the Barents Sea. Two mode waters in the Barents Sea branch are identified: a halocline water produced by surface cooling at shallow convective sites in the northern Barents Sea, and bottom water formed from NAW in the southeastern Barents Sea via full-depth convection and mixing. These two modes continue into the Nansen Basin along two separate routes: one through the northern Barents Sea shelf, and the other through the southeastern Barents Sea with halocline mode water dominating the outflow. Overall, less than half of the NAW coming into the Nordic Seas reaches the Arctic Ocean relatively unmodified, and the rest of it will have been modified in the Barents and Kara Seas with a large fraction re-circulating into the North Atlantic.<br/
The three-dimensional overturning circulation of the Southern Ocean during the WOCE era
No full textA box inverse model of the Southern Ocean during the World Ocean Circulation Experiment is constructed to investigate the three-dimensional structure of the regional overturning circulation in that era. The model has many features in common with various preceding inverse studies, but also contains several novel elements that make it well suited for addressing many of the significant uncertainties that surround the circulation at present. The net overturning circulation of the Southern Ocean is found to consist of two well-defined cells of similar strength. The upper cell consists of a northward transport of 18.8 ± 5.5 Sv of surface, mode and intermediate waters lighter than the 27.5 kg m−3 isoneutral, and an equivalent southward flow in the approximate 27.5–27.9 kg m−3 neutral density range, encompassing the bulk of the Upper Circumpolar Deep Water. The lower cell involves the northward export of 18.6 ± 0.9 Sv of Antarctic Bottom Water and Lower Circumpolar Deep Water denser than 28.08 kg m−3, and an opposing transport in the lighter classes of that water mass. Substantial structural differences between the overturning circulations of the Atlantic, Indian and Pacific basins are indicated by the model’s solution. Overall, the diagnosed Southern Ocean circulation shares many qualitative and some quantitative features with previous inverse estimates, particularly as regards the large-scale, depth-integrated lateral circulation and associated energy fluxes in the subtropics and in the Antarctic Circumpolar Current, and the strength of the upper overturning cell. However, it also suggests several significant adjustments to current views of the regional circulation. Most notable amongst these are: the subpolar circulation of the Southern Ocean is more vigorous and zonally interconnected than generally thought; the associated lower overturning cell is more intense than indicated by most preceding estimates; contrary to common perception, sub-surface mixing processes play a role of comparable importance to air–sea–ice exchanges of buoyancy in underpinning the dianeutral closure of the Southern Ocean overturning, even at shallow (mode and intermediate water) levels; and the connection between North Atlantic deep water formation and Southern Ocean upwelling is fundamentally three-dimensional, such that deep waters from the North Atlantic must upwell dianeutrally before being returned to the permanent pycnocline of the northern oceans
Polar outflow from the Arctic Ocean: A high resolution model study
No full textArctic fresh water plays a large role in both the global ocean circulation and the hydrological cycle. To identify sources and pathways of the Arctic outflow into the North Atlantic, results of a high resolution global model and observations have been examined. It is shown that continental runoff and snowmelt are the dominant sources of Arctic fresh water. The simulations demonstrated that the oceanic transports account for the majority of the fresh water export from the Arctic into the North Atlantic. The oceanic outflow from the Arctic is split between the western route through the Canadian Arctic Archipelago and the eastern route through the Nordic Seas. Two thirds of total oceanic fresh water comes via the western route. Arctic halocline waters are present on both the western and eastern routes with the western route providing most of the Arctic upper halocline water. Sea ice export contributes more than half the total freshwater flux east of Greenland and less than one fifth of the freshwater flux west of it. Pacific water constitutes about third of the outflow of the Arctic upper halocline water into the North Atlantic
Intensified turbulent mixing in the boundary current system of southern Greenland
No full textThe southern Greenland Deep Western Boundary Current (DWBC) advects major components of North Atlantic Deep Water. Turbulent diapycnal mixing rates of ?10?4 m2 s?1 commonly occur in the DWBC. These diffusivities, estimated with an internal wave strain-based parameterisation, are independently confirmed by budgets of density and dissolved O2. “Moderate” mixing rates (?10?4 m2 s?1) over the DWBC path between Denmark Strait and Cape Farewell (?1000 km) drive a similar net increase in volume transport by diapycnal entrainment to the more intense (?10?3 m2 s?1), localised mixing rates found near the Denmark Strait overflow sill. Therefore turbulent mixing associated with the DWBC south of Denmark Strait is instrumental in establishing the magnitude of the North Atlantic Meridional Overturning Circulation. Elevated mixing rates are also seen in the East and West Greenland Currents; this may be significant in preconditioning seasonal restratification of the intermediate layers of the Irminger and Labrador basins
Arctic sea surface height variability and change from satellite radar altimetry and GRACE, 2003-2014
Full text linkArctic sea surface height (SSH) is poorly observed by radar altimeters due to the poor coverage of the polar oceans provided by conventional altimeter missions and because large areas are perpetually covered by sea ice, requiring specialized data processing. We utilize SSH estimates from both the ice-covered and ice-free ocean to present monthly estimates of Arctic Dynamic Ocean Topography (DOT) from radar altimetry south of 81.5°N and combine this with GRACE ocean mass to estimate steric height. Our SSH and steric height estimates show good agreement with tide gauge records and geopotential height derived from Ice-Tethered Profilers. The large seasonal cycle of Arctic SSH (amplitude ?5 cm) is dominated by seasonal steric height variation associated with seasonal freshwater fluxes, and peaks in October–November. Overall, the annual mean steric height increased by 2.2?±?1.4 cm between 2003 and 2012 before falling to circa 2003 levels between 2012 and 2014 due to large reductions on the Siberian shelf seas. The total secular change in SSH between 2003 and 2014 is then dominated by a 2.1?±?0.7 cm increase in ocean mass. We estimate that by 2010, the Beaufort Gyre had accumulated 4600 km3 of freshwater relative to the 2003–2006 mean. Doming of Arctic DOT in the Beaufort Sea is revealed by Empirical Orthogonal Function analysis to be concurrent with regional reductions in the Siberian Arctic. We estimate that the Siberian shelf seas lost ?180 km3 of freshwater between 2003 and 2014, associated with an increase in annual mean salinity of 0.15 psu yr?1. Finally, ocean storage flux estimates from altimetry agree well with high-resolution model results, demonstrating the potential for altimetry to elucidate the Arctic hydrological cycle
The rapidly changing Arctic environment - Implications for policy and decision makers from the NERC Arctic Research Programme 2011-16
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