摘要:

North Atlantic sea surface temperature data from the Comprehensive Ocean‐Atmosphere Data Set were used to investigate the behavior of temperature anomalies on multiple‐year timescales during the period 1948–1992. Monthly anomaly time series for each 2° square from the equator to 70°N were low‐pass filtered at 4 years and normalized by the local standard deviation. Attention is focused on the extreme events, the upper and lower deciles, of the anomaly time series. A 45‐year sequence of January maps shows the already familiar phenomena of generally cold conditions prior to 1951, a long warm interval from 1951 through 1967, and again a cold period from 1968 through 1977. The years 1978 through 1982 were largely devoid of persistent strong anomalies, but moderate cold conditions returned during 1983–1986. Warm conditions dominated the North Atlantic from 1987 onward. Within these thermal epochs, however, a total of five cold anomaly features and nine warm anomaly features have been identified. These features have individual lifetimes of 3 to 10 years. A typical size is 20° of latitude or longitude, but they range from barely detectable to spanning the width of the basin, the latter especially in lower latitudes. Most of the anomalies move long distances along certain preferred paths. These paths generally follow the routes of the subarctic and subtropical gyres. Anomalies originating off North America along the boundary between the gyres move northeastward toward the Norwegian Sea along the approximate route of the North Atlantic Current. Midlatitude anomalies originating at the eastern boundary tend to spread both northward and southward along the coast. The speed of these movements (1–3 km d−1) is generally less than the expected speed of the near‐surface ocean circulation. Simple ideas about the effects of beta dynamics and air‐sea heat exchanges are briefly considered but do not provide a satisfactory explanation for the movements of the anomalies. The long timescale of these extreme events and the continuity of their movements suggest a useful degree of predictability of sea surface temperature based on persistence and p

ISSN:0148-0227    

DOI:10.1029/95JC03841

年代:1996

数据来源: WILEY

摘要:

Spatial and temporal variability in Geosat altimeter data is used to consider whether the Antarctic Circumpolar Current (ACC) responds on the large scale to global changes in wind forcing or whether its variability is primarily mesoscale. The Geosat data indicate a spatial decorrelation scale of 85 km and a temporale‐folding scale of 34 days. Using these scales to define autocovariance functions, the sea surface height variability is objectively mapped. The resulting maps indicate substantial evidence of mesoscale eddy activity. Over 17‐day time intervals, meanders of the Polar Front and Subantarctic Front appear to elongate, break off as rings, and propagate. Statistical analysis of ACC variability from altimeter data is conducted using empirical orthogonal functions (EOFs). The first‐mode EOF describes 16% of the variance in total sea surface height across the ACC; reducing the domain into basin scales does not significantly increase the variance represented by the first EOF, suggesting that the scales of motion are relatively short and may be determined by local instability mechanisms rather than larger basin‐scale processes. Likewise, neither complex nor extended EOFs indicate statistically significant traveling wav

ISSN:0148-0227    

DOI:10.1029/96JC00203

年代:1996

数据来源: WILEY

摘要:

Topographic Rossby waves propagating along the East Greenland Front between 77° and 72°N are simulated using a primitive‐equation ocean model of the Greenland Sea. The waves are prominent in both barotropic and baroclinic experiments, and their estimated wave period and wavelength are about 17.3 hours and 363 km, respectively. A possible mechanism for generation of topographic Rossby waves is presented. The dominant along‐slope component of the group velocity implies that the bulk of the energy propagates along the shelf slope of Greenland toward the northeast. Existence of the offshore component indicates that some energy is available for cross‐frontal mixing within the East Greenlan

ISSN:0148-0227    

DOI:10.1029/95JC03799

年代:1996

数据来源: WILEY

摘要:

The vertical structure of the surface layer of the ocean is examined at inertial and lower frequencies using observations made from the research platformFLIPin the late winter of 1990. The analysis isolates the shear supported by density gradients at the base of the mixed layer from that found within the layer. Near‐inertial frequency shear variability is most energetic at the base of the shallow, well‐mixed isothermal surface layer. At lower frequencies, significant shear is found within the surface layer. The wind‐driven transport is in good agreement with the Ekman transport and is divided almost equally between the surface layer and a less well mixed layer just below. The vertical structure of the wind‐driven flow within the surface mixed layer varies with frequency; it is slab‐like near the inertial frequency but strongly sheared at the lowest fr

ISSN:0148-0227    

DOI:10.1029/96JC00206

年代:1996

数据来源: WILEY

摘要:

Fresh water from summer ice melt and the total freshwater content of the Arctic Ocean water column above the thermocline are estimated from vertical profiles of temperature and salinity observed on theI/B Oden1991 cruise. The seasonal ice melt ranges from 0.5 m to slightly above 1 m and is moderately uniform over the observation area. Regions of lower melting are seen over the Morris Jesup Plateau. The freshwater content is calculated relative to the salinity just above the thermocline north of the Barents Sea. The freshwater content increases toward the interior of the Arctic Ocean, showing that fresh water is advected from other regions into the observation area. Regions of different freshwater content are separated by fronts over the Nansen‐Gakkel Ridge, over the Lomonosov Ridge, and in the western Eurasian Basin between waters derived from the Eurasian and Canadian Basins. Denser water, homogenized north of the Barents Sea, is recognized by a temperature minimum layer. The absence of the temperature minimum near the Nansen‐Gakkel Ridge indicates that heat is transferred from the Atlantic Layer over a longer time than the shortest route would allow. This observation can be explained if the layer circulates together with the Atlantic Layer, i.e., toward the east, and returns above the Nansen‐Gakkel Ridge and along the Amundsen Basin. North of the Laptev Sea, this water formed north of the Barents Sea becomes covered by low‐salinity shelf water. The increased freshwater content limits the winter convection, so it no longer reaches the thermocline and an intermediate halocline is formed. The halocline in the Eurasian Basin consists of water originating from winter convection in the Arctic Ocean north of the Barents Sea, which then circulates around the basin. Such a formation mechanism also explains the observed distribution of low NO water. The strong density increase limits vertical exchange, and the vertical diffusion coefficient in the halocline is small (∼1 × 10−6m2s−1). The increased temperature of the halocline shows that the heat lost upward by the Atlantic Layer, mainly by double‐diffusive convection, is trapped below

ISSN:0148-0227    

DOI:10.1029/96JC00143

年代:1996

数据来源: WILEY

摘要:

The Barents Sea is the site for one of the largest biomass productions in the world oceans. This is mainly due to the combined effect of winter thermohaline convection and a strong seasonal pycnocline during spring and summer. Secondary bloomings are due to wind‐induced entrainment of nutrients to the upper layer, which may be especially effective near 74.5°N, the critical latitude for the M2constituent. At this latitude, the inertial frequency equals the M2frequency, resulting in a strongly depth‐dependent tidal current and a thick benthic boundary layer. Our investigations of current meter measurements in the Barents Sea confirm these theoretical predictions. Statistical treatment of about 800 conductivity‐temperature‐depth stations between 70°N and 79°N reveals a stability minimum in the central part of the Barents Sea. This may be due to enhanced tidal mixing near the critical latitude. Observations of nutrients partly support this

ISSN:0148-0227    

DOI:10.1029/96JC00081

年代:1996

数据来源: WILEY

摘要:

Flow into the Bering Sea through Amchitka Pass in the central Aleutian Islands is shown to respond to the presence of a large meander/eddy in the Alaskan Stream south of the pass. Maximum flow into the Aleutian Basin of the Bering Sea occurs when the eddy center lies nearest to the axis of Amchitka Pass. A conceptual model of low‐frequency circulation variability within the Bering Sea, based upon results of empirical orthogonal function analyses of sea surface height anomalies measured by the Geosat radar altimeter, is presente

ISSN:0148-0227    

DOI:10.1029/95JC03806

年代:1996

数据来源: WILEY

摘要:

The heat flux through the snow and sea ice cover and at the ice/ocean interface were calculated at five sites in the western Weddell Sea during autumn and early winter 1992. The ocean heat flux averaged 7 ± 2 W/m2from late February to early June, and average ice/air heat flux in the second‐year floes depended on the depth of the snow cover and ranged from 9 to 17 (±0.8) W/m2. In late February, three of the five sites had an ice surface which was depressed below sea level, resulting, at two of the sites, in a partially flooded snow cover and a slush layer at the snow/ice interface. As this slush layer froze to form snow ice, the dense brine which was rejected flowed out through brine drainage channels and was replaced by lower‐salinity, nutrient‐rich seawater from the ocean upper layer. We estimate that about half of the second‐year ice in the region was covered with this slush layer early in the winter. As the slush layer froze, over a 2‐ to 3‐week period, the convection within the ice transported salt from the ice to the upper ocean and increased total heat flux through the overlying ice and snow cover. On an area‐wide basis, approximately 10 cm of snow ice growth occurred within second‐year pack ice, primarily during a 2‐ to 3‐week period in February and March. This ice growth, near the surface of the ice, provides a salt flux to the upper ocean equivalent to 5 cm of ice growth, despite the thick (about 1 m) ice cover, in addition to the ice growth in the small (area less than 5%

ISSN:0148-0227    

DOI:10.1029/95JC03675

年代:1996

数据来源: WILEY

摘要:

A new model is presented to reproduce the behavior of a solitary, circular, flexible ice floe brought into motion by the action of long‐crested sea waves. The intended application of the work is ultimately a fully three‐dimensional analogue of a marginal ice zone (MIZ) through which ocean waves propagate, allowing the attenuation and directional advance to be forecast and validated against observations. (Existing theory does not treat directional changes correctly.) To enable a check to be made on the model, two independent methods are developed: an expansion in the eigenfunctions of a thin circular plate, and the more general method of eigenfunctions used to construct a Green's function for the floe. Displacement and three‐dimensional scattering patterns in the water surrounding the floe are given for several floe geometries. The model is also used to investigate the strain field generated in the floe, its surge response, and the energy initiated in the water encircling it. Finally, with the aim of understanding how floes herd together to form cohesive structures in the MIZ, the force induced on floes of various thicknesses and diameters is pl

ISSN:0148-0227    

DOI:10.1029/95JC03706

年代:1996

数据来源: WILEY

摘要:

It has been observed that arctic coastal regions which receive river discharge show significant ice regression in advance of other coastal areas without this source of sensible heat. This phenomena has been particularly apparent on satellite imagery. The work reported here attempts (1) to quantify and assess the influence of springtime river discharge on the removal of nearshore ice and (2) to demonstrate the utility of satellite imagery as a source of data to “drive” a model assessing the influence of springtime river discharge on the removal of nearshore ice. Central to this work is a sequence of advanced very high resolution radiometer images of the Mackenzie Delta region, Northwest Territories, Canada. These images constitute a database establishing the temporal sequence of observable breakup events from which a thermodynamic model for the ice decay can be parameterized. The emphases of this model are the influence of the riverine sensible heat on the melting of sea ice and the ability of satellite imagery to monitor this process. For simplicity, climatological values rather than meteorological values for atmospheric and solar influences have been used. The results suggest that the river discharge supplies a significant amount of sensible heat to the base of the nearshore ice, accounting for nearly half of the energy required to melt the ice cover. As a result, the coastal ice is removed 7–14 days in advance of areas without significant river disc

ISSN:0148-0227    

DOI:10.1029/96JC00120

年代:1996

数据来源: WILEY