摘要:

Altimeter data from the Geosat Exact Repeat Mission (ERM) are analyzed with the aid of a simulation from an eddy‐resolving primitive equation model of the North Pacific basin in the region of the Kuroshio and Kuroshio Extension. The model domain covers the Pacific Ocean north of 20°S and has a resolution of 0.125° latitude and 0.176° longitude. The model is synoptically driven by daily 1000‐mbar winds from the European Centre for Medium‐Range Weather Forecasts (ECMWF) which encompass the Geosat time period. Model output is sampled along Geosat ground tracks for the period of the ERM. Additionally, the model and the Geosat data are compared with climatological hydrography and satellite IR frontal position analyses. Analyses compared include maps of sea surface height (SSH) mean and variability, eddy kinetic energy (EKE), seasonal transport anomaly, and time‐longitude plots of SSH anomaly. The model simulation provides annual mean SSH fields for 1987 and 1988 which reproduce the four quasi‐permanent meanders seen in hydrographic climatology (cyclonic at 138°E and anticyclonic at 144°E, 150°E, and 160°E). These are linked to the bottom topography. In the model simulation, Geosat altimeter data, and climatology, we observe four peaks in SSH variability associated with meander activity and two peaks in EKE, with the strongest about 3200 cm2s−2along the mean Kuroshio path in the Geosat data. The local maxima in SSH variability tend to occur where relatively strong, topographically steered meridional abyssal currents intersect the zonally oriented Kuroshio Extension. Westward propagation of SSH anomalies at phase speeds of 2 to 3 cm s−1in the region east of 155°E is observed in the model simulation and Geosat observations. A late summer maximum in the upper ocean transport anomaly of the Kuroshio Extension is inferred from changes in the cross‐stream differential in SSH from the simulation

ISSN:0148-0227    

DOI:10.1029/95JC02864

年代:1996

数据来源: WILEY

摘要:

A simple method for assimilating surface pressure data into a 21‐level, eddy‐resolving Cox model in a double‐gyre configuration [seeCox, 1985] is introduced. A conservation principle is used to derive the new water column structure based on rearrangement of the preexisting water masses. This respects the long timescales required to modify water properties on deeper isopycnals, while still allowing for immediate changes in isopycnal geometry and associated currents. Water columns are displaced vertically by an amount which reduces the surface pressure update to zero at the bottom. Current updates are then calculated geostrophically. An identical twin experiment is performed for 1 year with complete surface pressure data assimilated every 9 days. Thermocline temperature and current errors decrease rapidly after a single assimilation of surface pressure. Errors in subthermocline currents and isopycnal potential vorticity (stratification) within the thermocline decrease only after model integration. Deep current (3000 m) RMS errors are, after 1 year, reduced by up to 60%. A mixed layer scheme is added to the simple assimilation procedure to account better for changes in water properties near the surface (where property conservation is less realistic). Assimilation, with both surface pressure and surface temperature data provided, is also described. Surface temperature data have the biggest immediate impact on the circulation where the mixed layer is deep, e.g., in the subtropical gyre or in the far north, although after 1 year it does not contribute much over surface pressure assimilation alone. We speculate that the surface temperature data will contribute more for temporally varying surface boundary conditions, where it influences water properties subducted into the thermocline. This assimilation scheme should be easy to implement in any model framework and can be modified to incorporate an error analysis for use with real data

ISSN:0148-0227    

DOI:10.1029/95JC02902

年代:1996

数据来源: WILEY

摘要:

I investigate the response of the present‐day thermohaline circulation to a greenhouse gas‐induced global warming under different surface thermohaline conditions in a global Bryan/Cox [Bryan, 1969;Cox, 1989] ocean general circulation model with realistic bathymetry and geometry. Initially the model is spun up with surface temperature and salinity relaxed toLevitus[1982] climatologies. The forcing condition for salinity is then switched to a diagnosed flux, while that for temperature is provided by a restoration with a timescale of either 30 days (strong relaxation) or 300 days (weak relaxation). The present‐day ocean climate is obtained under these two sets of thermohaline conditions. Under the strong restoration, the modeled North Atlantic Deep Water Formation (NADWF) settles to an intensified state. Under the weak relaxation, the model solution hardly differs from that of the spin‐up. The ocean states are then subject to a global warming at a rate similar to that of the double CO2experiment in a fully coupled model described byManabe and Stouffer[1993, 1994]. The global thermohaline circulations under the weak relaxation show an initial weakening and shallowing, continued weakening upon the cessation of the atmospheric warming, and eventual reestablishment of their strengths. These behavior patterns are not unlike those found in the fully coupled model but are in sharp contrast to those in the case under the strong restoration, where NADWF eventually collapses. The processes responsible for these differences are di

ISSN:0148-0227    

DOI:10.1029/95JC03137

年代:1996

数据来源: WILEY

摘要:

A nonhydrostatic, Boussinesq, three‐dimensional model, the ocean large‐eddy simulation model(OLEM), has been developed to study deep oceanic convection. The model uses a subgrid‐scale parameterization of turbulence developed for large‐eddy simulation models, and the advection of scalars is accomplished using a monotonic scheme. A set of experiments was performed using OLEM to provide a direct comparison with laboratory results and aircraft measurements of the atmospheric convective boundary layer. The results from these experiments are in excellent agreement with laboratory and atmospheric convective boundary layer measurements of the mean profiles of zonal and vertical velocity variance, potential temperature variance, and heat flux. The horizontal wavenumber spectra of zonal and vertical velocity are also in good agreement with laboratory measurements and Kolmogorov's theoretical inertial subrange spectrum. A set of experiments using a potential temperature‐salinity profile from the central Greenland Sea for model initialization was conducted to study the effect of the thermobaric instability and rotation on the structure and evolution of deep oceanic convection. The artificial removal of the thermobaric instability suppresses penetrative convection, which is responsible for rapid changes in water properties at depths much greater than occurs for convective, mixed‐layer deepening. The vertical velocity and diameter,−0.08 m s−1and 300 m, respectively, of the penetrative plumes are in good agreement with observations from the Greenland Sea. A period of strong penetrative convection is followed by a gradual transition to convective, mixed‐layer deepening. During penetrative convection, the values of heat flux are about 2 times greater than convective, mixed‐layer deepening. In the absence of rotation, the evolution of penetrative convection occurs more rapidly, and vertical motions are more vigorous. The presence of the horizontal component of rotation forces asymmetries in the circulation around a penetrative plume. These experiments clearly demonstrate the importance of thermobaric instability and rotation on deep convection. To properly model large‐scale flows in regions of penetrative convection, it is necessary to include these effects in the vertical mixing parameterization.

ISSN:0148-0227    

DOI:10.1029/95JC02828

年代:1996

数据来源: WILEY

摘要:

A numerical simulation of the mixed‐layer circulation of the Arctic Ocean is presented usingOberhuber's [1993a] coupled sea ice‐mixed layer‐isopycnal ocean general circulation model. The model domain includes the Arctic Ocean and the Greenland‐Iceland‐Norwegian (GIN) Sea. The horizontal resolution is 2°. The vertical is resolved using five isopycnal layers, of which the uppermost layer is a turbulent mixed layer. The sea ice is modeled using a thermodynamic‐dynamic model which includes a viscous‐plastic rheology. Monthly climatological atmospheric forcing is used to spin up the model into a cyclostationary equilibrium. Model results are presented and discussed with respect to observational and previous modeling studies. The mixed layer shows a circulation pattern similar to that inferred from indirect observations and other modeling studies. In an attempt to determine the main driving mechanism for the mixed‐layer circulation as produced by the Oberhuber model, a set of sensitivity experiments is carried out. In particular, the relative importance of (1) ice cover, (2) atmospheric winds, (3) surface freshwater fluxes, and (4) initialization withLevitus[1982] data is examined to determine the contribution each makes to the modeled circulation. The key conclusion is that buoyancy forcing is critical to maintaining the mixed

ISSN:0148-0227    

DOI:10.1029/95JC02819

年代:1996

数据来源: WILEY

摘要:

An eddy‐resolving primitive equation model was used to simulate circulation on the shelf in the western Gulf of Alaska. This paper describes the development and sensitivity analysis of the model, while a companion paper [Stabeno and Hermann, this issue] demonstrates correspondence with measured currents in specific years. The model is initialized with a salinity field derived from local conductivity‐temperature‐depth surveys and forced with an idealized alongshore wind stress pattern. Buoyancy forcing is achieved by freshening the salinity field along a portion of the coastal wall. The model exhibits many of the patterns evident in observations, including mean flows and mesoscale eddies associated with the Alaska Coastal Current (ACC). Sensitivity studies suggest a strong dependence of the barotropic flux of the ACC on local winds and weak dependence on upstream buoyancy forcing. Both wind and buoyancy forcing significantly affect the generation of eddies. Strong northeasterly winds enhance cross‐shelf flow of the ACC to the west of Shelikof Strait, where isobaths veer southward. Strong buoyancy forcing also favors cross‐shelf flow in that region and enhances the formation

ISSN:0148-0227    

DOI:10.1029/95JC02681

年代:1996

数据来源: WILEY

摘要:

Currents generated by an eddy‐resolving, semispectral primitive equation model are compared with those measured by moored current meters (1989 and 1991) and satellite‐tracked drifting buoys (1987) from Shelikof Strait, Alaska. The model reproduced many of the dominant circulation features, including the cross‐sectional spatial structure of the Alaskan Coastal Current, the estuarine inflow at the bottom of the sea valley, and the mean transport. The first‐mode empirical orthogonal functions of the model and of the observed data represent similar spatial structures and were significantly correlated in time. While the model produced eddies (>20‐km diameter) at a similar rate as observed, the timing did not generally match the observations. As a result, correlations between the measured currents and model‐generated currents usually were not significant. Correlations between modeled and measured transport through the sea valley, however, were significant for

ISSN:0148-0227    

DOI:10.1029/95JC02682

年代:1996

数据来源: WILEY

摘要:

In sea ice of polar regions, high concentrations of microalgae are observed during the spring. Algal standing stocks may attain peak values of over 300 mg chlam−2in the congelation ice habitat. As of yet, the effect of additional heating of sea ice through conversion of solar radiation into heat by algae has not been investigated in detail. Local effects, such as a decrease in albedo, increasing melt rates, and a decrease of the physical strength of ice sheets may occur. To investigate the effects of microalgae on the thermal regime of sea ice, a time‐dependent, one‐dimensional thermodynamic model of sea ice was coupled to a bio‐optical model. A spectral one‐stream model was employed to determine spectral attenuation by snow, sea ice, and microalgae. Beer's law was assumed to hold for every wavelength. Energy absorption was obtained by calculating the divergence of irradiance in every layer of the model (Δz= 1 cm). Changes in sea ice temperature profiles were calculated by solving the heat conduction equation with a finite difference scheme. Model results indicate that when algal biomass is concentrated at the bottom of congelation ice, melting of ice resulting from the additional conversion of solar radiation into heat may effectively destroy the algal habitat, thereby releasing algal biomass into the water column. An algal layer located in the top of the ice sheet induced a significant increase in sea ice temperature (ΔT>0.3 K) for snow depths less than 5 cm and algal standing stocks higher than 150 mg chlam−2. Furthermore, under these conditions, brine volume increased by 21% from 181 to 219 parts per thousand, which decreased the physical streng

ISSN:0148-0227    

DOI:10.1029/95JC02687

年代:1996

数据来源: WILEY

摘要:

Temperature, salinity, nutrients, oxygen, and halocarbon data collected in the Arctic Ocean reveal a frontal structure previously unrecognized in the hydrography of the Canadian Basin. Samples were collected on a 1300‐km section extending from the Beaufort Sea in the Canada Basin to the East Siberian Sea in the Makarov Basin. These data, collected in 1993 aboard the CCGSHenry Larsen, reveal a lateral boundary between water masses of Atlantic and Pacific origin. The term “water mass assembly” is introduced to describe the basic arrangement or vertical stacking of water masses found in the Arctic Ocean, recognizing that water mass components within each assembly may differ from basin to basin. Using historical data, two primary water mass assemblies are defined, each consisting of three layers: an upper layer, an Atlantic layer, and a deep layer. These two assemblies are marked by important differences. One assembly, here defined as the Western Arctic (WA) assembly, is characterized by an upper layer of relatively fresh, high‐nutrient water of Pacific origin; below this, by an Atlantic layer with a core temperature generally below 0.5°C; and, finally, by a deep layer of higher salinities and colder temperatures (about −0.5°C) than found in the overlying Atlantic layer. The second assembly, here defined as Eastern Arctic (EA) assembly, is characterized by the absence of Pacific water in the upper layer; below this, by an Atlantic layer core as warm as 2° to 3°C; and by a colder (about −0.9°C) deep layer. Because the presence or absence of Pacific origin water is a key characteristic distinguishing the two assemblies, we will refer to the water mass boundary between the two assemblies as the Atlantic/Pacific front. Earlier research indicated that water masses in the Arctic Ocean were separated by a front above the Lomonosov Ridge into the Canadian and Eurasian basins. Although all Larsen‐93 stations from the Canada Basin (A1–D1) display classic WA assembly characteristics, the Makarov Basin station (E1) shows EA assembly characteristics in the upper and Atlantic layers and a WA assembly deep layer. This suggests a relocation in the position of the Atlantic/Pacific boundary away from the Lomonosov Ridge. Further, Larsen‐93 data show the transition region between the Atlantic and deep layers is fresher in the Makarov Basin than corresponding water in either the Canada or Eurasian basins, implying a source of cold, low‐salinity water, perhaps from the Laptev and East Siberian shelves. The front separating these two assemblies lies above the Mendeleyev Ridge and is marked by large lateral gradients in all measured properties. In particular, the penetration of anthropogenic halocarbons is 2 to 3 times deeper in the Makarov Basin than in the Canada Basin, implying enhanced rates of ventilation. This suggests that direct exchange between the Canadian and Eurasian basins has occurred recently near the perimeter and that physical and chemical properties, including contaminants, may have been transported by boundary currents more quickly fro

ISSN:0148-0227    

DOI:10.1029/95JC02634

年代:1996

数据来源: WILEY

摘要:

Thermistor cables have been deployed at two sites beneath Ronne Ice Shelf, Antarctica. One site is to the east of a submarine ridge that delineates the eastern boundary of the Ronne Depression, and the other is 100 km to the north, above the eastern slope of the depression. Long records from the cables (up to 22 months) indicate a large difference in the temperature variability at the two sites, being an order of magnitude greater in the Ronne Depression (site 2). Although the records appear otherwise similar, there is no significant correlation between them. The high variability in the site 2 record has allowed the construction of a simple descriptive model of the local oceanographic regime. Winter freezing in the open water north of the ice front generates Western Shelf Water (WSW), a type of High Salinity Shelf Water, which travels southwest beneath the ice shelf, appearing at site 2 as a slope‐trapped current at the bottom of the water column. Baroclinic instability in the flow manifests itself in the site 2 temperature record as oscillations on time scales of 5 to 15 days. The disturbances cause a periodic east‐west advection of water masses across the Ronne Depression. Site 2 is on the eastern slope of the depression, where the wave‐induced eastward motion forces Ice Shelf Water to rise, resulting in periodic ice‐platelet formation in the water column, as surmised from conductivity‐temperature‐depth measurements at the site. The depth of the WSW layer decreases by 40 to 60 m during a 100‐day period, starting some 4 months after the beginning of the summer. Assuming an absence of significant WSW production during the summer, the 4‐month delay implies a minimum average speed of WSW flow of about 0.02 m s−1. The WSW flux into the Ronne Depression is estimate

ISSN:0148-0227    

DOI:10.1029/95JC02679

年代:1996

数据来源: WILEY