摘要:

Sea ice drift in the Weddell Sea was studied on the basis of data from seven buoys deployed on ice floes in January–February 1996. Six of the buoys formed an array with initial mutual distances of 50 km, which by October 1996 were stretched to ∼400 km in an east–west direction. The differential kinematical parameters i.e., divergence, shearing rate, and vorticity, were estimated from the buoy array positions by applying a time‐dependent method. The large‐scale drift of the array was divergent until August 1996, but after the array came under the influence of the Antarctic Circumpolar Current the deformation was mainly shearing. Meteorological data from European Centre for Medium‐Range Weather Forecasts analyses and Special Sensor Microwave Imager derived ice concentrations were interpolated at the buoy sites. The drift velocity was highly wind‐dependent; it was also wind‐dependent in winter when the momentum balance was mainly between the internal ice stress and the air drag. The drift divergence and shearing rate were related to the wind forcing, while the array vorticity correlated better with the air pressure itself. We estimated the air‐ice drag coefficient to be 1.8×10−3at a height of 10 m over the ice and the geostrophic drag coefficient to be 6.1 × 10−4. The stability effects on the coefficients were mostly felt in conditions of light winds. The average ice‐water drag coefficient based on 5 day periods was 2.1×10−3. The ice transport through a transect crossing the Weddell Sea was computed on the basis of the geostrophic winds, observed wind‐drift dependence, SSM/I‐derived ice concentrations, and the literature on ice draft statistics. The resulting annual mean net

ISSN:0148-0227    

DOI:10.1029/1999JC900265

年代:2000

数据来源: WILEY

摘要:

We use a multicategory sea ice model coupled to the Princeton ocean model, which is driven by monthly climatological atmospheric forcing, to study the seasonal variation of ice cover in the Labrador Sea. Initial ocean conditions are derived from a gridded, objectively analyzed temperature‐salinity data set that provides improved resolution of gradients in the vicinity of the shelf break. The model produces a realistic seasonal variation of sea ice. There is ice growth over the inner shelf and ice melt over the outer shelf and slope. Over the inner shelf, advection and diffusion decrease the ice mass; over the outer shelf, advection and diffusion increase the ice mass, which maintains the location of the ice edge. Near the offshore ice edge the melt rate exceeds 1 m per month, and the heat to melt ice together with the heat lost to the atmosphere exceeds 500 W m−2. The heat lost at the ocean surface is compensated for by advection of heat from an offshore convective region. The dominant heat source for the spring retreat of ice in the south is shortwave radiation over the open water fract

ISSN:0148-0227    

DOI:10.1029/1999JC900264

年代:2000

数据来源: WILEY

摘要:

Two sets of data on currents in the near‐surface boundary layer are reanalyzed to more accurately determine shear stress and characteristics of the mean current profile (friction velocity and roughness length). The results are used to examine the mechanisms responsible for momentum transfer through the aqueous boundary layer (the top several meters of water). The relationship between friction velocity and roughness length found here is consistent with the prevailing concepts on wave‐generated mixing. This result conflicts with a commonly used parameterization of momentum transfer in the aqueous boundary layer, where molecular viscosity is treated as the dominant transfer mechanism. The shear‐related stress and current profiles are shown to followCharnock's[1955] relationship, which indicates that the shear‐related mixing is due to gravity waves. Charnock's constant for water is found to be within an order of magnitude of 850 (105times greater than that for the atmospheric boundary layer). Shear‐related stress in the aqueous boundary layer is compared to the wind stress: for conditions of near local wind‐wave equilibrium, the aqueous shear stress is ∼20% of the atmospheric stress, indicating that the majority of the vertical momentum flux is transferred through other mechanisms. These limited data sets suggest that this ratio increases (approaching 100%) for decayin

ISSN:0148-0227    

DOI:10.1029/1999JC900250

年代:2000

数据来源: WILEY

摘要:

Gas transfer velocities of SF6and3He were determined in a Kerguelen Islands lake at wind speeds in the range 0–10 m/s by injecting the two tracers into the water and measuring their concentrations over 40 days. Two methods are investigated for the determination of the relationship linking the gas transfer velocityKto the wind speedW. The first method postulates a power law relationshipK= βWα. This leads to the same exponent α = 1.5±0.2 for both gases. The second method is the classic determination of the gas transfer velocityKijbetween two tracer measurements at timestiandtjusing the well‐mixed reservoir assumption. This method proves to be less favorable owing to its nonlinearity bias and also because it induces much scatter in the gas transfer coefficient/wind speed relationship. This dispersion is shown to arise from the experimental scatter of the data and, above all, from the high sensitivity of the method to even small heterogeneities in the tracer vertical distribution. In the present experiment, the Liss and Merlivat correlation [Liss and Merlivat, 1986] is shown to underestimate the actual mean gas exchange rate by about 40%. Our results agree with the recent dual‐tracer experiment byWanninkhof et al.[1993] and are also consistent with CO2transfer coefficient data derived from the study of14C oceanic inventories. As expected from gas transfer theories and various experimental observations, the Schmidt number exponent in the comparison of3He and SF6transfer velocities is found to vary substantially with the transfer regime. However, its variation is found to be greater than that forecast by current gas transfer models, with values as high asn= −0.2 for intermediate to strong winds. This again raises the question of the validity of the normalization method forKCO2calculation from gas transfer experiments, especially in high

ISSN:0148-0227    

DOI:10.1029/1999JC900088

年代:2000

数据来源: WILEY

摘要:

The water‐air exchange of dimethylsulfide (DMS) has been measured in a laboratory wind‐wave tank in fresh and seawater. To understand the transport behavior of DMS, its exchange was measured simultaneously with that of O2, SF6, Ne, CH4, and He under varying wind speeds and hydrodynamic conditions. No unpredictable differences between fresh and seawater were found, indicating that DMS surface affinities do not exist in seawater. Results also indicate that Schmidt number corrections can be applied to DMS gas exchange. While the rate of transfer of relatively insoluble or sparingly soluble permanent gases between the ocean and the atmosphere is primarily controlled by the rate of flux through the aqueous boundary layer, interfacial mass balances indicate that increased DMS solubility increases the significance of the airside control of flux. The hypothesis that DMS transport across the water‐air interface is subject to waterside control for moderate environmental conditions is supported. However, for ocean‐atmospheric DMS exchange with low sea surface temperature or moderate wind speeds, there may be a significant influence by the atmospheric mass boundary layer. An atmospheric gradient fraction γaapplied to the waterside air‐sea gas transfer velocity will correct for these effects. Estimates of ocean‐atmospheric DMS transfer velocities for these conditions

ISSN:0148-0227    

DOI:10.1029/1999JC900243

年代:2000

数据来源: WILEY

摘要:

Turbulent properties of the surface layer of stratified lakes have been quantified from temperature microstructure measurements made during convective conditions. Large eddy simulations have been performed using corresponding surface boundary conditions. We compare rates of dissipation of turbulent kinetic energy and dissipation of temperature variance obtained from observations with those obtained from large eddy simulations. While profiles of the rate of dissipation of turbulent kinetic energy can be reproduced very well by the model, the rate of dissipation of temperature variance exhibits smaller values in the calculations. Other statistical characteristics of turbulence agree for both properties. We show that part of the discrepancy derives from large‐scale lateral advection, which is not represented by the model, while turbulence on the microscale is reproduced wel

ISSN:0148-0227    

DOI:10.1029/1999JC900266

年代:2000

数据来源: WILEY

摘要:

A new model for three‐dimensional mesoscale flow is used to simulate the vertical transport of nutrients into the euphotic zone from deeper waters. We use the model to diagnose the effects of mesoscale motions on the upper ocean biogeochemical cycles of NO3, CO2, O2, sea surface exposure tracers that resemble dissolved organic carbon (DOC) and H2O2, and the corresponding rates of new production and gas exchange in regions near the Joint Global Ocean Flux Study (JGOFS) oligotrophic time series sites of Hawaii and Bermuda during late summer. The physical model is nonhydrostatic and designed to handle open boundaries without excessive damping as described byMahadevan and Archer[1998]. It is initialized and driven at the open boundaries with flow fields from the 1/4° global circulation model ofSemtner and Chervin[1992]. We examine the effect of model resolution and the attending increase in mesoscale and frontal‐scale motions on the nutrient and carbon chemistry of the upper ocean. New production, approximated and diagnosed as an exponential uptake of nutrients within the euphotic zone, increases with model resolution. Nutrient is supplied for new production where isopycnals from the subsurface outcrop at the base of the euphotic zone, primarily at fronts. The rate and spatial scale of upwelling and nutrient uptake are more sensitive to model resolution than are temperature or the more slowly responding geochemical tracers such as

ISSN:0148-0227    

DOI:10.1029/1999JC900216

年代:2000

数据来源: WILEY

摘要:

A deep, nonlinear warm eddy advecting water that was also anomalously saltier, lower in oxygen, and higher in nutrients relative to surrounding waters was observed in moored current and temperature measurements and in hydrographic data obtained at a site ∼400 km off the coast of northern California. The eddy was reproduced using a nonlinear quasi‐geostrophic model, initialized by an iterative procedure using time series of 2‐day averaged moored current measurements. The procedure demonstrates how a data assimilative technique synthesizes and enhances the resolution of a relatively sparse data set by incorporating time‐dependence and model physics. The model forecast showed significant skill above persistence or climatology for 40 days. Our hypothesis, that the eddy was generated at the coast in winter and subsequently moved 400 km offshore by May, is consistent with the eddy movement diagnosed by the model and with the observations and coastal climatology. The model evolution significantly underpredicted the temperature anomaly in the eddy owing in part to unmodeled salinity compensation in trapped California Undercurrent water. Together, observations and model results show a stable nonlinear eddy in the California Current System that transported water and properties southwestward through the energetic eastern boundary region. Coherent features such as this one may be a mechanism for property transfer between the eddy‐rich coastal zone and the eddy desert of the eastern North Paci

ISSN:0148-0227    

DOI:10.1029/1999JC900284

年代:2000

数据来源: WILEY

摘要:

Moored observations of currents and temperatures made in the upper 600 m on eddy‐resolving scales over a 2‐year period are used to examine the spatial and temporal characteristics of the California Current mesoscale circulation. The observations were made at three principal longitudes: 124°W, 126°W, and 128°W in the vicinity of Point Arena. They bracket the 600‐km‐wide band of high mesoscale variability found along the eastern boundary of the North Pacific. At all locations, the mesoscale variability was larger than the mean flow, and the spatial modes of variability as determined from empirical orthogonal function analysis consisted of an alongshore mode, a cross‐shore mode, and a rotational mode. Observations made near the continental slope (124°W) were dominated by the poleward flowing California Undercurrent, with mesoscale eddies and meanders superposed. The nearshore eddy kinetic energy peaked in a band centered around 60 days. Observations made at 128°W, near the offshore boundary between the energetic mesoscale band and the “eddy desert” of the northeast Pacific, were characterized by small means, fewer eddy events, and a peak in eddy kinetic energy at 120–180 days. The good horizontal resolution of the current meter arrays allowed us to estimate the relative vorticity, horizontal divergence, and Rossby number and therefore to evaluate the relative strength and occurrence of anticyclones and cyclones. We found the mesoscale eddy field to be strongly nonlinear, with Rossby numbers ranging from 0.1 to 0.5. All of the eddies observed at the offshore site were nonlinear, deep, warm anticyclones. Shipboard hydrography revealed the origin of one of these anticyclones to be the California Undercurrent, and this eddy retained its strong anomalies after several months and several hundred kilometers of propagation. Despite the lower incidence of eddies as one moves west from the coast, the eddies that we observed offshore provide evidence for propagation and transport of properties from the coast to the central North Pacific across the Calif

ISSN:0148-0227    

DOI:10.1029/1999JC900252

年代:2000

数据来源: WILEY

摘要:

Eddies are generated in the eastern tropical Pacific (3°S–23°N, 75°–105°W) by winds blowing through Central American mountain passes from the Atlantic. We used Coastal Zone Color Scanner (CZCS) and advanced very high resolution radiometer (AVHRR) satellite imagery complemented with monthly in situ sea surface temperature and wind series from the Comprehensive Ocean‐Atmosphere Data Set (COADS) to study these eddies and their effect on pigment concentrations in the region. Pigment values in the Gulf of Tehuantepec generally reach higher values in November–March before those in the Gulf of Papagayo. The eddies generated in the Gulf of Tehuantepec are associated with passages of cold fronts across the Gulf of Mexico from the north, while the eddies off Papagayo and Panamá are associated with increases in trade wind intensity. CZCS images showed larger numbers of eddies per season than have been previously reported on the basis of in situ and AVHRR observations or numerical simulations. We counted 13 eddies in 1979–1980, 8 in 1984–1985, and 6 in 1985–1986. The eddies transfer both energy and biological constituents from the continental margin to the offshore tropical Pacific. The eddies frequently moved distances in excess of 1500 km from their point of origin. Both anticyclonic and cyclonic eddies are generated, but in general, there are more anticyclones. Anticyclonic eddies generally moved to the southwest. Some cyclonic eddies moved to the south and southeast along the Central American coast and appeared to be trapped by the cyclonic Costa Rica thermal dome. Eddies traveled at speeds varying between 9 and 21 cm s−1and had diameters of 100–500 km. Phytoplankton concentrations associated with the eddies varied from ∼2 to>10 mg m−3within ∼70 km of the coast to ∼1 mg m−3up to 600 km of the coast. Between late April and October, fewer eddies were observed, and phytoplankton concentrations were lower (<0.25 mg m−3

ISSN:0148-0227    

DOI:10.1029/1999JC900257

年代:2000

数据来源: WILEY