The Effect of Continental Slope on Buoyancy-Driven Circulation

ABSTRACT The effect of continental slope on buoyancy-driven circulation has been studied using a two-layer quasigeostrophic model. In the model, buoyancy flux is incorporated as interfacial mass flux, which consists of narrow intense detrainment in the north and broad entrainment in the south. The model explicitly shows that, in the presence of the continental slope, a small amount of buoyancy flux can drive a strong barotropic flow. This flow develops because the beta effect of bottom topography either reduces or deflects the buoyancy-driven deep flow so that it cannot compensate its overlying counterflow, thus generating a net transport. As a result, in a double gyre circulation with a western continental slope, a small amount of detrainment/entrainment water mass can substantially enhance the transport of the western boundary current through southwestern deflection of the deep subpolar circulation. For example, with a reasonable western continental slope, a 10 Sv (Sv ϵ 10 6 m 3 s Ϫ1 ) detrainment mass flux can increase the transport of the western boundary current from 40 Sv of the wind-driven transport to 148 Sv. Relevance to the North Atlantic is then discussed

Mean Atlantic Meridional Overturning Circulation Across 26.5° N From Eddy-Resolving Simulations Compared to Observations

Observations along 26.5 degrees N are used to examine the time mean structure of the Atlantic meridional overturning circulation (AMOC) in eddy-resolving simulations with the Hybrid Coordinate Ocean Model (HYCOM). The model results yield a 5 year mean AMOC transport of 18.2 Sv, compared to 18.4 Sv based on data. The modeled northward limb of the AMOC has a vertical structure similar to observations. The southward limb is shallower than observed but deeper than other ocean general circulation models and includes a secondary transport maximum near 4000 m corresponding to Nordic Seas Overflow Water. The modeled flow through the Florida Strait and the deep western boundary current (DWBC) east of Abaco, Bahamas, are also approximately consistent with observations. The model results are used to clarify the sources of the northward AMOC transport and to explore the circulation pattern of the southward transport in the western subtropical North Atlantic in the range 18-33 degrees N. About 14.1 Sv of the modeled northward AMOC transport is through the Florida Strait and the remainder through the mid-ocean, primarily in the Ekman layer, but also below 600 m. The modeled AMOC transport is about 2/3 surface water and 1/3 Antarctic Intermediate Water with no contribution from the thermocline water in between. In the western subtropical North Atlantic the model results depict a complicated deep circulation pattern, associated with the complex bathymetry. The DWBC flows southward then eastward in both the upper and lower North Atlantic Deep Water (NADW) layers but with different offshore recirculation pathways, and there exists a second, more northern branch of eastward flow in the lower NADW layer

Low-latitude circulation and mass transport pathways in a model of the tropical Atlantic

ABSTRACT An eddy-resolving numerical ocean circulation model is used to investigate the pathways of low-latitude intergyre mass transport associated with the upper limb of the Atlantic meridional overturning cell (MOC). Numerical experiments with and without applied wind stress and an imposed MOC exhibit significant differences in intergyre transport, western boundary current intensity, and mesoscale ring production. The character of interaction between low-latitude wind-and overturning-driven circulation systems is found to be predominantly a linear superposition in the annual mean, even though nonlinearity in the form of diapycnal transport is essential to some segments of the mean pathway. Within a mesoscale band of 10-100 day period, significant nonlinear enhancement of near-surface variability is observed. In a realistically forced model experiment, a 14 Sv upperocean MOC return flow is partitioned among three pathways connecting the equatorial and tropical wind-driven gyres. A frictional western boundary current with both surface and intermediate depth components is the dominant pathway and accounts for 6.8 Sv of intergyre transport. A diapycnal pathway involving wind-forced equatorial upwelling and interior Ekman transport is responsible for 4.2 Sv. Translating North Brazil Current rings contribute approximately 3.0 Sv of intergyre transport

Five years’ central pacific sea level from in situ array, satellite altimeter and numerical model: Research note

Temporal and spatial features of central equatorial Pacific Ocean sea‐level variation appear similar, in measurements from two very different systems (one in the ocean and one carried on a satellite), and in results from a numerical model of the region. In particular, there is an interannual cycle: during El Nino, Kelvin waves appear at the equator, and the sea‐surface ridge associated with the equatorial current system shifts southward; in non‐El Nino years, instability waves appear at 6°N (strongest around the end of each calendar year), and the ridge shifts to the north. This three‐way comparison gives support to both measurement systems and to the numerical model. © 1994 Taylor & Francis Group, LLC

Initial concepts for IAS-GOOS

The Intra-Americas Sea (IAS) (Caribbean Sea, Gulf of Mexico, Straits of Florida, and adjacent waters) is a semi-enclosed sea dominated by the throughflow of the Gulf Stream System that serves to link the physics, chemistry, ecology, and fisheries of the region. A regional GOOS, IAS-GOOS, has been conceived that exploits the geography and dynamics of the region. Euro-GOOS could play a role in the development of IAS-GOOS as well as with IOCARIBE-GOOS and individual national Coastal GOOS activities in the IAS

Temperature versus salinity gradients below the ocean mixed layer

We characterize the global ocean seasonal variability of the temperature versus salinity gradients in the transition layer just below the mixed layer using observations of conductivity temperature and depth and profiling float data from the National Ocean Data Center's World Ocean Data set. The balance of these gradients determines the temperature versus salinity control at the mixed layer depth (MLD). We define the MLD as the shallowest of the isothermal, isohaline, and isopycnal layer depths (ITLD, IHLD, and IPLD), each with a shared dependence on a 0.2 degrees C temperature offset. Data are gridded monthly using a variational technique that minimizes the squared analysis slope and data misfit. Surface layers of vertically uniform temperature, salinity, and density have substantially different characteristics. By examining differences between IPLD, ITLD, and IHLD, we determine the annual evolution of temperature or salinity or both temperature and salinity vertical gradients responsible for the observed MLD. We find ITLD determines MLD for 63% and IHLD for 14% of the global ocean. The remaining 23% of the ocean has both ITLD and IHLD nearly identical. It is found that temperature tends to control MLD where surface heat fluxes are large and precipitation is small. Conversely, salinity controls MLD where precipitation is large and surface heat fluxes are small. In the tropical ocean, salinity controls MLD where surface heat fluxes can be moderate but precipitation is very large and dominant