Isopycnal displacements within the Cape Basin thermocline as revealed by the Hydrographic Data Archive

The transfer of upper kilometer water from the Indian Ocean into the South Atlantic, the Agulhas leakage, is believed to be accomplished primarily through meso-scale eddy processes. There have been various studies investigating eddies of the “Cape Basin Cauldron” from specific data sets. The hydrographic data archive acquired during the last century within the Cape Basin region of the South Atlantic provides additional insight into the distribution and water mass properties of the Cape Basin eddies. Eddies are identified by mid-thermocline isopycnal depth anomalies relative to the long-term mean. Positive depth anomalies (the reference isopycnal is deeper than the long-term mean isopycnal depth) mark the presence of anticyclonic eddies; negative anomalies mark cyclonic eddies. Numerous eddies are identified in the whole region; the larger isopycnal displacements are attributed to the energetic eddies characteristic of the Cape Basin and indicate that there is a 2:1 anticyclone/cyclone ratio. Smaller displacements of the less energetic features are almost equally split between anticyclones and cyclones (1.4:1 ratio). Potential temperature, salinity and oxygen relationships at thermocline and intermediate levels within each eddy reveal their likely origin. The eddy core water is not solely drawn from Indian Ocean: tropical and subtropical South Atlantic water are also present. Anticyclones and cyclones carrying Agulhas Water properties are identified throughout the Cape Basin. Anticyclones with Agulhas Water characteristics show a predominant northwest dispersal, whereas the cyclones are identified mainly along the western margin of the African continent, possibly related to their origin as shear eddies at the boundary between the Agulhas axis and Africa. Cyclones and anticyclones carrying pure South Atlantic origin water are identified south of 30°S and west of the Walvis Ridge. Tropical Atlantic water at depth is found for cyclones north of the Walvis Ridge, west of 10°E and for stations deeper than 4000 m, and a few anticyclones with the same characteristics are found south of the ridge.     

Kinematic elements of Antarctic Intermediate Water in the western South Atlantic

The northward flowing Antarctic Intermediate Water (AAIW) is a major contributor to the large-scale meridional circulation of water masses in the Atlantic. Together with bottom and thermocline water, AAIW replaces North Atlantic Deep Water that penetrates into the South Atlantic from the North. On the northbound propagation of AAIW from its formation area in the south-western region of the Argentine Basin, the AAIW progresses through a complex spreading pattern at the base of the main thermocline. This paper presents trajectories of 75 subsurface floats, seeded at AAIW depth. The floats were acoustically tracked, covering a period from December 1992 to October 1996. Discussions of selected trajectories focus on mesoscale kinematic elements that contribute to the spreading of AAIW. In the equatorial region, intermittent westward and eastward currents were observed, suggesting a seasonal cycle of the AAIW flow direction. At tropical latitudes, just offshore the intermediate western boundary current, the southward advection of an anticyclonic eddy was observed between 5^°S and 11°S. Farther offshore, the flow lacks an advective pattern and is governed by eddy diffusion. The westward subtropical gyre return current at about 28°S shows considerable stability, with the mean kinetic energy to eddy kinetic energy ratio being around one. Farther south, the eastward deeper South Atlantic Current is dominated by large-scale meanders with particle velocities in excess of 60 cm s^-1. At the Brazil–Falkland Current Confluence Zone, a cyclonic eddy near 40°S 50°W seems to act as injector of freshly mixed AAIW into the subtropical gyre. In general, much of the mixing of the various blends of AAIW is due to the activity of mesoscale eddies, which frequently reoccupy similar positions.     

Stochastic study of the temperature response of the upper ocean to uncertainties in the atmospheric forcing in an Atlantic OGCM

The impact of errors in atmospheric forcing on the behaviour of ocean models is a fundamental issue for ocean modellers and data assimilation and one that has yet to be fully addressed. In this study, we use a stochastic modelling approach with 50 7-months (September–March) primitive equation eddy permitting (1/4°) integrations. We investigate the response of the oceanic circulation to atmospheric uncertainties, focusing principally on their impact on the upper oceanic temperature field. The ensemble is generated by perturbing the wind, atmospheric temperature and incoming solar radiation of the ERA40 reanalysis. Each perturbation consists of a random combination of the 20 dominant EOFs of the difference between the ERA40 and NCEP/CORE reanalysis datasets. The ensemble standard deviation of various interfacial and oceanic quantities is then examined in the upper 200 m of three distinct regions of the North Atlantic: in the Gulf Stream, in the Northern Tropical band and in the North East Atlantic. These show that even a very small perturbation of the atmospheric variables can lead to significant changes in the ocean properties and that regions of oceanic mesoscale activity are the most sensitive. The ocean response is driven by vertical diffusivity and eddy activity. The role of subsurface currents is also crucial in carrying the eddy signal away from the regions of mesoscale activity. Finally, the decorrelation time scale of the mesoscale activity is critical in determining the amplitude of the oceanic response.     

Wave mechanism of the seasonal variability of the largescale current field in the northern Tropical Atlantic

A spatial-temporal analysis of the density field in the large-scale hydrological observation areas in the Tropical Atlantic is carried out. The spectral maximum corresponding to the first baroclinic mode of the planetary Rossby wave is discriminated and studied. It is shown that the seasonal transformation of the large-scale circulation of the current field is connected with the propagation of this wave. A simple quasi-geostrophic model is suggested which describes the seasonal variability of the North Equatorial Countercurrent. The results obtained by this model are compared with the hydrological survey data.Translated by Mikhail M. Trufanov.     

On circulation in the Tropical Atlantic induced by a meridionally-inhomogeneous wind field

The paper discusses a non-linear barotropic model of large-scale circulation which incorporates a mechanism of lateral friction. Some numerically calculated data on the interaction of two large-scale gyres induced by a meridionally-inhomogeneous wind field are reported which enable modelling of the Equatorial Countercurrent (EC) formation in the Tropical Atlantic Ocean. Consideration of the coastline in the western part of the region leads to the meandering of the EC and the emergene of quasi-stationary eddies in the area of its origin, thereby providing good compatibility with the experimentally documented data.Translated by V. Puchkin     

Importance of eddy representation for modeling the intermediate salinity minimum in the North Pacific: Comparison between eddy-resolving and eddy-permitting models

In order to confirm the results of the authors’ previous work, which found that the existence of disturbances smaller than meso-scale eddies is important in large-scale mixing process between the Oyashio and Kuroshio waters in the intermediate layer, the results of an eddy-resolving model experiment are analyzed and compared with those of an eddy-permitting model. The intermediate salinity minimum given in the initial condition weakens as integration advances in the eddy-permitting model, while it recovers rapidly and is maintained thereafter in the eddy-resolving model, initialized from the unrealistic salinity distribution of the former. Filament-like fine structures in temperature and salinity develop actively in the latter, which are much smaller in horizontal width than meso-scale eddies, suggesting the importance of such disturbances in the large-scale mixing. The mixing ratio of the Oyashio water defined by the original Oyashio and Kuroshio waters shows that its value is generally higher in the intermediate lower sub-layer than in the intermediate upper sub-layer in the Mixed Water Region, and the salinity minimum exists between layers with low and high values of the mixing ratio with its strong vertical gradient. The eddy transports of the Oyashio and Kuroshio waters in an isopycnal layer are divided into four components, usual isopycnal mixing of temperature and salinity being dominant, followed by the component associated with the thickness flux. The southward eddy transport of the Oyashio water and the northward eddy transport of the Kuroshio water are not symmetric to each other because the thickness-flux-associated components are in the same direction (southward).     

Staircase and inversion structures of thermocline in the north-west Tropical Atlantic

The peculiarities of the vertical fine thermohaline structure of waters in the north-west Tropical Atlantic are considered on the data of STD surveys recorded in winter-spring 1984. The variability of the characteristics of staircase and inversion elements of stratification with depth is analysed over the horizontal as well as related to the mesoscale and large-scale dynamics of waters. The coefficients of horizontal turbulent exchange are estimated within the framework of Joyce's hypothesis on quasi-compensation of vertical and turbulent horizontal transport. The effects of double diffusion are considered to dominate in vertical transport.UDK 551.465.15Translated by Mikhail M. Trufanov.     

Measured and remotely sensed estimates of primary production in the Atlantic Ocean from 1998 to 2005

Primary production (PP) was determined using ¹⁴C uptake at 117 stations in the Atlantic Ocean to validate three PP satellite algorithms of varying complexity. An empirical satellite algorithm based on log chlorophyll-a had the highest bias and root-mean square error compared with measured ¹⁴C PP and tended to under-estimate PP. The vertical generalised production model improved PP estimates and was the most accurate algorithm in the Eastern Tropical Atlantic (ETRA) and Western Tropical Atlantic (WTRA), but tended to over-estimate PP in eutrophic provinces. A photosynthesis-light wavelength-resolved model was the most accurate over the Atlantic basin, having the lowest mean log-difference error, root-mean square error and bias, and exhibited a superior performance in six out of the nine ecological provinces surveyed. Using this algorithm and mean monthly SeaWiFS fields, a PP time series was generated for the Atlantic Ocean from 1998 to 2005 which was compared with Advanced Very High Resolution Radiometer (AVHRR) sea-surface temperature (SST) data. There was a significant negative correlation between SST and PP in the North Atlantic Subtropical Gyre Province (NAST), North Atlantic Tropical Gyre (NATR), and WTRA suggesting that recent warming trends in these provinces are coupled with a decrease in phytoplankton production.     

Vertical hydroacoustic structure of the Tropical Atlantic central and intermediate waters

Large-scale CTD survey data are used to calculate the acoustic velocity in the North Tropical Atlantic. It has been stated that a characteristic feature of the vertical hydroacoustic structure in this region is associated with the presence of numerous local sound channels in the subsurface and intermediate layers. Characteristics of these channels are given and possible causes of their origin are discussed. It is shown how local acoustic waveguides affect the sound beam trajectory.Translated by V. Puchkin.     

Distinctiveness of the mesopelagic fish fauna in the Gulf of Mexico

We quantify the similarity of the Gulf of Mexico mesopelagic fish fauna to that in adjacent oceanic regions, the Venezuelan and Colombian Basins of the Caribbean Sea and the North and South Sargasso Seas. The South Sargasso and Colombian are the least similar of the areas in terms of their faunal composition, and the Venezuelan and Colombian Basins are the most similar. The Gulf fauna lies somewhere in between, and is a composite of that in the Sargasso and Caribbean Seas. The Gulf of Mexico displays the greatest abundance, biomass and richness (S=140 species), and is an intermediate in evenness (J=0.66) and percent endemism (7.1%). Our findings support the view that the centrally located Gulf is an oceanic ecotone between the Atlantic Tropical and Subtropical faunal regions.     

On the simulation of temperature and salinity fields in the Arctic Ocean

This paper considers the formation of the thermohaline structure of the Arctic Ocean: the formation of the salinity field and a freshwater reservoir in the Beaufort Sea and the transport of warm Atlantic water into the central part of the Arctic Ocean. A new version of the Finite Element Model of the Arctic Ocean (FEMAO) with a low spatial resolution is used. The main distinctions of this version are the following features: a new equation of state, a more sophisticated parameterization of vertical turbulence, modified formulations for the boundary conditions on open boundaries (using satellite data on the sea level) and at the upper boundary of the ocean, and the use of a variable eddy diffusivity in the parameterization of the eddy transport of a scalar. Our experiments indicated that the use of the parameterization of the eddy transport of a scalar enhances the transport of warm Atlantic waters to the central part of the Arctic Ocean through the Fram Strait; the results are most realistic when a variable coefficient is used. The Neptune effect has a contradictory role and, in the future, a higher spatial resolution should be used instead of this parameterization. We revealed that a key factor in the thermohaline fields on a large time scale is the interaction with the Atlantic Ocean, which is the source of heat and saline water.     

Collocation numerical model for synoptic eddy evolution in the ocean

A quasi-geostrophic model for synoptic eddy evolution in a confined oceanic area is examined. The numerical scheme is based on the combination of spline collocation, splitting, and Fourier series expansion by the dynamic operator's eigenfunctions over the vertical. The functions and their derivatives are approximated with cubic splines in a basis constituted by B-spline functions. A series of experiments focusing on eddy transformation were conducted. The acquired data confirmed the inference regarding the oscillatory nature of the energy exchange between the primary eddy and the secondary ones and showed how vortical formations affect the currents. The evidence obtained indicates that, vertically, the pattern of eddy propagation is complicated.Translated by Vladimir A. Puchkin.     

Simulated South Atlantic transports and their variability during 1958–2007

South Atlantic transports, as simulated by a global ocean-sea ice model forced with the Coordinated Ocean-ice Reference Experiments version 2 (CORE-II) interannually varying air-sea reanalysis data sets, are analyzed for the period 1958–2007. The ocean-sea ice model is configured at three different resolutions: from eddy-permitting to coarsened grid spacing. A particular focus is given to the effect of eddy fluxes and inter-ocean exchanges on the South Atlantic Meridional Overturning Circulation (SAMOC), as well as on the main factors contributing to the interannual variability during the integration period. Differences between refined and coarsened grid spacing models are more evident in coastal areas and in regions of high eddy activities. Major discrepancies are associated to both the parameterization of eddy fluxes and the coarse representation of the bathymetry. The refined grid spacing model produces higher values of both SAMOC index, defined as the maximum of the zonally-integrated northward cumulative volume transport (CVT) from surface to bottom across ∼34° S, and meridional heat transport (MHT). All models show high correlations between SAMOC index and MHT, as well as a strengthening of the transports in the 1980–2007 period. The strengthening of the SAMOC index is mainly dominated by surface and mode waters in all models. In surface and intermediate layers, the regions contributing to this trend are located east of 40° W. These changes are compensated by the strengthening of the poleward transport in deeper layers, mostly in the western part of the basin. The MHT trend is connected with the combined effect of a heat transport increase through the Drake Passage and a reduction of the heat loss through the eastern section between Africa and Antarctica, mainly associated with a strengthening in heat entering into the basin through the Agulhas system.     

The mean flow field of the tropical Atlantic Ocean

The mean horizontal flow field of the tropical Atlantic Ocean is described between 20°N and 20°S from observations and literature results for three layers of the upper ocean, Tropical Surface Water, Central Water, and Antarctic Intermediate Water. Compared to the subtropical gyres the tropical circulation shows several zonal current and countercurrent bands of smaller meridional and vertical extent. The wind-driven Ekman layer in the upper tens of meters of the ocean masks at some places the flow structure of the Tropical Surface Water layer as is the case for the Angola Gyre in the eastern tropical South Atlantic. Although there are regions with a strong seasonal cycle of the Tropical Surface Water circulation, such as the North Equatorial Countercurrent, large regions of the tropics do not show a significant seasonal cycle. In the Central Water layer below, the eastward North and South Equatorial undercurrents appear imbedded in the westward-flowing South Equatorial Current. The Antarcic Intermediate Water layer contains several zonal current bands south of 3°N, but only weak flow exists north of 3°N. The sparse available data suggest that the Equatorial Intermediate Current as well as the Southern and Northern Intermediate Countercurrents extend zonally across the entire equatorial basin. Due to the convergence of northern and southern water masses, the western tropical Atlantic north of the equator is an important site for the mixture of water masses, but more work is needed to better understand the role of the various zonal under- and countercurrents in cross-equatorial water mass transfer.     

Combining Argo profiles with a general circulation model in the North Atlantic. Part 1: Estimation of hydrographic and circulation anomalies from synthetic profiles,over a year

Argo is a global array of profiling floats that provides temperature (T) and salinity (S) profiles from 2000 m to the surface every ten days with a nominal spatial resolution of 3°. Here we present idealized experiments where the adjoint method is used to synthesize simulated sets of Argo profiles with a general circulation model, over a one-year period, in the North Atlantic. Using a number of drifting profilers consistent with Argo deployment objectives, the simulated array permits one to identify large-scale anomalies in the hydrography and circulation, despite the presence of a simulated eddy noise of large amplitude. Model dynamics provide an objective means to distinguish eddy noise from large-scale oceanic variability, and to infer the absolute velocity field (including abyssal velocities and sea surface height) from sets of Argo profiles of T and S. In particular, our idealized experiments suggest that volume and heat transports can be efficiently constrained by sets of Argo profiles. Increasing the number of Argo floats seems to be an adequate strategy to further reduce errors in circulation estimates.     

Detection of cyclonic eddy generated by looping tropical cyclone in the northern South China Sea:a case study

A case study on the cyclonic eddy generated by the tropical cyclone looping over the northern South China Sea (NSCS) is presented, using TOPEX/POSEIDON altimeter data and AVHRR sea surface temperature (SST) data.Three cases relating to the tropical cyclone events (Typhoon Kai-Tak in July 2000, Tropical Storm Russ in June 1994and Tropical Storm Maria in August-September 2000) over the NSCS have been analyzed. For each looping tropical cyclone case, the cyclonic eddy with an obvious sea level depression appears in the sea area where the tropical cyclone takes a loop form, and lasts for about 2 weeks with a slight variation in location. The cold core with the SST difference greater than 2 ℃ against its surrounding areas is also observed by the satellite-derived SST data.     

Detection of cyclonic eddy generated by looping tropical cyclone in the northern South China Sea: a case study

A case study on the cyclonic eddy generated by the tropical cyclone looping over the northern South China Sea (NSCS) is presented, using TOPEX/POSEIDON altimeter data and AVHRR sea surface temperature (SST) data. Three cases relating to the tropical cyclone events (Typhoon Kai-Tak in July 2000, Tropical Storm Russ in June 1994 and Tropical Storm Maria in August-September 2000) over the NSCS have been analyzed. For each looping tropical cyclone case, the cyclonic eddy with an obvious sea level depression appears in the sea area where the tropical cyclone takes a loop form, and lasts for about 2 weeks with a slight variation in location. The cold core with the SST difference greater than 2℃against its surrounding areas is also observed by the satellite-derived SST data.     

Space-and-time variability of the meridional heat transport in the north atlantic

We analyze the space-and-time variability of the meridional heat transport in the North Atlantic. The contribution of various mechanisms to the integral meridional heat transport (MHT) is estimated. The key role played by the drift transport of the Tropical Atlantic in the formation of the meridional oceanic heat transport is confirmed. On the basis of the general analysis of estimations obtained by various authors according to the data accumulated for 1870–2008 and the results of numerical analyses based on the data of NCEP/NCAR reanalysis, we show that the long-term average meridional drift heat (mass) transport attains its maximum values equal to (1.6 ± 0.1) PW [(17.4 ± 1.5) Sv] in the vicinity of 12.5°N in the Tropical Atlantic. The contribution of the heat transport caused by the horizontal Sverdrup circulation to the integral meridional heat transport is maximum in the vicinity of 30° N. On the average, it is equal to ∼ 40%. In the Subtropical Atlantic, the meridional heat transport varies with a period of ∼ 50–70 yr. The minimum value of the integral meridional heat transport was attained in the mid-1960s and its maximum value was at attained at the beginning of the 1990s. The location of the center of Azores pressure maximum makes it possible to conclude that the intensification of the total meridional heat transport in the Subtropical Atlantic on these time scales is accompanied by the displacement of the center of the North Subtropical anticyclonic gyre in the southwest direction.