Bruno Blanke

Variability of the Tropical Atlantic Ocean Simulated by a General Circulation Model with Two Different Mixed-Layer Physics

Abstract The embedment of a 1.5 turbulence closure model in an ocean general circulation model of the equatorial Atlantic is presented. The eddy viscosity and diffusivity involved in the vertical mixing are defined as the product of a characteristic turbulent velocity—the root square of the turbulent kinetic energy—and a characteristic mixing length. The turbulent kinetic energy is defined through a prognostic equation while the turbulent length scales are defined by a diagnostic formulation. The results of an experiment that includes this closure scheme are compared to the results issued from another experiment that includes a Richardson number-dependent parameterization of the mixing coefficients. The two simulations were performed over the tropical Atlantic during the 1982–1984 period, which allows direct comparisons with data from the FOCAL and SEQUAL experiments. Obvious contrasts between the two experiments on the sea surface temperature and on the dynamics indicate that the turbulent vertical diffu...

Kinematics of the Pacific Equatorial Undercurrent: An Eulerian and Lagrangian Approach from GCM Results

Abstract Three-dimensional monthly velocity fields from an ocean general circulation model are used to study the annual mean mass balance of the Pacific Equatorial Undercurrent (EUC). Eulerian diagnostics are used to evaluate the various meridional, vertical, and zonal mass fluxes related to the EUC. There are several distinct regimes along the equator, showing clear asymmetries between the western and eastern parts of the basin, and between the northern and southern edges of the EUC. Meridional fluxes are decomposed into pure Ekman divergence and geostrophic convergence, and it is shown that the asymmetries are mainly related to the spatial structure of the Ekman divergence, and thus to that of the trade winds. Lagrangian calculations are used to evaluate accurately the mass transfers between various sections of the EUC and between the EUC domain and the Tropics. The authors show that geostrophic convergence only ventilates the upper layers of the EUC and that the EUC really is a tongue of water flowing ...

Short communication: A Lagrangian tool for modelling ichthyoplankton dynamics

Ichthyop is a free Java tool designed to study the effects of physical and biological factors on ichthyoplankton dynamics. It incorporates the most important processes involved in fish early life: spawning, movement, growth, mortality and recruitment. The tool uses as input time series of velocity, temperature and salinity fields archived from ROMS or MARS oceanic models. It runs with a user-friendly graphic interface and generates output files that can be post-processed easily using graphic and statistical software.

Warm Water Paths in the Equatorial Atlantic as Diagnosed with a General Circulation Model

Abstract Monthly mean velocity fields from a global ocean general circulation model are used to study the main circulation patterns within the upper 1200 m of the equatorial Atlantic. Some recently developed Lagrangian techniques are used to picture and quantify the routes followed in the model by distinct water mass classes, defined by their initial temperature on model transatlantic sections at 10°S and 10°N. The qualitative description in terms of equatorial pathways of this warm component of the so-called global “conveyor belt” is found coherent with the most recent circulation schemes inferred from direct measurements. Diagnostics emphasize the crucial role of the western boundary current system and that of the equatorial subsurface jets in distributing the flow in the equatorial domain, both for northward-flowing and southward-recirculating warm water masses. As the model tracer fields are constrained to remain close to the observed climatology outside the equatorial strip, the circulation calculate...

Estimating the Effect of Stochastic Wind Stress Forcing on ENSO Irregularity

Abstract One open question in El Nino–Southern Oscillation (ENSO) simulation and predictability is the role of random forcing by atmospheric variability with short correlation times, on coupled variability with interannual timescales. The discussion of this question requires a quantitative assessment of the stochastic component of the wind stress forcing. Self-consistent estimates of this noise (the stochastic forcing) can be made quite naturally in an empirical atmospheric model that uses a statistical estimate of the relationship between sea surface temperature (SST) and wind stress anomaly patterns as the deterministic feedback between the ocean and the atmosphere. The authors use such an empirical model as the atmospheric component of a hybrid coupled model, coupled to the GFDL ocean general circulation model. The authors define as residual the fraction of the Florida State University wind stress not explained by the empirical atmosphere run from observed SST, and a noise product is constructed by ran...

Tasman leakage: A new route in the global ocean conveyor belt

The existence of a new route that draws relatively cold waters from the Pacific Ocean to the North Atlantic via the Tasman outflow is presented. The new route materialises with comparable magnitude and characteristics in three independent numerical realisations of the global ocean circulation. Its realism is supported by hydrographic data interpolated via an inverse model. The “Tasman leakage” constitutes a sizeable component of the upper branch of the global conveyor belt and represents an extension to the prevailing views that hitherto emphasised the routes via the Drake Passage and the Indonesian Throughflow [ Gordon, 1986 ].

Extratropical sources of Equatorial Pacific upwelling in an OGCM

The extratropical sources of equatorial undercurrent (EUC) water have been identified for an ocean circulation model using Lagrangian trajectory analysis. It has been found that the EUC waters emenate from a wide range of latitudes in the Pacific basin, with its densest constituent watermass being Subantarctic Mode Water (SAMW) from 50°S. Further analysis of the basin-scale circulation fields has revealed significant advective diapycnal mass fluxes associated with intergyre exchange. As a result of these diapycnal mass fluxes, the EUC transport as a function of density at 151°W (an Eulerian diagnostic) looks quite different from the original subduction rate as a function of density for the same collection of water particles. This implicates diapycnal vertical mixing as an important player in determining the preferred density horizon of maximum EUC transport along the equator. In summary, these results illustrate an important interdependence between advective and diapycnal mixing processes associated with basin-scale inter-gyre and inter-basin exchange in determining the mean equatorial stratification and EUC structure.

Warm and cold water routes of an O.G.C.M. thermohaline Conveyor Belt

A global general circulation model analyzed with a Lagrangian methodology is used to describe and quantify the paths, transports, and characteristics of the “warm” waters forming the upper branch of the conveyor belt in the North Atlantic Ocean. The total transport for this branch turns out to be 17.8 Sv in the North Atlantic at 20°N: 11.8 Sv are composed of waters coming from the two classical origins, the Drake Passage and the Indonesian Throughflow, which contribute with 6.5 and 5.3 Sv respectively. The remaining 6 Sv find their origins partly in the passage between Antarctica and the Australian Continent (with 3.1 Sv) and partly in the Indo-Atlantic sector itself (i.e., with 2.9 Sv). The geographical structure of the different routes emphasizes the role of the Southern Ocean and large-scale current systems in water mass transformation and distribution.

Radiocarbon as a thermocline proxy for the eastern equatorial Pacific

An ocean model is used to test the idea that sea surface Delta(14)C behaves as a thermocline proxy in the eastern equatorial Pacific. The ORCA2 model, which includes Delta(14)C as a passive tracer, has been forced with reanalysis fluxes over 1948-1999, and the output is compared with a previously reported Galapagos Delta(14)C record. The model reproduces the abrupt increase in the seasonally minimum Delta(14)C in 1976/77 found in the data. This increase is associated with neither a shift of thermocline depth over the NINO3 region, nor a change in the relative proportion of Northern/Southern source waters. Rather, it is due to a decrease in the Sub-Antarctic Mode Water (SAMW) component of the upwelling water, thereby representing a decrease in entrainment of water from below the base of the directly ventilated thermocline.

Tracking coherent structures in a regional ocean model with wavelet analysis: Application to Cape Basin eddies

[1] This study is mainly aimed at proposing objective tools for the identification and tracking of three-dimensional eddy structures. It is conducted with a high-resolution numerical model of the ocean region around South Africa, and emphasis is put on Cape Basin anticyclones and cyclones thought to be actively implicated in the Indian-Atlantic interocean exchange. We settle on wavelet analysis for the decomposition and processing of successive maps of relative vorticity for a simulation run with 1/10 degrees resolution. The identification of three- dimensional coherent structures comes with the calculation of eddy trajectories and the time evolution of eddy properties. Instantaneous mass transport and momentum of eddies are calculated from the knowledge of instantaneous drift velocities, volumes, and diameters. The success of the regional model and of the analysis technique is assessed through comparisons with equivalent observations.