Alexandre Ganachaud

Improved estimates of global ocean circulation, heat transport and mixing from hydrographic data.

Through its ability to transport large amounts of heat, fresh water and nutrients, the ocean is an essential regulator of climate. The pathways and mechanisms of this transport and its stability are critical issues in understanding the present state of climate and the possibilities of future changes. Recently, global high-quality hydrographic data have been gathered in the World Ocean Circulation Experiment (WOCE), to obtain an accurate picture of the present circulation. Here we combine the new data from high-resolution trans-oceanic sections and current meters with climatological wind fields, biogeochemical balances and improved a priori error estimates in an inverse model, to improve estimates of the global circulation and heat fluxes. Our solution resolves globally vertical mixing across surfaces of equal density, with coefficients in the range (3–12) × 10-4 m 2 s-1. Net deep-water production rates amount to (15 ± 12) × 106 m3 s -1 in the North Atlantic Ocean and (21 ± 6) × 10 6 m3 s-1 in the Southern Ocean. Our estimates provide a new reference state for future climate studies with rigorous estimates of the uncertainties.

Large-Scale Ocean Heat and Freshwater Transports during the World Ocean Circulation Experiment

Hydrographic sections obtained during the World Ocean Circulation Experiment are combined using a geostrophic inverse model to estimate the global-scale horizontal transports and transport divergences of heat and freshwater with self-consistent error bars. The overall results are compared to bulk formula‐derived climatologies and estimates derived from atmospheric reanalyses. At 7.58N in the Atlantic, a previous estimate of the heat transport is modified. A recent atmospheric residual estimate from NCEP and the Earth Radiation Budget Experiment (ERBE) products is consistent with the present results for the heat budget, except at high northern latitudes where it falls outside error estimates. The freshwater transport divergence from hydrography is statistically significant only when integrated over very large areas and difficult to test—as extant climatological estimates differ substantially from each other. Hydrographic estimates can be improved through repeated observations to reduce the temporal aliasing, and by combining more detailed regional estimates using more data types. To permit a formal comparison and assimilation in ocean general circulation models, atmospheric estimates urgently require convincing error estimates for both heat and freshwater transports.

Pacific western boundary currents and their roles in climate

Pacific Ocean western boundary currents and the interlinked equatorial Pacific circulation system were among the first currents of these types to be explored by pioneering oceanographers. The widely accepted but poorly quantified importance of these currents—in processes such as the El Niño/Southern Oscillation, the Pacific Decadal Oscillation and the Indonesian Throughflow—has triggered renewed interest. Ongoing efforts are seeking to understand the heat and mass balances of the equatorial Pacific, and possible changes associated with greenhouse-gas-induced climate change. Only a concerted international effort will close the observational, theoretical and technical gaps currently limiting a robust answer to these elusive questions.

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 ].

The Southwest Pacific Ocean circulation and climate experiment (SPICE)

The Southwest Pacific Ocean Circulation and Climate Experiment (SPICE) is an international research program under the auspices of CLIVAR. The key objectives are to understand the Southwest Pacific Ocean circulation and the South Pacific Convergence Zone (SPCZ) dynamics, as well as their influence on regional and basin-scale climate patterns. South Pacific thermocline waters are transported in the westward flowing South Equatorial Current (SEC) toward Australia and Papua-New Guinea. On its way, the SEC encounters the numerous islands and straits of the Southwest Pacific and forms boundary currents and jets that eventually redistribute water to the equator and high latitudes. The transit in the Coral, Solomon, and Tasman Seas is of great importance to the climate system because changes in either the temperature or the amount of water arriving at the equator have the capability to modulate the El Nino-Southern Oscillation, while the southward transports influence the climate and biodiversity in the Tasman Sea. After 7 years of substantial in situ oceanic observational and modeling efforts, our understanding of the region has much improved. We have a refined description of the SPCZ behavior, boundary currents, pathways, and water mass transformation, including the previously undocumented Solomon Sea. The transports are large and vary substantially in a counter-intuitive way, with asymmetries and gating effects that depend on time scales. This paper provides a review of recent advancements and discusses our current knowledge gaps and important emerging research directions.

Implications of CMIP3 model biases and uncertainties for climate projections in the western tropical Pacific

Regional climate projections in the Pacific region are potentially sensitive to a range of existing model biases. This study examines the implications of coupled model biases on regional climate projections in the tropical western Pacific. Model biases appear in the simulation of the El Niño Southern Oscillation, the location and movement of the South Pacific Convergence Zone, rainfall patterns, and the mean state of the ocean–atmosphere system including the cold tongue bias and erroneous location of the edge of the Western Pacific warm pool. These biases are examined in the CMIP3 20th century climate models and used to provide some context to the uncertainty in interpretations of regional-scale climate projections for the 21st century. To demonstrate, we provide examples for two island nations that are located in different climate zones and so are affected by different biases: Nauru and Palau. We discuss some of the common approaches to analyze climate projections and whether they are effective in reducing the effect of model biases. These approaches include model selection, calculating multi model means, downscaling and bias correcting.

Contrasted geographical distribution of N2 fixation rates and nifH phylotypes in the Coral and Solomon Seas (southwestern Pacific) during austral winter conditions

Biological dinitrogen (N2) fixation and the distribution of diazotrophic phylotypes were investigated during two cruises in the Coral Sea and the Solomon Sea (southwestern Pacific) during austral winter conditions. N2 fixation rates were measurable at every station, but integrated (0–150 m) rates were an order of magnitude higher in the Solomon Sea (30 to 5449 µmol N m−2 d−1) compared to those measured in the Coral Sea (2 to 109 µmol N m−2 d−1). Rates measured in the Solomon Sea were in the upper range (100–1000 µmol N m−2 d−1) or higher than rates compiled in the global MARine Ecosystem biomass DATa database, indicating that this region has some of the highest N2 fixation rates reported in the global ocean. While unicellular diazotrophic cyanobacteria from group A (UCYN-A1 and UCYN-A2) and the proteobacteria γ-24774A11 dominated in the Coral Sea and were correlated with N2 fixation rates (p < 0.05), Trichodesmium and UCYN-B dominated in the Solomon Sea and were correlated (p < 0.05) with N2 fixation rates. UCYN-A were totally absent in the Solomon Sea. The biogeographical distribution of diazotrophs is discussed within the context of patterns in measured environmental parameters.

Influence of upwelling, subsurface stratification, and heat fluxes on coastal sea surface temperature off southwestern New Caledonia

[1] Strong cooling events off the western barrier reef of New Caledonia have been recently observed and attributed to wind-driven coastal upwelling. A simple one-dimensional model based on a heat budget in the mixed laxer is developed and calibrated to explain the daily variations of sea surface temperature (SST) observed at a coastal station off southwestern New Caledonia from 1992 to 2005. This model takes into account the daily wind-forced vertical advection and air-sea heat fluxes, as well as seasonal variations of the subsurface temperature stratification. It can explain a large part of the daily SST variations and helps to quantify the relative contribution of the main vertical processes involved in these variations. The model shows that upwelling is the dominant process at daily timescale, and its SST signature is strongly modulated by the seasonal variations of the subsurface stratification. The surface heat fluxes have a smaller influence than upwelling on daily SST variations. Coastal chlorophyll a concentration increases during upwelling events.

Bifurcation of the Subtropical South Equatorial Current against New Caledonia in December 2004 from a Hydrographic Inverse Box Model

The South Equatorial Current (SEC), the westward branch of the South Pacific subtropical gyre, extends from the equator to 30°S at depth. Linear ocean dynamics predict that the SEC forms boundary currents on the eastern coasts of the South Pacific islands it encounters. Those currents would then detach at the northern and southern tips of the islands, and cross the Coral Sea in the form of jets. The Fiji Islands, the Vanuatu archipelago, and New Caledonia are the major topographic obstacles on the SEC pathway to the Australian coast. Large-scale numerical studies, as well as climatologies, suggest the formation of three jets in their lee: the north Vanuatu jet (NVJ), the north Caledonian jet (NCJ), and the south Caledonian jet (SCJ), implying a bifurcation against the east coast of each island. The flow observed during the SECALIS-2 cruise in December 2004 between Vanuatu and New Caledonia is presented herein. An inverse box model is used to provide quantitative transport estimates with uncertainties and to infer the pathways and boundary current formation. For that particular month, the 0–2000-m SEC inflow was found to be 20 4S v (1 Sv 10 6 m 3 s 1 ) between Vanuatu and New Caledonia. Of that, 6 2 Sv bifurcated to the south in a boundary current against the New Caledonia coast (the Vauban Current), and the remainder exited north of New Caledonia, feeding the NCJ. The flow is comparable both above and below the thermocline, while complex topography, associated with oceanic eddy generation, introduces several recirculation features. To the north, the NCJ, which extends down to 1500 m, was fed not only by the SEC inflow, but also by waters coming from the north, which have possibly been recirculated. To the south, a westward current rounds the tip of New Caledonia. A numerical simulation suggests a partial continuity with the deep extension of the Vauban Current (this current would then be the SCJ) while the hydrographic sections are too distant to confirm such continuity.

Turbinaria ornata invasion in the Tuamotu Archipelago, French Polynesia: ocean drift connectivity

This paper focuses on the invasion by Turbinaria ornata (a brown algae) in the Tuamotu archipelago, French Polynesia [(5–35°S)/(200–230°E)]. Prior to 1980, this alga existed only in the Society and Austral archipelagoes. Between 1985 and 1990, it began to appear in the southern and northern parts of the Tuamotu archipelago. Genetic analyses have been shown not to be appropriate in determining the origin of this algae population. This study investigated the possible ocean drift of floating aggregates of algae. Ocean currents were calculated from satellite data from 1993 to 2001. Their spatial variations as well as their seasonal and interannual variations are described along with calculated drift trajectories. While it was found that mean currents cannot directly transport algae from the Society and Austral archipelagoes to the Tuamotu, the large interannual changes during the El Niño-Southern Oscillation phenomenon produce current reversals that are strong enough to create a transport pathway in a short enough time to allow their survival.