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Observed impact of upwelling events on water properties and biological activity off the Southwest coast of New Caledonia

Ganachaud, A., A. Vega, M. Rodier, C. Dupouy, C. Maes, P. Marchesiello, G. Eldin, K. Ridgway and R. Leborgne, 2009.

Marine Poll. Bul., accepted.


The upwelling events that follow strong trade wind episodes have been described in terms of their remarkable signature in the sea surface temperature southwest off New Caledonia. Upwelling brings deeper, and colder waters to the surface, causing 2°-4°C drops in temperature in a few hours, followed by a slower relaxation over several days. Upwelling may sporadically bring nutrients to the surface under certain conditions, and increase the biological productivity. Two multidisciplinary hydrographic cruises allow the impact of upwelling on the chemical and biological properties of the water to be documented. Both cruises took place in austral summer (December 2004 and December 2005), but the first cruise occurred during a strong upwelling event, while the second cruise occurred in calm conditions. The water properties and planktonic composition show important contrasts, with a strong southeastward current (the \textquotedblleft Alis Current of New Caledonia") competing with the upwelling system. Our analysis suggests that, while observed productivities are far less than those of typical upwelling systems, some wind events in New Caledonia may contribute to biological activity. A currentmeter mooring, deployed during the second cruise, documents the ocean response to a changing wind field and the local impact of upwelling on currents and temperatures on the water column. The results are discussed, with the help of climatology, Argo float profiler data, satellite data and of a high resolution numerical simulation.

topo_nc_3d_currentsSchematics of the two situations as observed during EMR1(Dec 2004, left) and EMR2 (Dec 2005, right) cruises. The limits of the ACNC core refer to its 20cm.s-1 isotach. (Left), the ACNC starts at 15km off the coast, with its core at 20 km off the coast. The green arrows indicate upwelling; the brown arrow indicate the possibility of water runoff. The green patch against the reef corresponds to colder waters with enhanced Chl-a, with enhanced copepod population. (Right) The ACNC core lies against the coast; a northwestward countercurrent appears 20km offshore; copepods are scattered. The empty red arrow to the northwest indicates a possible current suggested by the water properties.

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