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

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 degrees 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 "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.

Coastal upwelling, circulation and heat balance around New Caledonia's barrier reef.

An outstanding characteristic of New Caledonia upwelling is that most events appear limited to the southern half of the western barrier reef. This north-south difference cannot be explained by alongshore variability of the projected wind stress and no strong evidence for alternative explanations has been proposed. A major objective of this paper is to provide the first dynamical analysis of New Caledonia upwelling and its regional environment, based on numerical simulations. Coastal upwelling around New Caledonia is shown to be modulated by a system of geostrophic currents interacting with the island mass. Upwelling velocities are weaker than expected from the two-dimensional Ekman theory, as Ekman divergence is balanced by "coastal geostrophic convergence". The cooling effect of upwelling is also attenuated by alongshore transport of warm water by the Alis current, reminiscent of the Leeuwin current off Western Australia. Nevertheless, coastal upwelling can locally modify the large-scale surface water heat budget, dominated by meridional advection warming and surface cooling. The upwelled waters appear to be mostly of western origin and are transported below the surface by the Subtropical Counter Current before upwelling off New Caledonia. This appears in sharp contrast with the eastern barrier reef where the general warming by meridional advection of tropical surface waters is accentuated by the vigorous western boundary type Vauban current.

Weather regimes and orographic circulation around New Caledonia.

The local climate and island-scale circulation around New Caledonia is investigated using a 4-km resolution mesoscale atmospheric model in concert with QuikSCAT scatterometer winds at 12.5-km resolution. The mesoscale atmospheric weather regimes are first examined through an objective classification applied to the remote sensed winds for nine warm seasons from 1999 to 2008. Four main weather types are identified. Their corresponding synoptic-scale circulation reveals that they are strongly discernable through the position and intensity of the South Pacific Convergence zone (SPCZ), the mid-latitude systems, and the subtropical jet stream. The link between the mesoscale weather types and the two dominant large-scale modes of variability, namely the Madden-Julian Oscillation (MJO) and the El Niño-Southern Oscillation (ENSO), is also described in terms of their influence on the occurrence of each weather type. It shows that their occurrence is significantly controlled by both MJO and ENSO, through modulation of the SPCZ. The large-scale modes of variability are scaled down to island-scale circulation through synoptic and mesoscale regimes, and are eventually modulated by orographic and thermal control. The island-scale circulation is inferred in this study by applying the compositing method to both observed and simulated winds. Their comparison clearly shows the ability of the mesoscale model to capture the local circulation and its spatial and temporal variability. A scaling analysis conducted from the simulated atmospheric parameters shows that the mountain range of New Caledonia is hydrodynamically steep. As a result of trade-wind obstruction by the mountainous island, the flow is shaped by coastally trapped mesoscale responses, i.e., blocking, flow splitting and corner winds, with a spatial scale of about 150 km. Two main obstacles, Mont Panié and Mont Humboldt play a significant role on the dynamical behavior of the low-level flow, while the diurnal heating cycle in the vicinity of the Mainland strongly modulates the local circulation. Moreover, nocturnal drainage flow of cold air occurs on the leeside slope of Mont Humboldt and inhibits vertical mixing over the ocean, which results in a deceleration of surface winds.