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The Polynesian South Ocean: Features 120" E 180" 12O"W GOOW NORMAL PATERN Scale Oceanic Circulation That Is Directly Linked to the South Pacific Anticyclonic System

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.,ìq i I Aln~i~Polluriort Bullerin, Vol. 29, Nos 1-3. ..DD. 14-25. 1994 volume 29/Numbers 1-3 Pergamon 0025-326X(94)00095-6 Copyright Cí 1994 Elscvicr Science Ltd Printed in Great Britain. Ail rights reserved Tropospheric Jet Tuamotu. Drought conditions are experienced in the W25-326X/94 %7.00+0.00 Convection + , Subsidcncc western Pacific and in southern Polynesia while the Marquesas (8-1 2"s) record heavy increases in rainfall (6 m in 1992 compared with an annual average of 1.2 m). Oceanic circulation iiz the Central South Pacific Through surface friction, the trade winds induce a large The Polynesian South Ocean: Features 120" E 180" 12O"W GOOW NORMAL PATERN scale oceanic circulation that is directly linked to the South Pacific anticyclonic system. This geostrophic la and Circulation stream flows to the west and following classical nomencla- ture (Tabata, 1975), divides into the Equatorial Current (4ON-8"S band) and the Southern Equatorial Current FRANCIS ROUGERIE" and JACQUES RANCHERt O. R.S.T.Q.hl. FQM~Documentaire Tropospheric Jet (1 0-20"s band ). Because of the Coriolis force, Ekman's *ORSTOM, B.P. 529 Papeete, Tahiti, French Polyiiesia Subsidence 4 Convcction rotation produces a leftward deviation in these great TSMSRICEA, B.P. 208 Montlhéry, 91 31 1 France N" 5 1- currents which ends in the creation of a vast anticyclonic vortex or gyre, with its oriental part located under the high pressure system from Kermadec to the Easter Island The hydroclimatic pattern of the French Polynesian zone is Hydroclimate of the Central Southern Pacific \q sector (Fig. 1).The southern border of the gyre consists of a returning easterly current which flows in the subtropical determined mainly by the pressure difference between the The atmospheric driving forces consist of an anti- 12Ó0 E 180' 120" W 60" W Easter Island High and the Western Equatorial Low. Trade cyclonic high pressure zone located in the region of Easter ENSO (EL NINO SOUTHERN OSCILLATION) zone (25-35"S), favoured by the dominant westerly winds. wind activity is correlated with this gradient and drives Island (1 1O"W-28"s) coupled with a quasi-permanent The northern branch of the gyre consists of the South --WIND surface oceanic water westward to produce the South depressive system in the central and western equatorial L. __.- CURRENT Equatorial Current, with an average speed of Equatorial Current, which constitutes the northern part of a zone that reaches its lowest levels in the northern 10-20 cm s-'. North of 10"s appears the Equatorial vast anticyclonic gyre in the South Pacific. High evap- Australian-Indonesian zone during the Austral winter. 'i b Current; it is more rapid and constant with an average Fig. 1 Hydroclimatic features of the equatorial and South Pacific. (a) speed of 50cms-' (Wyrtki, 1981). This current is oration from the centre of the anticyclonic high results-in The pressure gradient between the southeastern sub- Normal pattern with dominant trade winds and correlative the formation of hypersaline surface water, the Southern tropical Pacific and the western equator causes a transfer vertical movements (Walker circulation). Oceanic circulation in primarily geostrophic south of the equator; however, in Tropical Water, which is dense enough to sink. This of air mass, in accordance with the global circulation the band 4"N-20°S is mainly westwards. (b) ENSO situation the equatorial band, the direct wind-driven flow plays a with inversion of the Walker circulation. Oceanic circulation in major role: due to the Coriolis force this flows to the left downwelling process removes plankton and particles from model of Walker (Ramage, 1970). This circulation, equatorial/south tropical zone is mainly eastwards. N.E.C.C., the euphotic zone and contributes to the strong oligotrophy orientated diagonally from the southeast to the north- North Equatorial Counter Current; S.E.C.C., South Equatorial (right) of the wind south (north) of the equator, while on of the gyre. Thermohaline pattern underlines the crucial west, is fed in the lower troposphere by the southeast trade Counter Current; E.C., Equatorial Current; E.N., El Nino; the equator it flows directly downwind. Consequently role of vertical mixing processes in the upper mixed layer, winds, and in the higher troposphere by a returning 'S.E.C., South Equatorial Current; E.D., Eastward Drift. there is a divergence of the surface waters and a vertical which is isolated from the deeper Antarctic Intermediate circulation (the westerly high altitude jet streams). In the compensation through upwelling of subsurface waters. Water by a stable pycnocline. Oceanic circulation meas- equatorial zone, a powerful vertical convection of warm I This upwelling zone can stretch 200-500 km each side of ured in the band 140-145"W between latitudes 8-28"s and humid air feeds the high troposphere (ascending on the Tuamotu Archipelago is generally limited to its the Equator and is easily identified by its temperature during six HYDROPOL cruises (1986-1989) showed a branch of the Walker model); this air returns to low western border. In the Easter Island High, the descending which is 2-5°C lower than neighbouring tropical waters. weak (5-10 cm s-') shallow (0-300 m) westward flow, altitudes in the returning circulation after being trans- dry rtir mass maintains permanent aridity and low cloud The strength of the upwelling, and the temperature South Equatorial Current, that was often dissected by small ported east and south to the subtropics (Fig. l(a)). cover. Kainfall is low (in the order of 1 m yr-') and the anomaly is directly dependent on the combination of eastward countercurrents. The eastward component was Because of the predominance of the austral pressure strong evaporation can surpass 2 m yr-l. This situation trade winds and Equatorial Current. An ENSO anomaly increased considerably during the ENSO event of 1987 field over the boreal pressure field, the southern hemi- occurs in the Tuamotu Archipelago, especially in its signifies an important restructuring of the equatorial indicating an aperiodic forcing superimposed on the sphere trade winds are stronger than their counterparts of 1 southeastern part. The Evaporation-Precipitation bud- dynamics and especially the disappearance of the diver- dynamic instability of the zone. the northern hemisphere; consequently, the southern get (E-P), is therefore strongly positive reflecting the gence at the surface in the Central Pacific. For example, Data gathered in the vicinity of barrier and atoll reefs Pacific trade winds cross the equator and reach 5"N. domiilance of evaporation: E-P is in the order of monitoring at 140"W showed that upwelling vanishea showed that oceanic conditions (that is oligotrophic mixed Between 5 and lWN, there is a windless zone where the + 80 cm yr-I around Mururoa Atoll and +40 cm yr-' from July 1982 to May 1983 as a result of anomalous layer) prevailed around these structures in spite of periodic trade winds of the two hemispheres converge called the around Rangiroa Atoll. westerly winds that created an inversion of the surface thetmal oscillations related to the tide regime. The Inter-Tropical Convergence Zone (ITCZ). Where the This coupled ocean-atmosphere equilibrium is current (rotation from west to east) and a convergence of Polynesian oceanic province being mainly in low dynamical trade winds of the two types have different concentrations influenced by profound, aperiodic disturbances known as warm waters towards the equatorial axis (Donguy et al., conditions, vortex and eddies created around islands are of water vapour and temperature, high levels of condensa- ENS3 (El Nino-Southern Oscillation) episodes. A very 1989). insufficient to modify the vertical stratification or to pump tion occur in this zonal band and rainfall can exceed strong event with many climatic and biological impacts Six HYDROPOL cruises (1986-1989) were made with up nutrients from the nutricline, that explains the absence 4 m yr-'; these are the famous 'doldrums', calm and was observed in 1982-1983; lesser events have since been the R.V. Murara (Rancher & Rougerie, 1993) in the Polynesian waters along a track Tahiti-North of Mar- of local upwelling or 'island mass effect'. The permanence rainy, dreaded by navigators. A second line of conver- , observed in 1987 (medium strength) and 1991-1993 of a clear oligotrophic, euphotic layer then allows coral and gence, called the South Pacific Convergence Zone (SPCZ) (strong) (Fig. l(b)).Each ENSO period lasts about 12-18 quesas (8--5"S)-Rapa (28"SbTahiti. The physical and reef communities to thrive and reach the atoll stage. stretches diagonally from Papua New Guinea to the months. Its onset is signalled by a weakening of the Easter chemical properties of water masses were determined by ,$ ,$ southeast of French Polynesia (Cauchard, 1985). The Island High and a proportional increase of the equatorial using the following techniques: a MORS SLS57 CTD to SPCZ is a convergence front between the two types of IOW pressure fronts (Hansen, 1990). When the normal measure conductivity, temperature and depth with a precision of 0.1%) O.O2"C, 0.1%) respectively, with The Great Southern Anticyclonic Gyre trade winds that blow in the southern Pacific: the presstire gradient across the Pacific is absent, the trade northeastern trade winds (controlled by the Easter Island winds weaken or stop, and the surface currents slacken. adequate calibrations before and after casts. Concentra- Situated more than 5000 km from the American, Austra- High) and the southeast trade winds, controlled by the The 1Tcz migrates south of the equator to 0-10"s and tions of nutrients were analysed on a Technicon Auto- lian and Antarctic continental coastlines, the Central high subtropical pressure ridge, east of the Kermadec heavy rains fall in that region.
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