Oceanic Circulation at the entrance of the Coral Sea (southwest Pacific Ocean), SPICE-France

Transcription

1 Oceanic Circulation at the entrance of the Coral Sea (southwest Pacific Ocean), SPICE-France Christophe Maes, Alexandre Ganachaud and Lionel Gourdeau Institut de Recherche pour le Développement, IRD Contact : Christophe.Maes@ird.fr ABSTRACT The water masses entering the Coral Sea are of primary importance in the long-term variations of the conditions affecting the equatorial Pacific Ocean through potential impact on El Niño phenomenon. The study of their circulation has been identified as a major priority in the international scientific program SPICE (CLIVAR-WCRP). The subtropical part of the South Equatorial Current enters the Coral Sea primarily through the gap between New Caledonia and Solomon Islands. Due to the reefs and islands the broad scale wind-driven oceanic gyre breaks into western boundary currents and narrow, predominantly zonal jets identified as the North Vanuatu Jet (NVJ) and the North Caledonian Jet (NCJ). Since 2003 a program composed by four oceanographic cruises allowed to provide quantitative transport estimates with uncertainties and to infer the pathways and boundary current formation. A survey of the circulation along 162 E using a Spray glider also revealed that the characteristics of the NVJ and NCJ entering the Coral Sea differ from those derived either from mean hydrography or from climate-scale oceanic models. The vertical extension of the jets remains largely unknown, even if the trajectories of the ARGO floats at their standard parking depth show that the waters transported by the NCJ do not have a unique source. Considering the ARGO collection of profiles allows to derive the averaged geostrophic zonal currents between 10 S and 20 S at some fine spatial resolution down to the 1/3 degree in latitude that was not previously considered. The patterns of the geostrophic circulation as well as other properties of the water masses will be compared and jointly analyzed with different numerical models using high horizontal resolution (1/12 degree). Measurements at the entrance of the Coral Sea will be a key component to the SPICE observing system, which will include observations of the waters inflows to the Solomon Sea and the Tasman Sea. modulate the El Niño-Southern Oscillation, ENSO, cycle and thereby produce basin-scale climate feedbacks. Despite its apparent importance to the climate system, few observations are available to diagnose the processes and pathways of transport through the complicated geography of the southwest Pacific. The South Pacific Convergence Zone is poorly documented; the region is remote, and the large temporal variability and strong narrow currents in a complex bathymetry (see the figure 1) pose serious challenges to an observing system. Numerical model results are sensitive to parameter choices and forcing, and the results are uncertain because of the lack of in situ data for validation. The existing observational network is beginning to provide a large-scale picture, but the complex circulation and western boundary currents require further dedicated study. This is the purpose of a regionally coordinated experiment, SPICE, the Southwest PacIfic Ocean Circulation and Climate Experiment [1], under the umbrella of the CLIVAR and the WCRP programs. Specifically, the work areas of SPICE include the South Pacific Convergence Zone (SPCZ), the South Equatorial Current inflow, the Tasman Sea circulation and East Australian Current, the North Coral Sea and Solomon Sea, as well as downscaling and environmental impacts of climate and oceanic environment changes such as cyclones, sea level rise, coral reef sustainability and coastal ecosystems. The areas of interest as well as main objectives of the SPICE program are recalled in the figure 2. More information on this program is available on this site: Keywords Physical Oceanography Ocean Circulation Water masses - Climate change Southwest Pacific. 1. INTRODUCTION South Pacific thermocline waters are transported in the westward flowing South Equatorial Current, SEC, from the subtropical gyre center towards the southwestern Pacific Ocean - a major circulation pathway that redistributes water from the subtropics to the equator and to the southern ocean. The transit in the Coral Sea is potentially of great importance to tropical climate prediction because changes in either the temperature or the amount of water arriving at the equator have the capability to Figure 1. The bathymetry of the southwest Pacific Ocean.

2 Thermocline waters originate from South Pacific Eastern Subtropical Mode Water [2] that forms in the dry and windy center of the Southeast Pacific gyre. This water mass is then advected to the west, forming the core of the South Equatorial Current with salinities in excess of 36. The SEC carries thermocline waters to the Coral Sea but upon its encounter with the island ridges of Fiji, New Caledonia and Vanuatu, it divides into three main jets; the South Caledonian Jet (SCJ, 24 S), the North Caledonian Jet (NCJ, 18 S) and the North Vanuatu Jet (NVJ, 13 S). The division of the SEC into jets is principally due to the island arcs (Godfrey's island rule, [3]) that are assumed to stabilize and enhance jets [4]. The bifurcation of the SEC, roughly at 18 S along the coast of Australia (western Coral Sea), separates water that flows into the equatorial current system from that which circulates in the subtropical gyre. and salinity (thermosalinograph). The different positions of the hydrological station are shown in figure 3. One objective of this program is to examine the mass transports of the ocean circulation in this region. As it could be noted from the figure 3, the northern legs of the Secalis-2 cruise form a closed box, allowing the setup of an inverse box model [5]. Transports between Vanuatu and New Caledonia during Secalis-2 are determined from geostrophy and S_ADCP and has been estimated at 20 ± 4Sv. Of that, 6 ± 2Sv 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 broad circulation as well as estimation of the mass transport for different parts of the water column as estimated by [5] is summarized in the figure 4. Such snapshot has its inherent limitations, and the next development of this work will be the analysis of variations of the circulation with time, based on all four SECALIS cruises and the few other existing CTD profiles in the area. As Argo profiles accumulate, they will also provide a complete picture of the SEC, but the large transports occurring in narrow boundary currents will continue to require direct surveys as provided by oceanographic cruises. A dedicated cruise to the NCJ is scheduled for 2010 (see last section) and will complete our view of the complex dynamics of this region. Figure 2. Main scientific issues addressed within the SPICE program. From [1]. Despite its direct climate implication for the Equator and the Tasman Sea surroundings, the estimates of the amount of thermocline water transported by the SEC into the Coral Sea are still subject to large uncertainty (in order of 20 Sv or more). Thus it needs to be observed comprehensively, and to some extent monitored. Also, the formation process of the jets on the east coasts of Vanuatu and New Caledonia, and the pathways and delays needed for this water to cross the Coral Sea determines part of the total time lag to the Equator or to the Tasman Sea. Those processes, delays and transports are not understood well and the spatial characteristics of the jets require observational description. These objectives represent the roots of several research programs dedicated to the Coral Sea and operated since 2003 from the IRD centre of Nouméa, New Caledonia. This paper is dedicated to present these different programs and the ongoing and future activities that continued within the SPICE context. 2. THE SECALIS CRUISES A first cruise staged in 2003 onboard the 92-ft R/V Alis (top of the figure 3) permitted the discovery of the North Caledonian Jet, thereby verifying analytical calculations and numerical model outputs. Since then, three more cruises have been made, sampling the deeper South Caledonian Jet and the jet structure between the islands of New Caledonia, South Solomon and Vanuatu archipelago. CTD profiles, Lowered-ADCP and water samples were collected during stations, along with continuous measurements of surface currents (shipboard ADCP), temperature Figure 3. Positions of hydrological stations staged during the 4 cruises of the SECALIS program since The R/V Alis is shown in the top part (@IRD, C. Maes)

3 3. THE SEC AS OCEANIC JETS The SEC entering the Coral Sea through the gap between New Caledonia and the Solomon Islands was also observed by an autonomous underwater vehicle, the Spray glider (top of the figure 5), during July October The measurements of temperature, salinity, and absolute velocity included highhorizontal resolution profiles to 600-m depth. These observations confirm the splitting of the SEC into a North Vanuatu Jet (NVJ) and North Caledonian Jet (NCJ), with transport above 600 m of about 20 and 12 Sv, respectively [6]. While the 300-km-wide NVJ is associated with the slope of the main thermocline and is thus found primarily above 300 m, the NCJ is a narrow jet about 100 km wide just at the edge of the New Caledonian reef (figure 5). It extends to at least a 1500-m depth with very little shear above 600 m and has speeds of more than 20 cm/s to 1000 m. 4. NUMERICAL MODELING Numerical models are used to study the role of topography and advection in modifying the SEC circulation in the southwest Pacific. A regional model based on the ROMS code has been embedded in the region in the thesis work of Xavier Couvelard in 2007 (Structure et dynamique des jets barotropes créés par les iles du Pacifique sud-ouest. Thése de l'université P. Sabatier, Toulouse). Such numerical experiment demonstrates that the regional topography drives a general equatorward shift of the SEC, which is beneficial to the North Fiji, North Vanuatu, and North Caledonian jets. A depth-integrated budget of the vorticity shows that this topographic effect is considerably attenuated by baroclinicity and advective processes [8]. Figure 4. SECALIS-2-estimated transports across selected sections (Sv), with Ekman transport excluded. The yellow numbers and arrows refer to transports above σ θ =26, and the dark ones between σ θ =26 and 2000 m (northern box, north of 22 S) or σ θ =27.75 (approximately 1000 m; lower box, south of 22 S). The uncertainty (1 std dev) is given in the parenthesis.positions of hydrological stations staged during the 4 cruises of the SECALIS program since From [5]. The trajectories of Argo floats have been also studied to investigate the mid-depth circulation in the southwest Pacific (Figure 6) that coincides with the spreading of the Antarctic Intermediate Waters (AAIW). Before entering the Coral Sea, the floats converge and join the NCJ south of 15 S. These observations thus suggest that the waters transported by this jet do not have a unique source [7]. Hydrologic parameters and oxygen concentration confirm that the characteristics of the AAIW in the NCJ result from a convergence and a mixing of waters from the northern limb of the subtropical gyre and from the south-eastern New Caledonian region (see also the figure 4). The northern contribution is associated with the water masses transported by the NVJ that re-circulate in the region between the D Entrecasteaux reefs and the Vanuatu archipelago. These analyses of in situ observations are also supported by the patterns of the mean circulation reproduced by a high 1/12 resolution configuration of the OCCAM model (see the next section). Figure 5. Absolute velocity (in red) averaged over the upper 600 m, measured by the glider for each dive, during 17 July-17 October 2005 (573 profiles). From [6]. The Spray glider is shown in the top (@IRD, C. Maes). The positions of the CTD operated during the SECALIS-3 cruise are shown in green.

4 Numerical models also exhibit a quite large sensitivity to horizontal resolution. This is illustrated by the figure 7 that represented the mean circulation at the 1000 m depth as simulated by the OCCAM model using different horizontal resolution. The main jets are simulated in both experiments and the total mass transport of the jets agrees within a 20% between the two experiments. In both experiments the general layout of the jets is similar: the NVJ results from the splitting of the SEC on the Vanuatu archipelago (between 14 S and 16 S) whereas the NCJ is mainly fed by a southern limb of the SEC. Note that, at the 1000 m depth, no current in either experiment crosses the Vanuatu ridge. The simulation at the 1/12 resolution exhibits small scale variability and stronger currents. Another important difference occurs in the re-circulation of the flow in the lee of the Vanuatu archipelago. The 1/4 simulation shows a re-circulation of the NVJ from the Coral Sea (near 16 S) and this flow turns southward near 164 E toward the Loyauté ridge with little or no eastward penetration toward the coasts of the Vanuatu archipelago. In the simulation at the 1/12 resolution this recirculation cell is stronger and located slightly northward. More importantly, it divides north of New Caledonia with the larger component extending to the east reaching the northern tip of Santo Island before to re-circulating toward the eastern edge of New Caledonia. The re-circulations of the NVJ jets in these model experiments are qualitatively consistent with several trajectories of the Argo floats as shown in figure 6. These re-circulations support the idea that water masses transported by the NVJ play a role in determining the water mass characteristics of the NCJ [7]. Figure 7. Mean velocity circulation at the 1000 m depth simulated by OCCAM model at the 1/4 (top) and 1/12 (bottom) horizontal resolution. The thin solid line represents the 1000 m isobath. From [7]. 5. ONGOING EFFORTS Figure 6: Trajectories of the Argo autonomous floats at their parking depth (~1000 m). The stars circled in black indicate the initial positions of the floats deployed during the Frontalis-3 cruise in April The background field represents the mean dynamic height (dyn. cm.) between 1000 db and the 2000 db reference as simulated by the OCCAM model. The dashed line is the 2000 m isobath. From [7]. To understand the volume transport of the highly variable incoming SEC, we start to measure it repeatedly from a commercial ship: deep XBT probes are released near the New Caledonia, Vanuatu and Makira shelves on a regular basis-about twice a year. The temperature profiles will be combined with data from Argo floats to reconstruct the corresponding salinities and density. From the end-point density profiles, the net SEC transport is calculated through geostrophy (only end-points against islands define the net geostrophic transports; regardless of the horizontal structure of the currents). Occasional high resolution XBT transects provide the detailed structure of the currents, showing the jet position and their characteristics (SECARGO project). The preliminary results of the first section staged in July 2008 are displayed on the figure 8. The position of the Argo floats deployed during the first stage of the SECARGO project is shown in figure 9. The parking depth of these specific floats [9] is set to 2000 m and their displacements show that the vertical extension of the jets is very deep. In fact we do not know, at the present times, what is the precise penetration in the vertical of the jets entering the Coral Sea. In the context of SPICE [1], we intend to monitor the mass transport entering the Coral Sea, especially on timescales longer

5 than seasonal. The main issue is related to the thermocline water inflow and to understand the jet formation, variability and characteristics. Means to do that will included the XBT drops, dedicated oceanographic cruise such as the SECARGO mission which is scheduled for 2010 in order to sample the NCJ at the tip of the D Entrecasteaux reefs, and release of Argo floats equipped with O 2 sensor as dissolved oxygen has been shown to be a remarkable tracer in this area. All these efforts are set up in order to provide the in situ observations necessary to interpret the southwest Pacific circulation and its effect on local and remote climate. 6. ACKNOWLEDGMENTS Contributions and participation to the different operations at sea from the technicians and engineers of IRD such as Jérôme Lefèvre, Jean-Yves Panché and David Varillon are greatly appreciated and acknowledged. During the PSI09 conference this work has been presented by Andres Vega. These research activities are supported by the LEFE/IDAO, the CNES, the GMMC and IRD. Figure 8. Position of XBT drops and Argo floats (top) deployed during July 2008 onboard the M/V Coral Islander 2, resulting in the temperature section between Santo and Honiara (bottom). The SECARGO program is set up for the period Figure 9. Trajectories of the Argo floats deployed during the SECARGO-1 transect between Nouméa and Honiara as available on June, (based on Google Earth software). 7. REFERENCES [1] Ganachaud, A., W. Kessler, S. Wijffels, K. Ridgway, W. Cai, N. Holbrook, M. Bowen, P. Sutton, B. Qiu, A. Timmermann, D. Roemmich, J. Sprintall, S. Cravatte, L. Gourdeau, and T. Aung, Southwest Pacific Ocean Circulation and Climate Experiment (SPICE) Part I. Scientific Background. International CLIVAR Project Office, CLIVAR Publication Series No. 111, NOAA OAR Special Report, NOAA/OAR/PMEL, Seattle, WA, 37 pp, [2] Hanawa, K., and L. Talley, Mode waters. In Ocean circulation and climate-observing and modelling the global ocean, G. Siedler, J. Church, and J. Gould, eds., Academic Press, , [3] Godfrey, J. S., A Sverdrup model of the depth-integrated flow for the world ocean allowing for island circulations, Geophys. Astrophy. Fluid Dyn., 45, , [4] Nakano, H. and H. Hasumi, A series of zonal jets embedded in the broad zonal flows in the Pacific obtained in eddypermitting ocean general circulation models, J. Phys. Oceanogr., 35, , [5] Ganachaud, A., L. Gourdeau, and W. Kessler, Bifurcation of the subtropical south equatorial current against New Caledonia in December 2004 from an hydrographic inverse box model, J. Phys. Oceanogr., 38, , [6] Gourdeau, L., W. Kessler, R. Davis, J. Sherman, C. Maes, and E. Kestenare, Zonal jets entering the Coral sea. J. Phys. Oceanogr., 38, , [7] Maes, C., L. Gourdeau, X. Couvelard, and A. Ganachaud, What are the origins of the Antarctic Intermediate Waters transported by the North Caledonian Jet, Geophys. Res. Let., 34, L21608, doi: /2007gl031546, [8] Couvelard, X., P. Marchesiello, L. Gourdeau, and J. Lefèvre, Barotropic Zonal Jets Induced by Islands in the Southwest Pacific, J. Phys. Oceanogr., 38, , [9] Riser, S., Profiling to 2000 m anywhere in the World Ocean: Advances with APEX Floats, ARGONAUTICS, 10, July issue, 2008.