Sophie Cravatte

Near-Surface Salinity as Nature’s Rain Gauge to Detect Human Influence on the Tropical Water Cycle

AbstractChanges in the global water cycle are expected as a result of anthropogenic climate change, but large uncertainties exist in how these changes will be manifest regionally. This is especially the case over the tropical oceans, where observed estimates of precipitation and evaporation disagree considerably. An alternative approach is to examine changes in near-surface salinity. Datasets of observed tropical Pacific and Atlantic near-surface salinity combined with climate model simulations are used to assess the possible causes and significance of salinity changes over the late twentieth century. Two different detection methodologies are then applied to evaluate the extent to which observed large-scale changes in near-surface salinity can be attributed to anthropogenic climate change.Basin-averaged observed changes are shown to enhance salinity geographical contrasts between the two basins: the Pacific is getting fresher and the Atlantic saltier. While the observed Pacific and interbasin-averaged sal...

About the role of Westerly Wind Events in the possible development of an El Niño in 2014

Similarities between early 1997 and 2014 has prompted climate scientists to wonder if an El Nino matching the 1997 “El Nino of the century” could develop in 2014. Until April 2014, the equatorial Pacific exhibited positive heat content anomalies along with an eastward warm pool displacement similar to those found during the onset of strong El Nino events. Yet in July 2014, the warm pool had retreated back to its climatological positions and equatorial temperature anomalies were much weaker than in mid-1997. Dedicated oceanic simulations reveal that these weak interannual anomalies can be attributed to differences in Westerly Wind Event (WWE) sequences. In contrast with 1997, the lack of WWEs from April to June significantly limited the growth of eastern Pacific anomalies and the eastward warm pool displacement in 2014. With the absence of additional WWE activity, prospects for a mature El Nino in late 2014 are fading.

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.

Intermediate Zonal Jets in the Tropical Pacific Ocean Observed by Argo Floats

AbstractArgo float data in the tropical Pacific Ocean during January 2003–August 2011 are analyzed to obtain Lagrangian subsurface velocities at their parking depths. Maps of mean zonal velocities at 1000 and 1500 m are presented. At both depths, a series of alternating westward and eastward zonal jets with a meridional scale of 1.5° is seen at the basin scale from 10°S to 10°N. These alternating jets, with mean speeds about 5 cm s−1, are clearly present in the western and central parts of the basin but weaken and disappear approaching the eastern coast. They are stronger in the Southern Hemisphere. Along the equator at both 1000 and 1500 m, a westward jet is seen. The jets closer to the equator are remarkably zonally coherent across the basin, but the jets farther poleward appear broken in several segments. In the western half of the basin, the 1000-m zonal jets appear to slant slightly poleward from east to west. At the western boundary in the south (east of Solomon Islands and Papua New Guinea), the al...

Mean circulation of the Coral Sea

The mean absolute geostrophic circulation of the Coral Sea is constructed from climatological hydrographic data referenced to a 1000 m velocity field derived from Argo float drift. Two branches of the South Equatorial Current (SEC) enter the Coral Sea between New Caledonia and the Solomon Islands: the broad, upper thermocline North Vanuatu Jet (NVJ), and the narrow North Caledonian Jet (NCJ) extending to at least 1500 m. Most of this incoming flow leaves to the Solomon Sea. Four distinct pathways through the Coral Sea are traced by their water properties: (1) The NCJ crosses the Sea to the coast of Australia and turns north at densities sigma 25-27.4 as the main source of the Gulf of Papua (GPC) western boundary current, eventually feeding the New Guinea Coastal Undercurrent; (2) part of the shallow NVJ turns into the Solomon Sea in midbasin, carrying high-salinity water above sigma 25.5; (3) another part of the NVJ continues to Australia, then turns north to join the GPC, extending it to the surface; (4) a shallow finger of NVJ water, traced by low oxygen above sigma 25, turns south along the coast, beginning the East Australian Current (EAC) at 15 degrees S. Total transport from the Coral to the Tasman Sea is small and shallow; instead, most of the EAC is fed from south of New Caledonia, consistent with the Island Rule. However, large transport fractions occur in narrow jets close to coastlines and reefs and are not well sampled, precluding a quantitative estimate of meridional redistribution of the incoming SEC.

Annual Reversal of the Equatorial Intermediate Current in the Pacific: Observations and Model Diagnostics

Abstract A large reversal of zonal transport below the thermocline was observed over a period of 6 months in the western Pacific Ocean between 2°S and the equator [from 26.2 Sv (1 Sv ≡ 106 m3 s−1) eastward in October 1999 to 28.6 Sv westward in April 2000]. To document this reversal and assess its origin, an unprecedented collection of ADCP observations of zonal currents (2004–06), together with a realistic OGCM simulation of the tropical Pacific, was analyzed. The results of this study indicate that this reversal is the signature of intense annual variability in the subsurface zonal circulation at the equator, at the level of the Equatorial Intermediate Current (EIC) and the Lower Equatorial Intermediate Current (L-EIC). In this study, the EIC and the L-EIC are both shown to reverse seasonally to eastward currents in boreal spring (and winter for the L-EIC) over a large depth range extending from 300 m to at least 1200 m. The peak-to-peak amplitude of the annual cycle of subthermocline zonal currents at ...

Sea surface temperature and salinity seasonal changes in the western Solomon and Bismarck Seas

We analyze mean and seasonal change of Sea Surface Temperature (SST) and Salinity (SSS) in the Solomon and Bismarck Seas, using 1977–2009 in situ data collected from Voluntary Observing Ships. Covariability of these two variables with surface wind, altimeter-derived and model-derived horizontal currents, precipitation, and Sepik River discharge are examined. SST and SSS show large annual oscillations in the Solomon Sea, with the coldest and saltiest waters occurring in July/August mainly due to horizontal advection. In contrast, they show large semiannual oscillations in the Bismarck Sea. There, the coldest and saltiest waters happen in January/February, when the northwest monsoon winds drive coastal upwelling, and in July/August, when the New Guinea Coastal Current advects cold and high-salinity waters from the Solomon Sea through Vitiaz Strait. The low SSS values observed in April/May, stuck between the January/ February and July/August SSS maxima, are further enhanced by the Sepik River discharge annual maximum. A high-resolution model strengthens the conclusions we derive from observations. The impacts of ENSO on SST and SSS are also discussed with, for instance, saltier-than-average and fresher-than-average waters during the 2002–2003 El Ni~no and 2007–2008 La Ni~na, respectively.

The Coral Sea: Physical Environment, Ecosystem Status and Biodiversity Assets

The Coral Sea, located at the southwestern rim of the Pacific Ocean, is the only tropical marginal sea where human impacts remain relatively minor. Patterns and processes identified within the region have global relevance as a baseline for understanding impacts in more disturbed tropical locations. Despite 70 years of documented research, the Coral Sea has been relatively neglected, with a slower rate of increase in publications over the past 20 years than total marine research globally. We review current knowledge of the Coral Sea to provide an overview of regional geology, oceanography, ecology and fisheries. Interactions between physical features and biological assemblages influence ecological processes and the direction and strength of connectivity among Coral Sea ecosystems. To inform management effectively, we will need to fill some major knowledge gaps, including geographic gaps in sampling and a lack of integration of research themes, which hinder the understanding of most ecosystem processes.

Pathways and Water Mass Properties of the Thermocline and Intermediate Waters in the Solomon Sea

AbstractThe semienclosed Solomon Sea is the final passage in the equatorward transit of the South Pacific western boundary currents (WBCs) that play a key role in heat and mass budgets of the equatorial Pacific. The Solomon WBCs and their associated water properties are examined using data from two oceanographic cruises undertaken during the contrasting trade wind seasons: July 2012 and March 2014. The mean circulation and associated transports with uncertainties is determined from the cruise data using a unique configuration of an inverse box model formulated based on measured shipboard acoustic Doppler current profiler velocities. An intense inflow of 36 Sv is found entering the Solomon Sea in July–August 2012 that falls by 70% to 11 Sv in March 2014. Large differences are also found in the total transport partitioning through each of the major exit passages during each season. Different water masses are found in the WBC stream northeast of the Solomon Islands that are likely related to a northern strea...

Spatial patterns of mixing in the Solomon Sea

The Solomon Sea is a marginal sea in the southwest Pacific that connects subtropical and equatorial circulation, constricting transport of South Pacific Subtropical Mode Water and Antarctic Intermediate Water through its deep, narrow channels. Marginal sea topography inhibits internal waves from propagating out and into the open ocean, making these regions hot spots for energy dissipation and mixing. Data from two hydrographic cruises and from Argo profiles are employed to indirectly infer mixing from observations for the first time in the Solomon Sea. Thorpe and finescale methods indirectly estimate the rate of dissipation of kinetic energy (ϵ) and indicate that it is maximum in the surface and thermocline layers and decreases by 2–3 orders of magnitude by 2000 m depth. Estimates of diapycnal diffusivity from the observations and a simple diffusive model agree in magnitude but have different depth structures, likely reflecting the combined influence of both diapycnal mixing and isopycnal stirring. Spatial variability of ϵ is large, spanning at least 2 orders of magnitude within isopycnal layers. Seasonal variability of ϵ reflects regional monsoonal changes in large-scale oceanic and atmospheric conditions with ϵ increased in July and decreased in March. Finally, tide power input and topographic roughness are well correlated with mean spatial patterns of mixing within intermediate and deep isopycnals but are not clearly correlated with thermocline mixing patterns.