Alexandre Ganachaud

Laboratoire d'Etudes en Géophysique et Óceanographie Spatiales

While the rise in global ocean temperature continues its course, reaching 1.45+/- 0.12 °C above pre-industrial level according to the World Meteorological Organization in 2023, marine heat waves frequencies and intensities increase. Consequently, coral reef ecosystems which are among the most vulnerable environments are strongly impacted with dystr...

Despite contributing minimally to global greenhouse gas emissions, Pacific Island Countries and Territories often shoulder an unequal burden of climate risks. To analyse pathways to sustainability, we conducted a leverage points analysis on the complex sustainability challenges facing the low-lying atoll Ouvéa in Kanaky New Caledonia (France). Leve...

This conference report synthesises the discussions and lessons learnt from a workshop with international and local experts and practitioners held in Nouméa, New Caledonia (France, Oceania) from 17th to 21st of October 2022. The workshop was part of a larger transdisciplinary process aimed to anticipate and react to marine heatwaves, coastal erosion...

With climate change, coastal communities in New Caledonia are or will be faced with displacement and degradation of their environment and quality of life, generating uncertainties. This qualitative assessment, drawn up during an expert workshop convened by the PACPATH project, aims to highlight the impacts of climate change on the coastal communiti...

The Copernicus Ocean State Report is an annual publication of the Copernicus Marine Service, established in 2014 by the European Commission for Copernicus 1 and renewed in 2021 for Copernicus 2. The report provides a comprehensive, state-of-the-art, scientific overview on the current conditions, natural variations, and ongoing changes in the global...

In the South Pacific Ocean, upper and lower Circumpolar Deep Water (UCDW and LCDW, respectively) occupy the deep layers; however, the presence and fate of both these water masses in the western equatorial Pacific have been mostly based on sparse measurements in both space and time. In this study, unprecedented deep measurements from three cruises c...

The Solomon Sea is a marginal sea in the western Pacific warm pool that contains the South Pacific low latitude western boundary currents. These low latitude western boundary currents chiefly exit the Solomon Sea through three channels (Vitiaz Strait, St. George's Channel, and Solomon Strait) and serve as the primary source water for the Equatorial...

Rising sea-levels are a critical concern in small island nations. The problem is especially serious in the western south Pacific, where the total sea-level rise over the last 60 years has been up to 3 times the global average (Becker et al. 2012). In this study, we aim at reconstructing sea-levels at selected sites in the region (Suva, Lautoka – Fi...

The satellite for Argos and AltiKa (SARAL/AltiKa) is the first ocean altimeter mission to operate in the Ka-band frequency. The objective of this article is to investigate the extent to which SARAL/AltiKa sea-level measurements provide valuable information in a complex bathymetric region, i.e., the semienclosed Solomon Sea. The data-editing procedu...

The semi-enclosed Solomon Sea in the southwestern tropical Pacific is on the pathway of a major oceanic circuit connecting the subtropics to the equator via energetic western boundary currents. Waters transiting through this area replenish the Pacific Warm Pool and ultimately feed the equatorial current system, in particular the equatorial undercur...

The semi-enclosed 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 t...

Climate models consistently project a substantial decrease in the Indonesian Throughflow (ITF) in response to enhanced greenhouse warming. On interannual timescales ITF changes are largely related to tropical Pacific wind variability. However, on the multi-decadal timescales investigated here we demonstrate that regional winds and associated change...

Biological dinitrogen (N2) fixation and the distribution of diazotrophic phylotypes were investigated during two cruises in the Coral Sea and the Solomon Sea (South Western Pacific) during austral winter conditions. N2 fixation rates were measurable at every station, but integrated (0-150 m) rates were an order of magnitude higher in the Solomon Se...

The regional and near-coastal circulation around New Caledonia is investigated using a compilation of more than 20 years of observations. Velocity profiles acquired by Shipboard Acoustic Doppler Current Profiler (SADCP) during 109 research cruises and ship transits since 1991 are analyzed and compared with absolute geostrophic currents inferred fro...

ReefTEMPS est un réseau de capteurs de température des eaux côtières de différents états insulaires du Pacifique Sud et Sud-Ouest pour le suivi à long terme du changement climatique et de ses effets sur l’état des récifs coralliens et de leurs ressources. Le système d’information SI-TEC-PSO permet la gestion et la valorisation de ces données d’obse...

Pacific Ocean western boundary currents and the interlinked equatorial Pacific circulation system were among the first currents of these types to be explored by pioneering oceanographers. The widely accepted but poorly quantified importance of these currents[mdash]in processes such as the El Nino/Southern Oscillation, the Pacific Decadal Oscillatio...

This paper presents the new international TPOS2020 project: why it has been established, what are its scientific objectives, its proposed organization, governance, and what the expected outcomes are. It is aiming at informing Coriolis, Mercator Océan, and the operational oceanography communities, all concerned, and involved in generating interest a...

An updated procedure for a CTD-oxygen calibration along with new data processing was applied to recent hydrographic cruise data in the Coral Sea during the Cruise "BIFURCATION" in September 2012. After a brief introduction with the scientific context (section 1), we describe the contents and acquisition models of hydrological and chemical data that...

ReefTEMPS est un réseau de capteurs de température des eaux côtières de différents états insulaires du Pacifique Sud et Sud-Ouest pour le suivi à long terme du changement climatique et de ses effets sur l’état des récifs coralliens et de leurs ressources. Le système d’information SI-TEC-PSO permet la gestion et la valorisation de ces données d'obse...

Biological dinitrogen (N-2) fixation and the distribution of diazotrophic phylotypes were investigated during two cruises in the Coral Sea and the Solomon Sea (southwestern Pacific) during austral winter conditions. N-2 fixation rates were measurable at every station, but integrated (0-150 m) rates were an order of magnitude higher in the Solomon S...

Ocean temperatures have warmed in most regions over the last century and are expected to warm at a faster rate in the future. Consistent with the view that marine species are thermally constrained, there is growing evidence that many marine species have already undergone poleward range shifts in line with warming trends. This study uses historical...

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...

A water mass analysis of the Coral Sea thermocline waters provides a description of their distribution, pathways and mixture based on recent oceanographic cruises in this region of strong western boundary currents. The Optimum Multiparameter method is used to determine the relative contribution of core water masses based on their measured temperatu...

The SPOT station (168°E-20°S,-4500 m) is a recent deep-sea and multidisciplinary marine research observation station which objective is the establishment of the first long time series of basic parameters in the South West Pacific. The main studies concerns the biodiversity, the productivity of key Plankton Functional Types, the biogeochemistry of c...

La campagne SECARGO représente une composante de la collecte d’observations in situ intégrée dans le cadre du projet scientifique SPICE (Southwest Pacific Ocean Circulation and Climate Experiment) soutenu par les instances internationales des programmes sur l’étude du climat et de sa variabilité, CLIVAR et WCRP, et par le volet scientifique du proj...

Regional climate projections in the Pacific region are potentially sensitive to a range of existing model biases. This study examines the implications of coupled model biases on regional climate projections in the tropical western Pacific. Model biases appear in the simulation of the El Niño Southern Oscillation, the location and movement of the So...

Future physical and chemical changes to the ocean are likely to significantly affect the distribution and productivity of many marine species. Tuna are of particular importance in the tropical Pacific, as they contribute significantly to the livelihoods, food and economic security of island states. Changes in water properties and circulation will i...

Pacific Island countries have an extraordinary dependence on fisheries and aquaculture. Maintaining the benefits from the sector is a difficult task, now made more complex by climate change. Here we report how changes to the atmosphere-ocean are likely to affect the food webs, habitats and stocks underpinning fisheries and aquaculture across the re...

Thermocline waters of the tropical southwest Pacific can be traced back to the center of the South Pacific basin and have a potential influence on equatorial surface conditions and on the characteristics of the El Niño Southern Oscillation on decadal timescales. The Solomon Sea is traversed by this influential flow, and therefore is an optimal plac...

EnglishHomogenised series of temperature and precipitation data for the late 20th century in New-Caledonia are presented. Annual mean minimum and maximum temperatures have increased by 0.3 and 0.2°C per decade respectively.A quantile-quantile downscaling method has been performed on IPCC AR4 numerical simulations in order to study the impact of cli...

Climate models participating in the third Coupled Model Inter Comparison Project (CMIP3) suggest a significant increase in the transport of the New Guinea Coastal Undercurrent (NGCU) and the Equatorial Undercurrent (EUC, in the central and western Pacific) and a decrease in the Mindanao current and the Indonesian Throughflow. Most models also proje...

En complément de l'ouvrage intitulé "Vulnerability of tropical Pacific fisheries and aquaculture to climate change", qui analyse de manière exhaustive les effets possibles du changement climatique sur les plans visant à maximiser les avantages économiques et sociaux tirés de la pêche et de l'aquaculture en Océanie, le présent document fait la synth...

Waters from the South Equatorial Current (SEC), the northern branch of the South Pacific subtropical gyre, are a major supply of heat to the equatorial warm pool, and have an important contribution to climate variability and ENSO which motivated the Southwest Pacific Ocean and Climate Experiment (SPICE, CLIVAR/WCRP). Initially a broad westward curr...

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. Up...

The Solomon Sea, in the western tropical Pacific, is part of a major oceanic pathway for waters connecting the tropics to the equator via low latitude western boundary currents. Shipboard Acoustic Doppler Current Profiler data from 94 various cruises and transits are used to describe the Solomon Sea mean circulation and its seasonal variability abo...

The El Niño–Southern Oscillation (ENSO) is a naturally occurring fluctuation that originates in the tropical Pacific region and affects ecosystems, agriculture, freshwater supplies, hurricanes and other severe weather events worldwide. Under the influence of global warming, the mean climate of the Pacific region will probably undergo significant ch...

The aim of this community white paper is to make recommendations for a glider component of a global ocean observing system. We first recommend the adoption of an ARGO-like (Global array of free-drifting profiling floats) data system for gliders. Then, we argue that combining glider deployments with the other components (ships, moorings, floats and...

The oceanic circulation in French Polynesia, central South Pacific, is investigated based on a high-resolution model validated with satellite data. We focus on the upper 500 m, where ocean current variability has been barely documented due to a lack of in situ measurements. Seasonal and El Niño–Southern Oscillation 1997–1999 variability of surface...

Afin de prévoir les modulations à basse fréquence du système climatique de l’Océan Pacifique tropical, il est impératif de comprendre le cheminement des masses d’eaux depuis les régions extra-tropicales vers la bande équatoriale et d’analyser les processus de ventilation qui contrôlent la variabilité et les caractéristiques des eaux de la thermocli...

The South Equatorial Current (SEC), the westward branch of the South Pacific subtropical gyre, extends from the equator to 30°S at depth. Linear ocean dynamics predict that the SEC forms boundary currents on the eastern coasts of the South Pacific islands it encounters. Those currents would then detach at the northern and southern tips of the islan...

South Pacific thermocline waters are transported from the subtropical gyre center in the westward flowing South Equatorial Current, towards the southwestern Pacific Ocean-a major circulation pathway that redistributes water from the subtropics to the equator and southern ocean. The transit in the Coral Sea is potentially of great importance to trop...

The trajectories of several Argo floats are studied to investigate the mid-depth circulation in the southwest Pacific that coincides with the spreading of the Antarctic Intermediate Waters (AAIW). Before entering the Coral Sea, the floats converge and join the North Caledonian Jet (NCJ) south of 15°S. These observations suggest that the waters tran...

This paper focuses on the invasion by Turbinaria ornata (a brown algae) in the Tuamotu archipelago, French Polynesia [(5–35°S)/(200–230°E)]. Prior to 1980, this alga existed only in the Society and Austral archipelagoes. Between 1985 and 1990, it began to appear in the southern and northern parts of the Tuamotu archipelago. Genetic analyses have be...

The CLIVAR program and many recent publications underline the importance of the oceanic connection between the south subtropical Pacific, where mode waters acquire their properties from the contact with the atmosphere, and the equator, where those same waters emerge after a 10 to 15-year travel-time below the surface. Recent field work and modeling...

1] Strong cooling events off the western barrier reef of New Caledonia have been recently observed and attributed to wind-driven coastal upwelling. A simple one-dimensional model based on a heat budget in the mixed laxer is developed and calibrated to explain the daily variations of sea surface temperature (SST) observed at a coastal station off so...

La Campagne FRONTALIS-2 fait partie d'une série de trois campagnes dans le cadre du projet « ENSO dans le Pacifique, variabilité à long terme et prévisibilité » du Programme National d'Etude du Climat. L'objectif de ces campagnes est d'observer et d'améliorer la compréhension de la zone frontale située sur le bord est de la « warm pool » du Pacifiq...

La campagne Frontalis 3 représente le dernier volet d'une série de trois campagnes à la mer effectuées dans le cadre du projet « ENSO dans le Pacifique, variabilité à long terme et prévisibilité » soutenu par le Programme National d'Etude Du Climat. L'objectif de ces campagnes était d'observer et d'améliorer la compréhension de la zone frontale sit...

ABSTRACT Linear inverse box models,based on hydrographic,data are widely used to estimate the ocean circulation and associated transports of heat and of other important quantities. The inverse method,permits calculation of a circulation that is consistent with basic conservation laws such as those for mass or salt along with uncertainties. Both unc...

Hydrographic sections obtained during the World Ocean Circulation Experiment are combined using a geostrophic inverse model to estimate the global-scale horizontal transports and transport divergences of heat and freshwater with self-consistent error bars. The overall results are compared to bulk formula-derived climatologies and estimates derived...

Large-scale oceanic transports of nutrient and oxygen are estimated from selected hydrographic sections from the World Ocean Circulation Experiment spanning the world ocean. A so-called geostrophic inverse box model is used to calculate the velocity field across the transoceanic sections. The circulation is required, a priori, to conserve mass, sal...

Wind friction at the ocean surface introduces energetic variations in the net mass transport across the oceanic basins. To conserve mass, the whole water column adjusts rapidly, introducing a depth-independent perturbation in the pressure field. This signal is analyzed in a high resolution numerical model of the Atlantic Ocean. While the pressure p...

We establish the fixed nitrogen budget of the Pacific Ocean based on nutrient fields from the recently completed World Ocean Circulation Experiment (WOCE). The budget includes denitrification in the water column and sediments, nitrogen fixation, atmospheric and riverine inputs, and nitrogen divergence due to the large-scale circulation. A water col...

Through its ability to transport large amounts of heat, fresh water and nutrients, the ocean is an essential regulator of climate. The pathways and mechanisms of this transport and its stability are critical issues in understanding the present state of climate and the possibilities of future changes. Recently, global high-quality hydrographic data...

The large scale Indian Ocean circulation is estimated from a global hydrographic inverse geostrophic box model with a focus on the meridional overturning circulation (MOC), The global model is based an selected recent World Ocean Circulation Experiment (WOCE) sections which in the Indian Basin consist of zonal sections at 32 degreesS, 20 degreesS a...

A new, global inversion is used to estimate the large scale oceanic circulation based on the World Ocean Circulation Experiment and JADE hydrographic data. A linear inverse "box" model is used to combine consistently the transoceanic sections. The circulation is geostrophic with an Ekman layer at the surface. Near conservation of mass, salt and top...