Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques

The Atlantic Multidecadal Variability (AMV) is a prominent mode of low-frequency climate variability, characterized by basin-scale sea surface temperature (SST) variations in the North Atlantic and strong global impacts. AMV can be forced externally by surface radiative fluxes or internally generated. The latter generation mechanism is commonly attributed to variations in the Atlantic Meridional Overturning Circulation (AMOC). Here we show that a robust AMV can arise and be sustained by large-scale atmosphere–ocean interactions, even in the absence of a dominant role for AMOC variations, in a fully coupled model—the Department of Energy’s Energy Exascale Earth System Model version 2 (E3SMv2). The simulated AMV is driven primarily by surface shortwave and turbulent heat fluxes across the North Atlantic. Essentially, an initial warming over the Gulf Stream region strengthens and spreads by reducing low cloud cover and enhancing surface shortwave radiation. This mechanism is enabled by the relatively narrow width of the North Atlantic, compared to the North Pacific. Our results broaden the conceptual understanding of AMV physics and underscore the importance of atmosphere–ocean interactions in sustaining it.

Plankton communities are an essential component of ocean biogeochemistry and play a key role in making oceans an important climatic buffer. In the oceans, the environmental control of planktonic activity is modulated by the composition and diversity of plankton physiological traits (e.g., size, temperature and light preferences, stoichiometry, etc.). Yet, very little is known about how plankton communities assemble in the ocean under the combined influence of biological (eco-evolutionary dynamics) and physical mechanisms (mixing, transport). Moreover, this key process is very crudely represented for in current ocean models. Here, I show how integrating Darwinian adaptation into ocean models allows simulating how the functional composition and diversity of plankton communities is shaped by adaptation and ocean physics, how it feeds back on ocean biogeochemistry, and what the implications are for the resilience of marine ecosystems under climate change.

<strong class="journal-contentHeaderColor">Abstract.</strong> Antarctic Sea Level Anomalies (SLA) remain poorly observed due to the presence of sea ice, which hampers conventional satellite altimetry. Recent advances in lead-based retracking techniques have enabled SLA estimation within ice-covered regions. However, existing products are temporally limited, with currently available dataset covering periods shorter than 10 years. In this study, we extend the time span of SLA reconstruction by processing multiple satellite missions: Envisat, CryoSat-2, SARAL/Altika, and Sentinel-3A. We produce a consistent and continuous SLA dataset spanning both ice-free and ice-covered areas of the Southern Ocean (south of 50&deg; S) from 2003 to 2021. This 19-year product provides the longest temporal coverage to date for SLA under sea ice. The resulting SLA fields resolve the large-scale variability and parts of the mesoscale signal, with smooth transitions across the sea-ice edge. We show that the product reliably captures physical signals on timescales longer than 10 days, with estimated uncertainties of 1.3 cm. in the subpolar ocean. Our satellite-based SLA reconstruction compares well with most independent in situ observations from tide gauges and bottom pressure recorders: mean correlation coefficients are 0.58 and 0.66, respectively. The reconstruction fills a key observational gap and offers new opportunities to study trends and interannual variability in sea level and ocean circulation in the Southern Ocean, particularly under sea-ice cover.

Abstract Diapycnal mixing in the ocean interior has diverse numerical representations in current global ocean models. These representations affect the simulated transport and storage of oceanic tracers in ways that remain little studied. Here we present the impacts of three different tidal mixing representations in thousand‐year‐long simulations with the NEMO global ocean model at one‐degree resolution. The first model experiment includes local bottom‐intensified mixing at internal tide generation sites and a constant background diffusivity. The second explicitly includes both local and remote tidal mixing, with no background diffusivity. The third experiment is identical to the second but has the added contribution of bottom‐trapped (subinertial) internal tides, known to be important in polar regions. The three simulations show broadly similar circulation and stratification but important regional differences. Explicit representation of remote tidal mixing strengthens the Atlantic Meridional Overturning Circulation by up to 1.5 × 10 6 m 3 s −1 . Inclusion of bottom‐trapped internal tides reduces heat reaching Antarctica by eroding Circumpolar Deep Water at southern high latitudes, and reduces the mean age of the global deep (&gt;2 km) ocean by 10%. The results call for more observational constraints on polar ocean mixing, and point to multi‐faceted climatic repercussions of tidal mixing representations.

Volcanic eruptions exert a profound influence on tropical hydroclimate, including interannual variability associated with the El Niño–Southern Oscillation. This review synthesizes recent advances in understanding these impacts through an energy framework that links radiative forcing to shifts in the Intertropical Convergence Zone (ITCZ) and global monsoon systems. While global responses are robust in climate models, regional expressions remain uncertain due to biases and limitations in both simulations and paleoclimate reconstructions. Nonetheless, consistent patterns emerge—such as ITCZ displacement and reduced monsoon precipitation—that align with energetic theory, offering a physically grounded explanation for observed hydroclimatic anomalies following eruptions. This framework also provides a basis for estimating potential hydroclimate impacts of future volcanic events. ▪ Volcanic eruptions strongly influence tropical hydroclimate, but the mechanisms linking radiative forcing to large-scale responses are not fully understood. ▪ We assess evidence from climate models and paleoclimate reconstructions, noting robust global responses but regional differences due to model biases and data limitations. ▪ The energy framework provides a physically based foundation for anticipating tropical hydroclimate responses to future volcanic eruptions. ▪ Consistent patterns emerge such as ITCZ shifts and weakened monsoons, explained by energy theory and underpinned by observed posteruption climate anomalies.

In oligotrophic regions like the Mediterranean Sea, vertical nutrient distribution plays a key role in controlling primary production via upward fluxes. In spring 2023, the BioSWOT-Med campaign explored how fine-scale ocean dynamics modulate nutrient availability in the North Balearic Front. Combining SWOT satellite observations with targeted Lagrangian sampling, the study focused on a 50 km domain encompassing contrasting water masses at and across the front, and a fine-scale anticyclonic eddy embedded within southern waters. High-resolution nutrient profiles revealed a clear two-level structure: at the regional scale, northern waters showed shallower nutriclines and steeper vertical nutrient gradients; at finer scale, the anticyclonic eddy exhibited even deeper clines and weaker gradients compared to its surroundings. A local relation between SWOT sea surface height and the nutricline highlighted the coupling between physical fine-scale structures and nutrient dynamics, with further implications for un- derstanding nutrient supply and biogeochemical functioning in oligotrophic marine sys- tems.

Abstract Winter near-surface temperature biases over Arctic sea ice in climate models contribute to uncertainties in future sea ice and Arctic climate projections. Mitigating these biases requires proper evaluation and understanding of their origin. We focus on the 1980-2014 near-surface air temperature simulated in the atmosphere-only and coupled configurations of the IPSL-CM6A-LR climate model. Model outputs are evaluated against multiple observational datasets, including the ERA5, in situ measurements, and satellite-based remote sensing products. To account for the known warm bias in ERA5 near-surface air temperatures during Arctic winters, we applied a simple linear correction fitted on in situ observations over the entire Arctic perennial ice zone, which served as the reference data. Within all observational datasets, a winter warm bias over sea ice is evident in both model configurations: a slight +0.4 °C in the atmosphere-only IPSL-CM6A-LR and +3.1 °C in the fully coupled configuration. A pan-Arctic energy budget analysis attributes this warm bias primarily to excess poleward atmospheric heat transport. In the coupled model, the bias is further amplified by an underestimated summertime sea ice cover, despite a deficit in net oceanic heat transport. The methods developed here could be used in multi-model evaluations to progress understanding and reduce Arctic biases in climate models.

Climate intervention research is expanding as current mitigation efforts to limit warming below crucial targets are falling short. • Substantial knowledge gaps exist on the potential impacts of climate intervention strategies on marine ecological systems. • We review the potential impacts of climate intervention on marine ecosystems, including biotic and abiotic factors.

Abstract In many regions, extreme temperatures increase faster than background global warming due to complex atmospheric circulation changes. This has been overlooked in Antarctica. Here, we examine extreme temperatures recorded at 16 weather stations since 1950, and identify their changes and drivers. Over the Antarctic Peninsula and the South Pole, 10-year return level temperatures increased by up to 1.19°C per decade since 1980, outpacing mean temperature trends. Most extreme events, including statistically implausible or black swan events, were driven by intense atmospheric rivers linked to Rossby wave trains forced by tropical Pacific convection. In contrast, 10-year return level temperature decreased over most of East Antarctica, despite increasing atmospheric river activity, as the mean cooling muted the impacts of warm intrusion until recently. Our results highlight how the entire continent could become vulnerable in a warmer climate or enhanced tropical convective activity, as illustrated by the March 2022 heatwave.

ABSTRACT Argopecten is primarily distributed in the Caribbean Sea and Western Atlantic coasts but includes two species found in the Eastern Pacific: Argopecten purpuratus and Argopecten ventricosus. Previous evidence drawing upon morphological characters from both fossil and extant Argopecten species coupled with ecological information suggested a closer evolutionary relationship between A. purpuratus and the extant Atlantic species A. gibbus and A. nucleus. In previous research, the estimated divergence of Pacific and Atlantic lineages occurred around 7–6 million Ma. Molecular studies, however, proposed a different relationship, suggesting a closer evolutionary relationship between A. purpuratus and A. ventricosus than with their Atlantic counterparts. To assess this controversy, we re-evaluated the phylogenetic relationships and divergence times of A. purpuratus and A. ventricosus, using a combination of updated fossil age ranges and protein-coding gene sequences from pectinid mitogenomes. We found that the divergence between A. purpuratus and A. ventricosus may have occurred earlier than previously thought, c. 26.6 Ma, likely in the Central American Seaway. Coexistence of different marine environments influenced by the potential presence of cold and warm oceanic water masses during the late Oligocene may have caused this earlier divergence. Furthermore, the phylogenetic analysis dated the most recent common ancestor of A. purpuratus to 3.77 Ma, probably within the Southeast Pacific. The progressive decline in sea surface temperatures during the Plio-Pleistocene may have led to extant A. purpuratus tolerating cold waters. In addition, the climate variability related to the El Niño Southern Oscillation during the Pliocene and onwards likely contributed to promoting A. purpuratus adaptations to changing upwelling conditions. These adaptations include tolerance to variable temperatures, as well as to acidic and hypoxic conditions, traits that are essential for the survival of this species within the Humboldt Ecosystem.