Deep mixed ocean volume in the Labrador Sea in HighResMIP models

Simulations from seven global coupled climate models performed at high and standard resolution as part of the high resolution model intercomparison project (HighResMIP) are analyzed to study deep ocean mixing in the Labrador Sea and the impact of increased horizontal resolution. The representation of convection varies strongly among models. Compared to observations from ARGO-floats and the EN4 data set, most models substantially overestimate deep convection in the Labrador Sea. In four out of five models, all four using the NEMO-ocean model, increasing the ocean resolution from 1° to 1/4° leads to increased deep mixing in the Labrador Sea. Increasing the atmospheric resolution has a smaller effect than increasing the ocean resolution. Simulated convection in the Labrador Sea is mainly governed by the release of heat from the ocean to the atmosphere and by the vertical stratification of the water masses in the Labrador Sea in late autumn. Models with stronger sub-polar gyre circulation have generally higher surface salinity in the Labrador Sea and a deeper convection. While the high-resolution models show more realistic ocean stratification in the Labrador Sea than the standard resolution models, they generally overestimate the convection. The results indicate that the representation of sub-grid scale mixing processes might be imperfect in the models and contribute to the biases in deep convection. Since in more than half of the models, the Labrador Sea convection is important for the Atlantic Meridional Overturning Circulation (AMOC), this raises questions about the future behavior of the AMOC in the models

Deep mixed ocean volume in the Labrador Sea in HighResMIP models

Simulations from seven global coupled climate models performed at high and standard resolution as part of the high resolution model intercomparison project (HighResMIP) are analyzed to study deep ocean mixing in the Labrador Sea and the impact of increased horizontal resolution. The representation of convection varies strongly among models. Compared to observations from ARGO-floats and the EN4 data set, most models substantially overestimate deep convection in the Labrador Sea. In four out of five models, all four using the NEMO-ocean model, increasing the ocean resolution from 1° to 1/4° leads to increased deep mixing in the Labrador Sea. Increasing the atmospheric resolution has a smaller effect than increasing the ocean resolution. Simulated convection in the Labrador Sea is mainly governed by the release of heat from the ocean to the atmosphere and by the vertical stratification of the water masses in the Labrador Sea in late autumn. Models with stronger sub-polar gyre circulation have generally higher surface salinity in the Labrador Sea and a deeper convection. While the high-resolution models show more realistic ocean stratification in the Labrador Sea than the standard resolution models, they generally overestimate the convection. The results indicate that the representation of sub-grid scale mixing processes might be imperfect in the models and contribute to the biases in deep convection. Since in more than half of the models, the Labrador Sea convection is important for the Atlantic Meridional Overturning Circulation (AMOC), this raises questions about the future behavior of the AMOC in the models.This work has been funded by the PRIMAVERA project, which is funded by the European Union's Horizon 2020 programme, Grant Agreement No. 641727PRIMAVERA. D. V. Sein was also supported by the state assignment of the Ministry of Science and Higher Education of Russia (theme No. 0128-2021-0014). PO was supported by the Spanish Ministry of Economy, Industry and Competiveness through the Ramon y Cajal grant (RYC-2017-22772). The global ocean heat flux and evaporation products were provided by the WHOI OAFlux project (http://oaflux.whoi.edu) funded by the NOAA Climate Observations and Monitoring (COM) program.Peer Reviewed"Article signat per 12 autors/es: Torben Koenigk, Ramon Fuentes-Franco, Virna L. Meccia, Oliver Gutjahr, Laura C. Jackson, Adrian L. New, Pablo Ortega, Christopher D. Roberts, Malcolm J. Roberts, Thomas Arsouze, Doroteaciro Iovino, Marie-Pierre Moine & Dmitry V. Sein "Postprint (published version

The potential role of wind variability on plankton retention in the Río de la Plata Estuary: a numerical study

The Río de la Plata is one of the most important estuarine systems of the world. It drains the waters of the Paraná and Uruguay rivers, which constitute the second largest basin of South America. As a result, it has a huge discharge with a mean of around 25,000 m3 s-1, and maximum values as high as 50,000 m3 s-1 under extreme conditions. Water stratification is controlled by the confluence of high buoyancy continental discharge advecting offshore, lying on denser shelf waters that intrude into the estuary as a topographically controlled wedge, typically between 100 and 250 km long. The upstream reach of the wedge defines a bottom salinity front, located over a submersed bar named Barra del Indio shoal following the 10 m isobath. Several authors have pointed out the importance of this bottom salinity front in structuring plankton communities as well as in the development of spawning grounds for several coastal fishes. It has been suggested that observed patterns result from retention processes associated to the bottom salinity front. However neither the occurrence of this retention nor the physical and/or behavioral mechanisms involved have been well studied. It has been classically thought that retention would be a natural consequence of the theoretical circulation associated to a salt wedge: salty water incoming from the bottom would push plankton upstream to the bottom salinity front. Nevertheless, several recent papers based on numerical simulations and ADCP current observations have shown that estuarine circulation does not necessarily follow this pattern and, moreover, that it is highly variable and essentially wind dominated. These papers indicate that estuarine scales of variability replicate atmospheric ones, and that currents response to changes in the wind field is very fast, occurring in a lapse of around 6 hours. They also demonstrated that estuarine response to winds can be explained in terms of two modes, both implying currents with a phase lag with respect to winds that depends on the location in the estuary, but that in any case, produce a response in the bottom layer. As a result of the described features, in the time scales relevant to biota, the Río de la Plata would display weather and climate, as the atmosphere does. Atmospheric circulation in the region is characterized in synoptic to intra-seasonal scales by a high variability. As winds in the region rarely blow from the same direction for more than a few days, currents present the same feature. A striking question is, then, whether this highly variable system can favor retention and if this is the fact, what are the involved mechanisms. The aim of this paper is to explore this matter. A set of process oriented numerical experiments in which neutral particles are released along the frontal zone of the Río de la Plata and its vicinity and tracked for different wind conditions in short time scales is conducted. Results of numerical experiments are complemented with an analysis of local wind statistics. As a result a possible mechanism for plankton retention is suggested.Pages: 1383-139

Internal multi-centennial variability of the Atlantic Meridional Overturning Circulation simulated by EC-Earth3

We report a multi-centennial oscillation of the Atlantic Meridional Overturning Circulation (AMOC) simulated by the ECEarth3 climate model under the pre-industrial climate. This oscillation has an amplitude of ~6 Sv and a period of ~150 years and signifcantly impacts the atmosphere. We fnd that it is a self-sustained low-frequency internal variability, driven by the accumulation of salinity anomalies in the Arctic and their release into the North Atlantic, afecting the water column stability and the deep convection. Sea ice plays a major role in creating the salinity anomaly in the Arctic, while the anomalous Arctic oceanic circulation, which drives the exchange of liquid freshwater between the Arctic and the open ocean, is the main responsible for its southward propagation. Interestingly, EC-Earth3 simulations with increased greenhouse concentrations, and therefore under a warmer climate, do not exhibit these strong AMOC fuctuations. We hypothesize that in a quasi-equilibrium climate with a global air surface temperature 4.5° higher than the pre-industrial period, the low amount of sea ice in the high latitudes of the North Atlantic is no longer able to trigger the mechanism

A Novel Initialization Technique for Decadal Climate Predictions

International audienceModel initialization is a matter of transferring the observed information available at the start of a forecast to the model. An optimal initialization is generally recognized to be able to improve climate predictions up to a few years ahead. However, systematic errors in models make the initialization process challenging. When the observed information is transferred to the model at the initialization time, the discrepancy between the observed and model mean climate causes the drift of the prediction toward the model-biased attractor. Although such drifts can be generally accounted for with a posteriori bias correction techniques, the bias evolving along the prediction might affect the variability that we aim at predicting, and disentangling the small magnitude of the climate signal from the initial drift to be removed represents a challenge. In this study, we present an innovative initialization technique that aims at reducing the initial drift by performing a quantile matching between the observed state at the initialization time and the model state distribution. The adjusted initial state belongs to the model attractor and the observed variability amplitude is scaled toward the model one. Multi-annual climate predictions integrated for 5 years and run with the EC-Earth3 Global Coupled Model have been initialized with this novel methodology, and their prediction skill has been compared with the non-initialized historical simulations from CMIP6 and with the same decadal prediction system but based on full-field initialization. We perform a skill assessment of the surface temperature, the heat content in the ocean upper layers, the sea level pressure, and the barotropic ocean circulation. The added value of the quantile matching initialization is shown in the North Atlantic subpolar region and over the North Pacific surface temperature as well as for the ocean heat content up to 5 years. Improvements are also found in the predictive skill of the Atlantic Meridional Overturning Circulation and the barotropic stream function in the Labrador Sea throughout the 5 forecast years when compared to the full field method

Understanding AMOC stability: the North Atlantic Hosing Model Intercomparison Project

International audienceAbstract. The Atlantic meridional overturning circulation (AMOC) is an important part of our climate system. The AMOC is predicted to weaken under climate change; however, theories suggest that it may have a tipping point beyond which recovery is difficult, hence showing quasi-irreversibility (hysteresis). Although hysteresis has been seen in simple models, it has been difficult to demonstrate in comprehensive global climate models. Here, we outline a set of experiments designed to explore AMOC hysteresis and sensitivity to additional freshwater input as part of the North Atlantic Hosing Model Intercomparison Project (NAHosMIP). These experiments include adding additional freshwater (hosing) for a fixed length of time to examine the rate and mechanisms of AMOC weakening and whether the AMOC subsequently recovers once hosing stops. Initial results are shown from eight climate models participating in the Sixth Coupled Model Intercomparison Project (CMIP6). The AMOC weakens in all models as a result of the freshening, but once the freshening ceases, the AMOC recovers in half of the models, and in the other half it stays in a weakened state. The difference in model behaviour cannot be explained by the ocean model resolution or type nor by details of subgrid-scale parameterisations. Likewise, it cannot be explained by previously proposed properties of the mean climate state such as the strength of the salinity advection feedback. Instead, the AMOC recovery is determined by the climate state reached when hosing stops, with those experiments where the AMOC is weakest not experiencing a recovery