Artic-North Atlantic interactions and multidecadal variability of the thermohaline circulation

Analyses of a 500-yr control integration with the non-flux-adjusted coupled atmosphere–sea ice–ocean model ECHAM5/Max-Planck-Institute Ocean Model (MPI-OM) show pronounced multidecadal fluctuations of the Atlantic overturning circulation and the associated meridional heat transport. The period of the oscillations is about 70–80 yr. The low-frequency variability of the meridional overturning circulation (MOC) contributes substantially to sea surface temperature and sea ice fluctuations in the North Atlantic. The strength of the overturning circulation is related to the convective activity in the deep-water formation regions, most notably the Labrador Sea, and the time-varying control on the freshwater export from the Arctic to the convection sites modulates the overturning circulation. The variability is sustained by an interplay between the storage and release of freshwater from the central Arctic and circulation changes in the Nordic Seas that are caused by variations in the Atlantic heat and salt transport. The relatively high resolution in the deep-water formation region and the Arctic Ocean suggests that a better representation of convective and frontal processes not only leads to an improvement in the mean state but also introduces new mechanisms determining multidecadal variability in large-scale ocean circulation

What causes the spread of model projections of ocean dynamic sea-level change in response to greenhouse gas forcing?

Sea levels of different atmosphere-ocean general circulation models (AOGCMs) respond to climate change forcing in different ways, representing a crucial uncertainty in climate change research. We isolate the role of the ocean dynamics in setting the spatial pattern of dynamic sea-level (zeta) change by forcing several AOGCMs with prescribed identical heat, momentum (wind) and freshwater flux perturbations. This method produces a zeta projection spread comparable in magnitude to the spread that results from greenhouse gas forcing, indicating that the differences in ocean model formulation are the cause, rather than diversity in surface flux change. The heat flux change drives most of the global pattern of zeta change, while the momentum and water flux changes cause locally confined features. North Atlantic heat uptake causes large temperature and salinity driven density changes, altering local ocean transport and zeta. The spread between AOGCMs here is caused largely by differences in their regional transport adjustment, which redistributes heat that was already in the ocean prior to perturbation. The geographic details of the zeta change in the North Atlantic are diverse across models, but the underlying dynamic change is similar. In contrast, the heat absorbed by the Southern Ocean does not strongly alter the vertically coherent circulation. The Arctic zeta change is dissimilar across models, owing to differences in passive heat uptake and circulation change. Only the Arctic is strongly affected by nonlinear interactions between the three air-sea flux changes, and these are model specific.Peer reviewe

OMIP contribution to CMIP6: experimental and diagnostic protocol for the physical component of the Ocean Model Intercomparison Project

The Ocean Model Intercomparison Project (OMIP) is an endorsed project in the Coupled Model Intercomparison Project Phase 6 (CMIP6). OMIP addresses CMIP6 science questions, investigating the origins and consequences of systematic model biases. It does so by providing a framework for evaluating (including assessment of systematic biases), understanding, and improving ocean, sea-ice, tracer, and biogeochemical components of climate and earth system models contributing to CMIP6. Among the WCRP Grand Challenges in climate science (GCs), OMIP primarily contributes to the regional sea level change and near-term (climate/decadal) prediction GCs. OMIP provides (a) an experimental protocol for global ocean/sea-ice models run with a prescribed atmospheric forcing; and (b) a protocol for ocean diagnostics to be saved as part of CMIP6. We focus here on the physical component of OMIP, with a companion paper (Orr et al., 2016) detailing methods for the inert chemistry and interactive biogeochemistry. The physical portion of the OMIP experimental protocol follows the interannual Coordinated Ocean-ice Reference Experiments (CORE-II). Since 2009, CORE-I (Normal Year Forcing) and CORE-II (Interannual Forcing) have become the standard methods to evaluate global ocean/sea-ice simulations and to examine mechanisms for forced ocean climate variability. The OMIP diagnostic protocol is relevant for any ocean model component of CMIP6, including the DECK (Diagnostic, Evaluation and Characterization of Klima experiments), historical simulations, FAFMIP (Flux Anomaly Forced MIP), C4MIP (Coupled Carbon Cycle Climate MIP), DAMIP (Detection and Attribution MIP), DCPP (Decadal Climate Prediction Project), ScenarioMIP, HighResMIP (High Resolution MIP), as well as the ocean/sea-ice OMIP simulations

Simulation of low-frequency climate variability in the North Atlantic Ocean and the Arctic

Low-frequency variability in large scale North Atlantic/Arctic properties like Meridional Overturning Circulation, heat transport, deep water formation, overflows, sea ice volume, thickness and extent, as well as the Arctic fresh water budget are studied by means of ensemble simulations with the global coupled ocean/sea ice model MPI-OM forced by realistic daily atmospheric forcing data from the NCEP/NCAR Reanalysis for the period 1948-2001. Major findings are that wintertime deep convection in the Labrador Sea is dominated by atmospheric forcing, in particular by the North Atlantic Oscillation. Intensified Labrador Sea convection induces substantial changes in the Labrador Sea Water (LSW) properties, in particular colder, fresher and denser LSW. The simulation links these changes to an increase in the Atlantic Meridional Overturning Circulation (MOC) strength. However, Labrador Sea deep convection is also strongly influenced by the presence of surface salinity anomalies, which originate from anomalous Fram Strait sea ice export events. These export events are shown to be mainly wind driven and are the most probable cause of the observed Great Salinity Anomalies of the 70th, 80th and 90th. In contrast to the Labrador Sea deep convection, the Greenland-Island-Norwegian (GIN) Sea deep convection shows a less clear imprint of the North Atlantic Oscillation variability. In the simulation, inter-annual to decadal variability in the Atlantic MOC circulation has its origin in the Labrador Sea, while longer term multi-decadal trends in the MOC are governed by the properties of the overflow waters from the GIN Sea. During the simulation period the strength of both overflows decreased, while the overflow water density increased. On one hand low-frequency variability of the Arctic sea ice volume is related to sea ice thickness changes, driven in equal parts through variability of atmospheric thermal and fresh water fluxes, and on the other hand through variability of the wind field. While there is a clear decrease of Arctic sea ice volume during the 1990s, there is no such trend present over the full simulation period. Arctic fresh water budget variability in the simulation is dominated by exports of sea ice via Fram Strait, while the sea ice exports are governed by variability of zonal planetary waves. Generally large parts of the observed low frequency variability in the North Atlantic/Arctic can be understood as a passive response of the ocean/sea ice system to variability of the large scale atmospheric forcing

Early detection of THC weakening: GCM and conceptual model simulations

Climate models show the possibility of abrupt climate changes caused by a collapse of the North Atlantic thermohaline circulation (THC). Strong THC fluctuations on interannual to interdecadal timescales and high erros in THC measurements hinder the detection of a possible THC slowdown.Our analysis shows that the temperature structure in the Atlantic Ocean can be a sensitive indicator identifying early THC weakening with a high signal-to-noise ratio. Simulations with the coupled atmosphere-ocean circulation model ECHAM5/MPI-OM emphasize the subsurface temperature signature in the Atlantic Ocean with its potential to trace THC changes. A part of this signature can be understood with the advective-diffusive balance which is confirmed in a stochastic low-order model of the Atlantic Ocean circulation. Finally, instrumental and proxy data are used to estimate THC fluctuations on decadal to multi-decadal time scales

Signal-to-noise ratios for the detection of ocean circulation changes

Our analysis shows that the temperature structure in the Atlantic Ocean can be a sensitive indicator identifying early Atlantic thermohaline circulation (THC) weakening with a high signal-to-noise ratio. Simulations with the coupled atmosphere-ocean circulation model ECHAM5/MPI-OM emphasize the subsurface temperature signature in the Atlantic Ocean with its potential to trace THC changes. A part of this signature can be understood with the advective-diffusive balance which is confirmed in a stochastic low-order model of the Atlantic Ocean circulation. It is shown that the integration of noise, which enters in a multiplicative way into the stochastic differential equation, is linked to the predictive skill of mid-depth temperatures for possible THC change

Arctic freshwater export and its impact on climate in the 20th and 21st. century

Climate simulations suggest that the emissions of carbon dioxide and other greenhouse gases will lead to strong climate changes in the 21st century. Here the resulting effects of the freshwater balance of the Arctic Ocean in the 21st century are analyzed using coupled Intergovernmental Panel on Climate Change simulations with the Max Planck Institute for Meteorology climate model. For the Arctic region, particularly strong warming and an almost complete removal of sea ice during summer time are predicted. Arctic river runoff and net atmospheric freshwater input ( P-E) are strongly enhanced. Most of this additional freshwater input is stored in the Arctic Ocean. While the total freshwater export out of the Arctic remains almost constant, significant changes occur in its distribution. The dominance of sea ice for the Fram Strait export disappears, while the liquid freshwater export is enhanced. The mean export shows therefore almost no changes, but its interannual variability is slightly reduced. In contrast, both the export through the Canadian Archipelago and its variability are increased in the 21st century. Therefore the importance of the Canadian Archipelago for the total Arctic export grows. Enhanced freshwater input into the Labrador Sea leads to a strong decrease in deep convection. Greenland Sea convection is reduced as well but mainly because of strong warming of the upper ocean layers. The meridional overturning circulation responds with a decline of about 6 sverdrups