Description: Observations from recent decades show significant salinity anomalies in the Arctic and the subpolar North Atlantic oceans. The evolution of their freshwater budgets has been the focus of many studies, most of which suggest a link between them. However, the nature and the significance of this link is still disputed, as are the driving forces behind it. Our aim was to perform a series of numerical simulations of the freshwater system of the Arctic and the subpolar North Atlantic oceans and to assess the role of wind stress in shaping it. For this we used the Max Planck Institute Earth System Model and ran model experiments in a partially coupled configuration applying the so called Modini-method with prescribed wind forcing. We constructed idealized scenarios of wind stress forcing associated with large-scale patterns of observed atmospheric variability. We present our results from scenarios representing prolonged positive or negative states of the AO/NAO. We also analyze the response to a sudden change from one state to another with particular focus on the Arctic and the North Atlantic freshwater reservoirs and the fluxes between them. This enables us to simulate the high freshwater content observed in the Beaufort Gyre concurrent with an unusually persistent anticyclonic wind pattern in the Arctic in recent years, and to study the effect of large-scale circulation shifts on Arctic freshwater export and thus salinity anomalies in the subpolar North Atlantic Ocean.

Description: Any use of observational data for data assimilation requires adequate information of their representativeness in space and time. This is particularly important for sparse, non-synoptic data, which comprise the bulk of oceanic in situ observations in the Arctic. To quantify spatial and temporal scales of temperature and salinity variations, we estimate the autocorrelation function and associated decorrelation scales for the Amerasian Basin of the Arctic Ocean. For this purpose, we compile historical measurements from 1980 to 2015. Assuming spatial and temporal homogeneity of the decorrelation scale in the basin interior (abyssal plain area), we calculate autocorrelations as a function of spatial distance and temporal lag. The examination of the functional form of autocorrelation in each depth range reveals that the autocorrelation is well described by a Gaussian function in space and time. We derive decorrelation scales of 150–200 km in space and 100–300 days in time. These scales are directly applicable to quantify the representation error, which is essential for use of ocean in situ measurements in data assimilation. We also describe how the estimated autocorrelation function and decorrelation scale should be applied for cost function calculation in a data assimilation system.

Description: high-frequency temporal variability (HFV) and meso-scale spatial variability (MSV) of winter sea-ice drift in the Southern Ocean simulated with a global high-resolution (0.1°) sea ice-ocean model. Hourly model output is used to distinguish MSV characteristics via patterns of mean kinetic energy (MKE) and turbulent kinetic energy (TKE) of ice drift, surface currents, and wind stress, and HFV characteristics via time series of raw variables and correlations.We find that (1) along the ice edge, the MSVof ice drift coincides with that of surface currents, in particular such due to ocean eddies; (2) along the coast, theMKE of ice drift is substantially larger than its TKE and coincides with the MKE of wind stress; (3) in the interior of the ice pack, the TKE of ice drift is larger than its MKE, mostly following the TKE pattern of wind stress; (4) the HFVof ice drift is dominated by weather events, and, in the absence of tidal currents, locally and to a much smaller degree by inertial oscillations; (5) along the ice edge, the curl of the ice drift is highly correlated with that of surface currents, mostly reflecting the impact of ocean eddies. Where ocean eddies occur and the ice is relatively thin, ice velocity is characterized by enhanced relative vorticity, largely matching that of surface currents. Along the ice edge, ocean eddies produce distinct ice filaments, the realism of which is largely confirmed by high-resolution satellite passive-microwave data.

Description: One of the key processes responsible for driving the circulation of ocean waters is the wind stress. This important air-sea interaction stands for the imparting of atmospheric momentum to the ocean. The prevailing wind patterns largely influence the velocity in the top Ekman layer in the ocean, sustaining the observed system of surface currents. Given the internal variability of the wind climate, these surface currents are subject to anomalies in space and time that can have large scale effects on oceanic processes. This is particularly true in the Arctic and the subpolar North Atlantic oceans that play a key role in the global ocean circulation, and are influenced by variations of wind stress forcing associated with large scale atmospheric modes in these regions. In this study we examine the sensitivity of surface currents, ice cover, freshwater and heat content in these ocean basins to wind stress forcing through numerical experiments. The tool for this is the Modini-system, a partial coupling technique that allows flexible experiments with prescribed wind stress fields for the ocean in the otherwise fully coupled Earth System Model of the Max Planck Institute. In this work we present our results investigating the role of wind stress forcing in shaping the distribution and exchanges of state variables in and between the Arctic and North Atlantic oceans by comparing our model results using external wind stress forcing with the Modini-system, and fully coupled runs.

Description: Observations from recent decades show significant salinity anomalies in the Arctic and the Subpolar North Atlantic oceans. The evolution of their freshwater budgets has been the focus of many studies, most of which suggest a link between them. Despite these efforts, the nature and the significance of this link is still disputed, and so are the driving forces behind it. Our aim was to simulate the freshwater system of the Arctic and the Subpolar North Atlantic oceans and to assess the role of wind stress in shaping it. For this we used the Max Planck Institute Earty System Model and ran model experiments in its original fully coupled configuration, and in the partially coupled configuration of the so called Modini-method with prescribed wind forcing. We analyzed the evolution of the distribution and the fluxes within this freshwater system and compared our results between model configurations. Our results showed that although there is a significant bias in modeled freshwater content (overestimation in the Arctic Ocean, underestimation in the Nordic Seas and in the Subpolar North Atlantic Ocean in comparison with observational data), anomalies in recent decades are similar to those derived from observations. The bias is somewhat reduced and the anticorrelation between the freshwater content of the Arctic and the Subpolar North Atlantic is also higher in the partially coupled runs with prescribed wind forcing. We suggest that this improvement is due to the role of wind stress in shaping their freshwater reservoirs.

Description: The Weddell Gyre plays a role in the climate system by advecting heat poleward to the Antarctic ice shelves and by regulating the density of water masses that feed the lowest limb of the global ocean overturning circulation. Warm Deep Water is the water mass that delivers heat to the Weddell Gyre, which is subject to complex modification processes to form Weddell Sea Deep and Bottom Water, providing an important sink for atmospheric heat and carbon dioxide. A fleet of Argo floats drifting in the currents of the upper 2000 m of the water column have recently provided data covering more or less the entire Weddell Gyre region in the horizontal, and Warm Deep Water in the vertical. Argo float data thus provide an opportunity to investigate surface to mid-depth ocean dynamics by providing both hydrographic and trajectory data. Here, Argo float profile and trajectory data are exploited in order to produce a full gyre scale view of the Weddell Gyre’s circulation from a purely observation-based dataset, whose pertinent features include the double-cell structure, with a stronger eastern core that intensifies with depth, and a weaker western core that remains invariant with depth. Considerable recirculation occurs within the gyre interior about the eastern core, before the water is able to fully traverse the zonal extent of the gyre, which causes considerable uncertainty when determining the gyre strength using single transects as is typical with ship-based surveys. Further investigation of the causes, temporal stability and the physical consequences of the recirculation cells along the central gyre axis, is required if we are to fully identify and understand the ways in which the Weddell Gyre may be changing in a rapidly changing climate.

Description: The Weddell Gyre plays a fundamental role in the climate system by advecting heat poleward to the Antarctic ice shelves and by regulating the density of water masses that feed the lowest limb of the global overturning circulation. Profile and trajectory data between 2002 and 2016 from a fleet of Argo floats between 50 and 2000 m are exploited in order to produce a full gyre scale view of the Weddell Gyre's circulation. The data exhibit a gradual cooling of the Warm Deep Water (WDW) as it circulates cyclonically around the gyre. A double-cell structure of the Weddell Gyre is revealed, with a stronger eastern core that intensifies with depth, and a weaker western core that remains invariant with depth. The deep outflow of Weddell Sea Deep and Bottom Water (WSDW, WSBW) at the western boundary of the gyre, formed from WDW by complex modification processes involving sea ice formation, and basal ice shelf melting, is not covered by the float observations. Using mooring array-based observations near the tip of the Antarctic Peninsula between 1989 and 1998, and between 2005 and 2014, a cooling of the WSBW plume is revealed over the observational period. This is in striking contrast to the WSDW in the interior Weddell Sea which has been undergoing a decadal warming. While the cause of the cooling of the WSBW plume is currently unclear, the mooring-based velocity observations indicate that the volume transport of the WSBW plume of 2.5 Sv has been stable over time.

Description: An automatic parameter optimization system for a coupled ocean-sea ice model is applied to investigate uniqueness of parameter set obtained from data assimilation. We set up a parameter optimization experiment, in which 15 model parameters are optimized simultaneously using a 23-years optimization window. A series of 11 independent experiments are conducted to examine spread of objective functions, optimized sea ice fields and associated optimal parameters. The result shows sufficiently small spreads of objective functions and ice fields, whereas a significantly large spread of optimized parameters. This indicates the system gives an unique solution regarding the simulated ice fields, whereas multiple solutions regarding the associated model parameters. A correlation analysis shows the optimal parameters are inter-related and covariant. A principal component analysis (PCA) reveals that the first 3 principal components explain 70% of the variation of the optimal parameter sets, indicating a contraction of the model parameter space.