Cornelia Köberle

The influence of numerical advection schemes on the results of ocean general circulation models

The dependence of results from coarse-resolution models of the North Atlantic circulation on the numerical advection algorithm is studied. In particular, the sensitivity of parameters relevant for climate simulations as e.g., meridional transport of mass and heat and main thermocline thickness is investigated. Three algorithms were considered: (a) a central difference scheme with constant values for horizontal and vertical diffusion, (b) an upstream scheme with no explicit diffusion, and (c) a flux-corrected transport (FCT) scheme with constant and strictly isopycnal diffusion. The temporal evolution of the three models on time scales of centuries is markedly different, the upstream scheme resulting in much shorter adjustment time whereas the central difference scheme is slower and controlled by vertical diffusion rather than advection. In the steady state, the main thermocline structure is much less diffusive in the FCT calculation which also has much lower heat transport. Both horizontal circulation and overturning in the meridional-vertical plane are strongest in the upstream-model. The results are discussed in terms of the effective vertical (diapycnal) mixing in the different models. A significant increase in vertical resolution would be required to eliminate the high sensitivity due to the numerical algorithms, and allow physically motivated mixing formulations to become effective.

Mechanisms Determining the Variability of Arctic Sea Ice Conditions and Export

Abstract In an ocean–sea ice model of the Arctic and the northern North Atlantic driven with 50-yr NCEP–NCAR reanalysis data, no appreciable trend in sea ice volume is found for the period 1948–98. However, rather long subperiods, for example, 1965–95, exhibit a large decline in Arctic sea ice volume. These results and the current data situation make connecting “global warming” to Arctic ice thinning very difficult because the large decadal and multidecadal variability masks any trend. Thermal and wind effects linearly contribute to the total sea ice volume variability. Wind stress forcing significantly contributes to the decadal variability in the Arctic ice volume, affecting both thermodynamic growth and the ice export rate. Ice export events are triggered by enhanced cyclonic wind stress over the eastern Arctic. However, large ice export events depend to a similar degree on the presence of thick ice that is generated in a preceding accumulation phase and do not depend on the local wind conditions aroun...

Effects of Increased Horizontal Resolution in a Simulation of the North Atlantic Ocean

Global mean and eddy fields from a four-year experiment with a 1/6° × 1/5° horizontal resolution implementation of the CME North Atlantic model are presented. The time-averaged wind-driven and thermohaline circulation in the model is compared to the results of a 1/3° × 2/5° model run in very similar configuration. In general, the higher resolution results are found to confirm that the resolution of previous CME experiments is sufficient to describe many features of the large-scale circulation and water mass distribution quite well. While the increased resolution does not lead to large changes in the mean flow patterns, the variability in the model is enhanced significantly. On the other hand, however, not all aspects of the circulation have improved with resolution. The Azores Current Frontal Zone with its variability in the eastern basin is still represented very poorly. Particular attention is also directed toward the unrealistic stationary anticyclones north of Cape Hatteras and in the Gulf of Mexico.

Variability of Arctic and North Atlantic sea ice: A combined analysis of model results and observations from 1978 to 2001

Ice cover data simulated by a coupled sea ice-oceanmodel of the North Atlantic and the Arctic Ocean are compared withsatellite observations for the period 1978 to 2001. The capability ofthe model in reproducing the long-term mean state and the inter-seasonalvariability is demonstrated. The main modes of variability of thesatellite data and the simulation in the summer and winter half yearsare highly similar.Using NCEP/NCAR reanalysis data and the results from the sea ice-oceanmodel, we describe the relationship with atmospheric and oceanicvariables for the first two modes of sea-ice concentration variabilityin winter and in summer. The first winter mode shows a time delayedresponse to the Arctic Oscillation due to advection of heatanomalies in the ocean. The second winter mode is dominated by anevent in the late 1990s that is characterized by anomalously highpressure over the eastern Arctic. The first summer mode isstrongly influenced by the Arctic Oscillation of the previouswinter. The second summer mode is caused by anomalous air temperaturein the Arctic. This mode shows a distinctive trend and is related to anice extent reduction of about 4 10^5 km^2 over the 23 years ofanalysis.

On the Influence of DSOW in a Numerical Model of the North Atlantic General Circulation

Abstract Two numerical experiments regarding the North Atlantic circulation are compared. The two experiments are initialized with climatological temperatures and salinities and are integrated for ten years with different surface boundary conditions in the lceland Sea. One case climatological wind stress, SST, and surface salinity to force the model. It reproduces the Atlantic circulation found in similar studies. A change in surface boundary values In the Iceland Sea in order to improve the Denmark Strait Overflow water properties in the second case results in major changes in circulation and hydrography. The meridional overturning more than doubles and horizontal gyres are intensified. Recirculation cells occur north and south of the Gulf Stream. The Gulf Stream separates from the North American coast in a more realistic manner, and its further course is in better agreement with observations than in the reference run. The subsurface temperature distribution differs by several kelvin in the subpolar and ...

Simulated variability of the arctic ocean freshwater balance 1948-2001

Abstract The Arctic Ocean freshwater balance over the period 1948–2001 is examined using results from a hindcast simulation with an ocean–sea ice model of the Atlantic and Arctic Oceans. Atmospheric forcing is taken from the NCEP–NCAR reanalysis and different terrestrial freshwater sources as well as the Bering Strait throughflow are specified as constant seasonal cycles. The long-term variability of the Arctic Ocean liquid freshwater content is determined by the variability of lateral exchanges with the subpolar seas. Surface freshwater flux variability is dominated by the thermodynamic growth of sea ice. This component of the freshwater balance has larger variability at interannual frequencies. The Arctic Ocean liquid freshwater content was at a maximum in the middle of the 1960s. Extremely low liquid freshwater export through Fram Strait caused this maximum in the freshwater content. The low export rate was related to weak volume transports in the East Greenland Current. Low volume transports were forc...

Validating satellite derived and modelled sea-ice drift in the Laptev Sea with in situ measurements from the winter of 2007/2008

A correct representation of the ice movement in an Arctic sea-ice–ocean coupled model is essential for a realistic sea-ice and ocean simulation. The aim of this study is to validate the observational and simulated sea-ice drift for the Laptev Sea Shelf region with in situ measurements from the winter of 2007/08. Several satellite remote-sensing data sets are first compared to mooring measurements and afterwards to the sea-ice drift simulated by the coupled sea-ice–ocean model. The different satellite products have a correlation to the in situ data ranging from 0.56 to 0.86. The correlations of sea-ice direction or individual drift vector components between the in situ data and the observations are high, about 0.8. Similar correlations are achieved by the model simulations. The sea-ice drift speed derived from the model and from some satellite products have only moderate correlations of about 0.6 to the in situ record. The standard errors for the satellite products and model simulations drift components are similar to the errors of the satellite products in the central Arctic and are about 0.03 m/s. The fast-ice parameterization implementation in the model was also successfully tested for its influence on the sea-ice drift. In contrast to the satellite products, the model drift simulations have a full temporal and spatial coverage and results are reliable enough to use as sea-ice drift estimates on the Laptev Sea Shelf.

Simulated history of convection in the Greenland and Labrador seas 1948-2001

Convection in the Greenland and Labrador seas is compared using a sea-ice-ocean model forced with NCEP reanalysis atmospheric data for the period 1948-2001. Model-derived convection rates for the Greenland Sea and the Labrador Sea show good agreement with previous estimates. Composites based on convection indices are used to identify important forcing processes and the relationship to oceanic, atmospheric, and sea-ice fields. Convection in the Labrador Sea is dominated by large-scale atmospheric forcing, especially by the heat fluxes associated with the North Atlantic Oscillation (NAO). On the other hand, we find no robust correlation between Greenland Sea convection and the NAO. In the model, local sea-ice formation, wind direction, and associated sea-ice drift are important parameters affecting convection in the Greenland Sea.

Simulating the long term variability of liquid freshwater export from the Arctic Ocean

The fresh water export from the Arctic has not been measured yet. The major problem lies in the transport over the shallow East Greenland shelf that is not easily accessible for oceanographic vessels and so far has been off-limits for moored instrumentation. Even if we would be able to start measurements now, we would have no statistics to evaluate trends and natural variability of the transport. For long time series and for predictions of future changes, there is no other means than numerical models of the oceanic circulation and the water mass distribution. For past times, models can perhaps be combined with observations of different variables to yield better reconstructions of long-term variability in fresh water fluxes between the Arctic and the sub-polar North Atlantic. The liquid fresh water export from the Arctic Ocean through the passages of the Canadian Archipelago, Fram Strait and the Barents Sea is constrained by the fresh water fluxes entering the Arctic Ocean and by changes in the fresh water contents in the Arctic halocline. If one knew the fluxes entering the Arctic Ocean and the changes in the salinity very precisely, the export rates could be determined as a residual. (We use this technique to derive export rates in a coupled climate model in Section 17.5.) Different components of the Arctic Ocean fresh water balance exhibit very different long-term variability. Serreze et al. (2006) provide a recent compilation of estimates of the interannual variability of river discharge, net precipitation, Bering Strait inflow, and Fram Strait ice flux. Fram Strait ice transport shows by far the largest standard deviation of these fresh water fluxes. River run-off into the Arctic Ocean has increased over the last 50 years by approximately 5% (Peterson et al. 2002). Interannual variability as shown by Peterson et al. is of similar or smaller magnitude. Compared to fluctuations in other components of the fresh water balance, this is a small variability. The variability in river discharge is also indicative of the variability of the total atmospheric moisture convergence at

Mechanisms for Spreading of Mediterranean Water in Coarse-Resolution Numerical Models*

Different processes have been proposed to explain the large-scale spreading of Mediterranean Water (MW) in the North Atlantic, however, no systematic study comparing the efficiency of different processes is yet available. Here, the authors present a series of experiments in a unified framework that is designed to quantify the effects of several physical processes on the spreading of MW in an idealized model of the North Atlantic. The common technique of restoring temperature and salinity to an observed distribution near the Mediterranean inflow fails to produce an adequate amount of MW because the eastern boundary region near the MW inflow is rather quiescent in models. Diapycnal processes like double diffusion and cabbeling turn out too inefficient to alone account for the large-scale MW anomaly. However, with a preexisting anomaly, double diffusion leads to a considerable northward and zonal redistribution of MW. The density anomaly induced by cabbeling curtails the zonal spreading of MW while it increases the northward spreading. With isopycnal mixing and the weak mean flow that prevails in the outflow region, a spatial distribution of the MW anomaly is obtained that is inconsistent with observations. Unrealistically high diffusion coefficients would be necessary to reproduce the observed salt flux into the Atlantic. The most effective process in the experiments is the volume flux associated with the Atlantic–Mediterranean exchange. The current system that is established in response to the inflow of MW into the Atlantic carries the anomaly almost 30° of longitude into the basin and along the eastern margin up to the northeastern corner of the domain and farther along the northern boundary.