Wind forcing of the Arctic and North Atlantic freshwater system

Oceanic processes in the Arctic and in the North Atlantic that play a key role in the global ocean circulation are often sensitive to density stratification of water, which is greatly shaped by salinity, or in another measure, by freshwater content. The freshwater budgets of these oceans are connected by currents that convey large volumes of water of different characteristics between one another. However, these budgets show spatial and temporal variations, and the fluxes between them cannot be considered constant either. The freshwater system of the Arctic linked to the North Atlantic is dynamic with changes and anomalies on different time scales, and the changes of this joint system seem to be in correlation with the evolution of atmospheric forcing patterns. Previous studies suggest the importance of wind stress forcing over key regions such as the Beaufort Sea or the Greenland Sea in influencing the distribution of freshwater. In this study we examine the sensitivity of freshwater distribution and fluxes between the Arctic and the North Atlantic oceans 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 the first results in investigating the role of atmospheric forcing in shaping freshwater reservoirs and exchanges between different oceanic subregions by comparing our model results using external wind stress forcing with the Modini-system, and fully coupled runs

Modeling the freshwater system of the Arctic and North Atlantic oceans

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

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

Drivers of freshwater distribution in the Arctic and Atlantic oceans

Oceanic circulation plays an important role in setting the climate of the Arctic and the Northern Atlantic regions. Currents conveying large volumes of water masses at various depths transport heat and salt to great distances, forming a global circulation system. In the Atlantic, the Meridional Overturning Circulation (MOC) is driven by exchanges of heat, freshwater and momentum with the atmosphere. Previous modeling studies suggest that the stability of the MOC is sensitive to different climate scenarios due to the sensitivity of the deepwater formation, a crucial component of the circulation to perturbations of freshwater content. Global climate models predict significant temperature rise in the future with larger trends at higher latitudes, and an enhanced hydrological cycle. Both of these trends act against the MOC, decreasing its strength by reducing meridional air temperature differences and freshening of ocean waters in key high latitude areas. The observed increase of the strength of the North Atlantic Oscillation (NAO) in recent decades, a trend that is predicted by many climate models to persist in the future, however, acts as a driver of the MOC. This duel of the evolution of fresh water fluxes and the development of the NAO is most likely going to define the strength of the MOC in the future. We examine the effects of different NAO scenarios using the Modini-system, a partially coupled spin-up that allows prescription of wind stresses for the ocean in the otherwise fully coupled Earth System Model of the Max Planck Institute. In our work we describe the processes affecting the circulation in more detail. We present our first results by investigating the role different wind stress patterns play in shaping fresh water reservoirs and exchanges between different subregions of the Arctic and the Atlantic Ocean

Sea-ice drag as a function of deformation and ice cover: Effects on simulated sea ice and ocean circulation in the Arctic

Many state-of-the-art coupled sea ice-ocean models use atmospheric and oceanic drag coefficients that are at best a function of the atmospheric stability but otherwise constant in time and space. Constant drag coefficients might lead to an incorrect representation of the ice-air and ice-ocean momentum exchange, since observations of turbulent fluxes imply high variability of drag coefficients. We compare three model runs, two with constant drag coefficients and one with drag coefficients varying as function of sea-ice characteristics. The computed drag coefficients range between 0.88 ×10−3 and 4.68 ×10−3 for the atmosphere, and between 1.28 ×10−3 and 13.68 ×10−3 for the ocean. They fall in the range of observed drag coefficients and illustrate the interplay of ice deformation and ice concentration in different seasons and regions. The introduction of variable drag coefficients improves the realism of the model simulation. In addition, using the average values of the variable drag coefficients improves simulations with constant drag coefficients. When drag coefficients depend on sea-ice characteristics, the average sea-ice drift speed in the Arctic basin increases from 6.22 cm s−1 to 6.64 cm s−1. This leads to a reduction of ice thickness in the entire Arctic and particularly in the Lincoln Sea with a mean value decreasing from 7.86 m to 6.62 m. Variable drag coefficients lead also to a deeper mixed layer in summer and to changes in surface salinity. Surface temperatures in the ocean are also affected by variable drag coefficients with differences of up to 0.06 °C in the East Siberian Sea. Small effects are visible in the ocean interio

Wind stress forcing in the Arctic and North Atlantic oceans

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

Climate response functions of the joint freshwater budget of the Arctic and North Atlantic oceans to changes in external wind forcing in an otherwise fully coupled earth system model

Ocean currents conveying large volumes of water can transport heat to great distances, through which they influence the climate. This is particularly true for the Arctic and the North Atlantic, the regions where water circulation has a significant impact on the atmosphere as well as on key oceanic processes. These processes are often sensitive to density stratification of ocean water, which is greatly shaped by salinity, or in another measure, by freshwater storage. Freshwater in the oceans is thus of particular importance. Being connected by a network of currents, the Arctic and North Atlantic oceans exchange a large volume of water of different characteristics. As a consequence, their freshwater budgets are also connected. However, these budgets show spatial and temporal variations, and the fluxes between them cannot be considered constant either. The freshwater system of the Arctic linked to the North Atlantic is dynamic with changes and anomalies on different time scales, and the changes of this joint system seem to follow the evolution of atmospheric forcing patterns. Previous modeling results suggest the importance of wind stress forcing over key regions such as the Beaufort Sea or the Greenland Sea in influencing the distribution of freshwater. In this study we examine the reaction of this linked freshwater system to changes in wind stress forcing through numerical experiments using 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. The aim of this work is to investigate the role of atmospheric forcing in shaping freshwater reservoirs and exchanges between different subregions of the Arctic and North Atlantic oceans by calculating and analyzing climate response functions to changes in wind forcing over key regions

The role of wind stress in the Arctic and North Atlantic freshwater covariability

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