Search Results

You are looking at 1 - 3 of 3 items for :

  • Author or Editor: Johan Nilsson x
  • Journal of Climate x
  • All content x
Clear All Modify Search
Johan Nilsson and Heiner Körnich

Abstract

A conceptual model of the salinity distribution in the oceanic Hadley cell is presented. The model pertains to the region of tropical easterly surface winds, where the surface salinity increases poleward from a local salinity minimum near the equator to a subtropical salinity maximum. A fundamental constraint is that the meridional freshwater transports in the atmosphere and the ocean have the same magnitude but opposite directions. A key assumption is that the strength of the meridional overturning cells in the atmosphere and the ocean is proportional and set by the surface layer Ekman transport. It is further assumed that, to the lowest order of approximation, the zonal-mean Ekman transports accomplish the meridional freshwater transports, that is, eddy fluxes and gyre-induced transports are ignored. The model predicts that the salinity variation in the oceanic cell is directly proportional to the specific humidity of the near-surface air, but independent of the meridional mass transport (as long as the atmospheric and oceanic mass transports remain proportional). If the relative humidity of the near-surface air is constant, the salinity variation in the oceanic Hadley cell varies essentially with the surface temperature according to the Clausius–Clapeyron expression for the saturation vapor pressure. Further, the model is compared to observations and a global warming simulation and found to give a leading-order description of the tropical surface salinity range.

Full access
Magnus Hieronymus, Jonas Nycander, Johan Nilsson, Kristofer Döös, and Robert Hallberg

Abstract

The role of oceanic background diapycnal diffusion for the equilibrium climate state is investigated in the global coupled climate model CM2G. Special emphasis is put on the oceanic meridional overturning and heat transport. Six runs with the model, differing only by their value of the background diffusivity, are run to steady state and the statistically steady integrations are compared. The diffusivity changes have large-scale impacts on many aspects of the climate system. Two examples are the volume-mean potential temperature, which increases by 3.6°C between the least and most diffusive runs, and the Antarctic sea ice extent, which decreases rapidly as the diffusivity increases. The overturning scaling with diffusivity is found to agree rather well with classical theoretical results for the upper but not for the lower cell. An alternative empirical scaling with the mixing energy is found to give good results for both cells. The oceanic meridional heat transport increases strongly with the diffusivity, an increase that can only partly be explained by increases in the meridional overturning. The increasing poleward oceanic heat transport is accompanied by a decrease in its atmospheric counterpart, which keeps the increase in the planetary energy transport small compared to that in the ocean.

Full access
Johan Nilsson, Peter L. Langen, David Ferreira, and John Marshall

Abstract

A coupled atmosphere–sea ice–ocean model is used in an aqua-planet setting to examine the role of the basin geometry for the climate and ocean circulation. The basin geometry has a present-day-like topology with two idealized northern basins and a circumpolar ocean in the south. A suite of experiments is described in which the southward extents of the two (gridpoint wide) “continents” and the basin widths have been varied. When the two basins have identical shapes, the coupled model can attain a symmetric climate state with northern deep-water formation in both basins as well as asymmetric states, where the deep-water formation occurs only in one of the basins and Atlantic–Pacific-like hydrographic differences develop. A difference in the southward extents of the land barriers can enhance as well as reduce the zonal asymmetries of the atmosphere–ocean circulation. This arises from an interplay between the basin boundaries and the wind-driven Sverdrup circulation, which controls the interbasin exchange of heat and salt. Remarkably, when the short “African” continent is located near or equatorward of the zero wind line in the Southern Hemisphere, the deep-water formation becomes uniquely localized to the “Atlantic”-like basin with the long western boundary. In this case, the salinification is accomplished primarily by a westward wind-routed interbasin salt transport. Furthermore, experiments using geometries with asymmetries in both continental extents and basin widths suggest that in the World Ocean these two fundamental basin asymmetries should independently be strong enough for uniquely localizing the Northern Hemisphere deep-water formation to the Atlantic Ocean.

Full access