Characteristics of the Nordic Seas overflows in a set of Norwegian Earth System Model experiments


GUO C., Ilicak M. , BENTSEN M., FER I.

OCEAN MODELLING, vol.104, pp.112-128, 2016 (Journal Indexed in SCI) identifier identifier

  • Publication Type: Article / Article
  • Volume: 104
  • Publication Date: 2016
  • Doi Number: 10.1016/j.ocemod.2016.06.004
  • Title of Journal : OCEAN MODELLING
  • Page Numbers: pp.112-128

Abstract

Global ocean models with an isopycnic vertical coordinate are advantageous in representing overflows, as they do not suffer from topography-induced spurious numerical mixing commonly seen in geopotential coordinate models. In this paper, we present a quantitative diagnosis of the Nordic Seas overflows in four configurations of the Norwegian Earth System Model (NorESM) family that features an isopycnic ocean model. For intercomparison, two coupled ocean-sea ice and two fully coupled (atmosphere-land-ocean-sea ice) experiments are considered. Each pair consists of a (non-eddying) 1 degrees and a (eddy-permitting) 1/4 degrees horizontal resolution ocean model. In all experiments, overflow waters remain dense and descend to the deep basins, entraining ambient water en route. Results from the 1/4 degrees pair show similar behavior in the overflows, whereas the 1 degrees pair show distinct differences, including temperature/salinity properties, volume transport (Q), and large scale features such as the strength of the Atlantic Meridional Overturning Circulation (AMOC). The volume transport of the overflows and degree of entrainment are underestimated in the 1 degrees experiments, whereas in the 1/4 degrees experiments, there is a two-fold downstream increase in Q, which matches observations well. In contrast to the 1/4 degrees experiments, the coarse 1 degrees experiments do not capture the inclined isopycnals of the overflows or the western boundary current off the Flemish Cap. In all experiments, the pathway of the Iceland-Scotland Overflow Water is misrepresented: a major fraction of the overflow proceeds southward into the West European Basin, instead of turning westward into the Irminger Sea. This discrepancy is attributed to excessive production of Labrador Sea Water in the model. The mean state and variability of the Nordic Seas overflows have significant consequences on the response of the AMOC, hence their correct representations are of vital importance in global ocean and climate modelling. (C) 2016 Elsevier Ltd. All rights reserved.