Eddy-permitting simulations of the sub-polar North Atlantic: impact of the model bias on water mass properties and circulation |
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| Authors: | Jieshun Zhu Entcho Demirov Fred Dupont Daniel Wright |
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| Institution: | (1) Memorial University of Newfoundland, St. John’s, Newfoundland, Canada;(2) Dalhousie University, Halifax, Nova Scotia, Canada;(3) Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada; |
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| Abstract: | Some previous studies demonstrated that model bias has a strong impact on the quality of long-term prognostic model simulations
of the sub-polar North Atlantic Ocean. Relatively strong bias of water mass characteristics is observed in both eddy-permitting
and eddy-resolving simulations, suggesting that an increase of model resolution does not reduce significantly the model bias.
This study is an attempt to quantify the impact of model bias on the simulated water mass and circulation characteristics
in an eddy-permitting model of the sub-polar ocean. This is done through comparison of eddy-permitting prognostic model simulations
with the results from two other runs in which the bias is constrained by using spectral nudging. In the first run, the temperature
and salinity are nudged towards climatology in the whole column. In the second run, the spectral nudging is applied in the
surface 30 m layer and at depths below 560 m only. The biases of the model characteristics of the unconstrained run are similar
to those reported in previous eddy-permitting and eddy-resolving studies. The salinity in the surface and intermediate waters
of the Labrador Sea waters increases with respect to the climatology, which reduces the stability of the water column. The
deep convection in the unconstrained run is artificially intensified and the transport in the sub-polar gyre stronger than
in the observations. In particular, the transport of relatively salty and warm Irminger waters into the Labrador Sea is unrealistically
high. While the water mass temperature and salinity in the run with spectral nudging in the whole column are closest to the
observations, the depth of the winter convection is underestimated in the model. The water mass characteristics and water
transport in the run with spectral nudging in the surface and deep layers only are close to observations and at the same time
represent well the deep convection in terms of its intensity and position. The source of the bias in the prognostic model
run is discussed. |
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