The Atlantic meridional overturning circulation and meridional heat transport (hereafter the AMOC and MHT) at 34 degrees S as simulated by global 1/16 degrees eddy-rich (henceforth GLOB16) and 1/4 degrees eddy-permitting (GLOB4) models are compared with observational estimates. Three different methods are used for calculating the modeled AMOC: the first method (MOCmod) is based on simulated velocity fields, while the second (MOCob) relies on the same assumptions as available observed-derived estimates. The third method (MOCob2) is also based on hydrostatic and geostrophic relationships, but relative to a barotropic circulation instead of the definition of velocity at a specific reference depth. All methods correctly reproduce the time-mean GLOB16 AMOC strength, but the value of the non-Ekman component of the GLOB16 AMOC is only about 75% of the observed-derived estimate. The GLOB16 MHT is also significantly less than observation value (slightly more than 60% of the observed). However, the mean AMOC and MHT values at 34 degrees S obtained with coarser resolution GLOB4 model are comparable with the observed-derived estimates. Possible causes for the differences between the eddy-rich model and observational data are studied. It is shown that the density field from the eddy-rich model has high temporal variability along 34 degrees S with spatial scale of about two hundred km that can be due to mesoscale variations, caused by Agulhas "leakage.'' This results in the decrease of the mean meridional geostrophic velocity, which leads to smaller values of the AMOC and MHT in GLOB16: subsampling GLOB16 density on 1/4 degrees or 1/2 degrees longitude grid along 34 degrees S for MOCob calculation significantly increases the AMOC values. The findings in this paper provide guidance in understanding AMOC and MHT dissimilarities between observation-based estimates and eddy-rich ocean models at 34 degrees S.
The impact of horizontal resolution of density field on the calculation of the Atlantic meridional overturning circulation at 34 degrees S
Storto Andrea;
2016
Abstract
The Atlantic meridional overturning circulation and meridional heat transport (hereafter the AMOC and MHT) at 34 degrees S as simulated by global 1/16 degrees eddy-rich (henceforth GLOB16) and 1/4 degrees eddy-permitting (GLOB4) models are compared with observational estimates. Three different methods are used for calculating the modeled AMOC: the first method (MOCmod) is based on simulated velocity fields, while the second (MOCob) relies on the same assumptions as available observed-derived estimates. The third method (MOCob2) is also based on hydrostatic and geostrophic relationships, but relative to a barotropic circulation instead of the definition of velocity at a specific reference depth. All methods correctly reproduce the time-mean GLOB16 AMOC strength, but the value of the non-Ekman component of the GLOB16 AMOC is only about 75% of the observed-derived estimate. The GLOB16 MHT is also significantly less than observation value (slightly more than 60% of the observed). However, the mean AMOC and MHT values at 34 degrees S obtained with coarser resolution GLOB4 model are comparable with the observed-derived estimates. Possible causes for the differences between the eddy-rich model and observational data are studied. It is shown that the density field from the eddy-rich model has high temporal variability along 34 degrees S with spatial scale of about two hundred km that can be due to mesoscale variations, caused by Agulhas "leakage.'' This results in the decrease of the mean meridional geostrophic velocity, which leads to smaller values of the AMOC and MHT in GLOB16: subsampling GLOB16 density on 1/4 degrees or 1/2 degrees longitude grid along 34 degrees S for MOCob calculation significantly increases the AMOC values. The findings in this paper provide guidance in understanding AMOC and MHT dissimilarities between observation-based estimates and eddy-rich ocean models at 34 degrees S.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
