An Intercomparison of a Bryan-Cox-Type Ocean Model and an Isopycnic Ocean Model. Part II: The Subtropical Gyre and Meridional Heat Transport

Abstract

In a companion paper, two ocean general circulation models were implemented in order to simulate and intercompare the main features of the North Atlantic circulation: the Atlantic Isopycnic Model (AIM) and the Hadley Centre Bryan-Cox-type ocean model (HC). Starting from the same initial state and using the same mechanical and thermohaline forcing datasets, both models were spun up from rest for 30 years. This paper examines the western boundary currents, meridional heat transport and subtropical gyre ventilation. AIM transports more heat poleward in the subtropics (with peak annual-mean meridional heat transport of 0.63 PW) than HC (which transports up to 0.48 PW), a difference that arises primarily due to surface-poleward and deep-equatorward flows, which are stronger, and at warmer and colder extremes, than in HC. However, HC displays stronger heat transport across the subpolar gyre (with a secondary maximum of 0.36 PW compared to 0.24 PW in AIM), consistent with stronger subpolar gyre heat gain (due to a more zonal North Atlantic Current path, leading to larger relaxation surface heat fluxes). To quantify the effect of diapycnic mixing and bathymetry two separate 30-year integrations of the isopycnic model, without diapycnal mixing and with the same bathymetry as HC, were undertaken. The isopycnic model is relatively insensitive to these two aspects of model setup on the 30-year timescale. Both models develop subtropical gyres of annual mean strength ?45 Sv (Sv ≡ 106 m3 s?1 (due to essentially identical Sverdrup responses), although AIM displays stronger seasonal cycles of Gulf Stream transport than HC (probably due to differences in topographic responses). At subtropical latitudes deep western boundary currents are weaker in AIM (?5 Sv) than in HC (?10 Sv). although in HC them is an approximate halving in strength of the DWBC as it progresses south of Florida, due to abyssal recirculation and upwelling. In the subtropical gyro AIM displays a clear pattern of ventilation, and potential vorticity is, to a large degree, conserved along particle trajectories inside the thermocline. Ventilation pathways are less sharply defined in HC and, compared to AIM, horizontal mixing of temperature and salinity more strongly limits the degree to which water properties (including potential vorticity) are conserved along isopycnals. Both models annually renew realistic quantities of subtropical mode water, AIM forming 15 Sv compared to 20 Sv in HC. Subsurface isopycnal warming in AIM is related to 30-year trends of surface cooling with little corresponding change in salinity. Subsurface isopycnal cooling in HC is due to surface cooling and freshening.