AN INTERCOMPARISON OF A BRYAN-COX-TYPE OCEAN MODEL AND AN ISOPYCNIC OCEAN MODEL .2. THE SUBTROPICAL GYRE AND MERIDIONAL NEAT TRANSPORT

In a companion paper, two ocean general circulation models were implem ented in order to simulate and intercompare the main features of the N orth 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 d atasets, both models were spun up from rest for 30 years. This paper e xamines 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 P W) than HC (which transports up to 0.48 PW), a difference that arises primarily due to surface-poleward and deep-equatorward flows, which ar e stronger, and at warmer and colder extremes, than in HC. However, HC displays stronger heat transport across the subpolar gyre (with a sec ondary 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 qu antify the effect of diapycnic mixing and bathymetry two separate 30-y ear integrations of the isopycnic model, without diapycnal mixing and with the same bathymetry as HC, were undertaken. The isopycnic model i s relatively insensitive to these two aspects of model setup on the 30 -year timescale. Both models develop subtropical gyres of annual mean strength similar to 45 Sv (Sv = 10(6) mJ s(-1)) (due to essentially id entical Sverdrup responses), although AIM displays stronger seasonal c ycles of Gulf Stream transport than HC (probably due to differences in topographic responses). At subtropical latitudes deep western boundar y currents are weaker in AIM (similar to 5 Sv) than in HC (similar to 10 Sv), although in HC there is an approximate halving in strength of the DWBC as it progresses south of Florida, due to abyssal recirculati on and upwelling. In the subtropical gyre AIM displays a clear pattern of ventilation, and potential vorticity is, to a large degree, conser ved along particle trajectories inside the thermocline. Ventilation pa thways 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 s ubtropical mode water, AIM forming 15 Sv compared to 20 Sv in HC. Subs urface isopycnal warming in AIM is related to 30-year trends of surfac e cooling with little corresponding change in salinity. Subsurface iso pycnal cooling in HC is due to surface cooling and freshening.