423
order of their difference). These results contradict the indirect estimates of very large
seasonal changes in meridional heat transport, for the tropical and subtropical regions,
by Oort and Vonder Haar (1976) and Carissimo et al. (1985). Such seasonal changes can
only be caused by two possibilities: (1) seasonal temperature changes within the upper
mixed layer several times larger than observed, which should also be associated with a
reversal of the Ekman transport, and/or (2) a reversal of the whole meridional geostrophic
velocity field. Fig. 2 illustrates that neither of these situations occur in the eastern NASG.
It is possible that Gill and Niiler's (1973) calculations were underestimates, caused by
omitting the correlation between seasonally oscillating quantities such as temperature
and mixed layer depth, but it seems clear that the heat transport reversals obtained
through indirect studies are too large. In order to properly estimate heat transport
divergence, as compared with the rate of heat storage within the mixed layer, we need
to improve our understanding of mixed layer physics. First, the thickness of the mixed
layer will determine over how much volume the incoming heat is distributed, controlling
the SST and hence the incoming heat flux density. Second, the seasonal evolution of SST
and mixed layer depth, in combination with the surface circulation, will determine the
possible asymmetry of the heating-cooling annual cycle. And third, the rate at which
mass and heat is transferred to the main thermocline will force the circulation therein,
and hence determine the rate of heat escape.
The main factors controlling mixed layer deepening are horizontal divergence within
this layer, vertical convection and mixing by wind events, and shear mixing associated to
frontal and mesoscale structures (Klein and Hua 1990, Pelegri and Csanady 1994, Sterl
and Kattenberg 1994). These processes, which result in mass and heat transfer to the thermocline layers, are usually parameterized in terms of vertical turbulent diffusion (eg. Colin
de Verdiere 1989, Suginohara and Aoki 1991), but this is only a numerical approximation of the real physical mechanisms. The importance of a meaningful representation of
these processes has become clear through the thermocline circulation studies by Stommel (1979), Luyten et al. (1983), Luyten and Stommel (1986), Huang (1989a), Marshall
and Nurser (1991,1992), Nurser and Marshall (1991), Pedlosky and Robbins (1991) and
Williams (1991). They have shown that convergence in the mixed layer (negative Ekman
pumping) and thermal/haline influx (buoyancy forcing) initializes the potential vorticity
(PV) of the flow going into the permanent thermocline.
Negative Ekman pumping, occurring over most of the eastern NASG (Leetma and
Bunker 1978, McClain and Firestone 1993), is a boundary condition at the base of the
Ekman layer which forces the thermocline circulation. On the other hand, buoyancy
influx results from the combination of heat and freshwater fluxes into the upper mixed
layer, the former being dominant over most of the N ASG (Schmitt et al. 1989). Buoyancy
forcing of the thermocline circulation has sometimes been specified as a positive (upward)
order of their difference). These results contradict the indirect estimates of very large
seasonal changes in meridional heat transport, for the tropical and subtropical regions,
by Oort and Vonder Haar (1976) and Carissimo et al. (1985). Such seasonal changes can
only be caused by two possibilities: (1) seasonal temperature changes within the upper
mixed layer several times larger than observed, which should also be associated with a
reversal of the Ekman transport, and/or (2) a reversal of the whole meridional geostrophic
velocity field. Fig. 2 illustrates that neither of these situations occur in the eastern NASG.
It is possible that Gill and Niiler's (1973) calculations were underestimates, caused by
omitting the correlation between seasonally oscillating quantities such as temperature
and mixed layer depth, but it seems clear that the heat transport reversals obtained
through indirect studies are too large. In order to properly estimate heat transport
divergence, as compared with the rate of heat storage within the mixed layer, we need
to improve our understanding of mixed layer physics. First, the thickness of the mixed
layer will determine over how much volume the incoming heat is distributed, controlling
the SST and hence the incoming heat flux density. Second, the seasonal evolution of SST
and mixed layer depth, in combination with the surface circulation, will determine the
possible asymmetry of the heating-cooling annual cycle. And third, the rate at which
mass and heat is transferred to the main thermocline will force the circulation therein,
and hence determine the rate of heat escape.
The main factors controlling mixed layer deepening are horizontal divergence within
this layer, vertical convection and mixing by wind events, and shear mixing associated to
frontal and mesoscale structures (Klein and Hua 1990, Pelegri and Csanady 1994, Sterl
and Kattenberg 1994). These processes, which result in mass and heat transfer to the thermocline layers, are usually parameterized in terms of vertical turbulent diffusion (eg. Colin
de Verdiere 1989, Suginohara and Aoki 1991), but this is only a numerical approximation of the real physical mechanisms. The importance of a meaningful representation of
these processes has become clear through the thermocline circulation studies by Stommel (1979), Luyten et al. (1983), Luyten and Stommel (1986), Huang (1989a), Marshall
and Nurser (1991,1992), Nurser and Marshall (1991), Pedlosky and Robbins (1991) and
Williams (1991). They have shown that convergence in the mixed layer (negative Ekman
pumping) and thermal/haline influx (buoyancy forcing) initializes the potential vorticity
(PV) of the flow going into the permanent thermocline.
Negative Ekman pumping, occurring over most of the eastern NASG (Leetma and
Bunker 1978, McClain and Firestone 1993), is a boundary condition at the base of the
Ekman layer which forces the thermocline circulation. On the other hand, buoyancy
influx results from the combination of heat and freshwater fluxes into the upper mixed
layer, the former being dominant over most of the N ASG (Schmitt et al. 1989). Buoyancy
forcing of the thermocline circulation has sometimes been specified as a positive (upward)