Seminar announcement

[Alessandro Crise]

Ph.D. in Environmental Fluid Mechanics
SEMINAR

Boundary Intensification of Vertical Velocity in a Beta-Plane Basin
by Prof. Joseph Pedlosky,
(Woods Hole Oceanographic Institution, Woods Hole MA, 02543, U.S.A.)

to be held on May 25 (Thursday) at 11.15
at the Seminar Room, OGS,
Borgo Grotta Gigante, "ex-albergo Cristallo"
(in front of the hotel Milic).

ABSTRACT

The buoyancy driven circulation of simple two-layer models on the beta 
plane is studied in order to examine the role of beta in determining the 
magnitude and structure of the vertical motions forced in response to 
surface heating and cooling. Both analytical and numerical approaches 
are used to describe the change in circulation pattern and strength as a 
consequence of the planetary vorticity gradient. The physics are 
quasi-geostrophic at lowest order but sensitive to small non 
quasi-geostrophic mass fluxes across the boundary of the basin.
The height of the interface between the two layers serves as a analogue 
of temperature and the vertical velocity at the interface consists of a 
cross-isopycnal velocity, modeled in terms of a relaxation to a 
prescribed interface height, as well as an adiabatic representation of 
eddy thickness fluxes parameterized as lateral diffusion of interface 
displacement. In the numerical model the lateral eddy diffusion of heat 
is explicitly represented by a resolved eddy field.
In the plausibly more realistic case when the lateral diffusion of 
buoyancy dominates the diffusion of momentum the major vertical 
velocities occur at the boundary of the basin as in earlier f-plane 
studies. The effect of the planetary vorticity gradient is to intensify 
the sinking on the western wall and enhance the magnitude of that 
sinking with respect to the f-plane models. The vertical mass flux in 
the Sverdrup interior exactly balances the vertical flux in the region 
of the strong horizontal transport of the western boundary current 
leaving the net flux to occur in a very narrow region near the western 
boundary tucked well within the western boundary current. On the other 
hand, if the lateral diffusion of heat is arbitrarily and 
unrealistically eliminated the vertical mass flux is forced to occur in 
the interior.
The circulation pattern is extremely sensitive to small net inflows or 
outflows across the basin perimeter. The cross basin flux determines the 
interface height on the basin's eastern boundary and affects the 
circulation pattern across the entire basin.
The time dependent response of the basin to the imposition of cooling 
(or heating ) is also considered. The focus is on the structure and 
magnitude of the vertical motion and its response to both a switch -on 
forcing and a periodic forcing.
The presence of time dependence adds additional dynamical features to 
the problem, in particular the emergence of low frequency, weakly damped 
Rossby basin modes. If the buoyancy forcing is zonally uniform the basin 
responds to a switch on of the forcing by coming into steady state 
equilibrium after the passage of a single baroclinic Rossby wave. If the 
forcing is non uniform in the zonal direction a sequence of Rossby basin 
modes is excited and their decay is required before the basin achieves a 
steady state.

-- 
Alessandro Crise
Department of Oceanography, Director
Istiuto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS
Borgo Grotta Gigante, 42/c - 34010 Sgonico (TS)
Italy

phone: +39 040 2140205
fax:   +39 040 2140266