Environmental Studies Program Direction:
Accelerated Deepwater Activities
Deepwater: Physical Oceanography
The
Gulf of Mexico is dominated by the Loop Current and its associated
anticyclonic and cyclonic eddies. A convenient marker for studying this system is formed
by the mixing of water masses. Circulation in the eastern Gulf is dominated by the Loop
current, which is a western boundary current in the Caribbean. Total transport in the Loop
current is approximately thirty million cubic meters per second with a variance of about
ten percent. Speeds may exceed 150 cm/s at the surface with velocities as high as 5 cm/s
still persistent at 1000 m. There are many studies of intrusion timing and eddy spawning.
To understand the fate of Loop rings, it is
necessary to review western Gulf background circulation. A few synoptic studies have been
undertaken, four in the 1960’s, and each found a weak anticyclonic feature in the
western Gulf with possibly an intensification along the Mexican coast. The mechanism is
under debate: wind stress, loop eddies, and ring-slope-ring interactions have all been
proposed. In the southwest Gulf, there is a semi-permanent cyclonic gyre forced by winds.
Eddies pinch off from the loop current and
migrate westward along a series of “preferred” paths. There has been
considerable effort expended in modeling ring separation and movement. One weakness has
been that the observed lifetime of rings exceeds that predicted in models. Deepwater
current measurements have captured the influence of these eddies. Current meters at 500 m
in the western Gulf showed velocity spikes to 30-40 cm/s.
There are three preferred paths. One goes
down the central Gulf to the western border, then north. Another goes through the central
Gulf, and a third skirts the continental slope off Louisiana. All three lead to the
“eddy graveyard.” These paths are caused by ring-slope interaction. Ring-slope
interaction in this graveyard area can result in very strong offshore transport.
Effects of atmospheric storms have also been
captured in current meters. For example, during Hurricane Andrew, currents at 100 m
reached 20 cm/s and at 490 m about 5 cm/s were experienced. Hurricane effects in deepwater
can be even more dramatic as in case of Hurricane Allen in 1980. At 700 m, currents were
15 cm/s. Cyclones also have an effect, giving rise to a characteristic current. Cold air
outbreaks are a third phenomena with three to four a month, fall through spring. These
force inertial oscillations. Thermal cycling causes some of the strongest surface currents
seen in the MMS-sponsored Texas-Louisiana Shelf Physical Oceanography (LATEX) study. These
are diurnal and associated with the summer.
For more information, contact Walter Johnson.
