Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1988
Description:
The highly energetic Agulhas Retroflection region south of the African
continent lies at the junction of the South Indian, South Atlantic, and Circumpolar
Oceans. A new survey of the Agulhas Retroflection taken in March 1985, plus
historical hydrographic data, allow its dynamical and water-mass characteristics,
and its role in exchanging mass, tracers, and vorticity between the three oceans, to
be extensively characterized. The 1985 survey is composed of three independent,
synoptic elements: a grid of closely-spaced, full-water-depth hydrographic
stations (the first entirely full-water-column survey in this area), including several
transects of the Agulhas and Agulhas Return Currents; a continuous survey of the
path of the currents (the first such survey in the Agulhas); and a contemporaneous
and relatively cloud-free sea surface temperature image derived from satellite
infrared measurements.
Mass transport balances within the closed grid boxes of the 1985
hydrographic survey provide information about current transport, recirculation
(transport in excess of estimated returning interior ocean transport), and the
overall Retroflection transport pattern. The current transport values exceed by as
much as a factor of 1.5 the maximum interior transport computed from observed
wind-stress curl and linear theory. Agulhas Current transports ranged from 56 to
95 x 106 m s-l at four 1985 transects crossing the current. Agulhas Return
Current transports at the two 1985 transects were 54 and 65 x 106 m s-l.
These transports are computed relative to 2400 dbar, which lies below the deep
oxygen minimum emanating from the South Indian Ocean, and above the North
Atlantic Deep Water salinity maximum.
The current retroflected in two distinct branches in 1985, with a cold ring
and a partially isolated warm recirculation cell found between the two branches.
The satellite-derived sea surface temperature (SST) image, in agreement with the
in situ measurements, showed that the cold ring lacked a cold SST anomaly; that
the subsurface current path, as represented by a survey of the 15 C isotherm and
200 dbar surface intersection, was closely followed by a sharp front in sea
surface temperature; and that most of the Agulhas's surface warm core retroflected
upstream of the second retroflection branch.
Anticyclonic curvature vorticity at sharp turns in the subsurface current
path was found to exceed the maximum allowed by gradient wind balance,
indicating that at these locations time-dependence and cross-frontal flow are
important. The current's density field is found to meet necessary conditions for
baroclinic and barotropic instability. These instability mechanisms may play a
role in ring formation and current meandering.
Top-to-bottom cross-stream spatial and isopycnal water-mass layering in
the Agulhas Current, Agulhas Return Current, and associated rings are presented
in two sets of sections, one contoured with pressure and the other with potential
density as vertical coordinate. Temperature, salinity, oxygen, potential density
and velocity sections are shown contoured versus pressure; and pressure, salinity,
oxygen, and planetary potential vorticity are shown contoured versus potential
density. These sections clearly illustrate water-mass structure both in space and
relative to isopycnal surfaces. Strong salt, oxygen, and potential vorticity fronts
on isopycnals in the upper -300 m across the Agulhas and Agulhas Return
Current are observed, as are deep western boundary filaments of (i) salty, low
oxygen water at intermediate depths traceable to Red Sea Water influences, and
(ii) salty North Atlantic Deep Water close round the tip of Africa.
The 1985 cold-core ring is the first cold-cored isolated feature to be
observed within the Retroflection itself. Its transport was 64 x 106 m s-1, its
integrated kinetic and available potential energy anomalies were 8.3 and
61 x 1015 J respectively, and its integrated planetary potential vorticity anomaly
was 2.8 x 10-12 m-1 s-1. The potential vorticity flux associated with the
exchange of 25 warm ring/cold ring pairs per year between the South Indian and
Southern Oceans would balance the potential vorticity input by the wind to the
entire South Indian Ocean.
Interbasin flow of warm thermocline water (warmer than 8 C) from the
South Indian to the South Atlantic Ocean is reconsidered in light of the 1985
hydrographic data. Thermocline water flow from the South Indian Ocean into the
South Atlantic in the 1985 and historical observations is found to range from 2.8
to 〈9.6 x 106 m s-I. These values are less than the S;10 x 106 m s·1 needed
to balance the Atlantic Ocean export of deep water, and implies that the deep water
export is balanced in part by water colder than 8 C.
Description:
Funding was provided by the Office of Naval Research under contract numbers
NOOOI4-84-C-OI34 (NR083-400), NOOOI4-85-C-OOOl (NR083-004), and
NOOOI4-87-K-0007 (NR083-004).
Keywords:
Ocean currents
;
Ocean temperature
;
Thomas Washington (Ship) Cruise 3
;
Knorr (Ship : 1970-) Cruise
;
Oceanus (Ship : 1975-) Cruise
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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