WHOI PIs: Joyce, Macdonald, Toole
The A22 section between Venezuela & New England was occupied in WOCE (Joyce et al., 2001)
and the basic zonal circulation explored using an inverse calculation using mass and silica
constraints, initialized by reference level velocities derived from LADCP measurements. Water
mass contrasts between the WOCE line and previous occupations were discussed in Joyce et al.
(1999). In the latter the changes at the A20 (52oW) line were also discussed. Recently, Hall et al.
(2004) presented an analysis of the zonal circulation of the WOCE section A20 similar to that for
the A22 section. In this proposal, Joyce’s main interests are to 1) complete a similar analysis of
the zonal circulation and its changes using the 2003 A22 data, 2) understand the substantial
changes in net heat and freshwater fluxes across the section compared with 1997, 3) work with
colleagues at WHOI and at other institutions, as part of this proposal, to document water mass
changes in the subtropics, and 4) rationalize estimates of large changes of salinity maximum
water subduction using CFCs (w/ R. Fine) and estimates of changing Ekman pumping using
NCEP and other suitable data products.
Figure xx. Some puzzling reductions in both net heat (left) and freshwater (right) fluxes across the 66W line have been
seen for the 2003 occupation of A22 (black, solid) compared to the 1997 (red, dashed). Most of these changes are to the
north of 34N, as can be seen in the cumulative contributions to the net fluxes, and cannot be explained by slight
differences in the northern part of the two cruise tracks.
Lowered acoustic Doppler profile (LADCP) data were obtained on CLIVAR repeat transects for
most stations as were underway shipboard ADCP (SADCP) data. Once rotated to obtain the
ADCP velocities perpendicular to the cruise track and averaged between adjacent stations pairs,
these data can be used to reference the geostrophic velocities determined from the CTD record.
SADCP profiles may be used to fill in gaps in the LADCP record. Using an inverse calculation
to ensure mass balance, the result will be an estimate of the absolute velocity field including
ageostrophic components. This calculation has been made for the 2003 A22 section. We are
seeing some indication that the zonal circulation of the Gulf Stream recirculation is less vigorous
compared to WOCE and that there are substantial changes in net heat and freshwater transport
across the A22 line (Figure XX). How much of these differences in net transports can be
explained by uncertainties and by decadal variability of heat and freshwater fluxes and/or storage
to the west of 66W is an important question to be addressed in this study.
Alison Macdonald will focus on obtaining absolute velocity estimates from the A20 dataset
which will be used in three different ways: 1) as the foundation for a comparison between large
scale circulation and property transports seen in 2003 and those seen in previous occupations, in
particular to see whether the temperature and/or freshwater flux differences found at 66oW are
also apparent at 52oW, and along with Joyce and Toole to determine what physical processes
might be responsible, 2) as the basis for the study of the potential vorticity budget in the western
margin as described below under the work of Toole, and 3) as the initial estimate of the velocity
field for the lines as they are included in the inverse box model investigation into the possible
variations of carbon flux and flux divergence of in Atlantic (NSF grant OC-0223421 and renewal
pending).
John Toole will focus on two aspects of the repeat section work in 2003: one engineering and one
scientific. The former will investigate what independent information about water mass,
stratification and circulation changes in the western subtropics on 5-10 year time scale was
obtained from the two meridional lines A20 and A22. The case to resample the Atlantic
hydrographic sections in future will depend on the balance of costs and results. Water property
and volumetric changes between the reoccupations of these two sections will be compared, and
together with Macdonald and Joyce, the changes in circulation will also be documented and
compared.
The second research element will focus on the potential vorticity (PV) budget of the western
subtropical Atlantic. Synoptic estimates of the PV budget will be constructed using the 2003 and
previous occupations of the A20 and A22 sections, related to basic western boundary current
models (e.g. Stommel, 1948; Munk, 1950; Fofonoff, 1954) and compared to a similar analysis of
a coarse resolution numerical model (Lu and Stammer, 2004). We anticipate a significantly
enhanced role of relative vorticity in the ocean data as compared to the coarse-resolution model.
The analysis will begin with mass-conserving, absolute velocity fields that Macdonald and Joyce
will develop. Following Pedlosky (1996), in a layered ocean framework, the potential vorticity,
qn, at any spot along a section is approximated as qn = ( f + ζn ) / hn for layer n with thickness hn
between specified isopycnals; f is the Coriolis parameter and ζn the relative vorticity. Along the
ocean sections, the acquired CTD data will provide the basis for layer thickness estimates, and for
these meridional sections that cross the major currents at near right-angle, the relative vorticity
may be approximated by -du/dy, the meridional (along-track) gradient in zonal (across-track)
velocity. SADCP, geostrophic and LADCP velocity estimates will be used to estimate du/dy in
the upper ocean; the first two are available for deeper layers. Combination of these fields gives
the vorticity advection along the open ocean line segment, and climatological wind stress data is
available to estimate the forcing. The vorticity budget for ocean regions is obtained by
integrating around closed boxes; boundary torques may be inferred as a residual.
Toole (together with Dr. K. Donohue) has previously carried out such an analysis for the SW
Indian Ocean that suggested inertial behavior within the Agulhas Current (small advective PV
flux divergence within a closed box of hydrographic stations at and south of Madagascar).
However, that study used data over a limited latitudinal extent with consequently small variation
in planetary potential vorticity. In contrast, the western Atlantic sections span a larger latitude
interval between the North Equatorial Current entering the western domain and Gulf Stream
leaving. We are keen to contrast the Gulf Stream system with the Agulhas and see if the
planetary PV change is also balanced by layer thickness and/or relative vorticity changes in the
Stream.
References
Fofonoff, N.P., 1954, Steady flow in a frictionless homogeneous ocean. J. Mar. Res., 13, 254262.
Hall, M. M., T. M. Joyce, R. S. Pickart W. M. Smethie Jr. and D. J. Torres, 2004. Zonal
circulation across 52°W in the North Atlantic. Journal of Geophysical Research, 109,
C11008, doi:10.1029/2003JC002103.
Joyce, Terrence M., Alonso Hernandez-Guerra, and William M. Smethie, Jr., 2001. Zonal
circulation in the NW Atlantic and Caribbean from a meridional World Ocean
Circulation Experiment hydrographic section at 66°W. Journal of Geophysical Research,
106(C10), 22,095-22,113.
Joyce, Terrence M., Robert S. Pickart, and Robert C. Millard, 1999. Long-term hydrographic
changes at 52° and 66°W in the North Atlantic subtropical gyre and Caribbean. Deep-Sea
Research II, 46, 245-278.
Lu, Y. and D. Stammer, 2004. Vorticity balance in a global ocean circulation model. J. Phys.
Oceanogr., 34, pp. 605–622.
Munk. W.H., 1950. On the wind-driven ocean circulation. J. Meteorology, 7, 79-93. Stommel,
H. 1948, The westward intensification of wind-driven currents. Trans. American Geophys.
Union, 29, 202-206.
Pedlosky, J., 1996. Ocean Circulation Theory, Springer-Verlag, Berlin, 450 pp.
Stommel, H. 1948, The westward intensification of wind-driven currents. Trans. American
Geophys. Union, 29, 202-206.
Sverdrup, H.U., 1947, Wind-driven currents in a baroclinic ocean, with application to the
equatorial currents of the eastern pacific. Proc. Nat. Academy of Science, USA, 33, 318326.