Variability in Southern Hemisphere Interocean Exchanges Part I: The Agulhas Current
Deirdre Byrne(1) and Julie McClean(2)
University of Maine,
360 Aubert Hall,
Orono, ME 04469,
(2) University of California San Diego, 9500 Gilman Drive, La Jolla CA, 92093-0230, United States
The heat, salt, and momentum fluxes carried by major ocean currents and embedded in interocean leakages constitute important benchmarks of ocean climate. In oceanography few time-series exist of sufficient duration and resolution to study this component of climate, in particular its variability, on seasonal to interannual time scales. While occurring over large spatial scales, these fluxes demonstrate significant variability at the mesoscale, presenting difficulties for in situ sampling programs. Moreover, fluxes at a variety of density horizons are important to the global heat and salt balances. Satellite altimetry (in particular the Jason-TOPEX/Poseidon missions) provides one of the most promising sources of observations for monitoring oceanic variability and long-term change in the ocean. This is particularly the case in undersampled regions such as the southern hemisphere.
In this paper we investigate interocean thermohaline fluxes into the South Atlantic Ocean via the Agulhas Retroflection region (the "warm water" route) and evaluate our ability to measure them using satellite altimetry. Our study synthesizes information from the Jason and T/P altimeter missions, information derived from the stand-alone 0.1° global Parallel Ocean Program (POP) model, and in situ profile and mooring data.
From the POP model output we evaluate length scales in the Agulhas Retroflection as well as the fully-resolved eddy fluxes along appropriate altimeter groundtracks, thereby gaining measures of their intrinsic variability in this location. We also use the model output to calculate fully-resolved model heat, salt and momentum fluxes in the region, both baroclinic and barotropic contributions. We subsample the model at altimeter overflight times and locations to determine the likely nature and extent of aliasing in the altimetric sea surface height anomaly (SHA) and in the fluxes calculated from them. For the quantities of interest, aliasing in the signal is less than might be expected.
We synthesize historic profile data into a streamfunction climatology, allowing heat, salt, and mass fluxes to be estimated from steric height (a major component of SHA) in our study region. When the climatology is used with altimeter data, the barotropic signal contained in SHA, another significant component, will introduce error in this method. Therefore we assess the relative magnitude and importance of the baroclinic and barotropic contributions to SHA using a variety of techniques, including in situ bottom pressure gauges and historic profile data. A variety of methods for removing the barotropic contribution to SHA are considered. Finally, the altimeter-derived thermohaline fluxes from the Agulhas Retroflection are compared to those from a recent field project, the Agulhas-South Atlantic Thermohaline Transport Experiment (ASTTEX, 2003-2005).