An altimetry‐based gravest empirical mode south of Africa: 1. Development and validation

• Published in: Journal of Geophysical Research
• Main Authors: Swart, Sebastiaan, Speich, Sabrina, Ansorge, Isabelle J., Lutjeharms, Johann R.E.
• Other Authors: Department of Oceanography Cape Town, University of Cape Town, Laboratoire de physique des océans (LPO), Institut de Recherche pour le Développement (IRD)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2010
• Subjects: [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography; Antarctic; The Antarctic; Antarc*
• Online Access: https://hal.archives-ouvertes.fr/hal-00495100; https://doi.org/10.1029/2009JC005299

International audience Hydrographic transects of the Antarctic Circumpolar Current (ACC) south of Africa are projected into baroclinic stream function space parameterized by pressure and dynamic height. This produces a two‐dimensional gravest empirical mode (GEM) that captures more than 97% of the total density and temperature variance in the ACC domain. Weekly maps of absolute dynamic topography data, derived from satellite altimetry, are combined with the GEM to obtain a 16 year time series of temperature and salinity fields. The time series of thermohaline fields are compared with independent in situ observations. The residuals decrease sharply below the thermocline and through the entire water column the mean root‐mean‐square (RMS) error is 0.15°C, 0.02, and 0.02 kg m−3 for temperature, salinity, and density, respectively. The positions of ACC fronts are followed in time using satellite altimetry data. These locations correspond to both the observed and GEM‐based positions. The available temperature and salinity information allow one to calculate the baroclinic zonal velocity field between the surface and 2500 dbar. This is compared with velocity measurements from repeat hydrographic transects at the GoodHope line. The net accumulated transports of the ACC, derived from these different methods are within 1–3 Sv of each other. Similarly, GEM‐produced cross‐sectional velocities at 300 dbar compare closely to the observed data, with the RMS difference not exceeding 0.03 m s−1. The continuous time series of thermohaline fields, described here, are further exploited to understand the dynamic nature of the ACC fronts in the region, and which is given by Swart and Speich (2010)