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Inter-comparison of simulations of the Agulhas Current - Part 2

 

While the volume transport of the Agulhas Current can be directly related to wind stress curl in the Indian Ocean, according to Sverdrup theory, its vertical and horizontal structure is far more complex due to all the non-linear processes that influence western boundary currents. (friction, viscosity, interaction with topography…). These processes ultimately determine the position of the retroflection, and possibly the leakage of South Indian Ocean water masses into the South Atlantic. Representing those in ocean models is a long-standing challenge.

 

Several models of the Agulhas Current have been developed using very different frameworks (NEMO, ROMS, HYCOM), with or without assimilation of observations, at various spatial resolution (1/4° to 1/12°). A detailed diagnostics of the intensity, location and depth of the Agulhas Current, as well as Agulhas leakage, using an ad-hoc software that avoids interpolation of ocean currents, suggested that:

1) ocean reanalysis products (with assimilation) of the Agulhas Current, despite being constrained to be close to available observations, remain largely dependent on their ocean modeling framework. Hence each of these products should be used with caution regarding the degree of uncertainty of the ocean state produced. Including more observations in the assimilation process is expected to reduce this uncertainty, largely due to the lack of realism of the non-assimilated ocean models in this region;

2) further analysis and development of ocean models for the Agulhas Current (with no data assimilation) should be carried on in order to identify the biases of these models hence improve their realism.

    OBS      NEMO HYCOM BlueLink
NMC 27 42 50 34
LOCO 16 25 26 19
SMC 37 22 36 37
ACT 79 58 71 N/A
Leakage 15 100 85 71

 Volume transports (Sv) around the Southern tip of Africa: in the North Madagascar Current (NMC), in the Mozambique Channel (LOCO), in the South Madagascar Current (SMC), in the Agulhas Current at ACT array (ACT) and in the Agulhas leakage, in observations (OBS), the ocean reanalyses of MyOcean-NEMO, HYCOM and the Bluelink simulation (without data assimilation). Source : MSc dissertation of Kyle Cooper (UCT).

Collaborations: J. Deshayes (CNRS-IRD), B. Backeberg (NTC), B. Loveday (UCT), J. Hermes (SAEON), J. Veitch (UCT), K. Cooper (MSc UCT).


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