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Article Type:
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The East Madagascar Current: Volume Transport and Variability Based on Long-Term Observations

Leandro Ponsoni Royal Netherlands Institute for Sea Research (NIOZ), Den Burg, Texel, Netherlands

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Borja Aguiar-González Royal Netherlands Institute for Sea Research (NIOZ), Den Burg, Texel, Netherlands

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Herman Ridderinkhof Royal Netherlands Institute for Sea Research (NIOZ), Den Burg, Texel, Netherlands

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Leo R. M. Maas Royal Netherlands Institute for Sea Research (NIOZ), Den Burg, Texel, and Institute for Marine and Atmospheric Research, Utrecht University, Utrecht, Netherlands

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Print Publication:
01 Apr 2016
DOI:
https://doi.org/10.1175/JPO-D-15-0154.1
Page(s):
1045–1065
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Abstract

This study provides a long-term description of the poleward East Madagascar Current (EMC) in terms of its observed velocities, estimated volume transport, and variability based on both ~2.5 yr of continuous in situ measurements and ~21 yr of satellite altimeter data. An array of five moorings was deployed at 23°S off eastern Madagascar as part of the Indian–Atlantic Exchange in present and past climate (INATEX) observational program. On average, the EMC has a horizontal scale of about 60–100 km and is found from the surface to about 1000-m depth. Its time-averaged core is positioned at the surface, at approximately 20 km from the coast, with velocity of 79 (±21) cm s−1. The EMC mean volume transport is estimated to be 18.3 (±8.4) Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1). During the strongest events, maximum velocities and transport reach up to 170 cm s−1 and 50 Sv, respectively. A good agreement is found between the in situ transport estimated over the first 8 m of water column [0.32 (±0.13) Sv] with the altimetry-derived volume transport [0.28 (±0.09) Sv]. Results from wavelet analysis display a dominant nearly bimonthly (45–85 days) frequency band of transport variability, which explains about 41% of the transport variance. Altimeter data suggest that this band of variability is induced by the arrival of westward-propagating sea level anomalies, which in turn are likely represented by mesoscale cyclonic and anticyclonic eddies. Annual averages of the altimeter-derived surface transport suggest that interannual variabilities also play a role in the EMC system.

Supplemental information related to this paper is available at the Journals Online website: http://dx.doi.org/10.1175/JPO-D-15-0154.s1.

Corresponding author address: Leandro Ponsoni, NIOZ Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands. E-mail: lponsoni@nioz.nl

Abstract

This study provides a long-term description of the poleward East Madagascar Current (EMC) in terms of its observed velocities, estimated volume transport, and variability based on both ~2.5 yr of continuous in situ measurements and ~21 yr of satellite altimeter data. An array of five moorings was deployed at 23°S off eastern Madagascar as part of the Indian–Atlantic Exchange in present and past climate (INATEX) observational program. On average, the EMC has a horizontal scale of about 60–100 km and is found from the surface to about 1000-m depth. Its time-averaged core is positioned at the surface, at approximately 20 km from the coast, with velocity of 79 (±21) cm s−1. The EMC mean volume transport is estimated to be 18.3 (±8.4) Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1). During the strongest events, maximum velocities and transport reach up to 170 cm s−1 and 50 Sv, respectively. A good agreement is found between the in situ transport estimated over the first 8 m of water column [0.32 (±0.13) Sv] with the altimetry-derived volume transport [0.28 (±0.09) Sv]. Results from wavelet analysis display a dominant nearly bimonthly (45–85 days) frequency band of transport variability, which explains about 41% of the transport variance. Altimeter data suggest that this band of variability is induced by the arrival of westward-propagating sea level anomalies, which in turn are likely represented by mesoscale cyclonic and anticyclonic eddies. Annual averages of the altimeter-derived surface transport suggest that interannual variabilities also play a role in the EMC system.

Supplemental information related to this paper is available at the Journals Online website: http://dx.doi.org/10.1175/JPO-D-15-0154.s1.

Corresponding author address: Leandro Ponsoni, NIOZ Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands. E-mail: lponsoni@nioz.nl
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