Role of Equatorial Basin-Mode Resonance for the Seasonal Variability of the Angola Current at 11°S.

Kopte, Robert, Brandt, Peter , Claus, Martin , Greatbatch, Richard John and Dengler, Marcus (2018) Role of Equatorial Basin-Mode Resonance for the Seasonal Variability of the Angola Current at 11°S. Open Access Journal of Physical Oceanography, 48 (2). pp. 261-281. DOI 10.1175/JPO-D-17-0111.1.

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Abstract

Multi-year moored velocity observations of the Angola Current near 11°S reveal a weak southward mean flow superimposed by substantial intraseasonal to seasonal variability, including annual and semiannual cycles with distinct baroclinic structures. In the equatorial Atlantic these oscillations are associated with basin-mode resonances of the fourth and second baroclinic modes, respectively. Here, the role of basin-mode resonance and local forcing for the Angola Current seasonality are investigated. A suite of linear shallow-water models for the tropical Atlantic is employed, each model representing a single baroclinic mode forced at a specific period. The annually and semiannually oscillating forcing is given by 1) an idealized zonally uniform zonal forcing restricted to the equatorial band corresponding to a remote equatorial forcing or 2) realistic, spatially-varying Fourier components of wind stress data that include local forcing off Angola, particularly alongshore winds. Model-computed modal amplitudes are scaled to match moored velocity observations from the equatorial Atlantic. The observed annual cycle of alongshore velocity at 11°S is well reproduced by the remote equatorial forcing. Including local forcing slightly improves the agreement between observed and simulated semiannual oscillations at 11°S compared to the purely equatorial forcing. However, the model-computed semiannual cycle lacks amplitude at mid-depth. This could be the result of either underestimating the strength of the second equatorial basin-mode of the fourth baroclinic mode or other processes not accounted for in the shallow-water models. Overall, our findings underline the importance of large-scale linear equatorial wave dynamics for the seasonal variability of the boundary circulation off Angola.

Document Type: Article
Funder compliance: info:eu-repo/grantAgreement/EC/FP7/603521
Keywords: SOUTH-ATLANTIC-OCEAN; BENGUELA UPWELLING SYSTEM; DEEP JETS; INTERANNUAL VARIABILITY; ALTIMETER DATA; WATER; WARM; NINO; FLOW; CIRCULATION
Research affiliation: OceanRep > SFB 754
OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-TM Theory and Modeling
OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-PO Physical Oceanography
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1175/JPO-D-17-0111.1
ISSN: 0022-3670
Projects: SACUS, PREFACE, SFB754, PIRATA, DRAKKAR
Expeditions/Models/Experiments:
Date Deposited: 22 Jan 2018 08:13
Last Modified: 01 Feb 2019 15:16
URI: http://oceanrep.geomar.de/id/eprint/41660

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