Thermocline circulation in the Solomon Sea: A modeling study

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In the southwest Pacific, thermocline waters connecting the tropics to the equator via western boundary currents (WBCs) transit through the Solomon Sea. Despite its importance in feeding the Equatorial Undercurrent (EUC) and its related potential influence on the low frequency modulation of ENSO, the circulation inside the Solomon Sea is poorly documented. A 1/12° model has been implemented to analyze the mean and the seasonal variability of the Solomon Sea thermocline circulation. The circulation involves an inflow from the open southern Solomon Sea which is distributed via WBCs between the three north exiting straits of the semi-closed Solomon Sea. The system of WBCs is found to be complex. Its main feature, the New Guinea Coastal Undercurrent, splits in two branches, one flowing through Vitiaz Strait, the other one, the New Britain Coastal Undercurrent (NBCU), exiting at Solomon Strait. East of the Solomon Sea, the encounter of the South Equatorial Current (SEC) with the Solomon Islands forms a previously-unknown current, which we call the Solomon Islands Coastal Undercurrent (SICU). The NBCU, the SEC and the SICU participate in the feeding of the New Ireland Coastal Undercurrent (NICU) which retroflects to the Equatorial Undercurrent, providing the most direct western boundary EUC connection, particularly active in June-August. The Solomon Sea WBC seasonal variability results from the combination of equatorial dynamics, of remotely forced Rossby waves north of 10°S, and of the spin up and down of the subtropical gyre as a response of Rossby waves forced south of 10°S.

Solomon Sea circulation vertically integrated over the thermocline (defined by the 24.0-26.5 isopycnal layer) from the 1/12° model (1986-2004 mean). Units are m²/s. The main undercurrents are labeled: NQC: North Queensland Current, NVJ: North Vanuatu Jet, SICU: Solomon Islands Coastal Undercurrent, SEC: South Equatorial Current, NGCU: New Guinea Coastal Undercurrent, NBCU: New Britain Coastal Undercurrent, VS: Vitiaz Strait transport, SGU: St Georges Undercurrent, NICU: New Ireland Coastal Undercurrent, EUC: Equatorial Undercurren

 

The annual cycle of the thermocline transport in the 1/12° model. The figure is based on a 1-cpy harmonic analysis of the thermocline circulation. The area of each vector indicates the magnitude of the 1-cpy harmonic, the direction points along the major axes of the corresponding variance ellipse (i.e. the direction of maximum variance, not the direction of mean flow a priori), and the color indicates the month of maximum transport in the direction of the vector. The choice of vector direction is arbitrary: each vector could be reversed and its phase advanced by 6 months, to show the opposite phase of annual anomalies.

Reference : Melet, A.; Gourdeau, L.; Kessler, W.; Verron, J. & Molines, J. Thermocline circulation in the Solomon Sea: A modeling study. Journal of Physical Oceanography, 2010, 40, 1302-1319.

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