Thermocline Circulation in the Solomon Sea: A Modeling Study*

Abstract

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 /128 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 semiclosed 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 and 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 the authors call the Solomon Islands Coastal Undercurrent (SICU). The NBCU, SEC, and 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, which is particularly active in JuneAugust. The Solomon Sea WBC seasonal variability results from the combination of equatorial dynamics, remotely forced Rossby waves north of 108S, and the spinup and spindown of the subtropical gyre as a response of Rossby waves forced south of 108S.

Variability in Solomon Sea circulation derived from altimeter sea level data

“…The highest level of variability, up to 16 cm rms, was quite localized in front of the Solomon Strait. High variability is in accordance with complex current recirculation that developed in the model for this area (Melet et al 2010). The western and eastern Solomon Sea SLA signals are highly correlated (0.91 correlation) with their variability that varies from 5.6 to 10.4 cm rms, respectively.…”

“…The 1/12°resolution model used in this study is described in detail in Melet et al (2010). Therefore, only a brief description is provided here.…”

“…In situ observational studies have shown that boundary currents have their core in the subsurface (Lindstrom et al 1987(Lindstrom et al , 1990Butt and Lindstrom 1994;Murray et al 1995). Using a highresolution model analysis, Melet et al (2010) proposed a scheme for thermocline circulation and detailed a double system of boundary currents. The main system flowing along the PNG coast was formed by the NGCU that permanently flows equatorward.…”

A 1/36° model of the Solomon Sea embedded into a global ocean model: On the setting up of an interactive open boundary nested model system

“…In a second step, the increase in resolution is achieved by setting up a 1/36° AGRIF nest (dedicated section in the following) over the Solomon Sea in the intermediate model. This two steps embedding strategy, comparable to that used at coarser resolution by Melet et al 3 but only briefly mentioned in their paper, is motivated by the need to preserve the upscaling effects of the 1/36° Solomon Sea on a greater area of the southwest Pacific and still not to carry on computations with the whole 1/12° global model. In the perspective of operational use, the use of the intermediate one-way embedment of an intermediate model in a global model at the same resolution (1/12°) has several advantages.…”

“…In the Solomon Sea, Melet et al 3 have shown that the circulation is notably constrained by the narrowness of Vitiaz Strait, which is partly responsible for a splitting of the main western boundary current into two branches upstream of the strait. Therefore, an accurate representation of the bathymetry is essential to correctly model the Solomon Sea circulation.…”