The analysis of large-scale turbulence characteristics in the Indonesian seas derived from a regional model based on the Princeton Ocean Model

Abstract

Abstract. Turbulence characteristics in the Indonesian seas on the horizontal scale of order of 100 km were calculated with a regional model of the Indonesian seas circulation in the area based on the Princeton Ocean Model (POM). As is well known, the POM incorporates the Mellor–Yamada turbulence closure scheme. The calculated characteristics are: twice the turbulence kinetic energy per unit mass, q2; the turbulence master scale, ℓ; mixing coefficients of momentum, KM; and temperature and salinity, KH; etc. The analyzed turbulence has been generated essentially by the shear of large-scale ocean currents and by the large-scale wind turbulence. We focused on the analysis of turbulence around important topographic features, such as the Lifamatola Sill, the North Sangihe Ridge, the Dewakang Sill, and the North and South Halmahera Sea Sills. In general, the structure of turbulence characteristics in these regions turned out to be similar. For this reason, we have carried out a detailed analysis of the Lifamatola Sill region because dynamically this region is very important and some estimates of mixing coefficients in this area are available. Briefly, the main results are as follows. The distribution of q2 is quite adequately reproduced by the model. To the north of the Lifamatola Sill (in the Maluku Sea) and to the south of the Sill (in the Seram Sea), large values of q2 occur in the deep layer extending several hundred meters above the bottom. The observed increase of q2 near the very bottom is probably due to the increase of velocity shear and the corresponding shear production of q2 very close to the bottom. The turbulence master scale, ℓ, was found to be constant in the main depth of the ocean, while ℓ rapidly decreases close to the bottom, as one would expect. However, in deep profiles away from the sill, the effect of topography results in the ℓ structure being unreasonably complicated as one moves towards the bottom. Values of 15 to 20 × 10−4 m2 s−1 were obtained for KM and KH in deep water in the vicinity of the Lifamatola Sill. These estimates agree well with basin-scale averaged values of 13.3 × 10−4 m2 s−1 found diagnostically for KH in the deep Banda and Seram Seas (Gordon et al., 2003) and a value of 9.0 × 10−4 m2 s−1 found diagnostically for KH for the deep Banda Sea system (van Aken et al., 1988). The somewhat higher simulated values can be explained by the presence of steep topography around the sill.