Seasonal variability in the vertical current structure and kinetic energy in the Central Indian Ocean Basin

Abstract

Seasonal variability in the currents at the subsurface (450–670 m), intermediate (1150–1370 m), deeper (3450–3670 m) and near-bottom (4270–5100 m) depths at three mooring stations in the Central Indian Ocean Basin (CIOB) were documented in 1995–1997. The current-meter moorings were deployed thrice at station 1 (MS-1, MS-1A and MS-1B) in the northern basin (∼10°S), and twice at stations 2 (MS-2 and MS-2A) and 3 (MS-3 and MS-3A) in the southern basin (∼15°S). In the northern basin, seasonal variability in the currents at 500 m and 1200 m depth is due to the seasonal north–south shift in the westward-flowing South Equatorial Current (SEC). Low-frequency (30–60 day) oscillation, superimposed on high-frequency (inertial, semi-diurnal and diurnal) fluctuations, is noticed at all the depths including near-bottom depth (5100 m) where the U-component of current was dominant. The variability of the currents at the intermediate depth range (1150–1300 m), encompassing the three stations of 500 km apart, indicates the existence of anticyclonic gyral circulation. The depth variation of kinetic energy (KE) emphasises the bottom intensification of currents with minimum KE at deeper depths followed by relatively higher KE at abyssal depths. In the northern basin, the total KE exhibits seasonal variation; it is high during southern spring–summer and low during southern fall–winter. The eddy KE constitutes a larger part of the total KE at synoptic scale (12–15 day) fluctuations in the northern basin and meso-scale (inertial, semi-diurnal and diurnal) oscillations in the southern basin.

Introduction

Information on the flow field at various depths, particularly near-bottom, is essential in any study of deep-sea environmental impacts. Various observational programmes such as Deep Ocean Mining Environmental Study (DOMES), Disturbance and ReCOLinisation Experiment (DISCOL), Benthic Impact Experiment (BIE) and Japan Deepsea Impact Experiment (JET) have been initiated in the northeast Pacific Ocean (Hayes, 1979; Theil and Schreiner, 1990; Klein, 1993; Trueblood, 1993; Fukushima, 1995). Surprisingly, the near-bottom currents have been found to be relatively strong (∼10 cm/s). Niller and Koblinsky (1985) and Koblinsky et al. (1989), found that the currents throughout the water column in the Northeast Pacific were in phase and correlated with the wind-stress curl during winter. Summers (1986) also found correlation between the wind-stress curl and deep currents off the coast of northern California. In the area of Gulf Stream, near-bottom currents as strong as 72 cm/s (50 m above the bottom, 4500 m) were measured during High Energy Bottom Boundary Layer Experiment (HEBBLE) programme (Richardson et al., 1981).

Under Indian Deepsea Environment Experiment (INDEX), direct measurements of the ocean currents were carried out in the potential manganese nodule mining area of the Central Indian Ocean Basin (CIOB) during 1995–97. The currents were obtained at three mooring stations from 5 depths covering the subsurface, intermediate, deeper and near-bottom depth ranges. The moorings at station MS-1 (9°56′S, 74°55′E), station MS-2 (14°46′S, 71°55′E) and station MS-3 (14°58′S, 76°58′E) were repeated twice (stations Ms-1A, MS-2A & MS-3A). The station MS-1 mooring was re-occupied third time (station MS-1B). The observational (Wyrtki, 1971; Donguy and Meyers, 1995; Murty et al., 2000; Peter and Mizuno, 2000) and theoretical (Woodberry et al., 1989; Bahulayan and Shaji, 1996) studies of circulation in the southern tropical Indian Ocean highlight that in the CIOB the westward flowing South Equatorial Current (SEC) is the permanent annual feature of circulation. The SEC, in general, extends between 10°S and 16°S and exhibits seasonal (north-south) shifts with its northern boundary moving up to 4°S occasionally during southern winter (Murty et al., 2000). The CIOB mooring stations lie within the north-south extent of the SEC. The detailed bathymetry of the CIOB is documented by Kodagali et al. (1992). Sharma and Nath (1998) presented the morphological variations in the vicinity of station. Murty et al. (1999) studied the abyssal current structure and kinetic energy in the lower 500 m of the water column at station MS-1B. In this paper, the seasonal variability of the currents and the associated kinetic energy patterns at the three mooring stations in the CIOB are presented.