Interannual to diurnal variability in the near-surface scattering layer in Drake Passage

Backscattering strength was estimated from 127 shipboard surveys with an acoustic Doppler current profiler (ADCP) made during Drake Passage transits from 1999 to 2004. The backscattering strength is used to determine the characteristics of the near-surface scattering layer, which south of the Southern Antarctic Circumpolar Current Front (SACCF) is dominated by Antarctic krill (Euphausia superba). Diel vertical migration in the upper 150 m was the dominant variability observed in any single transect. When averaged over depth, there was a well-defined annual cycle in backscattering strength, with a factor of four increase from a late-winter minimum to a spring-summer maximum over a period of four months, followed by a more gentle decline during late summer and autumn. in addition, there were significant differences in scattering strength north and south of the Polar Front (PF) on both seasonal and interannnual time-scales. The average summer maximum to the north of the PF was more than twice the maximum to the south, but the winter minima were about the same. On interannual time-scales, scattering strength south of the PF displayed a negative linear trend primarily attributable to a fourfold decrease in backscattering strength south of the SACCF. No significant long-term trend in the scattering strength north of the SACCF was observed

The Ekman temperature and salt fluxes at 8°30′N in the Arabian Sea during the 1995 southwest monsoon

The Arabian Sea Ekman transport is an important component of the meridional overturning circulation of the Indian Ocean. Chereskin et al. (Geophys. Res. Lett. 24 (1997) 2541) presented direct estimates of the Ekman transport across latitude 8°30′N in the Arabian Sea for June and September during the 1995 southwest monsoon. In this paper, we use these measurements to determine the Ekman depth and the resultant heat and salt fluxes. In June, at the monsoon onset, the Ekman temperature and salt fluxes were estimated to be southward, 2.4±0.4 PW and 0.71±0.1×10 9 kg s −1 . The transport-weighted Ekman temperature and salinity were 29.0±0.5°C and 35.31±0.03 psu , not significantly different from surface values, 29.2°C and 35.28 psu , respectively. In September at the end of the monsoon, the Ekman temperature and salt fluxes had decreased in magnitude but were still southward, 0.77±0.4 PW and 0.27±0.1×10 9 kg s −1 . The transport-weighted temperature, 25.8±0.5°C, was 1.1°C colder than the surface value, and the transport-weighted salinity, 35.83±0.03 psu , was not significantly different from the surface value of 35.86 psu . For this pair of sections, the top of the pycnocline appeared to be a better approximation for the Ekman depth than either the mixed layer or a fixed depth, and our estimates of the Ekman heat and salt fluxes were integrated from the surface to the top of the pycnocline. Although uncertainty in the Ekman mass transport dominates the error in the Ekman heat and salt fluxes, determining the Ekman depth is also important in estimating the Ekman contribution to the heat budget of the tropical Indian Ocean. A decrease in Ekman temperature by 1.1°C resulted in a 5% decrease in the temperature transport estimated for September

Variability of water properties, heat and salt fluxes in the Arabian Sea, between the onset and wane of the 1995 southwest monsoon

We investigate the variability of the circulation, water masses, heat and salt fluxes in the Arabian Sea over the course of the southwest monsoon. Two zonal sections taken along 8[deg]30'N in 1995 as part of the Indian Ocean WOCE hydrographic program are used. The first was occupied in early June at the onset of the southwest monsoon winds, the second in late September, at the wane of the monsoon. The September section was found to be generally warmer (+0.32[deg]C) and saltier (+0.04) than in June, despite a 50 mm drop in mean sea level. Therefore, the common assumption that an increase in sea-surface height follows an increase in heat content (the hydrostatic response) does not hold. Instead, we conclude that the heat content increases due to the advection of Arabian Sea Surface Water and Red Sea Water onto the section from the north, and the drop in sea level is due to a loss of mass, rather than heat, from the water column. There are large uncertainties involved in diagnosing the heat-flux divergence across the Arabian Sea, because the seasonal variability of the water masses and circulation in the basin mean that our data are not representative of a steady state. We treat each section separately and find an oceanic heat export of -0.72 PW in June and -0.19 PW in September, implying a basin cooling rate of about -0.36 PW in June and a slight heating of 0.12 PW in September. In June the mass and heat balances are dominated by the Ekman transport and the Somali Current, with very flat density surfaces resulting in a small interior geostrophic transport. By September the Ekman transport has reduced, and it is primarily the interior transport that balances a strong Somali Current. There are two main overturning cells in June and September: A shallow one of approximate magnitude 15 Sv in June and 0 Sv in September, which reaches depths of no more than 500 m and is driven by Ekman divergence at the surface; and a deep cell of magnitude 1 Sv representing a weak inflow and subsequent upwelling of Circumpolar Deep water. The deep cell implies a basin-averaged upwelling velocity of 3.2 x 10-5 cm s-1 through 2200