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.