Ocean bottom pressure variability in the Antarctic Divergence Zone off Lützow-Holm Bay, East Antarctica
We analyzed bottom pressure recorder (BPR) data obtained by the Japanese Antarctic Research Expedition (JARE) from 4500 m depth off Lützow-Holm Bay (66°50'S, 37°50'E) in the Antarctic Divergence Zone (ADZ). Data collected between December 16, 2004 and February 22, 2008 were processed after removing the tidal constituents with periods shorter than 1 month. The time series of monthly mean equivalent water height (OBP height) has a clear annual period. We correlated monthly GRACE Tellus OBP data with the JARE BPR heights. The GRACE data can account for about 38% of the variance of ocean signal observed by the JARE BPR. The variability in OBP height within the ADZ shows a maximum (2-4 cm) in the austral summer and a minimum (-2 to -4 cm) in the austral winter. In contrast, data from the coastal Syowa Station Tide Gauge (Syowa TG; 69.0°S, 39.6°E) have a maximum (7-8 cm) at the austral winter and a minimum (-5 to -6 cm) at the austral summer (opposite phase). We explained these OBP height variations in terms of the Ekman divergence mechanism using the wind velocity grids (2.5°×2.5°) of the NCEP/NCAR Reanalysis data. The monthly average of the vertical velocity (Ekman pumping) in the area of 65-67.5°S, 35-40°E is positive (upwelling) in all months, but varies seasonally, with its maximum in the austral winter and its minimum in the austral summer. This annual property is consistent with the variable differences observed between the JARE BPR and Syowa TG OBP heights. We subtracted this annual variation from the OBP monthly time series to obtain the non-seasonal OBP variability, and examined its correlation with the AAO index. The non-seasonal OBP variability of Syowa TG shows similar correlation strength (the coefficients of -0.50 to -0.52) for the short period (<7 months) and for the long period (7-19 months). The variability in the ADZ had stronger correlation for the long period (-0.79), compared with that for the short period (-0.32); this characteristic is attributed to zonal long-period response of the southern mode in the Southern Ocean. In conclusion, we presented the mechanisms of OBP fall in the ADZ and rise at the coastal Syowa TG during the austral winter, and relatively small OBP difference across the Antarctic Coastal Current (ACoC) region at the positive phase of the AAO index, related to north-south Ekman drifts.