Estimating the annual cycle of sea-ice thickness and volume in the Ross Sea

Antarctic sea-ice thickness and concentration derived from satellite-based ice charts are compared to contemporaneous ship-based estimates of the same parameters collected in the Ross Sea between 130°W and 160°E from 1995 to 1998. Thickness variability exists on the scale of individual ice-chart polygons (tens of kilometers), but found compatible with WMO thickness ranges except in areas with high concentration of ridging. Very good agreement is found between the thickness distributions on the regional scale of the Ross Sea with an annual (climatological) average thickness of 0.65 m for the ice charts in comparison to 0.76 m for the ship-based measurements. Given these analyses, we proceed by examining resulting annual cycle and interannual sea-ice thickness and volume estimates for the Ross Sea region resolved to weekly time intervals. The weekly thickness estimates show the influence of thicker ice advected from the Amundsen Sea, new ice produced adjacent to Ross Ice Shelf, and changes in the Ross Sea polynya. The relationship between weekly ice extent and volume indicate a lag (one to nine weeks) of volume (mass) due to thickness processes during the growth season which differs from lateral extent in 1996-1998, while both extent and volume are more similar during the melt season. The maximum extent and volume occur simultaneously in 1995-1997, but in 1998 the volume peak lags extent by almost 5 weeks. Moreover, thickness variability is found to be more sensitive than extent to climate events during the 1996 La Nina and 1997/98 El Nino episodes. We postulate, therefore, that ice thickening processes evolve somewhat independent of sea-ice extent. These results contradict some existing model studies which show that sea-ice thickness and extent co-vary. Hence, the thickness estimates here demonstrate the importance of measured ice thickness which responds distinctly different from extent in response to dynamic and thermodynamic processes during the annual cycle.