Abstract
AbstractThe yearly paired process of slow growth and rapid melt of some 15 million square kilometers of Antarctic sea ice takes place with a regular asymmetry; the process has been linked to the relationship of the position of the ice edge with the band of low pressure that circles the continent between 60° and 70°S. In autumn, winds to the north of the low-pressure band slow the advancing ice edge. In summer, Ekman divergence created by opposing winds on either side of the low-pressure band opens up warm water regions that rapidly melt sea ice. We use the 40 ensemble members from the CESM-LENS historical run (1920–2005) to examine the relationship between the asymmetry in the annual cycle and the position and intensity of the low-pressure band. CESM-LENS reproduces the magnitude of the annual cycle of Antarctic sea ice extent with a short lag (2 weeks). Melt rate is the characteristic of the annual cycle that varies the most. Our results provide evidence that lower pressure leads to increased melt rates, which supports the importance of the role of divergence in increasing the melt rate of Antarctic sea ice. The role of winds during the growing season remains unquantified.
Highlights
Each year, the largest seasonal cycle in surface cover occurs in the Southern Ocean when sea ice grows from a minimum of less than 5 million square kilometers in February to a maximum of over 17 million square kilometers in September (Parkinson, 2019)
Our results show that the growth of Antarctic sea ice, the semi-annual oscillation (SAO) index and the location of the circumpolar trough (CPT) remain consistent across model years
We compared the mean annual cycles during the overlapping period between the model and satellite and reanalysis datasets (1979– 2005) to confirm that the CESM-LENS outputs adequately capture the seasonal cycle in Antarctic SIE and the changes in the CPT that are understood to drive this cycle
Summary
The largest seasonal cycle in surface cover occurs in the Southern Ocean when sea ice grows from a minimum of less than 5 million square kilometers in February to a maximum of over 17 million square kilometers in September (Parkinson, 2019). The SAO is a twice-yearly change in the intensity and location of the CPT in which it moves south and deepens in March and September/October, and moves north and weakens in June and December/January (Meehl, 1991; van Loon, 1967; Meehl and others, 1998; Raphael, 2004; Ackerley and Renwick, 2010). This oscillation is linked to differential cooling and heating rates between the Antarctic continent and the mid-latitude polar oceans (van Loon, 1967) and it affects many components of the Southern Hemisphere atmosphere and ocean. The resulting pattern can be depicted as a half-yearly wave in the sea level pressure difference between 50° and 65°S that explains more than 50% of the variability in the southern hemisphere sea level pressure (Raphael and Holland, 2006)
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