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HARVARD EPS 5 - Non-annular atmospheric circulation change

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Non-annular atmospheric circulation change induced by stratosphericozone depletion and its role in the recent increase of Antarcticsea ice extentJohn Turner,1Josefino C. Comiso,2Gareth J. Marshall,1Tom A. Lachlan-Cope,1Tom Bracegirdle,1Ted Maksym,1Michael P. Meredith,1Zhaomin Wang,1and Andrew Orr1Received 29 January 2009; revised 11 March 2009; accepted 25 March 2009; published 23 April 2009.[1] Based on a new analysis of passive microwave satellitedata, we demonstrate that the annual mean extent of Antarcticsea ice has increased at a statistically significant rate of0.97% dec1since the late 1970s. The largest increase hasbeen in autumn when there has been a dipole of significantpositive and negative trends in the Ross and Amundsen-Bellingshausen Seas respectively. The autumn increase inthe Ross Sea sector is primarily a result of stronger cyclonicatmospheric flow over the Amundsen Sea. Modelexperiments suggest that the trend towards strongercyclonic circulation is mainly a r esult of stratosphericozone depletion, which has strengthened autumn windspeeds around the continent, deepening the Amundsen SeaLow through flow separation around the high coastalorography. However, statistics derive d from a climatemodel control run suggest tha t the observed sea iceincrease might still be within the range of natural climatevariability.Citation: Turner, J., J. C. Comiso, G. J. Marshall,T. A. Lachlan-Cope, T. Bracegirdle, T. Maksym, M. P. Meredith,Z. Wang, and A. Orr (2009), Non-annular atmospheric circulationchange induced by stratospheric ozone depletion and its role inthe recent increase of Antarctic sea ice extent, Geophys. Res. Lett.,36, L08502, doi:10.1029/2009GL037524.1. Introduction[2] Since the 1970s the two polar regions have experi-enced markedly different trends in sea ice extent (SIE)(Figure 1). In the Arctic, late summer ice reached recordminima in 2005 and 2007, with the ice in September 2007covering only 4.1  106km2, which was 39% belowclimatology. In contrast, Antar ctic SIE has actually in-creased over the same period. Zwally et al. [2002] showedthat over 1979–1998 the SIE had increased by 11.2 ± 4.2 103km2yr1or 0.98 ± 0.37% dec1. Regionally the trendswere positive in the Weddell Sea, Pacific Ocean and Rosssectors, and negative in the Indian Ocean and Amundsen-Bellingshausen Sea (ABS) sectors [see also Yuan andMartinson, 2000].[3] Basing its conclusions on data processed with theBootstrap algorithm [Comiso, 2003], the IntergovernmentalPanel on Climate Change (IPCC) noted that there had beena small positive trend in total Antarctic SIE of 5.6 ± 9.2 103km2yr1or 0.47 ± 0.80% dec1over 1978–2005, anincrease that they noted was not statistically significant.[4] A n improved version of the Bootstrap algorithm[Comiso and Nishio, 2008] also gave a positive trend inthe monthly anomalies of total Antarctic SIE, with the valueof 0.9 ± 0.2% dec1for 1978–2006 being very similar tothe value produ ced by Zwally et al. The study reconfirmedthe contrasting trends in the ABS and Ross Sea, which havebeen linked via model experiments to mean sea levelpressure (MSLP) across the ABS where lower values resultin enhanced northerly flow to the west of the AntarcticPeninsula and less SIE [Lefebvre et al., 2004]. Similarly, thestronger southerly winds over the Ross Sea promote greaterSIE.[5] The mean tropospheric flow pattern at high southernlatitudes has a strong wave number 3 pattern. Based onrotating tank experiments, Baines and Fraedrich [1989]proposed that the cyclonic eddies were forced by flowseparation around coastal irregularities, with the AmundsenSea Low (ASL) being present because of strong flowaround the northward extension of the orography near150° E and the presence of the Ross Sea embayment.[6] Lefebvre et al. [2004] linked this pattern of pressurechange across the ABS and ice increase/decrease to changesin the Southern Annular Mode (SAM). They found that, itsname not withstanding, the non-annular component of theSAM had the greatest impact in the ABS/Ross Sea areas.The SAM has become more positive in recent decades,primarily because of the combined effects of increasinggreenhouse gases and, most importantly, the development ofthe Antarctic ozone hole [Arblaster and Meehl, 2006].Although the ozone hole is a phenomena of the australspri ng, the impact on the tropospheric flow is greatestduring summer and autumn. Lefebvre et al. [2004] notedthat years when the SAM index was high there was more(less) sea ice in the Ross Sea (ABS) sector. However, theydid not find that the trend in the SAM was related to thetrend in the SIE.[7] The observed pattern of SIE change across the ABS/Ross Sea, and particularly the periods of ice advance andretreat, has also b een linked to the El Nin˜o-SouthernOscillation [Yuan, 2004], although the correlations wereless than those found with the SAM [Stammerjohn et al.,2008].[8] It is important to understand why Antarctic SIE hasincreased in recent decades and the potential role ofgreenhouse gas increase and stratospheric ozone depletion.GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L08502, doi:10.1029/2009GL037524, 2009ClickHereforFullArticle1British Antarctic Survey, National Environment Research Council,Cambridge, UK.2NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.Copyright 2009 by the American Geophysical Union.0094-8276/09/2009GL037524$05.00L08502 1of5Therefore, to gain insight into how ozone loss is influencingthe atmospheric circulation and SIE we present the result sof model experiments forced with different stratosphericozone concentrations.2. Data and Model Experiments[9] We use the SIE data produced by the Bootstrap 2algorithm of Comiso and Nishio [2008], which covers theperiod 1979 –20 07. The data are considered to be animprovement over the earlier Bootstrap algorithm data[see Comiso and Nishio, 2008]. Atmospheric circulationchanges since 1979 a re ex amined using the ECMWF40 year reanalysis data and the recent operational ECMWFanalyses.[10] We consider how the models of the IPCC FourthAssessment Report (AR4) have simulated recent atmospher-ic circulation changes and examine the relationship betweensea ice changes and the anthropogenic forcing applied tothese models. Our main focus is on model experiments thatgive insight into the linkage between Antarctic stratosphericozone depletion and SIE. Coupled atmosphere-ocean mod-els are the foundation for most climate initiatives. However,the models have many

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