Unprecedented Arctic ozone loss in 2011

Gloria L. Manney, Michelle L. Santee, Markus Rex, Nathaniel J. Livesey, Michael C. Pitts, Pepijn Veefkind, Eric R. Nash, Ingo Wohltmann, Ralph Lehmann, Lucien Froidevaux, Lamont R. Poole, Mark R. Schoeberl, David P. Haffner, Jonathan Davies, Valery Dorokhov, Hartwig Gernandt, Bryan Johnson, Rigel Kivi, Esko Kyrö, Niels Larsen, Pieternel F. Levelt, Alexander Makshtas, C. Thomas McElroy, Hideaki Nakajima, Maria Concepción Parrondo, David W. Tarasick, Peter von der Gathen, Kaley A. Walker & Nikita S. Zinoviev

Nature 478, 469–475 (27 October 2011) doi:10.1038/nature10556

Abstract 

   Chemical ozone destruction occurs over both polar regions in local winter–spring. In the Antarctic, essentially complete removal of lower-stratospheric ozone currently results in an ozone hole every year, whereas in the Arctic, ozone loss is highly variable and has until now been much more limited. Here we demonstrate that chemical ozone destruction over the Arctic in early 2011 was—for the first time in the observational record—comparable to that in the Antarctic ozone hole. Unusually long-lasting cold conditions in the Arctic lower stratosphere led to persistent enhancement in ozone-destroying forms of chlorine and to unprecedented ozone loss, which exceeded 80 per cent over 18–20 kilometres altitude. Our results show that Arctic ozone holes are possible even with temperatures much milder than those in the Antarctic. We cannot at present predict when such severe Arctic ozone depletion may be matched or exceeded.

Points

*  In the winter polar lower stratosphere, low temperatures induce condensation of water vapour and nitric acid (HNO₃) into polar stratospheric clouds (PSCs). PSCs and other cold aerosols provide surfaces for heterogeneous conversion of chlorine from longer-lived reservoir species, such as chlorine nitrate (ClONO₂) and hydrogen chloride (HCl), into reactive (ozone-destroying) forms, with chlorine monoxide (ClO) predominant in daylight.

*  In the Antarctic, enhanced ClO is usually present for 4–5 months (through to the end of September), leading to destruction of most of the ozone in the polar vortex between ~14 and 20 km altitude. Although ClO enhancement comparable to that in the Antarctic occurs at some times and altitudes in most Arctic winters, it rarely persists for more than 2–3 months, even in the coldest years. Thus chemical ozone loss in the Arctic has until now been limited, with largest previous losses observed in 2005, 2000 and 1996.

*  The 2010–11 Arctic winter–spring was characterized by an anomalously strong stratospheric polar vortex and an atypically long continuously cold period. In February–March 2011, the barrier to transport at the Arctic vortex edge was the strongest in either hemisphere in the last ~30 years

*   Daily minimum temperatures in the 2010–11 Arctic winter were not unusually low, but the persistently cold region was remarkably deep (Supplementary Figs 1, 2). Temperatures were below T_act for more than 100 days over an altitude range of ~15–23 km, compared to a similarly prolonged cold period over only ~20–23 km altitude in 1997; below ~19 km altitude, T < T_act continued for ~30 days longer in 2011 than in 1997 (Supplementary Fig. 1b). In 2005, the previous year with largest Arctic ozone loss⁷, T < T_act occurred for more than 100 days over ~17–23 km altitude, but all before early March

a–l, Maps (right) and vortex-averaged time series (left) at 485 K potential temperature (~20 km, ~50 hPa) for four different gases: HNO₃ (a, b, c), HCl (d, e, f), ClO (g, h, i) and O₃ (ozone; j, k, l); mixing ratios from Aura MLS are shown. Averaging for the time series is done within the white contour shown on the maps. Blue (purple) triangles on time series, 1995–96 (1996–97) values from UARS MLS. Line colours/shading as in Fig. 1, but shading is for Aura MLS measurements from 2005–10. Antarctic dates are shifted by six months (top axis on time series) to show the equivalent season. Vertical lines show dates of maps in 2011 (2010) in the Arctic (Antarctic). Black overlays on HNO₃ maps, T_act (~196 K at this level); HNO₃ may be sequestered in PSCs at lower temperatures. Dotted black/white contour on ClO maps, 92° SZA, poleward of which measurements were taken in darkness. Yellow/black triangles on ozone maps, locations of the profiles in Fig. 3.

Arabian Sea tropical cyclones intensified by emissions of black carbon and other aerosols

 Amato T. Evan, James P. Kossin,Chul ‘Eddy’ Chung & V. Ramanathan

Nature 479,94–97 (03 November 2011) doi:10.1038/nature10552

Abstract

  Throughout the year, average sea surface temperatures in the Arabian Sea are warm enough to support the development of tropical cyclones, but the atmospheric monsoon circulation and associated strong vertical wind shear limits cyclone development and intensification, only permitting a pre-monsoon and post-monsoon period for cyclogenesis. Thus a recent increase in the intensity of tropical cyclones over the northern Indian Ocean is thought to be related to the weakening of the climatological vertical wind shear. At the same time, anthropogenic emissions of aerosols have increased sixfold since the 1930s, leading to a weakening of the southwesterly lower-level and easterly upper-level winds that define the monsoonal circulation over the Arabian Sea. In principle, this aerosol-driven circulation modification could affect tropical cyclone intensity over the Arabian Sea, but so far no such linkage has been shown. Here we report an increase in the intensity of pre-monsoon Arabian Sea tropical cyclones during the period 1979–2010, and show that this change in storm strength is a consequence of a simultaneous upward trend in anthropogenic black carbon and sulphate emissions. We use a combination of observational, reanalysis and model data to demonstrate that the anomalous circulation, which is radiatively forced by these anthropogenic aerosols, reduces the basin-wide vertical wind shear, creating an environment more favourable for tropical cyclone intensification. Because most Arabian Sea tropical cyclones make landfall, our results suggest an additional impact on human health from regional air pollution.