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The Influence of Winter Cloud on Summer Sea Ice in the Arctic, 1983

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The Influence of Winter Cloud on Summer Sea Ice in the Arctic, 1983 PUBLICATIONS Journal of Geophysical Research: Atmospheres RESEARCH ARTICLE The influence of winter cloud on summer sea ice 10.1002/2015JD024316 in the Arctic, 1983–2013 Key Points: Aaron Letterly1, Jeffrey Key2, and Yinghui Liu1 • Arctic winter cloud cover causes changes in summer sea ice thickness 1Cooperative Institute of Meteorological Satellite Studies, University of Wisconsin-Madison, Madison, Wisconsin, USA, and area 2 • Data from the last three decades show NOAA/NESDIS, Madison, Wisconsin, USA that clouds impose regional and large-scale changes on sea ice • The 2007 sea ice minimum can be Abstract Arctic sea ice extent has declined dramatically over the last two decades, with the fastest decrease explained in part by anomalous winter and greatest variability in the Beaufort, Chukchi, and East Siberian Seas. Thinner ice in these areas is more cloud forcing susceptible to changes in cloud cover, heat and moisture advection, and surface winds. Using two climate reanalyses and satellite data, it is shown that increased wintertime surface cloud forcing contributed to the 2007 summer sea ice minimum. An analysis over the period 1983–2013 reveals that reanalysis cloud forcing Correspondence to: anomalies in the East Siberian and Kara Seas precondition the ice pack and, as a result, explain 25% of the A. Letterly, fi [email protected] variance in late summer sea ice concentration. This nding was supported by Moderate Resolution Imaging Spectroradiometer cloud cover anomalies, which explain up to 45% of the variance in sea ice concentration. Results suggest that winter cloud forcing anomalies in this area have predictive capabilities for summer sea ice Citation: Letterly, A., J. Key, and Y. Liu (2016), The anomalies across much of the central and Eurasian Arctic. influence of winter cloud on summer sea ice in the Arctic, 1983–2013, J. Geophys. Res. Atmos., 121, doi:10.1002/ 1. Introduction 2015JD024316. Over the last two decades, the Arctic has experienced a significant decline in sea ice extent. The decreasing Received 19 OCT 2015 ice extent corresponds to a warming of the Arctic at more than twice the global average, known as Arctic Accepted 15 FEB 2016 fi fi Accepted article online 18 FEB 2016 ampli cation. Though Arctic ampli cation is anticipated to cause warming through the year 2100 [Holland and Bitz, 2003; Zhang and Walsh, 2006; Overland and Wang, 2013] with a continuing decrease in sea ice extent, interannual variation in Arctic sea ice extent is difficult to predict. Arctic sea ice extent is forced by a number of feedback mechanisms, with ice-albedo and cloud-radiation feedbacks contributing significantly toward sea ice growth and melt [Serreze and Francis, 2006; Curry et al., 1996]. Large-scale atmospheric variability has also been shown to contribute to the sea ice decline, particu- larly changes in such modes of circulation as the Arctic Dipole Anomaly pattern [Overland et al., 2012; Wang et al., 2009] and the Northern Annular Mode [Deser and Teng, 2008; Ogi and Rigor, 2013]. The effect of Arctic clouds on the surface energy budget is substantial [Curry et al., 1996; Intrieri et al., 2002; Liu et al., 2008; Tjernström et al., 2008]. Arctic clouds warm the surface except during part of the summer [Intrieri et al., 2002; Stone, 1997; Schweiger and Key, 1994]. Seasonal changes in Arctic cloud amount have been observed and will also affect the surface energy budget [Liu et al., 2007, 2008, 2009; Schweiger, 2004; Wang and Key, 2003, 2005b]. The effect of sea ice changes on cloud cover is well documented [Vavrus et al., 2009, 2011; Schweiger et al., 2008a; Cuzzone and Vavrus, 2011; Kay and Gettelman, 2009; Palm et al., 2010; Liu et al., 2012], but the capability of cloud cover changes to influence sea ice is less studied. Pithan and Mauritsen [2013] examined a variety of climate models to determine the effect of clouds on surface temperature. Their results showed that climate models gave different answers regarding the net warming or cooling effect of clouds on the Arctic surface. Also uncertain is the effect of downwelling shortwave anomalies on sea ice. Kay et al. [2008] used climate rea- nalyses to determine that decreases in summer cloud cover increased the downwelling shortwave radiation at the surface and contributed to the 2007 ice extent minimum, citing 2.4 K of warming to the ocean by cloud and radiation anomalies alone. In contrast, Nussbaumer and Pinker [2012] found that ice areas receiving the largest amounts of downwelling shortwave radiation from January to June do not correspond to negative sea ice concentration anomalies. Schweiger et al. [2008b] also found that the downwelling shortwave flux and negative cloud anomalies over the summer months had no appreciable contribution to the sea ice extent ©2016. American Geophysical Union. minimum. Kauker et al. [2009] discussed the minor impacts of cloud reductions in summer on ice extent but All Rights Reserved. related the 2007 minimum primarily to wind conditions in May and June, surface temperature in September, LETTERLY ET AL. WINTER CLOUDS INFLUENCE SUMMER SEA ICE 1 Journal of Geophysical Research: Atmospheres 10.1002/2015JD024316 and ice thickness in March. Perovich et al. [2008] assigned the primary cause of enhanced ice loss in the Beaufort Sea during the summer of 2007 to be enhanced heating of the upper ocean, though cloud effects on this heating were not explicitly mentioned. Many of these studies are devoted to the radiative effects of spring or summer clouds, seasons in which downwelling shortwave radiation can be the largest contributor to the surface energy budget. During most of the year, the downwelling longwave flux dominates the surface radiation budget. Stone et al. [2005] found that a 5% increase in winter cloud cover during February at Barrow, Alaska, led to increased surface tempera- tures of 1.4°C, on average, over a 33 year period. Over sea ice, Kapsch et al. [2013] and Graversen et al. [2011] showed that increased downwelling longwave radiation caused by springtime cloud cover corresponds to years with negative sea ice anomalies in the summer. Similarly, Park et al. [2015] show that summers with anomalously small sea ice area are preceded by winters with above-average downward longwave radiation, particularly over the Eurasian portion of the Arctic Ocean. They attribute this primarily to anomalously strong moisture flux intrusions into the Arctic. The influence of wintertime cloud cover on surface radiation anoma- lies has been explored at length [Curry et al., 1996], but fewer studies have addressed the seasonally lagged contribution of winter clouds on summertime sea ice. Francis et al. [2009] explored Arctic atmospheric “memory” between autumn weather and summer sea ice. This system memory is further explored by Liu and Key’s [2014] use of climate reanalyses and satellite data to determine that below-average cloud cover in the winter of 2013 contributed to the summertime sea ice area rebound from the record low of 2012. This study extends the work of Liu and Key [2014] by examining the role of wintertime cloud anomalies on summertime sea ice concentration over three decades. Cloud radiative forcing from two climate reanalysis products, a satellite cloud cover product, and sea ice concentration from passive microwave satellite data are the primary data sets employed. The validity of using reanalysis results is supported by comparison to passive and active remote sensing products. The winter preceding the 2007 record (at the time) sea ice minimum is used as a case study to examine the lagged effect of cloud forcing on rapid sea ice change. Arctic clouds are analyzed in a climatological context by determining long-term radiative contributions and relationships with ice concentration over the satellite record. Though large-scale atmospheric heat transport and ice dynamics undoubtedly contributed to the significant decrease in sea ice extent over this time period, this study does not attempt to address them. Instead, it focuses on the increases and decreases in winter cloud cover that, year after year, may precondition the marginal sea ice zones for rapid changes in ice area. Understanding the relationship between wintertime clouds and summertime ice will aid in determining whether clouds play a significant role in abrupt year-to-year ice concentration changes, the long-term hemispheric sea ice decline, or both. 2. Data and Methods The geophysical variables of primary interest are cloud radiative effect, or “forcing,” and sea ice concentration. Cloud forcing refers to the difference in the net radiation flux between cloudy and clear-sky conditions. In winter, positive cloud radiative forcing creates a less negative surface radiation budget and inhibits ice formation, whereas a negative forcing leads to greater ice growth. Cloud radiative forcing is the integrated partial derivative of net radiative flux with respect to cloud fraction: Ac ∂S CS ¼ ∫ S da ¼ S ðÞÀA S ðÞ0 (1) S 0 ∂a S c S Ac ∂F CF ¼ ∫ S da ¼ F ðÞÀA F ðÞ0 (2) S 0 ∂a s c S CNETS ¼ CSS þ CFS (3) where CSS,CFS, and CNETS are the shortwave, longwave, and net cloud forcing at the surface, Ac is the total cloud amount, a is the cloud fraction, and SS and FS are the net shortwave and longwave fluxes at the surface. To represent the total seasonal cloud forcing anomaly, the monthly anomaly fields were summed over four winter months from November to February into a single value, hereafter referred to as the “cumulative cloud forcing.” There is no downwelling shortwave radiation during the winter months at very high latitudes (the “polar night”) and little at the lower latitudes of the Arctic. Nevertheless, the shortwave cloud forcing is included in the cumulative cloud forcing. LETTERLY ET AL. WINTER CLOUDS INFLUENCE SUMMER SEA ICE 2 Journal of Geophysical Research: Atmospheres 10.1002/2015JD024316 The primary data sets used in this study include satellite-derived sea ice concentration and cloud radiative forcing from climate reanalyses.
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