Changes in synoptic weather patterns in the polar regions in the twentieth and twenty-first centuries, part 1: Arctic

Embed Size (px)

Text of Changes in synoptic weather patterns in the polar regions in the twentieth and twenty-first...

  • INTERNATIONAL JOURNAL OF CLIMATOLOGYInt. J. Climatol. 26: 10271049 (2006)Published online 6 March 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.1306

    CHANGES IN SYNOPTIC WEATHER PATTERNS IN THE POLAR REGIONSIN THE TWENTIETH AND TWENTY-FIRST CENTURIES, PART 1: ARCTIC

    JOHN J. CASSANO,a,* PETTERI UOTILAb and AMANDA LYNCHba Cooperative Institute for Research in Environmental Sciences and Department of Atmospheric and Oceanic Sciences, University of

    Colorado, Boulder, CO, USAb School of Geography and Environmental Science, Monash University, Monash, Australia

    Received 30 April 2005Revised 30 November 2005Accepted 1 December 2005

    ABSTRACT

    An analysis of late twentieth and twenty-first century predictions of Arctic circulation patterns in a ten-model ensembleof global climate system models, using the method of self-organizing maps (SOMs), is presented. The model simulationswere conducted in support of the fourth assessment report of the intergovernmental panel on climate change (IPCC). Theanalysis demonstrates the utility of SOMs for climate analysis, both as a tool to evaluate the accuracy of climate modelpredictions, and to provide a useful alternative view of future climate change.

    It is found that not all models accurately simulate the frequency of occurrence of Arctic circulation patterns. Someof the models tend to overpredict strong high-pressure patterns while other models overpredict the intensity of cycloniccirculation regimes. In general, the ensemble of models predicts an increase in cyclonically dominated circulation patternsduring both the winter and summer seasons, with the largest changes occurring during the first half of the twenty-firstcentury. Analysis of temperature and precipitation anomalies associated with the different circulation patterns revealscoherent patterns that are consistent with the different circulation regimes and highlight the dependence of local changesin these quantities to changes in the synoptic scale circulation patterns. Copyright 2006 Royal Meteorological Society.

    KEY WORDS: Arctic; synoptic climatology; climate change; global climate model

    1. INTRODUCTION

    In 2001, at its eighteenth session, the Intergovernmental Panel on Climate Change (IPCC) agreed to prepare afourth comprehensive assessment report (AR4) of the scientific, technical, and socioeconomic understandingof anthropogenic climate change and its consequences. A key element of the physical science basis of thisassessment has been the development of global projections by climate system models from around the world.For these projections to contribute to our understanding of the sensitivity of the system, it is important tocharacterize in detail the range of performance of these models in simulating observed and future change.Such an undertaking, in the context of the complex system that is the earths climate, can only be attemptedthrough the collective effort of a community of scientists analyzing all aspects of model behavior. This paperis one such contribution to that effort, and we choose as our focus a description of changes in the circulationof the high northern latitudes simulated by an ensemble of global climate system models. The companionpaper (Lynch et al., 2005) addresses the circulation of the high southern latitudes.

    It is now well known that the Arctic region demonstrates many of the expected consequences of thepolar amplification of global climate change (Serreze et al., 2000; ACIA, 2004; Hinzman et al., 2005). Thesechanges have global implications, both as a model for the detection of anthropogenic climate forcing and in the

    * Correspondence to: John J. Cassano, Cooperative Institute for Research in Environmental Sciences, University of Colorado, 216 UCB,Boulder, CO 80309, USA; e-mail: cassano@cires.colorado.edu

    Copyright 2006 Royal Meteorological Society

  • 1028 J. J. CASSANO, P. UOTILA AND A. LYNCH

    broader sense through the effects on freshwater cycling, thermohaline circulation, the terrestrial carbon cycle,and biodiversity. One manifestation of this observed change is to be found in the atmospheric circulation ofthe northern high latitudes, which may be described in a number of ways (e.g. Walsh et al., 1996; Thompsonand Wallace, 1998; McCabe et al., 2001). This aspect of Arctic change is of particular importance becauseof its role in the modulation of Arctic sea ice distribution and North Atlantic ice export, and the subsequentimpacts on the global thermohaline circulation. For example, Walsh and Crane (1992) and Bitz et al. (2002)describe the sensitivity of simulated Arctic sea ice to changes in atmospheric circulation patterns. Bitz et al.(2002) highlight the importance of accurate simulation of high sea-level pressure (SLP) over the BeaufortSea in winter and low SLP over the Arctic Ocean in summer for accurate simulation of sea ice thickness inthe Arctic basin. The results presented here describe changes in Arctic circulation of the present and futureas represented in global climate models through the lens of synoptic climatology.

    The field of synoptic climatology provides a powerful method to study the climate of a region by stratifyinglarge volumes of data (daily or higher temporal resolution fields of the atmospheric state) into a small numberof categories on a physically meaningful basis. Such an approach provides important information on theweather processes that control the local climate, which may often be hidden by monthly or seasonal meanfields (Barry and Perry, 2001; Hanson et al., 2004). An important step in this type of analysis is developinga robust classification scheme that can be applied to large volumes of data. Barry and Perry (2001), andreferences therein, provide a detailed overview of synoptic climatology and its applications, but we summarizethe important points below.

    Most commonly, cyclone track and cyclone event climatologies have been developed using objectivealgorithms such as SLP, SLP Laplacian, vorticity, or potential vorticity minimization (e.g. Serreze et al.,1993; Sinclair, 1994; Serreze et al., 1997; Lambert et al., 2002; Paciorek et al., 2002; Cao and Zhang, 2004;Zhang et al., 2004). Some tracking schemes have been applied to both cyclones and anticyclones (Pezza andAmbrizzi, 2003). These approaches have been demonstrated to be physically consistent and reproducible, andhence create highly useful data sets. An alternative approach has been the application of synoptic timescalefilters to pressure or height data and analyzing the variance of the filtered data (Trenberth, 1991; Nakamuraand Shimpo, 2004). Other authors have argued for the use of unfiltered data (Berbery and Vera, 1996;Rao et al., 2002), but in any case the relationship of these variances to cyclone and anticyclone trajectoriesremains problematic (Wallace et al., 1988). A more general method for analyzing the circulation as a whole(as opposed to only cyclones, or cyclone and anticyclone centers) is the use of empirical orthogonal function(EOF) analysis (e.g. Kidson and Sinclair, 1995; Thompson and Wallace, 1998; Vera, 2003; Carvalho et al.,2005). Such approaches have been useful in identifying connections to large-scale modes of variability suchas the Arctic and Antarctic Oscillations and El Nino-Southern Oscillation (ENSO).

    Research efforts are underway to evaluate the IPCC global climate system model simulations with manyof the techniques discussed above as part of the larger analysis effort for the IPCC AR4. In this paper, weuse the method of self-organizing maps (SOMs) (Kohonen, 2001) to derive a synoptic climatology for theArctic from an ensemble of current and twenty-first century climate simulations conducted in support ofthe IPCC AR4. The SOM technique employs a neural network algorithm that uses unsupervised learning todetermine generalized patterns in data. We analyze the distribution of Arctic synoptic weather patterns in thelate twentieth century in a range of climate models and reanalyses, and changes in synoptic weather patternsover the twenty-first century on the basis of climate model predictions. The synoptic pattern classificationtechnique is used to create a continuum of 35 synoptic patterns on the basis of daily SLP data for the seasonsdefined by December, January and February (DJF) and June, July and August (JJA). The analysis is thenused as a framework to analyze trends in temperature and precipitation over the same time periods. Tenmodels participating in the IPCC Model Analysis1 project were selected for the study, and the future scenarioused is the Special Report on Emissions Scenarios (SRES) A1B (Nakicenovic and Swart, 2000). (The A1scenario family represents rapid economic growth, a global population that peaks in midcentury, and therapid introduction of new technologies. The A1B group is a representative scenario that postulates a balancebetween fossil-intensive and nonfossil energy sources.) We expect that useful comparisons between the SOM-based analysis presented here and other analyses using the methods discussed in the previous paragraph willbe carried out in the future and will highlight the advantages and disadvantages of the different analysis

    Copyright 2006 Royal Meteorological Society Int. J. Climatol. 26: 10271049 (2006)

  • ARCTIC SYNOPTIC WEATHER PATTERNS IN THE TWENTIETH AND TWENTY-FIRST CENTURIES 1029

    methods. By using a diverse set of analysis tools, the climate science community should be able to betterunderstand and evaluate the predicted climate change for the twenty-first century.

    The next section summarizes the data and methods used in this analysis, including a detailed discussionof the SOM algorithm, and presents the master synoptic pattern classifications that result for each season.Section 3 describes the ways in which the contemporary climate simulations are distri