1. INTRODUCTION

1. Cyclone Tracks From GISS GCM Simulation of 2049-2051 Cyclone Tracks From GISS GCM Simulation of 1999-2001 60ºN 60ºN 40ºN 40ºN 20ºN 20ºN 120ºW 60ºW 120ºW 90ºW 60ºW 90ºW number of cyclones in a given summer 1951-2004 mean of summer cyclones 1951-2004 standard deviation Cyclone Tracks of July 1960 from NCEP/NCAR Reanalysis Data 60ºN 40ºN 20ºN 120ºW 90ºW 60ºW Cyclone Tracks of July 1960 from GISS GCM Simulation 60ºN 40ºN 20ºN 120ºW 90ºW 60ºW Air Quality Degradation due to Greenhouse Warming Decreasing the Frequency of Mid-latitude Cyclones Eric M. Leibensperger (eleibens@fas.harvard.edu)1, Loretta J. Mickley1, Daniel J. Jacob1, Shiliang Wu1, Jean Lerner2, David Rind2 1 Harvard University2 NASA Goddard Institute for Space Studies 4. N. AMERICAN CYCLONES IN THE 2050 CLIMATE 1. INTRODUCTION Mid-latitude cyclones are a major agent in the ventilation of the U.S. boundary layer. Greenhouse warming is expected to decrease mid-latitude cyclone frequency by weakening the temperature contrast between the tropics and the poles [Lambert et al., 2006]. Analysis of observational datasets for the past 50 years has revealed a decreasing trend in the number of mid-latitude cyclones and a poleward shift of cyclone tracks in the Northern Hemisphere [McCabe et al., 2001].A decrease in the number of cyclones tracking through the northern United States and southern Canada or a northward shift in the mean storm track in the summer would degrade air quality in the United States by removing one of the primary mechanisms of pollution relief. Here we analyze long-term trends in North American cyclones for 1950-2050 using atransient-climate simulation with the NASA Goddard Institute for Space Studies (GISS) General Circulation Model (GCM) III using two horizontal resolutions (2ºx2.5º and 4ºx5º). The model is evaluated with present-day cyclone climatology from the NCEP/NCAR Reanalysis. We show that the GCM can reproduce at least qualitatively the observed decrease and shift of cyclone tracks in the second half of the 20th century (1951-2004). A future-climate simulation following the Special Report on Emission Scenarios (SRES) A1B emission inventory predicts a continuation of this decreasing trend, with significant negative implications for air quality. We then applied our cyclone-tracking analysis to diagnose the change in cyclone frequency between the present and future climates. The future climate is the result of a transient simulation with the GISS GCM III at 4ºx5º resolution, with greenhouse gas emissions following the A1B scenario. This simulation has been applied by Wu et al. (see poster) as part of the GCAP project to study the effects of climate change on air quality. 3. TRENDS IN THE LAST FIFTY YEARS 2. STORM TRACKING Twentieth-century trends in North American cyclones were diagnosed in the GISS GCM with 2ºx2.5º resolution by using the Storm Tracker to count individual cyclone tracks in the domain (40º-50ºN, 60º-75ºW) for the 1951-2004 summers. We see a significant decrease in cyclone frequency. The plot below shows the standardized departure for individual years from the 1951-2004 mean: We track cyclones using the NASA GISS Storm Tracking algorithm [Chandler and Jonas, 1999]. The algorithm tracks cyclones by analyzing time series of sea level pressure (SLP) fields. We use a temporal resolution of 6 hours for the SLP fields. For each time step the procedure searches for pressure minima extending 720km in radius. The low pressure center is tracked through time by assuming that the closest neighbor in the next time step within 720km is the same system. In order for a system to be considered a cyclone, it must exist for at least 24 hours (tracked 4 times) and have a central pressure no higher than 1010 hPa. The above plot shows the cyclone tracks in summer 1999-2001 and 2049-2051. Our analysis shows that there is a 14% decrease in the frequency of cyclones in the 2049-2051 time period. The study by Lambert et al. showcased a decrease in future cyclone frequency in all 15 climate models examined. Within their study, Lambert et al., found a decrease in cyclone frequency across all SRES emission inventory scenarios. The thinning of the storm tracks affects not only the northeastern United States, but also the midwestern states. The shift and change in frequency of mid-latitude cyclones is a likely factor in the increase of pollution events in the Northeast and Midwest found in the GCAP study (Wu et al., see poster) and corresponds to the results of Mickley et al. [2004]. Standardized Departure (c) We see from this metric (below, left) that the cyclone frequency has decreased by 2σ (a 40% decrease) over the 1950-2000 period (with a summer mean of 12 cyclones and standard deviation of 3 cyclones). This dramatic drop is similar in relative magnitude to the winter findings of McCabe et al (below, right). 5. CONCLUSIONS AND FUTURE WORK Mid-latitudes cyclones tracking across southern Canada are a crucial mechanism for ventilating pollution from the eastern United States. Analysis of a transient-climate GISS GCM III simulation for 1950-2004 reveals a significant decrease in the frequency of these cyclones, consistent with previous climatological analyses (McCabe et al., Gulev et al.) This trend can be understood simply in terms of greenhouse climate forcing, which decreases the thermal contrast between the tropics and the poles that provides the driving force for cyclogenesis. We are currently analyzing the NCEP-NCAR Reanalysis SLP data for 1950-2004 to better quantify this trend in the observations. Simulation of the 2000-2050 transient climate trend with the GISS GCM III (GCAP project; see poster by Wu et al.) projects a further decrease in the frequency of mid-latitude cyclones across North America. This decrease would be expected to lead to longer and more frequent stagnation episodes in the United States, with adverse consequences for air quality. Wu et al. show that 2000-2050 climate change has important consequences for surface ozone. We are presently analyzing the 2050 climate simulation further to diagnose changes in stagnation episodes seen in the accumulation of simple chemical tracers. McCabe’s Analysis of NCEP/NCAR GISS GCM To test our storm tracking procedure and to analyze the ability of the GISS GCM III to reproduce the North American storm tracks, we run our procedure on SLP data from the NCEP/NCAR reanalysis dataset and the GISS GCM simulation of the second half of the 20th century. An illustrative comparison for July 1960 is shown above (reanalysis) and below (GISS GCM). In a study of GCM data prepared for the Intergovernmental Panel on Climate Change Fourth Analysis Report, Lambert et al. analyzed the winter cyclone frequency of the participating climate models’ simulation of 1961-2000. The number of cyclones varied ~15% amongst the models, with the GISS models closely matching the observed quantities. The ability of the GISS models to match the frequency of cyclones, underscores the ability of the GISS climate models to accurately portray the frequency of cyclones. ACKNOWLEDGEMENTS REFERENCES The GCM and the reanalysis cyclones track eastward at comparable latitudes and frequency over North America. Inspection of other years indicates more generally that the GCM is successful in reproducing the observed climatology. Chandler et al. [1999] Atlas of extratropical storm tracks (1961-1998). NASA GISS Tech. Rep. [http://www.giss.nasa.gov/data/stormtracks/]. Gulev et al. [2001] Climate Dynamics, 795-809. Lambert et al. [2006] Climate Dynamics, 713-728. McCabe et al. [2001] J. Climate, 2763-2768. Mickley et al. [2004] GRL, L24103. This work was supported by the Electrical Power Research Institute. The GCAP data were produced with support of the EPA-Star grant R830959.