The high latitude blocking and impacts on Asia

Transcription

1 The high latitude blocking and impacts on Asia By Richard H. Grumm National Weather Service Office State College, PA INTRODUCTION An episode of high latitude blocking during the month of December 2009 and January 2010 created some unusual winter weather over the Northern Hemisphere. Some of the weather included: One of the longest cold episodes in over 25 years in the eastern United States Sustained cold weather and snow in Western Europe to include England and France. Cold and record snows in eastern Asia to include China and Korea. In addition to the blocking the Arctic Oscillation (AO: Wallace and Gutzler 1981; Hurrell 1995) and the North Atlantic Oscillation (NAO: Wallace and Gutzler 1981) went extremely negative during this period of unusual weather (Table 1). The average value of the AO for December 2009 was -2.43, the lowest value since records began surpassing the previous monthly value of set in 2000 (Table 2). The running mean AO value for the data in Table 1 was The lowest daily AO value was set on the 21 December 2009 (Table 1). Table 2 lists the bottom ten lowest AO December values by year. The data in Table 2 suggest that the NAO followed a similar evolution as the AO. The value of these indices in understanding the potential meteorological impacts and the relationship of the AO to high latitude of blocking are of interest here. Figure 1 shows the relationship of the AO to high latitude blocking. The upper panel shows the height anomalies from 65 to 90N and the lower panels shows the AO. A persistent episode of high latitude blocking is clearly evident in Figure 1 through the troposphere through most of the month of December. In November, positive anomalies were observed in the stratosphere. The negative and strongly negative AO values coincide well with large positive height anomalies in the troposphere. These data show strong positive height anomalies in the higher latitudes from about 12 December through 2 January High latitude blocking (Rex 1950a, b; Glickman 2000) can impact the value of the AO and NOA. The Rex block (Rex 1950a) is often characterized by a cut-off low beneath high latitude anticyclone. The omega block, so named due to its similarity to the Greek letter Ω is characterized by an anticyclone anchored by to cut-off cyclones on either side. Episodes of blocking can persist for periods of days to weeks. The impacts of the low AO, NAO, and high latitude blocking episode have brought cold and snow to eastern Asia. Seoul, South Korea, had a record snowfall 3-4 January 2010 with 25.8 cm of snow reported by the Korean Meteorological Agency (KMA). This was the largest snowfall in Seoul since records were begun in 1937 and surpassed the 25.6 cm snowfall recorded on 28 January The heavy snowfall impacted transportation and caused large impacts on air traffic across the region. In China, Beijing had 10 mm of snow on 2-3 January. The 24 hour snow total was the largest single day snowfall in the month of January since The snows lead to the cancellation of 90% of all flights in and out of 1 Soure Reuters and CMA, beijing-south-korea-record-snow

2 Figure 1. Climate Prediction Center (CPC) image showing the height anomalies in the northern hemisphere from 65 to 90 North latitude and a bar graph of the daily value of the Arctic Oscillation. Real-time link. the local airport. In addition to the snow, cold temperatures affected the region. Temperatures near -18C were reported around the Capital City. The Chinese Meteorological Agency (CMA) reported that the 10 mm of snow was the largest snowfall in one day since 1951 and exceeded the old record of 6.2 mm set on 6 January Snowfall amounts ranged from 10.6 mm to 27.2 mm in Juyongguan. This paper will show the conditions associated with the strong high latitude blocking episode of December 2009 and January The focus will be on the pattern and the impacts on the pattern and weather over the eastern Asia. It should be noted during this episode of blocking, unseasonably cool conditions affected Western Europe and the eastern United States. 2. METHODS and Data All NAO, AO, and Pacific North American index values were retrieved from the Climate Prediction (CPC) teleconnections website. The monthly mean values and the daily values were sorted using Microsoft excel. Plots of blocking and AO values were also obtained from the CPC. No attempt to replicate these images was accomplished.

3 Figure 2. NCEP GFS 00-hour forecasts of 500 hpa heights (m) and 500 hpa height anomalies (standard deviations) for the 9-day period from 1200 UTC 27 December 2009 through 5 January Return to text. The 500 hpa heights, 850 hpa temperatures and other standard level fields were derived from the NCEP GFS, GEFS, and the NCEP/NCAR (Kalnay et al. 1996) reanalysis data. The means and standard deviations used to compute the standardized anomalies were from the NCEP/NCAR data as described by Hart and deviations from normal, as standardized anomalies. All data were displayed using GrADS (Doty and Kinter 1995). The standardized anomalies computed as: SD = (F M)/σ ( ) Where F is the value from the reanalysis data at each grid point, M is the mean for the specified date and time at each grid point and σ is the value of 1 standard deviation at each grid point. Model and ensemble data shown here were primarily limited to the GFS and GEFS due to

4 Figure 3. As in Figure 2 except centered over the Atlantic basin. the global nature of the event and critical fields to examine. Displays will focus on the observed pattern and some forecast issues assoc iated with the pattern. Comparison cases were retrieved using the CPC AO and NAO data. Previous strongly negative NAO months are displayed using the NCEP/NCAR reanalysis data. For brevity, times will be displayed in day and hour format such at 29/0000 UTC signifies 29 December 2009 at 0000 UTC. Due to the transition from 2009 to 2010, times with such as 01/1200 and 03/1200 UTC refer to 1200 UTC 01 and 03 January Figure 5 shows the PW and PW anomalies during the same period. Note how the high PW AR is shunted to the south. 3. Overview i. Large scale pattern

5 Figure 2 shows the 500hPa pattern over Asia at 1200 UTC from 27 December through 5 January These data show the large positive height anomalies at higher latitudes, which increase in magnitude over the forecast period. Note the closed 5460m contour and the expansive area of 2-3SD above normal height anomalies by 04/1200 UTC (Fig. 2h). A series of troughs with negative height anomalies can be seen moving under the blocking ridge, one wave became a close 500 hpa cyclone as shown at 04/1200 UTC over Korea, the wave that produced the snowfall. The positive height anomalies shown here, contributed to those shown in Figure 1. At this time, there was a closed 5700 m contour over Greenland (Figure 3f-g). Thus, the large anomalies shown in Figure 2 were impacted by at least 2 large high latitude anticyclones. Similar to the high latitude block over Greenland, the block over northeastern Asia was associated with warmer than normal 850 hpa temperature values (not shown) and surge of high precipitable water air beneath the block (Fig.4). ii. Regional views The evolution of the surface and 850 hpa winds during the period of cyclogenesis and snowfall over Beijing and Seoul is shown in Figures 5-6. An area of low pressure with negative pressure anomalies can be seen by 03/0000 UTC (Fig. 5c). They cyclone deepened as it moved over water at 03/1800 UTC and then moved across the southern portion of the Korean peninsula (Fig. 5g-i). The 850 hpa winds and u-wind anomalies show the evolution of a strong low-level easterly jet north of the cyclone center. This feature is clearly present by 04/0000 UTC (Fig. 6g) and peaks with -5SD anomalies at 04/1200 UTC (Fig. 6i). The -4 to -5SD anomalies at 04/0600 UTC suggest heavy snows were observed over eastern North Korea, likely more snow than observed in Seoul. No data is available to confirm this. Figure 7 shows the 850 hpa temperatures over the region. These data show that it was around -8C over central South Korea during the time of the snowfall. It was clearly cold enough in the column for snow. Additionally, these data show a surge of extremely cold air with temperature anomalies on the order of -2 to - 3SDs below normal moving into eastern Asia with -20 to -22C air over Beijing. The PW (Figure 4 and Figure 8) field and anomalies during the time of the cyclone did show a surge of 1SD above normal PW air into the Korean Peninsula, suggesting an inflow of moisture into the storm. The PW values to the west were low and were on the order of -1SD below normal over the region. PW values below 4 mm are not common in the United States and the selected contour interval here likely masks the lower values apparently commonly observed over Asia. iii. Forecasts Not developed. 4. Summary and conclusions An episode of high latitude blocking and a period of unusually low AO and NAO values allowed cold air to penetrate into eastern Asia. A cyclone developed along one of the frontal systems associated with a surge of cold air. This cyclone brought snow to eastern China and Korea. The snowfall over Seoul and Beijing represented record events at both locations. Though there was considerably more snow over Korea then eastern China. Seoul set a new record for the most snow since records began in 1937 and Beijing had its largest one day snowfall since The deep cyclone and strong easterly jet north of the surface and 850 hpa cyclones are a common pattern associated with heavy snow in the eastern United States (Stuart and Grumm 2006). There was a major winter storm in northeast China in March 2007 that also exhibited strong low-level u-wind

6 anomalies (Grumm and Xuxuan). It would appear that there are some common signatures and some value in using anomalies to assess the potential for heavy snowfall in eastern China and Korea. Junker et al (2009), Grumm and Hart (2001) and Hart and Grumm (2001) showed the value of anomalies in identifying potentially significant weather events. This case, including the examination of the large scale blocking ridge, the strong cyclone, and anomalous 850 hpa easterly jet, all appeared to reinforce the concept and value of using anomalies to assess significant weather events. 5. Acknowledgements Discussions on the block were part of the SUNY-Albany map thread. Concepts and data sources were provided by the Climate Prediction Center. Jeremy Ross and Richard James provide links to data and images related to the CPC indices and raw data along with examples of previous low AO cases. Brian Tang and Lance Bosart provide information on the AO and links to the high latitude block page at the CPC. 6. References Ambaum, M.H.P., B.J. Hoskins, and D.B. Stephenson, 2001: Arctic Oscillation or North Atlantic Oscillation? J. Climate, 14, Doty, B. E., and J. L. Kinter III, 1995: Geophysical data and visualization using GrADS. Visualization Techniques Space and Atmospheric Sciences, E. P. Szuszczewicz and Bredekamp, Eds., NASA, Colucci, S.J. 1985: Explosive Cyclogenesis and Large-Scale Circulation Changes: Implications for Atmospheric Blocking Stephen.Journal of the Atmospheric Sciences, 42, Konrad, C.E, and S.J. Colucci, 1989: An Examination of Extreme Cold Air Outbreaks over Eastern North America. Mon. Wea. Rev.117, Glickman, T. 2000: Glossary of Meteorology, Second Edition; American Meteorological Society, 2000; ISBN Grumm, R.H. and R. Hart. 2001: Standardized Anomalies Applied to Significant Cold Season Weather Events: Preliminary Findings. Wea. and Fore., 16, Hart, R. E., and R. H. Grumm, 2001: Using normalized climatological anomalies to rank synoptic scale events objectively. Mon. Wea. Rev., 129, Higgins, R. W., A. Leetmaa, and V. E. Kousky, 2002: Relationships between climate variability and winter temperature extremes in the United States. J. Climate, 15, Hirsh, M.E, A.T. DeGaetano, S.J. Colucci,2000: An East Coast Winter Storm Climatology.J.Climate,14, Hurrell, J. W., 1995: Decadal trends in the North Atlantic Oscillation: Regional temperatures and precipitation. Science, 269,

7 , and H. van Loon, 1997: Decadal variations in climate associated with the North Atlantic Oscillation. Climatic Change, 36, Junker, N.W, M.J.Brennan, F. Pereira,M.J.Bodner,and R.H. Grumm, 2009:Assessing the Potential for Rare Precipitation Events with Standardized Anomalies and Ensemble Guidance at the Hydrometeorological Prediction Center. Bulletin of the American Meteorological Society,4 Article: pp characteristics and overland precipitation impacts of atmospheric rivers affecting the west coast of North America based on eight years of SSMI/satellite observations. J. Hydrometeor., 9, Pelly,J.L, and B.J. Hoskins, 2003: A new perspective on blocking. JAS,60, Ralph, F. M., P. J. Neiman, and G. A. Wick, 2004: Satellite and CALJET aircraft observations of atmospheric rivers over the eastern north Pacific Ocean during the winter of 1997/98. Mon. Wea. Rev., 132, Neiman, P.J., F.M. Ralph, G.A. Wick, J. D. Lundquist, and M. D. Dettinger, 2008: Meteorological

8 Rex, D. F., 1950a: Blocking action in the Stuart,N.A and R.H. Grumm Figure 4. As in Figure 2 except precipitable water (mm) and precipitable water anomalies. middle troposphere and its effect upon regional climate. I. An aerological study of blocking action. Tellus, 2, , 1950b: Blocking action in the middle troposphere and its effect upon regional climate. II. The climatology of blocking action. Tellus, 2, : Using Wind Anomalies to Forecast East Coast Winter Storms.Wea. and Forecasting, 21, Thompson, D. W. J., and J. M. Wallace, 1998: The Arctic Oscillation signature in the wintertime geopotential height and

9 temperature fields. Geophys. Res. Lett., 25, Wallace, J. M., 2000: North Atlantic oscillation/annular mode: Two paradigms one phenomenon. Quart. J. Roy. Meteor. Soc., 126, (patterns for AO look like NAO temps) Wallace, J.M. and D.S. Gutzler, 1981: Teleconncections and the geopotential height during Northern Hemisphere Winter. Mon. Wea. Rev.,109, Wettstein, J.J., and L.O. Mearns, 2002: The Influence of the North Atlantic Arctic Oscillation on Mean, Variance, and Extremes of Temperature in the Northeastern United States and Canada. J. Climate, 15, Zhou, S., A. J. Miller, J. Wang, and J. K. Angell, 2001: Trends of NAO and AO and their associations with stratospheric processes. Geophys. Res. Lett., 28,

10 Monthly Mean AO values December January February TABLE 2. Most negative monthly mean AO values sorted for the months of December, January, and February. For each month, the year and the AO value are provided. Data courtesy of the CPC. Return to text.

11 Year Month Day NAO AO Means Table 1. The daily values of the Arctic Oscillation and the North Atlantic Oscillation from 15 December 2009 through 5 January The mean of each index is provided at the bottom of the respective column. Return to text.

12 Figure 5. As in Figure 2 except showing mean sea-level pressure (hpa) and pressure anomalies for the period of 1200 UTC 3 January through 1200 UTC 4 January Return to text.

13 Figure 6. As in Figure 5 except showing 850 hpa winds and u-wind anomalies.. Return to text.

14 Figure 7. As in Figure 5 except showing 850 hpa temperatures (kts) and anomalies.. Return to text.

15 Figure 8. As in Figure 7 except precipitable water (mm) and anomalies.. Return to text.