The high latitude blocking and low arctic oscillation values of December 2009 By Richard H. Grumm National Weather Service Office State College, PA 16803 1. INTRODUCTION An episode of high latitude blocking during the month of December 2009 caused the Arctic Oscillation (AO: Wallace and Gutzler 1981; Hurrell 1995) and the North Atlantic Oscillation (NAO: Wallace and Gutzler 1981) to go extremely negative. The average value of the AO for December 2009 was -2.43, the lowest value since records began surpassing the previous monthly value of -2.354 set in 2000 (Table 1). The lowest daily AO value was -5.67 set on the 21 December 2009 (Table 2). Table 1 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. The impacts of the NAO and to a lesser degree, the AO, on winter weather in eastern North America and Western Europe is relatively well known. Ambuam (2001) showed that the AO has a similar impact as that shown by the NAO, though the AO was not as good as the AO in relation to the weather over North America. This correlation of the flow over the North Atlantic and weather over eastern North America and Europe was discussed by Walker and Bliss (1932). More recent research (Wallace and Gutzler 1981; Hurrell 1995;Zhou et. al. 2001 Higgens et. al 2002) have shown the impact of the NAO on weather patterns over North America and Europe. Although the two patterns are highly correlated, there is a clear distinction that could play a guiding role in how we attempt to understand physical mechanisms in the Northern Hemisphere variability (Wallace 2000). Most research clearly implies that the NAO may be a better indicator of the means to assess the variability of the patterns and weather over the Northern Hemisphere than the AO. The relationship of the NAO and temperatures patterns over North America, Europe, and Asia were presented by Asia (Hurrell and van Loon 1997). The negative phase of the NAO, the moisture plume or atmospheric river (AR: Neiman et. al 2008) is shunted into the Mediterranean. With few storms over Iceland the number of storms crossing the Atlantic is reduced. The eastern United States generally experiences more cold outbreaks (Thompson and Wallace 1998) and thus an increase in snowfall. The pattern is often colder and drier in the eastern United States. Hirsh et al (2000) found a correlation of an increase of East Coast Winter Storms (ECWS) associated with the pattern of the NAO. Wettstein and Means (2002) showed the correlations of the AO to temperature extremes in the eastern United States. 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 not include the AO or NAO data. AO was not 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. weeks. Colucci (1985) demonstrated how explosive cyclogenesis can impact blocking and lead to blocking episodes. Konrad and Colucci (1989) also demonstrated the association of rapidly cyclogenesis to blocking but also showed how blocking anticyclones can lead to increased cold episodes in the eastern United States. The case they presented began on 9 January 1977, when the AO was -3.97 and would reach -7.43 on the 15 th. They noted two consecutive outbreaks within a 7 day period and provided a list (Konrad and Colucci 1989: Table 1) of extreme cold outbreaks in the eastern United States 1977-1985. Their focus was on cyclogenesis and blocking and they did included in the original work, a comparison of their Table 1 to the CPC AO data suggest most of the cold episodes were observed during periods of negative AO values. This paper will show the conditions associated with the strong high latitude blocking episode of December 2009. The focus will be on the pattern and the impacts on the pattern and weather over the eastern United States. It should be noted during this episode of blocking, unseasonably cool conditions affected Western Europe. Reports of snow in France and England were reported in new stories. Near the peak of the first high latitude anticyclone, a major ECWS brought record
snows to many locations in the eastern United States on 19-20 December 2009. 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. 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 Grumm (2001). Anomalies were displayed in standard 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 the global nature of the event and critical fields to examine. Displays will focus on the observed pattern and some forecast issues associated 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 2010. 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 Figures 2 & 3 show the large scale 500 hpa pattern over North America from 12/0000 to 29/0000 UTC. The positive 500 hpa heights over Greenland and the height anomalies which evolve as this ridge retrogress eastward over northeastern North America a clearly visible. Height values over 5640m are evident and anomalies on the order of 3 to 4SDs above normal are present. Beneath the blocking anticyclone there were a series of 500 hpa cyclones. At times (17/0000 UTC:Fig. 2f) there is an appearance of a Rex block over the Atlantic with deep 500 hpa cyclone beneath the high latitude anticyclone. Figure 4 shows the pattern over the Atlantic Basin for the latter part of December 2009. These data show the deep trough over Western Europe on the eastern side of the high latitude block. The strongly negative NAO and AO at this time were directly related to the block. Additionally, the negative NAO pattern, where storms crossing the Atlantic are shunted into the Mediterranean is clearly visible in this 9 day progression of the 500 hpa pattern. The precipitable water (PW) and precipitable water anomalies (Fig. 5) show the sure of high PW displaced south and focused on southern Europe. As the successive troughs developed over southeastern North America, there were some surge of high PW and warm air up the
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 0000 UTC a) 12, b) 13, c) 14, d) 15, e)16, f) 17, g) 18, h) 19 and i) 20 December 2009. Return to text. East Coast. Figures 6 & 7 show the surges of warm air beginning on 9 December when a large storm winter storm, the transcontinental express, brought snow and heavy rain to much of the United States. Figures 6 & 8 shows the 18 day evolution of the PW pattern over North America and Figures 7 & 9 show the 850 hpa temperature evolutions. The surges of high PW air and warm 850 hpa temperatures clearly denote the major cyclonic episodes 9 December 2009, 13 December 2009, 19 December 2009, and the multi-day storm over the United States from 24-26 December 2009. ii. Previous significant negative AO events Figure 10 shows the mean 500 hpa heights and 850 hpa temperatures over North America for the month of January 1977. These data share many of the characteristics of the data shown in Figures 2 & 3. Note the anomalous 500 hpa height anomaly over Greenland and Baffin Bay with 1 to 1.5SD above normal values in that region with -1 to -1.5SD below
Figure 3. As in Figure 2 except focused over the North Atlantic for the 9 day period from 0000 UTC a) 21, b) 22, c) 23, d) 24, e)25, f) 26, g)27, h) 28 and i) 20 December 2009. Return to text. normal 500 hpa heights over the southeastern United States and adjacent western Atlantic. The 850 hpa thermal pattern shows warm air with a +1 to +1.5SD thermal anomaly under the blocking ridge and below normal 850 hpa temperatures over the eastern United States. over Baffin Bay and northeastern Canada. There is a hint of an above normal ribbon of high PW air across the south-central Atlantic. Clearly, these data are impacted by daily surges and cyclonic events presented by Konrad and Colucci (1989). The corresponding mean sea level pressure and PW patterns show a large anticyclone over Greenland and deep cyclone in the mean over the Canadian Maritimes. The PW fields shows above normal PW values beneath the ridge Figure 11 shows the pattern for the month of December 2000, which was the lowest mean AO for the month of December until December 2009. The 500 hpa pattern shows the strong 500 hpa ridge over Baffin Bay.
Figure 4. As in Figure 3 except the projection is over the Atlantic Basin. Return to text. Unlike January 1977, there was not negative height anomaly over the eastern United States, though an 850 hpa thermal anomaly, -0.5 to - 1.0SDs was present. This event showed a strong and more expansive cyclone displaced over northwestern North America with a deep cyclone in the mean, south of Greenland. A positive PW anomaly was present over Baffin Bay and there was a clearly defined plume of high PW focused across the south Atlantic. Figure 12 shows the mean pattern for January 1963, the second highest monthly mean AO month. These data show the ridge over Greenland at 500 hpa, but a sharp ridge over the West Coast of North America and the Gulf of Alaska. The surface pattern reflected a stronger northwestern North American anticyclone and high PW anomalies were over the Pacific. 4. Summary and conclusions An episode of high latitude blocking impacted the weather over North America and Europe during the month of December 2009. A series of deep cyclones over North America may have contributed to anomalous ridging over
Figure 5. As in Figure 4 except showing precipitable water (mm) and precipitable water anomalies. Return to text. Greenland and northeastern North America. This high latitude blocking was associated with a strongly negative AO value, produced the lowest monthly mean AO for December since the records began in 1950. The negative AO was also associated with a negative sign in the NAO. The latter is often more highly correlated with the weather over North America, Europe and Asia. No attempt is made to explain why the high latitude blocks developed and how they relate to the NAO and AO. But based on previous record low AO events, high latitude blocking plays a significant role in the sign and magnitude of the AO. Figures 6-9 suggest that several strong episodes of cyclogenesis may have played a role in developing or maintaining the high latitude block over Greenland and northeastern North America. This would support the original premise on block shown by Colucci (1985) and Konrad and Colucci (1989). These data show the surge of high PW air and warm 850 hpa temperatures with the major cyclonic episodes 9 December 2009, 13 December 2009, 19 December 2009, and the multi-day
Figure 6. As in Figure 2 except for precipitable water and precipitable water anomalies over North America from 0000 UTC 09 17 December 2009. Return to text. storm over the United States from 24-26 December 2009. Three of these storms, the 9 December, 19 December and 24-6 December storms were high impact winter storms. Despite the block, the pattern was cyclonically active. A fourth major storm developed in the Gulf of Maine on 30 December through 2 January 2010. In theory, these storms may have provided the surge of warm air and the high PW values observed over Greenland and northeastern Canada. Figure 13 shows the large scale pattern and the anomalies over the Atlantic basin for the month of December 2009. The anomalous 500 hpa ridge over Greenland is clearly visible along with above normal surface pressures in that region. The region of both negative height and pressure anomalies is suppressed to the south with a strong jet implied into southern Europe and northern Africa. The moisture plume across the Atlantic is also suppressed and focused into North Africa and the Mediterranean Basin. Below normal temperatures at 850 hpa were present over western Europe with above normal temperatures under the blocking ridge and over much of North Africa.
The features shown in Figure 13 are quite similar to the prototypical negative NAO pattern, though there was not significant cold air in eastern North America. Another aspect of this pattern is it is typically dry in eastern North America. Figure 14 shows the monthly precipitation over the United States. These data show a precipitation maximum from eastern Texas across Georgia and along the East Coast. A secondary axis up the Mississippi Valley was likely associated with the Christmas Eve storm of 23-25 December 2009 when 192 mm of precipitation fell over Arkansas. This pattern is similar to that projected of a negative NAO pattern and that associated with the positive phase of ENSO 1. The high latitude blocking episode of December 2009 was associated with a pattern that produced highly negative AO values and negative NAO values. The average AO value for December 2009 was the lowest value ever recorded. Additionally, the value -5.67 on 21 December was one of the lower values recorded. A value of -6 would be one of the top ten lowest recorded AO events and was predicted to occur in early January 2010 by the NCEP GFS. During the highly negative phase of the NAO and AO, there was at least one major ECWS. On 19-20 December 2009 a strong cyclone with a surge of high PW air (Fig. 8) moved up the East Coast and contributed to this significant winter storm. The increased chance of snow and ECWS has been linked to the negative phase of the NAO (Wettstein and Meams 2002; Hirsch et al. 2000). Thus, forecasters were anticipating the increased potential for a significant ECWS. 1 January 1977 was associated a large negative NAO and positive ENSO period. The daily record low AO events were observed in January 1977 and included the cold episodes document by Konrad and Colucci (1989). The 4 record lows from -6.52 to -7.43 occurred from 13-16 January 1977. The -5.67 value of 21 December 2009 will likely displace 20 January 1985 for the 15 th lowest daily value. Figures 10-12 showed the pattern over North America for 3 months with below normal AO values, January 1977 and January 1963 having the most negative AO values and December 2000 the lowest December AO value until 2009. These data suggest some similarities in the 500 hpa height fields between the January 1977, 1963, and December 2009. However, there were many differences suggesting each month is meteorologically rather unique. The common feature to all three and others (not shown) typically includes below normal PW air over the eastern United States and near or below normal 850 hpa temperatures. This generalized pattern suggests that these months should be colder and drier than normal. 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, 3495 3507. 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, 209 219. Colucci, S.J. 1985: Explosive Cyclogenesis and Large-Scale Circulation Changes: Implications for Atmospheric Blocking Stephen.Journal of the Atmospheric Sciences, 42, 2701 2717. Konrad, C.E, and S.J. Colucci, 1989: An Examination of Extreme Cold Air Outbreaks over Eastern North America. Mon. Wea. Rev.117, 2687 2700. Glickman, T. 2000: Glossary of Meteorology, Second Edition; American Meteorological Society, 2000; ISBN 1-878220- 34-9. Grumm, R.H. and R. Hart. 2001: Standardized Anomalies Applied to Significant Cold Season Weather Events: Preliminary Findings. Wea. and Fore., 16,736 754. Hart, R. E., and R. H. Grumm, 2001: Using normalized climatological anomalies to rank synoptic scale events objectively. Mon. Wea. Rev., 129, 2426 2442. 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, 1555-1572. Hirsh, M.E, A.T. DeGaetano, S.J. Colucci,2000: An East Coast Winter Storm Climatology.J.Climate,14,882-899. Hurrell, J. W., 1995: Decadal trends in the North Atlantic Oscillation: Regional temperatures and precipitation. Science, 269, 676 679., and H. van Loon, 1997: Decadal variations in climate associated with the North Atlantic Oscillation. Climatic Change, 36, 301 326. (Fig. 6 shows patterns of temps). Neiman, P.J., F.M. Ralph, G.A. Wick, J. D. Lundquist, and M. D. Dettinger, 2008: Meteorological 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, 22-47. Pelly,J.L, and B.J. Hoskins, 2003: A new perspective on blocking. JAS,60,743-755. 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, 1721-1745. Rex, D. F., 1950a: Blocking action in the middle troposphere and its effect upon regional climate. I. An
aerological study of blocking action. Tellus, 2, 196 211., 1950b: Blocking action in the middle troposphere and its effect upon regional climate. II. The climatology of blocking action. Tellus, 2, 275 301. Thompson, D. W. J., and J. M. Wallace, 1998: The Arctic Oscillation signature in the wintertime geopotential height and temperature fields. Geophys. Res. Lett., 25, 1297 1300. Wallace, J. M., 2000: North Atlantic oscillation/annular mode: Two paradigms one phenomenon. Quart. J. Roy. Meteor. Soc., 126,791 805. (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.,109,784-812. 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, 3586 3600. 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, 4107-4110.
Monthly Mean AO values 1948-2009 December January February 2009-2.43 1977-3.767 1969-3.114 2000-2.354 1963-3.311 1978-3.014 1995-2.127 1966-3.232 1986-2.904 2005-2.104 1969-2.967 1958-2.228 1976-2.074 1985-2.806 1960-2.212 1985-1.948 1960-2.484 1968-2.154 1950-1.928 1970-2.412 1965-2.084 1969-1.856 1979-2.233 1956-2.029 1952-1.827 1998-2.081 1977-2.01 1996-1.721 1980-2.066 1983-1.806 1958-1.687 1959-2.013 1952-1.747 1961-1.668 2004-1.686 1963-1.721 TABLE 1. 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.
Date AO NAO PNA 1-Dec-09-2.28E-01-7.35E-01 9.38E-01-9.80E-01 2-Dec-09-1.87E-01-7.09E-01 1.24E+00-3.62E-01 3-Dec-09-2.76E-01-1.04E+00 1.16E+00 3.20E-01 4-Dec-09-4.89E-01-1.21E+00 6.43E-01 6.11E-01 5-Dec-09-1.34E+00-1.56E+00 6.58E-01 5.37E-01 6-Dec-09-1.51E+00-1.10E+00 7.36E-01-6.42E-02 7-Dec-09-1.01E+00-4.19E-01 2.30E-01-3.16E-02 8-Dec-09-7.32E-01-3.68E-01-1.17E-01 1.77E-01 9-Dec-09-8.50E-01-7.55E-01-3.56E-01 3.32E-01 10-Dec-09-1.92E+00-1.55E+00-5.85E-01 7.61E-01 11-Dec-09-3.34E+00-2.09E+00-6.29E-01 8.72E-01 12-Dec-09-3.71E+00-2.07E+00-7.06E-01 7.83E-01 13-Dec-09-3.80E+00-1.98E+00-7.54E-01 7.80E-01 14-Dec-09-3.83E+00-2.16E+00-6.05E-01 8.75E-01 15-Dec-09-3.82E+00-2.05E+00-3.65E-01 5.03E-01 16-Dec-09-3.66E+00-1.78E+00-9.64E-02 8.04E-01 17-Dec-09-3.59E+00-1.62E+00 2.90E-01 1.57E+00 18-Dec-09-4.03E+00-1.78E+00 7.38E-01 2.12E+00 19-Dec-09-4.37E+00-1.74E+00 6.83E-01 2.30E+00 20-Dec-09-5.00E+00-2.05E+00 6.35E-01 1.91E+00 21-Dec-09-5.67E+00-1.87E+00 5.99E-01 1.25E+00 22-Dec-09-5.51E+00-1.26E+00 5.04E-01 7.36E-01 23-Dec-09-5.44E+00-9.41E-01 6.07E-01 8.91E-01 24-Dec-09-5.35E+00-9.72E-01 6.52E-01 7.74E-01 25-Dec-09-5.04E+00-1.02E+00 4.83E-01 6.61E-01 26-Dec-09-4.70E+00-9.99E-01 4.01E-01 7.68E-01 27-Dec-09-3.83E+00-7.45E-01 2.97E-01 6.58E-01 28-Dec-09-3.59E+00-5.22E-01 3.83E-01 4.15E-01 29-Dec-09-3.76E+00-5.01E-01 2.48E-01 1.73E-01 30-Dec-09-3.90E+00-6.94E-01-7.74E-02-2.77E-01 31-Dec-09-4.00E+00-1.08E+00-2.11E-01-8.70E-01 Table 2. Daily values of the AO,NAO, and PNA from the Climate Prediction website. Yellow shows AO below -5.0.
Figure 7. As in Figure 6 except for 850 hpa temperature and temperature anomalies over North America from 0000 UTC 09-17 December 2009. Return to text.
Figure 8. As in Figure 6 except for precipitable water and precipitable water anomalies over North America from 0000 UTC 18 26 December 2009. Return to text.
Figure 9. As in Figure 6 except for 850 hpa temperature and temperature anomalies over North America from 0000 UTC 09 17 December 2009. Return to text.
Figure 10. Mean values for the month of January 1977 showing (left side) a) 500 hpa height and anomalies and b) 850 hpa temperatures and anomalies, and on the right side a) mean sea level pressure and anomalies and b) precipitable water and anomalies. Data from NCEP/NCAR reanalysis data. Return to text.
Figure 11. As in Figure 10 except valid for the month of December 2000. Return to text.
Figure 12. As in Figure 11 except for January 1963. Return to text.
Figure 13. The mean pattern and standardized anomalies from the Japanesse reanalysis data for the period of 0000 UTC 1 December through 1800 UTC 31 December 2009 showing a) 500 hpa heights and anomalies, b) mean sea level pressure and anomalies, c) 850 hpa temperatures and anomalies and d) precipitable water and anomalies. Return to text.
Figure 14. Total liquid equivalent precipitation over the United States for the period of 1200 UTC 1 December 2009 through 1200 UTC 1 January 2010. Data from the unified precipitation data set. Return to text.