The Impact of the strong subtropical ridge of 2010

Transcription

1 The Impact of the strong subtropical ridge of 2010 by Richard H. Grumm and Jason Krekeler National Weather Service Office, State College, Pennsylvania And Kevin Lipton National Weather Service Office Albany, New York 1. Introduction The summer of 2010 will be remembered as one of the hottest summers in recent memory over most of the eastern United States, eastern Asia, and Russia (Grumm 2010). Global temperature data suggest that the period of June-August 2010 was one of the warmest on record in the northern hemisphere. NOAA reported that The July worldwide land surface temperature was 1.03 C (1.85 F) above the 20 th century average of 14.3 C (57.8 F) the warmest July on record. This record warmth came on the heels of a 3-month record warm period from April to June of Despite the warm temperatures, which began in January of 2010, the global sea surface temperature anomalies were decreasing as La Niña conditions set-up over the tropical Pacific Ocean for the summer months. By the summer La Niña conditions were fully in place and expected to dominate through the winter of How these conditions impacted the United States is of interest here. Overall, a modestly strong subtropical ridge dominated the pattern over the eastern United States (Fig. 1) from 1 June 2010 through 31 August During this period of time, several periods of sustained heat affected the eastern and southern United States. These heat waves shared many of the common characteristics of previously documented heat waves (Lipton et al. 2005) including a large and strong subtropical ridges with closed 5940 m contour, above normal 850 and 700 hpa temperatures, and a surge of deep moisture north and west of the heat affected region. During these heat waves, many new daily maximum records were set. Many locations across the eastern United States had one of the warmest summers on record. A sample of sites in the Mid-Atlantic region and northeastern United States which had the warmest or one of the warmest summers records is shown in Table 1. Heat waves are one of the most significant causes of weather related fatalities. Changnon et al. (1996) documented the 1995 Midwestern United States hear wave, which caused 525 deaths in Chicago and 830 deaths nationwide. Contributing factors related to the deaths in Chicago included the high dew points, the urban heat island effects, the aging population, and lack of ventilation. Kunkel et al. (1996) attributed two essential factors to the fatal affects of the heat wave including the high dew points and urban heat island effects. The large number of deaths during the 2003 European heat wave in France and Italy may have been related to population demographics and a lack of the widespread use of air conditioning. Strong subtropical ridges and extreme heat episodes also were present during the summer over central Europe and Asia. These ridges, too, persisted for most of the summer.

2 This paper provides a summary of the significant sustained heat episodes which occurred across North America during the summer of 2010, with emphasis on the role and strength of the subtropical ridges associated with these heat episodes through the use of climatological anomalies. 2. Methods Data The Japanese 25-year Reanalysis (JRA25: Onogi et al. 2007) data was used to makes images for the summer of 2010 and for years after Relevant examples prior to 1979 used the NCEP/NCAR data (Kalnay et al. 1996). All anomalies were computed using the NCEP/NCAR re-analysis data climatologies of the means and standard deviations The climate mean and standard deviations used to compute standardized anomalies were from the NCEP/NCAR re-analysis spanning 1980 to As described in Hart and Grumm (2001) and Grumm and Hart 2001). Standardized anomalies were computed for seasonal (June-August), monthly, and singe time periods. Images for cases and seasonal data were computed using GrADS (Doty and Kinter 1995). It should be noted that the JRA25 data is on a 1.25x1.25 degree grids and the NCEP/NCAR data is on at 2.5x2.5 degree grid. Comparison of NCEP/NCAR data to JRA data revealed no significant differences in upper air fields but minor differences (lower values) in the precipitable water fields in the coarser data. The NCEP GFS on a 35km grid was also used. These data revealed few differences in the upper-level fields relative to the JRA25 and NCEP/NCAR data. However, these finer scale data often had higher PW values. Thus, the narrow surge of high PW air around the ridge was underestimated by the reanalysis data by as much as 5-15mm when averaged over a month. Similar differences were seen when comparing daily images. Clearly, surface based variables can be greatly impacted by the horizontal resolution of the analysis data used. Tables of record high temperatures were obtained from National Weather Service Public information statements for the three events. Only limited data was used to illustrate the key points. Similarly, the records for the warmest summer was collected from National Weather Service reports and the Northeast Regional Climate Center. 3. Results i.) Large scale pattern composites The large scale pattern for the summer months of June-August 2010 is shown in Figure 1. The 500 hpa composite shows that over the course of the summer the dominant signal was a sub-tropical ridge centered over the southeastern United States. This feature was +0.5 to +1 SD above normal over most of the eastern United States. There was a small region of +1 to +2 SD anomalies over southern Alabama, Georgia and northern Florida. The dominant signal for the western United States was a weak trough along the West Coast centered over California. Though there was a trough in the mean, the heights in this feature were near normal. Clearly, the dominant signal was the subtropical ridge in the southeast with a weak trough in the west. The composites of the 250 hpa winds showed an anomalous and persistent 250 hpa jet over the northern Plains and western Great Lakes. This westerly jet was on the order of +0.5 to +1 SD and was the

3 dominant feature in the 250 hpa winds for the summer. The 850 hpa temperature composite showed the impact of the persistent ridge over the eastern United States with +0.5 to hpa temperature anomalies implying a warm summer over most of the eastern United States. Consistent with the weak trough in the west, there were slightly negative 850 hpa temperatures over the intermountain west. The PW composite does not show a significant anomaly for the summer. There were anomalously high values of PW over the Atlantic and Caribbean. There was a hint of a surge of high PW air into the central United States. But this variable did not show the signal in the JRA25 relative to the NCEP GFS. Figures 2 & 3 show the mean patterns for the months of July and August respectively. These two months were chosen as each had at least one prolonged heat wave within it. The 500 hpa composite for July (Fig. 2) shows a slight ridge over the western U.S. with a trough over the northeastern U.S. This pattern became dominant for the last two thirds of the month as frequent frontal passages occurred over that time in the northeastern U.S. The strong subtropical ridge was located over the eastern Gulf States where the 500 hpa heights averaged 0.5 to 2 SDs above normal (Fig. 2a). The trough to the north and the ridge to the south enhanced the subtropical jet (Fig. 2b). The 250 hpa composite shows a stronger than normal wind maximum from the northern Rockies into the western Great Lakes, denoted by 250 hpa winds of +0.5 to +1 SD. The composite 850 hpa temperatures (Fig. 2c) show above average temperatures for southeastern Canada extending into much of the northeastern U.S. and northern mid Atlantic states, with +0.5 to +1 SD anomalies. This included much of the megalopolis and coastal plain. The largest temperatures anomalies were over the Gulf and across Florida, beneath the subtropical ridge (Fig. 2a). Below normal temperatures dominated the southern Rockies and western Mexico, with some areas experiencing -1 to -2 SD temperature anomalies across southern New Mexico and western Texas. The PW composite shows above normal PW on the order of +1 to +2 SD over much of the Gulf of Mexico and Caribbean. This extended into southern Texas and adjacent parts of Mexico. Part of this enhanced signal may have been due to Hurricane Alex which brought record high PW values to coastal areas of Texas and northeastern Mexico in early July Figure 3 shows the pattern for August The 500 hpa composite shows a weak trough along the West Coast, and a broad ridge over the eastern U.S. Heights over the southern United States were generally between +0.5 and +1 SD above normal. A key difference between July and August was the westward shift of the ridge in August. The strong gradient north of the subtropical ridge kept the strong 250 hpa jet (Fig. 3b) in a similar location as it occupied in July (Fig 2b) with a slight shift in the orientation of this jet. The westward shift of the subtropical jet appeared to shift the warm air at 850 hpa westward too (Fig. 3c) and reduced the PW values over the central United States (Fig. 3d). The western United States had -0.5 to - 2SD below normal PW values for the month of August in the JRA25. The intermountain west was dry but warmer in August than in July. ii.) Episodic eastern heat waves

4 There were 3 episodic heat waves in the eastern United States during the summer of 2010 spanning the periods of 3-7 July, July and 29 August through 2 September (Figs. 4-6). Composites of each event are shown along with a few single time periods of note. Figure 4 shows the composite for the 3-7 July 2010 heat wave. The strong 500 hpa ridge (Fig. 4a) and warm 850 hpa temperatures beneath the ridge are clearly evident. The 850 hpa temperatures averaged over 16C over most of the eastern United States with an expansive area of +1 to +2SD anomalies over the northeastern United States and eastern Canada. During the 3-7 July 2010 heat wave, the 250 hpa jet was abnormally strong over the southwestern United States (Fig. 4b) and over the eastern plains. This produced lowlevel flow into these two jets and surges of higher PW air (Fig. 4d). Overall, PW values were below normal during the heat event in the eastern United States. These data do not show the surge of high PW air into the Midwest and into the enhanced 250 hpa jet that that the GFS analysis depicted (Fig. 7). The pattern during the heat wave of July 2010 is shown in Figure 5. The subtropical ridge was enhanced over the southeastern United States (Fig. 5a) with a closed 5940m contour over the southeastern United States and the adjacent western Atlantic Ocean. Not surprisingly, the 850 hpa temperature anomalies were maximized from southern New England southward to the Gulf States (Fig. 5c). The subtropical jet was above normally strong from Montana to Maine along the northern edge of the strong subtropical ridge (Fig. 5b). PW values were low beneath and west of the ridge. The pattern for the 29 August to 2 September heat wave is shown in Figure 6. Despite the strong 500 hpa ridge over the eastern United States (Fig. 6a) and the closed 5940 m contour over the Mid- Atlantic region, few daily temperature records were set with this event. During this late season heat wave, a deep trough was present over the western United States (Fig. 6a) with -2 to -3SD height anomalies. Cold air was also present at 850 hpa over the intermountain west (Fig. 6c). Above normally warm 850 hpa temperatures dominated most of the northeastern United States and Canada (Fig. 6c). The 250 hpa jet over the eastern United States shift farther north during this event (Fig. 6b) relative to the two previous heat waves. Thus, relatively weak westerlies were present over the eastern United States. The northward shift of the jet and the strong subtropical ridge was associated with an expansive area of dry air over most of the central and eastern United States and Canada (Fig. 6d). iii.) Comparison to previous events Ideally, one would like to make composites of the summer of 1936 and see how it compared to the summer of Many high temperatures records still stand east of the Rocky Mountains from this hot summer. Actually, the summer of 1936 was also known for warmth in central Europe and eastern Asia. In this section we will focus on comparisons of more recently warm summers. Figure 8 shows the pattern over the United States during the month of July The pattern during July 1988 showed weak positive height anomalies over the Gulf and southern Florida. Height were only slightly above normal over the United States near the Great Lakes (Fig. 9a) where the 850 hpa temperatures were 0.5 to +1SDs above normal (Fig. 9c). A similar feature between July 2010 and July 1988 was the relatively

5 cooler air over the western United States when there was relatively warmer air in the eastern United States. iii.) Impacts The strong subtropical ridge which dominated during the summer of 2010 modulated the rainfall patterns over the United States. This dominant feature brought warm weather providing one of the warmest summers on record in the northeastern United States (Table 1) and in the southern United States (Table 2). The subtropical ridge also impacted rainfall patterns over the United States. With the ridge farther east in July, MCS brought heavy rains to the western plains and Great Lakes in June and July which contributed to the overall 3-month rainfall totals (Fig. 9). Heavy rain and flooding was observed in Arkansas June 2010, over Texas and Oklahoma 14 June 2010, in Texas with the remnants of tropical storm Alex (29 June-2 July), and a ring of fire MCSs even over the upper Midwest and Great Lakes (20-23 July 2010) in late July. As the ridge shifted westward, there was a dramatic decrease in rainfall over the United States in August (Fig. 10b) in August relative to July (Fig. 10a). The rainfall in July (Fig. 10a) had several periods of active surges of high PW air into the Midwest and several heavy rain and convectively driven rainfall events. Many were classic ring-of-fire like events. This flow of moisture was greatly reduced in August with the westward shift of the subtropical ridge. 4. Conclusions The summer of 2010 was dominated by a strong subtropical ridge across much of the southern and eastern U.S., which was associated with several, sustained periods of extreme heat. 850 hpa temperature anomalies in the monthly means were generally +0.5 to +1 SD, with the greatest anomalies occurring within the megalopolis during July, then shifting slightly south and west in August. In addition, an enhanced jet stream was noted in the monthly means, traversing the northern Rockies, northern Plains and western Great Lakes. Another anomalous upper level jet was noted in the August mean 250 hpa winds across Florida and the Bahamas. It should be noted that 500 hpa height and 850 hpa temperature anomalies were much greater within individual heat episodes than depicted in the monthly means. In addition, the PW anomalies were also significantly greater on the north and west side of the subtropical ridge during these heat episodes than depicted in monthly means as well. The flow around the subtropical ridge appeared to play a role in the distribution of precipitation across the United States. With the ridge farther east in June and July, high PW air surged into the plains, contributing to heavy rainfall in that region (Figs 9 & 10a). In Augusts the ridge shifted westward and the influence of the Gulf of Mexico moisture was reduced as was the rainfall over the central United State. Thus, a key difference between July and August was the westward shift of the ridge in August (Figures 2a & 3a) which markedly changed the precipitation pattern (Fig. 10a-b). The issue with relatively low monthly mean PW anomalies on the north and west side of the subtropical ridge may be due to the fact that the JRA25 and NCEP/GR data are too coarse compared to what was depicted in the NCEP GFS (image). Interestingly, comparing the GFS upper air features to the JMA and NCEP/NCAR re-analysis data, the

6 differences were small. However, using the PW values it was clear that the coarse global re-analysis data often reduced the PW values by 5 to 15 mm over some areas. Thus, the surge of high PW air about the subtropical ridge is only weakly defined in both the JRA25 and the NCEP/NCAR re-analysis. Impacts on rainfall. August 2010 was a relatively dry month. 5. Acknowledgement The Japanese Meteorological Agency for access to JRA25 reanalysis data, specifically their administrator who helps with password issues. 6. References 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, Graham, Randall A., Richard H. Grumm, 2010: Utilizing Normalized Anomalies to Assess Synoptic-Scale Weather Events in the Western United States. Wea. Forecasting, 25, THE WESTERN UNITED STATES. Inpress. 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, Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40- Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, Onogi, K., J. Tsutsui, H. Koide, M. Sakamoto, S. Kobayashi, H. Hatsushika, T. Matsumoto, N. Yamazaki, H. Kamahori, K. Takahashi, S. Kadokura, K. Wada, K. Kato, R. Oyama, T. Ose, N. Mannoji and R. Taira (2007) : The JRA-25 Reanalysis. J. Meteor. Soc. Japan, 85,

7 Figure 1. The mean pattern and mean anomalies over the United States from 01 June 2010 through 31 August Data from the JRA25 include a) 500 hpa heights (m) and anomalies, b) 250 hpa winds (ms-1) and anomalies, c) 850 hpa temperatures and anomalies, and d) precipitable water and anomalies. Anomalies are standardized values shown in standard deviations from normal not raw departures. Return to text.

8 Figure 2. As in Figure 1 except for the period of 0000 UTC 1 July through 1800 UTC 31 July Return to text.

9 Figure 3. As in Figure 2 except for 0000 UTC 1 August through 1800 UTC 31 August Return to text.

10 Figure 4. As in Figure 1 except for the period of 0000 UTC 03 July 2010 through 1800 UTC 7 July Return to text.

11 Figure 5. As in Figure 4 except for the period of 0000 UTC 17 July through 1800 UTC 25 July Return to text.

12 Figure 6. As in Figure 4 except for the period of 29 August 2010 through 1800 UTC 1 September Return to text.

13 Figure 7. As in Figure 5 except computed from the 00-hour NCEP GFS data. Return to text.

14 Figure 8. As in Figure 1 except for the period of 0000 UTC 1 July through 1800 UTC 31 July Return to text.

15 Figure 9. Total accumulated precipitation from the 24-hour unified precipitation analysis data for the period of 1200 UTC 1 June through 1200 UTC 31 August Values are in millimeters as shown. Return to text.

16 Figure 10. As in Figure 9 except for the months a) June and b) August Return to text.

17 City or Town STATE 1st 2nd 3rd BRIDGEPORT CT WASHINGTON REAGAN DC DOVER DE WILMINGTON DE BALTIMORE MD SALISBURY MD PORTLAND ME ASHVILLE NC GREENSBORO NC RALEIGH NC WILMINGTON NC SALISBURY NH ATLANTIC CITY NJ ISLIP NY NYC CENTRAL PARK NY MT. POCONO PA PHILADELPHIA PA PROVIDENCE RI BLACKSBURG VA NORFOLK VA RICHMOND VA ROANOKE VA WAKEFIELD VA WALLOPS ISLAND VA NANTUCKET MA NEWARK NJ HARRISBURG PA ELKINS WV CHARLESTON WV WORCHESTER MA PRESQUE ISLE ME POUGHKEEPSIE NY STATE COLLEGE PA LYNCHBURG VA Table 1. List of cities by State and the years of the top 3 hottest summers on record. Data is limited to cities in the Mid-Atlantic and northeastern United States. The list is limited to climate sites used by Regional National Weather Service offices. Thus this list is not a complete list. Return to text.

18 CITY OR TOWN STATE 1ST 2ND 3RD LITTLE ROCK AR JACKSON MS BIRMINGHAM AL MONTGOMERY AL COLUMBIA SC CHARLESTON SC SAVANNAH GA ATHENS GA COLUMBUS GA ASHEVILLE NC HUNTSVILLE AL LAKE CHARLES LA BATON ROUGE LA LOUISVILLE KY PINE BLUFF AR TUPELO MS WEST PALM BEACH FL MIAMI FL MELBOURNE FL MOBILE AL CHATTANOOGA TN GATLINBURG TN PADUCAH KY CINCINNATI OH INDIANAPOLIS IN TALLAHASSEE FL COLUMBUS OH DAYTONA BEACH FL CHARLOTTE NC NEW ORLEANS LA MEMPHIS TN JACKSONVILLE FL VALDOSTA GA NASHVILLE TN NAPLES FL JACKSON KY ATLANTA ORLANDO MACON TEXARKANA GA FL GA AR

19 JACKSON TN SHREVEPORT LA Table 2. Listing of southern cities and State and the years of the warmest summer on record. Only the top 3 warmest years shown.