Special Climate Summary April-July 2004 Hot in Alaska, Cool Over Central North America, Wet in South-Central U.S

Special Climate Summary April-July 2004 Hot in Alaska, Cool over Central North America, Wet in South-Central U.S.

Gerald Bell Climate Prediction Center/NOAA/NWS/NCEP Table of Contents: 1. Overview p. 1 2. Surface Temperatures and Precipitation during April-July 2004 p. 2-4 3. Mean Atmospheric Circulation over North America during Apr.-Jul. 2004 p. 5 4. Mean Jet Stream over North America during April-July p. 5-8 5. Links to Hemispheric Circulation p. 8-10 6. No Link to the Madden-Julian Oscillation (MJO) p. 10 7. Summary p. 11-12

1. Overview Surface Temperature Departures a 70N The period April-July 2004 featured exceptionally warm and dry conditions from 60N Alaska to California and below-normal tempera- 50N tures across the eastern half of Canada and the 40N central United States. It also featured above- average precipitation from Texas northeastward to 30N New England (Fig. 1). These conditions were 20N 180 160W 140W 120W 100W 80W 60W 40W associated with a persistent blocking ridge over Alaska and northwestern Canada, and a west- -4 -3 -2 -1 -0.5 0 0.5 1 2 3 4 (C) Precipitation Percentiles b ward shift and strengthening of the mean trough 70N over central/ eastern Canada. Three factors that contributed to the 60N longevity of these conditions are discussed. These 50N include 1) A persistent zonal wave-3 pattern at 40N high latitudes; 2) A pronounced stratospheric extension of the high-latitude height anomalies, 30N with the blocking ridge over Alaska and the 20N 180 160W 140W 120W 100W 80W 60W 40W downstream amplified trough extending past the 30-hPa level; and 3) Anomalous wave and jet 10 30 70 90 (% ile) stream conditions over the western North Pacific, Fig. 1. April-July 2004: mean 300-hPa heights (contour including a reduced eastward extent of the East interval is 60 m) overlayed with (a) surface temperature departures (oC) and (b) precipitation percentiles. Asian jet core.

1 2. Surface temperatures and precipitation Surface Temperature Rankings 70N during April-July 2004 60N Most of western North America experi- 50N enced exceptionally warm (Fig. 1a) and dry (Fig. 40N 1b) conditions during April-July 2004, while 30N central North America experienced unusually cool 20N 180 160W 140W 120W 100W 80W 60W 40W conditions and the eastern United States experienced above-average precipitation. The most 1 3 5 10 15 20 40 45 50 53 55 anomalously warm and dry conditions were Fig. 2. April-July 2004 surface temperature rankings. A ranking of 1 indicates that April-July 2004 is the coldest observed in Alaska, northwestern Canada, and the such period, and a ranking of 55 indicates that it is the Pacific northwestern United States. In Alaska daily warmest such period, in the 55-year record 1950-2004. mean surface temperatures averaged 2°-5°C April-July Area-Average Temperature Departures Alaska a above normal, and precipitation totals were 3 generally in the lowest 30th percentile of occur- 2 rences. In northwestern Canada and southeastern 1 0 Alaska temperatures averaged 2°-3°C above -1 normal, and precipitation totals were in the lowest -2 th -3 10 percentile of occurrences. 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 Central Canada b Mean temperatures during April-July 2004 3 2 were among the warmest in the 1950-2004 1 record for much of Alaska and western Canada 0 (Fig. 2). For Alaska as a whole area-averaged -1 -2 temperature departures reached 1.8°C, exceeding -3 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 the previous record departure of 1.5°C observed Fig. 3. Time series of April-July surface temperature in 1993 (Fig. 3a). deparures (oC) from 1950-2004 in (a) Alaska and (b) central Time series of daily temperatures (Fig. 4) Canada. Departures are calculated from the 1950-2004 base and precipitation (Fig. 5) at Fairbanks in south- period means. Daily Mean Temperatures and Normals central Alaska and Yakutat in southeastern Alaska Fairbanks, Alaska a 25 25 illustrate the persistence of the exceptionally warm 15 15 and dry conditions. Above-normal temperatures 5 5 -5 -5 °C first appeared at Fairbanks in early April 2004 -15 -15 -25 -25 (Fig. 4a), and at Yakatut in early February 2004 -35 -35 -45 (Fig. 4b). This anomalous warmth continued at Sep Nov Jan Mar May Jul both stations through early August, with no sub- 2003 2004 Yakutat, Alaska b 20 20 stantial cold-air outbreaks during the period. 15 15 Significant precipitation deficits began in 10 10 5 5

September 2003 at both stations (Fig. 5), with °C 0 0 Fairbanks receiving no measurable precipitation -5 -5 -10 -10 for the 7 ½ month period from mid-September -15 -15 Sep Nov Jan Mar May Jul 2003 through early May 2004 (Figs. 5a, b). 2003 2004 Fig. 4. Daily mean temperatures and normals at (a) Beneficial rains were then observed during May, Fairbanks and (b) Yakutat, Alaska during September 2003 - and again in late July and early August (Fig. 5b). July 2004. Temperatures are shown by thin solid line and At Yakatut, the total September 2003- normals are indicated by dashed line. Departures from July 2004 precipitation deficit was 1400 mm (Fig. normal are shaded, with red (blue) indicating above (below) normal temperatures.

2 5c). Rainfall was particularly Precipitation Fairbanks, Alaska suppressed during May-July, inches mm inches mm 10 250 1.0 25 Accumulated Oberved: thick line Daily Observed b when daily totals reached 25 8 Accumulated Normal: thin line 200 0.8 20 mm on only three occasions 6 150 0.6 15 (Fig. 5d). 4 100 0.4 10 2 a 50 0.2 5 Looking at longer time 0 0 0 0 Sep Nov Jan Mar May Jul Sep Nov Jan Mar May Jul 2003 2004 scales, April-July surface 2003 2004 Yakutat, Alaska inches mm inches mm temperatures in Alaska have 160 4000 5 125 Accumulated Oberved: thick line Daily Observed d exhibited considerable low- 128 Accumulated Normal: thin line 3200 4 100 96 frequency variability since 2400 3 75 64 1600 2 50 1950. For example, during the 32 c 800 1 25 0 0 0 0 19-yr period 1959-1977 only Sep Nov Jan Mar May Jul Sep Nov Jan Mar May Jul one season (1969) was 2003 2004 2003 2004 above average and sixteen seasons were below average. During the following 27-yr Fig. 5. Daily and accumulated precipitation at (a, b) Fairbanks and (c, d) period (1978-2004) sixteen Yakutat, Alaska during September 2003 - July 2004. Accumulated departures from normal are shaded, with brown shading indicating precipitation deficits. seasons (59%) were above- average and nine seasons were below average. June 2004 a In contrast to the exceptionally warm and 55N dry conditions in Alaska during April-July 2004, daily mean temperatures were well below average 50N across central North America. The largest tem- 45N perature departures were observed across central 40N Canada (2°-3°C below normal) and the north- central United States (1°-2°C below normal). The 35N largest southward extent of the cool conditions 30N occurred during June and July, when maximum temperatures were below average across nearly 25N 120 110W 100W 90W 80W 70W the entire United States (Fig. 6). In central Canada, many regions experi- 55N July 2004 b enced their coldest April-July period in the 1950- 50N 2004 record (Fig. 2). Overall, area-average temperatures in central Canada were 1.4°C below 45N normal during April-July 2004, making this the 40N coldest such period since 1967 and the third coldest since 1950 (Fig. 3b). 35N A time series of temperature departures at 30N Waco, Texas (Fig. 7) illustrates the character of the cold conditions over the south-central United 25N 120 110W 100W 90W 80W 70W States during June and July. Temperatures at this station were below-average throughout the period, -4 -3 -2 -1 -0.5 0 0.5 1 2 3 4 (C) with the cold conditions even extending into mid- Fig. 6. Mean maximum temperature departures (°C) August. during (a) June and (b) July, 2004. Departures are calculated from the 1971-2000 base period normals.

3 Several significant cold-air outbreaks WACO, TEXAS ˚C Daily Average and Normal Temperatures ˚C occurred during mid-June through mid-August, as 35 35

cold fronts penetrated into the deep south. These 30 30

events also brought well below-average tempera- 25 25

tures to much of the eastern United States (shad- 20 20 ing, Fig. 8). 15 15 These cold-air outbreaks were associated 10 10 1APR 1MAY 1JUN 1JUL 1AUG 1SEP with an amplified upper-level ridge (contours) over 2004 western North America and an amplified trough extending southward into the central or eastern Fig. 7. Daily mean temperatures and normals at Waco, United States. In Texas, the exceptional south- Texas during Apr.-Aug. 2004. Temperatures are shown by ward penetration of the cold fronts during these thin solid line and normals are indicated by dashed line. Departures from normal are shaded, with red (blue) periods was also related to a strong southward indicating above (below) normal temperatures. funneling of cold air along the eastern slopes of the Rocky Mountains. This funneling is 300-hPa Heights and 1000-hPa Temperature Departures related to up-slope flow along the 18-24 June 2004 a 25 June - 1 July 2004 b southern (or leading) edge of the 80N surface pressure ridge, which is 70N generally located upstream of the 60N mean trough axes. 50N This combination of vigor- 40N ous upper-level troughs and strong 30N cold frontal passages also contrib- 20N 20-25 July 2004 c 26-30 July 2004 d uted to above-average precipitation 80N from Texas northeastward to New 70N England during June and July. 60N During June Texas recorded its 50N wettest such month in the historical 40N record dating back to 1895. 30N 1-9 August 2004 e 10-16 August 2004 f These conditions, especially 20N during the second half of June (Figs. 1-9 August 2004 e 10-16 August 2004 f 8a, b) and the latter part of July 80N (Figs. 8c, d), were associated with 70N a westward shift of the mean 60N summertime ridge that normally 50N extends from Texas northward to 40N south-central Canada. This mean 30N ridge not only influenced the sum- 20N 150W 120W 90W 60W 150W 120W 90W 60W mertime temperature and precipitation patterns over North America, -5 -4 -3 -2 -1 1 2 3 4 5 but also helped to regulate the Fig. 8. 300-hPa heights (controur interval is 60 m) and 1000-hPa stength of the Southwest U.S. monsoon temperature departures during selected periods from mid-July system. During June and July the anoma- through mid-August 2004. Anomalies are departures from the 1971- lous upper-level ridge was associated with 2000 base period means. a below-average strength of this monsoon system

4 (not shown). 500-hPa Percentage of Anomaly Days

3. Mean Atmospheric Circulation over North America during April-July 2004 The mean upper-level circulation over North America during April-July 2004 featured a blocking ridge over Alaska and northwestern Canada, an amplified trough over Hudson Bay, and above-average heights over the southwestern North Atlantic (Fig. 1). Each of these features was quite persistent, with the associated 500-hPa height anomalies evident during 65%-80% of the 4-month period (Fig. 9). Also during the period a very persistent jet stream was centered over the Great Lakes region within the base of the amplified upper-level trough (Fig. 10a). -80 -75 -70 -65 -60 60 65 70 75 80 The warm and dry conditions from Alaska Fig. 9. Percentage of days during April-July 2004 that to southern California coincided with the anoma- 500-hPa height anomalies were positive (red) and negative lous upper-level ridge, while the cool conditions (blue). Anomalies are departures from the 1971-2000 base over central North America were associated with period means. the anomalous northwesterly flow between the ridge and downstream trough axes. In the United April-July 2004 a States the below-average temperatures were 80N related to periodic significant southward penetrations of the anomalous upper-level trough and the 60N associated surface cold fronts during June and July 40N (Fig. 8). Also during June and July the above- 20N average precipitation from Texas northeastward to EQ New England coincided with the region upstream 90E 120E 150E 180 150W 20W 90W 60W 30W 0 of the mean ridge axis situated over the southeast- June 2004 b ern U.S., and with the right entrance region of the 80N mean jet stream located over the Great Lakes (Fig. 10b). For many areas the periods of 60N sgnificant precipitation were related to the strong 40N cold frontal passages and the associated southward penetrations of the amplified upper-level 20N trough/ jet system discussed in section 3. EQ 90E 120E 150E 180 150W 20W 90W 60W 30W 0

4. Mean jet stream over North America 20 25 30 35 (m/s) A series of diagnostics have been per- formed to better understand the relationships Fig. 10. 200-hPa heights (contour interval is 60 m), wind speeds (shading, m/s), and anomalous wind vectors, between the mean jet and wave features over during (a) April-July 2004, and (b) June 2004. Anomalies North America during April-July 2004. Similar are departures from the 1971-2000 base period means.

5 April-July 2004 Diffluence/ Confluence Speed Acceleration/ Deceleration 70N 70N 60N 60N 50N 50N 40N 40N 30N a 30N b 20N 20N 180 160W 140W 120W 100W 80W 60W 40W 180 160W 140W 120W 100W 80W 60W 40W June 2004 Diffluence/ Confluence Speed Acceleration/ Deceleration 70N 70N 60N 60N 50N 50N 40N 40N 30N c 30N d 20N 20N 180 160W 140W 120W 100W 80W 60W 40W 180 160W 140W 120W 100W 80W 60W 40W

-4 -3 -2 -1 1 2 3 4 -0.8 -0.6 -0.4 -0.2 0.2 0.4 0.6 0.8

Fig. 11. Mean 200-hPa heights (contour interval is 120 m) and ageostrophic wind vectors overlayed with (Left) mean diffluence (shading, x 10-5 s -1) and (Right) mean parcel speed accelerations (shading, x 10-2 m s -2) during (a, b) April-July 2004 and (c, d) June 2004. In (a) and (c) diffluence is shown red and confluence is blue. In (b) and (d) speed accelerations are shown red and decelerations are blue. Vector scale (m s-1) is shown on right between panels (b) and (d).

diagnostics are used in April-July 2004 Cross-Stream Frontogenesis Along-Stream Frontogenesis section 5 to examine the 70N 70N anomalous East Asian jet stream and its relationship 60N 60N to the persistent blocking 50N 50N ridge over Alaska. Over North 40N 40N America a prominent 30N 30N indicator of a link between a b 20N 20N the mean jet and wave 140W 120W 100W 80W 60W 140W 120W 100W 80W 60W features is the highly confluent (blue shading) -5 -4 -3 -2 -1 1 2 3 4 5 mean northwesterly flow Fig. 12. April-July 2004: Mean 200-hPa frontogenesis in the (a) cross-stream and between the upper-level (b) along-stream directions (shading, x 10-11 K (ms)-1). The mean 200-hPa heights ridge and downstream (contour interval is 120 m) are overlayed in both panels . Frontogenesis is shown red trough axes (Fig. 11a). The and frontolysis is blue. associated ageostrophic

6 wind vectors are directed toward lower heights, Anomalous Vertical Motion indicating strong speed accelerations ocurring in Forced by Frontogenesis the mean just upstream of the trough axis (Fig. 70N 11b). These conditions coincide with the left entrance region of the mean jet core situated over 60N the Great Lakes. Similarly the right jet entrance 50N region is situated in the southwesterly flow imme- diately upstream of the mean upper-level ridge 40N axis over the southeastern United States. These conditions are especially evident during June (Figs. 30N 11c, d), when the south-central U.S. recorded 20N well above-average precipitation. 140W 120W 100W 80W 60W During April-July 2004 the confluent flow between the mean ridge and trough axes was -2.5 -2 -1.5 -1 -.5 .5 1 1.5 2 2.5 highly frontogenetical (red shading) in both the Fig. 13. April-July 2004: Anomalous 300-hPa quasi- cross-stream (Fig. 12a) and along-stream direc- geostrophic vertical motion (shading, x10-4 hPa s-1) tions (Fig. 12b). This circulation forced an anoma- overlayed with mean 200-hPa heights (contour interval is lous descent of stratospheric air in the northwest- 120 m). Anomalous ascending motion is shown blue and erly flow over west-central Canada and along the anomalous descending motion is red. jet axis (Fig. 13). The large values of cyclonic vorticity produced by this descending motion were Over the south-central United States the then advected into the downstream trough axis, mean upper-level frontogenesis forced anomalous thereby helping to maintain both the mean jet itself ascending motion, which helps to account for the and the enhanced trough over Hudson Bay. above-average precipitation in that region. These conditions were particularly prominent during June (Fig. 14), when strong frontogenesis in the jet

June 2004 Vertical Motion Forced by Cross-Stream Frontogenesis Cross-Stream Frontogenesis 70N 70N

60N 60N

50N 50N 40N 40N

30N 30N a b 20N 20N 140W 120W 100W 80W 60W 140W 120W 100W 80W 60W

-5 -4 -3 -2 -1 1 2 3 4 5 -2.5 -2 -1.5 -1 -.5 .5 1 1.5 2 2.5 Fig. 14. June 2004: 200-hPa heights (contour interval is 120 m) overlayed with (a) frontogenesis calculated along mean parcel trajectories in the cross-stream direction at 200-hPa (shading, x 10-11 K (ms)-1), and (b) the associated mean quasi-geostrophic vertical motion at 300-hPa (shading, x10-4 hPa s-1). In (a) frontogenesis is shown red and frontolysis is blue. In (b) ascending motion is shown blue and descending motion is red.

7 entrance region covered the central United States 30-hPa and the resulting ascending motion extended from Texas northeastward to the mid-Atlantic region (blue, Fig. 14b).

5. Links to Hemispheric-Scale Circulation The North American circulation during April-July 2004 was linked to a persistent hemispheric-scale pattern of height anomalies charac- terized by an anomalous zonal wave-3 pattern at a high latitudes, and by above-average heights near Japan (Fig. 9). The positive anomalies over the 100-hPa Gulf of Alaska, the polar region, and near Japan, were the most persistent of these circulation features, and were evident for 75%-80% of the period. The hemispheric scale of this anomalous circulation likely contributed to its persistence. Another factor that likely contributed to the persistence of the anomalous circulation over North America is the deep stratospheric penetra- b tion past the 30-hPa level of the positive height anomalies over Alaska and the polar region, and -90 -60 -30 0 30 60 90 120 150 the negative anomalies over Hudson Bay (Fig. 15). Fig. 15. April-July height anomalies (m) at (a) 30-hPa A third contributing factor to the persistent and (b) 100-hPa. Positive anomalies are shown orange and North American circulation was the anomalous jet negative anomalies are blue. Anomalies are departures from the 1971-2000 base period means. and wave structure over the western North Pacific. This relationship is examined for the

Anomalous Outgoing Longwave Radiation Anomalous Sea-Surface Temperatures 40N 40N 30N 30N 20N 20N 10N 10N EQ EQ 10S 10S 20S 20S 30S a 30S b 40S 40S 60E 90E 120E 150E 180W 150W 60E 90E 120E 150E 180W 150W

-30 -20 -10 10 20 30 -2 -1.5 -1 -0.5 0.5 1 1.5 2 Fig. 16. April-May 2004 anomalous (a) Outgoing Longwave Radiation (OLR, W m-2) and (b) sea-surface temperatures (SSTs, oC). For OLR, green (brown) at low latitudes indicates enhanced (suppressed) tropical convection. OLR (SST) anomalies are departures from the 1979-1995 (1971-2000) base period means.

8 April-May 2004 April-May 2004 period, when enhanced convec- Mean Heights, Diffluence tion (Fig. 16a) and above-average sea surface 70N temperatures (Fig. 16b) over the western tropical 60N Pacific also likely influenced the East Asian jet. 50N During April-May a diffluent trough was situated over eastern Siberia (contours, Fig. 17a), 40N well west of its climatological position over the 30N Aleutian Islands. This trough and the persistent 20N positive height anomalies over Japan were associ- 10N a ated with above-average winds speeds (exceeding EQ 40 m s-1) in the core of the East Asian jet (Fig. 80E 100E 120E 140E 160E 180 160W 140W 17b). Mean Wind Speeds, Diffluence April-May 2004 70N Cross-Stream Frontogenesis, Vg 70N 60N 60N 50N 50N 40N 40N 30N 30N 20N 20N 10N b 10N a EQ EQ 80E 100E 120E 140E 160E 180 160W 140W 80E 100E 120E 140E 160E 180 160W 140W Anomalous -5 -4 -3 -2 -1 1 2 3 4 5 Wind Speeds, Diffluence, Vag 70N Vertical Motion Forced by Cross-Stream Frontogenesis, Vg 60N 70N 50N 60N 40N 50N 40N 30N 30N 20N 20N 10N c 10N b EQ EQ 80E 100E 120E 140E 160E 180 160W 140W 80E 100E 120E 140E 160E 180 160W 140W

-4 -3 -2 -1 1 2 3 4 -2.5 -2 -1.5 -1 -.5 .5 1 1.5 2 2.5 Fig. 17. April-May 2004: 200-hPa mean (a), (b) diffluence Fig. 18. April-May 2004: Anomalous (a) 200-hPa (shading, x 5 x 10-5 s-1) overlayed with (a) heights (contour geostrophic frontogenesis in the cross-stream direction interval is 120 m) and (b) wind speeds (contour interval is (shading, x 10-11 K (ms)-1), and (b) the associated anomalous 10 m s-1). (c) Anomalous diffluence (shaded), anomalous quasi-geostrophic vertical motion at 300-hPa (shading, wind speeds (contour interval is 2 m s-1) and anomalous x10-4 hPa s-1). The mean 200-hPa geostrophic wind speeds ageostrophic wind vectors. Diffluence is shown red and (contours, interval is 10 m s-1) are overlayed in both panels. confluence is blue. In (a) frontogenesis is shown red and frontolysis is blue. In (b) ascending motion is shown blue and descending motion is red.

9 These increased wind speeds were This significantly reduced eastward extent of the associated with an enhanced jet entrance region, jet core was linked to upper-level diffluence indicated by anomalous confluence along the jet throughout the Pacific basin (red, Figs. 17a, b), axis (blue, Figs. 17b, c) and anomalous southerly with the most anomalous diffluence observed ageostrophic flow extending from the deep downstream of the Siberian trough and in the Tropics northward to 45°N. The enhanced jet mean jet exit region (Fig. 17c). entrance region also featured anomalous upper- This combination of high-latitude diffluent level frontogenesis in the cross-jet direction (red, flow and a westward retraction of the East Asian Fig. 18a), which produced anomalous descending jet core and jet exit region is known to favor a motion north of the jet axis and ascending motion westward shift in the positions of the mean ridge south of the jet axis (Fig. 18b). and trough axes over North America, as is typi- In the subtropics the anomalous southerly cally seen during the winter season in association ageostrophic flow was also partly related to with Pacific cold episodes (La Niña). divergent outflow from enhanced convection over the western tropical North Pacific (Fig. 16a). In 6. No Link to intraseasonal variability associ- the mean this outflow is seen to accelerate north- ated with the Madden-Julian Oscillation ward into the mean jet entrance region, thereby (MJO) helping to maintain the above-average wind The period April-July 2004 featured speeds in the jet core. This anomalous outflow strong intraseasonal variability in tropical convec- also represents a source of anticyclonic vorticity tion associated with the Madden-Julian Oscillation for the amplified subtropical ridge, which likely (MJO) (Fig. 19). The corresponding time-longi- helped to maintain the ridge during the period. tude section of 300-hPa height anomalies aver- However, the East Asian jet core only aged between 60°N-70°N shows little relationship extended to 155°E, which is approximately 40 to the phase of the MJO. Therefore, it appears degrees of longitude shorter than its normal length. that the MJO did not have a major influence on

300-hPa Height Anomalies (m): 60N-70N a 200-hPa Velocity Potential Anomalies: 5N-5S b 1 Apr 1 Apr 16 Apr 16 Apr 1 May 1 May 16 May 16 May

1 Jun 1 Jun 16 Jun 16 Jun 1 Jul 1 Jul 16 Jul 16 Jul

0 60E 120E 180W 120W 60W 0 0 60E 120E 180W 120W 60W 0 -360 -300 -240 -180 -120 -60 60 120 180 240 300 360 -18 -15 -12 -9 -6 -3 3 6 9 12 15 18 Fig. 19. Time-longitude sections during April-July 2004 of (a) daily 300-hPa height anomalies averaged between 60oN-70oN, and (b) 5-day running mean 200-hPa velocity potential anomalies (x105 m2 s2) averaged between 5oN-5oS. Solid and dashed lines indicate opposite phases of the MJO. In (b) green indicates anomalous ascending motion and brown indicates anomalous descending motion. Anomalies are departures from the 1971-2000 base period daily means.

10 the persistent North American circulation during zonal wave-3 pattern at high latitudes, and a the period. pronounced stratospheric extension of the high- 7. Summary latitude height anomalies, with the blocking ridge over Alaska and the downstream amplified trough The period April-July 2004 featured extending past the 30-hPa level. They also include exceptionally warm and dry conditions from a sharply reduced eastward extent of the East Alaska to California, with record temperatures set Asian jet core, combined with a highly diffluent at many locations in Alaska. For the state as a trough over eastern Siberia. whole, this was the warmest April-July period in Key structural aspects of these circulation the 1950-2004 record. features have been examined to better understand April-July 2004 also featured well below- their role in maintaining the anomalous circulation average temperatures across the eastern half of over North America. For example, it is shown that Canada and the central/ eastern United States. the highly confluent flow configuration between the Central Canada experienced its third coolest such blocking ridge over Alaska and the downstream season in the record, with mean temperatures trough contributed to the persistent trough/ jet averaging 2°-3°C below normal. steam configuration, and to the anomalous tem- Across the central and eastern U.S., the perature and precipitation patterns over the central most significantly below-average temperatures and and eastern United States. above-average precipitation occurred during June Farther west, the mean circulation during and July. Several significant cold-air outbreaks April-July 2004 featured a highly diffluent trough occurred during this period, in association with an over eastern Siberia, and a westward retraction of amplified upper-level ridge over western North the East Asian jet core and jet exit region. These America and an amplified trough extending conditions favored the persistence of a blocking southward into the central or eastern United ridge over western North America. States. These conditions favored the southward The anomalous East Asian jet stream was pentration of cold fronts well into the deep south. also associated with an enhanced jet entrance In Texas, the exceptional southward penetration of region over extreme eastern Asia. This feature is the cold fronts was also related to a strong south- related to above-average confluence and upper- ward funneling of the cold air along the eastern level frontogenesis between the Siberian trough slopes of the Rocky Mountains. These frontal and an amplified subtropical ridge over the west- systems also brought below-average temperatures ern North Pacific. In the mean, enhanced speed to much of the eastern United States, and contrib- accelerations in this jet entrance region contributed uted to above-average precipitation from Texas to super-geostrophic wind speeds in the jet core, northeastward to New England. which then resulted in an anomalous ageostrophic In the mean, the anomalous temperature flow toward higher heights much farther west than and precipitation patterns over North America normal. This result is consistent with the pro- during April-July were related to a persistent nounced westward retraction of the jet core. blocking ridge over Alaska and northwestern It is found that conditions in the anomalous Canada, a westward shift and strengthening of the jet entrance region also likely helped to maintain mean trough over central/ eastern Canada, and a the anomalous East Asian jet/ Siberian trough persistent jet stream in the Great Lakes region. configuration, as well as the persistent ridge south While it is not possible to attribute a of the jet core. For example an enhanced descent specific cause or causes of this persistent circula- of stratospheric air in the left jet entrance region, tion, three factors that contributed to its longevity which represents a source of cyclonic vorticity, have been discussed. These include a persistent likely helped to offset the downstream advection

11 of the Siberian trough and jet stream, and therefore aided in the persistence of these features. As another example, during April and May the divergent outflow from enhanced convection over the western tropical North Pacific is seen to accelerate northward into the mean jet entrance region. This outflow represents a source of anticyclonic vorticity that may have helped to maintain the amplified ridge south of the jet core, and therefore may also have helped to maintain the anomalous East Asian jet during this period.