Extratropical Highlights

  Table of Indices  (Table 3)

  Global Surface Temperature  E1

  Temperature Anomalies (Land Only)  E2

  Global Precipitation  E3

  Regional Precip Estimates (a)  E4

  Regional Precip Estimates (b)  E5

  U.S. Precipitation  E6

  Northern Hemisphere

  Southern Hemisphere

  Stratosphere

  Appendix 2: Additional Figures

Extratropical Highlights

MAY 2010

1. Northern Hemisphere

The 500-hPa circulation during May featured above-average heights across the polar region, and generally below average heights in the middle latitudes (Fig. E9). This pattern reflected the negative phase of the Arctic Oscillation (AO). Regional circulation features during the month included above average heights over eastern North America, Greenland, northwestern Russia, and eastern Siberia, and below average heights across the central North Pacific, western U.S., high latitudes of the western North Atlantic, and central Europe. Over the North Atlantic, the anomalies projected strongly (-1.5) onto the negative phase of the North Atlantic Oscillation (NAO) (Table E1, Fig. E7). The negative NAO signal has been exceptionally persistent since July 2009. The circulation during May 2010 also reflected a strong negative phase (-3.1) of the West Pacific (WP) pattern, which contrasts with the positive phase of the WP pattern that had previously persisted from January through April.

The main surface temperature signals during May were consistent with the upper-level circulation, and included above average temperatures in eastern North America and northwestern Russia, and below average temperatures in the western U.S. (Fig. E1). The main precipitation signals included above-average totals in the northern Plains states, southwestern Canada, eastern Europe/ western Russia, and portions of China (Fig. E3). Below average totals were observed in the northeastern U.S., eastern Canada, and southern Alaska.

a. North Pacific/ North America

Over the Pacific sector, the circulation during May featured a north-south dipole pattern of 500-hPa height anomalies, with above average heights at high latitudes and below average heights in the middle latitudes (Fig. E9). This pattern reflected a strong negative phase of the WP teleconnection pattern (Fig. E7), and is opposite to that which persisted from January-April. This phase change of the WP pattern occurred coincident with the demise of El Niño, and with a corresponding shift back to Indonesia of the area of enhanced equatorial convection (Fig. T25).

Over the United States, the circulation during May featured a strong trough in the west and a ridge in the east. This pattern resulted in an east-west dipole pattern of surface temperature anomalies, with well below average temperatures in the west and above average temperatures in the east (Fig. E1). For much of the western U.S., temperatures were 2°-4°C below average, with departures generally in the lowest 10^th percentile of occurrences.

Above average precipitation occurred across the north-central U.S. and south-central Canada during May, with monthly totals in the upper 90^th percentile of occurrences (Fig. E3). This enhanced precipitation occurred in association with large-scale ascending motion and upper-level divergence immediately downstream of the mean trough axis. Conversely, below average precipitation occurred immediately downstream of the mean ridge axis, with totals in portions of eastern Canada in the lowest 10^th percentile of occurrences.

b. Europe

The 500-hPa circulation during May featured a north-south dipole pattern of height anomalies, with above average heights over Greenland and below average heights over the high latitudes of the western North Atlantic (Fig. E9). This pattern reflected the ongoing negative phase of the NAO, which has persisted since July 2009 (Fig. E7).

Also during May 2010, the circulation featured a trough over central Europe and a ridge over northwestern Russia. This pattern contributed to above average precipitation over eastern Europe and western Russia, with some areas recording totals in the upper 90^th percentile of occurrences (Fig. E3). It also contributed to well above average temperatures in western Russia, with some areas recording departures exceeding +4°C (Fig. E1).

  2. Southern Hemisphere

The 500-hPa circulation during May featured above average heights over the central ocean basins, and below average heights across the high latitudes of the eastern South Pacific and over eastern Australia (Fig. E15). At 200-hPa, the South Pacific jet stream was retracted westward and the jet exit region was located well west of the date line (Fig. T21). This jet stream configuration is consistent enhanced convection over Indonesia (Fig. T25).