1. Northern Hemisphere
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.
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
a. North Pacific/
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 Niņo, and with a corresponding shift back to Indonesia of the area
of enhanced equatorial convection (Fig. T25).
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 10th
percentile of occurrences.
average precipitation occurred across the north-central U.S. and south-central
Canada during May, with monthly totals in the upper 90th 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 10th
percentile of occurrences.
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.
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 90th 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
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.