1. Northern Hemisphere
The 500-hPa height
field during May 2009 featured positive anomalies over the high latitudes of the
central North Pacific and from the western
, and negative anomalies over eastern
, the high latitudes of the
, and north-central
the circulation continued to reflect a westward retraction of the East Asian
jet stream in association with the lingering La Niņa signal (Fig.
temperature signals during May included above average temperatures in the
, and below average temperatures across
The main precipitation signals included above average totals in the southeastern
quadrant of the
, and below average totals in southern
a. North Pacific/
Lingering La Niņa
signals included a stronger than average tough over the central subtropical
North Pacific, and above average heights over the high latitudes of the central
North Pacific (Figs. T22, E9).
Associated with this pattern, the East Asian jet stream was again retracted
westward toward the western Pacific (Fig. T21). Over
, the circulation during May featured above average heights at 500-hPa over the
southwestern and southeastern
, and below average heights over central and eastern
. This pattern was associated with below average surface temperatures across
, with departures in central
dropping into the lowest 10th percentile of occurrences (Fig.
E1). It was also associated with above average
temperatures in the southwestern
, with many areas recording values in the highest 90th percentile of
were accompanied by an enhanced storm track and 500-hPa trough situated over the
. The trough axis delineated areas of below average precipitation to the west
and above average precipitation to the south and east (Fig.
E3). Area-averaged totals were
significantly above average in the
, Southeast, and
regions, where departures exceeded the 90th percentile of
circulation during May featured a dipole pattern of 500-hPa height anomalies
with below average heights in the north and above average heights in the south (Fig.
E9). This pattern was associated with a focused
storm track across the central
These conditions contributed to below average precipitation across southern
where departures were generally in the lowest 30th percentile of
2. Southern Hemisphere
The circulation during May reflected a zonal
wave-2 pattern of 500-hPa height anomalies in the middle and high latitudes,
with above average heights over the central South Pacific and in the area south
of Africa, and below average heights over the high latitudes of the eastern
South Pacific and southern Indian Ocean (Fig.
T22). The anomaly pattern over the
extratropical South Pacific continued to reflect a lingering influence from La
The main temperature departures during May
were seen in central
, where values were in the upper 70th percentile of occurrences (Fig.
E1). The main precipitation anomalies reflected
below average totals in southwestern and southeastern
. Monthly totals in portions of the southeast were in the lowest 10th
percentile of occurrences.