500-hPa height pattern during May 2008 featured positive anomalies over the
eastern North Pacific, across the high latitudes of the
, and over eastern
, and negative anomalies over the high latitudes of the central North Pacific,
across the central
, and central
(Fig. E9). Prominent teleconnection patterns
during the month included the negative phase of the Arctic Oscillation (AO),
with regional anomalies reflecting the negative phase of the North Atlantic
Oscillation (NAO) and West Pacific pattern, and the positive phase of the
Pacific/North American (PNA) pattern (Table E1,
Fig. E7). This overall pattern also In the
subtropics the 200-hPa circulation remained consistent with La Niņa, with
enhanced mid-Pacific troughs in both hemispheres flanking the suppressed
convection over the central equatorial Pacific (Fig. T22).
main surface temperature anomalies during May reflected above-average
temperatures over northeastern
, and central
, and below-average temperatures over the northeastern quadrant of the
and portions of eastern Europe (Fig. E1). The
main precipitation anomalies included above average totals in the central
, and below-average totals in the
area of the
a. North Pacific/
La Niņa signal was again prominent across the subtropical
during May. La Niņa is associated with a westward retraction of deep tropical
, and a complete disappearance of tropical convection from the central
equatorial Pacific (Fig. T25).
These conditions result in a westward retraction of deep tropospheric heating,
and hence a westward retraction of the 200-hPa subtropical ridge toward
(Fig. T22). Over the central
equatorial Pacific, the reduction in convective heating contributes to an
increased strength of the mid-Pacific troughs in both hemispheres.
, the mean 500-hPa ridge-trough pattern was stronger than average, and there was
a pronounced split-flow configuration over the western US (Fig. E9).
This pattern was associated with an eastward extension of the East Asian jet
stream, which was related to large negative height anomalies across the high
latitudes of the central North Pacific (Fig. T21).
This overall pattern reflected the positive phase (+1.3) of the PNA
teleconnection pattern (Table E1, Fig. E7).
conditions contributed to below-average temperatures across the northeastern
quadrant of the
in the vicinity of the mean trough (Fig. E1).
They were also associated with increased storminess and above-average
precipitation in the central
, and with below-average precipitation along the
(Figs. E3, E5). This
marks the seventh consecutive month with below-average precipitation in the
region, which is consistent with La Nina (Fig. E5).
Ongoing precipitation deficits have led to a continuation of moderate-to-severe
drought in portions of the southeastern
the 500-hPa height anomaly pattern during May featured a north-south dipole,
with positive anomalies across the high latitudes and negative anomalies
extending from the eastern
. This pattern reflected a strong negative phase (-1.7) of the North Atlantic
Oscillation (Table E1). It was associated with
increased storminess and above-average precipitation over portions of southern
, and with below average precipitation in northern
2. Southern Hemisphere
The 500-hPa circulation during May generally featured above-average heights at
high latitudes and below-average heights in the middle latitudes (Fig. E15).
In the subtropics the ongoing 200-hpa pattern of positive streamfunction
anomalies over the central Pacific and negative streamfunction anomalies over
the western Pacific was consistent with La Niņa (Fig. T22).
Another La Niņa-related feature was an anomalous wave pattern emanating from
the western tropical Pacific, and characterized by negative height anomalies
and the high latitudes of the eastern South Pacific, and positive height
anomalies over the high latitudes of the western South Pacific.
This wave pattern again contributed to significantly cooler and drier than
average conditions in eastern Australia, which was located just upstream of the
mean trough axis (Figs. E1, E3).
was also situated upstream of the mean trough axis during May, and also
recorded well below average precipitation with monthly totals in the lowest 10th
percentile of occurrences. In southeastern
, precipitation has been below average for seven straight months in association
with La Nina (Fig. E4).