Beginning with this
month, all anomalies reflect departures from the 1981-2010 base period.
The 500-hPa circulation during May
featured above average heights over the central North Pacific Ocean, from
Newfoundland to central Europe, and across Siberia, and below average heights
over the high latitudes of the North Pacific and North Atlantic, the western
U.S., and the Mediterranean Sea (Fig. E9).
At 200-hPa, enhanced troughs in both hemispheres over the tropical and
subtropical central/eastern Pacific were consistent with La Niņa (Fig. T22).
The main surface temperature
signals during May included well above average temperatures in western Europe
and Siberia, and below average temperatures in the western and central U.S.,
western Russia, and western Canada (Fig. E1).
The main precipitation signals included above average totals across the northern
tier of the United States, Great Britain, and Scandinavia, and below-average
totals along the U.S. Gulf Coast, southern Alaska, and much of Europe (Fig.
a. North Pacific and North America
In the lower latitudes, the
200-hPa circulation during May featured amplified mid-Pacific troughs in both
hemispheres (Fig. T22).
This pattern is linked to the ongoing suppressed convection over the central
Pacific in association with La Niņa (Fig.
In the extratropics, the mean
500-hPa circulation during May featured a strong high-latitude trough that
extended from Mongolia to the western United States. This pattern was associated
with increased storminess and above average precipitation across the northern
tier of the United States, and with below average precipitation across the Gulf
Coast (Figs. E3,
E6). It was also associated with below average
temperatures across the western U.S. (Fig.
E1). In the U.S. Midwest, the above average
precipitation exacerbated flooding that began early in the month in response to
exceptionally heavy rainfall in April.
Conversely, the U.S. Gulf Coast
continued to experience exceptionally dry conditions during May. This marks the
8th consecutive month of well below average precipitation for this
region (Fig. E5).
The southeastern and mid-Atlantic regions of the U.S. also recorded well below
average precipitation during May, with area-averaged totals in both regions
falling into the lowest 10th percentile of occurrences.
b. North Atlantic and
The 500-hPa circulation during May
featured a zonally-elongated pattern of height anomalies, with positive
anomalies in the middle latitudes and negative anomalies at both high and low
latitudes (Fig. E9).
This pattern was associated with enhanced southwesterly winds across
Scandinavia, and with a strong poleward transport of heat into the high
latitudes of Eurasia. It was also associated with warmer (Fig.
E1) and wetter (Fig.
E3) than average conditions across Great Britain
and Scandinavia. Farther south, the ridge over southern Europe resulted in
exceptionally warm and dry conditions, with many areas recording temperatures in
the upper 90th percentile of occurrences and precipitation totals in
the lowest 10th percentile of occurrences.
For the past three months, the circulation has featured
an extensive southwesterly flow of mild air into the high latitudes of Eurasia (Fig.
E9). These conditions led to an early snow-melt,
and to a continuation of well above average temperatures during May across much
anomalies reflect departures from the 1981-2010 base period.
In the subtropics, the upper-level
(200-hPa) circulation during May reflected an amplified trough across the
central South Pacific and a marked westward retraction of the mean South Pacific
jet stream to west of the date line (Figs. T21, T22).
These conditions are consistent with La Niņa.
The 500-hPa circulation featured
above average heights over the central South Pacific and in the areas south of
both Australia and South Africa, and below average heights over the high
latitudes of the eastern South Pacific, eastern Australia, and southern Africa (Fig.
pattern was associated with below average precipitation across eastern
Australia, in response to large-scale descending motion upstream of the mean
trough axis. It was also associated with well above-average precipitation in
southern Africa, where monthly totals were generally above the 70th
percentile of occurrences (Figs. E3,