1. Northern Hemisphere
During September the 500-hPa
circulation pattern featured positive height anomalies over western and
, and the high latitudes of the central and western North Pacific, and
negative height anomalies from the north-central
and over central
(Figs. E9, E11).
For the second month in a row, the anomaly pattern from eastern
has reflected a strong negative phase of the North Atlantic Oscillation (NAO)
(Table E1, Fig. E7).
The main surface temperature
departures during the month reflected warmer-than-average conditions across
, and cooler-than-average conditions across the central
(Fig. E1). Above-average precipitation was
recorded in the Inter-Mountain,
, and Mid-Atlantic regions of the
, and across portions of southern
(Figs. E3, E6).
Below-average precipitation occurred in portions of the southern Plains and
Midwestern regions of the
, and southwestern
(Figs. E3, E4).
The mean 500-hPa circulation
pattern during September featured an amplified ridge over western and
and a broad trough across the central and eastern
(Fig. E9). This pattern contributed to
above-average temperatures over much of
, and to below-average temperatures across much of the
. The most significant positive temperature departures were observed in
portions of north-central and northeastern
, where values exceeded the 90th percentile of occurrences (Fig.
E1). In the
, the most significant negative temperature departures occurred in the
southern Plains region, where monthly temperatures were in the lowest 10th
percentile of occurrences.
The pattern of anomalous
precipitation was also consistent with the upper-air circulation, with
above-average precipitation in the
coinciding with regions of increased storminess in the vicinity of the mean
trough axis. The most significant precipitation surpluses occurred in the
, and Mid-Atlantic regions, where area-average totals generally exceeded the
80th percentile of occurrences (Fig. E6).
, large areas of below-average precipitation were observed within and
downstream of the amplified upper-level ridge (Fig. E3).
The 500-hPa circulation
pattern during September featured an anomalous ridge over
and an anomalous trough over the east-central
(Fig. E9). This circulation contributed to a
continuation of significantly above-average precipitation throughout
, with totals in many areas exceeding the 90th percentile of
occurrences. It also contributed to exceptionally warm surface temperatures
, with departures exceeding the 90th percentile of occurrences in
An anomalous north-south dipole pattern of
precipitation was also evident over
, with below-average precipitation coinciding with the high-latitude ridge,
and above-average precipitation across portions of southern
coinciding with broad upper-level westerly flow undercutting the ridge.
2. Southern Hemisphere
The mean 500-hPa
circulation pattern during September featured above-average heights across
, over portion of
and over the central and eastern
, and below-average heights over the high western longitudes of the three
ocean basins (Fig. E15). In southern
, this pattern contributed to anomalously warm and dry
conditions. Temperatures in that region generally exceeded the 70th
percentile of occurrences. Rainfall totals in portions of the southeast were
below the 10th percentile of occurrences for the second