– October 2014
1. Northern Hemisphere
The 500-hPa circulation during October
featured a zonal wave-4 pattern in the middle latitudes. This pattern included
above-average heights over western North America, eastern Canada, Scandinavia,
and the high latitudes of the North Pacific, along with below-average heights
over the eastern North Pacific, the eastern U.S., the eastern North Atlantic
and central Russia (Fig. E9). Also during October, above-average heights were
observed throughout the polar region.
The overall height anomaly pattern
projected onto several teleconnection patterns, including the positive phases
of the East Atlantic (+1.0), Pacific/ North American (+1.1), and Scandinavia
(+1.1) patterns (Table E1, Fig. E7).
Additionally, the overall anomaly pattern projected onto the negative phases of
the North Atlantic Oscillation (-0.9) and the Arctic Oscillation (-1.1) (Fig. A2.1).
The main land-surface temperature signals
during October included above-average temperatures in the western half of the U.S.,
eastern Canada, Europe, and China, and below-average temperatures across
northwestern and north-central Russia (Fig.
E1). The main precipitation signals included above-average
totals in the midwestern and northeastern U.S.,
southeastern Europe, and north-central Russia, and below-average totals in
southern Alaska and south-central Europe (Fig.
a. North Pacific/ North America
The mean 500-hPa circulation during
October featured an anomalous wave pattern across the eastern North Pacific and
North America, with above-average heights over the western U.S. and eastern
Canada and below-average heights over the eastern North Pacific and the eastern
U.S. (Fig. E9).
This pattern contributed to exceptionally warm temperatures over the western
half of the U.S., with many areas recording departures in the upper 90th
percentile of occurrences (Fig. E1). It also contributed to above-average
precipitation in the midwestern and northeastern U.S.
(Fig. E3), regions located within or
downstream of the mean trough axis.
According to the U.S. Drought
Monitor, a massive area of exceptional drought continued in central/ southern
California and western Nevada, with extreme drought extending northward into
Oregon. Exceptional or extreme drought was also observed in northeastern Texas
and south-central Oklahoma.
b. Europe/ Russia
The 500-hPa circulation featured an
anomalous wave pattern extending from the eastern North Atlantic to central
Russia, which included a strong ridge over Scandinavia and amplified troughs
over the eastern North Atlantic and central Russia (Fig. E9). This pattern impacted surface
temperatures and precipitation across the region.
The associated enhanced
southwesterly flow of mild marine air across Europe contributed to well
above-average surface temperatures, with most areas recording departures in the
upper 90th percentile of occurrences (Fig. E1). In contrast, surface
temperatures were well below average across northwestern and north-central
Russia, in response to strong northwesterly flow downstream of the mean ridge
axis. In addition, monthly precipitation totals were well above average in
north-central Russia in the region downstream of the mean trough axis (Fig. E3).
2. Southern Hemisphere
The mean 500-hPa circulation during
October featured an anomalous zonal wave-3 pattern, which was dominated by
above-average heights over southern Australia, the high latitudes of the South
Pacific, and south of Africa (Fig. E15).
In southern and eastern Australia,
the amplified ridge led to well above average surface temperatures, with much
of southwestern and southeastern Australia recording departures in the upper 90th
percentile of occurrences (Fig. E1). Most of southeastern and eastern Australia also
recorded exceptionally dry conditions (lowest 10th percentile of
occurrences) in response to anomalous sinking motion downstream of the mean
ridge axis (Fig. E3).
The Antarctic ozone hole typically develops during August
and reaches its peak aerial extent in September and October. Overall, the 2014
ozone hole was slow to develop during August, and was near the 2004-2013 mean
size during both September and October (Fig.