Extratropical Highlights
– October 2012
1. Northern Hemisphere
The 500-hPa circulation during October
featured above-average heights throughout the polar region and below-average
heights in the middle latitudes (Fig. E9). Regional aspects of this pattern included
above-average heights across the high latitudes of both the North Pacific and
North Atlantic Oceans, which indicates a disappearance of both the Aleutian and
Icelandic Lows. The mean 500-hPa circulation also featured below-average
heights over the central North Pacific, central North America, and the eastern
North Atlantic. The pronounced north-south dipole of height anomalies over the
North Atlantic reflected a strong negative phase (-1.7) of the North Atlantic Oscillation
(NAO).
The main land-surface temperature signals
during October included above-average temperatures in western Russia, and
below-average temperatures in western Canada and the north-central United
States (Fig. E1).
The main precipitation signals included above-average totals in western Canada,
the northwestern and Great Lakes regions of the United States, eastern Europe, and western Russia. Below-average totals were
recorded in the south-central U.S. and southeastern Alaska (Fig. E3).
a. North America
The mean 500-hPa circulation during
October featured a persistent north-south dipole pattern of height anomalies
over the North Pacific, with above-average heights at high latitudes and
below-average heights in the middle latitudes (Figs. E9, E11).
This pattern was associated with a confluent height field over the western U.S., and with an amplified trough across central North
America. Consistent with these conditions, below-average temperatures were
observed in western Canada and the north-central U.S., with areas of western
Canada recording departures in the lowest 10th percentile of
occurrences (Fig. E1).
Above-average precipitation was recorded
in western Canada and the northwestern and Great Lakes regions of the United
States, while well below-average precipitation was recorded in southeastern
Alaska and the south-central U.S. (Fig. E3).
Large portions of the central U.S.
continued to be impacted by extreme or exceptional drought. At the end of October,
the “U.S. Drought Monitor” indicated exceptional drought from Oklahoma
northward to central South Dakota, including portions of eastern Wyoming,
eastern Colorado, and northern Texas. Extreme drought persisted in Arkansas, Iowa,
and southern Minnesota.
b. Europe/ central
Asia
The mean 500-hPa circulation during
October featured a north-south dipole pattern of height anomalies over the
North Atlantic, with above-average heights at high latitudes and below-average
heights in the middle latitudes (Figs. E9). This pattern was associated with a complete
disappearance of the Icelandic Low, and also projected strongly onto the
negative phase (-1.7) of the NAO. This marks the sixth straight month with a
negative NAO index.
c. African Sahel
The west
African monsoon season lasts from May through October, and typically peaks
during July-September. The monsoon rains were again enhanced during October
2012 (Fig. E1).
The 2012 west African monsoon season featured
above-average precipitation throughout the entire season, with area-averaged
totals exceeding the 70th percentile of occurrences during all six
months (Fig. E4).
Overall, the west African monsoon system has been
enhanced since 1995, in association with the warm phase of the Atlantic
Multi-Decadal Oscillation (AMO). This combination of an enhanced west African monsoon system and a warm AMO has contributed
to significantly increased Atlantic hurricane activity seen since 1995.
For the Atlantic hurricane region,
6 Atlantic named storms were present during October, with three reaching
hurricane status. The most significant hurricane was Sandy, caused tremendous
flooding and damage in the mid-Atlantic and northeastern U.S. in late October.
By the end of the month, the 2012 Atlantic hurricane season has produced 19
named storms, with 10 becoming hurricanes and one (Michael) reaching major
hurricane status. Also, the seasonal ACE index was 142% of the 1981-2020 median.
2. Southern Hemisphere
The mean 500-hPa circulation during
October featured an anomalous zonal wave-3 pattern, with above-average heights over
the eastern South Pacific Ocean, across the central South Atlantic Ocean, and
in the area south of Australia (Fig. E15). Below-average heights were observed over the
western South Pacific and in the area south of South America.
In the lower atmosphere, the
subtropical high pressure system normally centered east of South America was
again enhanced along its westward flank (Fig.
T20). The associated anomalous poleward flow over eastern South America led to an
anomalous poleward position of the mean cold frontal
boundary, and resulted in a continuation of well above-average surface
temperatures across that region (Fig. E1). This marks the third straight month in which
large portions of eastern South America have recorded surface temperature
departures above the 90th percentile of occurrences.
Much of Australia also recorded
above-average temperatures in October, while exceptionally dry conditions were
present across the eastern half of the continent. The largest precipitation
deficits were observed in southeastern Australia, where totals were generally
in the lowest 10 percentile of occurrences (Fig. E3).
The South African rainy season
lasts from October to April. During October 2012, rainfall for the region as a
whole was above average, with much of southeastern Africa recording totals
above the 70th percentile of occurrences (Figs. E3, E4).
The Antarctic ozone hole typically develops during August
and reaches peak aerial extent in October and early October. During October,
the size of the ozone hole was below the 2002-2011 mean, decreasing from 15
million square kilometers at the beginning of the month to 5 million square
kilometers at end of the month. (Fig. S8)
Overall, the size of the 2012 ozone hole was comparable to
the smallest seen during the 2002-2011 period. This
reduced size coincided with a below-average aerial coverage of polar stratospheric
clouds throughout July- October, along with a below-average size of the SH
polar vortex during October (Fig. S8).