Extratropical Highlights – October 2013
1. Northern Hemisphere
The 500-hPa circulation during October
featured a zonal wave-4 pattern of height anomalies (Figs. E9, E11).
This pattern included above-average heights over the Gulf of Alaska, the high
latitudes of the North Atlantic, the Mediterranean Sea, and eastern Asia, and
below-average heights over the high latitudes of the central North Pacific, the
western U.S., the central North Atlantic and central Siberia.
The main land-surface temperature signals
during October included above-average temperatures across Alaska, eastern
Canada, Europe, and central Asia, and below-average temperatures in the northwestern
U.S. (Fig. E1).
The main precipitation signals included above-average totals in the southwestern
U.S., the north-central U.S., western Alaska, and Mexico, and drier-than
average conditions in western Canada, the southeastern and northeastern U.S.,
large portions of Europe, and eastern Siberia (Fig. E3).
a. North Pacific/ North America
The mean 500-hPa circulation during
October featured a strong ridge over the Gulf of Alaska, a broad trough across
the continental United States and Canada, and also a trough over the high
latitudes of the central North Pacific (Fig.
E9). This pattern contributed to well
above-average temperatures across Alaska and much of Canada, with much of
interior Alaska recording departures that exceeded +5.0oC and were
above the 90th percentile of occurrences (Fig. E1). In contrast, below-average
temperatures were observed in the northwestern U.S. in the vicinity of the mean
upper-level trough axis. The most significant negative departures ranged from -2oC
to -3oC and were in the lowest 10th percentile of
occurrences.
Several regions with significant
precipitation anomalies were also observed during October (Fig. E3). In the west, a pronounced
dipole pattern of rainfall anomalies reflected well above-average totals in
western Alaska and well below-average totals in western Canada and the
northwestern U.S. These regions coincided with the downstream portions of the
mean upper-level trough and ridge axes, respectively. Above-average
precipitation was also recorded in the north-central U.S., and near-average totals
were recorded in drought-stricken regions of the U.S. Midwest (Fig. E5) and southern Plains (Fig. E3). Also in October, below-average
precipitation was recorded in both the southeastern and northeastern U.S.,
where area-averaged totals were in the lowest 10th and 30th
percentile of occurrences, respectively (Fig.
E5).
In the western half of the U.S., average to
above-average precipitation during October again helped to lessen ongoing
drought conditions. However, much of the region still recorded severe or extreme
drought at the end of the month. Also, moderate or severe drought was present
across large portions of the upper Midwest, including Iowa, southern Minnesota,
Illinois, Wisconsin, and northern Missouri.
b. North Atlantic/ Europe
Across the North Atlantic Ocean, the
mean 500-hPa circulation featured a north-south dipole pattern of height
anomalies, with above-average heights at high latitudes and below-average
heights in the middle latitudes (Fig. E9). This pattern reflected the negative phase (-0.9)
of the North Atlantic Oscillation (NAO) (Table
E1). Previously, the positive phase of the NAO
had prevailed for the six month period spanning April-September (Fig. E7).
Despite the phase reversal in the
NAO, much of Europe remained exceptionally dry during October (Fig. E1), and
area-averaged totals in both southern and northern Europe were in the lowest 30th
percentile of occurrences (Fig. E4). This marks the fourth consecutive month of
below-average precipitation for both regions. Much of Europe also recorded
above-average temperatures during October, with portions of western and eastern Europe recording departures in the upper 90th
percentile of occurrences (Fig. E1).
2. Southern Hemisphere
The mean 500-hPa circulation during
October featured a zonal wave-1 pattern of height anomalies, with above-average
heights extending from the central South Pacific to the central South Atlantic Ocean,
and below-average heights spanning the southern Indian Ocean to southern New
Zealand (Fig. E15).
Above-average heights were also observed in the polar region and over
southeastern Australia.
Over Australia, the 200-hPa
circulation featured anticyclonic streamfunction
anomalies and an amplified ridge that spanned the entire continent (Fig. T22).
This pattern was associated with a southward shift of the mean jet axis, and
resulted above-average surface temperatures across the continent. The eastern
half of Australia experienced a continuation of exceptionally warm and dry
conditions, with many areas recording temperature departures above the 90th
percentile of occurrences (Fig. E1) and precipitation totals in the lowest 10th
percentile of occurrences (Fig. E3). In northeastern Australia, area-averaged
rainfall was below normal for the past six months, and was in the lowest 20th
percentile of occurrences for the past three months (Fig. E4).
The Antarctic ozone hole typically develops during August
and reaches its peak aerial extent in September and October. By the end of October
2013, the ozone hole (Fig. S6) spanned 7.5 million square kilometers, which is
less than the 2003-2012 mean of 11 million square kilometers (Fig. S8, top).
This reduced coverage reflected 1) a below-average size of the SH polar vortex
(Fig. S8, middle),
as indicated by well above-average heights across Antarctica extending upward
to at least 30-hPa (Fig. S1), and
2) a below-average extent of polar stratospheric cloud throughout the month (Fig. S8, bottom).
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