1. Northern Hemisphere
The 500-hPa circulation during October 2003 featured above-average
heights in the polar region, the high latitudes of both the North Pacific
and North Atlantic, and the western United States, and below-average
heights across the central North Pacific, eastern North America and Europe
(Figs. E10, E12).
Areas experiencing significantly warmer-than-normal temperatures during the
month generally coincided with the persistent positive height anomalies,
and included the western U.S., most of Alaska and Canada, and the North
Atlantic (Fig. E1). The main region of
below-average temperatures during October was Europe, which had previously
experienced significantly warmer-than-normal conditions during
March-September 2003. Prominent precipitation anomalies during October
included above-average totals over southern and eastern Europe, and
below-average totals over large portions of the U.S., eastern China and
Japan (Fig. E3).
a. North America
The mean circulation featured an amplified upper-level ridge over
western North America, and amplified troughs over the central North Pacific
and eastern North America (Fig. E10). This
circulation was associated with a pronounced eastward extension of the East
Asian jet stream to the northwestern U.S. (Figs. E11,
T21), and with an anomalous flow of relatively
mild air into western North America which contributed to a continuation of
above-average surface temperatures throughout the region (Fig.
E1). The most significant temperature departures were observed in
the inter-mountain region of the western U.S., where temperatures exceeded
the 90th percentile and averaged more than +2oC above
average during the month.
Above-average precipitation in the northwestern U.S. and southwestern
Canada during October (Figs. E3, E6)
is related to the upper-level divergence and increased storm activity found
upstream of the mean ridge axis and along the cyclonic-shear side of the
East Asian jet exit region. The northeastern states also experienced
above-average precipitation during October, due mainly to increased
storminess and large-scale ascending motion immediately downstream of the
mean trough axis.
In contrast, much of the remainder of the U.S. was situated within and
downstream of the mean upper-level ridge axis, and experienced near-normal
to below-normal precipitation during the month. For the central and eastern
states this represents a marked departure from the above-normal
precipitation totals observed during the past five months, and in some
areas for the past year (Fig. E5). For
example, the Ohio Valley, Mid-Atlantic, and Southeast regions recorded
precipitation surpluses in almost every month since September 2002, while
the Gulf Coast and Great Lakes regions recorded above-normal rainfall
during May-September, and June-September 2003, respectively.
A strong ridge-trough system spanned the North Atlantic and Europe
during October (Fig. E10), resulting in
below-average temperatures over most of Europe (Fig.
E1) and significantly below-average precipitation over portions of
Great Britain and Scandinavia (Fig. E3).
This circulation was also associated with a pronounced split flow pattern
over the eastern North Atlantic, with the strong southern branch
contributing to above-average precipitation across southern and eastern
Europe. Much of the remainder of Europe experienced near-normal
precipitation during the month. These overall conditions are in marked
contrast to the mean upper-level ridge and the associated exceptionally
warm and dry conditions (Fig. E4) that had
prevailed across Europe since the March.
2. Southern Hemisphere
In the Southern Hemisphere the 500-hPa circulation during October
featured an anomalous zonal wave-3 pattern, with above-average heights over
the central ocean basins and below-average heights generally situated
poleward of the three continents (Fig. E16).
Above-average heights also covered most of the polar region during October,
resulting in a slight weakening of the polar vortex in the region of
Antarctica (Fig. E17).
In the middle latitudes this circulation was associated with enhanced
jet stream winds across Australia (Fig. T21),
with the northern part of the country situated in anomalous anticyclonic
shear equatorward of the jet core and the southeastern part of the country
situated in anomalous cyclonic shear poleward of the jet core. This
anomalous cyclonic circulation extended down to 850-hPa, where it
contributed to an anomalous inflow of cold air across southern Australia (Fig.
T20). Surface temperatures across the region were below the 30th
percentile of occurrences, with temperatures in the southeast dropping
below the 10th percentile (Fig. E1).
The anomalous cyclonic circulation over southern South America
contributed to generally warmer and wetter-than-average conditions across
central and southern Argentina, with much of the region situated in broad
southwesterly flow immediately downstream of the mean trough axis.
b. Stratospheric Ozone Hole
The polar stratospheric circulation during October (Fig.
S1, top) reflected an overall shift of the Antarctic circumpolar
vortex and the associated ozone hole (Fig. S6,
bottom) toward eastern Antarctica. Mean 100-50-hPa temperatures within
the polar vortex averaged more than 8oC below-normal during the
month (Fig. S2).
The 2003 ozone hole reached record size from August through early
October (Fig. S8, top), when it covered
close to 25 x 106 km2. It then shrank considerably
during the second half of October, and was down to 10 x 106 km2
by the end of the month.
The large 2003 ozone hole is related to significantly below-average
stratospheric temperatures, which resulted in an increased polar
stratospheric cloud (PSC) cover (Fig. S8, bottom).
The decreased size of the ozone hole during October was associated with a
significant drop in PSC coverage. PSCís are comprised mostly of nitric
acid and water, and form when stratospheric temperatures drop below -78oC.
These clouds provide an ideal surface upon which inert chlorine compounds
can react with sunlight to form active chlorine compounds such as chlorine
peroxide. As sunlight reaches the polar region in August and September the
resulting reactions destroy nearly all stratospheric ozone, often to
altitudes of 20 km.