Extratropical Highlights – September 2013

 

1. Northern Hemisphere

The 500-hPa circulation during September featured above-average heights over central North America, the central North Atlantic, Scandinavia, and the western North Pacific. It also featured below-average heights over the Gulf of Alaska, the western North Atlantic, the Caspian Sea, and north-central Asia (Figs. E9, E11).

The main land-surface temperature signals during September included above-average temperatures across much of central and western North America, Scandinavia, south-central Russia, and portions of China (Fig. E1). The main precipitation signals included above-average totals in the western U.S. and Mexico, and drier-than average conditions in the upper Midwest and eastern U.S., eastern Canada, Scandinavia, and northwestern Russia (Fig. E3).

In the western U.S., above-average precipitation during September helped to lessen ongoing drought conditions. However, much of the region still recorded moderate or severe drought at the end of the month. Also, drought expanded and worsened across large portions of the upper Midwest.

 

a. North America

The mean 500-hPa circulation during September featured an amplified ridge over central North America and amplified troughs over the Gulf of Alaska and the east coast of the United States (Fig. E9). This anomalous wave pattern contributed to well above-average temperatures across much of the United States and Canada, with portions of central Canada and the north-central U.S. recording departures in the upper 90^th percentile of occurrences (Fig. E1). It also contributed to well above-average precipitation across the western U.S., with many areas recording totals in the upper 90^th percentile of occurrences (Fig. E3). For the Pacific Northwest and Intermountain region of the U.S., area-averaged totals were the largest for the month of September dating back to at least 1979 (Fig. E5).

In the western U.S., above-average precipitation during September helped to lessen the strength of ongoing drought conditions. However, according to the “U.S. Drought Monitor” much of the region still recorded moderate or severe drought at the end of the month. Conversely, severe drought conditions expanded in the upper Midwest, including much of Iowa, northern Missouri, northwestern Illinois, portions of southern Minnesota, and western Wisconsin.

 

b. Europe/ northwestern Russia

The 500-hPa circulation featured above-average heights over Scandinavia and northwestern Russia (Fig. E9), resulting in a continuation of exceptionally warm (Fig. E1) and dry (Fig. E3) conditions across the region. This marks the third consecutive month with well below-average totals in these regions, as well as in northern and southern Europe (Fig. E4).

 

2. Southern Hemisphere

The mean 500-hPa circulation during September featured above-average heights in the polar region, and over the central South Pacific and South Atlantic Oceans (Fig. E15). It also featured below-average heights across the southern Indian Ocean.

Over Australia, the 200-hPa circulation featured an amplified trough in the west at 200-hPa (Fig. T22), and the 850-hPa circulation featured anomalous southerly flow across the southern half of the continent. These conditions contributed to exceptionally warm surface temperature across the eastern half of Australia, with departures exceeding the 90^th percentile of occurrences (Fig. E1).

The Antarctic ozone hole typically develops during August and reaches its peak aerial extent in September and October. By the end of September 2013, the ozone hole (Fig. S6) spanned 18 million square kilometers, which is less than the 2003-2012 mean of 20 million square kilometers (Fig. S8, top). This reduced coverage reflected a below-average extent of polar stratospheric cloud during the second half of the month (Fig. S8, bottom).