Extratropical Highlights – October 2011

 

1. Northern Hemisphere

The 500-hPa circulation during October featured a zonal wave-4 anomaly pattern, with above average heights over the central North Pacific, eastern Canada, Europe, and central Russia (Fig. E9). Below average heights were observed over the Gulf of Alaska, the eastern U.S., and the high latitudes of the North Atlantic (Fig. E9). The main surface temperature signals during October included warmer-than-average conditions across Canada, southern Europe, Scandinavia, and central Asia (Fig. E1). The main precipitation signals included above-average totals in the both the eastern and northwestern U.S, and below-average totals in the central U.S. and central Europe (Fig. E3).

 

a. North Pacific and North America

The mean 500-hPa circulation during October featured above average heights over Hudson Bay and below average heights over the Gulf of Alaska and the southeastern U.S. (Fig. E9). This circulation was associated with enhanced jet stream winds entering the Pacific Northwestern U.S., and with a pronounced diffluent flow pattern across central North America (Fig. E10).

These overall conditions were associated with above average surface temperatures across Alaska and Canada, with much of eastern Canada recording departures in the upper 90^th percentile of occurrences (Fig. E1). They were also associated with a 3-celled pattern of precipitation anomalies over the U.S. (Fig. E3). This pattern featured above average totals in the northwestern U.S. in the area downstream of the mean trough axis located in the Gulf of Alaska. It also featured above average totals extending along the entire eastern sea-board, and below average totals extending from the Gulf Coast to the northern Plains. These north-south oriented bands of precipitation anomalies were largely related to the mean trough axis located over the eastern U.S.

In the Great Plains, area-averaged precipitation totals during October were again in the lowest 30^th percentile of occurrences (Fig. E5). Precipitation has been below average in this region during 11 of the last 13 months. These ongoing significant precipitation deficits led to a continuation of exceptional drought conditions (according to NOAA’s U.S. Drought Monitor) in Texas, Oklahoma, southern Kansas, and southeastern New Mexico.

 

b. North Atlantic and Europe

The 500-hPa circulation during October featured a deep trough over Greenland and the high latitudes of the North Atlantic, and a broad ridge over southern Europe (Fig. E9). This pattern was associated with a pronounced diffluent flow across central Europe, and with a northward shift to Scandinavia of the mean jet core. These conditions were associated with an enhanced northward transport of mild, marine air into Scandinavia, which was then transported eastward across central and northern Asia. The result was an extended area of above average surface temperatures from Scandinavia to north-central Asia, with departures exceeding the 90^th percentile of occurrences across large portions of Russia and Mongolia (Fig. E1).

 

2. Southern Hemisphere

 

The 500-hPa circulation during October featured above average heights in the middle latitudes and over Antarctica, and below average heights over the high latitudes of the eastern South Pacific and also extending from the eastern South Atlantic to the central Indian Ocean at lower latitudes (Fig. E15).  In the stratosphere, the height field featured a wave-1 anomaly pattern, with above average heights extending from the eastern South Atlantic to Australia, and below average heights covering the South Pacific basin (Figs. S1, S2).

This stratospheric pattern was associated with a record large size of the polar vortex during the latter part of the month (Fig. S8, middle). This pattern followed record low polar temperatures during August and September at both 2-hPa and 10 hPa (Fig. S4), along with a record amount of polar stratospheric cloud during September (Fig. S8, bottom). In response to this combination of conditions, the ozone hole covered approximately 20 million square kilometers in late October, which is a record size for that time of the year (Fig. S8, top).

Overall, the 2011 ozone hole developed very rapidly in mid-August, which is slightly later than its normal onset in early August (Fig. S8, top). The ozone hole then reached its peak extent from mid-September to early October, spanning approximately 24 million square kilometers.