Extratropical Highlights

  Table of Indices  (Table 3)

  Global Surface Temperature  E1

  Temperature Anomalies (Land Only)  E2

  Global Precipitation  E3

  Regional Precip Estimates (a)  E4

  Regional Precip Estimates (b)  E5

  U.S. Precipitation  E6

  Northern Hemisphere

  Southern Hemisphere

  Stratosphere

  Appendix 2: Additional Figures

Extratropical Highlights

OCTOBER 2010

1. Northern Hemisphere

The 500-hPa circulation during October featured above-average heights over western North America, Greenland, and across the western and central North Pacific Ocean, and below average heights over the Gulf of Alaska, eastern Canada and southern Europe (Fig. E9). Over the subtropical Atlantic basin, the 200-hPa streamfunction pattern continued to show a pronounced inter-hemispheric symmetry, with anticyclonic anomalies extending from the America’s to Africa in both hemispheres (Fig. T22). Conversely, anomalous troughs prevailed across most of the sub-tropical western and central Pacific in both hemispheres.

These signals are consistent with the combined influences of La Niña and an enhanced west African monsoon system (Figs. T23, T24), and with ongoing exceptionally warm SSTs across the tropical and high latitudes of the North Atlantic (Fig. T18). This combination of factors contributed to a continuation of exceptionally strong Atlantic hurricane activity.

The main surface temperature signals during October included warmer than average conditions across Canada and the western United States, southern Greenland, central Russia, and portions of the Middle East (Fig. E1). Below average temperatures were observed in eastern Europe. Monthly precipitation totals (Fig. E3) were above-average in the northeastern U.S., California, the eastern Mediterranean Sea region, tropical western Africa, and southern China. Monthly precipitation was below average over Mexico and across much of the southeastern and central United States (Fig. E6).

a. North America

The mean 500-hPa circulation during October featured an amplified wave pattern across North America, with a strong ridge over western North America flanked by troughs over both the Gulf of Alaska and eastern North America (Fig. E9). This pattern dictated the temperature (Fig. E1) and precipitation (Fig. E3) anomaly patterns across the continent. It was associated with well above average temperatures over much of western North America, with monthly departures in many regions exceeding the 90^th percentile of occurrences. It was also associated with increased storminess and above average precipitation in California and in the northeastern U.S., and with well below average precipitation across the southeastern and central U.S. Some of the largest precipitation deficits were observed in Texas, Florida, and Missouri, where monthly totals were generally less than 25% of normal (Fig. E6).

b. North Atlantic

Across the extratropical North Atlantic, the 500-hPa circulation featured an ongoing negative phase (-0.9) of the North Atlantic Oscillation (NAO) (Fig. E7, Table E1). This phase is characterized by above average heights over Greenland, and below average heights generally extending from eastern North America to southern Europe. The negative NAO has prevailed in every month since June 2009, with the exception of September 2009.

 Over the subtropical North Atlantic, anticyclonic streamfunction anomalies at 200-hPa extended from the America’s to Africa in both hemispheres (Fig. T22). This pronounced inter-hemispheric symmetry was associated with upper-level easterly wind anomalies that extended across tropical northern Africa and the tropical North Atlantic (Fig. T21). It was also associated with an extensive area of low-level westerly wind anomalies that extended across the eastern North Pacific and tropical North Atlantic (Fig. T20).

This combination of conditions is consistent with an enhanced west African monsoon circulation (Figs. T23, T24) and has been a prominent feature of the tropics-wide circulation since 1995. During October, the above conditions contributed to reduced vertical wind shear over the tropical North Atlantic, and to exceptionally conducive wind patterns that extended westward from Africa.  As a result, five Atlantic hurricanes developed during the month. Similar anomaly patterns have prevailed throughout the 2010 Atlantic hurricane season, which was extremely active during June-October with 19 named storms, of which 12 became hurricanes and 5 became major hurricanes.

The above conditions are typical of the high activity era for Atlantic hurricanes that began in 1995. They are also consistent with a continuation of exceptionally warm SSTs in the Atlantic basin, which have also prevailed since 1995 in association with the warm phase of the Atlantic Multi-decadal Oscillation (AMO). Since March 2010, record warm SSTs have persisted across the tropical North Atlantic.

 2. Southern Hemisphere  

The 500-hPa circulation during October reflected above average heights in the middle latitudes and below average heights over the high latitudes of the eastern South Pacific (Fig. E15). In the subtropics, the upper-level (200-hPa) streamfunction pattern reflected an anomalous trough across the western and central South Pacific, and an anomalous ridge extending from the eastern South Pacific to southern Africa (Fig. T22). A similar anomaly pattern was evident in the Northern Hemisphere. These conditions have been present since June, and likely reflect the combined influences of La Niña and an enhanced west African monsoon system. Another characteristic of La Niña seen during October was a marked westward retraction and weakening South Pacific jet core, as indicated by easterly wind anomalies centered along the jet axis near 30°S (Fig. T21).

The Antarctic ozone hole typically develops during August, reaches its peak aerial extent during September, and then slowly dissipates during October and November. During October, the size of the ozone hole was near the 2000-2009 mean, covering more than 15 million square kilometers during most of the month (Fig. S8).

The 2010 ozone hole did not form until late August, making this the latest formation date since 2000. This delay of onset reflected a significant decrease in polar stratospheric clouds that occurred during July and August in association with a sharp increase in polar stratospheric temperatures at both 10-hPa and 2-hPa (Fig. S4). During September, the ozone hole covered 14 million square kilometers early in the month, and expanded to 20 million square kilometers late in the month. The ozone hole spanned nearly 12.5 million square kilometers at the end of October.