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

DECEMBER 2010

1. Northern Hemisphere

The 500-hPa circulation during December exhibited two main features. The first was a hemispheric-scale pattern of anomalies characterized by above average heights in the polar region and below average heights in the middle latitudes (Fig. E9). This pattern reflects a negative phase of the Arctic Oscillation (AO), and also projects strongly (-1.8) onto the negative phase of the North Atlantic Oscillation (NAO) (Table E1, Fig. E7). A negative NAO index has prevailed for the last 18 months (since June 2009).

The second main circulation feature during December was a strong negative phase of the Pacific/ North American (PNA) teleconnection pattern (-1.8). This pattern reflects above-average heights over the high latitudes of the North Pacific, and below average heights over the central subtropical North Pacific and western North America. A negative PNA pattern is typical of La Niña.

La Niña was also associated with enhanced mid-Pacific troughs in both hemispheres over the tropical and subtropical Pacific basin (Fig. T22). In the NH, one consequence of this pattern was a pronounced westward retraction of the east Asian jet stream and associated jet exit region (Fig. T21). These conditions are consistent with the negative PNA pattern (Fig. E9).

The main surface temperature signals during December included warmer than average conditions across eastern Canada and south-central Russia, and cooler-than-average conditions in the southeastern U.S., Alaska, most of Europe, and north-central Russia (Fig. E1). Monthly precipitation was above-average in the western U.S. and southwestern Europe, and below average in the southeastern U.S., southern Alaska, and extreme western Canada (Fig. E3).

a. North Pacific and North America

The mean 500-hPa circulation during December featured a strong ridge over the high latitudes of the North Pacific and northeastern Canada, and troughs over the Gulf of Alaska and the eastern U.S. (Fig. E9). This pattern showed links to both La Niña and the negative phase of the AO/ NAO.

La Niña is associated with deep tropical convection focused over Indonesia and the eastern Indian Ocean, along with a disappearance of tropical convection from the western and central equatorial Pacific (Fig. T25). This westward retraction in the area of deep convection acts to amplify the mean mid-Pacific troughs at 200-hPa in both hemispheres (Fig. T22), which in the NH acts to amplify and retract westward the exit region of the east Asian jet stream (Fig. T21). This jet structure favors corresponding westward shifts in the downstream ridge and trough axes normally located over western and eastern North America, respectively. During December, this westward shift was indicated by a tendency for strong ridging over the high latitudes of the North Pacific and by a broad trough over the Gulf of Alaska. This trough resulted in well above average precipitation in the western and southwestern U.S. (Fig. E3). Typically, that region sees below average wintertime precipitation during La Niña.

The 500-hPa circulation over eastern North America continued to reflect a negative phase of the AO/NAO. Specifically, the pattern of positive height anomalies over eastern Canada extended well eastward to Greenland and the high latitudes of the central North Atlantic, and occurred in combination with negative height anomalies across the central North Atlantic. This north-south dipole pattern, along with its associated southward shift of the mean North Atlantic jet stream, reflects the negative phase of the NAO (Fig. T21).

Consistent with this pattern, average surface temperatures in northeastern Canada exceeded the 90^th percentile of occurrences (Fig. E1), while temperatures in the southeastern U.S. were in the lowest 10^th percentile of occurrences. The largest precipitation anomalies during December included above average totals in the western U.S., and continued below average totals in the southeastern and Mid-Atlantic regions of the U.S. (Fig. E5). The southeastern portion of the U.S. tends to record below average wintertime precipitation during La Niña.

b. North Atlantic

Across the extratropical North Atlantic, the 500-hPa circulation during December featured an ongoing negative phase of the North Atlantic Oscillation (NAO) (Fig. E7, Table E1). This phase is characterized by above average heights over Greenland, and below average heights 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.

A characteristic cool-season feature of the negative NAO is southward shift of the mean North Atlantic jet stream (Fig. T21). During December, the mean Atlantic jet stream entered the continent in northern Africa (Fig. E10), which is well south of its normal position near Great Britain. As a result, the normal northward heat transport associated with this jet stream was absent. Instead, much of Europe experienced an anomalous flow of cold arctic air during the month, which resulted in well below average temperatures across much of the continent (Fig. E1). The subsequent eastward transport of this colder air also contributed to well below average temperatures in north-central Russia.

 2. Southern Hemisphere  

Over the South Pacific Ocean, the 500-hPa circulation during December reflected above average heights in the middle latitudes and below average heights at high latitudes (Fig. E15). In the subtropics, the upper-level (200-hPa) streamfunction pattern reflected an anomalous trough across central South Pacific in association with La Niña (Fig. T22).

The main temperature anomalies during the month included well below average temperatures in eastern Australia, where departures were in the lowest 10^th percentile of occurrences. The main precipitation signals reflected above average totals throughout the eastern half of Australia, with extremely high totals and severe flooding in the northeast (Fig. E3).

The South African rainy season lasts from October to April. During December, rainfall was above average in southeastern Africa, with departures generally in the upper 70^th percentile of occurrences. To date, precipitation was slightly below average in October, near average in November, and above average in December (Fig. E4). Seasonal rainfall is typically above average in this region during La Niña.