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

JANUARY 2011

Beginning with this month, all anomalies reflect departures from the 1981-2010 base period.

1. Northern Hemisphere

The 500-hPa circulation during January featured a hemispheric-scale pattern of anomalies characterized by above average heights in the polar region and generally below average heights in the middle latitudes (Fig. E9). This pattern reflects a negative phase of the Arctic Oscillation (AO, Fig. S2), and also projects strongly (-1.5) onto the negative phase of the North Atlantic Oscillation (NAO) (Table E1, Fig. E7). A negative NAO index has prevailed for the last 19 months (since June 2009).

Also evident during January was a continuation of enhanced mid-Pacific troughs in both hemispheres over the tropical and subtropical central/eastern Pacific (Fig. T22).  In the NH, these conditions contributed to an enhanced East Asian jet exit region, and to above average 500-hPa heights across Alaska and the Gulf of Alaska. These features are consistent with the ongoing mature phase of La Niña.

The main surface temperature signals during January included warmer than average conditions across eastern Canada, the southwestern U.S., and south-central Russia, and cooler-than-average conditions in the eastern U.S., Alaska, most of Europe, and large parts of northern Asia (Fig. E1). Monthly precipitation was below average in the western and eastern U.S. and Canada, western Canada, portions of central Europe, and central Russia (Fig. E3). No significant regions of above average precipitation were observed this month.

a. North Pacific and North America

The mean 500-hPa circulation during January featured a strong ridge over the Gulf of Alaska and Alaska, and a deep trough over the eastern U.S. (Fig. E9). This pattern showed links to the mature La Niña conditions and to the ongoing strong 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 normally favors corresponding westward shifts in the downstream ridge and trough axes normally located over western and eastern North America, respectively. This latter feature was not present during January.

The 500-hPa circulation over eastern North America continued to reflect a strong 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, while the negative anomalies extended from the eastern U.S. to southern Europe. 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, monthly 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 January included below average totals in the western and eastern U.S., with many areas recording totals in the lowest 10^th percentile of occurrences (Fig. E5).  In the U.S., many areas recorded a multi-month continuation of below average precipitation. These areas included the Southwest, the Southeast, the Gulf Coast, the Great Plains, and the Great Lakes region.

b. North Atlantic

Across the extratropical North Atlantic, the 500-hPa circulation during January 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 January, the mean Atlantic jet stream entered the continent in northern Africa, which is well south of its normal position near Great Britain (Fig. E10).  As a result, the normal northward heat transport associated with this jet stream was greatly diminished, and was replaced by an extensive northwesterly flow of much colder air across Europe and western Russia. These conditions contributed to well below average temperatures across northern Europe, Scandinavia, and western Russia, with many locations recording totals in the lowest 10^th percentile of occurrences (Fig. E1). The subsequent eastward transport of this colder air, combined with a strong high pressure system and likely enhanced radiative cooling, also contributed to significantly below average temperatures across central Russia.

 2. Southern Hemisphere  

Over the South Pacific Ocean, the 500-hPa circulation during January 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 enhanced ridge across Australia and the western South Pacific, and an amplified trough across central South Pacific. This overall anomaly pattern is consistent with La Niña (Fig. T22).

The main temperature anomalies during January included a continuation of well below average temperatures in eastern Australia, where departures were again in the lowest 10^th percentile of occurrences. The main precipitation signals reflected above average totals in northwestern Australia, and across the entire South African monsoon region (Fig. E3).

The South African rainy season lasts from October to April. During January, rainfall for the region as a whole was the highest for any January dating back to at least 1979 (Fig. E4). For the 2010-11 rainy season, precipitation was slightly below average in October, near average in November, and above average in both December and January. Seasonal rainfall is typically above average in this region during La Niña.