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

NOVEMBER 2010

1. Northern Hemisphere

The 500-hPa circulation during November featured above-average heights over the high latitudes of the North Pacific, eastern Canada, Greenland, and south-central Russia. The circulation featured below average heights over the central North Atlantic, Europe, and central Siberia (Fig. E9). Over the Atlantic basin, the anomaly pattern reflected a continuation of the negative phase of the North Atlantic Oscillation (NAO) (Table E1, Fig. E7), which has been in place for the last 17 months (since June 2009).

Over the subtropical Atlantic basin, the 200-hPa streamfunction anomaly pattern continued to show a pronounced inter-hemispheric symmetry, with anticyclonic anomalies extending from the America’s to Africa in both hemispheres (Fig. T22). This pattern has persisted throughout the past several months in association with the combination of an enhanced west African monsoon system, La Niña, and well above average (or record) SSTs across the tropical Atlantic (Fig. T18). These conditions contributed to a very active 2010 Atlantic hurricane season, which produced 19 named storms, of which 12 became hurricanes and 5 became major hurricanes.

Over the tropical and subtropical Pacific basin, enhanced mid-Pacific troughs were evident in both hemispheres during November (Fig. T22), in association with the ongoing La Niña (Fig. T18). 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), which favored above average heights over the high latitudes of the central North Pacific typical of La Niña (Fig. E9).

The main surface temperature signals during November included warmer than average conditions across Canada, Alaska and nearly all of Eurasia (Fig. E1). Below average temperatures were observed in the northwestern U.S. and Scandinavia. Monthly precipitation was above-average over large portions of Europe and northwestern Russia, and (Fig. E3), below average along portions of the U.S. Gulf Coast, nearly all of the U.S. eastern seaboard (Fig. E6), Turkey, and eastern China.

a. North Pacific and North America

The mean 500-hPa circulation during November featured a broad ridge over the high latitudes of the North Pacific and a broad trough over western and central North America (Fig. E9). It also featured a disappearance of the mean Hudson Bay trough. This pattern showed links to both La Niña and the negative phase of the 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 shift westward the exit region of the east Asian jet stream (Fig. T21). As a result, that jet stream also retracts westward, which favors corresponding westward shifts in the downstream ridge and trough axes normally located over western and eastern North America, respectively. During November this westward shift was indicated by a tendency for broad ridging at 500-hPa over the high latitudes of the North Pacific and by a broad trough that covered western and central North America.

The 500-hPa circulation over eastern North America also showed consistency with the ongoing negative phase of the 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).

The absence of the mean Hudson Bay trough during November contributed to well above average surface temperatures in northeastern Canada, where temperature departures exceeded the 90^th percentile of occurrences (Fig. E1). The largest precipitation anomalies during November included below average totals in the Gulf Coast, Southeast, and Mid-Atlantic regions of the U.S. (Fig. E5). The southern portion of the U.S. tends to record below average precipitation during La Niña.

b. North Atlantic

Across the extratropical North Atlantic, the 500-hPa circulation 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 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.

A characteristic cool-season feature of the negative NAO is southward shift of the mean North Atlantic jet stream (Fig. T21). During November, the mean Atlantic jet stream entered Europe in the vicinity of Portugal (Fig. E10), which is well south of its normal position near Great Britain.

 Over the subtropical North Atlantic, anticyclonic streamfunction anomalies at 200-hPa extended from the America’s to Africa in both hemispheres (Fig. T22). This 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 was evident throughout the Atlantic hurricane season, which lasts from June through November. It has links to the enhanced west African monsoon circulation that was seen well into October (Fig. E4), La Niña, and well above average SSTs across the tropical Atlantic (Fig. T18). These conditions contributed to a very active 2010 Atlantic hurricane season, which produced 19 named storms, of which 12 became hurricanes and 5 became major hurricanes.

c. Eurasia

            The 500-hPa circulation during November featured a large amplitude trough over Europe and a broad ridge over south-central Russia (Fig. E9). This pattern was associated with an extensive flow of marine air into the continent, which subsequently extended across central Asia. As a result, much of the Eurasia recorded well above average temperatures (3+°C above average) during the month, with many areas recording departures in the upper 90^th percentile of occurrences (Fig. E1). This same flow pattern contributed to above average precipitation from central Europe to north-central Siberia, with most locations recording totals in the upper 70^th percentile of occurrences (Fig. E3).

 2. Southern Hemisphere  

        The 500-hPa circulation during November reflected above average heights in the middle latitudes and below average heights over the high latitudes of the eastern South Pacific (Fig. E15). A similar anomaly pattern was evident in October. 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).