Skip Navigation Links 
NOAA logo - Click to go to the NOAA home page National Weather Service   NWS logo - Click to go to the NWS home page
Climate Prediction Center

HOME > Expert Assessments > Climate Diagnostics Bulletin

Extratropical Highlights - November 1999

1. Northern Hemisphere

Very persistent and large-amplitude circulation anomalies covered the middle latitudes of the Northern Hemisphere during November (Figs. E9, E11), which contributed to anomalous temperature (Fig. E1) and rainfall patterns (Fig. E3) over large portions of the hemisphere. The most prominent circulation anomalies included large regions of above-normal heights over the central Pacific, North America and the eastern Atlantic, and below-normal heights over the eastern Pacific and the Caspian Sea sector. This predominance of above-normal heights is consistent with an ongoing pattern of anomalous anticyclonic circulations which have dominated large portions of the middle latitudes for the past several months.

a. North America

The circulation over North America was dominated by an amplified ridge that covered most of the continent, with the largest positive anomalies observed over central Canada and the central United States (Fig. E9). This pattern contributed to above-normal temperatures across the continent (Fig. E1), with departures averaging more than 4°C above normal over most of central North America. The amplified ridge also contributed to significantly below-normal rainfall over large portions of the United States (Fig. E3), with the largest percentile deficits observed in the following regions (Fig. E5): the Southwest, the Inter-mountain region, the Great Plains, the Midwest, the Gulf Coast, the mid-Atlantic, and the Northeast. For the midwestern region as a whole, rainfall totals have been significantly below normal throughout the July-November 1999 period (Fig. E5).

b. Europe/ Asia

Portions of western Europe, northern Africa and the Middle East experienced significantly above-normal temperatures during November (Fig. E1), with temperatures above the 90th percentile covering large parts of these regions. In northern Africa and the Middle East, this warmth represents a continuation of extremely warm conditions that have prevailed for the last several months. This persistence of well above-normal temperatures has been linked to an extremely persistent and anomalous anticyclonic circulation at upper levels (Fig. T22, bottom) which, in many months, has been associated with the previously-noted large-scale pattern of above-normal heights over the middle latitudes.

Farther north, the warmth that was observed over northwestern Europe during November also prevailed during September and October. This warmth has been linked to above-normal heights across the eastern North Atlantic, Scandinavia and northwestern Russia, which have persisted for much of the period. In the region downstream of this amplified ridge, anomalously cold temperatures prevailed during November across eastern Europe and much of western Russia (Fig. E1).

2. Southern Hemisphere

The Southern Hemisphere circulation during November featured a continuation of above-normal heights over large portions of the middle latitudes (Fig. E15), with the largest positive anomalies observed in the region poleward of Australia, and in the regions to the east of New Zealand and southern South America. Another prominent feature of the Southern Hemisphere circulation during November was a large region of negative height anomalies over the high latitudes of the eastern South Pacific. This overall pattern of height anomalies extending from Australia to the eastern South Pacific has persisted for the past four months. During the August-October period this pattern contributed to extremely warm conditions across large portions of Australia, New Zealand and southern South America, which continued during November across New Zealand and southern South America but replaced by anomalously cold conditions over nearly all of the Australia.

a. Australia

The transition to anomalously cold conditions over Australia during November was associated with a poleward shift of positive height anomalies to the region south of the continent, and to the development of negative height anomalies across the central latitudes of the country. This anomaly pattern was linked to an amplified jet stream (Fig. T21) with an anomalous jet entrance region across central and eastern Australia (Fig. T21, bottom). An anomalous thermally-direct transverse ageostrophic circulation, which is a dynamically required component of this anomalous jet entrance region, likely contributed to a strong dipole pattern consisting of anomalously dry conditions over southeastern Australia and extremely wet conditions over northeastern Australia during the month (Fig. E1).

b) South America

In South America, most of cental Argentina and southeastern Brazil experienced a continuation of anomalously dry conditions during November (Fig. E3). Rainfall deficits have been observed in these regions in each of the past four months. This dryness has been linked to an overall poleward shift of the main frontal boundary away from central Argentina and southeastern Brazil, in association with an overall poleward displacement of the mean storm track across the eastern Pacific and southern South America. This anomalous circulation has also contributed to anomalously warm conditions across southern South America during the period (Fig. E1), and is consistent with ongoing La Niņa conditions.

NOAA/ National Weather Service
NOAA Center for Weather and Climate Prediction
Climate Prediction Center
5830 University Research Court
College Park, Maryland 20740
Page Author: Climate Prediction Center Internet Team
Page last modified: August 24, 2007
Information Quality
Privacy Policy
Freedom of Information Act (FOIA)
About Us
Career Opportunities