In the subtropics, there was again considerable symmetry of the upper-level circulation anomalies in both hemispheres during January (Fig. T22, bottom). Notable aspects of this anomaly pattern included amplified troughs over the mid-Pacific of both hemispheres, and anticyclonic circulation anomalies extending eastward in the Northern Hemisphere from the Atlantic Ocean to Asia and in the Southern Hemisphere from the Atlantic Ocean eastward to Australia. This overall pattern was also prominent throughout 1999 in association with ongoing La Niña conditions.

In the Northern Hemisphere, the anomalous low-latitude height field over the North Pacific contributed to a confinement of the East Asian jet stream to the area well west of the date line (Fig. T21). It also contributed to an extremely strong diffluent flow in the exit region of the East Asian jet, along with an amplification of the mean thermally-indirect transverse ageostrophic circulation throughout the jet exit region. These dynamical changes in the wintertime East Asian jet represent a fundamental aspect of the extratropical atmospheric response to mature cold episode conditions.

a. North America

The atmospheric circulation, temperature and precipitation patterns over the central U.S. and Canada exhibited considerable variability during the month, in association with strong blocking activity over the high latitudes of both the North Pacific and North Atlantic (Figs. A2.3b, c). Farther south, above-normal heights dominated the southwestern U.S. and Mexico during the month (Fig. E9), which contributed to anomalously warm (Fig. E1) and dry (Fig. E3, bottom) conditions in these regions. Rainfall has been substantially below-average in the southwestern U.S. and northwestern Mexico since August (see box labeled Southwest in Fig. E5). This dryness reflects an early end to the regions’ monsoon rains in August, and a nearly complete absence of measurable rainfall since October. This region typically receives below-average rainfall during the cool season in association with La Niña conditions.The Gulf Coast region also recorded significantly below-normal rainfall during the November 1999 - January 2000 period (Fig. E5), which is also typical of mature La Niña conditions. However, it is likely that the extreme rainfall deficits observed during November and December in this region were also partly linked to the strong positive phase of the NAO (Table E1).

b. North Atlantic/ Europe

The atmospheric circulation featured strong blocking activity over the high latitudes of the North Atlantic during the second half of the month (Fig. A2.3c). This blocking activity contrasts with the strong positive phase of the North Atlantic Oscillation (NAO) observed during November and December (Figs. E6, E7, Table E1).

For the month as a whole, temperatures averaged near-normal over most of Europe, with below-normal temperatures confined to the extreme south. Much of Europe also recorded below-average precipitation for the month, which contrasts with the exceptionally stormy conditions observed in northern Europe in December.

Below-normal heights dominated northern Africa and portions of the Middle East during January, which contributed to below-average temperatures across portions of northern Africa and the eastern Mediterranean Sea, and to near-average conditions over large portions of the Middle East. They also contributed to near-to-above average rainfall over large portions of the Middle East and eastern Mediterranean Sea. These conditions contrast with the extremely warm temperatures and suppressed rainfall that had dominated these regions for the past several months, in association with an anomalous anticyclonic circulation at upper levels throughout the region.

2. Southern Hemisphere

In the Southern Hemisphere extratropics, the circulation during January featured a continuation of above-normal heights over large portions of the middle latitudes (Fig. E15), with the largest and most persistent positive anomalies observed in the region poleward of Australia and over the east-central South Pacific (Fig. E17, left) Over the central and eastern South Pacific, the above-normal heights were located immediately poleward of the La Niña-related subtropical cyclonic circulation anomalies, which is similar to the anomaly pattern observed in the Northern Hemisphere (Fig. T22, bottom). This strong inter-hemispheric symmetry of circulation features over the central and eastern Pacific is typical of mature La Niña episodes. At high latitudes, another prominent feature of the Southern Hemisphere circulation was a continuation of negative height anomalies throughout Antarctica (Fig. E17, right), in association with an amplified polar vortex.

a. Australia

Anomalously cool (Fig. E1) and wet (Fig. E3) conditions covered most of western Australia during January, while anomalously cool and dry conditions were observed in the east. Each of these regions has experienced cooler-than-normal conditions since November.

b. Southern Africa

The rainy season in southern Africa typically extends from October to April, with many regions recording more than 75% of their mean annual rainfall during this period. The heaviest rains are generally observed over southeastern Africa, while considerably less rainfall is observed farther west in areas such as the Kalahari Desert of south-central Africa and the Namib Desert of southeastern Africa. Rainfall throughout southern Africa exhibits a strong relationship to the ENSO cycle, with above-normal rains generally observed during La Niña episodes and below-normal rainfall observed during El Niño episodes.