CPC Banner
Expert Assessment
Monitoring & Data
El Niņo/La Niņa Home
Climate Diagnostics Bulletin Index


Extratropical Highlights - August 2002

1. Northern Hemisphere
The 500-hPa circulation during August featured above-average heights extending from the northeastern United States to Scandinavia, over central Asia, and across eastern Siberia, and below-average heights across the central North Pacific, southern Europe, central Russia, and eastern China (Figs. E10, E12). In the subtropics cyclonic streamfunction anomalies extended eastward from the central North Pacific to southeastern Asia (Fig. T22, bottom), with similar anomalies also evident in the Southern Hemisphere. This overall anomaly pattern is consistent with the strengthening El Niņo episode (Fig. T18), and with the eastward propagation of the ascending phase of the MJO across the tropical Pacific during the month (Figs. T11, T12).

Prominent temperature (Fig. E1) and precipitation (Figs. E3, E4, E5) anomalies during August included a continuation of warmer and drier than normal temperatures over the northeastern and southwestern United States, northern Europe, and Mongolia, and a continuation of exceptionally heavy rainfall across southern Europe and southeastern China.

a. North America

Above-normal 500-hPa heights covered the northeastern United States for a third consecutive month in association with an eastward expansion of the mean summertime ridge. During August this circulation was associated with a continuation of exceptionally warm and dry conditions across the region, with temperature departures exceeding the 90th percentile (Fig. E1) and rainfall departures falling in the lowest 10th percentile of occurrences (Fig. E3). Regionally, significantly below-average rainfall was again observed in the Great Lakes, Ohio Valley, and Northeast regions (Fig. E5) during August. Precipitation in the Ohio Valley has been below normal since June 2001.

Below-average rainfall also persisted in the Inter-Mountain and Southwest regions during August (Figs. E3, E5). Precipitation has been below normal in these regions since June 2001 (Fig. E5).

b. Europe

Most of northern Europe, western Russia, and eastern Asia experienced a continuation of well above-average surface temperatures during August (Fig. E1) in association with anomalously strong ridges at upper levels (Fig. E10). Monthly mean temperatures averaged 2-4°C above normal in these areas and exceeded the 90th percentile.

Below-normal rainfall was observed across northern Europe (Fig. E4) during August, while well-above-normal rainfall continued across the southern and central portions of the continent. This dipole pattern of rainfall anomalies was associated with a pronounced split-flow in the 500-hPa height field over the eastern North Atlantic (Fig. E10), which featured a persistent upper-level ridge across Scandinavia and northern Europe within the northern branch of the split flow, and enhanced jet stream winds across northern Africa and southern Europe within the southern branch.

C. China/ Mongolia

Above-average rainfall continued in southeastern China for a fourth consecutive month (Fig. E4), resulting in additional flooding in the Yangtze River Valley. This excessive convective rainfall has been associated with a very persistent upper-level trough situated over central China in association with an anomalously weak Asian monsoon ridge (Fig. T22).

Farther north well-below-average rainfall was again observed over large portions of Mongolia, in association with a southward shift of the main storm track toward China. The climatological rainy season for Mongolia is June-August. During 2002 June-August rainfall totals were less than 25% of normal over most of the country, which is well below the 10th percentile of occurrences. Mongolia also recorded considerably below-average rainfall during the 2001 rainy season, and has not recorded ample rainfall since 1998.

2. Southern Hemisphere

The upper level circulation during August featured cyclonic (i.e., positive) streamfunction anomalies in the subtropics from the eastern South Pacific eastward to the central Indian Ocean (Fig. T22 bottom), in association with strengthening El Niņo conditions (Fig. T18) and intra-seasonal activity linked to the MJO (Figs. T11, T12). In the extratropics above-average heights were observed over the high latitudes of the eastern South Pacific, the central Atlantic Ocean, and the central Indian Ocean (Fig. E16), and below-average heights were observed across the middle latitudes of the central and eastern South Pacific.

This circulation was associated with a pronounced eastward extension of the South Pacific jet stream to approximately 90°W (Fig. T21 top), and with a corresponding eastward displacement of the jet exit region to the eastern South Pacific. Increased storminess and enhanced rainfall were evident across southern South America along the poleward flank of this anomalous jet exit region, while exceptionally warm surface temperatures were observed over central South America along its equatorward flank.