The overall pattern of annual-average temperature anomalies during 1996 shows below-normal temperatures across the middle latitudes of the Northern Hemisphere and above-normal temperatures over large parts of the high latitudes (Fig. 3 ). In the middle latitudes, three primary areas of substantially below-normal surface temperatures prevailed during the year. The first region extended southeastward from eastern Alaska to the Gulf of Mexico. This area included all of western and central Canada, where annual temperatures averaged 1.0°_3.0°C below normal. It also included the contiguous United States east of the Rocky Mountains, where temperatures in many regions averaged 0.5°_1.5°C below normal for the year. Another area of below-normal temperatures extended from the eastern North Atlantic eastward across northern and central Europe to south-central Russia. The largest negative anomalies in this area were observed across northern Europe and south-central Russia and averaged 1.0°_1.5°C below normal for the year. The final region of negative anomalies extended from Japan eastward to the central North Pacific between 30° and 40°N, with temperatures averaging 0.5°_1.5°C below normal during 1996.
Above-normal surface temperatures covered the subtropical eastern North Pacific and the southwestern United States during the year, with much of the latter region recording annual mean temperatures more than 1°C above normal. Temperatures also averaged more than 0.5°C above normal across northern Africa, southwestern Europe, and much of the Middle East. Farther north, temperatures averaged 1.0°_2.5°C above normal across large portions of Siberia, the high latitudes of the North Pacific from eastern Siberia to the Aleutian Islands, and across much of eastern and extreme northern Canada.
Precipitation anomalies and percentiles for the 1995_96 "water year," defined as the period from October 1995_September 1996 and chosen because this 12month calendar period minimizes interruptions in the rainy seasons for most regions around the globe, are shown in Figs. 4a,b. In the Northern Hemisphere the primary regions of substantially above-normal precipitation included the northwestern and northeastern United States, southern Europe, and northern Africa. Substantially below-normal precipitation was observed across the high latitudes of the North Pacific and Alaska, eastern Canada, the central and southwestern United States, the central and subtropical North Atlantic, northern Europe, extreme eastern China, and southern Japan. Also evident is a well-defined pattern of anomalous rainfall across the Tropics, featuring above-normal precipitation from South America eastward to Indonesia and below-normal precipitation across the central and eastern equatorial Pacific (Fig. 4 ). This pattern reflects the positive phase of the Southern Oscillation (SO) during the year (see section 3a ).
The pattern of annual average height anomalies during 1996 over the Northern Hemisphere was generally dominated by above-normal heights across the high latitudes and below-normal heights across the middle latitudes (Fig. 5a). However, there was also considerable zonal variability in this pattern. The largest positive height anomalies were observed over the North Pacific and North Atlantic Ocean basins, and over the southwestern United States, and the largest negative anomalies were observed across the western and central North Pacific between 30° and 40°N, across the southeastern South Atlantic and southern Europe, and over central Russia. These anomalies were generally consistent with the annual temperature and precipitation anomaly patterns and, in many instances, reflected extremely persistent circulation features that lasted for several consecutive months.
Regionally, the pattern of height anomalies over the North Atlantic and Europe reflected the strong negative phase of the North Atlantic oscillation (NAO), which is a primary mode of low-frequency variability in the extratropics and which exhibits considerable interannual and interdecadal variability (see section 3c). The NAO pattern was most prominent during DJF 1995/96 and MAM 1996 and again during December 1996, and was associated with large-scale temperature and precipitation anomalies throughout Eurasia and northwestern Africa (see section 4a ).
Over the high latitudes of the North Pacific, the pattern of above-normal temperatures and below-normal precipitation reflected a recurring pattern of above-normal heights and substantial blocking activity in all seasons. Across the middle latitudes of the western and central North Pacific, the pattern of below-normal tem peratures and above-normal precipitation was associated with below-normal heights and an overall strengthening of the East Asian jet in all seasons. Collectively, these conditions were consistent with Pacific cold-episode conditions and the positive phase of the Southern Oscillation during the year.
The southwestern United States was dominated by above-normal heights in all seasons, while western Canada was characterized by below-normal heights in all seasons. This anomaly pattern reflected enhanced westerly flow, storm activity, and moisture transport into the northwestern United States throughout the year. These conditions contributed to abnormally cool conditions across western Canada and to significantly above-normal precipitation and periodic flooding in the Pacific Northwest [see section 4b(1)]. They also resulted in abnormally warm and dry conditions across the southwestern United States and central plains states, particularly during October 1995_ May 1996, and were associated with moderate to severe drought throughout the region [see section 4b(2) ]. These conditions contrasted with the above-normal rainfall, enhanced storm activity, and stronger than normal jet stream winds that characterized much of the Southwest during the first half of the 1990s, in association with recurring mature-phase warm-episode conditions in the tropical Pacific.
The 1996 annual surface temperature anomalies in the Southern Hemisphere extratropics (Fig. 3) showed above-normal temperatures over western and central Australia, Indonesia, the central latitudes of the South Pacific, and much of the region south of Africa. Below-normal temperatures were observed over the high latitudes between 50° and 70°S, across southern Africa, and over southeastern Australia.
The regions of significantly below-normal precipitation during the 1995_96 water year (Figs. 4a,b) included the east-central South Pacific, most of extratropical South America, the region between South America and the area immediately south of the African continent, and central Australia. In contrast, precipitation totals were above-normal during the water year across the central latitudes of the western and central South Pacific, and from southeastern Africa eastward to the Tasman Sea.
In southern Africa, the cooler and wetter than normal conditions during 1996 were observed mainly in the climatologically high precipitation areas of northeastern South Africa, southern Mozambique, and much of central Madagascar [see section 4c(2) ]. This above-normal rainfall is consistent with Pacific cold episode conditions during the year (Ropelewski and Halpert 1989) and contrasts with the suppressed rainfall observed during the 1994/95 rainy season in association with Pacific warm episode conditions. Above-normal rainfall also occurred in association with an elongated band of abnormally heavy precipitation that extended in all seasons from the central South Atlantic eastward to southeastern Australia and the Tasman Sea. This precipitation was associated with below-normal heights and stronger than normal westerlies at upper levels (Fig. 5b) and with enhanced storm activity across the region.
