1. Northern Hemisphere   

The 500-hPa height pattern during April featured positive anomalies over Europe and central Russia , and negative anomalies across the high latitudes of the North Pacific, the western North Atlantic , and western Russia (Figs. E9, E11). The 200-hPa streamfunction anomaly pattern showed a strong inter-hemispheric symmetry across the subtropical Pacific Ocean , with anomalous ridges west of the date line, and anomalous troughs east of the date line (Fig. T22). This pattern was also prevalent in March, and is consistent with enhanced equatorial convection west of the date line and suppressed convection east of the date line (Fig. T25).

The main surface temperature departures during April reflected well above-average temperatures across Europe and central Russia , and below-average temperatures over much of the United States (Fig. E1). The main precipitation anomalies included drier than average conditions in the southeastern U.S. , Florida , and throughout Europe (Fig. E3).

a. Pacific/ North America

In the subtropics, the upper-level circulation during April again reflected the pattern of anomalous convection across the equatorial Pacific, with enhanced subtropical ridges at 200-hPa flanking the region of enhanced convection and enhanced mid-Pacific troughs flanking the region of suppressed convection (Figs. T22, T25).

The 500-hPa circulation pattern featured an amplified trough over eastern North America and the western North Atlantic , which contributed to below-average temperatures over much of the United States (Figs. E9, E11). At 200-hPa, this trough was associated with an enhanced jet stream centered over the mid-Atlantic region and an anomalous anticyclonic circulation located near Florida (Fig. T21). The persistence of these circulation anomalies contributed to suppressed rainfall in both Georgia and Florida , with monthly totals in Florida falling into the lowest 30^th percentile of occurrences. Ongoing dryness in Florida has led to major water shortages and wildfires, especially in the south.

b. Europe and central Russia

The 500-hPa circulation pattern during April featured a persistent blocking ridge centered over western Europe (Fig. E9). This feature led to a continuation of exceptional warmth across much of the continent, with temperatures in many locations exceeding the 90^th percentile of occurrences (Fig. E1). This persistent ridge also led to a continuation of well below-average precipitation across much of Europe , with most regions recording totals in the lowest 10^th percentile of occurrences (Figs. E3, E4). Over central Russia , a persistent upper-level ridge also led to well above-average temperatures during April, with departures in many regions exceeding the 90^th percentile of occurrences.

2. Southern Hemisphere

In the subtropics and lower mid-latitudes the upper-level circulation featured an anomalous ridge over Australia and the western South Pacific, and an enhanced mid-Pacific trough east of the date line (Fig. T22). This anomaly pattern is consistent with the anomalous equatorial convection across the Pacific basin (Fig. T25).

In northern Australia , the persistent upper-level ridge was associated with exceptionally dry conditions, with totals in many areas falling into the lowest 10^th percentile of occurrences (Fig. E3). In southern Australia , the persistent ridge was associated with above-average surface temperatures, and above-average precipitation in the area upstream of the mean ridge axis.

In South America , the mean upper-level ridge normally centered over the Amazon Basin was enhanced during April, and extended farther south into the lower extratropics. This anomaly pattern appears to be associated with the amplified mid-Pacific trough father upstream, and does not reflect a response to local convection which was near-average during the month (Fig. T25). This amplified ridge contributed to well above-average surface temperatures in southeastern Brazil (Fig. E1), and to below-average precipitation over south-central Brazil in the vicinity of the mean ridge axis (Fig. E3).

In southern Africa , the rainy season extends from October to April. Precipitation was well above average during April, with the largest surpluses observed in eastern South Africa and southern Mozambique (Figs. E3, E4). For the entire 2006-07 rainy season, area-averaged totals were below-normal in October, February and March, near-normal in November and December, and above normal in January and April.