Extratropical Highlights – April 2014

 

1. Northern Hemisphere

The 500-hPa circulation during April featured above-average heights over Europe and eastern Asia, and below-average heights over the central North Pacific, central Canada, and the polar region (Fig. E9). The main land-surface temperature signals included well above-average temperatures in Europe and eastern Asia, and below-average temperatures in central Canada (Fig. E1). The main precipitation signals included below-average totals in the south-central U.S., Scandinavia, southern Russia and eastern Mongolia (Fig. E3).

 

a. North Pacific/ North America

The mean 500-hPa circulation during April featured an amplified and extended trough across the North Pacific, along with an increased strength of the Hudson Bay vortex (Fig. E9). This latter feature contributed to below average surface temperatures in central Canada (Fig. E1). It also contributed to an east-west dipole pattern of precipitation anomalies in the U.S., with above-average totals in the Midwest and southeast in the area downstream of the trough axis, and below-average totals over the southern Plains in the area upstream of the trough axis (Fig. E3). The Midwest, Southeast, Ohio Valley, and Great Lakes region all recorded their first month of well above-average precipitation since last summer (Fig. E5).

Conversely, ongoing precipitation deficits have contributed to an expansion and worsening of drought conditions in the U.S. southern Plains. According to the U.S. Drought Monitor (http://droughtmonitor.unl.edu), exceptional drought was present in the Texas Panhandle and western Oklahoma by the end of the month, and extreme drought was present from southern Texas northward to central Kansas. Much of eastern Kansas and central Nebraska recorded severe drought. Also during April, exceptional drought continued in central California, and extreme drought extended across the U.S. Southwest, California, and portions of the Pacific Northwest.

 

b. North Atlantic/ Eurasia

The mean 500-hPa circulation during April featured an amplified wave pattern across Eurasia, with above-average heights over Europe and northeastern Asia and below-average heights over central Russia (Fig. E9). This circulation was associated with above-average temperatures across Europe and northeastern Asia, with both regions recording recording departures in the upper 90^th percentile of occurrences (Fig. E1). The pattern also contributed to below-average precipitation in west-central Russia and eastern Mongolia, with many locations recording departures in the lowest 10^th percentile of occurrences (Fig. E3).

 

2. Southern Hemisphere

The mean 500-hPa circulation during April featured an anomalous zonal wave-3 pattern, with above-average heights over the central ocean basins and below-average heights concentrated around the polar region (Fig. E15). In northeastern Australia, precipitation was again near-average or below-average (Figs. E3, E4). This region has recorded precipitation deficits in nearly every month since last May.

The South African rainy season lasts from October to April. During April, much of the region recorded near- or above-average totals (Fig. E3). To date for the 2013-14 rainy season, area-averaged totals were above average from December to April (Fig. E4).