Extratropical Highlights - August 2014

 

1. Northern Hemisphere

The 500-hPa circulation during August featured above-average heights across Canada, Greenland, and central Russia, and below-average heights over southern Alaska, the central North Atlantic, and central Europe (Fig. E9). This pattern projected onto the strong negative phase of both the North Atlantic Oscillation (NAO, -2.28) and the east Atlantic/ West Russia (-1.69) teleconnection pattern  (Table E1, Fig. E7).

The main land-surface temperature signals included well above-average temperatures in northeastern Canada, southern Greenland, and western Russia, and below-average temperatures in the west-central U.S., the eastern U.S., and northern Europe (Fig. E1). The main precipitation signals included above-average totals in eastern Alaska, the north-central U.S., and northern Europe, and below-average totals in western Alaska, the northwestern U.S., and south-central Russia (Fig. E3).

 

a. North Pacific/ North America

The mean 500-hPa circulation during August featured troughs over southern Alaska and California, and an anomalous ridge across eastern Canada and Greenland (Fig. E9). This pattern was associated above-average precipitation in eastern Alaska and the north-central U.S., and with below-average precipitation in the extreme northwestern U.S. (Fig. E3). Area-averaged totals in both the Inter-Mountain and Great Plains regions reached the 80^th percentile of occurrences (Fig. E5), with many locations recording more than 175% of normal precipitation (Fig. E6).

According to the U.S. Drought Monitor (http://droughtmonitor.unl.edu), above-average precipitation helped to lessen drought conditions in central Plains states, with the most significant improvements observed in Nebraska and eastern Kansas. In contrast, a massive area of exceptional drought continued in central/ southern California and western Nevada, and severe or extreme drought extended across the U.S. Southwest, California, southern Oregon, central Washington, and southern Idaho.

 

b. Europe

The 500-hPa circulation during August featured above-average heights over Greenland and below-average heights across the central North Atlantic and Europe (Fig. E9). This pattern projected strongly onto the negative phase of both the North Atlantic Oscillation (NAO, -2.28) and the east Atlantic/ West Russia (-1.69) teleconnection pattern (Table E1, Fig. E7).

This anomaly pattern was associated with well above-average precipitation in central and northern Europe, and with below-average precipitation in southwestern Russia (Figs. E3, E4). It was also associated with well above-average temperatures in southern Greenland and western Russia, with many locations recording departures in the upper 90^th percentile of occurrences (Fig. E1).

 

c. Northern Africa

The West African monsoon, which lasts from June-September, has been well below average so far this year. Area-averaged totals during August were near average, following well below-average totals during June and July (Figs. E3, E4). Overall, the suppressed west African monsoon has likely contributed to exceptionally non-conducive conditions for hurricane formation across the tropical North Atlantic, including enhanced vertical wind shear, sinking motion, and atmospheric stability, and a southward shift of the African Easterly Jet (AEJ).  Another contributing factor has been an amplified Tropical Upper-Tropospheric trough (TUTT) over the western North Atlantic and Caribbean Sea (Fig. T22).

 

2. Southern Hemisphere

The mean 500-hPa circulation during August featured above average heights south of Australia and Africa, and below-average heights over the central ocean basins (Fig. E15). In southeastern Australia downstream of the mean ridge axis, exceptionally dry conditions continued during the month with regional precipitation totals in the lowest 10^th percentile of occurrences (Figs. E3, E4). Exceptionally cool conditions were recorded in north-central Australia, where monthly departures were in the lowest 10^th percentile of occurrences.

The Antarctic ozone hole typically develops during August and reaches its peak aerial extent in September and October. By the end of August 2014, the ozone hole (Fig. S6) spanned 10 million square kilometers, which is less than the 2004-2013 mean of 15 million square kilometers (Fig. S8, top). The aerial coverage of polar stratospheric cloud (Fig. S8, bottom) and the SH polar vortex (Fig. S8, middle) were near average during August 2014, while polar stratospheric temperatures were below average (Fig. S4).