Extratropical Highlights –August 2015

 

1. Northern Hemisphere

The mean 500-hPa circulation during August featured above-average heights across the high latitudes of the North Pacific, eastern Canada, and Scandinavia, and below-average heights over the eastern North Atlantic Ocean ad central Siberia (Fig. E9).

At 200-hPa, a significant El Niño response was evident in the streamfunction field throughout the global tropics and subtropics. This response featured a zonal wave-1 pattern of streamfunction anomalies in both hemispheres (Fig. T22), with anticyclonic anomalies over the subtropical North and South Pacific straddling the region of enhanced convection (Fig. T25), and cyclonic anomalies extending from the America’s to Australasia. 

The main land-surface temperature signals during August included above-average temperatures in the western U.S., eastern Canada, most of Europe, and Mongolia (Fig. E1). The main precipitation signals included above-average totals in the northern Europe, and below-average totals in the Pacific northwestern U.S., the northeastern U.S., northwestern Russia, and portions of Scandinavia (Fig. E3).

 

a. North Pacific/ North America

At 200-hPa, the circulation during August featured a sharper-than-average ridge over the inter-mountain region of the U.S., and sharper-than-average troughs over the extreme eastern North Pacific and the eastern U.S. (Fig. T21). This pattern contributed to a continuation of exceptionally warm surface temperatures in the western U.S., and also to well above-average temperatures in eastern Canada (Fig. E1). It also contributed to anomalous upper-level convergence and below-average precipitation over the northeastern quadrant of the U.S. (Fig. E3).

According to the U.S. Drought Monitor, severe or extreme drought expanded in Washington, Oregon, Idaho and western Montana. Exceptional drought continued across much of central California and western Nevada. Abnormally dry or moderate drought conditions developed across the U.S. Gulf Coast and in the southeast.

 

b. North Atlantic

In association with El Niño, the 200-hPa circulation featured cyclonic streamfunction anomalies extending across the tropical and subtropical North Atlantic to southern Asia (Fig. T22). Across the Atlantic hurricane Main Development Region (MDR, which spans the Caribbean Sea and tropical Atlantic Ocean between 9°N-21.5°N), this pattern contributed to an amplification of the Tropical Upper-Tropospheric Trough (TUTT), which now extends well southward into the western MDR.

Over the western MDR, these conditions were associated with above-average westerly winds at 200-hPa (Fig. T21) and enhanced low-level easterly trade winds  (Fig. T20). This wind pattern produced enhanced vertical wind shear, and was also associated with anomalous upper-level convergence (Fig. T23) and sinking motion, across large portions of the MDR. This combination of conditions is expected to continue, and to produce a below normal Atlantic hurricane season.

 

c. Europe

The 500-hPa circulation during August featured above-average heights over Scandinavia, and below-average heights over the eastern North Atlantic (Fig. E9). This pattern contributed to exceptionally warm surface temperatures across Europe and western Russia, with many locations recording departures in the upper 10^th percentile of occurrences (Fig. E1).  It also contributed to a dipole pattern of precipitation anomalies, with above-average totals in northern Europe and below-average totals in northwestern Russia and portions of Scandinavia.

 

2. Southern Hemisphere

The mean 500-hPa circulation during August featured an overall zonal wave-1 pattern of height anomalies, with above-average heights in the middle latitudes and below-average heights over Antarctica (Fig. E15). At 200-hPa, a significant El Niño response was evident in the streamfunction field throughout the global tropics and subtropics. This response featured a zonal wave-1 pattern of streamfunction anomalies in both hemispheres (Fig. T22), with anticyclonic anomalies over the subtropical North and South Pacific straddling the region of enhanced convection (Fig. T25), and cyclonic anomalies extending from the America’s to Australasia.

The El Niño response also included above-average heights over the high latitudes of the central South Pacific, and below-average heights over both the mid-central South Pacific and the high latitudes of the eastern South Pacific (Figs. E15, T22).  The resulting 4-celled anomaly pattern incorporated well-known features of El Niño: namely a strengthening and eastward extension of the South Pacific jet stream to well east of the date line, and an eastward shift of that jet exit region to the area immediately upstream of South America (Fig. T21). This wintertime jet stream pattern represents major dynamical and kinematic changes in the mid- and upper-level circulation during El Niño, and it also represents a fundamental manner in which the El Niño impacts are communicated downstream.

For example, the presence of the amplified trough and jet exit region immediately east of South America contributed to a poleward shift of the mean frontal boundary and to enhanced storminess across central South America, as indicated by exceptionally warm and wet conditions across the region. In many areas, surface temperature anomalies were in the upper 90^th percentile of occurrences (Fig. E1) and precipitation totals were above the 70^th percentile of occurrences (Fig. E3).

The Antarctic ozone hole typically develops during August and reaches its peak aerial extent in September and October. By the end of August 2015, the ozone hole (Fig. S6) spanned approximately 15 million square kilometers, which is close to the 2005-2014 mean (Fig. S8, top). The aerial coverage of polar stratospheric cloud (Fig. S8, bottom) and the SH polar vortex (Fig. S8, middle) were both slightly above average during August 2015, while polar stratospheric temperatures were below average (Fig. S4).