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.
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.
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 10th 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 90th percentile of occurrences (Fig. E1) and
precipitation totals were above the 70th 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).