Extratropical Highlights –July 2015
1. Northern Hemisphere
The mean 500-hPa circulation during
July featured above-average heights over the polar region, the Gulf of Alaska,
and southern Europe, and below-average heights over the high latitudes of the
North Pacific, the eastern North Atlantic, and Scandinavia (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 July included above-average temperatures in the Pacific Northwest and
Gulf Coast regions of the U.S., and across the southern half of Europe (Fig. E1). The
main precipitation signals during July included above-average totals in the
central U.S., and below-average totals in the Pacific Northwestern U.S. and
much of south-central Europe (Fig. E3).
a. North Pacific/ North America
At 500-hPa, the circulation during July
featured above-average heights over the Gulf of Alaska (Fig. E9), which contributed to
exceptionally warm (Fig. E1) and
dry conditions in the Pacific northwestern U.S. (Fig. E3). According to the U.S. Drought Monitor, these
conditions contributed to an intensification of drought conditions throughout
the region, with expanding and/ or developing regions of severe or extreme
drought in Washington, Oregon, Idaho and western Montana. Exceptional drought
continued during July across much of central California and portions of western
Nevada.
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; Goldenberg et al. 2001), this
pattern contributed to an amplification of the Tropical Upper-Tropospheric
Trough (TUTT), which now extends well southward into the western MDR.
These conditions were associated
with above-average westerly winds across MDR at 200-hPa (Fig. T21), along with enhanced low-level easterly trade winds over the
Caribbean Sea (Fig. T20).
This wind pattern produced record strength vertical wind shear, along with anomalous
upper-level convergence and anomalous 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-hPPa circulation during
July featured above-average heights over southern Europe, and below-average
heights over the high latitudes of the North Atlantic and Scandinavia (Fig. E9). This
pattern contributed to exceptionally warm (Fig.
E1)
and dry condition (Fig. E3) over
large parts of central and southern Europe, with many locations recording
temperatures in the upper 10th percentile of occurrences and
precipitation totals in the lowest 10th percentile of occurrences.
2. Southern Hemisphere
The mean 500-hPa circulation during
July featured above-average heights over the central ocean basins,
and below-average heights across 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. Over the
South Pacific Ocean, this pattern was associated with a strengthening and
eastward extension of the South Pacific jet stream to well east of the date
line (Fig. T21).