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 10^th percentile of occurrences and precipitation totals in the lowest 10^th 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).