Extratropical Highlights –March 2017
1. Northern Hemisphere
The 500-hPa height pattern during March
featured large anomalies over much of the NH extratropics
Above-average heights were present over the southwestern U.S., eastern Canada
and Greenland, from the Middle East to Japan, and over the middle and high
latitudes of the North Pacific. Below-average
heights were present over both the western and eastern
U.S., Europe, and northern Asia.
This anomaly pattern projected onto
a record positive phase of the East Atlantic/ West Russia pattern (+4.0 std.
dev) (Table E1,
Fig. E7). It also projected onto the negative
phases of both the North Atlantic Oscillation (NAO, -1.4) and the Pacific/
North American pattern (PNA, -1.2). A
negative phase of the PNA teleconnection pattern is typical of La Niña.
At 200-hPa, the circulation across
the subtropical Pacific Ocean in both hemispheres continued to reflect La Niña.
The La Niña signal included amplified troughs east of the date line in the
subtropics of both hemispheres (Fig. T22), in association with the disappearance of deep tropical
convection from the central and eastern equatorial Pacific (Fig. T25). The La Niña signal
also included a focusing of the subtropical ridges over Australasia (Fig. T22),
in association with enhanced convection over the western tropical Pacific and
The main land-surface temperature signals
during March included above-average temperatures in Alaska, Mexico, the
south-central U.S., and across southern Asia, and below-average temperatures in
western Canada, the eastern U.S., and much of northern Europe and northwestern
Russia (Fig. E1).
The main precipitation signals included above-average totals in the northwestern
U.S., western Canada, and throughout central and southern Europe (Fig. E3).
a. North Pacific and North America
The 500-hPa circulation during March
featured above-average heights over the middle and high latitudes of the North
Pacific and Mexico, and below-average heights over both the western and eastern
U.S. (Fig. E9).
This overall pattern projected onto the negative phase (-1.2) of the PNA
teleconnection pattern, which is typical of La Niña.
La Niña produces an amplified
mid-Pacific trough (Fig. T22), which act to retract westward the
East Asian jet steam (Fig. T21). This anomalous jet structure, in
turn, acts to retract westward the mean downstream ridge and trough positions,
resulting in the essence of the anomalous 500-hPa circulation pattern during March
(Fig. E7, Table E1).
During March, the overall circulation
pattern contributed to above-average surface temperatures
in Mexico and the south-central U.S., and to below average surface temperatures
in the eastern U.S. and western Canada (Fig.
E1). It also contributed to above-average
precipitation in the Pacific Northwest U.S. and western Canada (Fig. E3).
During the last few months, severe
or extreme drought has spread across the southwestern U.S. from Arizona to
northern Texas and south-central Kanas. This overall area recorded 50%-75% of
normal rainfall during March (Fig. E6).Within this region, exceptional drought has
developed in western Oklahoma, where less than 25% of normal rainfall was
recorded during March (Fig. E6). Severe drought was also present in southeastern
Georgia and southeastern South Carolina in response well below-average
precipitation during March.
height anomaly pattern featured a pronounced north-south dipole pattern
characterized by above-average heights extending from the Middle East to Japan
and by below-average heights extending from central Europe to eastern Siberia.
This pattern projected onto a record positive phase of the East Atlantic/ West
Russia pattern (+4.0 std. dev) (Table E1, Fig. E7).
In Europe, the amplified trough
contributed to well above-average precipitation, with area-averaged totals
across southern Europe in the upper 90th percentile of occurrences (Fig. E4).
This region also recorded well above-average totals in February.
2. Southern Hemisphere
The mean 500-hPa circulation during
March featured above-average heights over the Indian Ocean and the central
South Pacific, and below-average heights over the high latitudes of the eastern
South Pacific (Fig. E15). At 200-hPa, the subtropical circulation
featured an amplified trough over the central and eastern South Pacific Ocean,
and ridge over western Australia (Fig. T22). This anomalous
subtropical circulation is typical of La Niña.
The South African monsoon season
runs from October to April. This area recorded above-average precipitation
during March (Fig. E3), with area-averaged totals near
the 70th percentile of occurrences (Fig. E4). To date, the South African rainy season was
above average during February and March, and below
average during November and January.