Extratropical Highlights – April 2013
1. North Pacific/ Northern Hemisphere
The 500-hPa circulation during April
featured above-average heights over the northern and eastern Pacific Ocean, the
western North Atlantic, and southern Russia, and below-average heights over the
western and central North Pacific Ocean, Canada, and in the vicinity of Iceland
and Scandinavia (Figs. E9, E11). These
conditions projected strongly onto the negative phases of the West Pacific
teleconnection pattern (-1.9) and the polar/ Eurasia pattern (-1.6) (Table E1, Fig. E7).
The main land-surface temperature signals
during April included above-average temperatures over western Russia, and below-average temperatures across Alaska,
western Canada, and the north-central United States (Fig. E1). The main precipitation signals included
well above-average totals in the central and southeastern U.S (Fig. E3). Also
in the U.S., long-term precipitation deficits led to a continuation of extreme
and exceptional drought in the Great Plains states, with moderate or severe drought
also continuing in much of the West.
a. North America
The mean 500-hPa circulation during
April featured an amplified wave pattern across the Pacific/ North American
region (Figs. E9,
E11). This pattern reflected a strong ridge
extending southeastward from eastern Siberia to the western U.S., and another
ridge over the western North Atlantic. It also featured troughs over the
central North Pacific and Canada. This pattern was associated with a complete
disappearance of the normal Aleutian Low, and with an enhanced Hudson Bay
These conditions resulted in strong
northerly and northwesterly winds across western North America (Fig. E10),
which contributed to well below-average temperatures in Alaska, the western
half of Canada, and the north-central U.S. (Fig. E1). They also contributed to well
above-average precipitation in the central and southeastern U.S. in the region
downstream of the mean trough axis (Fig.
E3). Area-averaged totals in the Midwest were the
largest on record dating back to 1979 (Fig.
E5), and area-averaged totals in the Great Lakes
region reached the 90th percentile of occurrences. For both regions,
totals exceeding 200% of normal were widespread (Fig. E6).
Large portions of the U.S. Plains
region continued to be impacted by extreme or exceptional drought. At the end
of April, the “U.S. Drought Monitor” indicated exceptional drought from portions
of Texas northward to central South Dakota, including eastern Wyoming and
southeastern Colorado. Extreme drought
persisted in much of Wyoming and severe drought covered southwestern Minnesota.
In the southwestern quadrant of the U.S., many regions recorded moderate or severe
Drought, with extreme drought expanding across New Mexico (Fig. E3).
2. Southern Hemisphere
The mean 500-hPa circulation during
April featured above-average heights over the central South Pacific and western
South Atlantic Ocean, and below-average heights over the high latitudes of the
eastern South Pacific, the central South Atlantic, and southeast of Australia (Fig. E15).
At 200-hPa, the circulation over
Australia featured an anomalous wave pattern with an amplified ridge in the
west and a trough in the east (Fig. T21). This pattern contributed to well
above-average temperatures across the western half of Australia, with monthly
departures exceeding the 90th percentile of occurrences in both the
southwest and extreme west. Also, well below-average precipitation was observed
in the far west and the southeast. These regions were situated immediately
downstream of the mean ridge axis and immediately upstream of the mean trough
Over South America, the circulation
featured anomalous southwesterly flow downstream of an amplified upper-level
trough (Figs. E9,
T21). This pattern was associated
with a poleward shift of the mean surface frontal boundary, resulting in well
above-average temperatures across southern Argentina (Fig. E1).
The South African rainy season
lasts from October to April. April rainfall for the region as a whole was near
average (Fig. E4).
Regionally, rainfall during April was above average in eastern South Africa,
and below-average in northwestern South Africa, Madagascar, and northern
Mozambique (Fig. E3).
For the entire 2012-13 rainy season, rainfall was above-average during October
and January, near-average in December, February, and April, and below-average
in November and March.