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 trough.

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 90^th 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 90^th 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 axis, respectfully.

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.