Extratropical Highlights

  Table of Indices  (Table 3)

  Global Surface Temperature  E1

  Temperature Anomalies (Land Only)  E2

  Global Precipitation  E3

  Regional Precip Estimates (a)  E4

  Regional Precip Estimates (b)  E5

  U.S. Precipitation  E6

  Northern Hemisphere

  Southern Hemisphere

  Stratosphere

  Appendix 2: Additional Figures

Extratropical Highlights

MAY 2008

1. Northern Hemisphere

The 500-hPa height pattern during May 2008 featured positive anomalies over the eastern North Pacific, across the high latitudes of the North Atlantic and Greenland , and over eastern Siberia , and negative anomalies over the high latitudes of the central North Pacific, across the central North Atlantic , and central Russia (Fig. E9). Prominent teleconnection patterns during the month included the negative phase of the Arctic Oscillation (AO), with regional anomalies reflecting the negative phase of the North Atlantic Oscillation (NAO) and West Pacific pattern, and the positive phase of the Pacific/North American (PNA) pattern (Table E1, Fig. E7). This overall pattern also In the subtropics the 200-hPa circulation remained consistent with La Niña, with enhanced mid-Pacific troughs in both hemispheres flanking the suppressed convection over the central equatorial Pacific (Fig. T22).

The main surface temperature anomalies during May reflected above-average temperatures over northeastern Canada , central Europe , Siberia , and central Russia , and below-average temperatures over the northeastern quadrant of the U.S. and portions of eastern Europe (Fig. E1). The main precipitation anomalies included above average totals in the central U.S. and southern Europe , and below-average totals in the Gulf Coast area of the U.S. and northern Europe (Fig. E3).

a. North Pacific/ North America

The La Niña signal was again prominent across the subtropical Pacific Ocean during May. La Niña is associated with a westward retraction of deep tropical convection toward Indonesia , and a complete disappearance of tropical convection from the central equatorial Pacific (Fig. T25). These conditions result in a westward retraction of deep tropospheric heating, and hence a westward retraction of the 200-hPa subtropical ridge toward Indonesia (Fig. T22). Over the central equatorial Pacific, the reduction in convective heating contributes to an increased strength of the mid-Pacific troughs in both hemispheres.

Over North America , the mean 500-hPa ridge-trough pattern was stronger than average, and there was a pronounced split-flow configuration over the western US (Fig. E9). This pattern was associated with an eastward extension of the East Asian jet stream, which was related to large negative height anomalies across the high latitudes of the central North Pacific (Fig. T21). This overall pattern reflected the positive phase (+1.3) of the PNA teleconnection pattern (Table E1, Fig. E7).

These conditions contributed to below-average temperatures across the northeastern quadrant of the U.S. in the vicinity of the mean trough (Fig. E1). They were also associated with increased storminess and above-average precipitation in the central U.S. , and with below-average precipitation along the Gulf Coast and Florida (Figs. E3, E5). This marks the seventh consecutive month with below-average precipitation in the Gulf Coast region, which is consistent with La Nina (Fig. E5). Ongoing precipitation deficits have led to a continuation of moderate-to-severe drought in portions of the southeastern U.S. and Florida .

b. North Atlantic and Europe

Over the North Atlantic the 500-hPa height anomaly pattern during May featured a north-south dipole, with positive anomalies across the high latitudes and negative anomalies extending from the eastern U.S. to southern Europe . This pattern reflected a strong negative phase (-1.7) of the North Atlantic Oscillation (Table E1). It was associated with increased storminess and above-average precipitation over portions of southern and central Europe , and with below average precipitation in northern Europe and southern Scandinavia .

  2. Southern Hemisphere

      The 500-hPa circulation during May generally featured above-average heights at high latitudes and below-average heights in the middle latitudes (Fig. E15). In the subtropics the ongoing 200-hpa pattern of positive streamfunction anomalies over the central Pacific and negative streamfunction anomalies over the western Pacific was consistent with La Niña (Fig. T22). Another La Niña-related feature was an anomalous wave pattern emanating from the western tropical Pacific, and characterized by negative height anomalies over eastern Australia and the high latitudes of the eastern South Pacific, and positive height anomalies over the high latitudes of the western South Pacific.

      This wave pattern again contributed to significantly cooler and drier than average conditions in eastern Australia, which was located just upstream of the mean trough axis (Figs. E1, E3). Western Australia was also situated upstream of the mean trough axis during May, and also recorded well below average precipitation with monthly totals in the lowest 10^th percentile of occurrences. In southeastern South America , precipitation has been below average for seven straight months in association with La Nina (Fig. E4).