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

JULY 2009

1. Northern Hemisphere

The 500-hPa height field during July 2009 featured positive anomalies in the polar region, and negative anomalies from eastern North America to northern Europe and over the high latitudes of the central North Pacific (Fig. E9). Over the North Atlantic , the circulation reflected a strong negative phase (-2.2) of the North Atlantic Oscillation (NAO, Table E1, Fig. E7). The sea-level pressure field featured generally below-average pressure across the middle latitudes and above average pressure at high latitudes. This pattern reflected a record negative phase (-1.4) of the Arctic Oscillation for July dating back to 1950.

The 200-hPa streamfunction field indicates El Niño was impacting the upper-level circulation during July (Fig. T22). In particular, the combination of negative anomalies across the NH extratropics and positive anomalies across the SH extratropics, is consistent historically with past El Niño episodes. Earlier in the year La Niña was associated with an opposite pattern of streamfunction anomalies in both hemispheres.

The main temperature signals during July included above average temperatures in the southeastern U.S. , southern Europe , and China , and below average temperatures in the northern Plains States and central Canada (Fig. E1). The main precipitation signals included above average totals in the central and northeastern U.S. , and northern Europe , and below average totals in Alaska , western Canada , and Central America (Fig. E3).

a. North Pacific/ North America

The 500-hPa circulation during July featured strong troughs over eastern North America and the eastern North Atlantic , and an anomalous ridge over western Canada (Fig. E9). This circulation contributed to below average temperatures in the northern Plains States and central Canada (Fig. E1). Considerable variability within the base of the mean trough located over the eastern U.S. contributed to above average precipitation across the Gulf Coast and portions of the midwestern U.S. (Figs. E3, E5, E6).

b. North Atlantic and Europe

The 500-hPa circulation during July featured a dipole pattern of 500-hPa height anomalies over the North Atlantic , with above average heights at high latitudes and below average heights in the middle latitudes (Fig. E9). This pattern reflected a strong negative phase of the NAO (Table E1). The trough over the eastern North Atlantic , contributed to above average precipitation across northwestern Europe and Scandinavia (Fig. E3), and to above average temperatures across southern Europe .

c. China

  2. Southern Hemisphere

The 200-hPa streamfunction field indicates El Niño was impacting the upper-level circulation during July in both the subtropics and extratropics (Fig. T22). The subtropical ridge was stronger than average across the central South Pacific and weaker then average over the Indian Ocean . This circulation reflected an overall eastward extension/ shift of the mean subtropical ridge, and is consistent with the El Niño-related pattern of anomalous tropical convection (Fig. T25). These conditions were also associated with an eastward extension of the South Pacific jet stream, and with an eastward shift of the jet core to the date line (Fig. T21).

The 500-height field shows El Niño impacts extending well into the high latitudes, as indicated by negative anomalies immediately poleward of the jet core across the central South Pacific and by positive anomalies over the high latitudes of the eastern South Pacific (Fig. E15). Another El Niño impact is indicated by cyclonic streamfunction anomalies across much of the SH extratropics (Fig. T22). This pattern is opposite to that seen earlier in the year in association with La Niña.

The main temperature signals during July included warmer than normal conditions in southern Africa and southeastern Australia (Fig. E1). Precipitation was well above average in southeastern Brazil , and well below average in eastern Australia (Fig. E3).