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

APRIL 2010

1. Northern Hemisphere

The 500-hPa circulation during April featured above-average heights in the polar region, across the central North Pacific, and over Europe and northern Africa, and below average heights in the Gulf of Alaska, the western U.S., and northern China/ Mongolia (Fig. E9). These anomalies projected strongly onto several teleconnection patterns, including the negative phases of the North Atlantic Oscillation (NAO) and the East Pacific/Central Pacific (EP-NP) pattern, and the positive phases of the East Atlantic (EA), West Pacific (WP), and Pacific/North American (PNA) patterns (Table E1, Fig. E7). Each of these signals has been exceptionally persistent during the past several months, with the NAO, EA, and PNA patterns prevailing since July 2009. The negative EP-NP pattern has been present since November 2009, and the positive WP pattern has been present since January 2010.

The main temperature signals during April were consistent with the upper-level circulation, and included above average temperatures in Canada, the northeastern U.S., and Europe, and below average temperatures in China and Mongolia (Fig. E1). The main precipitation signals during the month included above-average totals in the western U.S., the Plains states, and eastern China, and below average totals across the eastern half of the U.S., most of Europe, and western Russia (Fig. E3).

a. North Pacific/ North America

The extratropical circulation during April featured a continuation of above average heights at 500-hPa over the central North Pacific Ocean and Canada, and below average heights over the high latitudes of the North Pacific, and over the eastern North Pacific/ western U.S. (Fig. E9).  This anomaly pattern projected onto three leading modes of variability, which included the positive phases of both the PNA and WP patterns, and the negative phase of the EP-NP pattern. These patterns have been exceptionally persistent for the past several months, with the positive PNA pattern prevailing since July 2009 in association with the ongoing El Niño.  Over North America, aspects of the PNA pattern during April (combined with the negative NAO) included a reversal in the normal ridge-trough positions, including a disappearance of the mean Hudson Bay trough.

These conditions again contributed to above average temperatures in Canada, with large portions of the country recording departures in the upper 90^th percentile of occurrences (Fig. E1). They also contributed to a continuation of below average temperatures along the U.S. Gulf Coast and in the Gulf of Mexico, where SST departures remained in the lowest 10^th percentile of occurrences.

The anomalous ridge-trough configuration also contributed to a pronounced east-west dipole of precipitation anomalies in the U.S., with above average totals in the west and below average totals in the east (Fig. E3). Area-averaged totals in the Pacific Northwest, Southern California, and Inter-Mountain regions were in the upper 90^th percentile of occurrences (Fig. E5), while area-averaged totals in the mid-Atlantic and Northeast states were in the lowest 10^th percentile of occurrences.

b. Europe

During April, conditions across the North Atlantic and Europe were associated with a 2-celled pattern of 500-hPa height anomalies (Fig. E9), and with a 3-celled pattern of 200-hPa streamfunction anomalies (Fig. T22). Over the North Atlantic, these anomalies reflected enhanced ridges at high latitude and in the subtropics, and a broad trough in the middle latitudes. This circulation projected strongly onto the negative phase of the NAO and the positive phase of the EA pattern (Table E1, Fig. E7). Both of these patterns have persisted since July 2009.

During April, regional aspects of these signals also included above average heights over Europe, and an extensive southwesterly and southerly flow into Europe (Fig. E9). These conditions contributed to above average temperatures across the continent, with portions of southern Europe recording departures in the upper 90^th percentile of occurrences (Fig. E1). They also contributed to below average precipitation across Europe and portions of western Russia, with Southern (Northern) Europe recording area-averaged totals in the lowest 10^th (20^th) percentile of occurrences (Fig. E4).

Another related circulation feature that has been observed since February is a nearly complete disappearance of the northeasterly trade winds that normally extend southward from the eastern extratropical North Atlantic into the tropical Atlantic. The result has been significantly reduced upwelling near the West African coast, the near-absence of cold water advection into the tropical Atlantic, and reduced oceanic heat flux into the atmosphere. These conditions contributed to record warm SSTs in the tropical Atlantic during March and April, with area-averaged departures during April reaching +1.3°C.

c. China and Mongolia

The extratropical circulation during April featured a deep 500-hPa trough centered over Mongolia and northern China (Fig. E9), which brought below average temperatures to the region (Fig. E1). Anomalous southerly flow and an enhanced jet entrance region downstream of the trough axis (Fig. T21) contributed to an axis of well above average precipitation that extended across eastern China and southern Japan (Fig. E3). For China as a whole, area-averaged precipitation during April was in the upper 90^th percentile of occurrences (Fig. E4).

  2. Southern Hemisphere

      The 500-hPa circulation during April featured above average heights in the area south of Australia, and below average heights across the high latitudes of the South Pacific and in the area southwest of South Africa (Fig. E15). At 200-hPa, the circulation also featured an enhanced ridge and anomalous anticyclonic circulation over tropical southern Africa and Madagascar (Figs. T21, T22).

In southern Africa, the official rainy season lasts from October to April, and rainfall is often below average during El Niño. During April 2010, rainfall totals for the monsoon region as a whole were in the upper 90^th percentile of occurrences (Fig. E4), with the most significant surpluses found in Mozambique and interior southeastern Africa (Fig. E3). For the entire 2009-10 South African rainy season, precipitation was above average in two months (November and April), generally near-average in four months (October and January-March), and below average in one month (December).