The 500-hPa height
pattern during March 2008 featured positive anomalies over the northwestern and
eastern North Pacific, and over much of
, and negative anomalies over
(Fig. E9). The circulation over the western
North Pacific was associated with a weakening and westward retraction of the
east Asian jet stream (Fig. T21).
The circulation across the eastern North Pacific remained consistent with La Niņa.
The subtropical circulation at 200-hPa was also consistent with La Niņa, with
enhanced mid-Pacific troughs in both hemispheres flanking the suppressed
convection over the central equatorial Pacific, and enhanced ridges over the
western Pacific and Australasia flanking the region of enhanced equatorial
convection (Fig. T22).
The main surface
temperature departures during March reflected significantly warmer than average
, and below-average temperatures over
(Fig. E1). The main precipitation anomalies
included above average totals from the upper Midwest to New England, and across
much of Europe, and below-average totals in the southwestern and southeastern US
and in the Middle East (Fig. E3).
a. North Pacific/
The La Niņa signal
was again prominent across the
during March. La Niņa is associated with a westward retraction of deep
tropical convection toward
, 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-hPasubtropical ridge toward
(Fig. T22). Over the central
equatorial Pacific, the reduction in convective heating contributes to an
increased strength of the mid-Pacific trough.
structure, and position of the East Asian jet stream are strongly linked to
conditions in the tropics and subtropics. For example, the jet core coincides
with the strongest north-south height gradient at 200-hPa, which is heavily
influenced by the height anomalies in the subtropics. The jet exit region
coincides with the area of strong diffluence between the subtropical ridge and
trough axes. During La Niņa, the core of the East Asian jet stream is often
retracted westward toward Asia and the heart of the jet exit region is shifted
westward to west of the date line (Fig. T21).
During March, an extensive area of above-average heights extending across
and the northwestern North Pacific also contributed to this jet structure (Fig.
, the circulation during March featured above-average heights extending into the
southwestern US, and below-average heights (and temperatures) across
. This pattern contributed to above-average precipitation from the
, and to below-average precipitation in both the southwestern and southeastern
US (Fig. E3). For the
region, this marks the fifth consecutive month with below-average precipitation
circulation during March featured a large-scale pattern of negative height
and positive height anomalies from the
(Fig. E9). This pattern was associated with
significantly above-average temperatures across most of
, with departures in many areas (> 5°C) exceeding the 90th
percentile of occurrences (Fig. E1). It was
also associated with above-average precipitation across northern
, and below-average precipitation in the
2. Southern Hemisphere
500-hPa circulation pattern during March generally featured above-average
heights over the central ocean basins and below-average heights in the higher
extratropics (Fig. E15). In the subtropics,
the ongoing pattern of negative 200-hPa streamfunction anomalies over the
and positive streamfunction anomalies over the central Pacific was consistent
with La Niņa (Fig. T22). In
, a persistent upper-level trough led to a continuation of cooler than average
temperatures and below-average precipitation (Fig. E3).
South African rainy season extends from October to April, and is often stronger
than average during La Niņa. The region received near-normal precipitation
during March, which followed below-average totals in February (Fig. E4).
Overall the 2007-08 rainy season has been above average, with significant
surpluses recorded from October through January.