1. Northern Hemisphere
During December the 500-hPa
height pattern featured positive anomalies over the subtropical North
, and central
, and negative anomalies over the
Gulf of Alaska
and high latitudes of the
(Figs. E9, E11).
In the subtropics, the 200-hPa streamfunction pattern featured anticyclonic
anomalies in both hemispheres flanking the regions of enhanced convection
over the central and east-central tropical Pacific (Fig. T22).
Anticyclonic anomalies were also evident across the subtropical
in both hemispheres.
Over the North Pacific and
, these conditions reflected the positive phase of the Pacific/ North
American (PNA) teleconnection pattern, and were consistent with El Niņo (Table
E1, Fig. E7). Over
, the circulation reflected the ongoing positive phase of the East Atlantic
(EA) teleconnection pattern, which has been prominent since April.
The main surface temperature
departures during December reflected warmer-than-average conditions across
(Fig. E1). For the entire Northern
Hemisphere monthly mean land surface temperatures were1.4°C above the
1971-2000 mean, and were the third warmest in the record. The main
precipitation anomalies during December included above-average totals in the
northwestern, Plains, and
states, and below-average totals in the southwestern
(Figs. E3, E5, E6)
Accompanying the positive PNA
pattern associated with El Nino, the East Asian jet stream was extended well
east of normal, and the associated jet exit region was shifted eastward to
the eastern North Pacific (Fig. T21). Also,
the Hudson Bay Low was nearly completely absent, and a strong zonal (west-to
east) flow was seen throughout the continent.
This circulation brought an extensive flow of marine air into
, which prevented arctic air masses from developing in
. It also prevented major cold-air outbreaks from occurring in the
. The result was significantly above-average temperatures across
, with the largest departures (> 5°C) observed across north-central
. In the
, the largest temperature departures (3°-4°C) were observed in the
region and the Northeast, where values exceeded the 90th
percentile of occurrences.
experienced significantly above-average precipitation during the month, in
response to enhanced storminess downstream of the mean upper-level trough
and the East Asian jet exit region (Fig. E3).
also experienced above-average precipitation within and downstream of a mean
upper-level trough situated over the central part of the country.
The 500-hPa circulation
pattern during December featured below-average heights near
, and above-average heights extending from the eastern
. This ongoing positive phase of the EA pattern was associated with a strong
southwesterly flow of mild, marine air into
. During December, these conditions contributed to well above-average
, with the largest departures (4°-7°C) observed in northern
, and central
. They were also associated with an enhanced storm track across northern
, which contributed to above-average precipitation in northern
and below-average precipitation across southern
Well above-average sea-surface
temperatures continued across the high latitudes and tropical latitudes of
during December. This pattern is associated with the ongoing warm phase of
the Atlantic multi-decadal mode that began approximately in 1995, and with
the warming trend in global temperatures during the past 100 years.
2. Southern Hemisphere
The mean 500-hPa circulation pattern during December
featured an anomalous zonal wave-4 structure, with above-average heights
and over the central longitudes of the three ocean basins, and below-average
and south of both
(Fig. E15). In the subtropics, anticyclonic
streamfunction anomalies at 200-hPa extended across the South Pacific
consistent with ongoing El Niņo conditions, and across the
, much of the region was situated in an area of enhanced storminess
immediately downstream of a mean upper-level trough axis. Precipitation
totals exceeded the 70th percentile of occurrences over much of
, with some areas of central
recording totals in the highest 90th percentile of occurrences.
, surface temperatures were again above average during December, and
precipitation was near average. The South African rainy season extends from
October to April. For the 2006-07 season, totals were below-average in
October and near-average in both November and December.
, the most significant precipitation anomalies again occurred in the east
and northeast, where totals remained in the lowest 30th
percentile of occurrences. This below-average precipitation is consistent
with El Niņo, and has been accentuated at times by large-scale subsidence
downstream of an anomalous upper-level ridge.