1. Northern Hemisphere
The 500-hPa circulation
pattern during December featured persistent positive height anomalies in the
, and from the high latitudes of the eastern
, and persistent negative anomalies over the western and central North
Pacific, the southeastern
, and south-central
(Figs. E9, E11).
The main surface temperature
departures reflected above-average temperatures across western
, the high latitudes and subtropical latitudes of the
, and below-average temperatures in the eastern
, and northern
(Fig. E1). The main precipitation departures
included above-average totals in the northwestern
and portions of eastern Europe, and below-average totals across the southern
, the high latitudes of the
, and northwestern
(Figs. E3, E5, E6).
During December, a persistent
pattern of below-average heights at 500-hPa covered the western and central
Pacific and the southeastern
, while positive height anomalies were observed in western
(Figs. E9, E11).
This pattern was associated with cyclonic streamfunction anomalies at
200-hPa across the middle latitudes of the North Pacific, and anti-cyclonic
anomalies in the subtropics over eastern
These overall conditions projected strongly onto the negative phase of the
West Pacific teleconnection pattern (-1.8 st. dev) and the positive phase of
the Pacific/ North American (PNA) teleconnection pattern (+1.4 st. dev) (Table
E1, Fig. E7).
These conditions were
accompanied by an amplified East Asian jet stream that extended well east of
normal, with the mean jet exit region also shifted eastward to just upstream
(Fig. T21). They also reflected an
amplified ridge-trough pattern across the
At 200-hPa, the anticyclonic
streamfunction anomalies over eastern
, along with their SH counterparts across the eastern
, were consistent with enhanced convection across
(Fig. T25). However, the circulation
anomalies over the eastern half of the North Pacific were opposite to those
normally associated with suppressed convection near the date line.
Therefore, while the amplified jet stream over eastern Asian has some links
to the enhanced tropical convection, the pronounced eastward extension of
the jet appears to be un-related to the tropical convection pattern.
This anomalous jet stream led
to a focused and persistent storm track over the eastern Pacific, which
resulted in series of major storms striking the northwestern
Area-average totals in the
were near the 100th percentile of occurrences (Fig. E5),
with many locations recording more than double their normal monthly amounts.
Conversely, monthly totals across the southwestern quadrant of the
were generally less than 25% of normal, with totals in the south-central
region falling in the lowest 10th percentile of occurrences.
The mean circulation was also associated with a strong onshore flow
of mild air into western
(Fig. E1), with monthly mean temperatures in
these regions generally ranging from 3° to 5°C above average. Conversely,
below-average temperatures over the eastern
coincided with the mean position of the amplified upper-level trough.
The 500-hPa circulation pattern during December featured above-average
heights from the high latitudes of the eastern
, and below-average heights across south-central
(Fig. E11). These conditions were
associated with above-average temperatures across
and portions of
, and with an east-to-west dipole of temperature anomalies flanking the
upper-level trough. Western Europe was situated between the mean upper-level
ridge and trough axes and recorded generally below-average precipitation
during the month, while southeastern Europe was situated downstream of the
mean trough axis and experienced above-average precipitation.
2. Southern Hemisphere
The mean 500-hPa circulation pattern during December
featured above-average heights across
, and below-average heights from southern
to the area south of
. At 200-hPa, anticyclonic streamfunction anomalies covered much of the
subtropics. The largest anticyclonic anomalies extended from southeastern
, and also across the central South Pacific (Fig. E15).
, the mean upper-level ridge axis was located over the middle of the
continent. Below-average rainfall occurred upstream of the ridge axis and
above-average totals occurred downstream over eastern
(Fig. E3). Much of eastern
also recorded significantly above-average temperatures (2+ °C above
average) during the month, with temperatures in many areas exceeding the 90th
percentile of occurrences (Fig. E1). This
warmth is consistent with an anomalous westerly flow of low-level air from
the interior of the continent, and with below-average precipitation
throughout the region (Fig. T20).
the rainy season normally lasts from October to April. Area-average rainfall
was above normal during December, thus marking three consecutive months of
above-average rains (Fig. E4). However, most
of the surplus rainfall was confined to
during December, with below-average totals observed across southeastern
where isolated areas recorded amounts in the lowest 10th
percentile of occurrences. This rainfall pattern was related to the
structure of the subtropical ridge, which was constricted in its poleward
extent partially in response to the persistent mid-latitude trough farther
south. As a result, southeastern
experienced a mid-latitude circulation regime and below-average
precipitation during the month.