– December 2014
1. Northern Hemisphere
The 500-hPa circulation during December
featured above-average heights across Alaska, Canada, the eastern U.S., and the
central North Atlantic, and below-average heights over the east-central North
Pacific and the high latitudes of the North Atlantic (Fig. E9). Regional aspects of this
circulation included an amplified ridge over western North America, a weaker
Hudson Bay trough, and a strong positive phase (+1.63) of the North Atlantic
Oscillation (NAO, Table E1, Fig. E7).
The main land-surface temperature signals
during December included above-average temperatures across Alaska, Canada, the
western U.S., and north-central Russia (Fig.
E1). The main precipitation signals included above-average
totals in the western U.S., and below-average totals in western
Europe (Fig. E3).
a. North Pacific/ North America
The mean 500-hPa circulation during
December featured above-average heights across Alaska, Canada, and the eastern
U.S. (Fig. E9).
This anomaly pattern reflected an amplified ridge in western Canada, along with
a marked weakening of the Hudson Bay trough. This pattern was associated with above-average
surface temperatures across most of North America (Fig. E1). The most significant departures
were observed in central Alaska and along the west coast of North America, where
monthly mean temperatures were in the upper 90th percentile of
Also observed during December was
an enhanced trough at 500-hPa over the east-central North Pacific (Fig. E9).
Increased storminess downstream of the trough axis resulted in above-average
precipitation in the western U.S., with much of the region recording totals in
the upper 70th percentile of occurrences (Fig. E3).
The U.S. Drought Monitor indicated
that despite the above-average precipitation, a large area of exceptional
drought continued in central/ southern California and western Nevada, with extreme
drought extending northward into Oregon. Exceptional or extreme drought also continued
in north-central Texas and western Oklahoma.
b. North Atlantic/ Europe/ Russia
The 500-hPa circulation featured above-average
heights extending from eastern Canada to western
Europe, with below-average heights over the high latitudes of the North
Atlantic. This north-south dipole pattern projected strongly onto the positive
phase (+1.63) of the NAO (Table E1, Fig. E7). It was
associated with amplified Icelandic Low, an enhanced ridge over the
east-central North Atlantic, and a split flow centered over eastern
Europe (Fig. E10).
This split flow configuration reflected enhanced jet stream winds extending
into northern Europe, and weaker than average winds across southern Europe.
The resulting enhanced flow of mild
marine air into the higher latitudes contributed to above-average surface
temperatures from Iceland to north-central Russia (Fig. E1). The most significant temperature
departures were observed in north-central Russia, exceeding the 90th
percentile of occurrences. Within the southern branch of the jet stream,
below-average precipitation was recorded across western
Europe in the region downstream of the mean ridge axis (Fig. E3).
2. Southern Hemisphere
The mean 500-hPa circulation during
December featured an anomalous zonal wave-4 pattern, with above-average heights
over the central South Atlantic and Indian Oceans, and also over the western
and eastern South Pacific Ocean (Fig. E15). Conversely, below-average heights were
observed in the area south of Australia, over the central South Pacific, across
the high latitudes of the South Atlantic, and throughout the polar region.
In Australia, a mean trough over
the eastern part of the continent brought above-average precipitation, with
much of the southeast coast recording departures in the upper 70th
percentile of occurrences (Fig. E3).
The South African rainy season
lasts from October to April. Rainfall for the region as a whole was above
average during December, (Fig. E4), with
the main surpluses recorded in the Mozambique and southern Madagascar (Fig. E3). To date, the South African rainy
season was below normal during October-November and above normal in December (Fig. E4).