December-February 2006-07: Seasonal Climate Summary

By 

Michelle L’Heureux  and Dr. Gerald Bell 
NOAA/ NWS/ Climate Prediction Center

TROPICS
Highlights 
1.	El Niño dissipates
2.	Madden-Julian Oscillation (MJO) and oceanic Kelvin wave influence the tropics

El Niño Dissipates
	The December-February (DJF) 2006-07 season was highlighted by the continuation of El Niño (Pacific warm episode) early in the season, and by its demise in February. El Niño episodes are often defined by the Niño-3.4 index, which measures the area-averaged sea surface temperature (SST) anomalies between 170°W-120°W, 5°N-5°S.  This index reached +1.3°C during the first half of December, with local SST departures reaching +2°C.  
	The El Niño then began to weaken in January, as indicated by the development of negative upper-ocean heat content anomalies in the east-central Pacific, and by cooler than average sub-surface temperatures at thermoline depth (50-200m). During this period the anomalously warm SSTs weakened throughout the central and eastern Pacific. In February, the sub-surface temperature and heat content anomalies became increasingly negative, and SSTs retuned to near average, as indicated by a monthly Niño-3.4 index value of +0.1°C. 
This rapid cooling was associated with the juxtaposition of two main factors. The first factor was the development of anomalously strong low-level easterly trade winds across the central equatorial Pacific. Enhanced easterlies produce increased oceanic upwelling and a cooling of ocean temperatures. The second factor was the up-welling phase of an equatorial Kelvin wave. Previously, the down-welling phase of that same wave had contributed to anomalously warm SSTs during December. The dissipation of El Niño was accompanied by a transition to drier than average conditions over the central equatorial Pacific and enhanced convection over portions of the western Pacific. 

Madden-Julian Oscillation (MJO) and oceanic Kelvin wave influence the tropics
The period from late December through mid-January was characterized by a moderate MJO event, which helped to strengthen the low-level easterlies across the central equatorial Pacific and contributed to the decline in the upper-ocean heat content anomalies.  Time-longitude sections of 200-hPa velocity potential and outgoing longwave radiation (OLR) show that this MJO emerged over the Indian Ocean with enhanced upper-level divergence and increased tropical convection.  As the enhanced convection moved eastward across the Maritime Continent, low-level easterly wind anomalies out ahead of the convection began to extend to the date line. These enhanced easterlies were likely one contributing factor to the demise of El Niño.
This period also coincided with the upwelling phase of an equatorial Kelvin wave, which contributed to an eastward progression of negative sub-surface temperature anomalies across the central and east-central equatorial Pacific. Associated with these combined conditions, subsurface temperatures at thermocline depth near 120°W dropped from +3°C above average in early December to -3°C below average by late January.

EXTRATROPICS
Highlights 
1.	Arctic Oscillation (AO) transitions from positive to negative
2.	El Niño-like pattern in December contributes to warmth in the U.S.
3.	Exceptional warmth in Eurasia during December and January
4.	Exceptional warmth in China during January and February

NORTHERN HEMISPHERE 

Arctic Oscillation (AO) transitions from positive to negative
	The mean 500-hPa circulation pattern during DJF 2006-07 was characterized by a large degree of zonal symmetry, with higher than normal heights in the middle latitudes and lower than normal heights at high latitudes. This pattern projected onto the positive phase of Arctic Oscillation (AO), and was associated with above average surface temperatures across much of the NH land mass. These conditions occurred mainly during December and early January. During this period, the mid-latitude jet streams were displaced poleward and high-latitude blocking was minimal, which acted to suppress cold air outbreaks into the middle latitudes. 
In contrast, the negative phase of the AO prevailed during the last half of January and February. This pattern was associated with increased high-latitude blocking, and strong cold-air outbreaks across both North America and the northern half of Eurasia. These conditions led to an abrupt end to the exceptionally mild winter conditions seen in these regions during December and January.

El Niño-like pattern in December contributes to anomalous warmth in the U.S.
	During December the 500-hPa height anomaly pattern featured above average heights over the subtropical Pacific, below average heights over the Gulf of Alaska, and above average heights across central Canada. This 3-celled pattern reflected the positive phase of the Pacific North American (PNA) teleconnection, and was consistent with ongoing El Niño conditions. 
Accompanying this pattern 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. Also, the Hudson Bay Low was nearly completely absent, and a strong zonal (west-to east) flow was seen throughout North America. This circulation brought an extensive flow of marine air into the continent, which prevented arctic air masses from developing in Canada. It also prevented major cold-air outbreaks from occurring in the United States. The result was significantly above-average temperatures across North America, with the largest departures (> 5°C) observed across north-central Canada. In the U.S., the largest temperature departures (3°-4°C) were observed in the Great Lakes region and the Northeast, where values exceeded the 90th percentile of occurrences. Also during the month, the northwestern U.S. and southeastern Canada experienced significantly above-average precipitation in response to enhanced storminess downstream of the mean upper-level trough and the East Asian jet exit region. 

Exceptional warmth in Eurasia during December and January
Over Eurasia above average heights at 500-hPa during December, combined with a strong southwesterly flow of mild, marine air into Europe, contributed to well above-average temperatures. The largest departures (4°-7°C) were observed in northern Europe, Scandinavia, and central Siberia. The circulation was also associated with an enhanced storm track across northern Europe, resulting in above-average precipitation in that region and below-average precipitation across southern Europe and Turkey.
	Although the anomalous 500-hPa circulation transitioned to the negative phase of the AO in mid-January, enhanced westerly flow north of the anomalous mid-latitude ridges helped to confine cold air to the polar region. This led to a continuation of above average temperatures across Eurasia. Monthly temperatures were 3-5°C above average from central Europe to Mongolia, with the largest departures (6°-8°C) recorded in central Russia.  
  
Exceptional warmth in China during January and February 
In Asia, a pronounced north-south dipole pattern of 500-hPa height and surface temperature anomalies was evident during February. Temperatures were generally 3°-6°C above average across east-central Asia, in association with a persistent 500-hPa ridge centered just north of the region. For central and eastern China, this was the second straight month with temperatures in the upper 10th percentile of occurrences. 

SOUTHERN HEMISPHERE 
1.	Below-average rainfall across South African monsoon region
2.	Hot and dry across Australia in February

The 500-hPa circulation was quite variable during DJF 2006-07.  In December, an anomalous zonal wave-four pattern prevailed, featuring above-average heights across southern Australia and the central longitudes of the three ocean basins, and below-average heights near New Zealand and south of both South America and South Africa.  During January 2007, the circulation anomalies were rather weak with the main features described by negative height anomalies over the high latitudes of both the central South Pacific and South Atlantic Oceans.  February 2007 featured a return of a zonal wave-four pattern, but with weaker amplitude than was observed in December.

Below-average rainfall across South African monsoon region
In southern Africa, the rainy season normally lasts from October through April. This region received below-average precipitation during DJF 2006-07, with many areas recording seasonal totals in the lowest 30th percentile of occurrences.  So far during the 2006-07 rainy season, area-averaged totals were below-normal in October and February, near-normal in November and December, and above normal in January.

Hot and dry across Australia in February
In Australia, surface temperatures and precipitation were quite variable during the season. Much of the continent recorded below-average temperatures during December and January, and above-average temperatures in February. The most significant temperature and precipitation departures were observed in February, in association with a persistent upper-level ridge in the west and trough in the east. This pattern contributed to extremely warm temperatures across the western half of the continent, with many areas recording values in the upper 10th percentile of occurrences. It also contributed to well below-average precipitation throughout the continent, with totals in the central and northwestern regions falling in the lowest 10th percentile of occurrences.