Hi Everyone,

March 2012 rewrote the record books on the warm side of the ledger with 25 states reporting 
their warmest March on record between 1895-2012. The first link below contains the details of 
the March 2012 climate summary from NCDC. The second link below contains a variety of images 
used in the discussion that follows.

NCDC Climate Summary for March 2012:  http://www.ncdc.noaa.gov/oa/climate/research/cag3/cag3.html

Link to images described in the discussion:  http://www.atmos.albany.edu/student/kgriffin/itsfriday/march2012_climate/

An impressive list of CONUS stations equalled or exceeded their all-time warmest mean month 
temperatures for March temperatures in 2012. A list of these stations can be found in an image 
downloaded from the Weekly Weather and Crop Bulletin for 3 April 2012. Many stations reported 
monthly departures of more than +10 F while some cities (e.g., Sioux City, IA, and Sioux Falls, 
SD, reported monthly departures of more than 15 F.  Although the attached list does not indicate 
station record length, it is noteworthy that many stations broke warmest March records that had 
stood since 1910 (102 years ago). Noteworthy examples include St. Cloud, MN, where the previous 
monthly record of 37.8 F, set in 1918, was eradicated by 6.2 F (44.0 F), and Chicago, IL, where 
the previous monthly record of 48.6 F, set in 1910 and 1945, was torched by 4.9 F (53.5 F). These 
mean temperature anomalies for March 2012 were so extreme that they would also rank among the warmest 
mean monthly temperatures for April as well.

Overall, the reported record-breaking positive temperature anomalies for March 2012 appear to 
be consistent with a centennial- (and longer) scale regional heat wave unlike anything anyone 
alive has experienced before. The 925 hPa mean temperature anomaly for March 2012 (source: NOAA/ESRL/PSD) 
shows that extreme western IA with a temperature anomaly of > 9 C was ground zero for what was a regional 
scale heat wave. Note also that across the southern Gulf of Mexico and much of the Caribbean and Central 
America that the 925 hPa temperatures were within 1 C of normal. This circumstance tells us that the extreme 
warmth over the CONUS in March 2012 was not a result of the poleward displacement of an extremely warm air mass 
from lower latitudes, but likely reflected the impact of a persistent southerly flow from a warmer source region 
that was abetted by local and regional conditions (e.g., a relative absence of snow cover and a cold bottom boundary 
and the presence of surface air warmed by downslope flow across the Rockies over relatively snow-free ground). The 
rest of the discussion will attempt to put the CONUS March 2012 extreme warmth into a larger-scale perspective. Imagery 
contained in the second link above used in support of this discussion was obtained from NOAA/ESRL/PSD and NCEP/CPC.

Mean and anomaly NH 850 hPa temperatures for March 2012 show that the extreme warmth was confined to North America 
east of the Rockies. Other lesser, but still prominent, anomalously warm regions were found over western Europe and 
part of northern Russia and the Arctic Ocean, and parts of the central North Pacific. Anomalously cold conditions at 
850 hPa are evident over parts of Russia, North Africa, northeast Asia, and eastward across Alaska and the northern 
Gulf of Alaska. The NH 850 hPa temperature anomaly pattern for March 2012 is consistent with a regionally confined extreme 
weather event. I was fascinated to see that the mean monthly 850 hPa temperature pattern shows a thermal ridge over the High 
Plains, a signature that is more typical of the warm season and one that I don't ever remember seeing in any previous March. 
Superlatives aside, there is nothing especially remarkable about the 850 hPa temperature anomalies around the rest of the NH when 
viewed from the perspective of departures from the new 1981-2010 climate normals. 

To address the large-scale circulation issues, mean and anomaly 850 hPa, 250 hPa, and 30 hPa geopotential height maps for the NH 
for March 2012 are included in the second link above. The 250 hPa geopotential height circulation features a wave number 4 pattern 
with strong midlatitude ridges located near the Dateline, the Great Lakes, western Europe northward and eastward along the north coast 
of Russia, and a weaker higher latitude ridge near 90 E. Deep troughs are apparent over western Russia and northeastern Russia eastward 
across the Gulf of Alaska. An anomalously strong and poleward-shifted subtropical jet (STJ) dominates much of Asia and the North Pacific. 
Anomalously strong 250 hPa anticyclones located near 90 W and the Greenwich Meridian northeastward along the north coast of Russia when 
combined with the deep trough over the Gulf of Alaska and Alaska ensures that the normal late winter and early spring jet over North America 
is shifted well poleward across northern Canada and parts of the Arctic Ocean, a pattern known to be unfavorable for allowing arctic air 
from northwest Canada to reach the CONUS east of the Rockies. This 250 hPa geopotential height and anomaly circulation pattern is also 
consistent with a relatively dry and warm CONUS east of the Rockies. 

The NH mean and anomaly 850 hPa geopotential height patterns for March feature a deep trough over northwestern Canada, the northeastern 
Gulf of Alaska, and Alaska. This height anomaly pattern favors an anomalously strong westerly flow across the northern Rockies and southern 
Canadian Rockies, with a resulting downslope flow that will tend to sour out any cold air east of the Canadian Rockies, and an anomalous 
southerly flow over west-central Canada that will work against the equatorward transport of any significant cold air masses from Canada 
toward the CONUS. Anomalous northwesterly flow at 850 hPa over the southern Gulf of Alaska helped to transport slightly colder than normal 
air toward the West Coast of the U.S. This Pacific air in turn was promptly warmed and dried during its passage over the central Rockies 
where it contributed to the absence of significant cold and helped to eliminate what little snow cover was present. Meanwhile, an anomalously 
strong 850 hPa anticyclone centered over the UK together with a deep 850 hPa trough over the central Atlantic allowed anomalously warm air 
to be steered well poleward across western and northwestern Europe, and across much of the Arctic north of Russia.

Finally, the mean and anomaly 30 hPa geopotential height maps for March suggest that the stratosphere is dominated by a prominent 
wave number 2 pattern arising from the displacement of the polar vortex off the North Pole toward Northern Canada, an anomalously 
strong ridge over northern Russia, and an anomalously amplified ridge over the Gulf of Alaska that links to the strong ridge over 
northern Russia. The 250 hPa ridge over the central Pacific extends well into the stratosphere and appears to be strongly phase-locked. 
A time series of normalized geopotential height anomaly averaged from 65 N to the North Pole (source: NCEP/CPC) shows that the AO was 
moderately positive for most of March, becoming weakly negative toward the end of the month when the persistent ridge over central North 
America finally broke down (the period of negative AO values in February was associated with the 1060+ hPa anticyclone over Russia and 
the extended European cold). 

The NH 250 hPa geopotential height pattern discussed above also suggests the existence of a prominent wave train that emanates 
northeastwards from the tropical central Pacific toward Alaska before it turns southeastward across central and eastern North America 
to the western Atlantic. This observed flow pattern motivates a look at tropical circulation and moisture anomalies. In support of this 
discussion, assorted mean and anomaly images (source: NCEP CPC) centered on the tropical Pacific (60 N to 60 S) for March 2012 are included 
in the second link above. These images include PW, OLR, 250 hPa winds, 850 hPa winds, and SLP, the Wheeler-Hendon 40-day MJO phase space 
diagram, and the CPC 5-day running mean MJO phase space diagram. The PW and OLR images show that anomalous negative OLR values and 
anomalously high PW values, indicative of anomalously active deep convection, are situated over the longitudes of the Maritime Continent 
from 20 S to 20 N with an extension southeastward along the South Pacific Convergence Zone. The active deep tropical convection observed 
over the Maritime Continent region during much of March is consistent with the overlapping observed high-amplitude MJO that occupied MJO 
phases 4-7 during most of the month and the ongoing La Nina. At 250 hPa, a strong subtropical anticyclone is situated downstream of the 
aforementioned region of anomalous tropical heating. This Pacific subtropical anticyclone is a part of an anomalous subtropical ridge axis 
that extends from India eastward to southeast China. Anomalous 850 hPa easterlies over the central Pacific and anomalous equatorial 
westerlies in the South Indian Ocean, consistent with the MJO and La Nina states discussed above, are supportive of the active region 
of observed deep convection over the longitudes of the Maritime Continent in both hemispheres.

So what caused the extreme warmth over interior North America in March 2012? My personal hypothesis is that persistent anomalous 
tropical heating over parts of the Indian Ocean, the Maritime Continent, and the western Pacific during much of March 2012 
contributed significantly to the warmth, given the quasi phase-locked, tropospheric-deep anticyclone situated over the central 
Pacific near the Dateline. The NH 250 hPa geopotential height anomaly map discussed above suggests that this phase-locked 
anticyclone over the central Pacific was a part of a Rossby wave train that propagated toward higher latitudes of the northeastern 
Pacific and western North America where it became associated with a strong quasi phase-locked anticyclone over central North America. 
The corresponding 850/250 hPa circulation patterns over North America with the jet displaced well to the north precluded any of the 
anomalously cold air trapped over Alaska during much of February and March 2012 from "escaping" equatorward toward the CONUS. 

Previous discussions of the NH circulation patterns for Dec 2011 through Feb 2012 have remarked upon how the 
winter just ended featured a combination wave number one and wave number two pattern (more wave number one in the 
eastern hemisphere and more wave number two in the western hemisphere). This pattern has featured above normal 300 hPa heights at 
high latitudes mostly over northeastern Canada and from northwestern Europe eastward to northern Russia, an anomalously strong and 
poleward shifted STJ from eastern Asia eastward across the North Pacific, an anomalously strong STJ from the eastern Atlantic eastward 
to central and eastern Asia, and a cross-polar from eastern Canada to the Arctic. With the eastern and western hemispheres featuring 
mostly below and above normal temperatures in the lower troposphere, respectively, and with the boundary separating these opposite 
temperatures regimes located near the east coast of Asia, to first order it is tempting to speculate that "constructive interference" 
of quasi phase-locked tropical heating anomalies associated with the MJO and La Nina in the vicinity of the Maritime Continent contributed 
significantly to the observed persistent midlatitude flow anomalies. 

At issue, and worthy of future investigation, is why this pattern was so stable. Dynamical forcing associated with tropical heating
 anomalies from the Indian Ocean to the western Pacific and locked-in cold air at higher latitudes over much of the eastern hemisphere 
 were likely "order one" contributors to the observed extreme warm weather over much of the CONUS east of the Rockies. As for "order two" 
 contributors to the observed extreme warmth, they likely included the persistent downslope westerly flow across the Rockies 
 (note the time-mean 850 hPa trough and associated thermal ridge over the High Plains) and the relative absence of snow cover over 
 much of the CONUS east of the Rockies that allowed surface-based deep mixed layers to form over bare ground under clear-sky conditions. 

The mean March 925/850 hPa temperature anomalies discussed above show that temperatures in the subtropics and tropics across 
Central America and adjacent regions are near or slightly below normal. This observation tells us that the extreme warmth over the 
CONUS associated with persistent southerly flow did not arise because the air in the subtropical/tropical source region was anomalously 
warm. Rather the extreme warmth likely arose in conjunction with: 1) the persistent poleward transport of climatologically warm air from 
the subtropics and tropics, 2) dynamically forced subsidence warming south of the jet axis east of the Rockies, and 3) deep (for March) 
surface-based mixed layers over the dry, bare soils devoid of snow cover east of the Rockies. Carefully constructed numerical simulations 
will  be needed to test to what extent these inferences have any applicability in fact.

Lance