Hi Everyone,

Friday map discussion for 30 March 2012 started out with a review of current weather and then quickly
turned into a spirited discussion of the extreme March weather observed over parts of North America 
and the broader implications of the large-scale circulation patterns associated with the observed
extreme weather. There was no shortage of viewpoints and ideas.

March 2012 has featured an amplified MJO pattern with active convection, maximizing south of the
equator, slowly shifting eastward from the Indian Ocean to the Maritime Continent. Over the North Pacific,
a strong zonally elongated upper-tropospheric anticyclone has been located near and east of the Dateline 
and downstream of the region of active connection farther west for what seems like weeks. This Pacific flow 
pattern has been associated with a highly amplified quasi-stationary upper-level anticyclone over central 
and east-central North America. 

Extreme weather usually requires an amplified flow pattern. Amplified flow patterns are typically associated with
 a weaker meridional temperature gradient and a weaker zonal jet. A weaker zonal jet may be more susceptible to amplified
flow breakdowns relative to a strong zonal jet. In the language of dynamicists, strong zonal jets at,
say, the equatorward edge of a strong polar vortex have a greater "stiffness" and may be less susceptible to breakdown
into an amplified flow pattern. Ice age simulations performed by climate scientists suggest that the NH flow pattern
was dominated by a stable, strong zonally oriented jet that was anchored near the ice edge where the maximum meridional
temperature gradient was situated. Presumably, the "stiffness" of the ice age jet made it relatively difficult for highly
amplified flow patterns to develop with any great frequency (but I am prepared to be wrong as usual). 

At issue is whether the ongoing reduction of the meridional temperature gradient associated with preferential high-latitude
warming and Arctic ice thinning and melting over the last few decades is contributing to reduced jet stiffness, 
especially over the North American continent where the meridional temperature gradient has (presumably?) weakened 
more than over the high-latitude North Pacific and North Atlantic, and an increase in the frequency of highly 
amplified flow patterns. The question arises as to whether reduced jet stiffness, if we hypothesize that it is 
occurring for the sake of argument, makes the jet more susceptible to periodic flow breakdown, anticyclonic 
wave breaking, and extended periods of the north-south exchanges of air masses in conjunction with jet-flow 
interactions associated with tropical heating anomalies that occur on interannual time scales (ENSO), intraseasonal 
time scales (MJO), and synoptic time scales (recurving and transitioning TCs), and jet-flow interactions that occur 
in conjunction when arctic disturbances are displaced equatorward during amplified flow regimes. This subject is ripe 
for further research and there is no better place to start than with extreme weather events that have occurred this past 
winter (e.g., European cold in Jan/Feb and CONUS March warmth) to take advantage of the opportunity to possibly better 
identify precursor signals associated with these extreme events.

Internal dynamical forcing (e.g., baroclinic instability) and boundary forcing 
(e.g., impact of SST/snow cover/soil moisture anomalies) dominate the large-scale 
flow pattern on synoptic/weekly and intraseasonal and longer time scales, respectively. 
The feedbacks between dynamical and boundary forcing on the atmospheric-oceanic circulation are highly nonlinear 
and difficult to sort out (even without the complicating effects of mountains). That said, multiscale observational 
analyses of real extreme weather in conjunction with idealized general circulation model investigations of the 
processes that govern the origin and onset of extended amplified flow patterns may prove to be enlightening. 

March 2012 concluded on a rather amazing note across the Plains. I have attached the 0000 
UTC 1 April soundings from Omaha (OAX), NE, and North Platte (LBF), NE. To see deep surface-based 
mixed layers that extend to near 600 hPa with surface potential temperatures near 40 C on the last 
day of March strikes this ancient weather observed as off the charts (several stations in NE, including 
Omaha, apparently set all-time high temperature records for any March day yesterday). To hear that a number 
of stations from the Plains eastward to the Midwest have apparently broken their all-time monthly mean temperature 
records for March by as much as ~5 F over the previous monthly record (e.g., Chicago) gives me pause and prompts 
me to ask whether our climate could be reaching a nonlinear tipping point associated with preferential high-latitude 
warming and the loss of Arctic sea ice, a reduction in the continental meridional temperature gradient in middle and 
higher latitudes, a weakening of the zonal jet over continents, and an increase in the frequency of more amplified flow 
associated with larger zonal variations in the strength of the upper-level flow associated with preferentially reduced 
meridional temperature gradients over continental regions. Let the debates (and preferably quantitative analyses) begin.

Lance