Photo is from June 26 2012 showing start of the June 26 Flagstaff fire near Boulder Colorado
I was alerted to an excellent presentation by Judy Curry [h/t to Don Bishop] which provides an informative explanation of the current hot and dry weather in the USA. The presentation is titled
Climate Dimensions of the Water Cycle by Judy Curry
First, there is an insightful statement by Judy where she writes in slide 5
CMIP century scale simulations are designed for assessing sensitivity to greenhouse gases using emissions scenarios They are not fit for the purpose of inferring decadal scale or regional climate variability, or assessing variations associated with natural forcing and internal variability. Downscaling does not help.
We need a much broader range of scenarios for regions (historical data, simple models, statistical models, paleoclimate analyses, etc). Permit creatively constructed scenarios as long as they can’t be falsified as incompatible with background knowledge.
With respect to the current hot and dry weather, the paper referenced by Judy in her Powerpoint talk
Gregory J. McCabe, Michael A. Palecki, and Julio L. Betancourt, 2004: Pacific and Atlantic Ocean influences on multidecadal drought frequency in the United States. PNAS 2004 101 (12) 4136-4141; published ahead of print March 11, 2004, doi:10.1073/pnas.0306738101
has the abstract [highlight added]
More than half (52%) of the spatial and temporal variance in multidecadal drought frequency over the conterminous United States is attributable to the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO). An additional 22% of the variance in drought frequency is related to a complex spatial pattern of positive and negative trends in drought occurrence possibly related to increasing Northern Hemisphere temperatures or some other unidirectional climate trend. Recent droughts with broad impacts over the conterminous U.S. (1996, 1999–2002) were associated with North Atlantic warming (positive AMO) and northeastern and tropical Pacific cooling (negative PDO). Much of the long-term predictability of drought frequency may reside in the multidecadal behavior of the North Atlantic Ocean. Should the current positive AMO (warm North Atlantic) conditions persist into the upcoming decade, we suggest two possible drought scenarios that resemble the continental-scale patterns of the 1930s (positive PDO) and 1950s (negative PDO) drought.
They also present the figure below with the title “Impact of AMO, PDO on 20-yr drought frequency (1900-1999)”. The figures correspond to A: Warm PDO, cool AMO; B: Cool PDO, cool AMO; C: Warm PDO, warm AMO and D: Cool PDO, warm AMO
The current Drought Monitor analysis shows a remarkable agreement with D, as shown below
As Judy shows in her talk (slide 8) since 1995 we have been in a warm phase of the AMO and have entered a cool phase of the PDO. This corresponds to D in the above figure. Thus the current drought and heat is not an unprecedented event but part of the variations in atmospheric-ocean circulation features that we have seen in the past. This reinforces what Judy wrote that
[w]e need a much broader range of scenarios for regions (historical data, simple models, statistical models, paleoclimate analyses
in our assessment of risks to key resources due to climate. Insightful discussions of the importance of these circulation features are also presented, as just a few excellent examples, by Joe Daleo and Joe Bistardi on ICECAP, by Bob Tisdale at Bob Tisdale – Climate Observations, and in posts on Anthony Watts’s weblog Watts Up With That.