1

Fourth IAP Meeting February 13 2008

° Extreme Event: Winter US Tornado Outbreak

--- Attribution challenge

°2007 US Annual Precipitation Extremes --- Southwest and Southeast Drought

° October-December 2007 Conditions ___ La Niña: What attributable effects?

° Record Warm 2007 Global Land Temperature ---- What role for the oceans/GHG forcing?

2

L

Super Tuesday

3

L

Q: What is the explanation for the active 2008 US Tornado Season To Date?

i) Strong and frequent synoptic weather disturbances sweeping across the Rockies into the southern Great Plains ---- there has been excellent 1-2 day predictability of tornado risk

ii) Global climate conditions in 2008 favorable for active US tornado season ---- Strong La Niña event since Fall 2007, with known historical precedence for a La Niña/enhanced tornado outbreak link (Bove 1998; Marzban and Schaefer 2001; Robinson and Schaefer 2008).

Based on the recent published studies, J. Schaefer stated “The two tornado outbreaks so far this year (January 7 and Super Tuesday) were

text book examples of organized La Niña tornado activity.”

• There is no evidence for a detected change in tornado activity to date due to greenhouse gas emission increases.

One of the Key Findings of the CCSP 3.3 Report on Extreme Convective storms is: “There is no trend in the frequency of tornadoes and other severe convective

storms when the data are adjusted for changes in observing practices”

4

L

Assessment of 2007 Annual US Precipitation Extremes

5

L

Assessment of 2007 Annual US Precipitation Extremes JFM AMJ

JAS OND

6

L

7

L

Assessment of 2007 Annual US Precipitation Extremes NOAA-CSI Team

1. The extreme low precipitation was inconsistent with east tropical Pacific SST variability during 2007, and thus was very unlikely caused by the ENSO cycle occurring during January-December 2007.

We estimate less than a 5% probability that the observed dryness was consistent with climate conditions driven from the tropical east Pacific in 2007.

2. An SST-induced dry signal did exist in 2007, spanning much of the southern U.S., and orignated from SST conditions outside the tropical Pacific. This dry signal overwhelmed the ENSO wet signal, and we estimate a large increase in the probability of U.S. drying having intensities as large as observed in

2007 due to such a global SST influence.

Regarding the SW and SE US:

8

OND 2007

9

OND Composite La Niña

10

October-Decmber 2007 Seasonal Anomalies Attributable to East Tropical Pacific SSTs?

11

Observed October-December La Nina Composite

12

Simulated OND 2007150-member multi-model

13

Simulated OND La Nina150-member multi-model

14

15

Observed October - December La Nina Composite

16

Observed October- December 2007 Anomalies

17

U.S. precipitation anomalies in La Niña Falls

Eight La Niña cases in wake of El Niño: out of 344 climate divisions, 82 show composite anomalies in excess of 0.43 standard deviations (33% tails) vs. 14 below -.43.

18

U.S. precipitation anomalies in La Niña Falls

ENSO correlations based on Niño 3.4 SST: out of 344 climate divisions, 54 show correlations in excess of +0.25 vs. 26 below -.25. These regions could be expected to have dry vs. wet anomalies, respectively, during La Niña conditions.

19

U.S. precipitation anomalies in OND 2007Compared to La Niña composites (below), 28 divisions hit OND anomalies in the upper tercile (left; out of 82 flagged divisions) - that is about what would be expected from random samples. On the expected dry tail (14 possible), only 3 hit that tercile, less than expected per chance.

20

U.S. precipitation anomalies in OND 2007Compared to the correlation-based flags, 12 out of 54 divisions hit OND anomalies in the lower tercile (for the positive correlations) and 6 out of 26 divisions scored in the upper terciles for the negative correlations, both less than expected by chance alone.

Bottomline: Another ‘underperforming’ENSO event - so far!

21

Simulated October- December 2007 Anomalies GOGA, 150-member multi-model

22

Simulated October- December 2007 Anomalies EPAC, 150-member multi-model

23

Simulated October- December La Niña Composite GOGA, 150-member multi-model

24

Simulated October- December La Niña Composite EPAC, 150-member multi-model

25

26

27

28

29

30

31

What are the Official Forecast for US winter climate?

1.5 month Lead 0.5 month Lead

32

What are the Official Forecast for US winter climate?

1.5 month Lead 0.5 month Lead