# Presentation: 640433

## Mechanisms of drought in present and future climate

- Gerald A. Meehl and Aixue Hu

## Definition of drought    (premise:  if it’s not raining it will be dry;  must be simple to apply to model analysis)

- “Megadrought” (long-lived multi-decadal extreme drought often mentioned in a paleoclimate context):  11 year running mean area-averaged precipitation anomalies over western U.S. less than zero for at least 20 consecutive years (Meehl and Hu, J. Climate, 2006)
- (one megadrought every 170 years in a 1360 year global coupled climate model control run, or 0.6 megadroughts per century)
- Is there an analogous simple definition for notorious shorter-lived decadal droughts that have had severe impacts in the U.S. (e.g. the 1930s and 1950s) for Great Plains and western U.S. areas?

## Severe decadal drought definition based on historical precedents (1930s and 1950s droughts):  negative values of 11 year running mean area-averaged precipitation that exceed one standard deviation for at least 5 consecutive years  (rate of 2 per century)

- Severe decadal drought definition based on historical precedents (1930s and 1950s droughts):  negative values of 11 year running mean area-averaged precipitation that exceed one standard deviation for at least 5 consecutive years  (rate of 2 per century)

-  1930s             1950s

- 1930s           1950s

## In another 150 year long dataset, the 1880s drought also qualifies (occurrence rate remains at 2 droughts per century)

- 1880s                      1930s         1950s

## Correlations of (11 year running mean) observed western U.S. and Great Plains precipitation with observed SSTs confirms connections to tropical Pacific and Atlantic

## Observed decadal pattern (1871-2000), “IPO”, >13 yr low pass

- Pacific pattern strongly resembles the “Interdecadal Pacific Oscillation” (IPO, Power et al., 1999)

## IPO correlated with observed low pass precip, 1901-2000

## EOF1 of low pass filtered (13 yr) SST

- EOF1 of band pass filtered (2-7 yr) SST

- Multi-decadal IPO pattern from long model control run

- ENSO

## Correlation of IPO (multidecadal EOF1 SST) with: 

- Correlation of IPO (multidecadal EOF1 SST) with:
- Precipitation
- Sea level pressure
- (over North America, connected to convective heating anomalies in Pacific)

## To look at future changes in decadal droughts:

-  analyze stabilized forcing multi-hundred year global coupled climate model simulations (PCM and CCSM3)
- “present-day”:  1990 control (300 years)
- “future”:  2100-2300 from stabilized (at year 2100) A1B scenario (200 years)
- Severe drought:  11 year running mean precipitation anomalies exceeding 1 σ for at least 5 consecutive years
- Extreme drought:  precipitation anomalies exceeding 2 σ for at least one year during a severe drought, or severe drought conditions for at least 10 consecutive years

## Great Plains area (wrt respective climatology; 1 σ = 0.05-0.07):

**Great Plains area** (wrt respective climatology; 1 σ = 0.05-0.07):
- PCM Present:  2 droughts per century; average duration 7.8 years; extreme: 0.7 per century
- (obs:  2/century, 9.0 yrs, 1.0/century)
- Future:  0.5 droughts per century;  average duration 9.0 years;   extreme:  0.5 per century
- CCSM3 present:  1.1 droughts per century;  average duration 11.0 yrs; extreme:  0.9 per century
- Future:  0.5 droughts per century;  average duration 9.0 years;   extreme:  0 per century

## Western U.S. area (wrt respective climatology; 1 σ = 0.05-0.06):

**Western U.S. area** (wrt respective climatology; 1 σ = 0.05-0.06):
- PCM Present (350 yrs):  2 droughts per century; average duration 7.0 yrs extreme: 1.0 per century
- (obs:  2/century, 9.5 yrs, 0.5/century)
- Future (200 yrs):  2 droughts per century;  average duration 5.3 years;   extreme:  1.0 per century
- CCSM3 present (300 yrs):  2.0 droughts per century;  average duration 8.0 yrs; extreme:  0.6 per century
- Future (200 yrs):  0.5 droughts per century;  average duration 8.0 years;   extreme:  0.5 per century

## Great Plains precipitation correlated with SSTs (11 yr running means)   dominant connection with tropical Pacific (cold tropical Pacific = dry Great Plains);

**Great Plains precipitation correlated with SSTs** (11 yr running means)   dominant connection with tropical Pacific (cold tropical Pacific = dry Great Plains);
- Future pattern somewhat weakened compared to present, but qualitatively similar

- present-day                                                                 future

- CCSM3
- PCM

## Western U.S. precipitation correlated with SSTs (11 yr running means)   dominant connection with tropical Pacific (cold tropical Pacific = dry Western U.S.);

**Western U.S. precipitation correlated with SSTs** (11 yr running means)   dominant connection with tropical Pacific (cold tropical Pacific = dry Western U.S.);
- Future pattern somewhat weakened compared to present, but qualitatively similar

- CCSM3
- PCM

## There are significant contributions of multi-decadal Pacific SST anomalies (IPO/PDO) to severe drought over both the Western U.S. and Great Plains

**Therefore, understanding the mechanism of severe drought over North America requires (at least) understanding the mechanism of the IPO/PDO that produces the multidecadal SST anomalies**

- ______

- Mechanism for multi-decadal SST and precipitation variability in the Indo-Pacific region (Meehl, G. A., and A. Hu, 2006, *Journal of Climate,* **19**, 1605–1623.)

- ______

## CMIP3 multi-model future changes of season-averaged precipitation show large seasonal changes in the pattern over North America (dry in far southwest U.S. and Mexico during winter; dry over the Pacific northwest and southern Great Plains in summer)

- Stippled areas are where more than 90% of the models agree in the sign of the change
- Precipitation increases very likely in high latitudes
- Decreases likely in most subtropical land regions
- This continues the observed patterns in recent trends

- Fig. SPM-6

## Average future JJA surface temperature change shows relatively greater warming in western U.S.

- due to changes in circulation (more ridging over western U.S.) associated with convective heating anomalies in tropical Pacific

## Proportionately greater increases of western U.S. temperature (and decreased precipitation) increase vulnerability to future drought in terms of precipitation and evaporation     

- Proportionately greater increases of western U.S. temperature (and decreased precipitation) increase vulnerability to future drought in terms of precipitation and evaporation
- (P minus E becomes more negative there in future)

## Western U.S.

**Western U.S.**
- P minus E mean base state climate change for JJA
- PCM:  -0.21 mm day-1
- CCSM3:  -0.08 mm day-1
- 1 σ for both about 0.01 to 0.02 mm day-1
- JJA base state change roughly an order of magnitude greater than one standard deviation, so virtually every year in future climate would be a severe drought year by present-day standards

## Great Plains 

**Great Plains **
- P minus E mean base state climate change for JJA
- PCM:  -0.12 mm day-1
- CCSM3:  -0.02 mm day-1
- 1 σ for both about 0.04 to 0.06 mm day-1
- JJA base state change produces a tripling of severe drought in PCM, and a doubling of severe drought in CCSM3

## Conclusions

- Severe droughts over two areas of North America (western US and Great Plains) defined in relation to historical severe droughts of 1930s and 1950s in terms of multi-decadal precipitation anomalies
- Similar patterns of SSTs that drive present-day droughts (relative cool tropical Pacific SSTs) also drive future droughts;  a warmer base state appears to qualitatively reduce drought severity somewhat with respect to the new future climate
- A mechanism proposed to explain the decadal pattern of SSTs in the tropical Pacific associated with droughts over North America involves coupled interactions between tropics-midlatitudes and ocean-atmosphere with wind-forced ocean Rossby waves  near 25N and 25S providing the decadal timescale
- In relation to present-day droughts, future summertime droughts more severe due to warmer mean temperatures and enhanced evaporation (P minus E becomes more negative)
- In western U.S. every year is a severe drought year in future compared to present drought regime;  over Great Plains there is a doubling to tripling of severe droughts in future compared to present

## Slide 24

## Can we predict North American droughts 10 to 30 years in advance?

- “Decadal prediction” regional skill could come from three sources:
- Climate change commitment from the forcing already in the system
- Climate change from the forcing from ongoing increases of GHGs
- Predicting time-evolving regional decadal phenomena whose mechanisms could be captured in an initialized climate state (e.g. PDO/IPO, MOC, AMO, etc.)

## An example:  

- An example:
- EOF1 from a model 20th century simulation is the forced trend
- Let’s see if we can predict EOF2 (the internally generated IPO pattern associated with North American drought)  in a perfect model ensemble experiment

## Decadal predictions of IPO index for the Pacific

- 9 out of 29 members (31%) show some predictive skill out to 20 years
- (CCSM3.0, T42, atmospheric initial state perturbed with same ocean initial state at year 2000;  one reference, 29 ensemble members)

## Slide 28

## CMIP5 Decadal Predictability/Prediction core model runs:

**1.1****	****10 year integrations with initial dates towards the end of 1960, 1965, 1970, 1975, 1980, 1985, 1990, 1995 and 2000 and 2005**
-   Ensemble size of 3, optionally to be increased to O(10)
-   Ocean initial conditions should be in some way representative of the observed anomalies or full fields for the start date
-   Land, sea-ice and atmosphere initial conditions left to the discretion of each group
-   Model run time: 300 years (optionally, an additional 700 years)
**1.2****	****Extend integrations with initial dates near the end of 1960, 1980 and 2005 to 30 yrs.**
-   Each start date to use a 3 member ensemble, optionally to be increased to O(10)
-   Ocean initial conditions represent the observed anomalies or full fields.
-   Model run time: 180 years (optionally, an additional 420 years)

## Slide 30