Mechanisms of decadal hydroclimate variability over the American SouthwestPedro DiNezio, Axel Timmermann, and Matthew Widlansky

Seasonal cycle of precipitation over N. American SW

The Great Basin area used by Meehl and Hu (2007) encompasses two regions with clearly distinct precipitation regimes.

CA and NE have their precip in winter.

AZ, NM, UT, CO have their precip in summer.

Great Basin (32°–42°N, 118°–106°W)

Seasonal cycle of precipitation over N. America

Cali and Nevada get precip during winter

AZ and NM get precip during late summer due to N. American Monsoon

Two precipitation regimes:

• Coastal (CA, NE): maximum precip in winter (winter storms)

• Continental (AZ, NM, UT, CO) maximum precip in summer (N. American monsoon)

The next slide shows that CAM T85 does NOT simulate the spatial extent of these regimes quite well.

• The coastal/winter regime extends excessively into the continent.

• AZ receives max precip in winter, opposite to observations, which show max precip during late summer

• The smooth T85 topography could be allowing more moisture to penetrate into AZ in CAM.

• We looked at one higher resolution model (MIROC4h, 0.5 x 0.5). This model simulates the maximum in precip during late summer over AZ and NM.

• We plan to explore this with higher resolution CAM experiments.

• CAM T85 does NOT simulate the spatial extent of the precipitation regimes quite well.

• The coastal regime extends excessively into the Continent. Possibly due to weak orographic effects due to smooth topography.

Winter precipitation extends excessively inland across the Rockies.

But insufficiently southward making SoCal too dry.

Good job simulating how storms reach equatorward as winter progresses.

Poor simulation of max precip during summer associated with NA Monsoon.

CAM T85 (~1x1)

Obs

MIROC4h does a much better job

Possibly because of its higher 0.5 x 0.5. Atm resol

too much precip

max precip occurs in LATE summer as in obs

Better simulation over SoCal

Obs

Observed decadal changes

R between decadal CA/NE precip and AZ/NM precip is 0.03

Different mechanisms driving the decadal changes?

Cali & Nevada Arizona & New Mexico

years with high SPI60

years with low SPI60

Cali & Nevada Arizona & New Mexico

Relationship with Pacific Climate (Observations)

Conclusions:

• CA & NE: high r with NP index (Aleutian Low) suggests that the decadal precip changes are related to atm changes in the mid-latitude Pacific atmosphere.

• AZ & NM: high r with SO index (tropics) suggests that the decadal precip changes are related to atm changes in the tropical Pacific.

Relationship with Pacific Climate (Models)

Model hierarchy used to explore the role of the ocean on decadal hydroclimate variability:

1. CAM forced with climatological SST

2. CAM coupled to mixed-layer ocean model

3. CCSM4

4. CMIP3 multi-model ensemble

Working hypotheses:

• CA & NE decadal precip changes can be explained by mid-latitude atmospheric processes alone.

• Models fail to simulate the connection between the tropical Pacific and the N. American monsoon suggested by observations.

Simulated decadal changes in precip over CA & NE

Coupling with the ocean mixed layer could make decadal anomalies persist longer or become larger?

mixed-layer oceanclim SST ocean dynamical ocean

Simulated decadal changes in precip over AZ & NM

Decadal changes in the summer precip are very small compared with obs in both coupling configurations.

I need to add a third plot with the changes simulated by CCSM3 or CCSM4.

mixed-layer oceanclim SST ocean dynamical ocean

Obs

CCSM4The coupled model simulates excessive precip everywhere in N. America, but with a more realistic seasonality.

Precip regressed on CA & NE SPI60

mixed-layer ocean

clim SST ocean

dynamical ocean

SST pattern associated with CA & NE precip

mixed-layer ocean

clim SST ocean

No ENSO-like pattern.

Just PDO pattern, but this pattern could be driven by the changes in atm circulation with no role for the ocean.

dynamical ocean

mixed-layer

clim SST

surface wind pattern associated with CA & NE precip

These surface winds can drive a PDO-like SST pattern.

What’s the role of the ocean on these decadal changes?

Role of the Ocean

The TOGA-ML experiments of Seager et al. (2005b) suggested that the “shoe horse” SST pattern in the North Pacific can arise just via integration of air-sea fluxes by the mixed-layer.

So, ENSO-like changes in the tropics can drive a PDO-like pattern via surface fluxes alone.

The spectrum of the NPi (Aleutian Low) becomes slightly redder in the mixed-layer ocean model and shows an ENSO peak in the fully coupled model.

mixed-layerclim SST ocean dynamical ocean

Experiments with Speedy

Speedy forced with cold SST patch off-California

(opposite to this warm SST anomaly)

Simulates drought over California (during winter)

Porting CESM to USC’s HPCC

• Compiled and run test experiments on HPCC

• Currently working on “load balancing” and validating experiments.

• The stott queue runs on nodes with dual dodecacore procesors, i.e. 24 processors per node.

• Many CESM grids can be parallelized on 2n processors.

• Idle processors.

• Sequential vs. concurrent load balancing.

• Next step: high-res control run with CAM.

• Creating a case:create_newcase_pdn -case /auto/ccr-01/pd_646/exp/b.e10.B_2000.f45_g37.007 -res f45_g37

-compset B_2000 -mach usc_cluster_stott -scratchroot /auto/ccr-01/pd_646/scratch

CESM 101