Atmospheric Circulation Atmospheric Circulation Changes in a Warming ClimateChanges in a Warming Climate

Brian SodenRosenstiel School for Marine and Atmospheric ScienceUniversity of Miami

Isaac Held, Gabriel VecchiGeophysical Fluid Dynamics LaboratoryNational Oceanic and Atmospheric AdministrationPrinceton, NJ

1) Identify those aspects of climate change which are:

Consistent across a large number of models

Are supported by simple physical arguments

2) Motivate observational studies to determine if these responses are currently detectable.

Objective

IPCC AR4 Model Archive

Held and Soden 2006

Change in Convective Mass Flux

Held and Soden 2006

P/P

– 0

.07

T

Background 500

Change in 500

Spatial Structure of Weakened Circulation(multi-model ensemble mean)

Changes in vertical velocity oppose mean state (except Central Pacific)

Weakening occurs primarily as a reduction in the Walker Cell, not Hadley Cell.

Vecchi and Soden 2006

Other El Nino “like” patterns:• Eastward shift of precipitation• Reduction in SST gradient• Reduction in thermocline tilt

GFDL CM2.1 Change in Daily Upward 500

Strongest updrafts become less common

Weakest updrafts become more common

Vecchi and Soden 2006

Pacific Zonal SLP Gradients Decrease

Ful

l oce

an G

CM

sS

lab

ocea

n G

CM

s

Vecchi and Soden 2006

Historical Trends in SLP: 1861-2000

Vecchi et al. 2006

Observational record shows similar pattern of SLP change. Only reproduced when anthropogenic forcings are added.

Model Projected Changes in Vertical Wind Shear

Increased wind shear over much of Tropical Atlantic.Vecchi and Soden 2007

The weakening of the tropical circulation is a robust projection of all climate models.

The slower circulation is manifest primarily as a weakened Walker Cell.

The weakening is associated with a shift of the mean climate to more “El Nino” like conditions in both atmosphere and ocean.

But it is not physically related to El Nino.

May have implications beyond tropical Pacific. E.g., Tropical Atlantic wind shear

Conclusions

EXTRA SLIDES

Weakening of Surface Easterlies

Vecchi and Soden 2006

Change in Convective Mass FluxF

ract

ion

al C

han

ge

in C

on

vect

ive

Mas

s F

lux

P = Mc q

Mc/Mc = P/P – q/q

P/P – 0.07T

Held and Soden 2006

Zonal vs Meridional Decomposition of Weakened Circulation

Weakening is manifest primarily as a reduction of the Walker cell, not Hadley cell Vecchi and Soden 2006

Ensemble-mean response of tropical thermocline.

Equatorial Pacific thermocline flattens and shoals.

Ensemble-mean response of equatorial subsurface temperatures.

Increased thermal stratification.

Ocean Response

Structure of long-term changes in SST depend on dataset used.

“La Niña-like” in Kaplan.

“El Niño-like” in Reynolds.

This is a problem that

needs to be resolved.

Vecchi and Soden 2006

Historical Trends in SST: 1861-2006