Climate Variability and Basin Scale Forcingover the North Atlantic

Jim Hurrell

Climate and Global Dynamics Division

National Center for Atmospheric Research (NCAR)

jhurrell@ucar.edu

BASIN Science Meeting12-14 March 2005

Reykjavik, Iceland

Climate Phenomena in the Atlantic

North Atlantic Oscillation (NAO)North Atlantic Oscillation (NAO)

Tropical Atlantic Variability (TAV)Tropical Atlantic Variability (TAV)

Meridional Overturning Circulation (MOC)Meridional Overturning Circulation (MOC)

• MOC, TAV and NAO interact, but in ways that are not well understoodMOC, TAV and NAO interact, but in ways that are not well understood• Understanding Atlantic climate variability/change requires a global viewUnderstanding Atlantic climate variability/change requires a global view

The Mean State and Stationary Waves (DJF)

L

H

LH

H

Sea Level Pressure

Mid Tropospheric Height

LOW

HIG

H

Change in Winter Surface Temperature since 1980

Human activities are superimposed on the background “noise” of natural variability

Cold

Warm

Change in Winter Sea Level Pressure since 1980(hPa)Dec-Mar

Pressure Falls

Pressure Rises

(related to)El Niño/Southern Oscillation (ENSO)

North Atlantic Oscillation (NAO)

The North Atlantic Oscillation

Spatial Structure and Seasonal Variability

DJF

SON

MAM

JJA

Alternative Definition: Cluster Analysis (Winter Only)NAO- 29%NAO- 29% NAO+ 20%NAO+ 20%

Scand 21%Scand 21%Ridge 30%Ridge 30%

NAO+ NAO+

NAO- NAO-

Scand Scand

RidgeRidge

Time History of Occurrence

The North Atlantic Oscillation: Winter

Spatial StructureDec-Mar 39% SLP (hPa)

••

Temporal EvolutionDec-Mar

r = 0.92

StrongerWesterlies

hPa

Winter Surface Temperature

Change since 1980

°C

Cold Warm

NAO influence (1 )

°C

Cold Warm

NAO Influence on Winter Precipitation

DRYWET

(Positive Index Phase)

Energy Supply and Demand (Example: Norwegian Energy Trade)

Export

Import

Positive NAO

Negative NAO

Visbeck et al. 2002

Ocean Response to NAO Variability

Leading patterns of North Atlantic climate variabilitySea Surface Temperature 500 hPa Geopotential Height

Ocean Response to NAO Variability

r = 0.72r = 0.72

Ocean Response to NAO VariabilityWinter Index (1864-2002)

WeakerWesterlies

StrongerWesterlies

SST

SST SST

Ocean Response to NAO Variability

Visbeck et al. (2003)Courtesy of Igor Yashayaev

Changes in Water Masses

Ocean Response to NAO Variability

Curry and McCartney (2001)

Circulation Changes

Oceanic NAO analogue: Eastward Transport Index

Deser et al. (2000)

+

-

Sea Ice Response to NAO Variability

What Climate Processes Govern NAO Variability?

Random and Unpredictable Variations

Simulated (Dec-Mar)

• 200 years of CCM3 without variations in “external” forcings• Basic structure & time scale arises from internal nonlinear atmospheric dynamics

EOF1 SLP(Dec-Mar)

What Climate Processes Govern NAO Variability?

Random and Unpredictable Variations

Consistent with Observations(Climate Noise Paradigm)

Observed

Simulated NAO Index

A role for external forcing?

• Basic structure & time scale arises from internal nonlinear atmospheric dynamics

• 200 years of CCM3 without variations in “external” forcings EOF1 SLP

(Dec-Mar)

r (1yr) = -0.07

r (1yr) = -0.06 Except for the latter half of the 20th Century

Observed

r (1yr) = 0.4

Rises

JFM 500 hPa Height Trend (1950-1999) m

Global SST Tropical SST

Observed

Falls

The Role of Ocean Forcing

JFM Trend in Tropical SST (1950-1999)

Cold Warm

Warm Pool (60°-170°E; 15°S-15°N)

0.62 ± 0.13°C

Tropics (15°S-15°N)

0.46 ± 0.25°C

Precipitation (mm day-1)

WETDRY

JFM Response to Indo-W. Pacific SST Trend

500 hPa Height (m)

FALLS RISES

r = 0.80

Change in Tropical SST (1950-1999)

0.62 ± 0.13°C

ObservedObserved Indian Ocean Temperature Indian Ocean Temperature

0.59 ± 0.14°C

Climate ModelForced with ObservedChanges in GreenhouseGas Concentrations

SimulatedSimulated Indian Ocean Temperature Indian Ocean Temperature

NAO is most prominent and recurrent pattern of atmospheric variability, driving upper-ocean and sea ice variations

Basic structure and time evolution results from internal, nonlinear atmospheric dynamics

Climate noise paradigm does not explain behavior in recent decades

Concerning observed winter North Atlantic climate change

Model experiments suggest North Atlantic climate change has been driven, at least in part, by warming tropical SSTs, with the Indo-Pacific region key

Nonlinear approaches reveal spatial asymmetries between the two NAO phases

Future Change?

Numerous modeling studies indicate two Atlantic phenomena that might change

NAO and MOC

Future Change

Most climate models simulate an increasing trend, with pressure decreases over the far North Atlantic and pressure increases in middle latitudes Details vary considerably from model-to-model, and the simulated trends are smaller than observed

NAO

Future Change

NAO

Observed (low pass filtered)Observed (low pass filtered)Simulated (7 models)Simulated (7 models)

Inability to capture mechanisms of by which stratospheric flow anomalies affect the tropospheric evolution Details of the simulated SST field are important

Concerning the discrepancy with observations Gillett et al. 2003

Future Change

Most, but not all, climate models project some weakening due to warming and freshening

MOC

Courtesy Jonathan Gregory

Future Change

Most, but not all, climate models project some weakening due to warming and freshening

Nature of changes and mechanisms vary considerably from model-to-model

MOC

Ongoing CMIP subproject to investigate model differences

Effect of weakening is to moderate regional warming