climate variability and basin scale forcing over the north atlantic jim hurrell climate and global...
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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
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