Climate Variability and Basin Scale Forcing

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Climate Variability and Basin Scale Forcing over
the North Atlantic
Jim Hurrell Climate and Global Dynamics
Division National Center for Atmospheric
Research (NCAR) jhurrell_at_ucar.edu
BASIN Science Meeting 12-14 March
2005 Reykjavik, Iceland
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Climate Phenomena in the Atlantic
North Atlantic Oscillation (NAO)
Tropical Atlantic Variability (TAV)
Meridional Overturning Circulation (MOC)

• MOC, TAV and NAO interact, but in ways that are
  not well understood

• Understanding Atlantic climate
  variability/change requires a global view

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The Mean State and Stationary Waves (DJF)
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Change in Winter Surface Temperature since 1980
Cold
Warm
Human activities are superimposed on the
background noise of natural variability
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Change in Winter Sea Level Pressure since 1980
(hPa)
Dec-Mar
Pressure Falls
Pressure Rises
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The North Atlantic Oscillation
Spatial Structure and Seasonal Variability
DJF
MAM
SON
JJA
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Alternative Definition Cluster Analysis (Winter
Only)
NAO- 29
NAO 20
Scand 21
Ridge 30
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Time History of Occurrence
NAO
NAO-
Scand
Ridge
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The North Atlantic Oscillation Winter
Spatial Structure
Dec-Mar 39 SLP (hPa)
Stronger Westerlies
hPa
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Winter Surface Temperature
Change since 1980
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NAO Influence on Winter Precipitation
(Positive Index Phase)
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Ocean Response to NAO Variability
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Ocean Response to NAO Variability
Leading patterns of North Atlantic climate
variability
Sea Surface Temperature
500 hPa Geopotential Height
r 0.72
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Ocean Response to NAO Variability
SST
Winter Index (1864-2002)
Stronger Westerlies
Weaker Westerlies
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Multi-decadal SST Variability

• Oceanic response to atmospheric decadal
  variability?

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Multi-decadal SST Variability

• Oceanic response to atmospheric decadal
  variability?

• What is the role of non-local dynamical oceanic
  processes?

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Ocean Response to NAO Variability
Changes in Water Masses
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Ocean Response to NAO Variability
Circulation Changes
Oceanic NAO analogue Eastward Transport Index
Curry and McCartney (2001)
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Sea Ice Response to NAO Variability
-

Deser et al. (2000)
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What Climate Processes Govern NAO Variability?
Simulated (Dec-Mar)
Random and Unpredictable Variations
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What Climate Processes Govern NAO Variability?

• 200 years of CCM3 without variations
• in external forcings

EOF1 SLP (Dec-Mar)

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

Simulated NAO Index
Random and Unpredictable Variations
r (1yr) -0.07
r (1yr) -0.06
A role for external forcing?
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The Role of Ocean Forcing
JFM 500 hPa Height Trend (1950-1999) m
Rises
Falls
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Reproducibility Individual Runs
Trend of JFM 500 hPa NAO Index (1950-1999)
GOGA (multi-model)
-200 -100
0 100 200
(m)
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JFM Trend in Tropical SST (1950-1999)
Cold
Warm
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JFM Response to Indo-W. Pacific SST Trend
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Change in Tropical SST (1950-1999)
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Concerning observed winter North Atlantic climate
change

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

• Nonlinear approaches reveal spatial asymmetries
• between the two NAO phases

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

• Climate noise paradigm does not explain behavior
• in recent decades

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

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Cautionary notes

• NAO explains only a fraction of the total
  variability

• No evidence for periodic behavior

• Nonlinear approaches give a different view of
  the
• dynamic signature of interannual North
  Atlantic
• climate variability

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Future Change
?
Numerous modeling studies indicate two Atlantic
phenomena that might change
NAO and MOC
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Future Change
NAO

• 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

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Future Change
NAO
Simulated (7 models)
Gillett et al. 2003
Concerning the discrepancy with observations

• Inability to capture mechanisms of by which
  stratospheric flow
• anomalies affect the tropospheric evolution

• Details of the simulated SST field are important

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Future Change
MOC

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

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Future Change
MOC

• 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

• Ongoing CMIP subproject to investigate model
• differences

• Effect of weakening is to moderate regional
  warming

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