Climate Variability and Basin Scale Forcing over the North Atlantic Jim Hurrell Climate and Global Dynamics Division National Center for Atmospheric Research (NCAR) BASIN Science Meeting March 2005 Reykjavik, Iceland

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

The Mean State and Stationary Waves (DJF) L H L H H Sea Level Pressure Mid Tropospheric Height LOW HIGH

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+ 20% Scand 21% Ridge 30%

NAO+ NAO- Scand Ridge Time History of Occurrence

The North Atlantic Oscillation: Winter Spatial Structure Dec-Mar 39% SLP (hPa) Temporal Evolution Dec-Mar r = 0.92 Stronger Westerlies hPa

Winter Surface Temperature Change since 1980 °C Cold Warm NAO influence (1  ) °C Cold Warm

NAO Influence on Winter Precipitation DRY WET (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 variability Sea Surface Temperature 500 hPa Geopotential Height Ocean Response to NAO Variability r = 0.72

Ocean Response to NAO Variability Winter Index ( ) Weaker Westerlies Stronger Westerlies SST

( ) – ( ) Annual SST (°C) Multi-decadal SST Variability Oceanic response to atmospheric decadal variability? Oceanic response to atmospheric decadal variability?

Multi-decadal SST Variability What is the role of non-local dynamical oceanic processes? What is the role of non-local dynamical oceanic processes? Oceanic response to atmospheric decadal variability? Oceanic response to atmospheric decadal variability?

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) = r (1yr) = Except for the latter half of the 20 th Century Observed r (1yr) = 0.4

Rises JFM 500 hPa Height Trend ( ) m Global SST Tropical SST Observed Falls The Role of Ocean Forcing

Reproducibility: Individual Runs GOGA (multi-model) Trend of JFM 500 hPa “NAO Index” ( ) CNTRL (CCM3) (m) Observed TOGA (CCM3)

JFM Trend in Tropical SST ( ) ColdWarm 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) FALLSRISES r = 0.80

Change in Tropical SST ( ) 0.62 ± 0.13°C Observed Indian Ocean Temperature 0.59 ± 0.14°C Climate Model Forced with Observed Changes in Greenhouse Gas Concentrations Simulated 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

Cautionary notes   NAO explains only a fraction of the total variability   Nonlinear approaches give a different view of the dynamic signature of interannual North Atlantic climate variability   No evidence for periodic behavior

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) 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