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Part II: Where are we going?

Part II: Where are we going?. Like an ocean. The waves crash down. Introducing OCEAN ATMOSPHERE INTERACTION. The North Atlantic Oscillation (NAO). Spatial Structure. more regional than hemispheric (AO)

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Part II: Where are we going?

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  1. Part II: Where are we going? Like an ocean . . . The waves crash down . . . Introducing OCEAN ATMOSPHERE INTERACTION

  2. The North Atlantic Oscillation (NAO) Spatial Structure • more regional than hemispheric (AO) • pressure variations of the Icelandic Low and the Azores High • shift in storm tracks and zonal wind HIGH INDEX PHASE

  3. The North Atlantic Oscillation (NAO) Temporal Structure • NAO is found in all seasons • NAO more pronounced during the winter (DJFM)- 37% of mthly 500hPa time series • time avg. pressure difference (Azores and Iceland) • srfc. spatial pattern is defined by regression with SLP

  4. The North Atlantic Oscillation (NAO) • 1900s – 30s: NAO high index • 1940s – 70s: NAO low index • 1980s – present: NAO high index • contributed to much of observed warming in past decades (Hurrell, 1995) HIGH INDEX PHASE

  5. The North Atlantic Oscillation (NAO) Spatial Structure • regression of SST on the winter NAO index • Tri-polar pattern: leading pattern of SST variability • SST responding on monthly to seasonal time scales (NAO forcing) • WHY? Fig 2a

  6. The North Atlantic Oscillation (NAO) Co-varying patterns • regression of SST on to srfc turbulent heat flux • Heat fluxes  Tri-polar pattern • SST tri-pole most energetic in late winter (needs a month to be forced by NAO) Fig 2b

  7. The North Atlantic Oscillation (NAO) Temporal Structure • wintertime NAO spectrum is red • tri- pole SST pattern has an even redder spectrum (thermal inertia) • increased power in the decadal band • cross-equatorial circulation thumbprint on SST ( ΔTGS) Fig 3

  8. The North Atlantic Oscillation (NAO) Co-varying Patterns • Labrador Sea: • NAO/ ocean covariance? • LSW has cooling/ warming trends consistent with NAO phase • variations are large and sustained Fig 5

  9. Tropical Atlantic Variability (TAV) Also known as the Tropical Atlantic SST Dipole • dominant low frequency pattern btwn tropical SST and trade winds around the ITCZ • positive SST anomalies north of the ITCZ  weak trade winds • cold SSTs south of the ITCZ  weak northern trades  smaller ITCZ displacement to the south

  10. Tropical Atlantic Variability (TAV) • SST variability north and south are NOT correlated • not a see- saw • trade wind response is to cross- ITCZ SST differences • SST-trade wind feedback? • SST and trade wind time series: decadal variability (ΔTEQ)

  11. TAV and the NAO • SST tripole (NAO) and interhemispheric SST gradients (TAV) share equatorial/ subtropical anomalies • TAV variability of SST is strongest in March- May lagging behind strongest NAO season (JFM) • NAO may be an extratropical forcing that can excite the TAV • interhemispheric SST gradients  tropical atmosphere • Subtropical SSTs  NAO variability?

  12. Meridonal Overturning Circulation (MOC) Also known as the Thermohaline Circulation • responsible for poleward transport of water • MOC function of upwelling, downwelling, and upper-wind forcing • air-sea interaction  cold North Atlantic Deep Water (NADW) • NAO is primary modulator of water mass transformation

  13. IMPACTS: The NAO • NAO is strongly linked to wintertime temperatures (SST over ocean, srfc air temperature over land) • change in synoptic eddy activity and shifts in storm tracks • High Index- • dry over central/ southern Europe, northern mediterranean • western N. Africa • (Alps snowcover) • wet from • Iceland • to Scandinavia • (Norway glaciers)

  14. IMPACTS: TAV • regions dependent on the north-south swings of the ITCZ especially sensitive • Nordeste, Brazil: • Dry when N. SSTs high, S. SSTs low • West Africa: • Wet when N. SSTs are positive, S. SSTs are negative

  15. IMPACTS: The MOC and abrupt climate change • GHG warming models suggest increased freshening and warming of subpolar seas • models suggest NAO/ AO anomalously high • combination  weakened MOC (abrupt change?) • potential for rapid cooling in northern Europe and NE America

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