Part II: Where are we going? Like an ocean... The waves crash down... Introducing OCEAN ATMOSPHERE...
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Transcript of Part II: Where are we going? Like an ocean... The waves crash down... Introducing OCEAN ATMOSPHERE...
Part II: Where are we going?
Like an ocean . . .
The waves crash down . . .
Introducing
OCEAN ATMOSPHERE INTERACTION
The North Atlantic Oscillation (NAO)
HIGH INDEX PHASE
• more regional than hemispheric (AO)
• pressure variations of the Icelandic Low and the Azores High
• shift in storm tracks and zonal wind
Spatial Structure
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
The North Atlantic Oscillation (NAO)
HIGH INDEX PHASE
• 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)
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
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
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
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
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
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)
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?
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
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)
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