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

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