North Atlantic Oscillation (NAO) Primary forcing mechanism of the NAO: internal atmospheric dynamics How can we account for the redness in the spectrum? Interaction with the stratosphere anomalies in stratospheric circulation propagate down to the surface are reflected as anomalies in NAO Remote forcing from the tropics Sensitivity of Hadley Circulation to meridional temperature gradients: act as bridge to mid latitudes influencing the NAO Strong signal of SST tripole in tropics/subtropics: southern node could affect variability of the jet stream AND………

Interactions with the Ocean Understand how the atmosphere impacts the ocean But does the ocean impact the atmosphere??

Observations Deser and Blackmon (1993) Examined low-frequency variability in North Atlantic surface climate 2 nd EOF of winter- time SST anomalies(12%) Dipole pattern with centers of action east of Newfoundland and SE US Time series exhibits long term trends(warming of 30s and 40s; cooling of 50s and 60s) Spectral peaks of ~2 and 12 yrs (not significant)

Deser and Blackmon(1993) 1st EOF(21%) and time series of wintertime SAT exhibit similar patterns to SST Spectral peaks at ~2-2.5yrs and 10-15yrs (significant) Local nature of dipole pattern is air sea fluxes (atmos. contributes to SST anomalies) Quasidecadal time scale a property of air-sea coupling?

Observations Kushnir(1994) Examined timescales of SST variability On interannual timescales, SST anomalies display coherent relationship with surface wind circulation First looked at variability on interannual timescales Used composite analysis on SST, SLP and wind anomalies

Kushnir(1994) Examines post World War II cooling trend Used difference between annual mean SST anomalies averaged from (warm years) to (cold years) On decadal timescales, ocean lacks distinctive relationship with corresponding atmospheric anomalies

Implications Covarying fluctuations between ocean and atmosphere on decadal timescales A coupled system? Variability on interdecadal timescales is forced by internal ocean dynamics (gyres, THC)

Interactions with Ocean Mixed Layer Barsugli and Battisti’s (B&B) model: how does a mode of atmospheric variability interact with ocean mixed layer? When ocean is in thermal equililbrium with atmospheric forcing, heat exchange at the ocean-atmosphere interface in coupled case is reduced compared to uncoupled case (when SSTs are fixed) Acts to reduce thermal damping of NAO Primary effect of ocean-atmosphere coupling in mid- latitudes Thermal coupling enhances variance and persistence in modes of variability(e.g. NAO)

Interactions with Ocean Mixed Layer Observations (Czaja and Marshall 1999) 25% of NAO winter variance explained by preceding large scale SST pattern Pattern projects strongly onto tripole SSTs suggesting a positive feedback Tripole pattern has winter-to-winter persistence (via re-emergence), Feedback could explain 1yr memory of NAO

Response of Atmospheric Models to Mid-latitude SST anomalies Often weak and inconsistent –Ambiguous response of atmosphere However, Rodwell et al (1999) produced an NAO- like signal by forcing AGCM with tripole SST pattern Implications for climate predictability Bretherton and Battisti use B&B’s model to show that behavior observed in above model could be interpreted as a weak response of mid-latitude atmosphere to SSTs without using ocean dynamics

Possible Coupling Mechanisms Shift the jet associated with NAO Drives anomalous wind curl (inter-gyre) -> affects trajectory of Gulf Stream and NAC ->change SSTs Buoyancy effects Strong NAO->polar oceans cool->increase THC

Active Ocean-Atmosphere Coupling On interdecadal timescales, variability could be the reflection of a coupled air-sea mode involving the gyres and THC

Role of the Gyres Interplay of meridional heat transport between ocean and atmosphere Invoke heat transport anomalies in gyres and compensate by atmospheric heat transport Gyre anomaly to be a result of atmospheric forcing (AO) from the past This delay(sets up oscillation of the system) is key aspect that joins the gyres with the jet stream above Oscillatory modes of variability in the gyres-> SSTs-> overlying circulation

Role of THC Decadal variability: change the strength of meridional circulation and associated heat transport->change SSTs->change overlying circulation Southward propagation of LSW could affect stability characteristics of western boundary current Affect down-stream intensity of Gulf Stream where air-sea interactions are important Variability in THC strength could affect SST variations on multidecadal timescales (~70yrs)

Tropical Atlantic Variability (TAV) Remote forcings NAO reaches down to tropics Covarying fluctuations in SST and trades could be caused by subtropical portion of NAO Strong relation between tripole and cross- equatorial gradient ENSO may affect tropical Atlantic On interannual timescales, ENSO affects Atlantic SST and wind fields Atlantic SSTs lag Pacific ENSO by 4-5 months

Feedback Processes over Land Could affect persistence in rainfall anomalies over Sub-Saharan Africa Bio-geophysical feedback mechanism Deterioration of of vegetation-> decrease in sfc net radiation->increases subsidence ->less cloudiness and precipitation-> less vegetation

TAV: Coupling with the Ocean Interhemispheric SST anomalies Increase in cross-equatorial gradient impacts low-level tropospheric flow on the western side of basin Warmer (colder) SST in north (south) tropics -> southerly wind anomaly along Brazilian seaboard-> ITCZ shifts northward-> reduced precip during boreal spring Circulation on both sides of the equator is linked

TAV: Coupling with the Ocean Subtropical-tropical Interactions Decadal SST variability may extend into tropics via subtropical gyre Link between TAV and convection in Labrador Sea Atlantic ENSO Appears to be stable, thus forced by external stochastic forcing (Pacific ENSO, NAO)

Prospects for Predictability Atmospheric Predictability: (SAT, precip) Needs oceanic predictability Invoke some kind of memory in boundary conditions Coherent response of circulation to SST anomalies Oceanic Predictability (SSTs, subsurface Ts, and Salinity) Already has memory Just needs spatially and temporally coherent variability on interannual and interdecadal timescales

Summary/Conclusions Ocean-Atmosphere Interactions SSTs have small impact on NAO (could drive the redness in NAO spectrum) Interannual timescales: stochastic models explain coupling well Reduced thermal damping caused feedback between SST and air temperature Enhanced persistence due to re-emergence mechanism Longer timescales: gyres and THC may have role in air-sea coupling TAV No single mechanism dominates the variability