The NAO and the Gulf Stream: Basin Scale Interactions to Regional Scale Variability Avijit Gangopadhyay University of Massachusetts Dartmouth

Outline Background on NAO The two phases of NAO Basin wide responses – wind driven and thermohaline Regional impact Past studies with simple models and statistical methods Future – basin-wide to regional nests

“It is generally recognized that an accentuated pressure difference between the Azores and Iceland in autumn and winter is associated with a strong circulation of winds in the Atlantic, a strong Gulf Stream, high temperature in the winter and spring in Scandinavia (Meinardus, 1898) and the east coast of the US, and with lower temperature in the east coast of Canada and the west of Greenland.” Sir Gilbert Walker (1924) Walker and Bliss (1932)

North Atlantic Oscillation (NAO) Large-scale atmospheric pressure anomaly between the north Atlantic subtropical high surface pressure at the Azores, and the sub-polar low over Iceland. It refers to an average of December to march. NAO is based on the difference of normalized sea level pressures.

NAO Phases Low Phase weak High and Low Decreased Pressure Difference Storms on a more EW track moist air in the Mediterranean and cold air to northern Europe US east coast: cold air outbreaks and snowy weather conditions. Greenland: milder winter temperatures Reduced production of LSW Labrador Current intensifies Gulf Stream north wall shifts South High Phase Increased Pressure Difference Strong High and Low Storms on a more northerly track eastern US: mild and wet winter warm and wet winters in Europe cold and dry winters in northern Canada and Greenland Enhanced production of LSW Labrador Current weakens Gulf Stream north wall shifts North

The Gulf Stream North Wall

Zonal Velocity (c) Latitudinal Grid Point Standard Model Run Wind Stress Latitudinal Grid Point (a) Air-Sea Temperature Differences Latitudinal Grid Point (b) Figure 1. Taylor et al. JGR

Latitude N Average Zonal Wind Velocity m s -1 High NAO years Low NAO years Figure 2. Taylor et al. JGR

Predicted Grid Location of Gulf Stream Observed GSNW (standardised units) Prediction with a 1 km Surface Layer Years Observed Predicted r = 0.59 (a) Predicted Grid Location of Gulf Stream Observed GSNW (standardised units) Years Observed r = 0.80 Predicted Prediction with a 1km Surface Layer and averaged NAO (b) Figure 6. Taylor et al. JGR

Figure 7. Taylor et al. JGR (a) Observed GSNW Joyce et al. Index Observed Position (standardised units) Year r = 0.59 (b) Observed Position (standardised units) Joyce et al. Index Predicted Gulf Stream Position Predicted Grid Location of Gulf Stream r = 0.42 (c) Year Predicted Gulf Stream Position Predicted Grid Location of Gulf Stream Year Observed Separation Latitude Observed Separation Latitude

Year Predicted Grid Location of Gulf Stream Year Predicted Grid Location of Gulf Stream Standard Model Run Prediction From Average NAO Over Two Years (a) (b) Figure 9. Taylor et al. JGR

Secular Changes in Temperature-salinity Distribution in the North Atlantic

Historical monthly mean temperature and salinity observations from 300 (+/- 25 m) depth within the Georges Basin subarea as defined by Petrie et al., (1996). Figure from Drinkwater (2001).

Low NAOHigh NAO LSW Intrusion -- GS moves SouthMore WSW-- GS moves North

Regional Impact

Georges Bank (GB) and Gulf of Maine (GoM) GoM 93,600 square km of ocean Cold water from the North Atlantic enters the GoM via the Northeast Channel Fresh water comes into the GoM over 60 rivers. counterclockwise around the Gulf creating a unique, self-contained oceanographic system Strong tidal currents GB 120x240 km large GB is more than 100 m higher than the sea floor of the GoM nutrient-rich Labrador current sweeps over most of the submarine plateau, and meets the warmer Gulf stream on its eastern edge Warm Core Rings of the GS hit the bank once in a while. oceanographic transition zone especially vulnerable to changes in climate

Atlantic Cod – Gradus Morhua gadoid species winter spawner Adults inhabiting inshore areas generally move offshore to reproduce Two different stocks at GB and GoM

Cod historical catch ( )

NAFO statistical unit areas Division 5Y (GOM) Division 5Z (GB) and Sub-area 6 (Southern N.E. Middle Atlantic Area)

Slow inverse relationship between the NAO and the Cod Over year long periods

Period=2л/л.f)* d T from Gangopadhyay & Taylor (submitted) d T=3yrs d T=5yrs

The Bio-Chemical Scenario Low NAO – high transport of LSW  cold and fresh conditions in upper slope  relatively low concentration of NO 3 (16  M) and Si(OH) 4 (10  M) High NAO – More WSW in the upper slope  warm and saline  relatively high NO 3 (24  M) and Si(OH) 4 (14  M) Flux of NO 3 across the Gulf Stream (at levels deeper than   >27.0)

How Do We Connect All These?

NAO and Gulf Stream Two effects influence the position of the GSNW: –Labrador Shelf Water penetration into GoM –Ekman Transport due to westwind stress.

(from Gangopahyay & Taylor,2002 submitted) A Synergistic Approach

North Atlantic Oscillation – Spectral Analysis

Gulf Stream excursion – Spectral Analysis

Shelf Slope Front excursion – Spectral Analysis

HADLEY Center Climate Model Result

HadCM3

Summary of Results The western section of the Gulf Stream has a dual-period response (8-10, and 3-5 years) The eastern segment (east of 60W) has a single-period response. O(4-7years) Supported by both Observation and Climate Model results Could these be related to wind-driven and thermohaline effects?

How do we study such climatic impact on our local environment?

Low NAOHigh NAO LSW Intrusion -- GS moves SouthMore WSW-- GS moves North

Basin-Scale Model Regional-Scale Model NPZ, IBM High-Resolution Local-Scale Model NPZ, IBM Climate -- NAO, ENSO Mesoscale Physics, Chemistry, Biology Ecosystem Dynamics Event and Process Studies

Basin-scale to Regional-scale Climate-scale to Plankton-scale to Fisheries NAO -- GS -- GOM -- Plankton -- Nutrients -- Topography Climate - Physics - Biology - Chemistry - Geology

A NSF/GLOBEC Proposal

Future Directions A Basin-to-Regional Scale Modeling System 50 year ( ) simulation for NAO impact studies -- Taylor and Gangopadhyay, JGR, Multidecadal variability -- 40s vs 90s -- Petrie and Drinkwater, Interannual variability -- boundary fluxes for GOM during smith et al., Regional high-resolution event-scale modeling Feature oriented initialization + NPZ + IBM simulations

CONCLUSIONS Sure! There is wind-driven as well as thermohaline response of the GS to the NAO Are they competitive? – Probably Synergistic! H1: Western boundary region and after separation – Wind-driving dominates! H2: Eastern end – large amplitude meanders – thermohaline (LSW inflow) effects dominate! 60-65W is the transition response zone! Need for simulation – !!

Figure 3. Taylor et al. JGR Observed GSNW (standardised units) NAO Index (standardised units) Years Standard Model Run (b) Predicted Observed r = 0.76 Observed GSNW (standardised units) Predicted Grid Location of Gulf Stream (a) Annual NAO Observed GSNW r = 0.10

Predicted Grid Location of Gulf Stream Predicted Observed r = 0.93 Predicted Grid Location of Gulf Stream Observed GSNW (standardised units) Years Latitudes of Maximum Zonal Velocity and Maximum Air-Sea Temperature Difference compared Without Thermal Feedback on Wind Stress ( = 0 ) (a) (b) r = 0.79 Maximum Zonal Velocity Maximum air-sea temperature difference Figure 4. Taylor et al. JGR

Without Thermal Feedback on Wind Stress ( = 0 ) 10 Air-Sea Temperature Differences Latitudinal Grid Point (a) Zonal Velocity Latitudinal Grid Point (b) Figure 5. Taylor et al. JGR

Gulf Stream excursion – Spectral Analysis