The NAO and the Gulf Stream: Basin Scale Interactions to Regional Scale Variability

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

2. 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

3. “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)

5. 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.

7. 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

8. The Gulf Stream North Wall

13. Standard Model Run Air-Sea Temperature Differences Wind Stress Zonal Velocity (b) (c) (a) 60 60 60 50 50 50 1994 1994 40 40 40 Latitudinal Grid Point Latitudinal Grid Point Latitudinal Grid Point 1994 30 30 30 20 20 20 1971 1971 1971 10 10 10 0 0 0 -0.1 0 0.1 -1 -0.03 0 0.03 0 1 Figure 1. Taylor et al. JGR

14. Average Zonal Wind Velocity 70 60 50 Latitude N 40 High NAO years 30 Low NAO years 20 -6 2 -4 -2 0 4 m s-1 Figure 2. Taylor et al. JGR

15. Prediction with a 1 km Surface Layer (a) 26.0 r = 0.59 2.0 25.5 Observed 1.0 25.0 Observed GSNW (standardised units) Predicted Grid Location of Gulf Stream 0.0 24.5 -1.0 24.0 Predicted -2.0 23.5 1965 1970 1975 1980 1985 1990 1995 2000 Years Prediction with a 1km Surface Layer and averaged NAO (b) r = 0.80 25.0 Predicted 2.0 Observed GSNW (standardised units) Observed 24.6 1.0 Predicted Grid Location of Gulf Stream 24.2 0.0 23.8 -1.0 23.4 -2.0 1965 1970 1975 1980 1985 1990 1995 2000 Years Figure 6. Taylor et al. JGR

16. (a) 26.6 37.5 Predicted Gulf Stream Position 37.0 26.2 Predicted Grid Location of Gulf Stream Observed Separation Latitude 36.5 25.8 36.0 25.4 Observed Separation Latitude 35.5 25.0 35.0 1950 1960 1970 1980 1990 2000 Year (b) r = 0.59 2 Joyce et al. Index Observed Position (standardised units) 1 0 -1 Observed GSNW -2 1950 1960 1970 1980 1990 2000 Year (c) 26.6 r = 0.42 2 Predicted Gulf Stream Position 26.2 Predicted Grid Location of Gulf Stream Observed Position (standardised units) 1 Joyce et al. Index 25.8 0 25.4 -1 25.0 1950 1960 1970 1980 1990 2000 Year Figure 7. Taylor et al. JGR

18. Standard Model Run (a) 27.5 27.0 26.5 Predicted Grid Location of Gulf Stream 26.0 25.5 25.0 24.5 1820 1840 1860 1880 1900 1920 1940 1960 1980 2000 Year Prediction From Average NAO Over Two Years (b) 26.0 25.5 25.0 Predicted Grid Location of Gulf Stream 24.5 24.0 23.5 23.0 1820 1840 1860 1880 1900 1920 1940 1960 1980 2000 Year Figure 9. Taylor et al. JGR

19. Secular Changes in Temperature-salinity Distribution in the North Atlantic

20. 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).

22. Low NAO High NAO LSW Intrusion -- GS moves South More WSW-- GS moves North

23. Regional Impact

24. 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

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

26. Cod historical catch (1893-2000)

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

28. Slow inverse relationship between the NAO and the Cod Over 30-35 year long periods

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

32. The Bio-Chemical Scenario • Low NAO – high transport of LSW  cold and fresh conditions in upper slope  relatively low concentration of NO3 (16M) and Si(OH)4(10M) • High NAO – More WSW in the upper slope  warm and saline  relatively high NO3 (24M) and Si(OH)4(14M) • Flux of NO3 across the Gulf Stream (at levels deeper than >27.0)

33. How Do We Connect All These?

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

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

37. North Atlantic Oscillation – Spectral Analysis

38. Gulf Stream excursion – Spectral Analysis

39. Shelf Slope Front excursion – Spectral Analysis

40. Shelf Slope Front excursion – Spectral Analysis

41. HADLEY Center Climate Model Result

42. HadCM3

43. 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?

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

45. Low NAO High NAO LSW Intrusion -- GS moves South More WSW-- GS moves North

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

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

48. A NSF/GLOBEC Proposal

49. Future Directions • A Basin-to-Regional Scale Modeling System • 50 year (1950-2000) simulation for NAO impact studies -- Taylor and Gangopadhyay, JGR, 2001. • Multidecadal variability -- 40s vs 90s -- Petrie and Drinkwater, 1993. • Interannual variability -- boundary fluxes for GOM during 1993-1997 -- smith et al., 2001. • Regional high-resolution event-scale modeling • Feature oriented initialization + NPZ + IBM simulations

50. 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 – 1950-2000!!