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 PhasesLow 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

0.030-0.03

40

30

Zonal Velocity

0

10

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60

(c)

0.10-0.1

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Standard Model Run

Wind Stress

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(a)Air-Sea Temperature

Differences

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10-1

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1971

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Figure 1. Taylor et al. JGR

La

titu

de

N

Average Zonal Wind Velocity

-4 -2 0 4

20

30

40

50

60

70

-6 2m s -1

High NAOyears

Low NAOyears

Figure 2. Taylor et al. JGR

-2.0

-1.0

0.0

1.0

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1965 1970 1975 1980 1985 1990 1995 2000

23.4

23.8

24.2

24.6

25.0

Predicted G

rid Location of Gulf S

treamObs

erve

d G

SN

W (

stan

dard

ised

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Prediction with a 1 km Surface Layer

Years

Observed

Predicted

r = 0.59

(a)

Predicted G

rid Location of Gulf S

tream

Obs

erve

d G

SN

W (

stan

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Years

Observed

r = 0.80 Predicted

Prediction with a 1km Surface Layer and averaged NAO(b)

-2.0

-1.0

0.0

1.0

2.0

1965 1970 1975 1980 1985 1990 1995 200023.5

24.0

24.5

25.0

25.5

26.0

Figure 6. Taylor et al. JGR

Figure 7. Taylor et al. JGR

(a)

Observed GSNW

Joyce et al. Index

Obs

erve

d P

ositi

on

(sta

ndar

dise

d un

its)

Year

r = 0.59

(b)

Observed P

osition (standardised units)

Joyce et al. Index

Predicted GulfStream Position

Pre

dict

ed G

rid L

ocat

ion

o f G

u lf

St r

eam

r = 0.42

(c)

Year

Predicted GulfStream Position

Pre

dict

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rid L

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ofG

ulf

Str

eam

Year

Observed S

eparation Latitude

Observed SeparationLatitude

-2

-1

0

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2

1950 1960 1970 1980 1990 2000

25.0

25.4

25.8

26.2

26.6

1950 1960 1970 1980 1990 2000

-1

0

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25.0

25.4

25.8

26.2

26.6

1950 1960 1970 1980 1990 200035.0

35.5

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37.5

Year

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Year

Pre

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eam Standard Model Run

Prediction From Average NAO Over Two Years

(a)

(b)

24.5

25.0

25.5

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26.5

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1820 1840 1860 1880 1900 1920 1940 1960 1980 2000

23.0

23.5

24.0

24.5

25.0

25.5

26.0

1820 1840 1860 1880 1900 1920 1940 1960 1980 2000

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 NAO High NAO

LSW Intrusion -- GS moves South More 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 (1893-2000)

NAFO statistical unit areas 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 CodOver 30-35 year long periods

Period=2л/л.f)*T

from Gangopadhyay & Taylor (submitted)

T=3yrs

T=5yrs

The Bio-Chemical Scenario

• Low NAO – high transport of LSW cold and fresh conditions in upper slope relatively low concentration of NO3 (16M) and Si(OH)4(10M)

• High NAO – More WSW in the upper slope warm and saline relatively high NO3 (24M) and Si(OH)4(14M)

• Flux of NO3 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

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 NAO High NAO

LSW Intrusion -- GS moves South More WSW-- GS moves North

Basin-Scale Model

Regional-Scale Model

NPZ, IBM

High-ResolutionLocal-Scale Model

NPZ, IBM

Climate -- NAO, ENSO

Mesoscale Physics, Chemistry, Biology

Ecosystem DynamicsEvent 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 (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

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!!

Figure 3. Taylor et al. JGR

-2.0

-1.0

0.0

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1965 1970 1975 1980 1985 1990 1995 2000

25.0

25.4

25.8

26.2

26.6

Obs

erve

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stan

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NA

O Index (standardised units)

Years

Years

Standard Model Run

(b)

Predicted

Observed

r = 0.76

Obs

erve

d G

SN

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stan

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redicted Grid Location of G

ulf Stream

(a)

Annual NAO

Observed GSNW-2.0

-1.0

0.0

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1965 1970 1975 1980 1985 1990 1995 2000-1.2

-0.8

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1.2r = 0.10

24.2

24.6

25.0

25.4

25.8

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1965 1970 1975 1980 1985 1990 1995 2000

Predicted G

rid Location of Gulf S

tream

Predicted

Observed

r = 0.93

Pre

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Years

Latitudes of Maximum Zonal Velocity and Maximum Air-Sea Temperature

Difference compared

Without Thermal Feedback on Wind Stress ( = 0 )

(a)

(b)

-2.0

-1.0

0.0

1.0

2.0

1965 1970 1975 1980 1985 1990 1995 2000

26.5

27.0

27.5

28.0r = 0.79

MaximumZonal Velocity

Maximum air-seatemperature difference

Figure 4. Taylor et al. JGR

Without Thermal Feedback on Wind Stress ( = 0 )

10-1

Air-Sea Temperature Differences

Latit

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(a)

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Zonal Velocity

Latit

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(b)

1994

1971

Figure 5. Taylor et al. JGR

Gulf Stream excursion – Spectral Analysis