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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
20
50
60
(c)
0.10-0.1
La
titu
din
al G
rid P
oin
t
Standard Model Run
Wind Stress
La
titu
din
al G
rid P
oin
t
0
10
20
30
40
50
60
(a)Air-Sea Temperature
Differences
La
titu
din
al G
rid P
oin
t
0
10
20
30
40
50
60
(b)
10-1
1994
1971
1994
1971
1994
1971
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
2.0
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
uni
ts)
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
dard
ised
uni
ts)
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
ed G
rid L
ocat
ion
ofG
ulf
Str
eam
Year
Observed S
eparation Latitude
Observed SeparationLatitude
-2
-1
0
1
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
1
2
25.0
25.4
25.8
26.2
26.6
1950 1960 1970 1980 1990 200035.0
35.5
36.0
36.5
37.0
37.5
Year
Pre
dict
ed G
rid L
ocat
ion
of G
ulf
Str
eam
Year
Pre
dict
ed G
rid L
ocat
ion
of G
ulf
Str
eam Standard Model Run
Prediction From Average NAO Over Two Years
(a)
(b)
24.5
25.0
25.5
26.0
26.5
27.0
27.5
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
1.0
2.0
1965 1970 1975 1980 1985 1990 1995 2000
25.0
25.4
25.8
26.2
26.6
Obs
erve
d G
SN
W (
stan
dard
ised
uni
ts)
NA
O Index (standardised units)
Years
Years
Standard Model Run
(b)
Predicted
Observed
r = 0.76
Obs
erve
d G
SN
W (
stan
dard
ised
uni
ts) P
redicted Grid Location of G
ulf Stream
(a)
Annual NAO
Observed GSNW-2.0
-1.0
0.0
1.0
2.0
1965 1970 1975 1980 1985 1990 1995 2000-1.2
-0.8
-0.4
0.0
0.4
0.8
1.2r = 0.10
24.2
24.6
25.0
25.4
25.8
26.2
26.6
1965 1970 1975 1980 1985 1990 1995 2000
Predicted G
rid Location of Gulf S
tream
Predicted
Observed
r = 0.93
Pre
dict
ed G
rid L
ocat
ion
of G
ulf
Str
eam
Obs
erve
d G
SN
W (
stan
dard
ised
uni
ts)
Years
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
udin
al G
rid P
oint
60
50
40
30
20
10
0
1971
1994
(a)
0.010-0.01-0.02
Zonal Velocity
Latit
udin
al G
rid P
oint
60
50
40
30
20
10
0
(b)
1994
1971
Figure 5. Taylor et al. JGR
Gulf Stream excursion – Spectral Analysis