Decadalscale hydrographic and circulation variability in the ScotianMaine region

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Decadal-scale hydrographic and circulation
variability in the Scotian-Maine region

• John W. Loder, Jennifer A.Shore, Charles G.
  Hannah, Brian D. Petrie

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1 Introduction--Domain
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1 IntroductionFormer research

• Increasingly recognized interannual and decadal
  variability in the upper ocean salinity and
  temperature in the northwest Atlantic
• ? atmospheric-ocean coupling
• ? ocean circulation contribution to this
  variability (major)
• ? spatial patterns in this variability are found
  to be apparently related to the competing
  influences of the subpolar Labrador Current and
  subtropical Gulf Stream.

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1 Introduction1960s cooling and refreshing events

• in the Scotian shelf and Gulf of Maine
• range of up to 4.6C for temperature and 0.7 for
  salinity (Petrie et al, 1993)
• Cold anomalies extended into Middle Atlantic
  Bight (MAB) ? southwestward geostrophic flow from
  along the Scotian Shelf edge (Marsh et al, 1999).

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1 IntroductionMotivations and Objectives

• The motivations
• ? the former research of the description and
  origin of decadal-scale variability in the
  Scotian-Maine region
• ? its implications for populations on Georges
  Bank and the Scotian Shelf where US/Canada GLOBEC
  (Global Ocean Ecosystem Dynamics) programs are
  conducting intensive studies.
• The objectives
• Main describe spatial structure of 1960s decadal
  variability and associated changes in seasonal
  circulation with particular focus on Georges
  Bank.
• The secondary objectives are to understand the
  origin of this variation and to identify other
  occurrences of such variability.

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2 Data and MethodsHistorical data

• The primary data
• ?historical data of ocean temperature and
  salinity assembled at Bedford Institute of
  Oceanography and from some datasets used in
  precursor studies.
• averaged into monthly mean
• subtracting the long-term means
• ?annual anomalies are obtained from this
  averaged monthly mean.
• Ancillary datasets
• ?sea surface temperature (SST) at Boothbay
  Harbor at Halifax
• air temperature at Halifax
• ?winter time atmospheric pressure difference
  between Iceland and Azores (NAO index)
• ?freshwater runoff into the Gulf of St.
  Lawrence
• ?wind stress at Sable Island and geostrophic
  ocean transport (under the assumption of
  geostrophic balance) around the Grand Bank.
• ?To examine the spatial structure and
  circulation anomaly in 1960s, 3-d seasonal mean
  temperature, salinity and density fields were
  estimated for two groups of yearsthe cold
  1960s (19591967) and the warm
  1970s(19721981)
• temporal grid points at 15 February,15 May,
  15 August for winter, spring and summer
  separately.

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2 Data and MethodsNumerical Models
QUODDY4 prognostic circulation model a false
bottom at 1200m a variable horizontal grid
spacing from order 2km over steep shelf areas to
30km in the deep ocean The initial velocity ?
FUNDY 5 harmonic model results initial
temperature, salinity and baroclinic pressure
gradients ? observational fields.
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3 Results Long term temporal variability
Increase 1C
To negative
Decrease
Decrease 2C
Decrease
This indicated that the characterized variability
in the Emerald Basin since the 1950s occurred
over a broad area of northwestern Atlantic shelf,
from the deepest waters of Cabot Strait to the
MAB slope. Similarly, in the 1930s this trend is
found in the longer time series from Halifax and
Boothbay Harbor in the late 1930s and early 1940s.
Decrease 4C
Late 1950s
Late 1960s
Similar
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3 Results--The comparison between NAO index for
larger scale atmospheric forcing and other
parameters

• Positive NAO (low temperature north and high
  temperature south )
• southwestwards strong winds
• increasing geostrophic transport of the Labrador
  Current on the eastern slope and tail of Grand
  Bank
• ?increased Labrador Current water was an
  important factor in the origin of the cold 1960s
  in the Scotian-Maine region
• ?the decadal hydrographic changes in the
  Scotia-Maine region are associated with changes
  in the North Atlantic subpolar gyres equatorward
  penetration

Air Temperature
Slope
Grand Bank Tail
Outflows??
Upper/not bottom
SST
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3 Results --volume-averaged hydrographic changes/
summary
gt
0.3

• the broad decadal variability showed below normal
  temperatures and salinities around 1940 and
  1960(consistent with increased transport of
  Labrador Current), and a rapid change to
  above-normal values around 1970s that have
  generally persisted into the 1990s.
• The variability was proven to be more influenced
  by circulation rather than via broad-scale
  air-sea coupling (Marsh et al, 1999)

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3 ResultsSpatial Structure/Winter
The results are consistent with the concept of
advective origin of bottom water from the shelf
slopethe slope water penetrated into the
near-bottom at depth and then mixed or upwelled
throughout the region, and eventually detached
from the slope edge southwestward and offshore
during the cold period.
1psu
4C
Widespread, 4C, 1psu advective origin of
bottom water from the shelf slope (detached)
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3 ResultsSpatial Structure/Summer

• Reduced Magnitude in surface but persistent

• The 3-d winter and summer fields indicate the
  year-round persistence of the 1960s hydrographic
  anomalies and also point to winter as the primary
  origin season

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3 Results-- Geostrophic transports during the
cold 1960s and warm 1970s

• The initial estimates of geostrophic flows in
  Southern Georges Bank and Halifax Section show
  increase in the transport of the same-order of
  magnitude by 1.22.0Sv
• Some model results also suggest barotropic
  transport of order 1Sv over the upper Scotian
  slope that isnt included in the geostrophic
  estimates.

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3 Results-- Circulation changes in the cold 1960s
3C
0.5psu

• Emerald Basin section shows reductions of lower
  layers temperature and salinity by over 3C and
  0.5psu and limited changes in density
• strengthening, deepening and broadening prominent
  southwestward current

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3 Results-Net transports from Model fields
Shelf Edge

• Limited variation from those away from edge

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3 Results--The depth-integrated transport
streamlines
Inner shelf

• The depth-integrated transport streamlines for
  the cold and warm periods show overall increase
  in the throughflow
• especially along the shelf edge and on the inner
  shelf (largest increase 1Sv)

Shelf edge
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3 Results--NEP section

• The vertical section from Georges Bank across
  Northeast Channel and Browns Bank to Nova Scotia
  coast in winter show similar decrease of
  temperature and salinity in the 1960s and 1970s
• at depth in 1960s the changes is the most
  striking
• outflow on its western side and inflow on its
  eastern side? depth-integrated flow are
  representative of most of the water column on the
  shelf

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3 Results--The Southern Georges Bank (SGB)
section

• shows similar structure for the 1960s and 1970s.
• in the upper slope at depth of 100m200m, we find
  large change (5C, 1psu)in temperature and
  salinity that is close to Halifax section in the
  cold period.
• The associated velocity show a broad
  southwestward flow(1.5Sv from the Tab 4) over the
  entire southern flank, a surface intensified
  shelf-edge jet until the 60m isobath.

5C
1psu
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3 ResultsThe spring vertical-integrated flow

• show overall similarity to winter fields
  (decrease in magnitude as GB), except for the
  reduced southwestward flow due to the less slope
  bottom isopycnal over the upper slope.

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3 ResultsModel result summary

• All the above results clearly point to the large
  and widespread changes in the magnitude of the
  predominant shelf-edge current.
• The comparison between geostrophic and model
  transport suggest that there may also be some
  combination of significant local adjustments in
  the density fields.

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3 ResultsInterpolations and Implications

• The temperature and circulation changes can also
  induce significant biological variability.
• ?temperature, transport of the organisms,
  chemical property
• ?less direct influence from density, mixing,
  stratification, circulation away from edge (might
  be related to interannual variability )

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4 Conclusions

• In this paper the decadal-scale
  hydrographic regime-shift events and associated
  circulation changes are carefully examined via
  using historical data, geostrophic computation
  and numeric circulation models.
• (1) The ocean temperature and salinity show
  multi-year periods of cooler and fresher
  conditions over the shelf and slope in 1940s and
  1960s, and this event is consistent with the
  increased Labrador Current transport.
• (2) The hydrographic computations show detailed
  structure of the hydrographic anomalies,
  including the largest magnitudes in winter at
  depth along the shelf edge and extend to the
  basin, and over the continental slope region at
  the surface.
• (3) The results from the numeric models indicate
  that there is predominant increase in the
  shelf-edge southwestward flow by 12Sv
  (comparable to mean value) in the cold 1960s
  compared to the warm 1970s. However, for those
  regions faraway from the shelf, there is limited
  decadal variability in the major shelf
  circulation features.
• (4) The relation between the Scotia-Maine decadal
  variability and larger-scale variability in the
  northwestern Atlantic and its potential
  implications for ecosystems are also discussed.

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• Thanks !