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Physical Variability Atlantic Shelves, Coastal Areas
Variables
• Background, general circulation
• Sea Level
• Ocean temperature/salinity
• Stratification
• Sea Ice
Circulation over Continental Shelf
Transport of heat and freshwater by ocean currents have an important effect on Atlantic regional atmospheric and ocean climate.
Life in the sea is dependent on the biogeochemical status of the ocean and is influenced by the mean and variable physical state and circulation
General CirculationGeneral north to south currents with major impact on region
Built on hydrographic surveys, current meter/drift bottles/buoy data, …, plankton distributions, chemical tracers …modelling
Sea Level
Global mean sea level has been rising by ~2 mm/year (0.2 m/century)
Sea level change varies substantially from area to area on a global scale, rising more rapidly than the mean in some regions, falling in others.
Sea Level in Atlantic Canada
Relative sea level is measured, ocean surface relative to a reference point on land.
For many issues, coastal erosion, storm surges, … RSL is the important variable.
Changes to RSL due to:
Crustal loading – post glacial rebound at Halifax has the land sinking at ~0.23 m/century; rising in the northern Gulf of St. Lawrence
Absolute sea level changes – changing ocean properties (temperature, fresh water content), currents, atmospheric pressure, winds.
Observed Trends of Relative Sea Level: over the last 100 years
Observed Trends of Relative Sea Level: over the last 100 years
• Sea Levels are generally increasing, with Halifax typical of the region for trend and variability
Halifax Sea Level
0.7
0.8
0.9
1
1.1
1890 1910 1930 1950 1970 1990 2010
An
nu
al S
ea
Le
ve
l Ha
lifa
x
Post-Glacial Rebound
Annual MSLLinear fit 1920ff
Overall trend 1920-present, rise of 0.32 m/century
RSL exceeds post-glacial rebound by 0.08 m over 90 y.
Two periods, 1920-70 & 1970-present, exhibit different long term trends.
1920-70 0.4 m/century
1970-present 0.19 m/century (<PGR model result of 0.23 cm/century)
How widespread?
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
1880 1900 1920 1940 1960 1980 2000 2020
Se
a L
ev
el
(m)
Halifax Saint John St. John's Charlottetown Rimouski
Portland Boston New York Atlantic City
-0.25
0
0.25
0.5
NJ
NY RI
Ma
Me
St
J
Hfx
Ch
rltw
n
Rim
SJ
's
Se
a L
ev
el T
ren
d (
m/c
en
tury
) <1970 1970ff Diffrnce
Decreasing sea level trend 1970-present, centered on Scotian Shelf-Eastern Gulf of Maine, moderating to the south along US East Coast and to the northeast into the Gulf of St. Lawrence and onto the Newfoundland Shelf.
Projected sea level trends
Used 1920-2009 Halifax sea level stats (-PGR) to project sea level to 2099 in 100 simulations (+PGR);
Used range of IPCC projections to construct 2009-2099 sea level rise.
0.4 m
0.6
0.8
1
1.2
1.4
1.6
1.8
1900 1950 2000 2050 2100
Sea
Lev
el
Observed
Projected
Range IPCC projections
Ocean Temperature and Salinity
The oceans are warming globally by about 0.1oC (1961-2003), 0-700 m; there has been considerable interannual and interdecadal variability.
IPCC assessment shows 1961-2003 cooling trend for deep ocean areas Lab-Nfld region, warming trend for NS region.
Large scale trends of salinity are seen globally with freshening characterizing subpolar latitudes. Data are not good enough to assess global average changes.
Atlantic region: 1. SST data, model; 2. Btm T vs NAO; . 3. Depth-averaged T/S
Sea Surface Temperature
Longest east coast series
Blue 5 year running mean obs;
red 5y rmf model
Trend
Halifax none
St Andrews +1oC/century
-2
-1
0
1
2
1920 1940 1960 1980 2000
An
nu
al T
em
pe
ratu
re A
no
ma
ly Halifax SST
-2
-1
0
1
2
1920 1940 1960 1980 2000
An
nu
al T
em
pe
ratu
re A
no
ma
ly St. Andrews SST
100 year Air Temperature Trends 1909-20091
+1
0
-1
North ~ 0oC
South – rise of 1oC
1Iqaluit, Cartwright, Sable <100y
Can Air Temperature predict SST?
Year19851990199520002005A
ZM
P A
rea
BravoHudStrLabShlfHamBankStAnthonyNENfldShlfFlemPassHiberniaAvlnChnSEShoalGreenStPNEGSLNWGSLGSLEstuaryMagShlwCabotStrESSWstrnBnkCSSWSSLurcherGrgesBankBoF
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Year19851990199520002005
AZ
MP
Are
a
BravoHudStrLabShlfHamBankStAnthonyNENfldShlfFlemPassHiberniaAvlnChnSEShoalGreenStPNEGSLNWGSLGSLEstuaryMagShlwCabotStrESSWstrnBnkCSSWSSLurcherGrgesBankBoF
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Observed SST Stat Model SST=f(Air T)
-2
-1
0
1
2
-2 -1 0 1 2
Obs Annual T Anomaly
Pre
d A
nn
ua
l T A
no
ma
ly
Model captures main features but smoothes them;
Underestimates anomalies = 0.76*Obs
R2 = 0.76
Are predictions acceptable for periods other than 1985-2008?
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
1945 1955 1965 1975 1985 1995 2005
An
nu
al S
urf
ace
T A
no
mal
y
Station 27PredictedStat Model PeriodHindcast Period
Avalon Channel predicted SST versus Station 27 observations:
Predicted = 0.84*Observation, underestimates anomaly
R2 = 0.66, captures variability well
Other areas: CSS 0.55*Obs, R2 =0.44; Fundy 0.39*Obs, R2 = 0.40.
If Air T were to rise by 2oC
0 0.5 1 1.5 2
Bay of FundyGeorges BankLurcher Shoal
WSSCSS
Western BankESS
Cabot StrMagdalen Shlw
GSL EstuaryNW GSLNE GSL
Green-St. PierreFlemish Pass
SE ShoalHibernia
Avalon ChnNE Nfld Shelf
St. AnthonyHamilton Bank
Nain BankHudson Strait
Bravo
An
nu
al S
ST
An
om
aly
Representative Annual SST rise ~ 1oC based on air temperature projections for 2050
NAO = Sea Level Atmospheric Pressure Difference between the Azores and Iceland
Positive NAO leads to severe winters over the Labrador Sea, Shelf and Grand Banks
Negative NAO leads to mild winters over the Labrador Sea, Shelf and Grand Banks
NAO Variability: the dominant meteorological pattern driving North Atlantic ocean climate
NAO Variability: the dominant meteorological pattern driving North Atlantic ocean climate
Negative NAO → warmer than normal bottom temperatures over the Labrador-Newfoundland Shelf, the Gulf of St. Lawrence and the eastern Scotian Shelf;
and
colder than normal conditions over the central and western Scotian Shelf and Gulf of Maine.
The pattern is reversed under positive NAO forcing.
Temperature Anomaly
Impact on Continental Shelf
Bottom temperature is strongly related to NAO
NAO tendencies
Over last 100+ years, the NAO trend has been towards longer periods with the anomalies having the same sign; cold and warm periods tend to last longer.
Model forecasts indicate that the NAO anomalies will tend to be more positive in the future.
NAO influence is more direct on Newfoundland-Labrador than on the Scotian Shelf-Gulf of Maine
Depth-Averaged T/S
4
5
6
7
8
1920
-24
1925
-29
1930
-34
1935
-39
1940
-44
1945
-49
1950
-54
1955
-59
1960
-64
1965
-69
1970
-74
1975
-79
1980
-84
1985
-89
1990
-94
1995
-99
2000
-04
2005
-09
Ave
rag
e A
nn
ual
T 0
-90
m
Bay of Fundy, Prince 5
31.8
31.9
32
32.1
32.2
32.3
32.4
1920
-24
1925
-29
1930
-34
1935
-39
1940
-44
1945
-49
1950
-54
1955
-59
1960
-64
1965
-69
1970
-74
1975
-79
1980
-84
1985
-89
1990
-94
1995
-99
2000
-04
2005
-09
Ave
rag
e A
nn
ual
T 0
-90
m
Bay of Fundy, Prince 5
Bay of Fundy
Temperature shows trend towards higher values at 1.2oC/century
Salinity shows weak trend of -0.18/century, i.e. the Bay is becoming fresher.
Corresponds to 3 weeks of Saint John River inflow
Depth-Averaged T/S
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1950-54 1955-59 1960-64 1965-69 1970-74 1975-79 1980-84 1985-89 1990-94 1995-99 2000-04 2005-09
Ave
rag
e A
nn
ual
T 0
-175
m
Avalon Channel Stn 27
32.35
32.4
32.45
32.5
32.55
32.6
32.65
1950-54 1955-59 1960-64 1965-69 1970-74 1975-79 1980-84 1985-89 1990-94 1995-99 2000-04 2005-09
Avera
ge A
nn
ual S
0-1
75 m
Avalon Channel Stn 27
Avalon Channel Observations illustrate difficulty of discerning trend (~0 for T) against strong interannual variability.
Salinity shows a weak trend of -0.21/century; again not significant against background variability.
Depth-Averaged T/S
-1
-0.5
0
0.5
1
1.5
1950-59 1960-69 1970-79 1980-89 1990-99 2000-09
An
nu
al 0
-150
m T
Ham
ilto
n B
ank Months 6-10
4.8
5
5.2
5.4
5.6
5.8
6
6.2
6.4
1950-59 1960-69 1970-79 1980-89 1990-99 2000-09
An
nu
al 0
-50m
T M
agd
alen
Sh
llo
ws Months 5-11
5.5
6
6.5
7
7.5
8
1950-59 1960-69 1970-79 1980-89 1990-99 2000-09
An
nu
al 0
-250
m T
CS
S
All Months
Trends for southern Labrador Shelf and Gulf of St. Lawrence, and central Scotian Shelf are obscured by decadal variability.
8 of 16 areas from Hamilton Bank to Gulf of Maine 2000-09 warmest
Labrador Scotian Shelf
Gulf of St. Lawrence
Stratification
-0.020
-0.015
-0.010
-0.005
0.000
0.005
0.010
0.015
1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010
dr
/dz
(kg
m-3
)/m
Long-term trend of increasing stratification on Scotian Shelf;
Trend equals an increase of density difference from 0-50 m of 0.4 kg m-3
Biological Impact of Stratification
Production models suggest relationship between spring bloom and mixed layer depth; figure compares stratification, a proxy for MLZ, and CPR greenness for 5 year periods
CPR Greenness – measure of chlorophyll concentration from transects of eastern Scotian Shelf; sampling is monthly but in practice the average number of months sampled was 8/y
R2 = 0.94
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
-0.008 -0.006 -0.004 -0.002 0 0.002 0.004 0.006
StratificationA
nn
ual
CP
R G
ree
nn
ess
96-00
91-95
01-06
71-76 66-70
61-65
No data 1977-90
Other Areas
-0.02
-0.015
-0.01
-0.005
0
0.005
0.01
0.015
0.02
1950 1960 1970 1980 1990 2000 2010
Str
atif
icat
ion
HamBnk Stn 27 Magdln Grgs Bsn
From 1950 to 2009, trends represent change of -0.17 for Hamilton Bank, +0.18 for Stn 27 Avalon Chn, +0.33 for Magdalen Shallows, +0.49 for Georges Basin
CausesSite Ratio S/T Difference
accounting for ρ(50 m)-ρ(0 m)
Series accounting for most ρ(50 m)-ρ(0 m) variance
Hamilton
Bank 3.2
Salinity 0 m
Avalon Channel
1.1 Salinity 0 m
Magdalen Shallows
2.4 Salinity 0 m
Misaine Bank
2.6
Salinity 0 m
Emerald Basin
2.9 Salinity 0 m
Roseway Basin
1.1
Temperature 0 m
Georges Basin
1.6 Temperature 0 m
Salinity (Freshwater) has had the largest impact in the region
Model projections indicate decreased freshwater inflow from St. Lawrence
Ice VolumeNfld Labrador
0
100
200
300
400
500
600
700
1950-54 1955-59 1960-64 1965-69 1970-74 1975-79 1980-84 1985-89 1990-94 1995-99 2000-04 2005-09
Av
era
ge
De
c-J
un
e Ic
e V
olu
me
45 - 55oN
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1950-54 1955-59 1960-64 1965-69 1970-74 1975-79 1980-84 1985-89 1990-94 1995-99 2000-04 2005-09
Av
era
ge
An
nu
al T
0-1
75
m
Avalon Channel Stn 27
0
5
10
15
20
25
30
35
40
1950-54 1955-59 1960-64 1965-69 1970-74 1975-79 1980-84 1985-89 1990-94 1995-99 2000-04 2005-09
Av
era
ge
CIL
are
a
Bonavista Section T<0oC
Ice volume shows a weak decreasing trend (270 km3/century);
ice volume is highly correlated with ocean temperature (r2 = 0.74, T increasing)
and
amount of subzero water (CIL) over northeast Nfld Shelf (r2 = 0.61, CIL area decreasing).
Summary
• Sea level increasing but since 1970 at ~PGR rate; coherent regional response
• Long (90 y) record indicates +1oC/century at St. Andrews; upper 90 m warming at same rate and freshening
• Air temperature in southern half of region rising for last century (~1oC); shelf SST can be constructed and hindcast well.
• Btm T responds coherently to NAO forcing with Nfld-Lab-GSL opposite to SSGoM
• Pentadal, decadal T/S are dominated by interannual variability but 8 of 16 areas show 2000-09 warmest.
• Stratification shows increasing trends south of Labrador with salinity dominating.
• Projected trends in forcing- air T increasing, (precipitation-evaporation)/runoff decreasing, NAO increasing