Interannual variability of Interannual variability of boundary fluxes and water boundary fluxes and water mass properties in the Gulf mass properties in the Gulf of Maine and on Georges of Maine and on Georges

BankBankPeter C. Smith, Robert W. HougPeter C. Smith, Robert W. Houghton, Richard G. Fairbanks, Dahton, Richard G. Fairbanks, Da

vid G. Mountianvid G. Mountian

1. Introduction—Earlier Research

Petrie and Drinkwater, 1993

Bigelow, 1927 fresh SSW, deep inflow of shelf water NECE;Local river runoff; heat fluxes The annual mean volumetric transport of SSW near

Cape Sable in winter was nearly double the mean but in summer was near the zero (Smiths, 1983)

the interannual variability of SSW was found influenced by atmospheric forcing and warm core rings offshore rather than St. Lawrence River runoff (Smiths, 1989b).

The peak deep inflow of slope water through Northeast Channel (NEC) occurred in late summer (Ramp et al, 1985), while in the deep basin in GOM, the downward mixing of cold intermediate water caused a maxima in May-June followed by gradually increasing T,S through December due to the advection of NCE inflows.

1 Introduction--Objective1 Introduction--Objective

to describe the interannual to describe the interannual variability of boundary fluxes and variability of boundary fluxes and water mass properties in GOM/GB water mass properties in GOM/GB on shorter terms of 1~3yr with on shorter terms of 1~3yr with reference to former observations, reference to former observations, and to investigate the sources of this and to investigate the sources of this variability. variability.

2 Data and Methods2 Data and Methods

Long-term (October 1993~September 1996) moored Long-term (October 1993~September 1996) moored measurements of current (surface, mid-depth, near measurements of current (surface, mid-depth, near bottom), temperature and salinity were made at Cape bottom), temperature and salinity were made at Cape Sable (C2), Northeast Channel East (NECE) and Sable (C2), Northeast Channel East (NECE) and Northeast Channel (NECW) sites. Northeast Channel (NECW) sites.

The current measurements were resolved into along-The current measurements were resolved into along-isobath and cross-isobath components and were isobath and cross-isobath components and were filtered to removed tides and other high-frequency filtered to removed tides and other high-frequency signals. All variables were averaged into months and signals. All variables were averaged into months and fit with a mean plus annual cycle. The local depth-fit with a mean plus annual cycle. The local depth-integrated monthly mean transport functions for integrated monthly mean transport functions for volume (q), fresh water (b) and temperature (f) are volume (q), fresh water (b) and temperature (f) are estimated using the monthly mean values of current estimated using the monthly mean values of current (U), salinity (S) and temperature (T) in appendix. (U), salinity (S) and temperature (T) in appendix. Missing data were interpolated from adjacent Missing data were interpolated from adjacent instruments with which correlations were typically instruments with which correlations were typically high. Finally the transport were interpolated into high. Finally the transport were interpolated into shallow (<75m) and deep (>75m) parts to reveal shallow (<75m) and deep (>75m) parts to reveal differences between the layers. differences between the layers.

2 Data and Methods2 Data and Methods

Hydrographic data includes GLOBEC broadscale surveHydrographic data includes GLOBEC broadscale survey cruises (Feb~Jul.1995, Jan~June 1996 and 1997) and fry cruises (Feb~Jul.1995, Jan~June 1996 and 1997) and from surveys conducted by Northeast fisheries Sciences om surveys conducted by Northeast fisheries Sciences Center (NEFSC). For each survey, the average surface lCenter (NEFSC). For each survey, the average surface layer temperature and salinity were calculated in each ayer temperature and salinity were calculated in each of four regions representing the three major basins (Wiof four regions representing the three major basins (Wilkinson, Jordan and Geoges) and Northeast quarter of lkinson, Jordan and Geoges) and Northeast quarter of Georges Bank.Georges Bank.

Coincident isotope and salinity samples were collected Coincident isotope and salinity samples were collected at standard depths on all GLOBEC broadscale surveysat standard depths on all GLOBEC broadscale surveys

Freshwater data were collected from BIO environmentFreshwater data were collected from BIO environmental data, NOAA data archive and the REDIMS distributeal data, NOAA data archive and the REDIMS distributed data archive for the Gulf of Maine. Local origin data id data archive for the Gulf of Maine. Local origin data included Boston precipitation (NOAA), Maine River discncluded Boston precipitation (NOAA), Maine River discharge (REDMIS), St. John River discharge (BIO) , RIVSharge (REDMIS), St. John River discharge (BIO) , RIVSUM (BIO), Gulf of St. Lawrence Ice cover (BIO), LabradUM (BIO), Gulf of St. Lawrence Ice cover (BIO), Labrador Shelf ice cover south of 55°N (BIO), and Station 27(47or Shelf ice cover south of 55°N (BIO), and Station 27(47°33’N,52°35’N) salinities at 0 and 50m.°33’N,52°35’N) salinities at 0 and 50m.

2. Data and Methods—GLOBEC long term moorings

2. Data and Methods—NESFC stations/Oxygen Isotope

3 Results—Annal cycle/NECE　 Axial an cross-channel currents

Early Spring

Late Summer, similar

surface opposed to bottom. the different governing processes of shallow and deep flows There is significant annual cycle in NECE cross-channel current Bottom Axial

Surface crossSurface Axial

Bottom Cross

3 Results—Ｃ 2 and NECE volumetric and freshwater transports

Early Winter

May~June , different

Early Winter

April, even earlier

C2 volume

C2 freshwater

NECE volume

NECE freshwater

C2 surface peak inflow peaks in early winter (not shown)

3 Results-- NECE surface/bottom volumetric and freshwater transports

Apr~MayJul, consistent

NECE surface volume

NECE surface freshwater

NECE bottom volume

NECE bottom freshwater

Surface volume consistent with V transport

surface freshwater transport reflect the entire freshwater transport

coherentcoherent Obscured winterObscured winter

Three eventsThree events a period of a period of

enhanced warm, enhanced warm, salty inflow at the salty inflow at the beginning (October beginning (October 1993- September 1993- September 1994)1994)

followed by a followed by a sharp reduction of sharp reduction of the inflow and the inflow and transition to cooler, transition to cooler, fresher than fresher than normal conditions normal conditions during the winter during the winter of 1994/1995of 1994/1995

and a second and a second period of greatly period of greatly reduced inflow and reduced inflow and cold, fresh cold, fresh conditions during conditions during the first nine the first nine months of 1996. months of 1996.

3 Results-- NECE deep layer(100m,150m,190m) inflow current, temperature, salinity

Inflow current

temperature

salinity

3 Results-- NECE deep layer(100m,150m,190m) T-S diagram

100m the water is a mixture of mid-depth SSW and Warm Slope Water (WSW)150m/190m the mixtures include some Labrador Slope water (LSW) and Gulf Stream (GS).

100m (diamonds), 150m(squares) and 190m (triangles)

66 clearer threclearer thre

e events near te events near the bottom at Nhe bottom at NECE site ECE site

an out-of-phan out-of-phase between dease between deep NECE and Cep NECE and C2 inflow transp2 inflow transports orts

the NECE suthe NECE surface transport rface transport transits into retransits into reduced flow earlduced flow earlier in 1994, the ier in 1994, the 1996 reduction1996 reductions are weaker, as are weaker, and 1995 featurend 1995 features are enhanced s are enhanced

3 Results-- The 5-month filtered transport C2/NECE shallow/Deep

C2 Transport

NECE shallow

NECE bottom

NECE In phase NECE In phase with volumetric with volumetric in surfacein surface

Out of phase in Out of phase in bottombottom

transport transport salinates the salinates the Gulf in the Gulf in the deep water deep water

||Net transport ||Net transport trend are trend are similar to similar to surface surface transport/Fresh transport/Fresh inflowinflow

3 Results-- The 5-month filtered freshwater volumetric transport C2/NECE shallow/Deep

C2 surface freshwater

NECE surface freshwater

NECE bottom freshwater

Positive Positive only in only in 1994 for 1994 for inner inner basinbasin freshwater freshwater reducedreduced

persist on persist on NWGB NWGB until early until early 1995 1995

overall overall period the period the water is water is fresher fresher during the during the 1990s 1990s

3 Results– Water properties in GOM/NESFC Surface salinity anomalies

Similar Similar events are events are detected in detected in the bottom the bottom water layers water layers of the Gulfof the Gulf

most years most years decreasedecrease

return-to-return-to-normal 1997normal 1997

Also found in Also found in BOFBOF

3 Results– Water properties in GOM/NESFC Deep-layer salinities

fluctuation of fluctuation of water propertiwater properties during the 1es during the 1990s is mainly 990s is mainly due to the varidue to the variable mixing raable mixing ratio between SStio between SSW and WSW rW and WSW rather than the ather than the effect from LSeffect from LSW as in 1960W as in 1960s’ cold events s’ cold events

3 Results– Water properties/T-S diagram in GOM (NESFC)

3 Results– Water properties/GLOBEC in NWGB

Similar Similar salinity salinity decrease decrease from from GLOBECGLOBEC

3.Results--Oxygen Isotope data3.Results--Oxygen Isotope data

The The freshwater freshwater invaded into invaded into Georges Bank Georges Bank is derived is derived largely from largely from Scotian Shelf Scotian Shelf WaterWater

is devoid is devoid Maine Coastal Maine Coastal Water Water (unclear)(unclear)

Suggesting Suggesting strong strong variations in variations in mixing ratio mixing ratio of SSW and of SSW and MCWMCW

EvidenceEvidence1:1:

Warm Warm core core observed observed from from satellitessatellites

Not a Not a reliable reliable indicator indicator of of enhanced enhanced inflow of inflow of slope slope waterwater

4 Discussions—Warm Core effects(Oct.1993~Sep.1994)

Evidence 2:Evidence 2: ContemporaneoContemporaneo

us hydrographic us hydrographic T/S sectionsT/S sections

Warm core streWarm core stream: 15°Cam: 15°C，， 35.535.5psupsu

transport throutransport throughout the deep ighout the deep inflow and mixenflow and mixed vertically into d vertically into surface surface

4 Discussions—Warm Core effects(Oct.1993~Sep.1994)

Using a simple Using a simple box model driven box model driven by observed by observed boundary fluxesboundary fluxes

in in 1995~1996, the 1995~1996, the volumetric flow volumetric flow rate increased rate increased by 17% of total by 17% of total transporttransport

increased increased freshwater freshwater inflow at C2 and inflow at C2 and NECE induced a NECE induced a decrease of decrease of 0.73psu in 0.73psu in salinity of the salinity of the outflow. outflow.

4 Discussions- Box model4 Discussions- Box model

4 Discussion—source for 1995~1997 4 Discussion—source for 1995~1997 freshwater inflows?freshwater inflows?

Local runoff not Local runoff not exceptionalexceptional

Nor is St. Nor is St. Lawrence River Lawrence River runoffrunoff

Ice-cover Ice-cover suggests suggests northern originnorthern origin

Newfoundland Newfoundland the most likely the most likely originorigin

Good correlation Good correlation between between

PositivePositiveC2/NECE/GB/BOFC2/NECE/GB/BOFNegativeNegativeSJ/Nfd/Scotian ShelfSJ/Nfd/Scotian Shelf

/NECE/C2/NECE/C2 8~9 month lag8~9 month lag

4 Discussions—Sources/ Salinity anomalies

GSA in 1980s and GSA in 1980s and 1990s are associated 1990s are associated with high NAO index.with high NAO index.

Icelandic low Icelandic low deepens and deepens and intensifies, producing intensifies, producing strong, cold wind strong, cold wind eventsevents

along with possible along with possible contribution of contribution of freshwater through freshwater through Canadian ArchipelagoCanadian Archipelago

Lead to cold, fresh Lead to cold, fresh surface waters surface waters transporting around transporting around

Effect of cold 1960sEffect of cold 1960sQuasi-decadal Quasi-decadal

fluctuationfluctuation

4 Discussions—Impact from climate aspect?

5 Conclusions5 Conclusions

(1)The annual cycles in NECE are found different for surface (with peak in spring) and deep inflow (with peak in later summer), which suggest different dynamic control factors for shallow and deep layers. In later winter there is a maximum near-surface cross-channel flow toward Georges Bank from NECE, which suggests a climatological “cross-over” tendency during that season.

(2)There is no clear annual cycle in the salinity variation over most of the water column.

(3) The inflow rates from C2 and deep NECE are generally out of phase, with increase C2 inflow and associated reduced inflow at deep NECE (C2), while both sites show cooler and fresher conditions. Freshwater inflow into GOM is largely negative over the first half of observation period with maximum transport near the surface. The hydrographic data in GOM and GB shows consistency with timing of the freshwater inflows, with peak salinity in 1994 and declined salinity in 1995 and 1996.

Oxygen data suggest that in 1997 freshwater on central cap of GB is from Scotian Shelf in 1997 as opposed to 1994 and 1995, when Maine river water contributed 38% and 26%.

(4) box model driven by observed boundary fluxes, the author finds in 1995~1996, the volumetric flow rate increased by 17% of total transport, and that the increased freshwater inflow at C2 and NECE induced a decrease of 0.73psu in salinity of the outflow.

(5) The origin of 1996~1997 freshwater is proven in the northern Labrador Sea/Baffin Bay and results from exceptionally cold winters in the early 1990s. Analysis of similar event in the early 1980s suggests that the occurrence is part of quasi-decadal climate signal that follows NAO.

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