Recent Variability in Western Boundary Currents on the Atlantic Slope from Moored Measurements and Altimetry

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1 Fisheries and Oceans Canada Pêches et Océans Canada Canada Recent Variability in Western Boundary Currents on the Atlantic Slope from Moored Measurements and Altimetry John Loder 1, Yuri Geshelin 1, Igor Yashayaev 1, Guoqi Han 2, Miguel M. Maqueda 3, Shane Elipot 3 and Chris Hughes 3 1 Fisheries and Oceans Canada (DFO), Bedford Institute of Oceanography 2 Fisheries and Oceans Canada, Northwest Atlantic Fisheries Centre 3 National Oceanography Centre (NOC), Liverpool, United Kingdom Focus on: Labrador Current and Deep Western Boundary Current Drawing on: - Projects jointly supported by DFO, PERD, CSA and industry - DFO s Atlantic Zone Off-Shelf Monitoring Program (AZOMP) - Collaborative project with NOC as part of the UK NERC Rapid Climate Change Program (RAPID) Thanks to all those involved in these field programs! Large-Scale Upper-Ocean Temperature and Circulation in the NW Atlantic Temperature at 50m Schematic of Mean Circulation Labrador Current Gulf Stream Predominant influence of the subpolar and subtropical gyres and their western boundary currents the Labrador Current and Gulf Stream Mid-Latitude Confluence or Transition Zone I. Yashayaev (BIO) 1

2 Transports (Sv) from Moored Measurements & Geostrophic Estimates (Loder et al 1998) 30 NADW /NABW ANNUAL MEAN TRANSPORT (Sv) Deep Labrador Current NADW/LSW Gulf Stream Large (& Variable) Diminution of Labrador Current Transport West of Grand Bank Strong Influence on Oceanography of Scotian Shelf/Slope & Gulf of Maine Large-Scale 3-D Circulation in the North Atlantic: Atlantic Meridional Overturning Circulation (AMOC) Schematic of AMOC Deep Western Boundary Current (DWBC) Upper-Ocean Circulation contributes to the AMOC Multiple Branches of the DWBC LSW Labrador Sea Water DSOW Denmark Strait Overflow Water ISOW Iceland-Scotland Overflow Water NEADW Northeast Atlantic Deep Water I. Yashayaev (BIO) 2

3 DFO-BIO Offshore Moored Measurement Sites, Supported by - PERD, PERD & Industry, CCTII Frontier Gas, DFO IGS, DFO - National Oceanography Centre (UK) Past & Present BIO Moorings Planned 2012 Makkovik Bank (PERD): BIO/Kiel Labrador Slope : * + Existing German (Kiel or Bremen) Orphan Basin PERD/Industry Orphan Knoll: Flemish Pass PERD/Industry Laurentian Channel/Fan: PERD/Industry CCTII PERD Scotian Slope/Rise PERD/Industry PERD/RAPID RAPID * * Presently deployed Upper-Ocean (0-200m) Transport Anomalies from Altimetry, between 200 & 3000m isobaths (except m on Scotian Slope) Different decadal-scale variability in NL and SS regions - Weakening of the Labrador Current since early 1990s (declining NAO) - Intermittent periods of stronger flow on Scotian Slope Lab Slope AR7W Orphan Basin C Scotian Slope G. Han (NAFC) W Scotian Slope

4 DFO Moored Measurements on Labrador Slope (AR7W, AZOMP) 1992 Seasonal & Vertical Variations on 1000-m Isobath Near-Bottom Variation on 1000-m Isobath from 1995 to 2011 Temperature Trend 0.05 o C/yr Near-bottom maxima on upper slope (LC) and near 3000m isobath (DWBC) Along-Slope Velocity German Moored Measurements on the Southern Labrador Slope (53 o N): Along-Slope Currents and Transports Near-bottom maxima on upper slope (LC) & near 3000m isobath (DWBC) Temperature 4

5 Orphan Basin (PERD/Industry ) Mooring Sites & CTD Section 2006 Mooring Sites & CTD Sections 2009 Moored Arrays Orphan Basin Annual-Mean Currents: June May 2010 Upper Water Column Mean velocities generally equatorward along isobaths: - LC & DWBC Magnitudes of m/s Mid Depths Lower Water Column Upper/mid depths: Strongest flows at A-C: m(?) isobaths - Labrador Current Near-bottom: :Strongest flows in mid-basin (C-F): m isobaths - Deep Western Boundary Current 5

6 Flemish Pass Seasonal-Mean Currents by Depth DEPTH 50 M 100 M 200 M LATITUDE NEWFOUNDLAND FLEMISH CAP 300 M 500 M 800 M 1000 M 1500 M 2000 M 2500 M 3000 M 3500 M GRAND BANK A B LONGITUDE FP-A (400m): Core of Labrador Current FP-B (1100m): Middle of Flemish Pass Variability of Seasonal-Mean Along-Shelf Flow Stronger on 400-m isobath and in Fall-Winter Moored Current Measurements on Laurentian Fan : LC: 400m , : Annual Mean Velocities LF-A: 1100m LF-B: 2000m 25m above bottom 500m below surface 20cm/s LC LF-A LF-B Earlier Deployment Sites Persistent equatorward near-bottom flow at LF-A, but weak and variable at LF-B 6

7 Extended Halifax Line (PERD/Industry) and (RAPID) (3 sites): Tall moorings (+) on 300-, 1100-, 2000-m isobaths C A + PERD (6 sites): Short moorings (x) on 1100-, 1600-, 2300, 2700-, & 3900-m isobaths B LC Extension and DWBC Seasonal-Mean Along-Slope Velocities from Moorings By Site and Instrument Depth C Persistent SWward flow below 500m at A & B, with barotropic component of 1-4 cm/s Disruptions of SWward flow above 500m at A & B during springsummer 02 & summer-fall 03 A B Mix of Seasonal and Interannual Variability 7

8 Along-Slope Transports & Gulf Stream Influences Disruptions of equatorward transport in Feb-Jun 2002 and Aug-Oct 2003 Feb- Jun 02 Aug- Oct 03 Persistent N excursion of Shelf/Slope front and anomalously warm SST in winter-spring 2002 Persistent N excursion of Shelf/Slope front and anomalouslywarm SST in Jul-Oct 03: possible influences on Hurricane Juan? Gulf Stream variability causes disruptions of equatorward flow of subpolar water Variability of Near-Bottom Temperature and Along-SlopeVelocity at m Temperature Velocity positive equatorward Increasing T at all sites with differing seasonal cycles; No trend in V Increasing T downstream; Large decrease in V from LS/OB to LF/SS 8

9 Moored Measurements on Extended Halifax ( RAPID Scotia) Line as part of the RAPID Western Atlantic Variability Experiment (WAVE): BPR data & density/velocity data return Objective of RAPID WAVE: To monitor the zonally-integrated AMOC overturning transport through an approximate and efficient geostrophic-based measurement method, involving bottom pressure estimates on the western boundary relative to those on the eastern boundary, obtained from a combination of moored temperature, salinity, pressure and velocity sensors. Complementing the more detailed RAPID/MOCHA AMOC measurement program at 26.5 o N RAPID/MOCHA Array at 26.5N RAPID WAVE Estimates of the AMOC from Oct 2008 to Dec 2010 Cumulative transports (Sv) relative to 1000m with layer increments Total transport between approx. 1000m and 3800m from western boundary measurements - mean southward transport of 13 Sv - variability on time scales of months to years - decreasing magnitude during short measurement period but origin and -20 implications unclear Elipot, Hughes et al 9

10 Summary Points Large diminution in Labrador Current transport west of Grand Bank but spatial and temporal variability difficult to quantify from observations Weaker downstream increase in bottom temperature at 1000m apparent in moored measurements Increasing bottom temperature at 1000m since 1995 on Lab Slope and since early 2000s on Scotian Slope - but see Yashayaev et al poster and Loder et al talk re- warmer earlier No indication in moored measurements of trend in along-slope current at 1000m Arrays for Deep Western Boundary Current in place: - since 1997 by Germans at 53N (Lab Sea exit), but no detectable trend - since 2008 by UK/BIO (RAPID) on Scotian Rise, but record too short for trend Need for circulation models with adequate resolution of interaction of Labrador Current and DWBC with Gulf Stream to complement moored and altimetry observations 10