- not to be cited without prior reference to the authors .
"
,.
ICES 'CM 1996/0:9
Theme Session 0
nc661ing
of
the Northern
A.lexander Sy and Klaus Peter Koltermarin
für
seeschirrabrt und Hydrographie
, Bundesamt
Bernhard-Nocht-Str. 78, D-20359 I1amburg, Germany
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Tel:
Fax:
email:
+40 - 3190 3430
+40,-: 3190 5000
Sy@Hamburg.BSH.d400.de
Abstract
0.
Seyerai rcpetiÜonsöf transoceanic sections ,\.i/AR' and A2/Aiü9 froni i991 ihroügli
1996 reycal significant and rapid' changes of interniediate ,waterrriass property
characteristics. A cascade of new modes of Labrador Sea 'Vater (LS\V) is traced
throughout the ,riorthcrn North AtIaritic after aseries of increased deep \"iritertime
con\'ection events started in 1986, that renewed the intermediate watcr mass in the
Labrador Sea~ TIl(~, arrival of new LS\V. vintages in other paris of the North Atlantic is
illarked by a significant cooling of the locai LS\V core as weil as its deepcning and its
densification. Thc trayel time from thc SOUfCe region to the Eriropean continental slope
is esÜlriated to be 7 - 8 ~'ears, )'ielding a srirprisingly high meari speed of some 1 em/s.
This outstariding eyent, as,. a eonehisiye _, evidence for ocean c1imatt~, yariability
conipa~able to the ;Great Salinity Ariomaly' of the mid 70s (Dicksön et aI., 1988), is
curreittly in progress. Chäriges reported at 36~ N arid 24° N sho,v a consistent pattern
with mir observations. 'Ve conclude that coolirig, and also ."·arming episodes produce
T/S anomalies that seeni to follow to distinctly differe#t pathways: one cj'c1oiiic aroulid
the subpolar gyre arid a secorid confined to the North Atlantic and mainly the
subtropical gyre. Thc t"·o path\vays have a distinctly different impact on its meridional
oyerturriing circulation.
'
Convectively vertical mixing 'of the meridi,onal overturmng cell of the no~hern North
Atlantic Oceari transforrns wami, salty arid light surface water to cold, fresh and dense
intermediate and deep ,vaters. In the Labrador Sea,. convective overturning in late winter
cn::ates the relatively homogeneous Labrador Sea 'Vater (LSW) with its unique
characteristics of low salinity, high oxygen content, low potential vorticity. and high
anthropogenie tracer concentrations, which ean be traced at depths oetween 1000 m and
2000 m as it spreads to other parts of the, ocean '(Fig. 1). Three pdmary pathways a\vay
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from the fonnation region are distingiIishable (Talley and McCartney, 1982; Schmitz and
McCartney, 1993) (Fig. 2): (i), southward flow.as part of the deep \Vestem bouridary
current, (DWBC), (ii) eastward flow near 50 0 N laiitiIde (beneath or parallel to the
North, Atlantic Ctirrent) with several bifurC,ltiorls east of the Mid-Atlantic Ridge (MAR),
and (iii) riorth-eastward flow into the Irrninger Basin. As the dorriinating fraction. of the
Upper North Atlantic Deep \Vater, LS\V is orie of ihe main water masses of the North
AtlanHc, arid depending, on the. intensity of its fonnatiori process it is one of the
controlling sources ,a(fecting the North Atlantic meridional overnirning system (Schmiti
and McCartney, 1993; \Veisse et a1., 1994;'Rahrrisiorf and \Villebrarid; 1995).
The formation of LS\V is _not a process of constant interisity nor of constant
characteristics every winter. Hydrographic time serh~s colIected in the central Labrador
Sea reveal that deep convectiori occurs discontinuously on a decadal' timescale (Lazier, .
1996). Only two periods of intensified LS\V production \v~re identified dudng the, last
30 years, from 1972 to 1976 with elS chanictefistics in the range of 2.9 °C_ 3.2 °Cand
34.83 - 34.85 and approxiinateiy .from i986 onward. It appears that the discontinuous
LSW fonnation record is coritrolIed by the combined, remote response to the cotipled
cells of the North Atlaniic Oscillation (NAO) ,with the effed, that the evolution of
,convedive activity in the Labrador and Greenland Seas during this centlu)t was in
.
opposite phase (Dickson et a1., 1996).
•
The first indication for the begiiming of the recent convection pei:iod came from
chlorofluorocarbon (CFC) measurements in the south central Labrador Sea in July 1986
(\Vallace and Lazier, 1988), although at one station only. The follO\ving years corifiriried
this observation when Üi\V was' found to be significantly cooler and deriser (eiS in the
range of 2.6 oe - 3.0 oe and 34.82 - 34.84) in the entire central Labrador Sea Ü..azier,
1996). Finally, ,the 1990s vintages are the coldest, densest and freshest ever observed in
the' Labrador Sea (P. Rhines, pers. comm). The first appeanince of recently renewed
LSW outside its source region \VaS observed 1991 in the subpolar North Atlaritic by
comparison with historIcal data (Read and Gould; 199i). Age estimates of LS\V vintages
encountered in the Northeast Atlantic, however, varied in ages" over 15 years, Le.
spanning the vintages from 1972 ta 1986 in 1991 (Top et a1., 1987; Read an~ Gould,
1992; Cuiuiingham and Haine; 1995).
As a, contribiition _to the \Vorld Ocean Circulatiori Experiment (WOCE) _we \vorked
section Al/AR7 in'1991; 1992, 1994 and 1995 and section 'A2/AR19 in 1993, 1994 arid
1996 (Fig. 2). Ta trace nie temporal change of LSW properties we ,calculaied its
differences~ at the•.uniquely defiried salinity minimum of the LS\V core (Fig.' 3) which . ' . ,
represents. the 'purest' Üi\V in our data. This waier-mass-follO\ving method is insensitive
to depih or density variations of ihe LS\V vintages and thus has not the disadvantage of
differences calculated from gridded section data (Fig. 4).
Thecomparison of LSW core temperatures along section AlIEast reveals a cascade-Üke
cooling in the Inninger Sea (" 1986-LSW cascade") of the order of -0.28 oe in 4 years
(Fig. 5a, Tab. 1); with the annual arrival of a new LS\V mode. Cooling in the Iceland
Basiri of -0.19 °c and in the Rockall Trough area of -0.11 oe also indicates renewed
Üiw. Although \.ve firid a cascade-like signal in the Iceland Basin, in the Rockall Trough
area, ho\.vever" ihe .only signal of rene\ved LSW appears in the 1994 data, indicative of
2
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the arrival of the 1986-LS\V cascade at the European continental sIope. SubstantiaI
cooling results in an inerease of density (Fig. 5d) anel in deepening of the core iayer
· (Fig. 5c), which are greatest in the Inninger Sea (0.018 kg/m 3 and 240 dbar). Since there
· is. no sigriifieant signal in LSW core salinity (Fig. 5b) a compensaiion of density cannot
take pIaee. A striking but not sui-prising featUre (Fig. 5a, b) is the separation of the LSW
core läyer by the Reykjanes Ridge (at 31 O\V) into two differerit hydrographie regimes,
the Irminger Sea, and the IceIand Basin, reflecting the mixing aIC?ng the Ionger pathway
irito the eastern basins rind enlüinced mixirig over topography. Also note' that the area
between 25°\V and 27°\V is marked, bythe ,lowest LS\V core temperatures , and salinities
observed in the eastern basiris of the North Atlantic: äfter crossing the I\iAR, a branch
· of LS\V flows at this Iatitude into the Icelarid Basin (Fig. 2) as part of the Subpolar
Gyre circulation.
•
The characterisiic property ch·anges of the 1986:LSW cascade are co~ling, densification,
deepening arid only minute or no freshening (Tab. 1). These n:sults corresporid weil with'
observations in the central Labrador Sea (Fig. 6). From 1988 to i993 the LS\V formation is charaeterized by a tempeniture deerease of the order' of -004 °c, without
significani salinity changes arid aeeompanied bya density inerease of aboiit O.O~ kg/m 3
(Lazier, 1996). This is in contrast to the properties generated dtifing the preceding
eonveetion period (1972 - 1976), \vhere strong cooling was aecompanied bya substantial
freshening of the order of 0.08 psu. and consequeritly only very small densifieation.
Comparing eiS vaiues for Ls\V from the central Labrador Sea with those ,f~om the
adjoinirig Irminger Sea (Fig. 6), the Labrador Sea observations ,are grotiped in tight
clusters representing the homogeneous 6/S signature of each LSW. viritage; \vhereas the
Irminger Sea vaIues are more scattered arid generally higher. This derrionstrates nie
influence of rriixing along the spn:iading path. Ho\vever, ail data are in the same density
range and in botl1 bäsins they show the same discreet increase in densitY. As we found
no indications of deep .convection hi. the Inninger. Sea 'we believe this layer is a direct
iritrusion of LS\V which spread ,from the source region alorig its characteristic, isopycriaIs.
The aIignment of core values ~llong isopycnaIs suggests that the arinual intrusion of LSW
in the Irminger Sea originated in t11e Labrador Sea during the conveetiori season in
March of the same or the previous year. Qnly the 1993 vintage in the Labrador Sea data
has rio counterpart in t11e irriiinger Sea, because there no observations are. available.
Also, due, to a lack of appropriate Labrador Sea data iri 1991 it is not clear \yhether the
1990 or the missing 1991 yintage would have a counterpart in the 1991 )nninger Sea
data. FinaIly" for the 1995, Inniriger Sea cluster a eoimterpart in the Labrador Sea
cannot be assigned at this time.
. ,.
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\Ve conCIude that the intrusive events observed in the Inninger Sea are a resliIt of
convection in the Labrador Sea of the same year resulting in a maximum tnivel time of
6 months. The alternative of a maxirImm travel time of 18 months, hO\vever; is not
supported by CFC observations (M. Rhein, pers. comrn.). Convection in the Labrador
Sea takes pIaee, based ·ori direct observations and large scale surveys, in the ,vestern
eentral, region (Lazier, 1996). This is abotit 700 km from where the \vater was observed
in the Irminger Sea some 6 months after eonvection; yielding a mean speed of 4.5 em/s.
Our firiding, thai intermediate ,waters move rapidly from the Labrador to the, Irminger
Sea suggests these regions to be considered as a single entity in discussions of the roIe
of LS\V as source of intefrriediate water to other· ~egions of the No'rth Ätliiiiiic:·
3
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New LS\V arrives in the eastem basins only years later because of the longer pathways.
Adopting the circuhition scheine proposed by Talley and McCartney (1982) and by
Sclunitz and McCaftney (1993). the LS\V in the Iceland Basin should be of more recent
origin than thai at the eastem bouridary ~ In aÜ our reeords ,ve find the freshest LSW at'
26/27~ \V and a signifieant erosion of the core' saliriity east\varrl of that area. (Fig. 5b).
This doeuments th6 length of the path froin the source. Thtis; we assurne that the LS\V
observed iri the area south of the Rockall Trough in Deeember 1994 manifests the
arrival of the 1986-LS\V ensea,de. beeatise in 1991 imd 1992 no' signifie:mt property
changes. were detectabl~. In eontrast; we ,observed signifieant ehanges in the Iceland
Basin \vhere new LS\V had arrived earlier. For the events observed east of the
Reykjanes Ridge, we eonclude thai the new LS\V took 7 to 8 years to propagate from
its souree, region to the eastem boundary.· With an assumed lerigth of the pathway. of
2500 kin this is equivaleni to a mean speed 'of about 1 crri/s. at least 2 times faster than
assumed until now (Read and Gould; 1992; Cunningham and Haine. 1995). Dur result
is consisterit \vith the iridependent estimate derived from CFC measurements only aiong
section Al in 1991 arid 1994 (M. Rheiri. pers. coinin.) .
•
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These observations are supported by results from our southe'm sectiori (A2 at 48 ° N).
whieh we supplemented by the 1982 "Hudsori". section as a reference for ,a period, of
'normal' LSW produetion. Along the entire seeHon we find a dramatic eoolirig ofLS\V
between 1982 and 1993 (Fig. 7). with the most prominent anomalies west of the MAR
amounting to -0.45 °C (-0.15 °C east of theMAR). AssoCiated with eooling ,we find a
densificatiori by 0.043 kg/m 2 arida deepening by more than 600 dbar for only the
,v.estern basin (Table 1). In contrast., saliriity remains almost unchanged west of the
MAR but was fourid to be signifieantly fresher (by about -0.03) in the eastem bäsin.
Similar to the ncirthem seetiori. the eooling continues. iridicative for the passage of the
1986-LS\V caseade as weIl. Between 1993 arid 1996 the temperattire decreased in the
west by another -0.16 °C and. in the east by about -0.14 °C: the ne\vly forrned LS\V has
already invaded the eentral \Vest-European Basin and the ventilation of the LSW layer
is eurrently hi progress.
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The attempi to assign the 8/S ccire values observed 1993 and 1996 iri the Newfoundland
Basiri to an equivalerit couriterpart iri ttie Labrador Sea;, hc)\vever. does not lead to a
similar clear correlation as was the case for the Inninger Sea (Fig. 8). That ean be due
to the more effeetive mixing along the mueh longer path\vay. In 1993 we observed a
very patchy eore layer which suggest the adveetion of LSW iri tenns of smaller seale
billows (Fig. 9). These are more susceptible to transfonnation by mixing than a largeseaie uniform l3.yer.
'
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a
Between 199,3 arid 1996 we observe
temperature drop in t116 narrmv band of the
DWBC some 600 m shaliower than the deep LsW cores further east. This eannot be
assigried to an equivalent signal in the Labrador Sea (Fig. 8). Following Pickart (1992)
\ve suggest, the. 3 fres11 and isolated 8/5 core values from 1993to be a' part of the "ncinclassical LSW" produeed, iri the southem Labrador Sea. This irriplies that the elassieal
LSW has arrived at the Tail of the Grand Banks probably within one year. suggesting °a
mueh higher advection within the DWBC system.
\Ve have discüssed so fär the impact of surface c.00ling jn the, Labrador Seä on' the
production and the T/S eharaeteristies of LSW sinee 1991. Individual year classes üf
4
LSW
have been ideritified iri agenerally, period . of wann; positive sea surface teinp'erature anomalies in the mid-Iatitude North Atlantic and negative SST anomalies inthe
Labrador Sea (Harnen arid Bezdek; 1996). Earlier occupatioris of particularty the 48 0 N
secHon allow us to go back in time to other periods where the meridional SST gradients
were smaller, or of opposite sigri. The '1982 occupatiori by CCS "Hudson", falls into a
period of moderate, if not wanner positive anomalies in the Labrador. Sea. For. the only
other occupation by RRS "Discovery" in 1957 Hansen and Bezdek ideritify. a lang and
\vann interval. \Ve have described so far the progression of LSW as part of the subpolar
gyre. \Vith adequate data from reoccupied transoceanic sections further .south, it seems
opportune, to invesiigate. the overall influence of the changes of the LSW on the longtemi and large-sale meridional circulation of the North Atlariti<?
•
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\Ve no\v hilVe five occupations of the 48 0 N secHon since 1957; makirig it the most
sampled transoceanic section. Vaguely, statistics of these occupations can be builtup.
Igrioririg the seasorially. influenced top 1000dbar, the teIriperrinire differerices are ca. 0.5
~Chet\veeri 1000 and 2000 dbar, and ca. 0.2 in the \Vestern and < 0.1 °C in the Easterri
Basin (Fig. 10) iridicating the measure of variability across the section. The \Vestern
Basin appears mtich more variable tban the eastern one..\Ve have traced the fate of the
LS\V by followirig the changes of its core properties. These have been identified in
temperat1.ire, salinity, density and subsequently the depths of the core. Comparing these
cllanges section by section, the changes .in the depths of the core cari lead to differences
that not necessarily agree with those obtained by the water-mass-follo\ving metllod.. This
is a methodological artifact. These section by section differences on the other harid can
give a yery useful iridication of the large-scale chariges iri the full section and the
underlying physics. '
The Eastern Basin' ,vas the wamiest overall iri the 1957 (Discovery) anct 1982 (Hudson)
occupations, particularly for the intennediate layers arid the centres of the two basins
(Fig. 11 a-d). The \Vesterri Basiri has warined since arid was found to he wannest in the
1993/1994 occupaÜons by Gauss (1993) and Meteor (1994). The 150 nin ;,vide band of
the D\VBC system has partly wannest during 1957 (Discovei-y) arid 1993 (Gauss). The'
1994 and i996 occupations show, thatpart of the D\VBC cores have, coolect arid
freshened since 1993. The lowest temperatures have beeri observed since .1993 for most
of the Eastem Basiri, leavirig oniy the boimdarY. curients west of the MAR and the farfield of the DWBC system colder in 1957. The saliriities in the top 1000 dbar have
increased west of the MAR to the preserit ,maximum. Below .and 'east of the MAR the
highest salinities were observed in the 1957 and 19820ccupatioris., On both sides of the'
MAR at depths' greater than 2000 dbaf \ve find the highest saliriities in 1982, iridicatirig
a predominantly northerri influence at both t~e intemiedhite .and deep layers; 1994 and
1996 (riot shown)' are the freshest years :11 this section, with only the far-field of the
DWBC between 2500 arid '3500 dbar being fresher in 1957 and Ü)82. Agairi, the top
1500 dhar west of the MAR have been fresher hefore and dudng i982. This. clear
picture of large-scale and iorig-time ch~mges invites to look at nie impact of the highly
variabie iritermediate \vaters on the iritegrated heat and freshwater transports.
The drastic changes since i982 iri the LSW propei-ties, paiticularly in temperature; ,have
b~en noted befere (Fig. 7). T~e year 1982 (Htidson) indicates the wamies~ arid.a sIightIy
saltier LS\V on record. Iri 1957 LSW was slightly colder ar,id of .31most the same salinity.
For a similar comparisori of transoceanic seciioris sp<inriirig the last 40 years at 36 0 N;
5
Dobroliubov .et al. (1996) have shown a similar ~hange from a slow and long warming
period for LS\V peakirig abou! 1982 to a drarriatic and strorig cooling period since the
mid 1980s.
For the deep and bohom iayers we find for both seciions at 36° N arid 48° N a clear
cooling and freshening. Together with a strong increase in siiicate concentrations, this
indicates a riorthward intrusion of waters of Antärctic origin. The strongest signals are
fOlirid on the west side of th~ MAR and in the \Vest-Etiropean Basiri.· In contrast, depthaveraged temperatl.lres arid salinities (Fig. 12) show for the deep, basins for the 1957 and
19820ccupations the Northamedcan Basin to be colder arid fresher. The differences gei
smqller when moving east to the MAR; east of the MAR in the\Vest-European Basin
1957 and 1982 are slightly ,,,anner and saltier than· the later occuiJations ofthe 1990s.
Strong signais are seen also in the bouridary currents at both sides of the MAR. For the
\Vestem Basins the cooling of the intenriediate arid deep waters must have been overcomperisated by warmer and saltier mixed layer and thennocline ,vaters for the entire
40 years period.
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•
DobroÜubov at a1. (1996) compared the 1959, 1981 and 1993 occupations of the 36° N
section arid find for 1981 the largest mass and heat flux estimates for this period arid
subsequently the largest overturning rate of the meridionar circulation. This is confirmed
for the' 48°N section for 1982. For periods of low production of intermediate waters the
south,vard flO\v at intermediate ,,depths is reduced and the overttirning meridional circu:lation has only one sirigle cell: the north\vard flow of surface and thermocline waters and '
the southward flow of NAD\V and its cOlltfibutioris from the, Overflow area. For periods
of strong production of intermediate waters such as the ,LSW, this implies a strong
southward arid eastward transport of newly formed intermediate \vaters which are replaced by regionai surface waters. Iri this case we have a t\vo-cell meridional circulatiori
,vhere the 100ver' cell is fed by the riorthward penetraiion of AABW \vhich is
subsequenily entraine~ into the southward flowing NAD\V.
At ihis time we can only' speculate about ihe impact of these circulation changes on the
total meridional circulatiori. It seems feasible that during periods of high prodtictiori
rates of interrriediäte waters in the northem North Atlantic and the hvo-cell meridional
oveituming, the northward ,flO\virig warm waters of the surface and thermoclirie Iayers
are short-circuited south of Iceland into the production loop. At limes of low production,
arid the single-cell mei-idional overiUrning; the wann waters continue to be advected into
the Norwegian and Greenland Seas where they are available for the production of
ultimately the overflow waters (Dickson ct a1., 1996).
From our data ,ve conclude that ~ew modes of LS\V arrive in other parts of the North
Atlantic in the following order: 1. Irrriinger Sea after 6 months, 2. Newfoundland , Basin,
3. Iceland Häsin, 4. western part of the \Vest-European , Basin and 5. Rockall Abyssal
Plain after 7 to 8 years. The new and partly surprisingly fast spreading rates give rise to
address new questions as e.g. about the transport mechäriisms. The two proposed
meridional circuI:ition modes, single cell versus two cells, show a strong linkage of the
intermediate waters of the Subpolar Gyre with ihe North Atlaniic. They are seen to
have a direct impact on the meridional transports of heat and fresh\vater. These findings
highlight the iritimate relationship , betweeri cooling in the Labrador Sea arid· its impact
on tl1e interior of the oceal1 - an important piece of the global climate puzzle.
6
References
Cunningham, S.A.and T.W.N.Haine (1995): Labrador Sea Water in the eastern North Atlantic. Part I: A
synoptic circulation inferred from a minimum in potential vorticity.J. Phys. Oceanogr., 25,649-665.
Dickson, R.R., J. Meincke, S.-A. Malmberg and A.J. Lee (1988): The "Great Salinity Anomaly" in the
Northern North Atlantie 1968-1982.Prog. Oceanogr., 20, 103-151.
Dickson, R.R.,J. Lazier, J. Meincke, P. Rhines and J. Swift (1996): Lang-term coordinated changes in the
convective activity of the North Atlantie. Prog. Oceanogr., submitted.
Dobroliubov, S.A., V.P. Tereschenkov, A.V. Sokov (1996): Water mass transport and heat flux at 36°Nin
the Atlantic Ocean. ICES C.M. 1996/0:1
Gordon. A.L. (1986): Inter-ocean exchange of thermohaline water. J. Geophys. Res., 91,5037-5046.
•
Hansen, D.V. and H.F. Bezdek (1996): On the nature of decadal anomalies in North Atlantie sea surface
temperature. JGR, 101, C4, p.8749-8758
Lazier, J. (1996): The salinity decrease in the Labrador Sea over the past thirty years. In: Climate on
. decade-to-century time scales, National Academy of Sciences Press. Washington. D.C., in press. '
.
Pickart R.S. (1992): Water mass components of the North Atlantic deep western boundary current. DeepSea Res., 39, 1553-1572.
Rahmstorf, S. (1994): Rapid c1imate transitions in a coupled ocean-atmosphere model. Nature, 372,82-85.
Rahmstorf S. and J. Willebrand (1995): The role of temperature feedback in stabilizing the thermohaline
circulation. J. Phys. Oceanogr., 25, 787-805.
Read. J.F. and W.J. Gould (1992): Cooling and freshening of the
1960s.Nature, 360,55-57.
subpol~
North Atlantic Ocean since the
Schmitz, W.J. and M.S. McCartney (1993): On the North Atlantie Circulation. Rev. of Geophysics, 31,29-49.
•
Talley, L.D. and M.S. McCartney (1982): Distribution and circulation of Labrador Sea Water. J. of Phys.
Oceanogr., 12, 1189-1205.
Top, Z., W.B. Clarke and W.J. Jenkins (1987): Tritium and primordial 3He in the North Atlantic: A study
in the region of Charlie-Gibbs Fracture Zone. Deep-Sea Res., 34, 287-298.
.
.
Wallace D.W.R. and J.R.N. Lazier (1988): Anthropogenie chlorofluoromethans in newly formed Labrador
Sea '\Yater. r-:~tur~. ~~2, 61-63.
Weisse, R., U. Mikolajewicz and E. Maier-Reimer (1994): Decadal variability of the North Atlantie in an
ocean general circulation model. J. Geophys. Res., 99, 12411-12421.
7
....
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Figure Captions:
Fig. 1:
Vertical sections of
a) salinity and
b) oxygen content
obtained from R.V. "Meteor" along line AlIEast in September 1991
revealing the wide spread abundance of Labrador Sea Water in mid
depths . by means of its characteristie intermediate salinity minimum and
oxygen maximum.
Fig.2:
Main topography and schematics of primary pathways of the Labrador Sea
Water according to Talley and McCartney (1982) and Sclunitz and
McCartney (1993). LS: Labrador Sea; IS: Irminger Sea; IB: Ieeland Basin;
RT: Rockall Trough; WEB: West European Basin; NF: Newfoundland
Basin.
The data· along the hydrographie sections were acquired on the following
cruises:
July 1990
RV "Dawson" No. 90012/1
Al/West
June 1992 (incomplete)
RV "Hudson" No. 92014/1
"
June 1993
RV
"Hudson"
No.
93019/1
"
May/June 1994
RV "Hudson" No. 94008/1
"
Nov. 1994 (incomplete)
RV "Meteor" No. 30/3
"
June 1995
RV "Hudson"
"
Sept. 1991
RV "Meteor" No. 18
AlIEast
Sept.
1992
RV
"Valdivia"
No.
129
"
Nov.lDec. 1994
RV "Meteor" No. 30/3
"
June
1995
RV
"Valdivia"
No.·
152
"
July 1993
RV "Gauss" No. 226/2·
A2
Oct./Nov. 1994
RV
"Meteor"
No.
30/2
"
MaylJune 1996
RV "Gauss" No. 276/2
"
Fig.3:
Potential temperature versus salinity along section AlIEast compiled from
all data of cruise "Valdivia" 129 (9/1992) for the intermediate and deep
subpolar North Atlantic. The colder and fresher group of minima at about
e = 2.9 °C and S = 34.85 characterizes the LSW core in the .Irminger
Sea (a), the warmer and saltier group of minima at about e = 3.2 °Cand
S = 34.88 characterizes the LSW core in areas east of the Mid-Atlantie
Ridge (b).
Fig.4:
Temperature differences (8(1992) - 8(1991) of the LSW core along
seetion AlIAR7 calculated by different methods. The mean temperature
decrease of the LSW core along the entire seetion was calculated with the
water-mass-following method as ~e = -0.08 oe (bold line) whereas the
corresponding number calculated with the depth depending method
(differences at fixed depths z=z(1991» was ile = ± Ö.Ö "·C(dotte"d line).
•
•
,
.
8
Fig.5:
Change of LSW core properties along trans-ocean sections AlIEast (For
better clarity only values from 1991, 1992 and 1994 are shown).
a) Potential temperature at LSW salinity minimum
b) Same as a) but for salinity (PSU) ,
c) Same as a) but for pressure (dbar)
d) Same as a) but for potential density anomaly
0)
«(J
•
.
Fig.6:
elS diagram of LSW cores for adjacent areas. Data collected between
1990 and 1996 in the central parts of the Labrador and Inninger Seas
(except for LabS-11/94 with values only from the northern part of section
AlIW). Potential density anomaly «(J1.~ referenced to 1500 dbar. Each
value is calculated as the mean of a 200 dbar thick layer centred at the
elS peak of the LSW core. From each cruise the five most central profiles
were used.
Fig.7:
Change of Potential temperature at LSW salinity minimum along transocean section A2 and with data from CCS "Hudson" from 1982 for
,comparison. (For better clarity values from 1994 are not shown).
Fig.8:
Same as Fig. 6 but for Labrador Sea and Newfoundland Basin' between
40 0 \Vand 49°W. The 3 isolated fresh and wann values observed in 1993
in the DWBC band are suggested to be part of the "non-classical LSW"
according to Pickart (1992).
Fig.9:
Two casts carried out ~t station # 30 of cruise "Gauss" 226 (1993) in the
centre of the Newfoundland Basin at 43° 07' N, 41 ° 10' W indicate a
billow-like structure of the LSW layer. Within 8 hours only the double
layered LSW core (thin line) was found being replaced bya single layered
LSW core (bold line) .
Fig. 10:
Differences of potential temperature for four occupations of the 48°N
section from 1957, 1982, 1993 and 1994. Differences between the extrema
calculated for each grid point (see Fig. 11).
Fig. 11:
.Extreme values for the foul' occupations of the 48°N section. Each
gridpoint is given a symbol indicating the relevant cruise/year when it was
assigned the particular extreme value
a) maximum potential temperature
b) minimum potential temperature
c) maximum salinity
d) minimum salinity
Fig. 12:
Vertically averaged temperatures. ~n:-', nli;-:ties along the 48°N seetion
from 1957 (Discovery), 1982 (Hudson), 1993 ~Gauss), 1994 (Meteor)
'
9
Tab. 1:
Change of mean values of LSW core properties (intermediate eIS-Minimum) for Irminger Basin (>33° W), Iceland Basin (28-22° W) and
Rockall Trough entrance ( < 22° W) along section AllE, and for
Newfoundland Basin (> 32°W) and West-European Basin « 32°W) along
seetion A2.
~.:
AlIEast:
.
A = "Valdivia" 9/1992 - "Meteor" 9/1991
B = "Meteor" 12/1994 - "Valdivia" 9/1992
C = "Valdivia" 6/1995 - "Meteor" 12/1994
A2:
D = "Gauss" 7/1993 - "Hudson" 4/1982
E = "Meteor" 1011994 - "Gauss" 7/1993
F = "Gauss" 511996 - "Meteor" 10/1994
Units of calculated differences: Pressure p [dbar], potential temperature
e [OC],salinity s [PSU], potential density anomaly 0 8 [kg/m 3] •
•
Section
AllE
I1t [month]
Longitude
>33°W
<28-22°W
<22°W
A
12
B
C
27
6
A
B
A
C
C
A
I1s
118
I1p
B
B
11 (J 8
128 122 -15 -.092 -.140 -.050 -.002 -.002 -.006 .007 .011
47
61
83 -.068 -.038 -.082
103 112 -56 -.003 -.096 -.010
C
-.002 .007 .004
.0
.0
.0
.006
.002
.002 -.004 .002 .010 -.002
Section
D
;.A2
t [month] 135
>32°W
<32°W
641
E
F
15
18
19
58 -.450 -.057 -.103 -.002 -.001 -.009
27 125
D
E
F
D
E
38 -.154 -.096 -.042 -.026 -.006
10
F
D
E
F
.043 .005 .003
.004 -.006 .005 .007
..
a)
•
o
500
2~O
RV Meteor
'8 SALINITY/
pSol
GSH
x-
70
Areo in X. by no. of profiles Areo in Y. by physicol units
No of GRQ-Po:,.,h in Y-
70
Order of Orthogonal Surfoce
Gr'dd,n~
No of
Parameter:
C~D-Po:"t'S
!
in
2
100
-
Homburg
,
!
:52.00 :5:5 00 34,00
I
I
I
II
035 ..30 .35.40 35 SO
~ ... U')
l")'tlOU)r-.alO'lOooN"lotltl lD '" <0 0\ 0 ~lD""'tOO) 0
,....."' ..... "'''''''r-.a:-'O)lI)C)OJ ::0 QJ co IX) (11 O:lJlQ\O\O\ 0
U')1/)ll)tOlOI/'Ht'llOu:TlIOl/110 10 U1 10 10 10 ,,'h(h()lf'ld1 tO
tO ..... lJ)0' 0
\0 lO COlD "
ll'l 10 llllil III
ull.t::1illlOlDl.D
ulrll:llva()lCltll
Nt")
0
1.0
00
ID \0
v lfl(jlO~ N r")
0 00........
10 IOtDtOfD \0 U)
10 tOl'al:O-N
U)
...~t'\l
fD~l()
11
500·
1000 -
•
1500 -
2000·
2500 -
:l000·
:l500 -
4000 [ObO r ]
b)
o
250
500
RV Meteor'8
Cr'dd.I"'g Po,.ometer:
No of GRD-Po;nts in X=
No of GR:>-Points in Y=
I
4.00 .
I
I
650
I
4.20
I
660
I
4.40
,-
6.70
'8 OXYGEN/ mI/I
GSH
Areo in X. by no. of profilesArea in Y. by pliys;col lJl"tits
Order of Orthogonal Surfoce
70
70
t
4.60 ,,,.4.BO
,.5.00
'F;:"'~!'
5.20 .. 5.40
m#Mt
I
I
I
I
,
680
690
700
7.50
B 00
!
..
.,
2
IOD
1
..
I
5.60 " 5.80 .• 6.00
. .
Homburg
I
6.10
I
620
Ij
6.:50
I
6.40
B 50 - 900
Fig.l:
.
•
60·
50'
O'
30'
"O~JI!O'-?'~~~"T':TI~=r±_-:T-""'_:"I-=I~!O;;·;':l_E-='_""-E-:!=E~~~~~=!3=:l~20~.==_t==~~"""";~3=~Ss!:E'~
•
WOCE
Seelion
Al
o
WOCE
S@'cUon
-----_._---- -----------
A2
.'OUudeon"
Seelion
1982
- - - - - - - - - - - - -----
. Fig.2:
•
0
. .--
••••
Fig.3:
•
I
"' . • • •
--- - - - - - - - 1
Property Change ofLabrador Sea Water
"Valdivia" 129 (1992) - '~Meteor" 18 (1991)
-e...
I
ee
u
~
~
~
l
e
~
E-
•
0.4
,~ .
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
Q
LSWCore
·····u····· z. z(1991)
40
35
30
25
Longitude [OW)
20
15
Fig.4:
•
·Temllerature ofL~Drador Sea 'Vater
Depth of Labrador Sea Water
1000
1100
Ü
1200
2..
1300
t
B
f
'i:"
llII
X.
a
.....
~
~
.Q
::!.
3.2
3.1
3
1500
ofi
1600
~
1700
co.
...
1400
1800
'S
~
1900
I
2000
40
35·
30
25
20
15
.a)
2100
40
Longltude (0 W)
35
30
25
20
15
c)
Longltude (0 WJ
Salinity of Labrador Sea Water
Density of Labrador Sca Water
3-4.92 "---'---r--rr-~-- ....- - - . - - - . . . ,
27.74 ..---,--,--.-----r---..,.----,----,
27.75
;;;
~
34.88
27.76
.......
CD
34.87
'"
34.86
_..
34.85
S
t>
'1'
....._
__.._
- - NMeteor" 18
__
"Vald1VIa 129 (199~J)
- - "Meteor" 30/3 (1 ...
__
~
(1991)
N
27 79 -
b)
35
30
25
Longilude (0 WJ
20
15
.
27.8
_. -=:= ~~:~N ~~9 g::~~
JI
"Meteor" 30/3 (1994)
L....o..--.J~ ........'"""""'L-.........~J-o.._...........J.-.~...........I-........_ . . J
35
30
25
Longltude (0 WJ
20
d)
15
Fig.5:
-
,....I-.-..-..~f.ö-...-···-····-···..,..r"r.'~·r----.
3.1 r----.,-i--..,..,--,;.,..1....-....-...-........,
.. 1-····-···-···.....,···.·..
i
0
L..
34.65.....···
!
i
........!
_~-----..
I
e
•
T
•
•
8-
o
(!)
~
LabS-719O
LabS-6192
LabS-6193
LabS-5194
(LabS-11194)
LabS-6195
IrmS-9/91
IrmS-9/92
IrmS-11/94
IrmS-6/95
+
Ä
E
~
'V
H3
-
:!
•
c
S0
a.
i
·······1.J.......................L................
I
I
i
i
i
2.6 [...............~".;.;;.;.
-J...;................L...o...............-...L.............................................
34.83
34.84
34.85
Fig.6:
34.86
Salinity
Temperature of Labrador Sea Water
3.8
•
-f
U
0
.e
f
!.
3.7
3.6
3.5
3.4
e
3.3
f-4
3.2
:j
3.1
~
-=
5
l:lo.
3
2.9
2.8
50
45
40
35
30
25
Longitude [0 W]
20
15
10
Fig.7:
--
-------------------------
.'
3.5 . - - - - - - - - - - - - - - - - - - ....-...-::
...:""
.. -~
.........
!
3.2
!..
3.1
E
3
l
~.
~
J
~,
•
LabS-719O
T
•
LabS-6192
LabS-6/93
•
LabS-5194
(LabS-11194)
..
LabS-6/95
NewB-7/93
NewB-6196
+
..'
....
....
.....·TT
....
.. . •
~AI~ ....
::: e.:::;..::::..: : : : :.: : : : ·
+.:r.
2.7~:::::::~
····
34.70
ES
.....:
2.6
34.83
o
34.85
,
34.87
Salinity
I
34.89
34.91
34.93
Fig.8:
Fig.9:
.'
Min) - Pot~Temperature [Oe]
(1957, 1982, 1993, 1994)./ 48°N
(Max -
,.....,
>co
.c
"
"'0
......
e
1.0
Cl)
','
L.,
::I
(J)
(J)
Cl)
.
,~,
~;
0.5
-3000
L.,
c.
0,2
0.1
0,0
-5000
-5000
-4000
-4500
-3500
-3000
-2500
-2000
-1500
-1000
Grid: 80x80
Melhod: Kriging
Filename: swtmm48n,ps
Date: 13,08.1996
Mean. 0.8, 1.0, Min • O. Max • 15.8
Fig. 10:
..
•
....'
.:
'
.....
;
..
•
~
..
. 1:
"
'-"
•
.~
...
''lI
Maxhnum ~Pot Tempefäture [OC]
" (~§5~,i§8~,i9Ö3,1~94)/48°N
'i:'
ca
.c
"0
",
-2000
~
Q)
'-
:J
rn
rn
·3000
C1.l
'-
a.
.
.400C
......
"
·5000
-5000
,
·4500,
.,
-'
·4000
Filename: tmax48n,ps
Date: 13.08.1995
"
·3500
·3000
~
J
.
. -2000
-2500
•
·1500
.1000
•
longitüde [OW] •
Oiscovery 1957
Hudson 1982
A G~)Uss 1993
Meteor 1994
.
o
Minimum- Pot; Terriperature" [OC]
""
{~§§i,i§~2,1993,1994ij4äo~
•
..
·4500
Fiiename: Imiri48;;.p~
Date: 13.08.1998
.
·4000
·3500
·3000
·2500
.
.
~
·2000
·1000
·1500
• Discovery 1957
" Hudsori 1982
A' Gau~s·t9l:13·
o Meteor 1994
b),'
Fig. 11:
.
MC!xlmum;. Salini!y [psJj .
.
rr 11;l'lI'
(1957, 1982,1993, 1994)/48°N
I
u -otil-
1I1 11
I
111::
'ill
';"j
!11!~'i'
'I
-400e
·500e
-5000
-4500
-4000
Filename: smax48n.ps
Date: 13.08.1998
-3500
-3000
.
-2500
Longitude [OW]
-2000
-1500
•
Oiscovery1957
A
Hudson 1982
Gauss 1993
o
Meteor 1994
-1000
c) .
, Minimum - Salinity [psu]
'(1957; 1982, 1993,'1994) /48°N
_'00:
•
~
i
·li~:!I-illl!I.!I!IIIIIIII:I,!I. i!I ;~§ §i! lil
I
-2000
I.
8
;,.1
111
I •
!II UUnnnUjUnU~
(1)
~
a.:
·4000
-5000'o-------"!!!"!!"li--~~
-5000 . '. -4500
-4000
. ·3500
-3000
·2500
i=ilen~~e: ~miri48n.ps
-2000
•
Date: 13.08.1998
......
...•.
.
.
~
·1500
Oiscovery1957
Hudson 1982
A Gauss 1993
o
Meteor 1994
·1000
..
16
14
·1000
Cruise:
-
12
-~
Discovary 57
0
-+-
Hud.on 82
'i:'
(/)\0
-+-
GaulJS221
.c
C»
I-
-e-
=0
fG
-2000
Metaor 30/2
~
Q)
"::J
-
e... a
Cf)
Cf)
CU
e
Cl)
.c
I-
-3000
0-
6
-4000
Topography
1':'_~~::::::::::t-..b-='i=-=~-=!.........,..---:t----"--~-5OO0
20
10
0+
50
40
30
.Longitude [0W)
358
-t--..:.-----,-----'-----T----"""/f
35.6
Cruise:
354
•
-.....
:J
Cf)
-~
Ciscovery 57
-+-
Hudson 82
-+-
Gau•• 226
-e-
'i:'
fG
.c
.2000~
Meleor3Ol2
Q)
35.2
":J
C-
Cf)
CI)
.?:'
·c
f!
350
-30000-
iü
Cf)
34.8
-4000
34 6
..u--\----+I-V--==----f-=----y~r\r...:::!loo'rt-~
Topography
34.4
~I --.;...~~::::::::::;--!;=T==~~~r-~_j;;---r---:~.5000
30
20
10
Longitude rW)
Fig. 12: