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					   Ocean advection, Arctic-Atlantic Connections, Climate
                 P.B.Rhines, University of Washington
                 Sirpa Hakkinen, NASA Goddard SPC
    with David Bailey, Wei Cheng, Jerome Cuny, Trisha Sawatzky
                  WUN Climate teleseminar, 26iii2003



             North Atlantic Oceanic Heat
             Advection is Important to the
               Wintertime Storm Track,
             Eurasian climate and weather
                  and Global Climate

www.ocean.washington.edu
/research/gfd/gfd.html
                  TABLE OF CONTENTS
• Polar amplification of global warming
• Storms and weather….ocean to stratosphere: continuity of the
        Atlantic storm track/Icelandic Low from subtropics to Arctic
• Atmospheric and oceanic transports, strongly coupled, supply
        continental and polar warmth, fresh water
• The oceanic heat source, strongly channeled, local and imported heat:
         driving the 3 scales atmospheric circulation,
• Transports by ocean circulation: connecting the Arctic and Atlantic
   – Erika Dan, 600N winter in the N Atlantic: deep, shallow and
                   shelf water masses
   – Atlantic – Arctic exchange: Θ-S-transport (~heat,
                    fresh-water transport) diagrams
   – Rapid and slow modes of response of Atlantic overturning:
                   Labrador Sea vs. Nordic Sea overflows
   – What matters to the global overturning circulation
        • The freshwater cap and its movement: liquid and solid
        • Layering of deep-ocean water masses, and its
                    representation in models
        • Shallow continental shelves and communication with
                   the deep ocean
• Arrays and instruments: establishing the baseline and sustaining
        it in time
Global surface temperature has seen two major warmings in
  the 20th Century: in the 1920s-30s, when it was very
  concentrated around Greenland, and since the 1980s, when
  it is much more global, yet still concentrated in high
  northern latitudes.

  Cod and herring fisheries responded to the much warmer
  ocean temperatures, which lasted for more than 25 years.

  Ironically we anticipate the ocean circulation slowing
  during the current warming, whereas it is possible that an
  accelerated oceanic meridional overturning circulation was
  a factor in driving the 1920s warming.
Surface Air Temperature                      Delworth+Knutson, Nature 1999




              Uppernavik, West Greenland SAT anomaly (Imke Durre)
The standing-wave structure with wintertime Icelandic and
Aleutian Lows gives the atmosphere some of the east-west
structure and gyre circulation familiar to the ocean. The
rapid radiative cooling in fall sets up a cold dome which
slumps under gravity, tending to create a surface polar
vortex which is anticyclonic, the convergence overhead
strengthening the polar cylonic vortex…yet mountains and
vertical momentum transport can intervene, opposing the
surface high.


In particular, the Atlantic storm-track forms a continuous
connection between subtropics and the high Arctic.
Surface air temperature shows the imprint of both warm
ocean and the orographic component of the circulation. The
ice free region of the Arctic and subArctic is marked by warm
surface air.                          NCEP reanal 2 Jan 1993
Standing Rossby wave models of the circulation suggest a
  cyclonic trough in the lee of the Rockies, which transmits
  wave track to the atmosphere

The hemispheric baroclinity (cold air dome) provides energy
  for both standing and transient eddies, while ocean heating
  augments both (e.g., Held et al., J.Clim 2002).

The surface low lies east of the 500 mb. low in this winter
  JFM averaged 1993 data, expressing both poleward heat
  flux and diabatic heating by the ocean (note particularly
  the Labrador Sea and subpolar Atlantic).
The atmospheric circulation is increasingly zonal with height,
  and variance-based maps of storm tracks are quite zonal in
  the upper troposphere yet the low-level tracks of storms fills
  out an extensive meridional ‘gyre’ centered on the Icelandic
  Low. (Hoskins and Hodges, JAS 2002).

Storms amplify at several sites along the track, as over the
   warm water of the Nordic and Barents Seas.

The low-pressure storm centers circulate round the Icelandic
  Low and spill heat and moisture onto the Eurasian continent
  over a broad range of latitudes.
Lagrangian storm track density – Hoskins+Hodges JAS 02
based on sea-level pressure (ECMWF 1979-2000 period).
 NAO/AO positive index is prevalent during this period.
Storm activity propagates remarkable distances round the
   northern hemisphere, with one storm seeding the next, with
   group velocity exceeding the speed of individual storms
5-week Høvmüller plot
  following the principal
  storm tracks (Chang,
  Lee and Swanson J
  Clim 2002). This is
  variance of meridional
  velocity at 300 mb.

       Atlantic
        Pacific
• Heat flux between ocean and atmosphere is inferred from
  many sources: atmospheric reanalysis fields (plus top-of-
  atmosphere radiation in the brief ERBE period), ship-board
  observations, aircraft boundary layer observations, satellite
  altimetery and thermometry, water-column heat storage,
  and ocean circulation.

• NCEP and ECMWF net heat fluxes appear to be less
  accurate than re-analyzed products guided by the aircraft
  observations.
           NCEP
           Aircraft    latent
sensible
           (Xue et al           Renfrew+Moore, 2002
           JPO95)
           NCEP +
           Smith-deCosmo
• Maritime storms grow ‘explosively’, and move along the
  Gulf Stream front and the lower atmospheric temperature
  front lying near the coast;

In mesoscale model simulations (Kuo, Reed and Low-Nam
   MWR 1991) of 7 explosive storms ocean heating caused
   SLP deepening (13.5 mb in 48 hours of 30-40 mb total
   deepening) for long forecasts begun when the storms were
   weaker.

APE generation is
      -favored by latent heat in the warm sector (warming
             warm air),
      -damped by sensible heat in the cold sector (warming
             cold-outbreak air: Orlanski et al. J.Clim.2002_
Lau’s JAS 1979 winter diabatic heating of the 700mb-
1000mb lower atmosphere. Peak values are 100-150
watts/m2 in the subtropical storm track regions
• This net atmospheric heating by the ocean, plus or minus
  radiation, is less than the surface upward flux, Qnet,
  inferred by most methods (as the atmosphere cools to
  space). The net surface heat flux peaks with winter-
  average values as great as 300 watts/m2 near the Gulf
  Stream and Labrador Sea. The Barents Sea in the Arctic is
  not far behind with peaks exceeding 200 watts/m2

• In the map that follows the annual-average plotted values
  of Qnet are less than these wintertime values, roughly by a
  factor of 4. This may be explained by the radiative cooling
  of this air mass to space, although accuracy of the
  observations is also unknown.
Qnet, net atmosphere-ocean heat flux, watts/m2 (Keith Tellus 95)
                       (annual average)




 It should be noted that because the sun heats the ocean, O,
 but does not cool the atmosphere, A, the most useful maps
 of Qnet for A will differ those for O by the short-wave insolation.
• Keith’s analysis uses top-of-the-atmosphere radiation
  observations from the ERBE satellite together with
  atmospheric reanalaysis data to infer the surface heat flux
  as a residual. Errors can immediately be seen in the large
  values over land, which cannot support significant net heat
  flux.
• The Arctic in winter is ice-free in the Barents Sea and in
  regions of the Arctic rim warmed by circulation of tropical
  heat as part of the oceanic MOC. The relationship of ice,
  fresh-water and heat is an essential part of the climate
  system. Shrinkage of ice-cover in both winter and summer
  has been observed, yet there are dynamical as well as
  thermodynamic causes (Rigor and Wallace, 2003).
Cryosphere today (Feb. 2003)
• Oceanic mixed-layer depth mirrors the Atlantic storm-
  track (here, Levitus records the depth at which the density
  is greater than the surface density by an amount equivalent
  to 0.50 C, which is deeper than the actual depth of
  wintertime mixing). Deep regions of weak stratification
  lie beneath the storm track.
Mixed layer depth (March, in m) based on 0.5C
• The evidence so far suggests a relationship between the
  heating originating in the oceanic overturning circulation,
  energizing the atmospheric storm track. Energy released
  from the polar cold-air dome through baroclinic instability
  is perhaps the primary source for transient eddy energy
  (Chang et al., J Clim 2002). Held et al. (J Clim 2002)
  however argue that the oceanic heat source produces
  significant forcing for the standing wave energy in the
  northern hemisphere circulation, comparable with the
  Himalayan plateau/Rocky Mountain orographic source.
  Decadal NAO variability is another issue, of course, with
  the oceanic high latitude heat source being less prominent
  (e.g. Kushnir et al. J Clim 2002).

•    The meridional heat transport curves, averaged zonally
    and in time, show this, yet with the atmosphere doing the
    majority of the ‘work’.
Trenberth J Climate 2001 Global meridional heat transport
    (based on TOA radiation, atmospheric reanalysis)
                                                ocean + atmosphere




                             atmosphere ocean
• Bryden and Imawaki, 2001, emphasize that the
  atmospheric contribution to meridional heat flux should be
  separated into sensible-heat and latent-heat components. 1
  Sverdrup of water vapor (the scale amplitude of the
  hydrologic cycle) moving poleward carries 2.5 petawatts
  (2.5 x 1015 watts) of thermal energy.

   They plot the break-down using Keith’s 1995 data.
            Global meridional heat transport:                         atmos (latent)
  (residual method, TOA radiation 85-89 atmos                      atmos (sensible)
  and ECMWF/NMC atmos obs)                                                ocean (sensible…)




   Oceanic heat flux is convergent north of here
                Atmospheric flux is divergent south of here                         Ice-cover north of here
                                                                              data of Keith (Tellus 95)
Error est.: ± 9% at mid-latitude; Bryden est 2.0 ±0.42 pW at 24N
• The latent heat flux is intrinsically a coupled atmosphere-
  ocean mode, with the fresh-water carried north by the
  atmosphere returning in the ocean (Bryden et al. 2001).

• A part of the poleward moisture flux switches from
  atmosphere to land, eventually draining in the massive
  outflow of Russian rivers.
     Arctic river outflows (cubic km per year) Aagaard+Carmack
                               JGR 1989

Delivery of moisture to
continental interiors:
sensitivity to storm-tracks,
NAO/AO (e.g. Tigris-
Euphrates, Cullen et al. 2001)

Arctic river outflows provide ~
4200 km3 yr-1=0.12 Sverdrup of
fresh-water; P-E, Bering Strait
inflow (~0.8 Sv at ~32.5 ppt) and
distillation by freezing supply the
rest of the exported fresh-water.
(Vuglinsky 1997 ACSYS-Orcas Isl)
Traditionally, a dominant atmospheric moisture pathway
  takes evaporation from the subtropical Atlantic and sends
  it eastward across Eurasia; however with high NAO/AO
  index, a sizeable part of this moisture takes a more
  northern route, some of it entering the Arctic.
    Impact of oceanic heat/fresh-water transport
Despite its apparent smallness at higher latitudes, oceanic
  meridional heat flux is important:
• Its divergence in the subtropics is the dominant source of
  fresh-water for the higher latitudes
• Heat transport is channeled geograpically in both ocean
  and atmosphere, and is seasonally enhanced (sensible heat
  in winter exceeds that in summer by a factor of about 2.5
  (Trenberth 2001), and latent heat/moisture flux is more
  complexly enhanced in winter (Peixoto and Oort, 1992).
• Observations of ocean circulation have definitively altered
  the ratio of oceanic and atmospheric heat transports (see
  Bryden and Imawaki 2001), and we may not have heard
  the last word.
Atlantic meridional heat flux: Sato + Rossby JPO 2000
Seasonal cycle: how is the wintertime upward heat flux
  supplied?

   - locally stored summertime heating
                      or
   - heat imported from the south by ocean circulation?
        Atlantic north of 250N: air-sea heat flux calculated from
        reanalyzed COADS data of da Silva et al. 1994. Black:
       monthly heat loss Red: it’s cumulative sum starting in April


Monthly flux
Reaches 4.5 pW




 Heating of ocean (black) integrated from 1 April returns to zero by early Dec
 (orange)                                      Rhines+Hakkinen 2003
In the Atlantic north of 25N, on average the previous
   summer’s heating is withdrawn from the ocean by early
   December.

Subsequently, December through March, the heating of the
  atmosphere by the ocean is supplied almost entirely by
  laterally advective ocean circulation.

Thus it is the geography and seasonality (concentrated
  arteries, steadier in the ocean, cold-season in the
  atmosphere) that raises the profile of oceanic heat
  transport to dominate in winter.
We can plot the year-day when last summer’s local heating is
 exhausted by fall-winter cooling.

For orange areas this day falls in early December or earlier,
  suggesting that in these regions, for most of the winter, the
  oceanic warming of the atmosphere is entirely due to
  advection by the circulation.

For blue areas it is later, often suggesting local one-
  dimensional closure of the heat storage with no need for
  advective heat-flux convergence.

{In the Southern Hemisphere the year-days are shifted by 6
   months so as to have the same connotation.}
by early Dec. local,                 In blue regions local 1-d
seasonal heat storage                Mixed layer heat storage dominates
is used up here




         Year-day when seasonal heat is used up (+ 6 mo in SH)
• There are several other ways to reach the same conclusion, for
  example mapping the ratio of seasonal cycle to annual mean heat flux
  divergence. Data sources range from da Silva/Levitus (1994)
  reanalysis of COADS, with calibration by meridional heat transport
  inferred from oceanic hydrography, and with constraints on net global
  vertical flux.

• As an example, Seager et al. 2002 describe some 0.8 petawatts of heat
  transported northward across 35N in the Atlantic Ocean. On average
  this produces 37 watts/m2 of heating of the atmosphere over the ocean
  surface to the north. Yet the wintertime average Qnet is 135 watts/m2
  substantially larger.
     However the fraction, say γ, of this ocean-circulation heat flux lying
  deeper than about 100m has to wait for winter with its deep mixed
  layers to escape. Its contribution to the wintertime upward heat flux is
  about 4 γ (taking winter to be 3 months long in this sense). Thus maps
  of ocean-atmosphere heat flux (as seen by the atmosphere) should
  compare wintertime heat flux Qnet with annual-average ocean heat-
  flux convergence maps multiplied by 4 γ. If γ ~ 0.5 we are comparing
  2 x 37 = 74 watts/m2 from ocean circulation with 135 watts/m2
  observed wintertime Qnet. In this case about 1/2 of the surface area of
  the northern Atlantic can be fully supplied with heat in winter by the
  lateral circulation, after locally stored ‘mixed-layer’ heat is removed.
• Thus the conclusion is that warming and moistening of the
  atmosphere by the circulating ocean in middle latitudes as
  well as tropics is crucial to 3 scales of atmospheric motion
  and climate: winter storms, the stationary winter gyres of
  sea-level pressure (Icelandic and Aleutian Lows), and the
  zonally averaged poleward transport so vital to maintain
  equable temperatures and rainfall at higher latitude.

• While this may seem obvious to many oceanographers, it
  has become a point of contention in the literature:
North Atlantic heat balance vs. month and
latitude Carton+Wang jpo 2002
ABSTRACT
“Here, seasonal heat transport in the North
Pacific and North Atlantic Oceans is compared
using a 49-year-long analysis based on data
assimilation. In midlatitudes surface heat flux is
largely balanced by seasonal storage….”

          Surface flux Qnet
                (daSilva 94)

Heat storage rate         Small but it
                          accumulates
                  for release in winter


Ocean heat transport
  divergence                   Contour interval
                               2.5 x 108 w m-1
Several recent papers all argue that oceanic heat storage is
 ‘local’, and requires only the thin upper mixed layer to
 describe it. The advocates often cite Gill and Niiler (JPO
 1973), who do a scale analysis based on large ocean gyres
 with weak mid-ocean currents (and disclaim any
 conclusions about more active western boundary current
 regions). Wang and Carton (JPO 2002), Seager, Battisti,
 Gordon, Naik, Clement and Cane (QJ Roy Met Soc 2002),
 Voison and Niiler ( JPO 1998) are among these.
Seager et al. state in their abstract
      “Further, analysis of the ocean surface heat budget shows that
    the majority of the heat released during winter from the ocean to
    the atmosphere is accounted for by the seasonal release of heat
    previously absorbed and not by ocean heat-flux convergence.
    Therefore, the existence of the winter temperature contrast
    between western Europe and eastern North America does not
    require a dynamical ocean.“
Wintertime (DJF)
average atmosphere-
ocean heat flux


 From Seager et al. 2002
 who use da Silva et al.
 1994 COADS derived
 Qnet data


 Annual mean
 atmosphere-ocean
 heat flux



Smaller but it
accumulates
Effect of ocean heat transport convergence on GISS T42 atmosphere
                         (Seager et al. 2002)
The geographic pattern of wintertime ocean-to-atmosphere
  heating by itself suggests involvement of the ocean
  circulation; it seems unlikely that a non-dynamical mixed-
  layer ocean could exhibit the large values coinciding with
  the northward track of the major currents.
Northward heat transport by ocean circulation would be easy
  to identify if it were all well beneath the seasonal
  thermocline: simply look for deep mixed layers in winter;
  yet it is partitioned between deep and shallow levels.
Deep mixed layers are a sufficient but not necessary symptom
  of dominant oceanic heat transport.
Direct observations of ocean currents and hydrography
  complement local water-column heat storage and altimetric
  observations in constraining heat transport. In these
  studies, oceanic heat advection has been shown to be
  dominant in interannual variability of the heat budget in
  the Gulf Stream/Sargasso Sea (the ‘Bunker-Budyko
  Bullet’): (Dong and Kelly, JPO submitted ix2002).
Winter convective mixing has reached 110m (mild winter so far)




                                        Igor Yashayaev, BIO Canada
                      Bravo mooring: heat content annual cycle by layer




Lilly et al. JPO 99




 deep warming due by
 ocean heat transport
Heat content and fresh-water content of the oceanic water column,
Bravo site, central Labrador Sea (1964-74 and 1990-98, Lilly et
al. JPO 1999): the annual heat storage range is 2.5 to 3 GJ/m2;
offsets between 1960s and 1990s reflect the NAO/AO+ period of
intensely cold winters and declining ocean salinity.
Heat content in central Labrador
Sea, 1997-8 (ECMWF flux integral,
mooring, acoustic tomography)
Fischer et al. IFM Kiel Germany

 Annual cycle is close to 2.5 GJ;
 of this, about 1 GJ is bound up
 In the advective, annual mean

Note time of maximum heat content
lagged 3 months relative to SAT and
integrated air-sea heat flux and
6 months relative to insolation
The vertical structure of meridional heat transport is not
  unique because of the uncertain choice of reference
  temperature (though the heat transport divergence is
  unique); yet plotting the ‘temperature transport’, ρCPTv,
  integrated zonally across the Atlantic, is of interest. The
  following plot from the Hakkinen model suggests a
  substantial transport beneath the upper mixed layer (note
  the shallow southward Ekman driven transport between
  35N and 50N).
  Annual-mean meridional temperature transport: vertical
structure (terawatts per m); Hakkinen Atlantic/Arctic model
Impact of the extensive meridional extent of the Atlantic
  storm-track on the warming and moistening of Eurasia,
  and its decadal variability: Thompson and Wallace (J.Clim
  2000) plot the 30-year trend in temperature advection,
  which closely resembles the pattern associated with the
  NAO/AO positive phase: strong eastward advection
  implicating the N. Atlantic and Arctic ocean sources
30 year trend in advection of time-averaged
 winter temperature (925-500 Hpa av.)
                                                   (from Thompson+Wallace J Clim 2000)
..anomalous velocity and advection, cooling the
ocean while warming the land in both Atlantic
and Pacific sectors. Could these oceanic
advective heat sources be the root cause of this
                                                            U T / x
contribution to global warming over Eurasia?
• Heat and the hydrologic cycle are coupled in the
  atmosphere by latent and sensible heat flux
• In the ocean the coupling is evident in the potential
  temperature/salinity relation that defines water masses and
  their density (Stommel and Csanady, JGR 1980). Many
  facets of the MOC, are involved:
   – Buoyant low-salinity water can mediate the deep convection
     process, and modulate the MOC. ..the Arctic as an estuary
   – Evaporation/precipitation and freezing/thawing and run-off control
     these upper-ocean effects while also affecting atmospheric climate
   – Sea-ice cover divides regions of weak and strong air-sea
     interaction: a strong climate feedback.
   – Polar amplification of global warming: even though mid-latitude
     ocean circulation is expected to slow, high-latitude circulation is
     predicted to increase as ice-cover diminishes; (Holland and Bitz, J
     Clim submitted).
   –
  Erika Dan temperature section, 600N
  Labrador-Greenland-Rockall-Ireland                   warm, saline water moving
  Worthington+Wright, 1970                             north from the subtropics




                                                             deep winter mixing, sensitive
shallow continental shelf circulation (unresolved in CCMs)   to upper ocean low-salinity
        deep overflows from Denmark Strait/I.S. Ridge        waters
  Stommel’s cooling spiral at OWS Juliette (52.5N, 20W) is a
  symptom of upward Qnet




Plot of (u,v) velocity
components with depth
as a parameter from
hydrographic data
   Stommel, PNAS 1979
       Hudson Strait salinity at 50m August 1965 (J.Lazier)
     This is part of the only 3-dimensional hydrographic survey
              ever made of the Labrador/Irminger seas.




   Cuny et al. 2003                                     Northward
                                                        moving
                                                        Greenland
                                                        shelf water
Southward
                                                  Irminger
moving low-
                                                  Current water
salinity water
                                                  circumnavigating
from Baffin Bay
                                                  the Labrador Sea
Earlier we described the coupled heat- and fresh-water transport of the
   atmosphere; in the ocean the potential-temperature/salinity relation
   is remarkable in its stability and simplicity round the Earth. If we
   could plot volume transport as a 3d variable on this plane it would
   express the coupled heat- and fresh-water (a.k.a. salinity anomaly)
   transport for the ocean.
For a section across a strait (Davis-, Fram-) these plots sum up the
   movement of water masses. If the strait enters a cul-de-sac (or
   approximately does, as with the AR7W section from Labrador to
   Greenland, across the Labrador Sea) then the transport/Θ/S diagram
   sums up conversion by mixing and air-sea interaction as water
   masses enter at on region of Θ/S and leave at another.
The heat-flux is expressed by taking this diagram’s transport density
   and multiplying by ρCpT (the 1st-moment with respect to Θ, nearly).
   Fresh-water flux is expressed by multiplying the transport by (S-So).
   The effect of different choices for So is readily observed through
   shifts on the diagram.
Finally, the familiar overturning diagram, transport vs. potential density,
   σ, is found by summing the transport/Θ/S along curved σ surfaces.
   This is done in model examples below.
The Cuny et al. 2003: transport through Davis Strait in
temperature-salinity space; blue bars are southward flux,
peaking in shallow Arctic low-salinity water; red bars are
northward flux of Irminger (warm, saline) water from the
Labrador Sea and farther south.
• The shallow continental shelves are particularly active
  parts of the transport, mixing and water-mass modification
  of ice-laden, buoyant surface waters. Yet climate models do
  not resolve continental shelves!

• Transports of low-salinity water on the Labrador and
  Greenland shelves are not well-known, but estimates
  suggest for the Labrador Current, seen in the earlier
  satellite ice-image, at least 0.2 Sverdrup fresh-water
  (relative to 34.8 ppt), with volume transports of roughly 3
  Sverdrups (Loder et al., The Sea, 1998). This is a crucial
  part of the Arctic ‘estuary’.
• Observations are wanting in the important regions that
  connect Arctic and Atlantic. The ASOF program is
  seeking to instrument the Arctic Rim with sustained
  moorings, hydrography, tracers and supporting satellite
  observations. In the Canadian Archipelago, for example
  this is a challenging problem due to ice-cover, difficult
  access, and a vast complex of passages.
ASOF (Arctic-SubArctic Ocean Flux): www.asof.npolar.no
   moored array sites in Canadian Arctic Archipelago




                   Melling, in FW Budget of the Arctic, E.L. Lewis Ed. 2001
• We need better climate models and ocean circulation
  models. High-latitude processes are not well represented,
  or altogether absent. Here is a meridional section from a
  new climate model built by Wei Cheng and Rainer Bleck,
  based on the MICOM ocean and CCM3 atmosphere, with
  a thermodyanamic ice model. Note how the density
  surfaces lift radically up topographic slopes where there
  are deep boundary currents. This and the attendant sinking,
  passage flows and deep convection are represented
  differently than in oceanic level-models.
                         Cheng et al. Clim Dyn subm 2003
Atlantic subpolar gyre
Entrainment and mixing occurs as dense waters sink to the deep ocean. In
   this experiment from our GFD lab, you see dense water sinking at the
   right; the colors are injected into the sinking branch at intervals…they
   are time-lines. The sinking branch entrains water, driving a subsurface
   overturning cell. This is realistic, in that the transport of the observed
   Atlantic high-latitude overflows more than doubles on its way round
   the subpolar gyre.
In most numerical ocean models, mixing is too large and sinking too
   inefficient (unless one tries to ‘fix’ it with a Beckmann-Doescher-type
   benthic layer). We need to preserve the articulate stratification of
   water masses that make up the huge North Atlantic Deep Water mass.
boundary                                                                     entrainment
mixing/
upwelling




            broad upwelling, yet much of it recirculates below the mixed layer
                            The remainder of the talk
                            describes analysis of the
                            Häkkinen ocean/ice model
                            carried out by David Bailey
                            at UW:

                            20 level σ-coordinate
                            0.70 x 0.90 resolution
                            Arctic + Atlantic to 350S
                            where relaxed to Levitus
                            NCEP forcing for 1950-
                            2000

                            elastic-plastic ice model
                            specified Bering Strait flux

                            smoothed topography
                             (Bailey et al. 2003)

mean barotropic transport
       The meridional circulation penetrates farther north in density spac
       Than in the physical y-z plane.



MOC vs y,z




MOC vs. y,σ
If the volume transport measured on an oceanic
   section is plotted on the Θ-S (potential-
   temperature/salinity) diagram, it shows
   individual water-mass contributions; between
   sections it shows water-mass conversion
Red shades are transport into the
Labrador Sea, blue shades exit the Sea;
Water mass conversion is represented by
the movement toward fresher, colder (red
=> blue)




                         Labrador Sea cross-section
                         AR7W line: 15 Sverdrups
                         of conversion (Bailey et al.
                         2003)
                                     Labrador Sea
                                     inflow/outflow/conversion
Greenland-Scotland Ridge overflows
Here at 25N one has a wind-driven gyre with both northward
and southward transports at low density. The deep southward
        flows are prominent at the highest densities.




        Sargasso Sea 25N
  Transports at various sections in Hakkinen model. The
  model’s Labrador Sea responds quickly to atmospheric
   forcing and strongly influence the MOC at 24N. The
overflows from Iceland-Scotland Ridge evolve more slowly,
                   as has been observed.
• The headwaters of MOC formation involve dense
  overflows from the Greenland-Iceland-Norwegian seas,
  and the mid-depth waters from the Labrador Sea. These
  multiple sources produce a well-stratified North Atlantic
  Deep Water that occupies much of the Atlantic between
  1200m and 5500m depth.
• Models need to find the right balance between these
  sources, yet diapycnal mixing resulting from resolution
  contraints make this difficult to achieve.
• Cold winds from Arctic Canada drive deep convection in
  the Labrador Sea, but buoyant low-salinity waters mediate
  and weaken the convective deepening; again a challenge to
  modelling.
These plots show how much buoyancy must be removed from
the top of the Labrador Sea to yield convection to a given depth.
The contributions of temperature and salinity to this resistance-to-
convection are shown in red and green: low salinity near the surface
dominates the stratification.




                                  Blue: total buoyancy flux needed;
                                  Green: salinity contribution to buoyancy
                                  Red: thermal contribution to barrier
CSS Hudson cruise, May 1994
Buoyancy barrier to convection: 0-500m difference between
thermal and haline components: the low-salinity cap is stronger in
the observed Labrador Sea yet weaker in the observed Greenland
Sea.
    Levitus November           Hakkinen model November
                          Conclusions
• Atmospheric storm-tracks, stationary winter low-pressure
  centers and hemispheric circulation all show the imprint of
  thermal ocean forcing. The Atlantic storm-track
  continuously connects the subtropics with the high Arctic,
  and its non-zonally oriented image extends vertically from
  beneath the oceanic mixed layer up to the stratosphere.
• Air-sea heat-flux observations, direct inference of oceanic
  meridional heat transport from hydrographic sections, and
  the residual method (TOA radiation minus atmospheric
  analysis) all give heat-flux convergence which, if released
  substantially in winter, accounts for most of the upward
  wintertime heat flux in the western subtropical Atlantic, the
  entire subpolar Atlantic and ice free regions of Labrador,
  Nordic and Barents Seas.
• Fresh-water and heat are transported interactively in both
  atmosphere and ocean; at high latitude particularly the
  movement of buoyant low-salinity water and changing ice-
  cover exert strong climate feedbacks. A mixed-layer ocean
  is not sufficient for such climate studies.
• The vertical structure of the oceanic heat transport
  determines the seasonal ‘delivery curve’ and its partition
  between local mixed-layer heat storage and storage of
  advected heat.

• The water-column heat-storage annual cycle (~ 3
  GJoules/m2 in Labrador Sea, Gulf-Stream ‘hot spots’) has
  the potential to more accurately give Qnet (based on
  sustained time-series of in situ hydrography or satellite
  altimetry).
   – We have ignored seasonal modulation of the ocean heat transport,
     for example in the wind-driven Ekman layer, believing that its
     divergence will be important mostly in frontal regions
• Coupled heat- and fresh-water transport occurs in both
  ocean and atmosphere: analyzing ocean models one finds
  that in the northern Atlantic both shallow and deep
  outflows from Arctic and Labrador Sea shape the
  meridional overturning and its thermodynamic transports.

•    Oceanic transport diagrams drawn on the Θ-S plane
    express the heat/fresh-water interaction, show water-mass
    movement across sections, and water-mass conversion
    between sections. They help to diagnose, in the example
    given here, the loss of identity of high-latitude (Greenland-
    Scotland-Ridge) overflows and the strong role played by
    the Labrador Sea in the Häkkinen model. In particular,
    shallow low-salinity waters from the Arctic strongly resist
    deep convection and their model representation is an
    ongoing challenge.
The solution? Instrument the Arctic Rim. Do ASOF.
            Deploy the Eriksen Seaglider:
THE END

				
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