Mode (Eighteen Degree) Water

1
Mode (Eighteen Degree) Water

• V.Y. Chow
• EPS 131
• 12 Dec 2005

2
Outline

• What are Mode Waters?
• Eighteen Degree Water
• Location and properties
• Formation of 18 Water
• Rates and Mechanisms
• Importance of 18 Water

3
What are Mode Waters?

• Mode waters are upper ocean waters
• Uniform temperature over thickness of few 100-m
• Short renewal timescales
• Strong property correlations with atmospheric
  indices

4
Eighteen Degree Water

• North Atlantic Subtropical Mode Water (STMW)
• Name from dominant T 18 degrees C thickness
  250 m
• Uniform T?, S, O2, density (T?, S varied since
  Worthington 1959 )
• 18 Water canonical example of STMW (all
  STMW found in regions of large air-sea exchange)

• one of most studied mode water
• most obs one-time hydrographic surveys or
  single point long time- series inside 18 Water
  region
• Station S near Bermuda

5
Uniform Properties

• Vertically homogenous waters (66ºW )
• A. T? (µ 17.88C)
• B. S (µ 36.5 psu)
• C. s? (µ 26.45 kg m-3)
• D. O2

6
18 Water in the North Atlantic

• Thick layer south of the Gulf Stream in the
  Sargasso Sea
• Lies above the permanent pycnocline

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18 Water Potential Vorticity
The presence of 18 Water is reflected in a
substantial 1000 km diameter bowl of low
potential vorticity at depths of 200-500m found
just to the south of the Gulf Stream
Red high PV, Purple low PV
8
18 Water Formation part I

• Due to a mix of processes

• cross-Gulf Stream fluxes
• transformation processes within the Gulf Stream
• impact of the recirculation region on
  stratification,
• diapycnal mixing and subduction, and
• buoyancy loss
• Relative roles not conclusive b/c no
  comprehensive observations collected in any STMW
  region!!

9
18 Water Formation part II

• Surface, late winter, max. air-sea exchange in N.
  Atlantic
• Large winter heat loss same area w/ warm
  surface waters carried by Gulf Stream
• Late winter SST ? 18?C, water parcelsmove east
  under this cooling
• Ocean buoyancy loss triggersconvection
• Convected water mass 18 Water
• Interannual variability due to climate
  variability (North Atlantic Oscillation)

10
Rates Mechanisms

• Best available estimates of formation and
  dissipation rates
• ?15-20 Sv yr-1 (indirect methods from air-sea
  fluxes)
• only about 5 Sv yr-1 inferred to be injected
  seasonally into the subtropical gyre
• 18 Water enters North Atlantic thermocline
• Ekman pumping, and
• Eddy driven subduction

11
18 Water Dissipation

• starts w/ subduction advection into the western
  subtropical gyre
• also due to lateral flows, diapycnal mixing and
  fluxes
• 48 observed winter volume destroyed annually.

• Sann 1.23 x 1014 m3
• µann destruction 1.06 0.16 x 1014 m3
• Turnover 3.54 0.54 yr

12
Importance of 18 Water

• Dominant baroclinic and potential vorticity
  signal in the subtropical North Atlantic
• Substantial contribution to interannual
  variability in oceanic CO2 uptake
  (0.03-0.24 Pg C yr-1 from 1998-2001)
• Can prohibit deep-ocean nutrients from directly
  upwelling to the "euphotic" zone (wedge of cool,
  nutrient-poor water)

13
18 Water and the Nutrient Gradient
14
References

• http//www.nature.com/nature/journal/v420/n6915/ab
  s/nature01253.html
• http//sam.ucsd.edu/sio210/lect_5/lecture_5.html
• http//www.nature.com/nature/journal/v437/n7059/fu
  ll/nature03969.html
• http//www.whoi.edu/science/PO/people/tjoyce/cliva
  r/clivarindx.htm
• http//www.climode.org/documents/climode_science_f
  inal_overview_REFs.pdf
• http//ams.allenpress.com.ezp1.harvard.edu/amsonli
  ne/?requestget-documentissn0894-8755volume013
  issue14page2550
• http//www.uib.no/jgofs/Final_OSC/Bates.pdf
• http//sam.ucsd.edu/sio210/gifimages/shallowovertu
  rn.gif
• http//www.climode.org/Meetings/NOV_04/kwon_nov04.
  pdf