1 / 14

Mode (Eighteen Degree) Water

Mode (Eighteen Degree) Water. V.Y. Chow EPS 131 12 Dec 2005. Outline. What are Mode Waters? Eighteen Degree Water Location and properties Formation of 18 ° Water Rates and Mechanisms Importance of 18 ° Water. 26.2. 26.5. 25.2. 24-25.4. 25.5. 25.5. 26.0. 26.2-26.7. 26.0.

faunus
Download Presentation

Mode (Eighteen Degree) Water

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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. 26.2 26.5 25.2 24-25.4 25.5 25.5 26.0 26.2-26.7 26.0 26.2-26.3 26.85 Eighteen Degree Water 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 Tθ, S, O2, density (Tθ, 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. A - Potential Temperature B - Salinity C - Density D - Oxygen Uniform Properties • Vertically homogenous waters (66ºW ) • A. Tθ (μ = 17.88°C) • B. S (μ = 36.5 psu) • C. σθ (μ = 26.45 kg m-3) • D. O2

  6. 4°C 18°C 25°C Gulf Stream 18°C Generic winter location of STMW formation Geostrophic recirculation pathways of STMW 18° Water in the North Atlantic • Thick layer south of the Gulf Stream in the Sargasso Sea • Lies above the permanent pycnocline

  7. 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 • 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!! Due to a mix of processes:

  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/abs/nature01253.html • http://sam.ucsd.edu/sio210/lect_5/lecture_5.html • http://www.nature.com/nature/journal/v437/n7059/full/nature03969.html • http://www.whoi.edu/science/PO/people/tjoyce/clivar/clivarindx.htm • http://www.climode.org/documents/climode_science_final_overview_REFs.pdf • http://ams.allenpress.com.ezp1.harvard.edu/amsonline/?request=get-document&issn=0894-8755&volume=013&issue=14&page=2550 • http://www.uib.no/jgofs/Final_OSC/Bates.pdf • http://sam.ucsd.edu/sio210/gifimages/shallowoverturn.gif • http://www.climode.org/Meetings/NOV_04/kwon_nov04.pdf

More Related