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 Eighteen Degree Water 26.0 25.2 26.5 26.2-26.7 25.5 26.2 24-25.4 26.2-26.3 26.0 26.85

4. Eighteen Degree Water North Atlantic Subtropical Mode Water (STMW) Name from dominant T= 18 degrees C; thickness = 250 m Uniform T θ, S, O 2, density (T θ, 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 A - Potential Temperature B - Salinity C - Density D - Oxygen Vertically homogenous waters (66ºW ) A. T θ (μ = 17.88°C) B. S (μ = 36.5 psu) C. σ θ (μ = 26.45 kg m -3 ) D. O 2

6. 18° Water in the North Atlantic 18°C 18°C25°C4°C Generic winter location of STMW formation Geostrophic recirculation pathways of STMW Gulf Stream 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 Due to a mix of processes: (1)cross-Gulf Stream fluxes (2)transformation processes within the Gulf Stream (3)impact of the recirculation region on stratification, (4)diapycnal mixing and subduction, and (5)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 parcels move east under this cooling Ocean buoyancy loss triggers convection 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. S ann = 1.23 x 10 14 m 3 μ ann destruction = 1.06 ± 0.16 x 10 14 m 3 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 CO 2 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_REF s.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