Mode (Eighteen Degree) Water V.Y. Chow EPS 131 12 Dec 2005.

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Presentation transcript:

Mode (Eighteen Degree) Water V.Y. Chow EPS Dec 2005

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

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

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 = kg m -3 ) D. O 2

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

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 m found just to the south of the Gulf Stream Red = high PV, Purple = low PV

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!!

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)

Rates & Mechanisms Best available estimates of formation and dissipation rates:  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

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 m 3 μ ann destruction = 1.06 ± 0.16 x m 3 Turnover = 3.54 ± 0.54 yr

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 ) Can prohibit deep-ocean nutrients from directly upwelling to the "euphotic" zone (wedge of cool, nutrient-poor water)

18° Water and the Nutrient Gradient

References s.pdf document&issn= &volume=013&issue=14&page=