Class 8. Oceans Figure: Ocean Depth (mean = 3.7 km) bounded by continents deep: difficult to make observations Figure: Ocean Depth (mean = 3.7 km)

Ship-measurements Only a limited area covered bounded by continents deep: difficult to make observations Only a limited area covered

SST from buoys drifter: can freely drift moored: anchor

Ocean Surface Temperature from Remote Sensing (NOAA) -2.0 34.2 0C 16.1 cold water sinks warm - maximum insolation - albedo of water ~ 7% cold water sinks strong gradient towards the poles some structures: green cold tongue off the coast of california, Peru source NOAA http://www.osdpd.noaa.gov/ml/ocean/sst/sst_anim_full.html

Ocean Surface Salinity Prep>Evap Evap>Prep strong gradient towards the poles some structures: green cold tongue off the coast of california, Peru

ARGO: profiling the interior of the ocean (up to z=-2000 m) drifter: can freely drift moored: anchor

ARGO: profiling the interior of the ocean (up to z=-2000 m) drifter: can freely drift moored: anchor Data products: Temperature, salinity and density

Zonal average temperature in deep ocean warm salty stratified lens of fluid abyss z>1000 m homogeneous mass of very cold water

Schematic of vertical structure convection in the upper layer causes a vertically well mixed layer strong vertical temperature gradient defines the thermocline note: analogy to thermal inversion in the atmosphere very cold water present below z<1000 m

Thermal expansion: Sea-level transgression scenarios for Bangladesh

Density (anomaly s), Temperature and Salinity higher density salty water has a higher density fresh water show dip in density Fig. 9.2: Contours of seawater density anomalies (s=r-rref in kg/m3) rref = 1000 kg/m3 PSU = Practical Salinity Unit ≈ g/kg grams of salt per kg of solution

Simplified equation of state (defined with respect to s0(T0,S0))

Simplified equation of state (defined with respect to s0(T0,S0))

Schematic of vertical structure tendency due to radiative heating T = temperature F = heat flux (Wm-2) rw = density of water cw = heat capacity of water μ

1000 depth (m) cold water - deep convection cold water upwelling 900S 900N 00 latitude

P>E P<E 1000 depth (m) 900S 900N 00 latitude Low salinity if precipitation (P) exceeds evaporation (E)

Thermohaline circulation arctic sea ice

Sea level height

Which balances do apply in the ocean? Hydrostatic balance -> yes Geostrophic balance? Thermal "wind"? Ekman pumping/suction?

Rossby and Reynolds number in the ocean Far away from the equator, e.g. latitude = 400, North-South length scale L = 2000 km (east-west larger) Velocity scale U = 0.1 m/s

Pressure in the ocean mean density in water column high pressure low pressure geef eventueel dp/dz

Which sea level tilt is needed to explain U=0.1 m/s? werk uit op bord Example 1: assume density is constant

Geostrophic flow at depth Example 2: assume density is NOT constant, but varies in the x,y directions => r(x,y)=rref+s(x,y) 1000 depth (m) 900S 900N 00 latitude 23 24 25 26 26.5 27 1. Taylor Proudman 2. Thermal wind

Estimating the geostrophic wind from the density field: The dynamic method This method allows for assessing geostrophic velocities relative to some reference level One can assume that at a "sufficiently" deep height ug=0 1. Taylor Proudman 2. Thermal wind

Geostrophic flow at depth z Example 3: I) assume density is NOT constant, but varies in the x,y directions => r(x,y)=rref+s(x,y) II) surface height is NOT constant 1. Taylor Proudman 2. Thermal wind

Geostrophic flow Example 1: In the ocean geostrophic flow applies (not too close to equator) Pressure induced by surface height variations η Example 2: Horizontal density gradients cause a vertical change in the geostrophic flow velocity ("thermal" wind) Example 3: In principle both height and density variations may apply 1: p184, above 9-11 2: 7-16 3:

Determining the ocean flow from floating plastic ducks? 1. Taylor Proudman 2. Thermal wind

1. Taylor Proudman 2. Thermal wind

1000 depth (m) cold water - deep convection cold water upwelling 900S 900N 00 latitude

Ekman pumping/suction 1. Taylor Proudman 2. Thermal wind

Wind-driven ocean flow Equations with wind-stress

Wind-driven ocean flow Equations with wind-stress Split velocity in geostrophic ('g') and ageostrophic parts ('ag') e.g.

Ekman transport

Ekman pumping (downwards)/suction X wind into the screen

Ekman pumping (downwards)/suction elevated sea level height in convergence area tropics midlatitudes

Ekman pumping/suction due to wind stress 1. Taylor Proudman 2. Thermal wind

Ekman pumping/suction Explanation mass conservation

Ekman pumping/suction Explanation 1. we do not assume that f is constant, but f=f(y) 2. variations in wind stress are much larger than in f

Ekman pumping/suction Example = 32 m/year

Ekman pumping/suction from wind stress climatology downward upward f=0