Pressure Pressure changes provide the push that drive ocean currents Key is the hydrostatic pressure Hydrostatic pressure is simply the weight of water acting on a unit area at depth Total pressure at depth will be sum of the hydrostatic & atmospheric, or p t = p h + p a

Hydrostatic Pressure Hydrostatic pressure is simply the weight of water acting on a unit area at depth Mass seawater in column =  A D = [kg] – A = cross-sectional area of column [m 2 ] – D = depth of water column [m] Weight column = (  A D) * g – Mass * acceleration gravity (g = 9.8 m s -2 )

Hydrostatic Pressure Hydrostatic pressure is the weight per unit area p h =  g A D / A p h =  g D Holds for  = constant Often p h = -  g z (z+ up) D p h =  g D

Let, D = 100 m &  = 1025 kg m -3 Hydrostatic Pressure, p h =  g D = (1025 kg m -3 ) (9.8 m s -2 ) (100 m) = 1,004,500 kg m -1 s -2 [=N/m 2 ] Pressure is a stress (like  w ) but normal to the surface not along it Hydrostatic Pressure Example

p h = 1,004,500 N m -2 1 N m -2 = 1 Pascal pressure 10 5 Pa = 1 bar = 10 db p h = 1,004,500 Pa (10 db/10 5 Pa) = db Example Cont. (or unit hell)

First, 100 m depth gave a p h = db Rule of thumb: 1 db pressure ~ 1 m depth 1 db ~ 1m

Total pressure = hydrostatic + atmospheric p t = p h + p a p a varies from 950 to 1050 mb ( db) p a = p h m) Mass atmosphere = mass top 10 m ocean Total Pressure

Dealing with Stratification Density is a f(depth) Taking a layer approach dp =  (z) g dz dz = layer thickness [m] Summing over D p h =   (z) g dz (where  over depth, D) D

Example with Stratification  1 = 1025 kg m -3  2 = increases from 1025 to 1026 kg m -3 What is p h (100m)??

Example with Stratification Sum over the top 2 layers p h (100 m) = p h (layer 1) + p h (layer 2) Layer 1: p h (1) = (1025 kg m -3 ) (9.8 m s -2 ) (50 m) = 502,250 N m -2 (or Pa) 10 5 Pa = 10 db p h (1) = db

Example with Stratification Layer 2: Trick: Use average density!! p h (2) = ( kg m -3 ) (9.8 m s -2 ) (50 m) = 502,500 Pa = db Sum over top 2 layers p h (100 m) = p h (1) + p h (2) = = db

Hydrostatic Pressure Hydrostatic relationship: p h =  g D Links water properties (  ) to pressure Given  (z), we can calculate p h Proved that 1 db ~ 1 m depth Showed the atmospheric pressure is small part of the total seen at depth