Since the first two digits of seawater density (when expressed as kg m-3) are always the same, it is very common for oceanographers to drop these first two digits and express density in terms of a value called “Sigma-t (σt )”. Seawater Density σs,t,p = ρ(s, t, p) - 1000.0 (salinity and temperature effects) ignoring the pressure effect… σt = ρ(s, t, 0) - 1000.0 Example, ρ = 1026.34 kg m-3 is the same as σt = 26.34 potential temperature (θ) is the temperature a water parcel will reach if a water parcel is brought from to the surface adiabatically sigma-theta (sθ) is the same as sigma-t except potential temperature replaces temperature σθ = ρ(S, θ, 0) - 1000.0 1 Temperature Salinity (T-S) Diagram Linear Equation of State an approximation of the full nonlinear equation of state a = 0.15 kg/m3/oC b = 0.78 kg/m3/part per thousand k = 4.5x103 kg/m3/decibar Major Constituents of Seawater 1. Salt (typical value) = 3.5% = (wt salt/wt salt water)*100 Composition of Salt (percent of total salt by weight) Cl- (chloride) 55% Na+ (sodium) 31% - SO4 (sulfate) 8% Mg2+ (magnesium) 4% Ca2+ (calcium) 1% K+ (potassium) 1% - HCO3 (bicarbonate) 0.4% sea water density increases until the freezing point is all other ions 1% reached. This means that to form ice, the entire mixed layer must be cooled to the freezing point before ice formation can 2. dissolved gases: N , O , CO , He, Ar begin (more info at: http://www.usna.edu/Oceanography/courses/ 2 2 2 SO426_maksym/text/chapter3_iceformation.htm). 2 Formal Definition of Salinity (see Stewart Chapter-6) Salinity Based on Chlorinity S = 1.80655 Cl Practical Salinity Scale of 1978 Stewart 6.1 where C (S, T, 0) is the conductivity of the sea-water sample at the range of salinity for most of the ocean's water is temperature T and standard atmospheric pressure, and C (KCl, T, 0) is from 34.60 to 34.80 parts per thousand, which is 200 the conductivity of the standard potassium chloride (KCl) solution at temperature T and standard atmospheric pressure. parts per million. The variability in the deep North Pacific is even smaller, about 20 parts per million. Salinity variation with Zonal Average Salinity depth • Curves for high and low latitudes begin at different surface salinities • Halocline = layer of rapidly changing salinity • At depth, salinity is uniform 3 Fine details of how salinity changes with time are not well understood because of limited observations, but that should change in the near future with new satellite technology Consequences of Sea Ice Surface salinity variation Formation • High latitudes have low surface salinity – High precipitation and runoff • Ice formation excludes salts and makes – Low evaporation “freshwater” ice and leaves behind more • Tropics have high surface salinity saline seawater – High evaporation – Low precipitation • Equator has a dip in surface salinity • Ice melting releases freshwater to form – High precipitation partially offsets high evaporation thin low salinity lens on the ocean surface 4 Vertical Temperature Profiles of Tropical, Mid Latitude and Polar Oceans Seasonal variations in the ocean surface - at a location in the North Pacific (50N, 145W) Note variations in the mixed layer depth and the thermal contrast between surface and deeper waters (when is the mixed layer deepest? And coldest? Why is there this seasonal variation?) SST Seasonal Amplitude 5.