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Density and Stratification Density of Water Ocean Surface Temperature

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Density of water density = mass/volume units: g/cm 3 (= g/ml = kg/L) Effect of Temperature on Density density of water - @ 4 oC and 1 atm pressure fresh:1.000 g/cm 3 (by definition!) Density and Stratification sea: 1.027 g/cm 3 (on average) what determines water density? temperature – inverse relationship the major players of the ocean’s layers lower temp = higher density higher temp = lower density salinity – direct relationship lower sal = lower density higher sal = higher density pressure water is essentially (but not exactly) incompressible but at very high pressures (deep depths) – pressure increases density sea level would be ~30-50 m higher without pressure effect Ocean surface temperature often called sea surface temperature or SST strongly correlates with latitude because insolation (amount of sunlight striking Earth’s surface) is high at low latitudes & low at high latitudes surface ocean isotherms (lines of equal temperature) generally trend east-west except where deflected toward poles or equator by currents warm water carried poleward on western side of ocean basins Gulf Stream, Kuroshio Current – Northern Hemisphere Brazil Current, East Australia Current – Southern Hemisphere cooler water carried equatorward on eastern side of ocean basins Canary Current, California Current – Northern Hemisphere Benguela Current, Peru Current – Southern Hemisphere SST overall pattern highest in the tropics (~25-29 oC) where insolation is highest decreases poleward with decreasing insolation negative temperatures in Arctic Ocean & around Antarctica 1 Ocean abyssal temperature cold and dense colder than 4 oC (remember Challenger expedition) colder = more dense remember temperature-density relationship for seawater temperature of maximum density is right at freezing point (~ -1.91 oC) smaller temperature range than SST <1 oC – 2.5 oC @ 4000 m depth overall, from ~1000 m and below, most temperatures are 2 o – 4oC more uniform distribution than SST isolation from insolation (read that twice!) still, some patterns can be discerned colder near Antarctica warmer in North Atlantic warmest in isolated basins (due to diffusion of heat over time) Caribbean Sea basin Philippine Sea basin Predict-a-profile Produce-a-profile kinda like whack-a-mole How did we do? except you don’t actually warm surface waters ≈ 2% of ocean volume hit anything thermocline waters ≈ 18% of ocean volume deep waters ≈ 80% of ocean volume near near near near Alaska Antarctica Alaska Antarctica Polar Temperate Region Tropics Temperate Region Polar Polar Temperate Region Tropics Temperate Region Polar ("high ("mid-latitudes") ("low latitudes") ("mid-latitudes") ("high ("high ("mid-latitudes") ("low latitudes") ("mid-latitudes") ("high latitudes") latitudes") latitudes") latitudes") 0 0 0 warm surface waters warm surface waters 0 ve r cool to warm warm to very warm cool to warm ld pycnocline and permanent thermocline y o 1000 co 1000 1000 1000 ld to 2000 co 2000 2000 2000 ld very cold deep waters very cold to c 3000 3000 3000 3000 Water Depth (m) very cold very cold very cold Water Depth (m) 4000 4000 4000 4000 5000 5000 5000 5000 70 oN 60 ooooooooooooooN 50 N 40 N 30 N 20 N 10 N 0 10 S 20 S 30 S 40 S 50 S 60 S 70 S 70 oN 60 ooooooooooooooN 50 N 40 N 30 N 20 N 10 N 0 10 S 20 S 30 S 40 S 50 S 60 S 70 S 2 Explain-a-profile thermocline in tropics What does this mean? Tropical Waters The permanent thermocline (low latitudes) warm, less dense surface layer over very cold and dense deep waters extends from the base of 32 41 50 59 68 77 86 °F the permanent thermocline is the interval through which temperature the surface mixed layer decreases rapidly with increasing water depth 0 5 1015 20 25 30 °C near near (~75-150 m) to ~1000 m 0 Alaska Antarctica water depth. Polar Temperate Region Tropics Temperate Region Polar seasonal ("high ("mid-latitudes") ("low latitudes") ("mid-latitudes") ("high latitudes") latitudes") The depth of the mixed (summer) thermocline 0 warm surface waters warm surface waters 0 layer is a function of mixing pycnocline and permanent thermocline (homogenization) of the 500 permanent 1000 1000 thermocline warmed surface waters (~150-1000 m) 2000 2000 downward by day-to-day very cold deep waters winds and storms, waves 3000 3000 1000 and surface currents. Water Depth (m) cold deep waters 4000 4000 below thermocline 5000 5000 (below pycnocline) o oooooooooooooo 70 N 60 N 50 N 40 N 30 N 20 N 10 N 0 10 S 20 S 30 S 40 S 50 S 60 S 70 S 1500 Growth of thermocline in mid-latitudes Seasonal Thermocline Temperate Waters Winter storms tend to be (mid-latitudes) NOTE: applies to mid-latitude temperate regions only bigger/stronger than 32 41 50 59 68 77 °F summer storms. March – winter cooling of surface waters has destroyed 0 5 10 15 20 25 °C seasonal thermocline, vertical mixing is taking place Therefore the mixed layer 0 summer mixed layer (0-75 m) 75 m May – surface waters winter mixed layer (0-150 m) tends to be deeper during 150 m seasonal begin to warm, weak winter months. (summer) thermocline forms Summer heating causes thermocline 500 permanent June – surface layer the creation of a seasonal thermocline increasingly shallow thermocline (a steeper (~150-1000 m) and isolated from temperature gradient than deeper waters; ther- during the winter). 1000 1000 m mocline strengthens pronounced seasonality cold deep waters August – thermocline below thermocline reaches maximum is the hallmark of (below pycnocline) the mid-latitudes Decay of thermocline occurs August – January, as surface 1500 waters increasingly cool and mix with deeper waters 3 thermocline in polar regions Salinity and the halocline A permanent thermocline is Polar Waters Salinity changes with latitude due In some areas of the ocean, (high latitudes) to variations in precipitation and surface water and deep water are absent in polar regions 32 41 50 °F evaporation with latitude. separated by a halocline , a zone of rapid change of salinity with because surface waters are Highest ocean salinity is between water depth. 0 5 10 °C 20-30 o north and south of the very cold and deep waters 0 The thermocline and halocline are very cold. equator, because evaporation combine to form the pycnocline exceeds precipitation there. (which is mighty fine). Therefore, there is little Low salinity at the equator and Water stratification (layering) temperature contrast (or poleward of 30 o results from within the ocean is most 500 evaporation being less than pronounced at the latitudes gradient) between polar no permanent precipitation. between 40 oN and 40 oS. surface and deep waters. thermocline (surface waters A small seasonal (summer) are cold, deep waters are cold) thermocline forms but (m) depth water 1000 vertical mixing occurs basically year-round. temperature salinity density 1500 = “thermo” = “halo” = “pycno” Fun with dots on plots o Temperature ( C) Density ( σ t ) 0 5 10 15 20 25 23 24 25 26 27 28 0 warm surface 0 less dense surface waters waters 500 thermocline 500 pycnocline 1000 1000 1500 1500 very cold dense 2000 deep waters 2000 deep waters 2500 2500 Water Depth (in meters) Depth Water (in Water Depth (in meters) Depth Water (in 3000 3000 3500 3500 4000 4000 32 41 50 59 68 77 1.023 1.024 1.025 1.026 1.027 1.028 Temperature ( oF) Density (g/cm 3) 4.
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