Lecture 4b (Ch. 5 of text) Properties of Seawater (Part II)

1. Lecture 4b (Ch. 5 of text) Properties of Seawater (Part II) Density and Pressure

2. Why is the deep ocean cold?

3. Vertical Structure of Temperature Thermocline Thermocline is a permanent hydrographic feature of temperate and tropical oceans.

4. Seasonal evolution of thermocline at the mid-latitudes Growing period Downward heat transport from Sep. to Jan. Decaying period

5. Vertical Structure of Temperature Outstanding question: what sets the depth of the thermocline?

6. Transfer of Heat to the Ocean (heat flux) Absorption of solar radiation decreases rapidly with depth

7. What controls the ocean’s salinity? Salinity variations are determined by the addition or removal of H2O from seawater Processes such as evaporation and sea ice formation will increase the salinity Processes such as rainfall, runoff, and ice melting will decrease the salinity

8. halocline Salinity Become unchanged with time How do the water masses move? c.f. Fig.5.13b Temperature

9. Pressure in the Ocean (water is not absolutely incompressible) Hydrostatic Equation Hydrostatic Balance

10. Seawater density is a function of both temperature and salinity (so-called TS diagram) A ρA < ρB ρB < ρC B C

11. OCEAN WATER MASSES

12. Vertical profiles DENSITY: controls the movement and stability of the ocean water masses

13. Vertical circulation driven by density  Thermohaline Circulation Density stratification (18%) Tropical oceans: pycnocline ≈ thermocline Mid-latitudes: pycnocline ≈ halocline High latitudes: no pycnocline formation Why? (important)

14. More on the DENSITY Density: amount of mass per unit volume Units: kg m-3 Linear Equation for “in situ” Density

15. But water is slightly compressible

16. Density is actually a non-linear function of Temperature, Salinity and Pressure ! Kg m-3

18. Taking into account compressibility effects Potential Temperature

19. Taking into account compressibility effects Potential Density

20. In situ Temperature Temperature of a particle of water measured at a particular depth and pressure (no correction for compressibility effects) At the ocean surface In Situ and Potential Temperature are the same! T1 θ1 T1=θ1 Surface T2 T2≠θ1 θ1 Deep ocean Potential Temperature Temperature that a particle would have if raised adiabatically to the surface of the ocean (corrects for the effects of compression occurring at great depth  make the particle warmer)

21. In situ Density Potential Density

22. Histograms of Temp. and Salinity in the Oceans Temperature Natural thermostate mechanism tropical cirrus clouds resulting from deep convection contribute to long-wave radiative heating of the tropospheric column, and at the same time reduce solar insolation at the sea surface, in this way cooling the ocean. This dual tropospheric, long-wave radiative heating and surface, short-wave radiative cooling role of cirrus is called the thermostat mechanism. The deep convection occurs only when the SST exceeds 27 C, which is associated with the so-called super-greenhouse effect Salinity

23. TS Diagram Kg m-3 Temperature Salinity

24. Distribution of T and S in the Ocean

25. Tracking Water Masses on TS diagrams AABW: Antarctic Bottom Water NADW: North Atlantic Deep Water AAIW: Antarctic Intermediate Water

26. Tracking Water Masses on TS diagrams

27. Worlds ocean Water Masses

28. Properties of Seawater Mixing (supplements of Ch.5.6)

29. How to mix water masses in the ocean? Molecular diffusion Turbulent diffusion

30. Horizontal Stirring and Mixing

31. Horizontal Stirring and Mixing

32. VerticalStirring and Mixing Mixing of two water masses with same Density O1T1 S1 O2T2 S2

33. Mixing along surfaces of Constant Density y + z Surfaces of constant density (i.e. isopycnal) _

34. Mixing along surfaces of Constant Density y + z Surfaces of constant density Along - Isopycnal diffusive mixing _

35. Mixing across surfaces of Constant Density y + z Surfaces of constant density Along - Isopycnal diffusive mixing Across - Isopycnal diffusive mixing _

36. Definitions of Mixing y + z Surfaces of constant density the “skew flux” Diapycnal Mixing _

37. Definitions of Mixing y + z Surfaces of constant density the “skew flux” advection Diapycnal Mixing turbulent diffusion _

38. 非絕熱 Diabaticexchanges with the atmosphere at the surface T1 S1 T2 S2 Adiabaticchanges and Mixingin ocean interior 絕熱

39. Summary of major mixing processes in the Ocean • Surface: • Wind stirring and vertical mixing in the surface layer • Surface fluxes of heat and salt  buoyancy fluxes • Surface Waves • Interior: • Along Isopycnal • eddies and fronts • Across Isopycnal • internal wave breaking • Bottom: • Breaking internal waves over rough topography (Important concepts)

40. Ocean Circulation and Climate Mixing energy and dissipation of tides Mixing rates in the ocean govern the rate at which the ocean absorbs heat and greenhouse gases, mitigating climate. Global climate change forecasts are uncertain in part due to uncertainty in the global average ocean mixing rate. Mixing rates in the ocean vary geographically depending on bottom roughness. Shown are mixing rates observed during an oceanographic survey across the Brazil Basin in the South Atlantic Ocean. Low mixing rates (purple) were found over the smooth topography to the west, and higher mixing rates (colors) over the rough topography to the east (Mauritzen et al. 2002, JGR)

41. Properties of Seawater Dissolved Gases (Ch.5.6) (focus on O2 and CO2)

42. Dissolved Gases (ml l-1) Air Total pressure = sum of partial pressures Seawater

43. Oxygen Saturation curve

44. Main regulator is the activity of organisms (biological oceanography later)

45. compensation depth Anoxic environment Respiration: Animal, plants and microbial decomposition Dissolved Gases in the Ocean Oxygen profile

46. Main sources of O2 in the surface layer: photosynthesis and diffusion across the air-sea interface Why does the O2-minimum layer coincide with the pycnocline layer? (important) Why does the concentration increase with depth toward the deep seas? (important)

47. Why is the pH of seawater close to neutral? (Seawater pH=7.5-8.5) pOH ?

48. Carbon Dioxide and Carbonate system Why is this important (important)? • Regulates temperature of our planet 2. Important for the ocean biota 3. Regulates the acidity of sea water The pH of water is directly linked to the CO2 system

49. Carbonate (碳酸鹽) Carbon Dioxide and Carbonate system Sources for acidity in the ocean Carbonic Acid Bicarbonate Ion

50. At the pH of normal seawater, HCO3- makes up about 80% of the carbon species less H+ ions need to be released More H+ ions need to be released