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

Why is the deep ocean cold?

Vertical Structure of Temperature Thermocline Hydrographic:水文 Thermocline is a permanent hydrographic feature of temperate and tropical oceans.

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

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

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

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 Why not plot the salinity in the polar seas?

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

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

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

OCEAN WATER MASSES

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

 Thermohaline Circulation 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)

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

But water is slightly compressible

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

Taking into account compressibility effects Potential Temperature

Taking into account compressibility effects Potential Density

HW#1: Application of Isostasy/Buoyancy Concept (Due date: 17 April) There is a huge lake with constant depth 100 cm and extension of 500 km. The water surface is still and undisturbed, that is, nothing moves. Now objects A, B, and C (see below for their configurations) are dropped separately and we wait until everything is quiet again. How many cm will the objects be sticking out above or beneath the water surface, if (a) density of the water is constant at 1.03 g/cm3, (b) density of the water, for some reasons, increases linearly with depth from 1.03 g/cm3 at the surface to 1.43 g/cm3 at the bottom

Potential Temperature In situ Temperature Temperature of a particle of water measured at a particular depth and pressure (no correction for compressibility effects) Surface Deep ocean T1 T2 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) T1=θ1 T2≠θ1 At the ocean surface In Situ and Potential Temperature are the same! θ1

In situ Density Potential Density

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

TS Diagram Kg m-3 Temperature Salinity

Distribution of T and S in the Ocean

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

Tracking Water Masses on TS diagrams

Worlds ocean Water Masses

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

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

Horizontal Stirring and Mixing

Horizontal Stirring and Mixing

Vertical Stirring and Mixing Mixing of two water masses with same Density O1T1 S1 O2T2 S2 Object1; object2

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

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

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

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

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

Diabatic exchanges with the atmosphere at the surface 非絕熱 Diabatic exchanges with the atmosphere at the surface T1 S1 T2 S2 Adiabatic changes and Mixing in ocean interior 絕熱

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)

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)

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

Dissolved Gases (ml l-1) Air Seawater Total pressure = sum of partial pressures Seawater Note the log scale of figs

Oxygen Saturation curve Metabolic:新陳代謝的

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

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

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)

Why is the pH of seawater close to neutral? (Seawater pH=7.5-8.5) Hydrochloric:氯化氫的 Acetic:醋的,酸的 vinegar Borax:硼砂 Magnesia:苦土,氧化鎂 Lye:灰汁;灰水;洗濯用鹼水 Sodium:鈉 Hydroxide:氫氧化物 pOH ?

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

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

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

(b) Photosynthesis and respiration

Why are the CaCO3 shells dissolved in the cold, deep water, but not in the warm, shallow water (important) ? Carbonate Buffer self-regulating system

As temperature is low, The cold water has a higher gas-saturation value As the water becomes deeper, The higher pressure also has a higher gas-saturation value Thus, the dissolved CO2 amount increases and makes the water acidic, and melts the CaCO3 shells that sink to the deep-sea floor. →NO Calcareous oozes at high latitudes

Carbon Dioxide and Carbonate system Why is it important? Regulates temperature of our planet Important for ocean biota Regulates the pH value of sea water

CO2 70 ppm Temperature Deuterium:重氫;氫的同位素

CO2 changes in the last 300 yr 70 ppm Industrial Revolution

CO2 changes in the last 50 yr Oceans Biosphere Rock Weathering

How much CO2 can be dissolved by the ocean (role of ocean uptake in regulating the global climate)? Chemical Biological Physical Process that control CO2 absorption in the ocean Carbon Cycle

Grand Carbon Cycle

The Carbonate System sources of inorganic carbon from dissolved CO2 gas from dissolution of Calcium Carbonate NOTE: Biology and Physics participate in the equilibrium of the carbonate system

Total dissolved inorganic carbon this is very small not found in this form

(1) Total dissolved inorganic carbon (2) formation and decomposition of organic matter (1) from dissolution of Calcium Carbonate (2) Total dissolved inorganic carbon

Carbon Dioxide and Carbonate system + High pH - Low pH

Distribution of Carbon species in water + -

Control of pH very rapid reaction in seawater at equilibrium Equilibrium constant hydrogen ion concentration

hydrogen ion concentration + -

Concept of Alkalinity (鹼度)

Alkalinity

Why is the pH of seawater close to neutral?

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