Introduction to Ocean Physics

Transcription

1 Introduction to Ocean Physics Alberto R. Piola Departamento Oceanografía, Servicio de Hidrografía Naval and Departamento Ciencias de la Atmosfera y los Océanos, Universidad de Buenos Aires, Argentina apiola@hidro.gov.ar

2 Why care about ocean physics? Beware, this list is biased and incomplete Together with the atmosphere the ocean redistributes heat on Earth

3 Heat unbalanced North South Net Gain Net Loss Net Loss 250 W/m poleward poleward W -5 Meridional Heat Flux

4 Meridional Heat Flux (PW) Redistribution of heat on Earth Q m = 0 C p T v A v A atmosphere ocean total Eq Fasullo & Trenberth, J. Climate, 2008

5 Why care about ocean physics? Beware, this list is biased and incomplete Together with the atmosphere the ocean redistributes heat on Earth Water's high heat capacity, climate modulation at long time scales

6 Earth radiation balance and ocean heat content CERES: Clouds and the Earth s Radiant Energy System Argo upper ocean profiling floats Douglass & Knox, Phys. Lett. A, 2009 F TOA (t)da o F geo (t)da i o i d dt o E O (t)dv O dh O dt

7 Argo network 3235 active floats as of 30 Aug 2011

8 Why care about ocean physics? Beware, this list is biased and incomplete Together with the atmosphere the ocean redistributes heat on Earth Water's high heat capacity, climate modulation at long time scales About 50% of the Earth chlorophyll is produced by marine organisms Vertical circulation and mixing drive the upward nutrients flux required to support phytoplankton growth

9 Annual mean surface chlorophyll distribution Life in the Ocean

10 Why care about ocean physics? Beware, this list is biased and incomplete Together with the atmosphere the ocean redistributes heat on Earth Water's high heat capacity, climate modulation at long time scales About 50% of the Earth chlorophyll is produced by marine organisms Vertical circulation and mixing drive the upward nutrients flux required to support phytoplankton growth The ocean can store constituents, such as greenhouse gases during long periods of time

11 Outline Basic properties of seawater: Heat capacity, pressure, temperature, salinity and density Drivers of the ocean circulation: density and wind Rotation Wind induced circulation, Ekman and linear theories Upwelling: coastal, equatorial, others Thermohaline alterations, heat and mass balance Overturning circulation

12 Cationes g/kg meq/kg Sodio Potasio Magnesio Calcio Sr Total Aniones Cloro Bromo Fluor Sulfato Bicarbonato Total Salinity is the mass of dissolved salts (in grams) per kilogram of seawater. Mass of dissolved salts (g) Mass of seawater (Kg) Principales Constituyentes (35 ppm) Major constituents of seawater S ~ 35 por m 3 (aprox Kg) de agua de mar NaCl Kg MgCl MgSO CaSO K 2 SO CaCO Kbr SrSO H 2 BO Total Salinity of seawater Salinity changes the ocean density. In short time scales salinity changes due to evaporation, precipitaion and runoff. Excellent indicator of changes in Earth water cycle Away from the sea surface salinity is a useful quasi-conservative watermass tracer. Changes in salinity produce changes in the electrical conductivity of seawater. Conductivity measurements lead to the most accurate measurements of ocean salinity

13 Surface salinity NODC World Ocean Atlas 2009 You can build your own fields here:

14 Density of seawater Density of seawater is a function of its pressure, temperature and salinity ρ = ρ (p, T, S) Compressibility κ Δp = -ΔV/V κ = κ (p, T, S) Thermal expansion α ΔΤ = ΔV/V α = α (p, T, S) Haline contraction β ΔS = -ΔV/V β = β (p, T, S)

15 Temperature (ºC) Density of seawater Salinity κ Surface α β ~ 4000 m Surface ~ 4000 m Changes of compressibility (κ, 10-4 kg/m 3 /dbar), thermal expansion (α, 10-4 kg/m 3 /ºC) and haline contraction (β, 10-4 kg/m 3 /psu) as a function of T and S at the sea surface and at 4000 m.

16 Sea surface temperature The non-linearity of the equation of state of sea water gives rise to several important phenomena: 1) At low temperature the effect of salinity changes over density is relatively more important than the temperature changes 2) At low temperature seawater is more compressible than warm water (thermobaricity) 3) Mixtures of two waters with different temperature and salinity but the same density is denser than the original waters (cabbeling)

17 Temperature vs. Salinity A B Nonlinearity: Cabelling The density σ at the sea surface (0 dbar) plotted as a function of T and S. Note the change in the slope of isopycnals, The gray dots represents two water types of equal density ( = 1026 kg/m 3, σ = 26 kg/m 3 ). The segment is the T-S space of mixtures of A and B, which is denser than the original water masses.

18 Effects of nonlinearity: Evaporation The Mediterranean case Atlantic Ocean Mediterranean The warm-salty water flowing from the Med into the North Atlantic (red arrow) has density similar to the cold-fresh waters outflowing from the Northern Seas (blue arrow). T and S distributions at 1000 m in the North Atlantic

19 Depth (m) Depth (m) Effects of the nonlinearity: longitude The Mediterranean case Why do Med waters sink to about 1000 m while the NADW, its Northern Seas counterpart sinks further deep? NADW Med %O 2 saturation Med %O 2 saturation and salinity section across the N. Atlantic at 35ºN NADW Salinity Salinity

20 Temperature (ºC) Temperature (ºC) Salinity A (Med) Nonlinearity The density σ at the sea surface (0 dbar) (a) and at 4000 dbar (b) as a function of T and S. Note the change in the slope of isopycnals, B (NS) A Temperature vs. Salinity Colder and fresher water is more compressible The red pair (2) in both panels illustrates the comparison of warm-salty water (A) with cold-fresh water (B). If these waters move from the surface (a) to 4000 dbars (b) the warmer water (A), which was denser than B near the surface becomes lighter at 4000 dbars B Adapted from Talley et al., 2011, Descriptive Physical Oceanography, 6th Edition, Academic Press / Elsevier

21 TEOS-10 New Thermodynamics and Equation of State of Seawater In 2010 several International bodies such as IOC, IAPSO and SCOR jointly adopted a new standard for the calculation of the thermodynamic properties of seawater. This new standard, called TEOS-10, supersedes the old EOS80 standard which has been in place for 30 years, and should henceforth be the primary means by which the properties of seawater are estimated. TEOS-10 has constructed a Gibbs function of seawater, from which its density, sound speed, specific heat capacity, specific enthalpy and specific entropy can be calculated. S P (Practical Salinity) S R (Reference Salinity) S A (Absolute Salinity) what we measure with a salinometer, until now was used to determine derived properties, such as density mass fraction of solute in Standard Sea Water mass fraction of solute in the sample, what matters to calculate the sample density Oceanographers don t panic BUT visit (asap)

22 Effects of rotation Ω = 7.29x10-5 s -1 Coriolis force N Pole t = 0 60ºN t = 28h N Pole 60ºN 34ºN Equator 0ºN Object thrown over sphere at 1389 km/h Object thrown over sphere rotating at 1 revolution per day at 1389 km/h (relative velocity = 556 km/h) Coriolis, 1835, J. Ec. Polytech. Available at However, also take a look at (in Spanish)

23 Drains, tornados and other popular myths No way. Water doesn't obey your rules: it goes where it wants...like me, babe! (B. Simpson, Bart vs. Australia, 1995) Even Bart can be right, sometimes (though most likely for the wrong reasons). Before you jump to conclusions check the Rossby Number: the ratio of the inertial to the Coriolis terms in the momentum balance Ro = U/L f. If the Earth rotation plays a role Ro << 1.

24 Surface winds Satellite derived wind speed (m s -1 ) from SCOW climatology July (Risien and Chelton, J.Phys.Oceanogr., 2008). You can browse maps and download data at

25 Wind stress = ρ a C D U 2 10 C D : drag coefficient

26 From D. Reed SJSU Oceanography course, reproduced from M. Tomczak Surface wind stress

27 Wind stress Current velocity vectors plotted at increasing depths The boundary layers Ekman layers: The balance between vertical turbulent mixing and the Coriolis force Ekman, Ark. Mat. Astron. Fys., 2 (11), 1-52, Available at

28 The boundary layers Ekman transport The vertical integral of the velocity vector (the Ekman transport, T Ek ) is perpendicular to the wind direction. Opposing wind directions create regions of mass convergence and divergence in the Ekman layer, which are compensated by vertical motions. T Ek

29 Sverdrup circulation A linear wind induced circulation Sverdrup balance circulation (Northern Hemisphere). Westerly and trade winds force Ekman transport, creating Ekman pumping and suction and hence Sverdrup transport. β V = x / The meridional transport (V) can be determined from the curl of the wind stress Copyright 2011 Elsevier Inc. All rights reserved Adapted from Talley et al., 2011, Descriptive Physical Oceanography, 6th Edition, Academic Press / Elsevier

30 Sverdrup circulation Realistic winds TALLEY FIGURE 5.17 Sverdrup transport (Sv), where blue is clockwise and positive is counterclockwise circulation. Wind stress data are from the NCEP reanalysis (Kalnay et al., 1996). The mean annual wind stress and wind stress curl used in this Sverdrup transport calculation are shown in Figure 5.16a and in the online supplement, Figure S5.10. Copyright 2011 Elsevier Inc. All rights reserved Adapted from Talley et al., 2011, Descriptive Physical Oceanography, 6th Edition, Academic Press / Elsevier

31 Wind induced vertical velocities Ekman pumping Satellite derived upwelling velocity from SCOW wind climatology July (Risien and Chelton, J.Phys.Oceanogr., 2008). You can browse maps and download data at

32 no rotation Asymmetric circulation A wind induced circulation with linear (bottom) friction and the western boundary currents constant df/dy (Beta plane) constant f (f plane) Stommel, Transactions Amer. Geoohys. Union, 29, , Available at Also check Munk, J.Meteor., 7, 79-93,

33 Meridional Overturning Circulation Temperature observations in the deep ocean HMS Earl of Halifax, 1751 Observations from Captain Ellis: 28.9 C at the surface and 11.7 C at 1630 m depth The experiment, which seem d at first but mere food for curiosity, became in the interim very useful to us. By its means we supplied our cold bath, and cooled our wines or water at pleasure; which is vastly agreeable to us in this burning climate It appears to me to be extremely difficult, if not quite impossible, to account for this degree of cold at the bottom of the sea in the torrid zone, on any other supposition than that of cold currents from the poles. Count Rumford (Benjamin Thompson, ), 1800

34 Temperature (ºC) Meridional Overturning Circulation Meridional and vertical temperature changes surface 500 m 3000 m 4000 m Latitude Data from World Ocean Atlas

35 heat flux through the sea surface Net winter heat flux through the sea surface (W/m 2 ) Dec-Feb (NH) Jun-Aug (SH) ECMWF : ERA-40 Atlas

36 global water reservoirs and fluxes % 2.3 Sv 3.6 Sv 13.1 Sv 11.8 Sv 86% 78% 1,335,040 4% 96% Courtesy Ray Schmitt, WHOI, In red: updated from Trenberth et al., J. Hydromet., 6, , 2007

37 Evaporation minus Precipitation mm/day ECMWF : ERA-40 Atlas:

38 World Ocean Atlas Winter sea surface salinity

39 Surface thermohaline modification Surface Buoyancy Flux Annual mean air sea buoyancy flux converted to equivalent heat fluxes (W/m 2 ), based on Large and Yeager (2009) air sea fluxes. Positive values indicate that the ocean is becoming less dense. Contour interval is 25 W/m 2. TALLEY FIGURE S5.8 Copyright 2011 Elsevier Inc. All rights reserved Adapted from Talley et al., 2011, Descriptive Physical Oceanography, 6th Edition, Academic Press / Elsevier

40 Potential density at the sea surface

41 Overturning Heat exchange South Equator North

42 Sustaining the vertical structure w T/ z = 2 T/ z 2 (and the MOC) Downward turbulent diffusion of heat (RHS) is compensated by upwelling of cold water (LHS). Globally averaged turbulent heat diffusivity needs to be ~ 10-4 m 2 s -1 or about three orders of magnitude larger than molecular diffusivity. This balance provides the upwelling necessary to balance water formation at high latitude. Munk, Deep-Sea Res., 13, , Available at Munk and Wunsch, Deep-Sea Res., 45, , 1998.

43 Non homogeneous diffusivities Microprofiler estimated diapycnal diffusivity (m 2 /s 2 ) along 32 S in the Indian Ocean. Microprofiler estimated diapycnal diffusivity (m 2 /s 2 ) along 32 S in the Indian Ocean. Adapted from Talley et al., 2011, Descriptive Physical Oceanography, 6th Edition, Academic Press / Elsevier

44 From Kuhlbrodt et al., Rev. Geophys, 45, 1-32, Schematic MOC

45 Surface buoyancy fluxes rising warming/ precipitation expansion density decrease contraction density increase cooling/ evaporation sinking Density variations can effectively generate vertical circulations