Transfer of Heat Across the Ocean Surface (part 2)

1. Transfer of Heat Across the Ocean Surface (part 2) Lecture 5 OEAS-604 September 21, 2011 • Outline: • The Steady State Balance • Heat Content vs. Heat Flux • Heating and Cooling and the Seasonal Thermocline • The Diurnal Warm Layer • Long Term Changes in Solar Constant—Milankovic Cycles

2. Over long time scales and averaged over the whole ocean, the heat budget is roughly in steady state: Flux in Flux out At shorter time scales there is a change in the heat content of the ocean Change in Heat content with time Flux in Flux out

3. Heat Content in the Ocean Recall the change in the amount of heat due to a change in temperature ΔT: Change in temperature Specific heat Change in Heat Content mass Normalize by Volume

4. Change in heat Content: cp = 4.186 J/gC° = 4186 J/kgC° ρ = 1028 kg/m3 10°C 20°C h = 10m A 10 m thick column of water warms by 10°C in 10 days. a) What is the change in heat content? b) If no heat was transferred through the sides or the bottom, what was the surface heat flux?

5. Change in heat Content: cp = 4.186 J/gC° = 4186 J/kgC° ρ ~ 1028 kg/m3 Dimensionally, change in heat content has units: But to get the total heat content over the whole water column, you have to vertically integrate: h = 10m 10°C 20°C The rate at which heat content is changing is a function of time. 10 days This has same units as heat flux!

6. Heat Budget Change in Integrated Heat Content per time Flux In = Flux Out - Qin Qout 1m 1m

7. Heat Flux HEAT FLUX = Q is defined as the amount of heat passing through a unit area in a unit time Q 1m 1m So the amount of energy entering the ocean surface through area A (m2) in time Δt is: Joules

8. Heat Flux This input of heat goes into a volume (V = A×h), where h is the depth of the surface mixed layer, changing its temperature by ΔT QT A hML

9. Thermocline Depth Changes in thermocline intensity and position indicate changes in the the heat content of the ocean

10. What is the total change in heat content? • If the profiles were taken 10 days apart, what was the net surface heat flux?

11. What is the total change in heat content? • If the profiles were taken 10 days apart, what was the net surface heat flux? • Is there any heat flux within the water column?

12. What is the total change in heat content? • If the profiles were taken 10 days apart, what was the net surface heat flux?

13. Heating and Cooling • Integrated over the entire year, the heat losses and gains roughly balance. • So, the area under these two curves is equal. • However during from roughly March to Aug there is a net gain; and from Sept to March a net loss. Heat Gain > Heat Loss Heat Loss > Heat Gain

14. Diurnal Heating and Cooling Over the course of the day, the incoming shortwave radiation is cyclic while the heat loss is roughly steady.

15. Diurnal Warm Layer

16. This balance is approximate and does not hold for any given location in the ocean or over short time periods. Advective Loss/Gain

17. Temperature of Earth (and oceans) is not really constant.

18. Solar Radiation Variability: Eccentricity Earth-Sun orbit varies from elliptical to circular with a period of 100,000 years.

19. Solar Radiation Variability: Inclination The angle of inclination of the Earth is 23.4°, which gives us the seasons. This angle changes between 22 and 24.5° with a period of 41,000 years.

20. Solar Radiation Variability: Precession Precession refers to a change in the direction of the axis of a rotating axis. Earth’s precessional arc is repeats about every 25,000 years.

21. Precession is the change of direction of the axis, not its tilt. Inclination or Obliquity is a change in the tilt.

22. Milankovic Cycles: Milanković mathematically theorized that variations in eccentricity, axial tilt and precession of the Earth’s orbit resulted in 100,000-year ice age cycles. Wobble in the spin about it axis Variations in the tilt of the Earth’s axis Changes in the Earth’s Orbit from circle to ellipse Few glaciers Ice age