In Part One we looked at the absorption of solar and atmospheric radiation (aka “back radiation”) in the ocean.
About half of the solar energy is absorbed in the first meter, but most of the DLR (downward longwave radiation), or “back radiation”, from the atmosphere is absorbed in the first few microns of the ocean surface. And all of the DLR is absorbed in the first 0.1mm of the ocean.
Warmer water is less dense (more buoyant) than cooler water. Less dense water rises to the top over more dense water and so a surface being heated at the very top is “stratified”. This really means that there is no tendency for water from below to displace water above. How then can heat from the top layer of the atmosphere make it into the ocean?
In Part Two we looked first at what would happen if the “back radiation” was absorbed and immediately consumed in the process of evaporation. The ocean would be much colder.
Then we looked at simple models of heating from solar radiation and DLR if there was no convection (within the ocean). Because the conductivity of water is quite low the results are strange – the water boils around 1m down. Of course, this doesn’t happen – the water heats up, expands and rises. This is “natural convection”. The article touched on this and also on “forced convection” via the effects of the wind “stirring” the water.
The ocean is a fascinating place, very complex, but of course the basics of heat transfer still stay the same.
How Does Heat Move Through the Ocean?
There are competing forces, as you might expect in a dynamic system.
- the sun heats the ocean, mostly in the first few meters
- the ocean heats the boundary layer of the atmosphere via convection
- the ocean heats the lower atmosphere via radiation
Huge amounts of energy also move from the equator to the poles, approximately 50:50 via the ocean and the atmosphere.
To consider the complete picture of the energy transfer – the atmosphere also radiates to the ocean (see The Amazing Case of “Back Radiation” -Part One and the following parts). The issue that was probably implied in the original question was “if back radiation increases due to more CO2 (or other trace gas increases) will that energy be mixed into the ocean?”.
If we “dive in” to a little more detail, as in Part Two, then we see that solar radiation is absorbed in the top few meters of the ocean and as a consequence, this ocean heat rises to the surface.
In this article we will try to understand more about how the upper layer of the ocean mixes.
Temperature – Depth Profile vs Wind Speed and SST Variation
There is a huge wealth of experimental results in ocean “heat budgets” and temperature profiles.
First, 35 vertical temperature profiles during the warming and cooling phases of the diurnal cycle at low wind speed from Soloviev & Lukas (1997):
Note: Each successive temperature profile is artificially shifted in temperature for comparison purposes
From Price and Weller (1986) we see another example of how the vertical temperature profile goes from “well-mixed” under higher winds to a definite temperature profile (during the day) under light winds:
With these temperature profiles we also see the effect of the day-night variation. Regardless of wind speed, at night the temperature profile shows that the ocean has become well mixed.
Now temperature profiles under different wind speeds from Soloviev & Lukas (1997):
COARE is the coupled ocean-atmosphere response experiment, and the area of study is in the Western Pacific from the R.V. Moana Wave. What this diagram shows is very interesting, but not surprising:
- in higher wind speeds (A) the ocean is very well mixed (the temperature is the same at the surface as 10m depth)
- in very calm conditions (C) there is a very pronounced temperature profile with depth
- in between (B), the ocean temperature profile shows a transition between these two results
Another dataset from Soloviev & Lukas (1997) – time-series of temperature, salinity and density on one day:
Note: there was a rain event at 19:00
The variation in surface temperature (SST) from day to night indicates the same effect taking place. During light winds in tropical waters very large temperature variations can be seen (up to 2-3°C and even 5°C), whereas more generally the SST diurnal variation is less than 0.5°C.
The SST measurements were done by the HCMR (Heat Capacity Mapping Radiometer) and the study area is East of Sardinia in the Mediterranean.
From Kawai & Wada (2007) an interesting time-based dataset of temperature vs depth against solar flux and wind speed:
Wind-Induced Convection and Diurnal Cycle
At night the surface of the ocean cools rapidly via radiation – but is not receiving any heat via solar radiation. Therefore, the top surface of the ocean cools – and so it sinks.
Therefore, every night, the ocean heat in the top few meters becomes very well mixed.
During the day (and the night), when the wind picks up over the surface two effects take place. One is that the top few meters of the ocean become well-mixed due to wind-induced stirring. The other effect is that the surface cools more rapidly than normal due to convective cooling (convective heat transfer to the atmosphere), and therefore is more likely to sink.
A Brief Digression on Cameos
Not only do climate scientists do a lot of experiments, but sometimes they even come together for a cameo role just because the stars align:
Let’s not pretend this paper was anything other than what it obviously appears to be..
The aim of this article is just to show some field research about how the top layer of the ocean mixes. Wind and diurnal cooling mix heat from the surface into the top few meters of the ocean.
In the next article we will analyze the possible effect of a little more “back radiation” in the light of these results.
Observation of large diurnal warming events in the near-surface layer of the western equatorial Pacific warm pool, Soloviev & Lukas, Deep Sea Research Part I: Oceanographic Research Papers (1997)
Large Diurnal Heating of the Sea Surface Observed by the HCMR Experiment, Deschamps & Frouin, Journal of Physical Oceanography (1984)
Diurnal Sea Surface Temperature Variation and Its Impact on the Atmosphere and Ocean: A Review, Kawai & Wada, Journal of Oceanography (2007)
Diurnal cycling: Observations and models of the upper ocean response to diurnal heating, cooling, and wind mixing, Price & Weller, Journal of Geophysical Research (1986)
Note 1 (added Dec 6th) - To avoid upsetting the purists, when we say “does back-radiation heat the ocean?” what we mean is, “does back-radiation affect the temperature of the ocean?”
Some people get upset if we use the term heat, and object that heat is the net of the two way process of energy exchange. It’s not too important for most of us. I only mention it to make it clear that if the colder atmosphere transfers energy to the ocean then more energy goes in the reverse direction.
It is a dull point.