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Caldeira Lab

Sensitivity of temperature and precipitation to frequency of climate forcing: Doug MacMynowski

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The title of the paper is Frequency Response of Temperature and Precipitation in a Climate Model. The basic idea here is that, if you’ve got a large range of time scales in a climate’s response, then it’s very useful to look at the response in the frequency domain instead of in the time domain.

What we did is we took this coupled climate model and put in sinusoidal variations in the solar forcing as basically a change in the radiative forcing and looked at how the climate responded on different time scales. In particular, we looked at the temperature response and the precipitation response.

When you look at it in a frequency domain, there’s a couple of really interesting things that you can observe quite clearly. So one of those is that the global mean temperature in this particular model--this is HadCM3L--fits semi-infinite diffusion much, much better than it does, say, a one-box energy reservoir model or a two-box energy reservoir model.

One of the consequences of that is that there’s very, very long equilibration times to changes in radiative forcing. Several people have actually tried using one- or two-box energy models just to get something simple that they can use in policy analysis when, for example, they want to look at what’s the response of the planet going to be to different profiles of greenhouse forcing. Well, this basically shows that using a semi-infinite diffusion as a simple model would be much, much better at capturing the long time scales of the planet.

The other really interesting thing that you observe is that not all of the climate variables respond the same way. In particular, if you look at, say, precipitation over land, that’s strongly influenced by the land/sea contrast. And the land/sea contrast itself reacts very quickly to changes in radiative forcing. So while the ocean takes a long time to heat up, the difference in temperature between the ocean and land responds rapidly, and that means that the precipitation response changes rapidly. So even if you have a very rapid perturbation, in our case that was sinusoids, but this would apply similar conclusions to, say, a volcanic eruption that has very, very quick change in the radiative forcing on the planet, you get a lot of the change in precipitation right away, but you don’t get a lot of the change in temperature until you’ve had that perturbation there for a long time. So if you look at the response to short-period forcing, volcano, you can get quite a different pattern of response than you do if you’re looking at long-term changes, say, from changing greenhouse forcing.


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