Guest Post by Willis Eschenbach
To continue my investigations utilizing the CERES satellite dataset of top of atmosphere radiation, here is a set of curious graphs. The first one is the outgoing (upwelling) longwave radiation at the top of the atmosphere (TOA) versus the sea surface temperature, for the northern hemisphere, at the times of the solstices and equinoxes.
Figure 1. Northern Hemisphere TOA outgoing longwave, versus sea surface temperature. Colors represent latitudes, as follows: dark blue, 10°; red 30°; yellow 50°; sky blue 70°. Vertical dashed line is at 30.75°C. Horizontal dashed line is at 300 W/m2. Black solid line shows the surface upwelling longwave radiation (calculated at emissivity = 0.95). Click to enlarge.
I find this graph both interesting and puzzling.
The first puzzle to me is, why is outgoing radiation in July about 230-250 W/m2 from the pole to the Equator? I mean, the upwelling radiation from the surface (solid black line) increases by 50% from the coldest to the warmest areas … but the upwelling longwave is all about the same regardless of the sea surface temperature. How bizarre!
The second puzzle is that there seems to be a fairly hard limit of about 300 W/m2 of TOA upwelling LW. Not only that, but it doesn’t vary much month to month.
The third puzzle is that even up in the Arctic regions, there’s little seasonal change in the upwelling LW. It only swings about 30 W/m2 at the most variable point, and less as you move away from the poles.
Now, what I think is happening at the warmest temperatures is the same thing that the TOA reflected solar showed in my last post—a significant increase in clouds. Let me explain why more clouds means less upwelling longwave radiation. Remember that this is upwelling longwave radiation. Suppose we have some amount X of upwelling radiation coming from the ground. If we interpose a layer of cloud between the surface and the TOA, the cloud will absorb that upwelling LW radiation, and then re-radiate it, half upwards and half downwards. This reduces the amount of upwelling longwave at the TOA, as we see happening at the warm end of the scale above.
Here is the same analysis, but this time for the southern hemisphere.
Figure 2. Southern Hemisphere TOA outgoing longwave, versus sea surface temperature. Colors represent latitudes, as follows: dark blue, 10°; red 30°; yellow 50°; sky blue 70°. Vertical dashed line is at 30.75°C. Horizontal dashed line is at 300 W/m2. Black solid line shows the surface upwelling longwave radiation (calculated at emissivity = 0.95). Click to enlarge.
Want to know what is surprising to me about the southern hemisphere?
I’m surprised at how little the TOA upwelling longwave changes from season to season. The sun comes and goes … but the southern hemisphere upwelling LW is largely unaffected. Every season of the year it’s about the same, 200 W/m2 around the icy antarctic, rising to 300 W/m2 at about 28°C, and then dropping from there. What’s up with that?
My goodness, the amount there is to learn about this incredibly complex system has no end, I can only shake my head in awe …