I know I was going to have a bit of a break but this is just a short post to try and explain something to Willis Eschenbach. He has a new post on Watts Up With That (WUWT) called Upwelling Longwaver Over the Ocean. Willis has recently been playing around with CERES data and here he shows a set of curious graphs …. [showing the] outgoing (upwelling) longwave radiation at the top of the atmosphere (TOA) versus the sea surface temperature. The graph for the Southern Hemisphere is below and is just a plot of outgoing flux (Wm-2) on the y-axis and sea surface temperature (oC) in the x-axis.
Willis comments that
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?
Well, I think that’s because Willis has plotted outgoing flux against temperature. Just because it’s summer, winter, autumn or spring, if a particular region of the sea has a particular surface temperature, I suspect that this largely determines the outgoing flux. The sea surface temperature may vary with season, but the outgoing flux (as a function of temperature) does not change.
But Willis could actually do quite a lot more if he really wanted to. He could go to the MODTRAN radiation transfer code. If you set the CO2 ppm to 390 (i.e., roughly today’s value) and the Ground T Offset to -27 (i.e., OoC) you get an upward IR heat flux of 200.05 Wm-2. If you change the Ground T Offset to 3 (i.e., 30oC) and redo the calculation you get 299 Wm-2. That seems remarkably similar to the values in Willis’ graph (i.e., about 200 Wm-2 when surface temperatures are 0oC and about 300 Wm-2 when surface temperatures are 30oC). You can change the the type of atmosphere (I was using the Tropical Atmosphere) but then you’ll need to adjust the Ground T Offset to set the surface temperature to be what you want it to be.
So, as far as I can tell there is nothing particularly surprising about these figures. What Willis appears to have essentially shown is that radiation transfer codes – used to determine how CO2 and other greenhouses gases influence the outgoing longwavelength flux – are largely consistent with observations.