Guest Post By Willis Eschenbach
Often I start off by looking at one thing, and I wind up getting side-tractored merrily down some indistinct overgrown jungle path. I was thinking about the difference in the strength of the sunshine between the
apogee aphelion, which is when the Earth is furthest from the sun in July, and the perigee perihelion in January, when the Earth and the sun are nearest. On a global 24/7 average value, there is a peak-to-peak aphelion to perihelion swing of about twenty-two watts per square metre (22 W/m2). I note in passing that this is the same change in downwelling radiation that we’d theoretically get if starting in July the CO2 concentration went from its current level of 400 ppmv up to the dizzying heights of 24,700 ppmv by January, and then went back down again to 400 ppmv by the following July … but I digress.
Now, because the Earth and sun are nearest in January when the southern hemisphere is tilted towards the sun, there is a larger swing in the solar strength in the southern hemisphere than in the northern.
While I was investigating this, I got to looking at the corresponding swings of the ocean surface temperature. I split them up by hemisphere, and I noticed a most curious thing. Here’s the graph of the annual cycle of solar input and sea surface temperature for the two hemispheres:
Figure 1. Scatterplot, top-of-atmosphere (TOA) solar input anomaly versus ocean surface temperature. Northern hemisphere shown in violet, southern hemisphere shown in blue. Monthly data has been splined with a cubic spline. Data from the CERES satellite dataset.
So … what is the oddity? The oddity is that although the swings in incoming solar energy are significantly larger in the southern hemisphere, the swings in ocean temperature are larger in the northern hemisphere. Why should that be?
The difference is impressive. As a raw measure, the northern hemisphere ocean surface temperature changes about seven degrees C from peak to peak, and the TOA solar varies by 216 W/m2 peak to peak. This gives a change of 0.032°C per W/m2 change in solar input.
In the southern hemisphere, on the other hand, the ocean surface temperature only swings 4.7°C, while the solar input varies by 287 W/m2 peak to peak. This gives a change of .0162°C per W/m2, about half the change of the northern hemisphere.
So that’s today’s puzzle—why should the ocean in the northern hemisphere warm twice as much as the southern hemisphere ocean for a given change in solar forcing?
Part of the answer may lie in the depth of the ocean’s mixed layer. This is the layer at the top of the ocean that is mixed regularly by a combination of wind, waves, currents, tides, and nocturnal overturning. As a result, in any given location the mixed layer all has about the same annual average temperature. (However, monthly changes are still largest and the surface and decrease with increasing depth.) This mixed layer worldwide averages about 60 metres in depth. But the mixed layer is deeper in the southern hemisphere, averaging about 68 metres in the southern hemisphere versus about 47 metres in the northern.
However, two things argue against that conclusion. One is that the mixed layer in the southern hemisphere is about 45% deeper than in the northern … but the northern hemisphere sensitivity of temperature to incoming solar is double, not 40% larger.
The other thing that argues against the mixed layer difference is that the thermal lags in the two hemispheres are very similar, with peak temperatures in each hemisphere occurring almost exactly two months after peak solar. In a previous post entitled “Lags and Leads” I discussed how we can use the difference in time between the peaks of solar power and temperature shown in the scatterplot in Figure 1 to calculate the time constant “tau” of the system. This two month lag is equivalent to a time constant tau in both hemispheres of 3.3 months.
Then, using that time constant tau we can calculate the equivalent depth of seawater needed to create a thermal lag of that duration. A time constant tau of 3.3 months works out to be equivalent to 25 metres of seawater with all parts equally and fully involved in the monthly temperature changes (or a deeper mixed layer with monthly temperature swings decreasing with depth).
But since the time constant “tau” is the same for both hemispheres, this means that the equivalent depth of water that is actually involved in the annual cycle is the same in both hemispheres.
Or in other words, the more of the ocean that is involved in monthly temperature swings, the greater the lag there will be between solar and temperature peaks. But in this case, the thermal lags are the same in both hemispheres, meaning the equivalent depth of ocean involved is the same.
Then, I thought “Well, maybe it’s because one pole is underwater and the other pole is on land”. So I repeated the calculation of the temperature and solar swings using just the range from 60° North to 60° South, in order to eliminate the effect of the poles and the ice … no difference. The northern hemisphere non-polar ocean warms twice as much for a given change in solar energy as does the southern non-polar ocean. The difference is not at the poles.
So my question remains … why is the northern hemisphere ocean surface temperature twice as sensitive to a change in solar input as is the southern hemisphere ocean temperature?
My best to all. Here, we have had a rare June rain, most welcome in this dry land, so for all of you today, I wish you the weather of your choosing in the fields of your dreams …
My Usual Request: Misunderstandings can be minimized by specificity. If you disagree with me or anyone, please quote the exact words you disagree with, so we can all understand the exact nature of your objections. I can defend my own words. I cannot defend someone else’s interpretation of some unidentified words of mine.
My Other Request: If you believe that e.g. I’m using the wrong method or the wrong dataset, please educate me and others by demonstrating the proper use of the right method or the right dataset. Simply claiming I’m wrong about methods or data doesn’t advance the discussion unless you can point us to the right way to do it.
UPDATE: Here are two maps of the same data, which is the change in ocean temperature per 0ne watt/metre squared (W/m2) change in top of atmosphere (TOA) solar radiation …
The gray contour lines show the global average value of 0.02 °C per W/m2.