AMSU Temperature Change per year

plotting the variation in temperature per year for each day produces this plot:



Interestingly the t-rise is not a constant over the year but has a pronounced dip at the beginning of May for lower air temp.

The sea surface temperature is practically flat. There is a peak during the NH summer. Why is this? Isn't the SH ocean bigger and therefore more likely to show an increase during it's summer when averaged globally. Is evaporation producing a stabilising effect which is less effective over the NH?