Guest Post By Ben Herman of the University of Arizona
Roger, I have recently read your posting on your blog site on Jan. 6 of this year, referring to a posting of yours in 2009, titled “Two Hypotheses with Respect to Lower Tropospheric Temperature Changes“. Your hypothesis #1 was that the long term temperature changes were due mostly to atmospheric circulation changes, while hypothesis #2 was that the observed changes are due mostly to greenhouse gas forcing. You then closed the current posting by saying evidence to date would favor hypothesis #1, that the changes are due to circulation changes.
Several years back I suggested to you (Roger) and Dr Tom Chase from CU that there might be a correlation between cold/warm winters on the circulation patterns over land and water. The idea was that more than normal northerly wind components at 500 mb over land in mid-latitudes with returning southerly flow over oceans would create a stronger than normal transport of cold surface air over the land, at least in mid latitudes. This would occur because the cold air masses over land would would be warmed much more slowly in traveling south than than they would be over water, resulting in a net cooing. The reverse would be true if the wind regimes were reversed, resulting in a net warming. We performed a simple test of this by computing average 500 mb winds over the Northern Hemisphere for the winter months for the decade 1950-1960 (a cooling period) and 1990-2000 (a warming period). While we did not actually calculate the net north-south transport of air for the 2 decades, it did appear from observations of the mean winds that there was a small tendency for there to be a stronger north-south component of flow over land in the earlier decade. If this were shown to be true with a more rigorous study, it would lend further confirmation of your “hypothesis #1).