Note: This post resulted from the claim by Dr. Garth Paltridge that the observed surface warming could have been caused by internal ocean fluctuations. — Andreas Schmittner
Could the Earth’s observed surface warming of the last 50 years have been caused by ocean circulation?
Let’s phrase this question as a hypothesis: Changes in ocean circulation have caused the observed surface warming of the past 50 years. We can test this hypothesis because if it were true there must be regions in the subsurface ocean where it has cooled. This follows from energy conservation. Observations, however, show no evidence for cooling of the subsurface ocean. The upper ocean heat content (top 2 km) has clearly increased as several independent estimates show (here is one). How about the layers below that? Could the heat come from there? Here is a figure estimating heat content changes for the decade from the 1990’s to the 2000’s showing that the deepest layers of the oceans have also warmed. I’m not aware of observations that show that deep ocean layers have cooled over the last 50 years. Thus, existing evidence does not support the hypothesis. Therefore we have to answer the question with “No”.
Note that the layers below 2.5 km depth have much smaller changes in heat content compared with the surface. This is because the deepest layers are very old and mix very slowly with the surface. Therefore I think that it is extremely unlikely that ocean circulation has caused the surface warming of the last 50 years.
UPDATE: April 28, 2014:
Mauri: thanks. Here is the figure. This is from an ocean re-analysis, which is essentially a global ocean circulation model driven with (or nudged to) observations. Thus it predicts the changing state of the ocean, including fluid flow, consistent with the existing observations. You could regard this as a reconstruction of past ocean circulation and heat content or a dynamically consistent extrapolation (filling of data gaps) of existing (relatively sparse) observations. Similar to the NOAA Ocean Heat Content figure this estimate also shows an increase over the past 50 years.
Here is another paper with observations of deep ocean heat content changes. Purkey and Johnson 2010 Journal of Climate, vol. 23, p. 6336. These authors use observations from the 1980 to 2010 to estimate the changes from the 1990s to the 2000s. Here is the global figure expressed as heat gain per meter (W/m^3). You can see heat gain at all depths consistent with the Kouketsu et al. figure above.
These authors also show a map of deep (below 4 km) ocean heat flux changes.
Note that bottom waters are warming in areas where sinking from the surface takes place around Antarctica. Particularly in the Atlantic sector along the east coast of South America. This is where Antarctic Bottom Water flows north. So, the pattern of bottom water warming is consistent with heating from the surface.
Note also in the figures above that warming is largest at the surface and small near the sea floor. This distribution is inconsistent with warming from below (the sea floor). It is consistent with warming from above (the atmosphere).
The following figure was added on July 28, 2014.