Younger Dryas cooling event not caused by shut-down of the Gulf Stream (e.g., by the rapid draining of Laurentide pro-glacial Lake Agassiz), but instead by changes in atmospheric energy transport.
Is the Gulf Stream responsible for Europe’s mild winters?
By R. SEAGER, D. S. BATTISTI, J. YIN, N. GORDON, N. NAIK, A. C. CLEMENT and M. A. CANE
Is the transport of heat northward by the Gulf Stream and North Atlantic Drift, and its subsequent release into the midlatitude westerlies, the reason why Europe’s winters are so much milder than those of eastern North America and other places at the same latitude? Here, it is shown that the principal cause of this temperature difference is advection by the mean winds. South-westerlies bring warm maritime air into Europe and north westerlies bring frigid continental air into north-eastern North America. Further, analysis of the ocean surface heat budget shows that the majority of the heat released during winter from the ocean to the atmosphere is accounted for by the seasonal release of heat previously absorbed and not by ocean heat-flux convergence. Therefore, the existence of the winter temperature contrast between western Europe and eastern North America does not require a dynamical ocean. ….
…In the current paper we demonstrate that transport of heat by the ocean has little influence on the contrast between the mild winters of western Europe south of 60±N and the harsh ones of eastern North America. North of 60±N the OHT accounts for about a quarter of the contrast by restricting winter sea-ice cover. The dominant cause of the contrast, at both latitudes, is atmospheric advection around the Icelandic Low and the simple maritime–continental climate distinction. The exact positioning and strength of the Icelandic Low is important to the climate contrast and is shown to be greatly influenced by the orographic forcing of the Rocky Mountains. Therefore, the difference in the winter climates arises fundamentally through atmospheric processes and the seasonal storage and release of heat by the ocean mixed layer. This is also all that is required to establish the difference in winter climates between the west coast of Europe and the west coast of North America at the same latitudes.
…Clearly, the atmosphere is doing the lion’s share of the poleward heat transport required to ameliorate climates at mid latitudes. This will be even more so in northern winter when the atmospheric heat transport is greater than its annual mean while the OHT appears to be less than its annual mean (see later). …The dominance of the atmosphere is far greater than earlier estimates (e.g. Peixoto and Oort 1992) which gave more weight to the ocean. Trenberth and Caron (2001) have compared their results to direct estimates in the ocean and those derived using an inverse method by Ganachaud and Wunsch (2000), and show that the NCEP-derived estimates fall within the error bars of those estimates in the subtropics while the ECMWF-derived estimates are clearly too low. North of 40±N, NCEP and ECMWF estimates agree with each other and with independent direct estimates. Interestingly, these recent estimates are in quantitative agreement with the early estimates of Houghton (1954) and Sverdrup (1957) as presented by Bjerknes (1964).
Reference https://www.physicsforums.com/threads/younger-dryas-and-the-gulf-stream-urban-myth.830052/ citing Seager et al. (2006)