This project is directed at a quantitative understanding of the ocean circulation during the last glacial maximum, a period during which the climate was a radically different state from today with an ocean inferred to also have behaved quite differently. The central approach is to combine an ocean general circulation model found to be useful in the modern world, with the paleo-oceanographic proxy data for the North Atlantic during the last glacial maximum. The numerical technique for combining the two is based upon the least squared method of Lagrange multipliers as implemented using an adjoint model. Proxy data will be individually weighted according to best estimates of the relative errors. Calculation of the oceanic state involves adjusting "first-guess" initial and boundary conditions that are then modified to reduce the model-proxy-data misfits. The most abundant useful proxy data include isotopic oxygen and carbon, inferred pore water salinities and various sea surface temperature reconstructions. Estimates will be of an assumed perpetual yearly cycle. Supporting work is directed at inferences, mainly global ones, of circulation properties in the modern ocean that can most readily be compared to the much more poorly constrained last glacial maximum period.
Intellectual Merit: At one level, determination of the ocean circulation is a fundamental intellectual challenge in the earth sciences - apart from any practical applications. Determination in the modern world is very difficult, even given the comparatively large data sets available. The circulation of the last glacial maximum presents a far greater intellectual challenge, dependent as it is on indirect data sets (called "proxies") and which are sparse in space and time. In addition to this scientific curiosity aspect, the ocean is widely regarded as a major contributor in determining past and future climate states differing radically from the present one. As long as the oceanic state during the last glacial maximum remains as obscure as it is, it will be extremely difficult to claim understanding how the climate system works, and it will undermine predictions of what the future might bring as anthropogenic forcing increases.
Broader Impacts: This project is a part of a wide community effort aimed at understanding the very different climate state known to have occurred in the past, including periods of full glaciation and ones of essentially no ice. A consensus exists that the influence of the ocean in bring about these different states and/or sustaining them for extended periods of time is very important, but extremely poorly understood. This effort is unique in attempting to understand the ocean of the last glacial maximum by the fully quantitative use of all the proxy data plus a general circulation model thought to well describe the modern world. With a successfully outcome, this approach could be used to the deglacial periods as well as other intervals in the past, perhaps inter-glacial, where an adequate data exists. Most to the proposed research will be done by a graduate student as part of her Ph.D. thesis.
Changes in the Earth's climate through time, past, present and future, are of intense basic scientific interest in the same way that understanding stars, galaxies and the evolution of the universe are studied for their pure intellectual interest and excitement. But in contrast to astronomical phenomena, present and future climate change are of intense political, economic, and social interest. For both reasons, depicting and understanding of past climate becomes a pathway to inferring the extent to which Earth could be drastically different, with potential human and natural costs, and as the answer to a purely scientific challenge. The purpose of this grant was to continue the process of attempting to understand how the ocean circulation might have differed during the last glacial period, in particular at the maximum extent (LGM) as the first-step in determining the degree to which, and how, that circulation might have controlled the overall climate. In practice, determining the circulation at any time requires making inferences about what the atmospheric state was, as well. If definitive answers can be found, they would illuminate the difficult problem of determining what a future circulation, and hence climate state, would or could be. In a continuation of that central question, the work on the grant began to attempt to define how the ocean circulation changed during the deglaciation from the radically different climate state of the glacial maximum to that of today. For practical reasons of computational costs, and the reality of observational data relevant to the last glacial maximum, the focus was placed on the Atlantic Ocean. A summary of the work is that a reasonably complete inferences could be made about the upper ocean circulation and the consequent interactions with the atmosphere, as detailed in the more extended report below. The sea surface temperature as an annual average, a winter, and a summer average during the glacial period are shown in the attached figure along with a calculated change from today. It was found that the wind field was generally strengthened in many areas, and that the corresponding circulation was measurably different from that of today, although not in any radical sense. Because the deep ocean takes so long to adjust to changes in surface conditions, numerical costs precluded more than very rough estimate of the changed abyssal circulation. Work on that problem continues today by a PhD student who completed a master's thesis on this grant. The results answer the question of whether the ocean circulation was observably different during the LGM and permits some inferences about what will happen, at least in the Atlantic Ocean, as the world continues to warm in the trend away from glacial conditions. Because the most conspicuous change in the ocean from glacial to modern conditions was the rise in sea level by about 130m, particular importance can be attached to the processes which relate to ocean warming (and water column expansion), as well as the way the ocean state influences, and responds to, glacial melting. Continued work will exploit the results already found. The bulk of the work on the project is documented in a PhD thesis and a subsequent published paper; and a Master's thesis, and a paper recently submitted for publication.
