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Modeling Ocean Behavior: The Key to Understanding Our Future Climate
Scientists have long recognized the importance of oceans in our climate. In fact, the unique physical characteristics of our oceans are largely responsible for making the Earth a livable environment. Oceans are major "climate-controllers" because of their large heat capacity. For instance, it requires four times the amount of energy to raise the temperature of water by one degree than it does soil. As a result, over a long period, oceans can store and transport heat from one location to another. Furthermore, water reacts slowly to the surrounding atmosphere. While this means our oceans may exert relatively little influence in short-term weather, they have a large effect on long-term climate.
Of particular interest is the ocean's Conveyor Belt or thermohaline circulation, since both temperature and salinity are at the root of its existence. People in the U.S. know it as part of the "Gulf Stream" that carries warm waters north along the U.S. eastern seaboard and across the north Atlantic Ocean.
It then flows south from the European side of the Atlantic, crosses the equator, joins another ocean circulation, and eventually reaches the Pacific, a trip that lasts 1000 years! At high latitudes, cold, dry air from land lowers the average temperature of the warm waters coming from the equator. Evaporation then removes water free of salt and ice formation leaves behind the salty ocean waters. These processes go to increase the ocean's water density, forcing it to sink, leading to the formation of what is known as the North Atlantic Deep Water (NADW).
A major player in the thermohaline circulation is a process known as "deep convection" (DC) that mixes very efficiently warm and cold waters, affects sea ice melting and how much solar radiation is absorbed. DC forms in the Labrador Sea during winter when polar air blows from Canada, removes heat and salt from the waters, and causes it to sink and mix. It is a key feature since it represents the initial stages of the global-scale ventilation loops of the world's oceans. Outside of the Labrador Sea, DC is confined to small portions of the Weddell and Western Mediterranean Seas.
Computer models are the tools employed by scientists to simulate present oceanic-atmospheric behavior as well as future and past climates. But, due to the limitations of today's computers, it is not possible to explicitly represent all the important physical processes that govern the climate. Deep convection is one such process that must therefore be modeled in ocean general circulation models (OGCM).
While lab and numerical simulations have brought to light several key features of DC, the translation of this information into a reliable model usable in OGCMs has not yet been achieved, with the result that Deep Convection is still poorly understood.
Dr. Vittorio M. Canuto, a senior scientist at NASA's Goddard Institute for Space Studies (GISS) and Columbia University's department of applied physics in New York, has recently focused on how well DC is represented, or modeled, in OGCMs. In a research paper, "Modeling Ocean Deep Convection", published in the April 2004 issue of Ocean Modelling, Canuto and several colleagues assessed the accuracy of several well-known mixing models widely used by the scientific community to represent DC in the Labrador Sea. They compared the model predictions with data on Labrador DC that has recently become available.
Canuto and colleagues examined three specific mixing models. They found that while their NASA-GISS mixing model simulated the observed DC data more faithfully than the other models, it still suffered from problems. For example, it overestimated the depth of DC. Canuto and collaborators are presently trying to determine how the inclusion of rotation may improve the model's performance.
This study is particularly timely since in recent years, considerable concern has been expressed about the fragility of the thermohaline circulation to climate change. Increased greenhouse gases would add fresh water to our oceans by melting glaciers. Increased rainfall at high latitudes, as predicted by OGCMs, would further lower ocean's salinity, inhibiting water from sinking. The net result would be a slow-down of the NADW, depriving much of Europe and eastern North America of warmth. The Younger Dryas, a period of ice-age-like conditions characterized by a partial collapse of the Conveyor Belt around 11,000 years ago, is believed to have been initiated in this manner.
Whether or not human-induced greenhouse gases will affect the thermohaline circulation is strongly dependent on the future temperature distribution and fresh water supply over the North Atlantic. Most models predict an increase in precipitation in high latitudes and some warming over the North Atlantic within the next 70 years, assuming a doubling of carbon dioxide. However, the extent of projected weakening of the thermohaline circulation varies considerably among the models, with some even indicating little to no change. The details and long-term effect — more than 100 years — of such changes to the ocean circulation have only been explored by a few studies.
"While it is unlikely that climate change will lead to a collapse of the thermohaline circulation, it is possible that DC in the Labrador Sea might be severely affected," said Canuto. "Such an event would have a significant impact on the climate in Europe," he added.
One thing is for certain: the climate system is extremely complex, and many questions remain. Current computer climate models offer a "best guess" as to how scientists believe different climate processes interact. Since scientists do not know exactly how human industry, including transportation and agriculture, will change over the next 100 years, the uncertainty associated with computer climate models will not be much reduced even if today's climate models were perfect.
Other changes to our oceans, spawned by climate change, may also have a considerable impact on human populations. "If we recall that nearly 100 million people live within one meter of the mean sea level, we can easily understand the societal and economic impacts of a rising sea," said Canuto. "One of the most immediate consequences will be a huge migration of people in the tens of millions, a phenomenon that will seriously burden the host nations."
Inevitably, the discussion turns back to the influence of human behavior. A few scientists believe that the changes we are seeing, such as those in the Arctic, are consistent with large, slow natural cycles in the oceans. But many scientists believe there is a greater human component that is also impacting climate. The one overwhelming question is how significant greenhouse gases are against the backdrop of larger climate changes, and how "self-correcting" the Earth system will be to these changes. Scientists hope that improved climate and ocean models will provide clearer answers to such questions in the near future.
Canuto, V.M., A. Howard, P. Hogan, Y. Cheng, M.S. Dubovikov, and L.M. Montenegro 2004. Modeling ocean deep convection. Ocean Modelling 7, 75-95, doi:10.1016/S1463-5003(03)00038-6.
Mike Bettwy, Goddard Space Flight Center, Greenbelt, Md., (Phone: 301/286-3026)
This article was derived from the NASA Goddard Space Flight Center Top Story.