January 10, 2012
The influence of oceanic circulation on climate is immense. These global flows, technically known as meridional overturning currents (MOC) carry warm water towards the northern and southern polar regions, where it cools down, sinks to deeper levels and returns towards temperate and tropical latitudes. The amount of heat transported in the Atlantic Meridional Overturning Current (AMOC), for example, has a direct impact on the climate along Europe’s Atlantic coasts. Climate researchers believe that the unusually frosty winter of 2009-10 was partly due to a weak heat supply from the Atlantic circulation; however, even the frequency of hurricanes in the Gulf of Mexico and droughts in the Sahel region of West Africa depends largely on the surface temperature of the ocean, which is determined, in turn, by the strength of Atlantic circulation.
“Given its relevance to climate, it is very important that we should now be able to predict when the AMOC will be weaker or stronger over a number of years”, affirms Jochem Marotzke, Director at the Max Planck Institute for Meteorology and Leader of the study. The scientists predict the fluctuations in Atlantic circulation over the next four years; according to their findings, there will be no changes other than the usual seasonal variations.
“We can now also say for certain that the weakening of the Atlantic circulation in March 2010 was only a transient phenomenon”, says Daniela Matei, who played an important part in the research. Some took the slackening of the overturning current in spring 2010 as a sign of long-term weakening. This prospect is repeatedly discussed as a possible consequence of climate change, as not only is ocean circulation heavily involved in the climate mix, it also reacts sensitively to climate change.
Data measurements enable the predictions to be tested
The team asserts that the validity of the predictions is mainly due to the fact that real data on the strength of Atlantic circulation is now available. It is extremely costly to acquire such data, which is not currently available for any other ocean outside the Atlantic, and even there is only recorded at one latitude, i.e. 26.5 degrees north. During his time at the National Oceanography Centre in Southampton, England, Jochem Marotzke played a key role in driving this idea forward. His former colleagues now monitor the Atlantic circulation at latitude 26.5 degrees north continually.
“The measured data enables us to test our simulation”, explains Daniela Matei. “Starting from different points in the past, we first generate predictions for the years for which data is available.” If the forecast of the past matches up well with the real-world measurements, the researchers know that their model delivers useful prognoses of Atlantic circulation at 26.5 degrees north – for the future as well as the past. However, comparison of the model’s calculations with the real data has shown that the simulation cannot generate reliable predictions beyond the fourth year into the future.
A step towards reliable climate predictions for a few years
The Hamburg-based scientists hope to improve their simulations in the future to make reliable forecasts for periods of more than four years. “I am confident that we can do it”, says Jochem Marotzke with a smile. He refers, by way of comparison, to the weather forecasts, which now give reliable predictions for three-day periods, whereas 20 years ago they could only manage an acceptable hit rate for one day at a time. “To achieve this, we need to be able to better define the initial conditions for our calculations”, he says.
The quality of Atlantic circulation predictions and their extension in time will impact more than just climate prediction. Jochem Marotzke envisages a new time horizon: “We can predict weather well for a few days and we can calculate long-term climate change, but our ability to safely predict the climate effect of human activities in particular for the coming years is very limited.” Natural and human impacts are approximately equal and therefore difficult to distinguish for this period, he explains, but the distinction is important in order to identify the role of humans. “The fact that we can now predict Atlantic circulation for several years signifies a major advance for climate predictions in just a few years”, concludes the researcher.