As Earth’s atmosphere and surface temperature change in response to greenhouse gas emissions, scientists who study the ocean are intent on predicting the effect climate change will have on ocean systems. Researchers are therefore looking to historical ocean data to investigate long-term climate trends and understand the context of any changes to sea temperature, circulation, or biodiversity.
Unfortunately, a large part of historical ocean observations is missing because humans did not consistently document ocean or atmosphere observations until recently. For example, there are only a few recorded ocean observations before the year 1880, and even then, the records are mostly from ship routes between Europe and North America, which do little to illuminate the global history of ocean climate. As a result, scientists are using other methods to peer into the past, including “reconstructing” ocean climate data.
In their latest paper, Giese et al. created multiple ocean reanalyses in order to look at long-term trends in ocean variability from 1815 to 2013. Understanding the patterns of sea surface temperature change, ocean circulation, and salinity can help scientists better understand the ocean’s connection to atmospheric climate change and predict future changes that could impact ocean life, the fishing industry, and recreation.
For their reanalysis, the authors used the Simple Ocean Data Assimilation sparse input (SODAsi) methodology based on the Parallel Ocean Program (POP) model. This method combines an ocean reanalysis algorithm with an ocean circulation model.
As part of SODAsi, the scientists used a “loosely coupled” strategy to produce their ocean reanalysis spanning from 1815 to 2013. This process uses alternating iterations of atmosphere-ocean models. In each iteration data and observations are added to the models: atmospheric and ocean circulation, surface wind stress, sea level pressure, and more.
The final result is a much closer approximation of the ocean climate history over the last 2 centuries. For example, from the final iteration of atmospheric-ocean reanalysis, the authors were able to look more closely at the long-past 1877–1878 El Niño event, which was not well documented at the time. From their reanalysis, the team could identify the series of wind anomalies that initiated the strong warming of the 1877–1878 El Niño.
However, the scientists are most interested in the long-term trends. Their research shows there was a global ocean cooling period from 1880 through 1910, followed by a warming until the mid-1940s. More recently, there has been a continued warming trend from 1980 to the present, with an average warming of approximately 1.4°F (0.8°C). There are also regions that show cooling: the central equatorial Pacific and eastern tropical Atlantic Oceans, the Gulf Stream, the region around Antarctica, and the Kuroshio region east of Japan.
Not only is the temperature increasing, but the amount of heat stored in the upper ocean, called heat content, has also increased. The change of heat from the atmosphere into the upper 700 meters of the ocean requires an increase of 0.47 watt per square meter since 1920.
The results provide yet another view into the recent history of ocean-atmosphere interactions, and the trends identified here could help researchers build more accurate climate models. (Journal of Geophysical Research: Oceans, doi:10.1002/2016JC012079, 2016)
—Alexandra Branscombe, Freelance Writer