Long term changes in the deep sea: examples from two Mediterranean Channels

Climate change is one of the key topics of our century. The study of processes related to climate change in the atmosphere, the open ocean, the deep sea or even in shallow coastal waters require sustained long-term observations, often deploying sophisticated and expensive equipment. According to the Deep-Ocean Observing Strategy (DOOS, http://deepoceanobserving.org/), the deep ocean (below 200 m water depth) is the least observed, but largest habitat on our planet by volume and area. With more than 90% of anthropogenic heat imbalance absorbed by the oceans, monitoring long-term changes of its heat content, and over its full depth, is essential to quantify the planetary heat budget.

The Mediterranean Sea is a mid-latitude marginal sea, particularly responsive to climate change as reported by recent studies. Straits and channels divide it into several sub-basins and the continuous monitoring of these choke points allows to intercept different water masses, and thus to document how they changed over time. This monitoring, in many cases, is done under the umbrella of the CIESM Hydrochanges program (http://www.ciesm.org/marine/programs/hydrochanges.htm). Here we report the long-term time series of physical data collected in two of these choke points: the Sardinia Channel (1900 m) and the Sicily Channel (400 m).

The Sardinia Channel allows the Western Mediterranean Deep Water (WMDW) to enter the Tyrrhenian Sea (depths > 3000 m), connecting it with the Algerian Sea (depths > 2500 m). This water mass has experienced a significant increase of heat and salt content over the past decades, due both to a gradual process and to and abrupt event, called Western Mediterranean Transition (WMT). The monitoring at the sill (1900 m) of the Sardinia Channel since 2003 shows this very clearly, and the interannual trends are significantly stronger than the global average trends.

The Sicily Channel (sill at 400 m) separates the Mediterranean in two main basins, the Eastern Mediterranean Sea and the Western Mediterranean Sea. Here the thermohaline properties of the Intermediate Water (IW) are monitored since 1993, showing increasing temperature and salinity trends at least one order of magnitude stronger than those observed at intermediate depths in the global ocean.

We investigate the causes of the observed trends and in particular discuss the role of a changing climate over the Mediterranean, especially in the eastern basin, where the IW is formed. The long-term records in two Mediterranean channels reveal how fast the response to climate change can be in a marginal sea compared to the global ocean, and demonstrates the essential role of long time series in the ocean.