EU Science Hub

Could the Cold Intermediate Layer of the Black Sea disappear in the near future?

By transporting heat and energy, seas and ocean bodies are powerful regulators of the Earth's climate.
©divedog –
Aug 14 2018

A recent JRC article investigates how regional climate warming is heating the Cold Intermediate Layer (CIL) of the Black Sea. This finding could explain why the Black Sea surface waters have apparently bucked global warming trend, and could have major implications for thermohaline circulation.

On a global scale, thermohaline circulation (also known as the ocean conveyor belt) is part of the ocean circulation driven by global density gradients that are created by surface heat and freshwater fluxes. It is a powerful transporter of heat and matter, and therefore is a significant regulator of the Earth's climate.

The thermohaline circulation transport process of the Black Sea is similar, but on a smaller regional scale. The specific structure of its thermohaline circulation makes the Black Sea of special interest, as its components respond in different ways to regional, climate-induced changes.

One of the unique features of the Black Sea is its Cold Intermediate Layer (CIL), a remnant of the cold winter water masses, which in summer are covered by warmer surface water.

This unique and important feature of the Black Sea is rapidly decreasing, according to the results of a recent JRC study. The research uses the JRC's specific modelling techniques to predict long-term trends in the Black Sea’s hydrodynamics.

The Black Sea has undergone significant ecological degradation since the 1970s, due largely to pollution, overfishing and natural climatic variations. It is essential to map trends in its ecosystem and simulate future scenarios in order to understand how the Sea’s properties may develop in the future as a result of climate change and policy decisions.

Main findings

The simulations in this study, covering five decades, demonstrate a substantially large multi-annual CIL variability and confirm two major generation mechanisms: (i) a cooling of the surface waters in the central and deep interior part of the Black Sea Basin (the 'basin interior') in winter and (ii) the transportation of cold water masses (formed in winter on the North Western Shelf - NWS) by the main cyclonic current and by mesoscale eddies on the shelf break.

Graphical representation of the "cold content" (negative heat content) in the Black Sea, showing a marked drop in the past decade. © Stips, EU
Graphical representation of the "cold content" (negative heat content) in the Black Sea, showing a marked drop in the past decade.

© Stips, EU

The novel element of the JRC study is the quantification of the relative importance of these mechanisms during a prolonged time period. In particular, (i) plays a key role in the CIL formation in the basin interior, while (ii) controls CIL renewal along the main cyclonic current and along the shelf rim (the 'basin exterior'), as well as in the south-eastern region.

In order to isolate the effects of basin circulation and the contribution of the NWS cold water masses, the JRC studied the distribution of a passive tracer originating in the NWS. Tracer distribution in the basin interior indicates that a large fraction of the cold NWS water is transported via the main cyclonic current to the eastern convergence and anticyclonic areas. A smaller fraction of the cold NWS water is transported to the central part of the basin.

The temporal cooling capacity of the CIL is highly variable and has decreased drastically in the last decade of the simulation, approaching nearly zero (which means that the CIL has practically disappeared). This is likely due to changes in regional weather conditions in the NWS.

This seems to show that the additional heat from regional warming is transported downward to greater depths and warming the CIL, instead of contributing to surface water warming. This could potentially explain the missing increasing temperature trend in the Black Sea surface waters reported last year (Black Sea water temperatures may buck global trend).

Further information

Formation and changes of the Black Sea cold intermediate layer

JRC wide hidden block