North Atlantic variability and its links to European climate over the last 3000 years

By Paul Homewood

 

A newly published paper has linked changes in European climate to North Atlantic variability over the last 3000 years:

ABSTRACT

The subpolar North Atlantic is a key location for the Earth’s climate system. In the Labrador Sea, intense winter air–sea heat exchange drives the formation of deep waters and the surface circulation of warm waters around the subpolar gyre. This process therefore has the ability to modulate the oceanic northward heat transport. Recent studies reveal decadal variability in the formation of Labrador Sea Water. Yet, crucially, its longer-term history and links with European climate remain limited. Here we present new decadally resolved marine proxy reconstructions, which suggest weakened Labrador Sea Water formation and gyre strength with similar timing to the centennial cold periods recorded in terrestrial climate archives and historical records over the last 3000 years. These new data support that subpolar North Atlantic circulation changes, likely forced by increased southward flow of Arctic waters, contributed to modulating the climate of Europe with important societal impacts as revealed in European history.

INTRODUCTION

Ocean circulation has a key role in the Earth’s climate, as it is responsible for the transport of heat but also its storage in the ocean’s interior. Because of the large heat capacity of water and hence its thermal inertia, the ocean is potentially amongst the most predictable components of the Earth’s climate system on time scales from decades to centuries. The subpolar North Atlantic, specifically, is a key region for understanding climate variability, as it is one of the world’s main areas of deep water formation. Here, northward flowing warm and salty surface waters lose their heat to the atmosphere, become denser and eventually sink, as part of the Atlantic Meridional Overturning Circulation (AMOC), forming around half of the global deep water¹. This process is not only important for the ventilation of the oceans abyss but its attendant northward heat transport¹ contributes to shaping the climate of northwest Europe.

Changes in the strength of the deepwater formation process have widely been proposed as the mechanism behind multidecadal sea surface temperature variability in the North Atlantic^2,3, which has been shown to have important impacts on atmospheric patterns and the weather in Europe^4,5. Recent work indicates that specifically the strength of deep water formation in the Labrador Sea is key component for driving variability in the strength of the AMOC, and hence for modulating recent and also decadal North Atlantic climate variability^6,7. Observational studies have shown that interannual to decadal changes in the formation of deepwater in the Labrador Sea, namely Labrador Sea Water (LSW), are driven by changes in the upper ocean density gradients controlled by heat removal from winds and/or buoyancy forcing from freshwater input⁸. However, because of the lack of oceanographic measurements beyond the last 100 years, our understanding of the oceans’ role, particularly centennial changes in the strength of LSW formation and associated subpolar gyre strength, in European climate over longer time scales remains fairly limited.

Several model studies have suggested that centennial-scale climate variability in the North Atlantic over the current interglacial was largely driven by changes in the formation of LSW responding, albeit non-linearly, to freshwater inputs from the Arctic Ocean into the Labrador Sea^9,10. Centennial timescale increases in the export of polar waters into the subpolar North Atlantic spanning the last 10,000 years, have been recorded in the abundance of ice-rafted debris deposited in marine sediment cores¹¹. These records have been widely used to establish a temporal framework for cold climatic events recorded in the circum-North Atlantic region by invoking ocean–land linkages¹². Yet, there are very limited data that support the mechanism by which these pulses of ice-laden, fresh, Arctic Ocean waters impacted the ocean circulation in the North Atlantic and specifically, the strength of LSW formation and the surface circulation around the subpolar gyre, which are very likely candidates for the modulation of the northwest European regional climate. This is largely because of the lack of high sediment accumulation sites for proxy reconstructions at key locations, such as the areas of active deep water formation in the centre of the Labrador Sea.

In this study, we present a suite of subdecadally to decadally resolved proxy records from across the subpolar North Atlantic from which we can infer changes in the formation of deep waters in the Labrador Sea and its associated gyre strength across the last 3000 years. This interval spans several important periods within European history, which have often been related to climate variability such as the warm intervals during the Roman Empire expansion (colloquially referred to the Roman Warm Period ~250 years Before Common Era (BCE)—400 years Common Era (CE)) and Medieval times (Medieval Climatic Anomaly ~900–1200 years CE) and the cold periods such as the one centred around ~2700 years Before Present (BP) known as the Iron Age Cold Epoch, the short-lived Dark Ages Cold Period (~500–750 years CE) and the Little Ice Age (~1450–1850 years CE). Studying the ocean changes over the last 3000 years at high temporal resolution thereby provides a unique opportunity to investigate the potential linkages between ocean circulation changes and European climate variability and its impacts on societies. Our new findings suggest centennial changes in the circulation of the subpolar North Atlantic, likely modulated by the input of Arctic Ocean waters to the Labrador Sea, with similar timing to climate variability recorded on land in historical and terrestrial proxy data in Europe for the last 3000 years.

https://www.nature.com/articles/s41467-017-01884-8

 

 

Two tables in the paper stand out.

The first charts many of the known climatic events, and correlates with the proxy sediment cores:

 

 

The second postulates ocean circulation patterns during centennial warm and cold periods.

 

 

Climatologists have long been aware of these centennial events, and this paper adds a little more to the understanding of them.

As we know, oceans play a key role in the Earth’s climate, because of their massive heat content, which dwarfs anything GHGs can do.

Even during the 20thC , we have seen the effect of North Atlantic cycles, which brought rapid warming to the Arctic in the 1920s and 30s, followed by the sea ice years of the 1960s to 80s, in turn succeeded by the recent warming.

There is no evidence at all that GHGs have had any effect at all on any of these changes.