There are many circulating ocean currents in the North Atlantic,
particularly the well known Gulf Stream. The "Stream" as we call it
around here is the dual product of warm, expanding tropical waters,
and cold, sinking salt water south of Greenland and Iceland. The
combined effect causes a north easterly flow which warms eastern
Canada, Iceland and north western Europe.

Even though the year 2015 has produced many record high temperatures,
a cold patch of water has developed south of Greenland due to
increased glacial melt. Since the glacial melt is predominantly fresh
water, it is lighter than the surrounding sea water. The resulting
reduced density also reduces the sinking effect which tends to pull
water northward. If the north eastward flow of the Gulf Stream were
significantly reduced over the long term, the climates of eastern
Canada and western Europe would become much colder. There would also
be large changes in fish populations and other marine ecology.

It's all documented in greater detail he

https://www.washingtonpost.com/news/energy-environment/wp/2015/09/24/why-some-scientists-are-worried-about-a-surprisingly-cold-blob-in-the-north-atlantic-ocean/?tid=hybrid_experimentrandom_2_na