The Gulf Stream, or the Atlantic Overturning Meridional Circulation is slowing down:
Quote:The Atlantic meridional overturning circulation (AMOC)—a system of ocean currents in the North Atlantic—has a major impact on climate, yet its evolution during the industrial era is poorly known owing to a lack of direct current measurements. Here we provide evidence for a weakening of the AMOC by about 3 ± 1 sverdrups (around 15 per cent) since the mid-twentieth century. This weakening is revealed by a characteristic spatial and seasonal sea-surface temperature ‘fingerprint’—consisting of a pattern of cooling in the subpolar Atlantic Ocean and warming in the Gulf Stream region—and is calibrated through an ensemble of model simulations from the CMIP5 project. We find this fingerprint both in a high-resolution climate model in response to increasing atmospheric carbon dioxide concentrations, and in the temperature trends observed since the late nineteenth century. The pattern can be explained by a slowdown in the AMOC and reduced northward heat transport, as well as an associated northward shift of the Gulf Stream. Comparisons with recent direct measurements from the RAPID project and several other studies provide a consistent depiction of record-low AMOC values in recent years.
https://www.nature.com/articles/s41586-018-0006-5This slowing down has consequences: a more rapid sea level rise in the Atlantic opposite the East coast of the US, significant warming of the Gulf Stream region of the North Atlantic. For the UK and West Europe it will make summers hotter and winters a bit cooler:
Quote:Movement of warm Equatorial waters northward and their subsequent overturning and sinking in the North Atlantic drives a number of key weather and climate features. The first is that it tends to keep Europe warm during winter and to moderate European temperatures during summer. The second impact is that a fast moving current off the U.S. East Coast pulls water away from the shore keeping sea levels lower. The third is that warm water in the North Atlantic during winter time tends to keep the regional jet stream relatively flat. And the fourth is that a more rapid circulation keeps the ocean more highly oxygenated — allowing it to support more life.
A slowing down of ocean circulation in the North Atlantic therefore means that Europe will tend to cool during winter even as it heats up during summer. Sea level rise will accelerate faster for the U.S. East Coast relative to the rest of the world due to a slowing Gulf Stream combined with the effects of melting land glaciers and thermal ocean expansion. The North Atlantic jet stream will tend to become wavier — with deep troughs tending to form over Eastern North America and through parts of Europe. These trough zones will tend to generate far more intense fall and winter weather. Finally, a slowing ocean circulation will tend to increase the number of low-oxygen dead zones.
https://robertscribbler.com/2018/04/23/why-a-15-percent-slow-down-in-north-atlan...The obvious fingerprint of a slowing AMOC:
Quote:(Cool pool formation near Greenland juxtaposed by a warming and slowing of the Gulf Stream as it is forced southward is an early indication of ocean circulation slow-down. During recent years, this phenomena — which is related to larger human-forced climate change — has become a prevalent feature of North Atlantic Ocean climate and weather patterns. An indicator that climate change and ocean system changes for this region are already under way. Image source: Earth Nullschool.) Increasing amounts of freshwater from the melting of Arctic sea ice and Greenland ice sheet is one factor slowing the Gulf Stream:
Quote:With rates of Greenland melt increasing, there is a risk that the historic observed North Atlantic circulation weakening will increase further and more radically — producing still more profound results than we see today. In the event of large melt outflows coming from Greenland during abnormally warm summers or due to warming deep water melting glaciers from below — a possibility that rises with each 0.1 C of global temperature increase — we could see a very rapid weakening of ocean circulation above and beyond that which has already been recorded.
The reason Antarctic and Greenland ice sheets increase the amount of meltwater and calved icebergs reaching the sea is that many of the big glaciers in those places are in valleys that get deeper as you go further inland, i.e. the ice faces exposed to warm currents increase in size as the glaciers retreat inland.
So if AGW continues the slowdown of the AMOC is going to slow down more and more.