Effects on weather
Hansen et al. 2015 found, that the shutdown or substantial slowdown of the AMOC, besides possibly contributing to extreme end-
Eemian events, will cause a more general increase of severe weather. Additional surface cooling from ice melt increases surface and lower tropospheric temperature gradients, and causes in model simulations a large increase of mid-latitude
eddy energy throughout the midlatitude troposphere. This in turn leads to an increase of
baroclinicity produced by stronger temperature gradients, which provides energy for more severe weather events.
Many of the most memorable and devastating storms in eastern North America and western Europe, popularly known as superstorms, have been winter cyclonic storms, though sometimes occurring in late fall or early spring, that generate near-hurricane-force winds and often large amounts of snowfall. Continued warming of low latitude oceans in coming decades will provide more water vapor to strengthen such storms. If this tropical warming is combined with a cooler North Atlantic Ocean from AMOC slowdown and an increase in midlatitude eddy energy, we can anticipate more severe baroclinic storms. [h=2]Observations[/h] [h=3]2010 and earlier[/h]
In April 2004, the
hypothesis that the Gulf Stream is switching off received a boost when a retrospective analysis of U.S. satellite data seemed to show a slowing of the
North Atlantic Gyre, the northern swirl of the Gulf Stream.[SUP]
[15][/SUP]
In May 2005,
Peter Wadhams reported in
The Times (London) about the results of investigations in a submarine under the Arctic ice sheet measuring the giant chimneys of cold dense water, in which the cold dense water normally sinks down to the
sea bed and is replaced by warm water, forming one of the engines of the North Atlantic Drift. He and his team found the chimneys to have virtually disappeared. Normally there are seven to twelve giant columns, but Wadhams found only two giant columns, both extremely weak.[SUP]
[16][/SUP][SUP]
[17][/SUP]
In 2005 a 30% reduction in the warm currents that carry water north from the Gulf Stream was observed, from the last such measurement in 1992. The authors noted uncertainties in the measurements.[SUP]
[18][/SUP] Following media discussions, Detlef Quadfasel pointed out that the uncertainty of the estimates of Bryden
et al. is high, but says other factors and observations do support their results, and implications based on
palaeoclimate records show drops of air temperature up to 10 °C within decades, linked to abrupt switches of ocean circulation when a certain threshold is reached. He concluded that further observations and modelling are crucial for providing early warning of a possible devastating breakdown of the circulation.[SUP]
[19][/SUP] In response Quirin Schiermeier concluded that natural variation were the culprit for the observations but highlighted possible implications.[SUP]
[13][/SUP][SUP]
[20][/SUP]
In 2008, Vage et al. reported "the return of deep convection to the subpolar gyre in both the Labrador and Irminger seas in the winter of 2007–2008," employing "profiling float data from the
Argo program to document deep mixing," and "a variety of in situ, satellite and reanalysis data" to set the context for the phenomenon. This might have a lot to do with the observations of variations in cold water chimney behaviour.[SUP]
[21][/SUP]
In January 2010, the Gulf Stream briefly connected with the
West Greenland Current after fluctuating for a few weeks due to an extreme negative phase of the
Arctic oscillation, temporarily diverting it west of Greenland.[SUP]
[22][/SUP][SUP]
[23][/SUP]
Their results at least imply that strong cooling in the North Atlantic from AMOC shutdown does create higher wind speed. The increment in seasonal mean wind speed of the northeasterlies relative to preindustrial conditions is as much as 10–20 %. Such a percentage increase of wind speed in a storm translates into an increase of storm power dissipation by a factor ∼1.4–2, because wind power dissipation is proportional to the cube of wind speed. However, the simulated changes refer to seasonal mean winds averaged over large grid-boxes, not individual storms.[SUP]
[14][/SUP]
