When The AMO Turns, Forget Global Warming

By Paul Homewood

http://www.esrl.noaa.gov/psd/cgi-bin/gcos_wgsp/tsanalysis.pl?tstype1=91&tstype2=0&year1=&year2=&itypea=0&axistype=0&anom=0&plotstyle=0&climo1=&climo2=&y1=&y2=&y21=&y22=&length=&lag=&iall=0&iseas=1&mon1=0&mon2=11&Submit=Calculate+Results

The Atlantic Multidecadal Oscillation, or AMO, is an ongoing series of long-duration changes in the sea surface temperature of the North Atlantic Ocean, with cool and warm phases that may last for 20-40 years at a time and a difference of about 1°F between extremes. These changes are natural and have been occurring for at least the last 1,000 years.

It is known to have significant climatic impacts on the Northern Hemisphere. According to NOAA:

Recent research suggests that the AMO is related to the past occurrence of major droughts in the Midwest and the Southwest. When the AMO is in its warm phase, these droughts tend to be more frequent and/or severe (prolonged?). Vice-versa for negative AMO. Two of the most severe droughts of the 20th century occurred during the positive AMO between 1925 and 1965: the Dustbowl of the 1930s and the 1950s drought. Florida and the Pacific Northwest tend to be the opposite – warm AMO, more rainfall.

The AMO has a strong effect on Florida rainfall. Rainfall in central and south Florida becomes more plentiful when the Atlantic is in its warm phase and droughts and wildfires are more frequent in the cool phase. As a result of these variations, the inflow to Lake Okeechobee – which regulates South Florida water supply – changes by 40% between AMO extremes. In northern Florida the relationship begins to reverse – less rainfall when the Atlantic is warm.

During warm phases of the AMO, the numbers of tropical storms that mature into severe hurricanes is much greater than during cool phases, at least twice as many. Since the AMO switched to its warm phase around 1995, severe hurricanes have become much more frequent and this has led to a crisis in the insurance industry.

What drives the AMO? NOAA explain:

Models of the ocean and atmosphere that interact with each other indicate that the AMO cycle involves changes in the south-to-north circulation and overturning of water and heat in the Atlantic Ocean. This is the same circulation that we think weakens during ice ages, but in the case of the AMO the changes in circulation are much more subtle than those of the ice ages. The warm Gulf Stream current off the east coast of the United States is part of the Atlantic overturning circulation. When the overturning circulation decreases, the North Atlantic temperatures become cooler.

As the graph at the top indicates, we are currently in the warm phase of the AMO, and probably will be for a further 5 to 10 years. In addition to the effects already mentioned, the AMO also has a big effect on Northern Hemisphere temperatures, so much so that it alternately obscures and exaggerates global warming.

The solid blue curve shows the observed northern Hemisphere temperatures and the dashed blue curve is a smoothed version. The red curve is the temperature history for a model that responds to the external forcing of greenhouse gases and solar variability but not to natural climate variations. The blue alternations about the red curve represent the natural AMO oscillations. When the AMO decreases, as from 1950 to 1975, global warming may appear to be reversed. When the AMO increases, as from 1975 to the present, the global warming (red) is exaggerated .

http://www.aoml.noaa.gov/phod/faq/faq_fig2.php

We can see just how closely AMO and Northern Hemisphere temperatures have tracked since the index bottomed out in the 1970’s.

http://www.esrl.noaa.gov/psd/data/correlation/amon.us.long.data

http://www.metoffice.gov.uk/hadobs/hadcrut4/data/current/time_series/HadCRUT.4.3.0.0.annual_nh.txt

There has been a rise in temperature of a full degree since 1976. Note, though, how temperatures have flatlined in the last decade.

The question now is just how much temperatures will fall when the AMO index begins to track downwards, as it soon will. Regardless of what happens in the Southern Hemisphere, when this happens, we can then expect 30 years of cold in the north.

It will happen, it’s just a matter of when.

FOOTNOTE

Jennifer Marohasy is concerned that using HADCRUT numbers may not give a true picture. I have therefore added the plot below using UAH satellite data.

As you can see, there is still a close track between the AMO and temperature.

http://vortex.nsstc.uah.edu/data/msu/t2lt/uahncdc_lt_5.6.txt