Has The Atlantic Ocean Cycle Flipped To The Cool Phase?

  • Date: 13/09/15
  • Judith Curry, Climate Etc.

The AMO seems to be headed for the cool phase, I think we need to start watching for a cool flip around 2020 (I think it is early yet to see a flip).  But I am prepared to be surprised.

Klotzbach and Gray ask whether the active Atlantic hurricane era has ended, owing to the negative values of the AMO.

For background on the AMO index, see the Wikipedia, including the different methods for calculating the AMO.  Previous CE posts on the AMO:

The AMO also plays a dominant role in the stadium wave.

Klotzbach and Gray

Klotzbach and Gray published a Comment in Nature Geoscience: Active Atlantic hurricane era at its end? [link; behind paywall].  The comment is getting some press, which  is discussed   in a post at WUWT.

Some excerpts from the paper below, focused on the AMO (see WUWT for additional excerpts related to hurricanes):

The Atlantic hurricane seasons in 2013 and 2014 were quieter than average, and there are indications that hurricane activity in 2015 will also be below normal. Here we investigate whether the active Atlantic hurricane era that began in 1995 may have ended. To this end, we assess hurricane variability in the Atlantic since 1878, along with a proxy for the Atlantic multidecadal oscillation (AMO), whose positive phases have been noted to be closely linked to active periods for Atlantic hurricanes. We find that the AMO proxy values are currently at their lowest values since the early 1990s, when Atlantic hurricane activity was well below average.

The AMO, an indicator of sea surface temperature (SST) variations in the North Atlantic, has been argued to arise from natural climate variations in the thermohaline circulation. Alternatively, it could be primarily driven by alterations in levels of sulfate aerosols. We argue that the weight of the evidence points towards natural oceanic variability being the principal driver of the AMO. The AMO phase was classified as being positive from 1878–1899, 1926–1969 and 1995–2012, and negative from 1900–1925 and 1970–19942. Positive AMO phases are characterized by above-average far North and tropical Atlantic SSTs, below-average tropical Atlantic sea level pressures (SLPs), and reduced levels of tropical Atlantic vertical wind shear. All three of these conditions are known to create a more favourable environment for Atlantic hurricane formation and intensification.

Atlantic SSTs from 50–60° N, 50–10° W and SLPs from 0–50° N, 70–10° W has been utilized to monitor the strength of the AMO in real time. When the AMO is positive, SSTs in the far North Atlantic tend to be warmer, while SLPs throughout the tropics and subtropics tend to be lower. This index has decreased since 2012: SST anomalies in the tropical and far North Atlantic have become cooler and SLP anomalies throughout most of the Atlantic have increased. The decrease in far North Atlantic SSTs in the past three years has been associated with a weaker thermohaline circulation. Annual mean SSTs in the North Atlantic have cooled in 2013 and 2014 compared with values averaged from 1995 to 2012.

A large variety of other climatic factors have also been shown to be linked to phase changes of the AMO including frequency and intensity of El Niño and likelihood of Sahelian drought. Consequently, the impacts of a potential phase change of the AMO extend well beyond its impacts on Atlantic basin hurricane activity.

[…]

AMO index

Here is the most up to date (conventional) AMO plot I can find, by van Oldenborgh:

Slide1

Hmmm . . . not looking too negative in recent years.  If you look at the ‘official’ NOAA AMO monthly index data page [link], you don’t see too much in the way of negative monthly AMO values (4 of the months in 2014 had slightly negative values).  Reading the fine print, I see that they use the Kaplan SST v2 to make the calculations – I do not regard the Kaplan SST as one of the better SST data sets (I prefer the HADSST, or OISST).

Klotzbach and Gray have developed a new method for calculating the AMO (referred to as ‘AMO proxy values’), that relates better to the Atlantic hurricanes:

A proxy using a combination of North Atlantic SSTs from 50–60° N, 50–10° W and SLPs from 0–50° N, 70–10° W has been utilized to monitor the strength of the AMO in real time

So, not only does the method of determining the AMO matter, but presumably also the underlying SST data set used in the calculation.

Whither the AMO?

So, does the Klotzbach/Gray version of the AMO combined with low Atlantic hurricane activity, along with a slow down in the AMOC, portend a flip to the cold phase of the AMO?

Here is the way that I have been looking at this, which is encapsulated by the stadium wave wheel.  According to the stadium wave wheel (recall the indices are smoothed with a 13 yr filter), we passed peak AMO circa ~2010, so we are arguably in a declining phase, heading to a time in the ~mid 2020’s when the index switches to the cool phase (I’ve seen other estimates of the switch around 2030).

The smoothed stadium wave says nothing about short term fluctuations, such as was seen in the mid 1940’s.  Perhaps we are encountering an analogous cool blip, to return in a few years to warm values.

Or perhaps we are headed for a surprise.  If the AMO is truly an unforced internal oscillation, the oscillation can cease, change frequency or amplitude at any time.  If external forcing (e.g. solar) plays an important role, then such a dramatic change is less likely.  Since we are still trying to figure out the AMO, we really don’t know.

With regards to sea ice, and I have been predicting this for several years, I see a recovery occurring in the Atlantic sector of the Arctic.

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