by Judith Curry
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:
Critique of Mann’s new paper characterizing the AMO
Climate model simulations of the AMO
Two contrasting views of multidecadal climate variability in the 20th century
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.
McCarthy et al.
Ocean impact on decadal Atlantic climate variability revealed by sea-level observations
Gerard McCarthy, Ivan Haigh, Joel Hirschi, Jeremy Grist, David Smeed
Abstract. Decadal variability is a notable feature of the Atlantic Ocean and the climate of the regions it influences. Prominently, this is manifested in the Atlantic Multidecadal Oscillation (AMO) in sea surface temperatures. Positive (negative) phases of the AMO coincide with warmer (colder) North Atlantic sea surface temperatures. The AMO is linked with decadal climate fluctuations, such as Indian and Sahel rainfall1, European summer precipitation, Atlantic hurricanes3 and variations in global temperatures. It is widely believed that ocean circulation drives the phase changes of the AMO by controlling ocean heat content. However, there are no direct observations of ocean circulation of sufficient length to support this, leading to questions about whether the AMO is controlled from another source. Here we provide observational evidence of the widely hypothesized link between ocean circulation and the AMO. We take a new approach, using sea level along the east coast of the United States to estimate ocean circulation on decadal timescales. We show that ocean circulation responds to the first mode of Atlantic atmospheric forcing, the North Atlantic Oscillation, through circulation changes between the subtropical and subpolar gyres—the intergyre region. These circulation changes affect the decadal evolution of North Atlantic heat content and, consequently, the phases of the AMO. The Atlantic overturning circulation is declining and the AMO is moving to a negative phase. This may offer a brief respite from the persistent rise of global temperatures, but in the coupled system we describe, there are compensating effects. In this case, the negative AMO is associated with a continued acceleration of sea-level rise along the northeast coast of the United States.
Published in Nature.
The paper is discussed in good overview article in the Conversation: The Atlantic is entering a cool phase that will change the world’s weather. Excerpts:
Scientists have widely hypothesised that ocean circulation, and in particular the Atlantic meridional overturning circulation which sends warm surface water northward (the Gulf Stream) and deeper cold water southward, drives the phases of the AMO by moving heat around. However, we do not have direct observations of ocean circulation of sufficient duration to support this theory, which has lead some to question whether the AMO is actually controlled by the ocean.
We have measurements of the strength of the Gulf Stream flow in the Straits of Florida since 1982 and the flow across the Greenland-Scotland ridge since the mid-1990s. Since 2004, we also have continuous, full-depth, basin-wide measurements of the Atlantic overturning circulation with the RAPID monitoring project at 26ºN. However, none of these records are long enough to directly link ocean circulation with the decadal climate variations such as the AMO.
Sea-level measurements from tide gauges on the other hand extend back more than 100 years in places. Many studies, dating back to 1938, have used this data to study variations in ocean circulation.
In our latest research we were able to show that differences in sea level along the US east coast provide a measure of the strength of ocean circulation.
Sea-level fluctuations from Florida to Boston can be divided by Cape Hatteras, where the Gulf Stream leaves the coast to flow eastward. The difference (south minus north) is representative of ocean circulation, and more circulation means more heat is transported.
By comparing our sea-level index against the AMO index we were able to provide, for the first time, observational evidence of the widely hypothesised link between ocean circulation and the AMO.
Our results show that ocean circulation responds to the first mode of Atlantic atmospheric forcing, the North Atlantic Oscillation, through circulation changes between the subtropical and subpolar gyres – the intergyre region. This a major influence on the wind patterns and the heat transferred between the atmosphere and ocean.
The observations that we do have of the Atlantic overturning circulation over the past ten years show that it is declining. As a result, we expect the AMO is moving to a negative (colder surfer waters) phase. This is consistent with observations of temperature in the North Atlantic.
Vencore Weather
Paul Dorian of Vencore has an interesting article The Atlantic Ocean is showing signs of a possible significant long-term shift from warm-to-cold. Worth reading, this paragraph about Arctic sea ice in particular caught my eye:
If the Atlantic Ocean is indeed slipping back into a colder-than-normal phase (i.e., negative AMO) then this would quite likely have a significant impact on Northern Hemisphere (NH) sea ice areal extent. The NH sea ice areal extent was generally at above-normal levels before the middle 1990’s (arrow in plot below) which is when the Atlantic Ocean temperature phase change took place from cold-to-warm. Once the warm phase of the Atlantic Ocean became established in the late 1990’s, the NH sea ice areal extent trended sharply downward from positive levels into well below-normal territory. In recent years, there has been a jagged, but generally sideways trend in NH sea ice areal extent at those well below normal levels. However, if these recent signs of a possible long-term Atlantic Ocean temperature phase change from warm-to-cold are “real and sustained” (and sometimes there are false starts), then the NH sea ice areal extent will very likely return to above-normal levels in the not too distant future – just as it was during the last cold phase pre-mid 1990’s.
AMO index
Here is the most up to date (conventional) AMO plot I can find, by van Oldenborgh:
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.
With regards to hurricanes, I took a look at the number of Atlantic hurricanes during the period in the mid 1940’s with cool AMO [link], and I don’t see a dip. Klotzbach and Gray have their work cut out in terms of trying to figure out why we have had 3 dud Atlantic hurricane seasons in a row. The El Nino’s obviously played a role, but new complexities seem to be emerging.
JC forecast: 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.
Filed under: Oceans 
