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More On The AMO

June 13, 2016

By Paul Homewood 


h/t Ron Clutz



I mentioned the Atlantic Multidecadal Oscillation earlier, and Ron Clutz has sent me this article,  written by Gerard McCarthy, which describes the processes and effects quite nicely:



The Atlantic Ocean’s surface temperature swings between warm and cold phases every few decades. Like its higher-frequency Pacific relative El Nino, this so-called “Atlantic Multidecadal Oscillation” can alter weather patterns throughout the world. The warmer spell we’ve seen since the late 1990s has generally meant warmer conditions in Ireland and Britain, more North Atlantic hurricanes, and worse droughts in the US Midwest.

However a colder phase in the Atlantic could bring drought and consequent famine to the developing countries of Africa’s Sahel region. In the UK it would offer a brief respite from the rise of global temperatures, while less rainfall would mean more frequent summer barbeques. A cold Atlantic also means fewer hurricanes hitting the southern US.

The good news is our latest research, published in the journal Nature, gives us a much better understanding of these Atlantic oscillations. We now know that accelerations in sea-level rise in cities like New York and Boston on the north-east coast of the US are linked to a cold spell in the Atlantic.

The bad news, at least if you’re an African farmer or have a coastal property in New England? We’re about to go into a cold phase.


Atlantic Multidecadal Oscillation

Once we adjust for overall global warming trends, an oscillation in Atlantic sea-surface temperatures emerges. The ocean went through a warm period in the 1930s/1940s and again in the 1990s/2000s. However the 1970s/1980s were much cooler and there are hints of a transition to a relatively cold period at the moment.


The black line represents the ten year average.
van Oldenborgh et al. / ERSSTv3b, CC BY-SA


This is known as the Atlantic Multidecadal Oscillation (AMO), and the transition between its positive and negative phases can be very rapid. For example, Atlantic temperatures declined by 0.1ºC per decade from the 1940s to the 1970s. By comparison, global surface warming is estimated at 0.5ºC per century – a rate twice as slow.

In many parts of the world, the AMO has been linked with decade-long temperature and rainfall trends. Certainly – and perhaps obviously – the mean temperature of islands downwind of the Atlantic such as Britain and Ireland show almost exactly the same temperature fluctuations as the AMO.

Atlantic oscillations are associated with the frequency of hurricanes and droughts. When the AMO is in the warm phase, there are more hurricanes in the Atlantic and droughts in the US Midwest tend to be more frequent and prolonged. In the Pacific Northwest, a positive AMO leads to more rainfall.

A negative AMO (cooler ocean) is associated with reduced rainfall in the vulnerable Sahel region of Africa. The prolonged negative AMO was associated with the infamous Ethiopian famine in the mid-1980s. In the UK it tends to mean reduced summer rainfall – the mythical “barbeque summer”.

Because the AMO influences a range of climate conditions in different parts of the world, it is important that the mechanisms driving it are properly understood.


What drives the phases of AMO?

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.


Author: The ConversationGerard McCarthy is Senior research scientist in Physical Oceanography at National Oceanography Centre. Ivan Haigh is Lecturer in Coastal Oceanography at University of Southampton.



The article underplays the AMO’s effect on global temperatures, which is real and not just some regional variation.

When we look at the conjoining of AMO and PDO cycles, we can see that the warm phases of both coincided with the warmth of the 1930s and 40s.

Later, both AMO and PDO came together again at the bottom of their cycle in the bitter 1970s.

More recently, the AMO has remained high, while prior to the current El Nino the PDO began to turn negative. This has coincided with the pause in global warming since 1998.



Once this El Nino event is over, there is no reason why the PDO will not return to its natural cool state for the next couple of decades. If this coincides with a change to a cold AMO, as is likely, global temperatures will plummet.


NOAA have a useful FAQ on the AMO here.

  1. June 13, 2016 7:51 pm

    There are also implications for Arctic Ice to grow rather than decline, as this is also affected by water temperatures. An analysis by Yeager et al. at NCAR led them to conclude:

    “Consequently, projected 10-year trends in winter Arctic winter sea-ice extent seem likely to be much more positive than has recently been observed, with the possibility of actual decadal growth in Atlantic sea-ice in the near future.”

    • June 13, 2016 9:25 pm

      Thats a great point. Arctic ice recovery is a more potent warmunist counter than the paise because so much has been made of it. Al Gore’s drowning polar bears. And there are other lines of evidence suggesting Arctic ice is cyclical: essay Northwest Passage, Wyatt and Curry’s stadium wave, Akasofu 2010…

      • nigel permalink
        June 14, 2016 6:12 pm

        “Arctic [sea] ice”

        It melted faster than usual this Spring. But in the last three weeks it has expanded a little in the Beaufort Sea and the Central Arctic. The actual Northern Hemisphere extent, with 4 sq km resolution, is the same as in 2012. It would be rather amazing if the present 10.8 million sq km of ice were to completely disappear over the remaining two and a half months of Summer! If it happens the warmunistas will be shouting about it. When it doesn’t happen they will be howling about some other crazy nonsense.

        We only have MASIE, of course, due to technical difficulties.

        Meanwhile, temperature anomalies in the Southern Hemisphere are plummeting – the latest ‘2-meter anomaly’ is now 0.25 C below the 1981-2010 average.

  2. June 13, 2016 9:32 pm

    The key observation from my point of view, and it represents a great step forward for which I congratulate the researchers, is this: ‘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.’

    But what follows is nonsense: ‘This a major influence on the wind patterns and the heat transferred between the atmosphere and ocean’.

    The ocean has heat capacity and it accumulates energy according to the extent of cloud cover. The atmosphere does not have a heat capacity and it functions to efficiently remove energy from the surface and deliver it to space. Where the heat manifests in terms of the surface layers of the ocean depends upon the winds.The extent of cloud cover that determines the rate of energy uptake depends upon the state of the North Atlantic Oscillation that is simply the local Atlantic manifestation of the Arctic Oscillation.

    The Arctic Oscillation measures the change in the relationship in atmospheric pressure between the mid and high latitudes. Since the mid 1990’s surface pressure has been increasing in high latitudes and declining in mid latitudes. This is a function of the temperature of the stratosphere and the energy sustaining the development of polar cyclones. As the stratosphere cools polar cyclone activity weakens and atmospheric pressure increases from about 50° of latitude northwards. As this happens the warm wet westerlies fall away and the cold dry easterlies begin to flow southwards. This is observed as the jet stream swinging further southward more frequently.The north Atlantic drift that depends upon the strength of the westerlies consequently weakens. Cold waters tend to stay where cold waters are normally found and warm waters stay in those zones where warm waters are normally found and the temperature gradient from equator to pole steepens.

    As atmospheric pressure falls in the mid latitudes the extent of cloud free sky shrinks. The rate of uptake of energy from the sun by the ocean simply falls away.

    The chain of causation begins in the stratosphere. Functionally, the rate of descent of mesospheric air that is ozone poor into the stratosphere modulates the partial pressure of ozone in the stratosphere. The Jet stream links and actually gives rise to a chain of polar cyclones that vary in there intensity according to density differentials driven by ozone heating of the air at and above the tropopause.

    To go back one more step and explain what drives the variable flow of mesospheric air into the stratosphere involves complications that are too many to deal with here.

    • June 13, 2016 9:41 pm

      But what follows is nonsense: ‘This a major influence on the wind patterns and the heat transferred between the atmosphere and ocean’.

      I’d actually read it as transferring heat from ocean to atmosphere, rather than the other way round, Erl

      But perhaps that is confirmation bias!!

    • June 13, 2016 10:50 pm

      Right on erl. It should have read: “This a major influence on the wind patterns and affects the amounts of heat transferred from the ocean to the atmosphere.”

  3. Ben Vorlich permalink
    June 14, 2016 7:20 am

    A cold Atlantic also means fewer hurricanes hitting the southern US.

    I thought that there’s been no hurricanes in the USA for a record number of years. Does this mean that the current low hurricane count will continue at least for southern states?

    • RAH permalink
      June 14, 2016 1:32 pm

      No Major (CAT III or higher) Atlantic hurricanes have hit the shores of the 48 contiguous states since Wilma came ashore at Cape Romano, Florida Oct 24, 2005 as a CAT III storm with 120 mph winds. And that is a record that continues but if the projections are correct there is a pretty high probability that hiatus will end this coming season.

      According the Weatherbell conditions have turned favorable for near in tropical storm development off both the Gulf and Eastern coasts of the US so if they have it right, then the southern gulf shore is just likely to be hit as the south eastern seaboard this season.

      Anyway the weather record sure indicates that colder times are stormier times. And the earth is cooling now.

  4. June 14, 2016 12:24 pm

    I had thought that the computer models only failed at their input end, where the radiant forcing functions that drive all heat input are truly radiant fiddle factors with very weak (i.e. simple grey-gas theory based) science behind them. Reading this, it is apparent that the oceanic movements and their cycles are not at all understood: I suppose that this is discounted in the same way as clouds are in the atmospheric models. It is the oceans that drive the atmosphere, so how can any model that over-simplifies this factor be credible?

    The Wizard of Oz was more credible than the models that underwrite global warming: at least although he was a bad wizard, he was a good man!

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