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Atlantic multidecadal oscillation

Contents

Idea

The Atlantic multidecadal oscillation or AMO is a fluctuation in the sea surface temperature in the North Atlantic Ocean. It seems to occur with a ‘period’ of roughly 70 years. However, accurate temperature records only extend back about 140 years, which is not enough to give us much certainty — and this oscillation may be irregular instead of periodic in the strict sense.

In this graph based on data from NOAA?, the AMO is visible after the linear trend has been subtracted. This subtraction is designed to remove effects due to global warming. However, if global warming is non-linear as a function of time, variations due to global warming may be misinterpreted as the AMO. Conversely, the AMO may affect estimates of global warming.

Effects

The AMO is correlated to air temperatures and rainfall over much of the Northern Hemisphere. There are more severe Atlantic hurricanes during the warm phase of the AMO, and also more droughts in Midwest and Southwest United States. On the other hand, in central and south Florida, rainfall becomes more plentiful when the Atlantic is in its warm phase, while droughts and wildfires are more frequent in the cool phase.

Right now, the AMO is causing weather changes that may be making striped bass populations drop, despite tougher catch limits:

A quote:

Here’s how Wood and his team think the AMO is messing with fish food. When it’s in a warm phase, springtime along the East Coast actually tends to be wet and cool — more rain, more water, more food. In the years following that phase, striper numbers tend to go up. Then the AMO flips — drier springs, less rain, less food. After a lag, it looks like striper numbers start to decline.

Wood says the past 100 years of fishing records show that very trend. And currently?

“It hasn’t been so good in say the last five years,” Wood says. “And it just so happens this is also the time when the Atlantic Multidecadal Oscillation seems to be switching phase.”

Wood suspects it’s switching into a “bad for stripers” phase, and he thinks it was also a down cycle that caused the striper crash in the 1980s. When that cycle ended, stripers recovered — not just owing to the fishing limits but because the weather bcame more favorable.

Janet Nye, who studies fish stocks for the Environmental Protection Agency, thinks this research could help fisheries managers.

“We would be able to say, ‘OK, for the next 35 years or so we’re pretty certain that the AMO is going to be more positive or warm,’ and we would be able to say, ‘These are the fish that respond favorably to that — you might be able to fish those more,’ ” she says.

Conversely, fish less in a down cycle, Wood says. “If we know that there is this cycle coming up,” he says, “a trend that we are beginning to enter, we can keep that in our heads as we set limits.”

If Wood’s research is correct, it may take tougher catch limits to bring striper numbers back up again.

Causes

In some climate models, the AMO is associated with small changes in the Atlantic meridional overturning circulation, a current which is part of the worldwide thermohaline circulation. For example:

Abstract: Analyses of global climate from measurements dating back to the nineteenth century show an ‘Atlantic Multidecadal Oscillation’ (AMO) as a leading large-scale pattern of multidecadal variability in surface temperature. Yet it is not possible to determine whether these fluctuations are genuinely oscillatory from the relatively short observational record alone. Using a 1400 year climate model calculation, we are able to simulate the observed pattern and amplitude of the AMO. The results imply the AMO is a genuine quasi-periodic cycle of internal climate variability persisting for many centuries, and is related to variability in the oceanic thermohaline circulation (THC). This relationship suggests we can attempt to reconstruct past THC changes, and we infer an increase in THC strength over the last 25 years. Potential predictability associated with the mode implies natural THC and AMO decreases over the next few decades independent of anthropogenic climate change.

Modelling the AMO

The long time period of the Atlantic multidecadal oscillation makes it tricky to find this behavior in climate models. A ‘minimal model’ of the AMO was formulated in these papers:

  • R. J. Greatbach and S. Zhang, An interdecadal oscillation in an idealized ocean basin forced by constant heat flux, J. Climate 8 (1995), 81–91.

  • F. Chen and M. Ghil, Interdecadal variability in a hybrid coupled ocean-atmospheric model, J. Phys. Oceanogr. 26 (1996), 1561–1578.

This uses an idealized Atlantic Ocean of constant depth bounded by the longitudes 74° W and 10°W, and latitudes 10° N and 74° N. The equations for a model of this sort, and a discussion of stochastic aspects, can be found here:

  • Leela M. Frankcombe, Henk A. Dijkstra and Anna S. von der Heydt, The Atlantic Multidecadal Oscillation: a stochastic dynamical systems view, in Stochastic Physics and Climate Modelling, eds. Tim Palmer and Paul Williams, Cambridge U. Press, Cambridge, 2010.

References

The graph above is taken from:

See also:

category: climate