Monday, December 12, 2011

AMO, NAO and predicting Atlantic Coast temperatures

Several months ago, after finishing the individual state temperature evaluations, I started putting the state data together in regional groups, starting with the Atlantic States. What motivated that start was the discovery that, along the East Coast, the individual states all displayed a drop in temperature, over the period from about 1950 to 1965 on the order of 4 degrees F. This was a surprisingly large number, in terms of the consistency over both space and time, with which the fall in temperature occurred. But there was some physical evidence of the impact of these changes since, following a piece on bird populations, it was possible to tie the fall in temperture to the migration of the Black Capped Chickadee, as an example, which moved south as the temperature fell, appearing for the first time in North Carolina at the height of the cold spell. When the average plot for all the states (excluding Florida) was derived the average fall was reduced to about 3 degrees F, with the drop in temperature reducing for states further South.

Averaged Time of Observation corrected (TOBS) temperatures for the Atlantic states, with state averages weighted by area in the overall average of the data.

Today’s post shows how this can now be explained, and how it might, from existing data, be possible to predict the future shape of this plot.

Looking at the range of states over which this fall occurred it was clear that it while the temperature drop made it across the Adirondacks into the Midwest it got no further and the Midwest had some differences in the pattern of temperature recovery. The weather normally travels from West to East, and so the logical cause seemed to be to look first at what happened in the Pacific, to see if US temperature variations could be explained.

Over the last couple of weeks this has taken the course of looking at the impact of the El Nino Southern Oscillation (ENSO) with its roughly 5-year cycle, first on regional temperatures and then on that of the individual states. While changes in the ENSO impacts moisture and drought patterns in the states, it did not, comparing temperatures, seem to have as great an impact on the temperatures along the West Coast, where the current flows impact. And further it was restricted to coastal states, in what correlation was immediately visible.

Coming back to the Atlantic Coast there is a similar oscillation in the sea surface temperatures (SST) called the Atlantic Multidecadal Oscillation (AMO). Looking at a plot of this average over time, it is immediately clear that there is a concurrent drop in these temperatures, in around the same time period, that might answer the question at to what had caused the temperature drop along the East Coast.

Sea Surface Temperatures in the Atlantic showing the AMO (NOAA )

One can then, in the same way as for the Pacific Coast States, overlay this curve on the average of the Atlantic States variation (with the Midwest added to show the limit of range) to validate the above statement. However in the first step the overlay was kept at the same vertical scale, so that the size of the anomaly in SST’s that forms the oscillation, could be compared with the size of the temperature changes on land.

Comparison of the fluctuation in AMO anomaly with the temperatures on the Atlantic and Midwest regions. Note that the AMO plot has been scaled and randomly placed vertically to allow distinction from the two other curves.

The immediate first observation is that the land temperatures have fluctuated over twice the scale (roughly) of the SST values, though it should be remembered that the latter are averaged over a much greater area.

Looking over the entire history of the temperature record, which for now is taken as post 1895, the plot for AMO values is doubled, to make it easier to see how the peaks and troughs may coincide, and then 52 deg F as added to the anomaly values to set them below the Atlantic States, and the values are averaged for each year.

Comparison of the form of the AMO (which has been doubled in size) with the changes in the average temperatures of the Atlantic States over the past 112 years.

So now the question comes as to the cause of the changes in the AMO, and one finds that this is currently a work in progress. The AMO itself was only described in 1994, and work on what is the cause, and what it correlates to is ongoing. But there are are several different other cycles in the Atlantic, and it is interesting to see what folk have already found by way of correlation.

One of these other cycles is the drift of the InterTropical Convergence Zone, or ITCZ. Back when I was reading “Stories for Boys” about sailing ships in the Atlantic the ITCZ was called the “Doldrums.”

Location and characteristics of the ITCZ (Da Silva 1994 ).

The Doldrums drift North in the summer. However the movement and nature of the ITCZ behavior has been suggested to be dependent on the AMO, rather than the cause (see for example Knight, Folland and Scaife)

What is actually more interesting, particularly from the “climate prophet” point of view is a suggested correlation with another Atlantic Cycle, that of the North Atlantic Oscillation.
North Atlantic Oscillation – NAO is defined as the fluctuation in the difference of atmospheric pressure at the sea level between two specific locations: Ponta Delgada, Azores and Stykkisholmur/Reykjavik.
Although the NAO is now undergoing a name change to the Northern Annular Mode, the relevant changes in atmospheric pressure (which are shown in color and globally at the Climate Prediction Center can be illustrated, to allow easier layman comprehension with this picture from JISAO).

Pressure patterns in the North Atlantic showing the pressure difference that builds up to value the NOA. (JISAO via Gosselin)

Comes now this new paper from Vukcevic which suggests that there is a correlation between the AMO and the NAO, but that the correlation becomes much better if the values for the NAO are delayed by eleven years. Further that the correlation becomes better when only the fluctuations in the pressures at Reykjavik are considered.

Correlation between an eleven year lagged Icelandic pressure and the 3 year moving average SSTs in the Atlantic – the AMO. (Vukcevic )

What is riveting about this is the current correlation not only the AMO and NAO, but that it is time lagged. And as Vukcevic notes, there is an eleven year time lag between the behavior of parts of the NAO and the consequent change in the AMO. Which means, if this is correct, then we know what the future behavior of the AMO is going to be, since the end of the red line above indicates the SST trends over the next eleven years. That in turn, as was shown earlier, means that the temperature trends for the Atlantic Coast can also be surmised for the next eleven years, and as has been noted elsewhere this holds true not only for the United States but also Europe in that the NAO also has been shown to influence the Central English Temperature (CET).

This is obviously quite intriguing and will no doubt be the object of some discussion over the next few months, particularly since a second paper is promised. It is unlikely that it is the drop in pressure itself that is the cause, but it is conceivable that this is an indicator (dare one say proxy) for some other event and as such it is indicating how this other forcing event is behaving, and through that route becomes a marker itself for the temperature changes that are coming. But then that raises the question as to what it is a proxy for?


  1. So it is a good time to open a sweater store in Georgia. Hey this is off topic but I would love to see a thread by bloggers in the peak oil camp on Exxon's latest Energy Outlook. From what I have read it does not see a peak in oil production occurring for decades. So who is right - The Oil Drum or Exxon?

  2. Give me a little time and I'll see what I can do. These projections differ between oil companies, and I reviewed some of the differences between them last year.