Monday, November 21, 2011

Observed SST and model trends

Bob Tisdale has a post at WUWT comparing sea surface temperature trends predicted by a mean of IPCC models versus HADISST observed trends. He notes that the 17 year (204 month) trends do not agree very well.
Update - I've added an appendix showing how the all-trend plot can be used to understand the arithmetic behind the current drop in 17-year trend.

Tamino pointed out that the model mean that Bob used had far less variability than individual model runs, and could not be expected at all to reproduce the decadal variation of observations.

You can see some of this in the following plot, which includes two other SST measures, HADSST2 and HADSST3 in the mix. These of course are far more interdependent than model runs, but you can already see that the model mean is within the variation of the observations, with the exception of an oscillation between about 1930 and 1960.

I'm interested in this, because I have been writing a series of posts here, here. and here, which try to give a wider view of how calculated trends are part of a larger picture, which can indicate whether the choice is special in some way.

To see decadal trend variation in greater breadth, I made an interactive plot similar to the one described here. It's below the jump.

This color plot shows all the trends that you could have created over the period since 1901, over periods greater than 4 years. The white diagonal lines are tracks of constant trend period, shown on the right axis, and you can estimate the 17-year line and follow it to see how it behaves, as indicated in the plots above. Then you can look at nearby trend periods.

You can click on this plot to move the red and blue balls on the plot on the right. There are also controls on the plot - the red and blue bars, and the "nudgers", marke dwith <<<<>>>>. The purple one of these is particularly useful, because if you have set the interval to 17 years (or 30, or whatever), then it moves the balls preserving this interval. Then you can see how the trends are responding to the various features of the plot. Every time you create a trend, the numerical result is written below the graph.

One thing that stands out is the big peak in 1998. That's the reason for the current decline in trend in HADISST particularly, but also visible in HADSST2. It is a deviation from the models, because they do not hindcast such features. The present drop in trend reflects the fact that the peak is nearly 17 years ago, and is reaching its maximum leverage in the trend. That is more influential than the current observations.

So here is the gadget. You may find that it doesn't work in Internet Explorer, which is usually set to disable Javascript (you can change this). You can select the different time periods and datasets with the radio buttons on the right. There is more information on it here.


Appendix - further explanation of the 204-month trend effects.

Here is one of the still plots from the interactive gadget. It is for HADISST, focussing on effects since 1960. I've emphasised the white line corresponding to 17-year trends. In Bob Tisdale's Fig 3 represents the transect along this line. But we can learn more from the full picture.

It's a mass of colors, but horizontal and vertical bar effects are a feature. These correspond to unusual years, and 1998 is a big one. There is a vertical bar of warming trend, for trend periods which lag 1998. Having a warm year at the "future" end of the trend period augments the trend. But if it is at the other end, it has a negative effect, and you see this with the horizontal bar.

Bob made mention of the current dip in HADISST. With this plot, you can see what is influencing that. It's close to the big horizontal bar from 1998, which lowers the trend. And it's close to the vertical bar from 2008/9, which also lowers the trend. There's a small increment from current cooling. The plot gives you a feel for how these add together. 1998 is dominant. If you move down to the 30-year trend, as Bob showed, there is very little recent dip. The reason is that we've moved away from the 1998 horizontal, but not from the 2008 vertical.