Wednesday, April 21, 2010

What Can We Expect for Summer 2010?

Of course predictions of future climate is always wrong, to some degree. But it is often said by meteorlogists you see on TV things like "we had a bad summer last year, so it will be good this year". Or "we had a hot spring so expect a cold summer" and so on.

But what does the data actually tell us? Does what happen in the spring affect what happens that summer? That is, is the deviation from spring's average affect the deviation from the summer average?

Spring average is tough since the temperature is increasing during the months of March, April and May. So to get an "average" for spring and see the deviation of each year (spring anomaly) I did an average for each week, subtracted each week's average from that and averaged the anomalies for those weeks into one number for the spring. This way I could easily break spring up into smaller segments if need be.

Summer deviation from the average was simply the subtraction of each day June, July and Aug from the average temp for those months.

In all cases above the max temp was used, not the mean. Station 4333 (Ottawa) was used.

This is the scatter plot of the spring anomaly vs the summer anomaly.

Notice that there are equal number of points in each quadrant. Thus the spring has no effect on what the summer will be. Hotter springs can have hot or cooler subsequent summers.

But does a hot summer mean the following year's summer will not be as hot? To do that you again have to plot each summers max temps from the average of the summer max temps to see deviations (anomalies). Does a negative deviation in one year mean a positive deviation in the following year? Yes it generally does.

From just 1930 onward, shows a flatter trend.

Checking against the mean and lowest of the max temps we see the same back and forth swinging around the baseline for the anomaly.

Is this surprising? Not really. Any time there is an average, with a variation in each year, it will have to swing back and forth around that average.

So what will happen this summer? Is it possible to calculate or at best estimate if it will be hot or not?

Plotting only those years above the baseline against the next years anomaly shows this scatter.

It shows that a hot summer has just as much of a chance to produce another hot summer in the following year as a cool one.

However, things are a little different if you look at the mean temps in this manner. This plot is only those summer years where the deviation is positive (above normal) plotted against the deviation of the following year. When the deviation has a small positive, there is equal chance the next year will be positive or negative. But those years with a high anomaly, the following year tended to be cooler. 2C+ anomaly for a summer mean and the following year will be below the baseline. This needs to be checked with other stations, but it would be nice if we had more than a few points of data to confirm this.

2009 summer highest max temps was below the baseline, 2008 was even lower. If you look at the means, 2005 was the last above the baseline, with each subsequent year below. Looking only at that graph one could claim that this summer should be at or above the baseline giving us an average max temps between 25 and 27C, and a max temp of around 35.

This could be a good tomato season (last year was too cool).

Come Sept we will know for sure.

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jrwakefield (at) mcswiz (dot) com