Question 1. (Note: All figures and illustrations are at bottom of this page).

I have attempted to reproduce the findings reported upon in the news. In particular, I sought to obtain evidence that the strongest relationship between one of the Pacific Indices and Southern California rainfall was that between the Pacific Decadal Oscillation Index (PDO). I did this in two ways: (a) first, I simply obtained the correlation between the 1948-2003 PDO indices and rainfall by climate division across the United States; and (b) second, I actually correlated the rainfall at Los Angeles Civic Center and PDO indices across the same period.

If the news reports were correct, then there should be a positive correlation between PDO and rainfall, the correlation coefficient should be large (to account for a large part of the variance in the record), and statistically significant (meaning that if there is a relationship, that it is not due to chance alone).

Figure 1 shows the results I obtained for (a). The light orange colors indicate that PDO is indeed positively correlated with Southern California rainfall (when PDO increases, rainfall increases, when PDO decreases, rainfall decreases). However, the correlation coefficient of 0.25 or so indicates that that correspondence only occurs for 25% of the paired seasons in the period, and that correspondence only accounts for about 6% of the total variance (the rainfall variations from the mean) in the record. In addition, the correlation is barely statistically significant at the 5% level.

The results I obtained in (b) (correlation of 0.26, not statistically significant) substantiate those I got for (a). The important finding is that no matter what trends are observable in the PDO indices at current time, the relationship between those trends and rainfall trends in Southern California certainly do not appear to be important enough to warrant the strong statement that appeared in the news reports. In fact, most scientists examining these statistical results would dismiss PDO as an important contributor to rainfall trends in Southern California.

On the other hand, the same procedure applied to several Pacific Indices that are related to El Niño (the Southern Oscillation Index -- SOI; and, NINO3) suggest that warming of ocean temperatures in the tropical Pacific is a much more significant contributor to variations in rainfall in Southern California. For example Figure 2 shows the results I got by correlating NINO3 with climate division rainfall over the United States. The correlation coefficient was 0.55, and was significant at the 1% level. This correlation accounts only for about 33% of the variance observed in the precipitation record over Southern California. The conclusion is inescapable, of the indices mentioned in the newspaper article, the only ones that are reasonable "predictors" of seasonal southern California are those that deal with El Niño. But even those should not be used to account for all of the rainfall trends there.

The second portion of the first part of this contract also involves characterizing whether precipitation is increasing or decreasing in Southern California, no matter what the PDO indices are trending at this time. Since both rainfall amounts and PDO indices have year to year variability, the consultant applied a "filter" so that the longer term trends are apparent. In this case, I used the 10-yr running means of PDO indices and LA Civic Center seasonal rainfall since 1921 (Figure 3). The reasons for the very low correlation between PDO indices and Southern California rainfall immediately become apparent. While the correspondence mentioned in the press does appear through about 1970 or so, the corresopndence has reversed since. In short, PDO indices ARE indeed decreasing at present time, but rainfall in Southern California rainfall is increasing over the same period, exactly the opposite of what was described in the newspaper articles.

Based upon these observations, the consultant advises Conspicuous Construction to ignore the accounts about PDO and Southern California rainfall that appeared in the press.

Question 2. (Note: All figures and illustrations are at bottom of this page)

Since Pacific indices, even those associated with El Niño, do not account well for the total variations in the precipitation record of Southern California (for example, PDO and NINO3 indices combined only account for about 40% of the variance), it is best to examine the precipitation record itself for trends that might be used to anticipate rainfall over the construction site over the next 5 years. Figure 4 shows the 25 year running mean of seasonal rainfall and coefficient of variation at LA Civic Center (Figure 4). A comparison of Figs. 3 and 4 shows that both on a 10 y and 25 y running mean basis, rainfall at LA Civic Center can be characterized as increasing in recent years.

In addition, the coefficient of variation, a measure of the year to year swings in seasonal rainfall, has also seen a large and consistent increase in the last 25 years or so. Assuming that these trends are statisically significant and persist, the consultant advises Conspicuous Construction to expect large fluctuations around the average over next 5 to 10 years. In essence, very dry years alternating with very wet years. There is no evidence that rainfall will, on average, decrease.

In short, the consultant advises Conspicuous Construction to completely ignore the news reports.