I’ve read predictions that this winter will be a strong La Niña period, which means that the tropical eastern Pacific Ocean temperature will be colder than normal. The National Weather Service has a lot of information on how this might affect the lower 48 states, but the only thing I’ve heard about how this might affect Fairbanks is that we can expect colder than normal temperatures. The last few years we’ve had below normal snowfall, and I was curious to find out whether La Niña might increase our chances of a normal or above-normal snow year.
Historical data for the ocean temperature anomaly are available from the Climate Prediction Center. That page has a table of “Oceanic Niño Index” (ONI) for 1950 to 2010 organized in three-month averages. El Niño periods (warmer ocean temperatures) correspond to a positive ONI, and La Niña periods are negative. I’ve got historical temperature, precipitation, and snow data for the Fairbanks International Airport over the same period from the “Surface Data, Daily” or SOD database that the National Climate Data Center maintains.
First, I downloaded the ONI index data, and wrote a short Python script that pulls apart the HTML table and dumps it into a SQLite3 database table as:
sqlite> CREATE TABLE nino_index (year integer, month integer, value real);
Next, I aggregated the Fairbanks daily data into the same (year, month) format and stuck the result into the SQLite3 database so I could join the two data sets together. Here’s the SOD query to extract and aggregate the data:
pgsql> SELECT extract(year from obs_dte) AS year, extract(month from obs_dte) AS month, avg(t_min) AS t_min, avg(t_max) AS t_max, avg((t_min + t_max) / 2.0) AS t_avg, avg(precip) AS precip, avg(snow) AS snow FROM sod_obs WHERE sod_id=’502968-26411’ AND obs_dte >= ’1950-01-01’ GROUP BY year, month ORDER BY year, month;
Now we fire up R and see what we can find out. Here are the statements used to aggregate October through March data into a “winter year” and load it into an R data frame:
R> library(RSQLite) R> drv = dbDriver("SQLite") R> con <- dbConnect(drv, dbname = "nino_nina.sqlite3") R> result <- dbGetQuery(con, "SELECT CASE WHEN n.month IN (1, 2, 3) THEN n.year - 1 ELSE n.year END AS winter_year, avg(n.value) AS nino_index, avg(w.t_min) AS t_min, avg(w.t_max) AS t_max, avg(w.t_avg) AS t_avg, avg(w.precip) AS precip, avg(w.snow) AS snow FROM nino_index AS n INNER JOIN noaa_fairbanks AS w ON n.year = w.year AND n.month = w.month WHERE n.month IN (10, 11, 12, 1, 2, 3) GROUP BY CASE WHEN n.month IN (1, 2, 3) THEN n.year - 1 ELSE n.year END ORDER BY n.year;" )
What I’m interested in finding out is how much of the variation in winter snowfall can be explained by the variation in Oceanic Niño Index (nino_index in the data frame). Since it seems as though there has been a general trend of decreasing snow over the years, I include winter year in the analysis:
R> model <- lm(snow ~ winter_year + nino_index, data = result) R> summary(model) Call: lm(formula = snow ~ winter_year, data = result) Residuals: Min 1Q Median 3Q Max -0.240438 -0.105927 -0.007713 0.052905 0.473223 Coefficients: Estimate Std. Error t value Pr(>|t|) (Intercept) 2.1000444 2.0863641 1.007 0.318 winter_year -0.0008952 0.0010542 -0.849 0.399 Residual standard error: 0.145 on 59 degrees of freedom Multiple R-squared: 0.01208, Adjusted R-squared: -0.004669 F-statistic: 0.7211 on 1 and 59 DF, p-value: 0.3992
What does this mean? Well, there’s no statistically significant relationship between year or ONI and the amount of snow that falls over the course of a Fairbanks winter. I ran the same analysis against precipitation data and got the same non-result. This doesn’t necessarily mean there isn’t a relationship, just that my analysis didn’t have the ability to find it. Perhaps aggregating all the data into a six month “winter” was a mistake, or there’s some temporal offset between colder ocean temperatures and increased precipitation in Fairbanks. Or maybe La Niña really doesn’t affect precipitation in Fairbanks like it does in Oregon and Washington.
Bummer. The good news is that the analysis didn’t show La Niña is associated with lower snowfall in Fairbanks, so we can still hope for a high snow year. We just can’t hang those hopes on La Niña, should it come to pass.
Since I’ve already got the data, I wanted to test the hypothesis that a low ONI (a La Niña year) is related to colder winter temperatures in Fairbanks. Here’s that analysis performed against the average minimum temperature in Fairbanks (similar results were found with maximum and average temperature):
R> model <- lm(t_min ~ winter_year + nino_index, data = result) R> summary(model) Call: lm(formula = t_min ~ winter_year + nino_index, data = result) Residuals: Min 1Q Median 3Q Max -10.5987 -3.0283 -0.8838 3.0117 10.9808 Coefficients: Estimate Std. Error t value Pr(>|t|) (Intercept) -209.07111 70.19056 -2.979 0.00422 ** winter_year 0.10278 0.03547 2.898 0.00529 ** nino_index 1.71415 0.68388 2.506 0.01502 * — Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 Residual standard error: 4.802 on 58 degrees of freedom Multiple R-squared: 0.2343, Adjusted R-squared: 0.2079 F-statistic: 8.874 on 2 and 58 DF, p-value: 0.0004343
The results of the analysis show a significant relationship between ONI index and the average minimum temperature in Fairbanks. The relationship is positive, which means that when the ONI index is low (La Niña), winter temperatures in Fairbanks will be colder. In addition, there’s a strong (and significant) positive relationship between year and temperature, indicating that winter temperatures in Fairbanks have increased by an average of 0.1 degrees per year over the period between 1950 and 2009. This is a local result and can’t really speak to hypotheses regarding global climate change, but it does support the idea that the effect of global climate change is increasing winter temperatures in our area.
One other note: the model that includes both year and ONI, while significant, explains a little over 20% of the variation in winter temperature. There’s a lot more going on (including simple randomness) in Fairbanks winter temperature than these two variables. Still, it’s a good bet that we’re going to have a cold winter if La Niña materializes.
Thanks to Rich and his blog for provoking an interest in how El Niño/La Niña might affect us in Fairbanks.