The warmest winter on record in San Francisco Bay

Here in San Francisco, the common sauna wisdom is that we just experienced one of the warmest winters in recent memory. The Dolphin Club’s Polar Bear Challenge was hardly challenging, and the South End’s “Dreaded 9th” of February swim was hardly dreaded.

Just how warm was it, though? I crunched the numbers from the NDBC, because, well, why not?

Here we see the last 15+ months of data from the Crissy Field station (FTPC1) inside San Francisco Bay, plotted in solid black. The dashed green, red, and blue lines show the long-term average, maxima, and minima for each day of the year, summarized over the eight years of available data from that station.

From July 2014 until just the past few days (early April 2015), Bay waters have been hovering 2-3 degrees (F) above the all-time highs (going back to 2006), and about 5 degrees above the long-term averages.

Eight years isn’t much data, unfortunately. Can we do better?

A bit: Lightstation 46026 - about two-thirds of the way out to the Farallones - has data going back to 1982. Almost 33 years of data! This is colder, hairier water than inside the Bay, but it’s as close as we’re going to get.

Here’s a similar chart for this much longer-term data set. Unfortunately, the water temp sensor at Station 46026 was out of commission for the first eight months of 2014, so we can compare “recent data” starting from late August.

The story is similar, though: This was one of the warmest winters on record. Through most of December, we were ~4 degrees above the all-time highs going back over 30 years.

Indeed, when I took the average sea temps for each full winter season (December 1 through February 28), I found the following five warmest winters since 1982-83:

  • 2014-15:     57.3 (F)
  • 2004-05:     55.0
  • 2002-03:     54.7
  • 1982-83:     54.6
  • 1983-84:     54.3

Only time will tell if the recent drop in Bay temps is a momentary aberration, or a longer-term regression to the mean.

No doubt, this summer’s Farallon swimmers will be watching this closely. 2014 saw the first two Farallon solos since 1967 — Craig Lenning and Joe Locke. And from a water temp perspective, 2014 was probably the best year to swim the Farallons in over a generation. We won’t always be so fortunate.