San Diego’s warm spell is broken.
When the clock struck midnight on Thursday, a 54-month run without a cooler-than-normal month ended.
The last time a month was cooler than normal in town was October 2013 — 4½ years ago. It was a remarkable, rare run of warmth. There have been other extended warm spells in the city’s past, most notably in the early 1980s, but none were quite like the one that just ended.
What caused the extended warm spell, and what does it mean?
“There are a number of factors,” said Dan Cayan, a climate researcher at the Scripps Institution of Oceanography at UC San Diego. “There’s been a natural variability that has promoted the warmth in the majority of the months during this period. But global change is pretty much inescapable. I don’t think we can avoid that as an ingredient of what we’re seeing in San Diego County.”
May ends the warm streak, but the preceding period appears to represent a pattern shift, Cayan said. “We’re starting to see development of a new normal,” he said.
Parade of heat records
Since that last cooler-than-normal month in 2013, the city experienced its:
• Warmest year on record (2014)
• Second-warmest year on record (2015)
• Warmest 4-year period on record (2014-17)
• Warmest February (2016), March (2015), May (2014) and October (2015), and second-warmest January (2018), April (2015), September (2015) and November (2016)
May of this year, by comparison, seemed incredibly cool. The last 26 days of the month never topped 70 degrees.
But in actuality, the month was just barely cooler than normal — by only 0.5 of a degree.
“After all these above-normal years, it seems cold to us,” National Weather Service forecaster Brandt Maxwell said.
The 2014-17 years were similar to the 1981-84 period, which was only 0.3 of a degree cooler. Before this decade, San Diego’s hottest years were 1981, ’83 and ’84. In those years, the record heat came mainly in the warmer summer months, from June to September, which may have made them stand out a little less.
The longest stretch without a cooler-than-normal month in the 1980s was 35 months, compared to the recent streak of 54. (In May 2015, San Diego’s average temperature, which weighs the combined highs and the lows, was exactly normal. Every other month since October 2013 was warmer than normal.)
Common to both warm periods were some exceptionally warm nights, when many daily records for highest minimum temperature were set.
It’s important to note what is considered “normal.” Today’s normal is warmer than what was normal in the ’80s.
The numbers are revised after every 10 years, and they’re based on the temperatures that occurred during the preceding 30-year period. What was considered “normal” in the ’80s was based on what occurred during the cooler ’50s, ’60s and ’70s. The normals in the 2010s are higher than they were in the 1980s, because the warm ’80s are part of the calculation.
The recent 54-month warm period included a powerful, long-lasting El Niño (from late 2014 through early 2016), marked by elevated sea-surface temperatures in the equatorial Pacific. El Niños usually result in increased precipitation and temperatures in Southern California. The temperatures did climb during the El Niño, but the precipitation didn’t develop.
The period also included two La Niñas (late 2016, and late 2017 to early 2018), which generally have the opposite effect and tamp down precipitation and temperatures locally. Still, the streak of warm temperatures continued.
“It’s hard to point to any (ocean) circulation regime,” said Mike Halpert, deputy director of the Climate Prediction Center in Maryland. “La Niñas oftentimes cool down the Pacific north of the tropical areas. I don’t really have much of an explanation.”
Local sea-surface temperatures are also part of the equation. When the waters off the San Diego coast are warm, as they were for much of the 54-month streak, some of that warmth transfers to nearby land, raising temperatures.
When the waters are cool, as they were for much of May, it’s easier for low clouds to form, especially in the spring. Those low clouds help keep down the nearby daily highs.
Cayan, the climatologist at Scripps, points to a fairly persistent ridge of high pressure over much of the West as an important factor during the 54-month warm stretch. Ridges generally preclude storms from reaching the region, which leaves it drier, and drier usually translates into warmer.
In May, the high-pressure ridges relented. Other than a couple of days early in the month, the weather was dominated by low-pressure troughs, which brought a fairly consistent dose of May gray and a cool, onshore flow — cool enough to end the warm streak.
The current short-term pattern doesn’t show signs of a major shift soon, although the weekend should warm up a few degrees before another trough of low pressure is expected to bring June gloom next week.
According to the National Oceanic and Atmospheric Administration, April was Earth’s 400th warmer-than-normal month in a row. The last cooler-than-normal month globally was February 1985.
Cayan said there is more variability on smaller, regional scales. San Diego’s thermometer is symptomatic of the warmth over a much broader area.
“There’s no mistaking we’ve had this large footprint of warmth across the western U.S. in this same period,” Cayan said. “It’s quite clear we’re part of that bigger picture.”
And the pattern in the West is just part of a global change, Cayan said. Greenhouse gases have continued to build in the atmosphere, which traps radiation and increases the chances for record-breaking warmth.
“We are participating in what is going on in a much larger scale,” he said.