Eric Holthaus continues his entertaining El Nino coverage in Slate:
El Niño means the tropical Pacific is warmer than normal, which improves the chances that typhoons will form. This week, a series of typhoons on both sides of the equator are helping to reinforce a big burst of westerly winds along the equator. These westerly wind bursts are a hallmark of El Niño, and help push subsurface warm water toward the coast of South America. If enough warm water butts up against Peru, the normal cold water ocean current there can get shut off, exacerbating the pattern.
Exactly how this all gets kicked off is an area of active research, but it’s clear that big El Niños need deviant trade winds to maintain the feedback loop. During especially strong El Niños, like this year’s promises to be, the trade winds can sometimes reverse direction—and this week’s off-the-charts wind surge is at record-strengthfor so early in an El Niño event. Since all this takes place in the tropical Pacific Ocean—the planet’s biggest bathtub—a fully mature El Niño has the power to shift rainfall odds worldwide and boost global temperatures. That’s exactly what’s happening this year.
Don’t worry, says Michelle L’Heureux at NOAA:
xWe are now nearing 1.5 degrees Celsius in the Niño-3.4 index for a 7-day or weekly average. Among the post-college age crowd who can remember it (yes, this officially means you’re old), the level of warmth in sea surface temperatures this time of year harkens back to 1997-98 El Niño, which ended up becoming a record strength event.
Are these weekly numbers impressive? Yes. But when a weekly value hits 1.5°C is El Niño instantly considered strong? I’d argue no. While a short-term (daily or weekly) number might be striking, it shouldn’t be used as an indicator of El Niño strength unless it is carefully placed into a larger context…
Weekly averages bounce around because… you know, weather. It is not unusual to see jumps of several tenths of a degree from week-to-week. This is because there are shorter-term changes in the ocean and atmospheric circulation that can be related to faster, non-ENSO phenomena, such as the Madden Julian Oscillation, which happens to be currently in place and isinfluencing the tropical Pacific. With short-term changes, we cannot be sure they reflect El Niño, which evolves more slowly.