Guest Post by Bob Tisdale
AUSTRALIA’S BOM UPGRADES ENSO TRACKER STATUS TO EL NIÑO ALERT
On November 18, Australia’s Bureau of Meteorology (BOM) upgraded the conditions in the tropical Pacific from El Niño “watch” to “alert” levels, “indicating at least a 70% chance of El Niño occurring”. See the rest of their update here.
NOAA’S WEEKLY NINO3.4 DATA
The sea surface temperature anomalies of the NINO3.4 region of the equatorial Pacific are a commonly used ENSO index. NOAA’s Oceanic NINO Index is a form of the data of that region. According to NOAA’s weekly sea surface temperature data for the NINO3.4 region, as of the week centered on November 12th, El Niño conditions (+0.5 deg C or greater) have existed in the NINO3.4 region for 5 weeks and they are presently well above the threshold, now at 0.8 deg C for two weeks. This is still a far cry from the NOAA requirements to declare an “official” El Niño has taken place (5 consecutive periods of 3-month averages with NINO3.4 anomalies equal to or above the +0.5 deg C threshold). But it’s a start.
BE CAREFUL HOW YOU INTERPRET ANOMALIES ON MAPS
Over the past month or so, the unusual warming of the eastern extratropical North Pacific (known as “the blob”) has dissipated. Whether or not it will return next year is still uncertain. But as a result of the blob, there was a large volume of warm water on the surface in the northeastern North Pacific, and presently there are residuals (leftovers) that are now hugging the west coast of North America. See the map of sea surface temperature anomalies in Figure 1. It’s from the CMC Environment Canada webpage here.
Some of that warm water along the North American coast, according to the recent interviews with Trenberth and Timmermann, came from a coastally trapped Kelvin wave that formed after the large downwelling equatorial Kelvin wave hit the coast of Ecuador back in April 2014. Also see the discussion of coastally trapped Kelvin waves in the post Axel Timmermann and Kevin Trenberth Highlight the Importance of Natural Variability in Global Warming… And some of the warm is left over from the blob.
That raises a few questions: is that warm water feeding back down to the tropics? If so, will it enhance the El Niño conditions in the tropics?
If we look at animations of sea surface temperature anomaly maps, it APPEARS that, yes, the warmer waters along the west coast of North America might be migrating southward along the California Current.
Animation 1 presents the 6 cells from the maps available as part of today’s animation from the CMC Environment Canada webpage here. That animation runs early October to today. The visual effect shows up pretty well on those maps. [Note: It could also be that the warming sea surfaces along the equator are creating a more typical El Niño spatial pattern.]
Unfortunately, there is a large seasonal component to hemispheric sea surface temperature anomaly data, especially in the Northern Hemisphere. As a result, on sea surface temperature anomaly maps, the warmest anomalies can cycle yearly, tracking the change of seasons. Some of the seasonal appearance (not the blob) results from the seasonal loss and gain of Arctic sea ice, but there can still be large seasonal cycles in small regions…such as along the west coast of North America, where the California Current exists. See Figure 2.
That brings us to the questions: is the appearance of warm water migrating southward simply a product of the climatology used to determine the anomalies? Or is the warmer water actually heading south along the coast of North America as one might expect in the California Current?
Well, there’s something else to keep an eye on.