Australia raises alert on stealthy El Niño

Don't look now, but the phenomenon formerly known as El Niño is knocking.

In fact tropical Pacific Ocean sea surface temperatures have increased so much in the past four weeks that a new El Niño event may already be here.

As you can see on the loop below, the majority of the tropical Pacific Ocean near the equator is now at or above the National Oceanic and Atmospheric Administration's +0.5C "El Niño threshold."

The recent vigorous and widespread warming, along with continued forecast model projections of an El Niño this winter, prompted Australia's Bureau of Meteorology to raise the status of it's El Niño Southern Oscillation Tracker to alert status from watch status today.

The Aussie Bureau now predicts a 70 percent chance of an El Niño this winter. The El Niño Southern Oscillation status is raised to alert level when three of the following criteria are met.

The aggressive warming in the tropical Pacific begs the questions of why an El Niño has not yet been declared, and what it will take for El Niño to be officially declared?

Here's a good explanation from NOAA on what it takes to "officially" declare an El Niño.

El Niño “events” versus El Niño “conditions”

However, the folks at NOAA CPC and the IRI issue their ENSO Diagnostics Discussion and update the ENSO Alert System every month.  If it’s issued monthly, then will it take three months to say El Niño has arrived?

The short answer is no. Knowing that ENSO is a coupled climate pattern between the atmosphere and ocean (see earlier post) gives us an ability to provide information on a monthly timescale. We will declare the onset of El Niño conditions (not yet a full-blown El Niño event) when three criteria are met:

1. Departures in the Niño-3.4 index equal to or exceeding +0.5C for a single month.

2. The tropical Pacific atmosphere should be consistent with El Niño.  In particular, rainfall should be enhanced near the Date Line and suppressed near Indonesia, and the surface winds across parts of the equatorial Pacific should be anomalously westerly.

3. A forecast that the ONI will equal or exceed +0.5C for several seasons in a row.

So it is possible that NOAA will officially declare an El Niño event when it issues it's next Monthly El Niño/La Niña Diagnostics Discussion On Dec. 4. The Australian Bureau of Meteorology issues updates on ENSO every two weeks.

Odds of a milder winter increasing?

The developments in the Pacific may be increasing the odds of an average, or even milder than average winter Minnesota and the in the northern US.

Looking back at past El Niño events, statistical odds of a milder than average winter in Minnesota approach 70 percent if a full blown El Niño develops.

Winter snowfall patterns are somewhat less predictive for Minnesota in El Niño winters.

Changes already in progress?

It's too soon to tell for sure, but I am already seeing some atmospheric circulation patterns that may be a sign that El Niño is taking hold.

In spite of our early cold in Minnesota and the U.S. the overall upper air and jet stream patterns appear to be about to change from a "high amplitude arctic invasion" into a more mild west to east or "zonal flow" regime in the coming two weeks.

Zonal flow patterns draw Pacific air into the U.S., and temps will moderate to above average levels across much of the eastern half of the U.S. as early as this weekend.

It appears the recent dreaded negative phase of the Arctic Oscillation that has delivered an early season November cold wave is about to flip to a milder, positive AO phase.

The result should be a much milder weather pattern for Minnesota, and most of the eastern U.S. in the next two weeks.

Here are the numbers from NOAA's Global Forecast System extended range output.

A milder than average late November and early December in Minnesota?

An early sign of the winter to come?

Stay tuned.