Milwaukee – It is official. We are in La Niña. It just started. And guess what? It looks like it is already ending. Allow me to explain. The Climate Prediction Center (CPC) of NOAA monitors water temperatures in the tropical Pacific Ocean. CPC publishes weekly and monthly updates and forecasts on the tropical Pacific Ocean. This area is closely monitored by meteorologists because water temperature swings in the Pacific Ocean impact global weather. These swings have names that are common to the ears of most people. Are you familiar with El Niño and La Niña? Those are the names for these water temperature swings in the tropical Pacific.

What Qualifies La Niña or El Niño

CPC monitors a specific region in the Pacific Ocean for La Niña and El Niño conditions. This region is known as Niño 3.4. It is a rectangle with longitudes of 120°W as the eastern border and 170°W as the western border. The southern and northern borders are latitudes 5°S and 5°N. It looks like this:

Nino Regions

It is far from any continent. There are other Niño regions, but the overlap of Niño 3 and Niño 4 create Niño 3.4. When water temperatures in this very large rectangle deviate more than one half of a degree Celsius from the long term average, La Niña or El Niño conditions are possible. The temperature threshold for La Niña is a half of a degree Celsius below average. El Niño? It is half of a degree Celsius above average. We were in El Niño in 2015 and 2016. By July 2017, La Niña thresholds were reached in Niño 3.4. So, we have a specific temperature in a specific region to monitor. There is one last qualifier. NOAA averages the anomalies in Niño 3.4 over time. It is an average of temperature anomalies over consecutive three month periods. But, one three month average doesn’t qualify. It takes five consecutive three month averages with anomalies greater than the threshold to qualify. This is a process and not a light switch. Here is a sample:

Nino Index Sample

Across the top of this above chart are the months in consecutive rolling blocks (DJF is December, January, February). Towards the end of 2004, El Niño was developing as water temperatures average more than positive .5° anomaly for consecutive months. These are the red blocks above. In January 2005, El Niño became official as we had five consecutive three month averages of .5° or warmer. The blocks in blue are La Niña conditions. Note that in 2005 and 2006 there were four consecutive rolling averages of La Niña thresholds. Again, it takes five consecutive. So, those blocks are not highlighted in blue and NOAA did not recognize La Niña, by definition. To summarize, Niño conditions are based on defined temperature anomalies, in a specific region of the Pacific, averaged over a specific period of time. Here is a chart showing we have now reached the five consecutive three month averages:

Current La Nina Episode Highlighted

The bold box highlights the five blue time periods showing the current La Niña. By the way, the Australian Bureau of Meteorology and the Japanese Meteorological Agency have slightly different definitions, but we will save that for another article.

La Niña is Happening

In July 2016, water temperature anomalies in Niño 3.4 dipped below -.5° Celsius. These colder than normal anomalies continued through Fall and into Winter. Here is how that process looks on a time-series plot.

Niño 3.4 Anomalies

I inserted the bold horizontal line to highlight the La Niña threshold. Since July, Niño 3.4 has been mostly below the threshold. In fact, the greatest anomaly occurred in October. Since late December, the anomaly has weakened. In fact, water temperatures are warming above the threshold. Here is the latest temperature analysis from February 6th, 2017.

February 6 2017 Niño Analysis

Note that Niño 3.4 temperature anomaly is lower than the -.5° Celsius La Niña threshold. Niño regions 1+2 and 3 are actually warmer than average. This next image shows the temperature trend over time in the Pacific Ocean. The date is on the y-axis and longitude on the x-axis. Blue is below average temperatures with orange and red above average.

Pacific Temperature Time-Series

You can see the end of the 2016 El Niño at the top of the chart  (the areas of red and orange). Blue, or below average temperatures, begins to appear in June and July 2016. I highlighted the bottom of the chart to show warmer temperatures developing. What does this show. It shows that just as La Niña became official this week, it is also ending. Here is another image that shows heat anomalies in the Pacific Ocean to a depth of 300 meters.

300m Heat Anomalies in Pacific Ocean

This goes beyond surface water temperatures. It looks at the total column of water down to a depth of 300 meters. The chart shows below average heat anomalies from March 2016 through early January 2017. Since mid January, a change. We now have above average heat anomalies. This reinforces that La Niña is ending just as it started.

What is Coming

This comes as no surprise. A weak and short La Niña was forecast. Going forward, a neutral pattern is most likely. Neutral means that temperatures in Niño 3.4 don’t deviate more than .5° Celsius. The official Niño forecast from mid January favors neutral through summer.

Niño Forecast

In this forecast, blues columns denote La Niña, green columns Neutral and red columns El Niño. The forecast shows that we are already in Neutral and that Neutral is most likely through August, September and October 2017. Interesting because this coincides with the peak of the Atlantic Hurricane Season. That is quite significant. There are many factors that influence tropical development in the Atlantic Ocean, but the Niño phase is a powerful influence. Neutral phases do favor slightly above average hurricane seasons in the Atlantic Ocean. The next forecast comes out February 9th and likely reinforces the January forecast based on recent observations.

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