Saturday, June 13, 2015

June 2015 El Niño update

June El Niño update: Damn the torpedoes, full speed ahead


El Niño continues to pick up steam. NOAA CPC/IRI forecasters are now very confident that the event will continue through the fall (over 90% chance) and into the winter (~85% chance). Now that we’re emerging from the spring barrier, this month’s update provides a first guess of the potential strength of El Niño. It’s harder to predict the strength of the event than it is to predict its duration, so we are less confident about that, but forecasters currently favor a “strong” event for the fall/early winter. By “strong” we mean it’s expected that the three-month average sea surface temperature in the Niño3.4 region will peak at more than 1.5°C (2.7°F) above normal.

What’s happening right now?

During the month of May, we saw increases in a lot of the ENSO indicators. Sea surface temperature anomalies (the departure from average) were up all across the tropical Pacific, and the most recent Niño3.4 Index was 1.2°C. Both the lower-level and upper-level winds along the equator were substantially weaker than average last month, characteristic of El Niño’s weakened Walker Circulation. This feedback between the higher sea surface temperature anomalies and the atmosphere is critical to both perpetuating and strengthening an El Niño event, and to communicating the effects of El Niño to other areas of the globe.

Signs of another downwelling Kelvin wave have begun to appear in the upper levels of the equatorial Pacific. This reinforcing source of warmer-than-average waters follows the strong Kelvin wave that has been slowly moving east since February. The consistently warmer waters are reflective of the slower changes we’d expect to see when the ocean-atmosphere system has settled into an ENSO event, as opposed to the shorter-term changes that dominate the rest of the time.

What signs are suggesting a possible strong event?

The physical observations I described above are a source of confidence that this event is continuing to build. Also, most climate models are forecasting a continued increase in Niño3.4 sea surface temperature anomalies, and many forecast a peak in the early winter above 1.5°, some above 2.0°C.

It’s important to remember, though, that we only have a handful of strong El Niño events in the historical record—seven since 1950. And only three of those saw Niño3.4 index values of 2.0°C or higher: 1972-73, 1997-98 and 1982-83)*. Imagine if we’d only had seven hurricanes in the history of record-keeping; it would make it much harder to understand and predict how a future hurricane could develop. So, while we’re confident that this El Niño event will continue, there’s still plenty of uncertainty about how it will evolve.

That said… what would a strong event mean?

As Tony pointed out in our last post, ENSO impacts on North American weather and climate are most noticeable in the winter. This is largely because winter weather is governed more by large-scale processes (e.g. nor’easters) than summer weather, when local effects (e.g. isolated thunderstorms) tend to be more important. El Niño events affect the strength and position of the jet stream, and tilt the odds toward more rain than average along the West coast and in the Southeast during the winter.

El Niño loads the dice in favor of certain impacts, but it doesn’t guarantee them. However, stronger events tend to lead to more predictable effects. My brother asked me if he should repair the leaks in the roof of his campervan before he goes to Florida this winter… based on the ENSO forecast, I said yes. (I would probably have said yes anyway —who wants to stay in a drafty camper?— but that’s beside the point.)

Globally, this El Niño event is likely to lead to higher global temperatures, possibly record-breaking. On this, check out Deke Arndt’s post on our new sister blog, Beyond the Data. Also, as I mentioned last month, the tropical Pacific hurricane season is already breaking records, while the Atlantic hurricane season is forecast to be quieter than average—both effects linked to El Niño. For the coming summer (June-August), check out Tony’s post on potential temperature and precipitation impacts linked to El Niño.

Many, many different components are at work in the global climate system, making exact predictions impossible. However, the development of a climate phenomenon like El Niño can make some outcomes more likely than others, which is why we follow it so closely. We’ll keep you posted as this event continues.

Spring 2015 Weather Summary for Northern and Eastern Maine

..Spring weather summary for northern and eastern Maine...

For the meteorological spring (March through May) of 2015 temperatures and precipitation were below normal. Temperatures averaged from 1 to 2 degrees below normal and precipitation averaged from 50 to 90 percent of normal. At both Caribou and Bangor it was the driest spring since 2001.

At Caribou, the average temperature of 37.3 degrees was 0.9 degrees below normal. At Bangor the average temperature of 40.8 degrees was 1.4 degrees below normal. A total of 6.75 inches of rain (and melted snow) was observed at Caribou, which was 78 percent of normal. At Bangor, a total of 6.16 inches of rain (and melted snow) was observed, which was 65 percent of normal.

For the second year in a row March was a very cold month. Temperatures averaged from 4 to 6 degrees below normal. It was the 4th coldest March on record at Bangor and the 11th coldest at Caribou. The big story was the persistent cold as there were only 5 days the entire month in most areas where the average temperature was above normal. Precipitation was below normal and ranged from 40 to 75 percent of normal.

April also featured below normal temperatures and precipitation. Temperatures averaged from 1 to 3 degrees below normal. Snowfall was close to normal in most areas, but the total precipitation (rain and melted snowfall) was 60 to 90 percent of normal. The highlight of the month was the morning of 6th when the temperature dropped to 4 below at Caribou, which set an all-time record low for the entire month of april. A low of 20 below was observed at Big Black River that morning, and from all available data sources was the lowest temperature ever observed in the state of Maine during the month of April.

May was the first month since December 2014 with above normal temperatures. Temperatures averaged from 2.5 to 4.5 degrees above normal. Precipitation in most areas ranged from 40 to to 75 percent of normal, but was highly variable due to the showery nature of the precipitation.

This past May was only the 3rd time since weather records began at Caribou in 1940 that the temperature did not fall below freezing during the month of may. The last freeze of the season at Bangor was observed on May 2nd, which was close to normal.

The most noteworthy event occurred on the morning of the 23rd when 3 tenths of an inch of snow was observed at Caribou. It was the 2nd latest measurable snowfall on record. An amazing 5.2 Inches of snow was observed 2 miles north of Portage.