Saturday, December 19, 2015

El Nino

Note this program first aired December 19, 2015.

The last two winters here in Maine have been long, cold and snowy thanks to persistent upper atmosphere air masses and the loopiness of the polar vortex. Many people I know are gearing up for more of the same this winter, but I am not ready to make any bets yet, and that is because of one big difference between last winter and this winter, El Nino.

Surely you have heard by now that this winter, the winter of 2015/16 is shaping up to be one of the strongest El Nino events in recent history, vying for the top spot with the winter of 1982/83 and 1997/98. El Nino is the common name for a oceanographic and atmospheric phenomenon or set of phenomena that occur in the Pacific Ocean but influence weather world wide. Meteorologists and climate scientists refer to the whole package as ENSO, short for El Nino Southern Oscillation, a name that gets us a little closer to what is actually happening in the Pacific Ocean this winter.

We take for granted that the ocean plays a huge role in our weather patterns and long term climate, we take it so much for granted that we never think about it, until something like a strong El Nino comes along. In the equatorial Pacific the typical ocean pattern runs east/west. Warm surface water is found in the western Pacific, held there by the prevailing winds, the trade winds which blow east to west. That net movement of surface water from east to west drives upwelling in the eastern Pacific, which means cool deep water comes to the surface. So, warm in the western ocean off Papua New Guinea, cool in the eastern ocean off South America. That warm water warms the atmosphere above it, creating low pressure and bringing lots of precipitation to the western Pacific region. The cold water in the eastern Pacific keeps the air mass there cool, maintaining high pressure and relatively dry conditions.

When an El Nino happens, this pattern totally reverses. It isn’t clear exactly whether the chicken or the egg comes first but several things happen that culminate in what we call El Nino. The trade winds diminish, so the wind doesn’t blow as strongly from east to west, and in some cases actually reverses and blows from west to east.  Without the wind holding it there, the warm surface water in the western Pacific moves back across the ocean towards the eastern Pacific. Because the wind drives that upwelling of cold water off Peru I mentioned, no wind means no upwelling. With warm water heading east, and no cold water to cool the eastern ocean, the Pacific ocean off south America gets warm, and that is the hallmark sign of El Nino. That warm water has profound effects locally, dramatically diminishing the productivity of the waters off Ecuador and Peru. Oceanic food chains start with phytoplankton, and with no nutrient rich deep water coming to the surface, the phytoplankton don’t survive. And if there are no phytoplankton that ripples all the way up the food chain.

As far as we are concerned, it isn’t the warm water in the eastern Pacific that changes our winters, but the warm air mass above that warm water. Warm moist air leads to low pressure, in an area where there is usually high pressure. This switch in pressure regime can drive changes in the way air masses travel across north America in the winter, strengthening the east/west flow of the jet stream, reducing the chance of the north/south loopiness of the polar vortex we’ve experienced the last couple of winters. Typical effects are increased precipitation in the southeast and west, and warmer winters across the northern tier of the US and southern Canada.

So is El Nino responsible for this extraordinarily warm December we’re having? Its hard to tell. It is staying warm because the air masses that are dominating the continental US are coming from the Pacific Ocean, rather than the Canadian Arctic, that warm Pacific air is blocking the jet stream from coming south, and it may be related to El Nino. We should remember however, that last December was pretty warm too, without an El Nino, it was 54 degrees on Christmas Day at my house last year. El Nino doesn’t mean that it will be an effortless balmy winter. If you think back to January of 1998, you might remember a little event we like to call the Ice Storm. So we will have to wait and see how this all plays out. I’m beginning to think though, it might have been the wrong winter to buy new skis. I hope I’m wrong.


NASA/Goddard Space Flight Center. "NASA studying 2015 El Nino event as never before." ScienceDaily. ScienceDaily, 19 October 2015.

Accuweather’s White Christmas prediction map
The straight talk from NOAA

Even straighter talk from the Weather Channel

There’s even an El Nino blog (it is really good, check it out)