Sunday, August 17, 2014

Probability of Negative NAO During Upcoming Winter Increasing

The likelihood that we will see high pressure over Greenland this winter, a sign of the negative North Atlantic Oscillation, is on the rise.

The image above shows sea surface temperature anomalies for the day of August 16, 2014. In this image, we can see quite a few areas of interest that we are monitoring for the upcoming winter, but today we'll focus in on Greenland and around the Canadian Maritimes. Looking towards that part of the world, we see a swath of well above normal SST anomalies stretching from the waters south of Greenland towards the Arctic circle. The warm waters also extend to the east, immediately offshore of Greenland.

It is well known that sea surface temperature anomalies can exert a significant influence on the presence of high or low pressure in a given area. For example, areas with warmer than normal waters tend to observe high pressure more often that low pressure, while colder than normal SST anomalies usually result in stormy conditions prevailing over quiet conditions. This was observed well last winter, when the body of positive SST anomalies in the northeast Pacific brought about strong high pressure along the western coast of North America, which permitted the cold weather to penetrate deep into the US.

In Greenland, with the presence of a rather large swath of warmer than normal waters, I wouldn't be surprised to see persistent ridging over this part of the world this winter. If such a scenario does play out, we would enter the negative phase of the North Atlantic Oscillation (NAO), which would allow cold and stormy conditions to prevail over the Central and East US, the latter of which may experience episodes of intense snowstorms, as can happen in negative NAO events.

In the graphic above, we see SST anomalies over four different parts of the Pacific Ocean, centered along the Equator. Nino region 4 is located near the Oceania region, while Nino region 1+2 is centered just offshore Ecuador. By looking at these panels, we can identify a body of warm water on the surface in the west-central Pacific in Nino region 4, as well as the far east Equatorial Pacific in Nino regions 1+2 and region 3.

It is expected that all of these regions may experience additional warming as we progress into Fall, which is when we anticipate the emergence of a Weak El Nino (SST anomalies +0.5 to 1.0 degrees above normal).

Weak El Nino's affect the United States in many ways, but the primary effect is through manipulation of the jet stream. When this happens, the jet stream is forced south along the Eastern Seaboard, allowing cold air to flow deep into the Central and East US. Storm systems are then pushed south as well, and end up following the jet stream along the South US before shooting north as the jet stream is pulled up towards Greenland, where a ridge (Negative NAO) resides. Consequentially, snowstorms pose threats to the Northeast and East Coast, in addition to the cold weather.

The negative NAO and weak El Nino generally tie into each other, as they seem to work in a tandem. If current projections verify for this winter, and the SST anomalies near Greenland persist, the likelihood of a negative NAO throughout the winter significantly rises.