Wednesday, May 28, 2014

Could Unusual Pacific Ocean Patterns Make for Another Cold Winter?

It is possible that an unusual Pacific ocean water pattern could increase the risk of a second straight cold winter.

Shown above are two composite images. The top row shows sea surface temperature anomalies during an El Nino event (top left), and during a La Nina event (top right). The bottom row shows sea surface temperature anomalies during a positive PDO event (bottom left), and during a negative PDO event (bottom right). You may notice that the images look similar. This is because typically, when there is an El Nino, below normal waters form in the North Pacific. Similarly, during a La Nina, warmer than normal waters tend to be observed in the northern Pacific. This is not a 100% correlation, it is just what usually happens. This year, we are seeing this correlation broken.

The above image shows observed sea surface temperature anomalies (SSTAs) on May 25th. We see two items of interest. The first is the presence of significantly above normal water temperatures in the northeast Pacific. This mechanism is what created sometimes-relentless bouts of ridging over the west coast of North America this past winter, greatly aiding in the cold weather that hit North America. This body of warm water still remains here this summer, and if it stays, may once again produce bouts of ridging resulting in a chilly winter for many in the United States.

Going by the composite images at the top of this post, we would typically watch out for below normal water temperatures in the Equatorial Pacific, more specifically the waters west of Ecuador. However, that is not the case. We have been discussing the incoming El Nino for months now, which is allowing SSTAs in that area to rise. This means that we have warmer than normal waters in both the North Pacific and Equatorial Pacific, breaking the correlation we saw at the top of this post.

So, what could that mean for winter? Let's find out.

The image above shows correlation values between surface temperatures in December-January-February (the winter season), and the observed Pacific-Decadal Oscillation (PDO). In other words, if the PDO is positive (see example in composite image at top of post), areas in positive correlation areas (warm colors) would see warmer than normal temperatures, since a positive PDO is positively correlated with surface temperatures. Inversely, a positive PDO being negatively correlated with surface temperatures means a positive PDO would result in colder weather for those in the negative correlation areas/cool color shades. Since these warm waters in the Northeast Pacific appear to be proclaiming a positive PDO regime right now, we could be in for a warm West US and very cold South/East US this winter if this correlation comes to fruition.

This image above shows correlations between the current El Nino or La Nina state, and surface temperatures across December-January-February. If we take positive water temperatures to mean El Nino, areas in the North and West US would see warmer than normal temperatures this winter, as an El Nino is positively correlated with surface temperatures. Similarly, a La Nina would bring colder weather to those Northern states. However, with the expected El Nino, we can expect warmth in the North and cold in the South, as the South is in that negative correlation area again. 

These two correlation comparisons can be put together to mean that a positive PDO and El Nino this winter may result in some very cold weather in the South and East US, with warmer temperatures stretched out across the North and West US.

The PDO and El Nino are not the only two indices in the atmosphere, which may mean other factors may lessen this potential cold winter foreshadowing. We will get a better handle on this situation this summer and fall.