October is an important month for climatologists and meteorologists to analyze trends and variables to aid in providing an idea of what may come during the winter season. There are certain areas around the world we can look to, to provide a ‘snapshot’ of what the winter season may look like. One area of significance importance is the equatorial Pacific. The state of this region is a large piece of the winter outlook puzzle. There are three very important variables to analyze when looking at the equatorial Pacific: I) sea-surface temperature (SST) anomalies, II) the positioning of the anomalously warm or cool SSTs, and III) the depth of the anomalously warm or cool SSTs.
As of mid-October, the state of the equatorial Pacific continues to trend anomalously warm, which is signaling El Nino conditions developing (see Fig. 1). In fact, the trade winds (normally flow from east to west) in the central and eastern equatorial Pacific have started significantly weakening. This is only reinforcing the anomalously warm temperatures that have persisted for several weeks; another sign of a developing El Niño. This is supported and is expected to continue per the majority of the models (see Fig. 2). We are currently in a neutral condition (not El Niño or La Niña); however, due to this region continuing to warm since June, the chances of El Niño developing are increasing. The Climate Prediction Center has increased the odds of El Niño developing in the winter to 70-75%.
Fig. 1: Current SST anomalies (Tropical Tidbits)
Fig. 2: Models ENSO predictions (IRI and CPC)
So what is El Niño? El Niño simply put a climate pattern that occurs when SSTs (in the equatorial Pacific) are above-normal for a long period of time. This climate pattern is a cycle (ENSO) in which below-normal SSTs can occur, too. When SSTs are below normal for an extended period of time in this region, the development of La Niña can occur. This climate cycle play an important role in shaping weather patterns around the entire World. Of significance, are the impacts of this cycle on the winter in the U.S. El Niño is typically associated with an extended Pacific jet stream and amplified storm track for parts of the southern U.S. (see Fig. 3). This can cause an increase in cloud-cover and precipitation for the southern U.S. In return, due to the cloud-cover and precipitation, temperatures tend to be below-average in this region. With the hyperactive storm track across the south, the chances are increased that at some point a phase between the northern and southern jet stream will occur, leading to the possibility of southern winter storms. It can also lead to above-average temperatures in northern parts of the U.S. The impacts on temperatures and precipitation, however, are dependent upon the strength of the El Niño and the positioning of the warm SSTs.
Fig. 3: El Niño winter pattern in North America (CPC)
Based on the current trends and available data, it appears a weak (to possibly moderate) El Niño may develop by winter. It should be noted, the strength of El Niño does not necessarily reflect its impacts on global weather patterns that develop. The type of warming and cooling pattern in the tropical Pacific alters the normal winter pattern in the U.S. and has different implications on temperatures and precipitation. Historical weak El Niños have produced these temperature and precipitation anomalies in the lower-48 (see Fig. 4 and Fig. 5):
Fig. 4: December-February temperature anomalies
Fig. 5: December-February precipitation anomalies
While the aforementioned figures show below-average precipitation across parts of the southern U.S. during a weak El Niño, El Niño that tend to be on the strong side of weak (close to moderate) lead to wetter conditions in this region.
Firsthand Weather is in the process of developing its 2018-2019 Winter Outlook but I wanted to give some insight into one of the pieces of the puzzle we analyzing for the winter season. Firsthand Weather’s 2018-2019 Winter Outlook tentative release date is October 26th, 2018!!