July and August are important months for climatologists and meteorologists to begin analyzing certain trends and variables to start the preliminary process of obtaining an idea of what may come during the winter season. There are certain areas around the World we can look to, to aid in providing a ‘snapshot’ of what the winter season may look like for the U.S. One area that is important to look at is the equatorial Pacific. The state of this region can play one of the puzzle-pieces (an important puzzle-piece) into how the winter will shape up. 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-July, the state of the equatorial Pacific is trending anomalously warm which is signaling El Nino conditions developing (see Fig. 1). 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 March, the chances of El Niño developing are increasing. The Climate Prediction Center has increased the odds of El Niño developing in the winter to 65%.
Fig. 1: Current SST anomalies (Tropical Tidbits)
Fig. 2: Models ENSO predictions (IRI and CPC)
So what is El Niño? El Niño is simply 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, in which 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. During an El Niño, the storm-track tends to shift significantly southward (see Fig. 3). This causes 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. 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 moderate Modoki El Niño may develop by late fall/winter. I am sure you’re wondering what differences, if any, are there between Modoki El Niño and El Niño. And yes, there are differences. An Modoki El Niño is slightly different than the conventional El Niño (see Figures: 4 and 5). Modoki El Niño features stronger warming, and at a great depth, of the central equatorial Pacific and cooling in the eastern and western tropical Pacific. This is what is occurring based on the latest SST anomalies (see Fig. 1). Also, notice the cooler temperatures along the West Coast of South America. This is a signature, when paired with the central warming, of Modoki El Niño.
Fig. 4: El Niño SST anomalies (JAMSTEC)
Fig. 5: Modoki El Niño SST anomalies (JAMSTEC)
This type of warming and cooling pattern in the tropical Pacific alters the normal winter pattern in the U.S. and has different implications than the typical El Niño on temperatures and precipitation (see Figures: 6, 7, 8 and 9). Instead of the Southwest seeing an increase in rainfall, as expected with El Niño, Modoki El Niños can cause an increase in temperatures and lack of precipitation in this region. An increase in storminess and cool temperatures can occur for South-central and Southeastern parts of the U.S. during a Modoki El Niño.
Fig. 6: El Niño temperatures (JAMSTEC)
Fig. 7: Modoki El Niño temperatures (JAMSTEC)
Fig. 8: El Niño precipitation (JAMSTEC)
Fig. 9: Modoki El Niño precipitation (JAMSTEC)
Updates will be needed as we head into August and September, but I wanted to give some insight into what I am currently analyzing for the winter season in the U.S. Please note, this is only one puzzle-piece in a medium to long range forecast. Other teleconnections can influence fall and winter patterns that have big implications on temperatures and precipitation.