Winter 2018 2019 preliminary thoughts.

These are just my detailed preliminary thoughts on winter 2018-2019, everything is subject to change. But If I didn't think what I talk about was likely I wouldn't post it.

I talk a lot about the teleconnections and oscillations in the Atlantic and North Pacific, but seldom about the oscillations in the South Pacific. If we look to the South Pacific, oscillations and pressure patterns suggest if an El Nino forms this year, it will be most likely a Central Pacific (Modoki) event. I've been talking about the Modoki for quite some time. I've showed other data on my Facebook weather pages. But I want to show something new here.

Education on a new teleconnection/oscillation.

The South Pacific Oscillation (SPO) has a impact on if an El Niño will be East Based or Central Based.

Here are some images showing how the SPO works. Positive (negative) values of SLP anomalies denoted by red (blue) solid (dashed) contours. Warmer than average Sea Surface Temperatures (SSTs) are in yellows and reds while colder than average SSTs are in blues.

 

 

The two centers of the SPO represent different physical processes in the Southern Hemisphere climate system. For this post, we will concentrate on  the northern part of the SPO (Off the west coast of South America). This deals with changes in the strength of the South Pacific subtropical high, a semi-permanent area of high pressure in the South Pacific, similar to the Bermuda High in the Atlantic . This high pressure center provides much of western South America with a mild and relatively dry climate, much like the climate of coastal California, which is also controlled by a subtropical high.

The negative departures from average (dashed blue contours in the image above) indicate a weaker-than-normal subtropical high, meaning a weakening of the southeasterly trade winds. This leads to  subsurface cooler water coming up to the surface (upwelling) in the tropical Pacific, and warmer-than-average SSTs appear (shaded contours in the image above). The SPO is well correlated with tropical Pacific SST anomalies, which is the first clue that this pattern is important for ENSO development. The SPO is most active during the winter months, which for the Southern Hemisphere is June – August (JJA)

Now let’s consider the evolution and development of our coming El Niño event. The slightly warmer than average SSTs anomalies are already present in the central tropical Pacific. For development of our possible El Nino to continue, we have to warm the tropical Pacific even more. This year we've had strong easterly trade winds across the Atlantic into the Pacific. This has caused the warmer water to pile up in the western Pacific. For an El Nino, the warm  SSTs in the western Pacific have to build eastward and amplify.   This eastward propagation (movement) of anomalies is driven by westerly winds and the formation of Kelvin waves in the ocean in the eastern tropical Pacific, thus allowing waters to get warmer there. We have warmer than average water just under the surface in the tropical Pacific. As the easterly trades relax, those westerly winds will help upwell these warmer waters to the surface.

Pacific Oceanic Kelvin Waves work like this,  warm water from the western Pacific moves east and over the top of the cooler water in the central and eastern Pacific.  The warmer SST's will continue to surge eastward, while the colder SST under the surface will surge westward. The Kelvin Wave will slosh back and forth.   A Kelvin Wave  isn't a surface wave, instead it is a deepening of the mixing layer, the boundary between the warm and cold water.  

 

Since the SPO modulates the strength of the South Pacific trade winds in the eastern tropical Pacific, and it is most active during JJA, the phase and magnitude of the SPO can either help or hurt those Kelvin waves and the winds during the critical growth phase for ENSO. If the SPO is in the positive phase ,  a weaker South Pacific subtropical high is in place, then the southeasterly trade winds weaken, which reduces the cold-water upwelling in the eastern tropical Pacific and allows for easier eastward propagation of the warm waters from the central tropical Pacific to the eastern tropical Pacific. However, if the SPO is in the negative phase (i.e., a stronger South Pacific subtropical high), then the southeasterly trade winds intensify, and the cold-water upwelling in the eastern tropical Pacific also increases. These two factors create an environment hostile for eastward expansion of the warm waters. Thus, the warm SST anomalies tend to remain in the central tropical Pacific.

 

A  strongly positive SPO during JJA means that the event will likely be a strong/ East Pacific El Nino.

 

A near-neutral or negative SPO during JJA means that the event will likely be a weak/CP El Nino.

 

The SPO has been  slightly negative. This goes along with my idea of an El Nino Modoki setting up for Fall into Winter 2018-2019. IMO, we should see a lot more in the way of warming in the Nino 3.4 region (Central Pacific) over the next 30-40 days.

 

The factors that look to shape this coming winter: We have very cold SSTs around Greenland, which has contributed to above average snowfall for this time of year over Greenland. The ENSO in the Pacific is in a neutral mode. We look to have a predominate positive PNA. We also have a negative (cold) AMO, this will most likely carry on through winter 2018-2019.

So I tried to find sensible analog years. The years I've been playing with are: 1957-1958, 1977-1978 1994-1995, 2002-2003, 2007-2008, 2009-2010, 2014-2015. Many of these years saw a lot of Greenland Blocking; but, high latitude blocking over Greenland is going to be a big question mark. Those cold SST in the northern Atlantic could make high latitude blocking more difficult to form than normal. But there are other factors that improve the likelihood of high latitude blocking; I will touch on some of those in my next update.

Based on what I see right now:

As we move into Fall and Winter we should see ridging in the Gulf of Alaska and over the West Coast and Western Canada. We should see a trough undercutting to the east over Eastern Canada, Great Lakes, into the East Coast.

Based on the current trend and available data, It appears that this December through February  will be overall cold in the Midwest, Great Lakes and into the Northeast.  Those in western New York State, Pennsylvania, the Middle- Atlantic, could see well below average temperatures. The rest of the Northeast would still see below average temperatures. 

 

As for precipitation  eastern (especially Southeast) Pennsylvania, Middle-Atlantic, and Southern New England would see overall precipitation well above average. The rest of Pennsylvania, much of New York State and Central into Southern New England would see above average precipitation.  Northern New York State , Northern Vermont, Northern New Hampshire, and Southern Maine would see average precipitation, while Northern Maine would see below average precipitation.

These precipitation and temperature maps don't show a plus or minus numbers, only above or below chances. It is simply too early to show more detail.  

Remember, This isn't really a forecast, just my first thoughts on what looks to be shaping up. This could and very well might change at least somewhat as we get closer. I will post updates to this as we get into Fall. But this should give you an idea of where things are going.

Part 2 of this preliminary outlook.

Part 3 of this preliminary outlook.