Teleconnections What the Heck Are They Part 3

 I have a confession. I had started this series a long time ago, but never finished it. A recent question on my Facebook Weather Page, led me to find I needed to finish part 3. So here it is.

This isn't designed to give you a complete understanding. It is only intended to give you a quick understanding of the processes involved; so you can have a better understanding of teleconnections, when you hear or see them used by myself and other weather outlets.

This is written  as simple and clear as I can make it. As always, I'm willing to answer questions to try and clear up any confusion. Here are links to the other parts.   

Part 1

Part 2 

El Nino Southern Oscillation (ENSO):

 

The ENSO deals with the relative sea surface temperatures (SST) in the equatorial Pacific Ocean. There are three phases… EL Nino, Neutral, and La Nina.

El Nino (La Nina) is a phenomenon in the equatorial Pacific Ocean characterized by a five consecutive 3-month running mean of sea surface temperature (SST) anomalies in the Nino 3.4 region.

An El Nino is declared when the SST is equal to or above +0.5°C

A Neutral ENSO is 0.5°C above down to -0.5°C below the center line.

An La Nina is declared when the SST is equal to or below -0.5°C.

 SST are monitored in four regions along the equator:

Nino 1 (80°-90°W and 5°-10°S)

Nino 2 (80°-90°W and 0°-5°S)

Nino 3 (90°-150°W and 5°N-5°S)

Nino 4 (150°-160°E and 5°N-5°S)

These regions were created in the early 1980s. Since then, continued research has led to modifications of these original regions. The original Nino 1 and Nino 2 are now combined and is called Nino 1+2. A new region, called Nino 3.4 (120°-150°W and 5°N-5°S) is now used as it correlates better with the Southern Oscillation Index and is the preferred region to monitor sea surface temperature.

 The changes in sea surface temperatures during El Niño and La Niña are caused and helped along by changes in the trade winds, which normally blow from east to west across the tropical Pacific Ocean.

El Nino and La Nina episodes typically last 9-12 months. They both tend to develop during the spring (March-June), reach peak intensity during the late autumn or winter (November-February), and then weaken during the spring or early summer (March-June).

Both El Nino and La Nana can last more than a year, but it is rare for El Nino events to last longer than a year or so, while it is common for La Niña to last for two years or more.

El Nino:

During an El Nino event, warmer than normal SSTs occur across the central and eastern equatorial Pacific. During El Nino the trade winds weaken or may even reverse. As a result, convection (thunderstorms) shifts east. This causes weaker upwelling, which keeps the warmer water on the surface.

When temperatures are warmer-than-normal it is called an El Nino, which is normally wetter and cooler across the Northeast. Outbreaks of very cold Arctic air are much less common during an El Nino winter.

During El Nino, the jet stream along with vertical windshear and other factors become more hostile during the Atlantic hurricane season which runs from June to the end of November. This helps lessen the number and strength of Tropical Cyclones.

 The neutral phase:

In the neutral state (neither El Nino nor La Nina) tropical Pacific SSTs are generally close to average.  The trade winds blow east to west across the surface of the tropical Pacific Ocean, bringing warm moist air and warmer surface waters towards the western Pacific and keeping the central Pacific Ocean relatively cool. The thermocline is deeper in the west than the east.

During the neutral phase, the jet still can bring colder air into the Northeast. However, storms don't get as much help from the southern subtropical jet. One thing I've noticed is there is a lot of Greenland blocking during an ENSO neutral winter. Moisture out of the Gulf tends to move to the SE coast where it can move up the coast.  In a neutral ENSO the southern jet stream plays a huge rule in ice/snow events in the Northeast, timing of cold air outbreaks and storms is critical.

 La Nina:

During an La Nina event, warmer than normal SSTs occur across the central and eastern equatorial Pacific.  This leads to stronger trade winds that shift the convection farther west.  The conditions allow for stronger upwelling, which brings colder deep ocean water to the surface.

When in a La Nina, typically we see a warmer and drier winter in the Northeast. During La Nina the polar jet stays up around Alaska and then drops down into the Northern part of the US. During La Nina Arctic and Siberian air will at times intrude into the Northeast. 

During hurricane season (June to November), upper-level winds are much lighter, and therefore more favorable for tropical cyclone development in the Caribbean and Atlantic.

These ENSO phases peak during the winter and early spring, but weaken as summer approaches. So even if an El Nino developed in January or February it wouldn't make a difference for snow in the Northeast.

 These ENSO phases peak during the winter and early spring, but weaken as summer approaches. So even if an El Nino developed in January or February it wouldn't make a difference for snow in the Northeast.

The Southern Oscillation Index (SOI):

The SOI is a measure of the atmospheric pressure difference between sea level pressures at Darwin, Australia, and Tahiti. Large negative values of the SOI are associated with El Nino and large positive values associated with La Nina. The SOI index, being a simple measure of pressure differences, is highly variable, much more so than other indices such as ocean temperatures. So, the SOI will at times be positive during El Nino events and negative during La Nina events. As such, a multi-week running average of the SOI can provide a better indicator of overall ENSO conditions.

 

The Eastern Pacific Oscillation (EPO):

The EPO is a dipole pattern similar to the NAO in the Atlantic, but located in the eastern Pacific. There is a tendency for heights/pressures/temperatures to be higher to the north and lower to the south in the negative phase and lower to the north and higher to the south in the positive phase. The negative phase corresponds to widespread cooling over central and eastern North America and the positive phase to warming. During the positive phase we typically see negative geopotential height anomalies, and a negative phase in which the opposite is true.

Geopotential height approximates the actual height of a pressure surface above mean sea-level. Therefore, a geopotential height observation represents the height of the pressure surface on which the observation was taken. Since cold air is more dense than warm air, it causes pressure surfaces to be lower in colder air masses, while less dense, warmer air allows the pressure surfaces to be higher. Thus, heights are lower in cold air masses, and higher in warm air masses.

A negative EPO regime corresponds to ridging over the northeastern Pacific, and a positive EPO regime corresponds with a trough in the same location. The negative phase corresponds to widespread cooling over central and eastern North America and the positive phase to warming. Most major cold waves in winter are associated with a negative EPO.

 

Negative EPO:

The EPO pattern opens the door to very cold Arctic and Siberian air into the US. Here's an image that depicts the negative phase of the EPO.

Positive EPO:

The weather pattern in the positive phase is just the opposite of the negative, and so you have low pressure over high pressure which gives the central and eastern US a different outcome.

  

The Atlantic Multidecadal Oscillation (AMO):

The AMO is a series of quasi-periodic (not completely regular) variations, The AMO signal is based on spatial patterns in SST variability after removing the effects of anthropogenic forcing on temperature, revealing natural long-term patterns in SST. The AMO is characterized by warm and cool phases with periods of approximately 20-40 years. The AMO index is related to air temperatures and rainfall over North America and Europe and is associated with changes in the frequency of droughts in North America and the frequency of severe hurricane events. A positive phase of the AMO is associated with above average water temperatures across the northern half of the Atlantic basin and increased tropical activity, while a negative phase of the AMO is associated with cooler water temperatures and decreased tropical activity.

 

In the positive (warm) phase of the AMO, sea surface temperatures are typically above normal in the Atlantic Main Development Region (MDR), the eastern subtropical Atlantic, and the far North Atlantic. Sea surface temperatures are typically below normal or near normal in the western subtropical Atlantic near the United States East Coast.

  

 In the negative (cold) phase of the AMO, sea surface temperatures are typically below normal in the Atlantic MDR, the eastern subtropical Atlantic, and the far North Atlantic. They are typically near or above normal in the western subtropical Atlantic.

  

  

Pacific Decadal Oscillation Index (PDO):

The PDO is often described as a long-lived El Nino-like pattern of Pacific climate variability. As seen with the better-known ENSO, extremes in the PDO pattern are marked by widespread variations in the Pacific Basin and the North American climate.

 

 

The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of 20° N. During the positive phase, the west Pacific becomes cool and part of the eastern ocean warms. During the negative phase, the opposite pattern occurs. It shifts phases on at least inter-decadal time scale, usually about 20 to 30 years.

When SST's are anomalously cool in the interior North Pacific and warm along the Pacific Coast, and when sea level pressures are below average over the North Pacific, the PDO has a positive value. When the climate anomaly patterns are reversed, with warm SST anomalies in the interior and cool SST anomalies along the North American coast, or above average sea level pressures over the North Pacific, the PDO has a negative value.

The impacts of the PDO are similar to those associated with ENSO events. When in the positive phase during the winter, the Aleutian low is deepened and shifted southward, warm/humid air is advected along the North American west coast and temperatures are higher than usual from the Pacific Northwest to Alaska but below normal in Mexico and the Southeastern United States. Winter precipitations are higher than usual in the Alaska Coast Range, Mexico and the Southwestern United States but reduced over Canada, Eastern Siberia and Australia. McCabe showed that the PDO along with the AMO strongly influence multi-decadal droughts pattern in the United States, drought frequency is enhanced over much of the Northern United States during the positive PDO phase and over the Southwest United States during the negative PDO phase in both cases if the PDO is associated with a positive AMO.

 

 

The Quasi-Biennial Oscillation (QBO):

The QBO is a measure of the oscillation in wind direction about 15 miles up (generally where atmospheric pressure is between 10 and 100 mb) around the equator. These winds reverse direction (oscillate) roughly every other year (quasi-biennially) between easterly (negative-phase QBO) and westerly (positive-phase QBO).

Effects of the QBO include mixing of stratospheric ozone by the secondary circulation caused by the QBO, modification of monsoon precipitation, and an influence on stratospheric circulation in northern hemisphere winter (mediated partly by a change in the frequency of sudden stratospheric warmings). Eastward phases of the QBO often coincide with more sudden stratospheric warmings, a weaker Atlantic jet stream and cold winters in Northern Europe and eastern USA whereas westward phases of the QBO often coincide with mild winters in eastern USA and a strong Atlantic jet stream with mild, wet stormy winters in northern Europe. In addition, the QBO has been shown to affect hurricane frequency during hurricane seasons in the Atlantic and research has also been conducted investigating a possible relationship between ENSO and the QBO.

The Western Pacific Oscillation (WPO):

During winter and spring, the pattern consists of a north-south dipole of anomalies, with one center located over the Kamchatka Peninsula and another broad center of opposite sign covering portions of southeastern Asia and the western subtropical North Pacific. Like the EPO, the negative phase favors colder weather across the eastern portions of North America. However, on average, the westward extent of cold is greater during a negative WPO compared to a negative EPO. At times, the WPO and EPO will be in phase, both being negative or positive at the same time.

Well I hope that helps in your understanding of the teleconnection patterns that effect our weather patterns. But remember all these teleconnections work to together, so little changes in one teleconnection can allow other teleconnections to have greater influence on the weather pattern.