El Niño Southern Oscillation (ENSO)

• Teleconnections are significant relationships or links between weather phenomena at widely separated locations on earth.
• These relationships typically entail climate patterns that span thousands of miles.

El Niño Southern Oscillation (ENSO)

• ENSO refers to the variation of sea surface temperatures, wind patterns, and surface air pressures over the equatorial Pacific Ocean.
• The cycle has an average period of three to five years, but can range anywhere between two and seven years.
• The Southern Oscillation describes the shift between below-average and above-average air pressures in the western Pacific Ocean around Indonesia and the eastern tropical Pacific Ocean.

Phases of ENSO

• There are three phases of ENSO.

El Niño

• This phase refers to a warming of the ocean surface in the central and eastern tropical Pacific Ocean.
• Rainfall tends to be reduced over Indonesia and increased over the tropical Pacific Ocean.
• Low-level surface winds, which normally blow from east to west along the equator ("easterly winds"), weaken or, in some cases, start blowing the other direction (from west to east or "westerly winds").

La Niña

• This phase refers to a cooling of the ocean surface in the central and eastern tropical Pacific Ocean.
• Rainfall tends to increase over Indonesia, with associated rainfall decreases over the central tropical Pacific Ocean
• The normal easterly winds along the equator become even stronger.

Neutral

• Neither El Niño or La Niña is occuring
• Tropical Pacific SSTs are generally close to average.
• There are some instances when the ocean can look like it is in an El Niño or La Niña state, but the atmosphere is not playing along (or vice versa).

El Niño Effects

• The warming effects from El Niño begin in December of the first year of the cycle and peak during the late fall of the following year.
• During a strong El Niño cycle, the weather effects are felt globally, causing heavy rainfall and mudslides in regions like Peru and Southern California, and droughts in Indonesia, Africa, and Australia.
• Waters cool slowly but naturally in the year following the peak of the natural phenomena.
• Temperatures in the center and tropical regions of the Pacific Ocean extending from the subtropical latitudes of North America through the western coast of South America rise.
• Atmospheric circulation is affected , causing the jet stream to strengthen over the Pacific Ocean, which in turn impacts storm paths, and can even cause extratropical storms in the Northern Hemisphere
• Mid-latitude low-pressure systems increase in strength in the Northern Hemisphere.
• Storms also increase in frequency due to the low-pressure systems around the Gulf of Mexico, causing wetter conditions in the United States.

Indicators of ENSO

• The Southern Oscillation Index (SOI) measures the intensity or strength of the Walker Circulation and is one of the key atmospheric indices for gauging the strength of El Niño and La Niña events and their potential impacts on the Australian region.

SOI Index

• The SOI measures the difference in surface air pressure between Tahiti and Darwin.
• Averages are best represented by monthly (or longer) daily or weekly averages, as SOI values can fluctuate markedly due to short-lived weather patterns.
• Positive SOI values above +8 indicate a La Niña event, while sustained negative values below -8 indicate an El Niño.
• The negative phase of the SOI represents below-normal air pressure at Tahiti and above-normal air pressure at Darwin.
• Prolonged periods of negative SOI values coincide with abnormally warm ocean waters across the eastern tropical Pacific typical of El Niño episodes.

Oceanic Niño Index (ONI)

• The ONI is a measure of sea surface temperature anomalies in degrees Celsius in four different regions along the Pacific Ocean.
• The four regions are known as Niño 1, Niño 2, Niño 3, and Niño 4.
• The ONI values are normally taken at the El Niño 3.4, which is between region 3 and 4.
• ONI values need to exceed +0.5 or -0.5 for a full effect of the Southern Oscillation.
• When the ONI value exceeds +0.5, an El Niño is occurring, and when the ONI values exceed less than -0.5, a La Niña episode ocurrs
• Warm and cold phases are defined as a minimum of five consecutive 3-month running averages of SST anomalies in the Niño 3.4 region surpassing a threshold of +/- 0.5°C.
• Negative values of SOI correspond to positive values of ONI.
• Higher pressure in the eastern region of the Pacific Ocean correlates to above-average sea surface temperatures shown through positive ONI values.

Pacific Decadal Oscillation (PDO)

• The Pacific Decadal Oscillation is described as long-term fluctuations of the ENSO variability.
• Characterized by fluctuations in sea temperatures in the northeastern and tropical Pacific Ocean regions resulting in a warm and cool phase like ENSO.
• ENSO can last for 6 to 18 months, the PDO can last anywhere from 20 to 30 years.
• When the PDO is in warm phase, the effects of El Niño to weather and climate patterns are strong and frequent.
• When the PDO is in cold phase, effects of El Niño are weakened.
• The strength of ENSO is dependent on PDO state.
• When PDO and ENSO are in phase, their combined effect causes stronger climate effects and opposite if they are unaligned.
• Phases of PDO are measured through PDO index.
• A positive index value corresponds to a warm phase and a negative index value corresponds to a cool phase.

Arctic Oscillation (AO)

• The AO refers to natural fluctuations in mean sea level pressure north of 20°N.
• Measures differences in sea level pressures between the Arctic region and the Northern Pacific and Atlantic regions
• Has two phases: a positive phase associated with warmer temperatures, and a negative phase associated with colder temperatures.
• The AO can change phases anywhere from days to months.
• A better indicator of short-term weather patterns rather than long-term climate change.
• The AO index measures the extent of the arctic oscillation.
• Established by D. Thompson and M. Wallace and comprised monthly mean sea level pressure in the Northern Hemisphere for each year
• Positive AO Index = positive phase and Negative AO Index = negative phase.
• During the positive phase of AO, the sea level pressures in the northern latitudes of the Arctic are lower than in the lower latitudes, causing stronger westerly winds.
• During the positive phase, cold air masses are restricted to the Arctic region.
• During a negative phase, sea level pressure differences in the Northern Hemisphere are smaller
• During negative phase, polar vortex can extend to the south

North Atlantic Oscillation (NAO)

• Describes variations in air pressure between regions in the Northern Atlantic that have implications on climate around the Northern Hemisphere.
• The NAO causes shifts in weather patterns and conditions between the Greenland region and Northwestern Europe.
• There is lower air pressure in the northern latitudes of the Atlantic Ocean near Iceland and Greenland with air flowing counterclockwise.
• There is a zone of higher air pressure in the central latitudes of the Atlantic Ocean near the Azores with air flowing clockwise.
• Creates a north-south pressure gradient and causes westerly flow of wind through the Northern Atlantic region

NAO Index

• Measures the exact intensity of the oscillation each month of the year.
• When the NAO index is below 0, the oscillation is experiencing a negative phase, and when the NAO index is above 0, it is in a positive phase.
• During a positive phase, both the sub polar low and the subtropical high are stronger than normal
• During a positive phase regions near Iceland experience lower than average air pressure
• During a positive phase, regions near the Azores experience higher than average air pressure.
• During a negative phase the exact opposite occurs.positive

Atlantic Multidecadal Oscillation

• The AMO refers to the variation in sea surface temperatures in the Atlantic Ocean that has implications on climate around the world.
• Describes the change in the circulation of water and heat in the Atlantic Ocean.

AMO phases

• Warm and cool
• Each phase could last for about 20 to 40 years.
• During a warm phase, moist air is displaced northward over the tropical Atlantic Ocean and the westerlies strengthen, causing a reduction in rainfall over the United States and the northeastern regions of Brazil while creating an increase in precipitation over the Sahel region in Africa and northwestern Europe.
• During a cold phase, the opposite occurs, and the United States and northeastern regions of Brazil experience increased amounts of precipitation and the Sahel region experiences lower amounts of rainfall and hurricane formations.

Pacific-North American Pattern (PNA)

• The Pacific-North American teleconnection pattern, influences regional weather by affecting the strength and location of the East Asian jet stream, and the weather in North America.
• Is a pattern of air pressure anomalies at four locations over the Pacific Ocean and North America correlate with regional temperature and precipitation anomalies across North America.

PNA Positive Phase

• Pattern of higher-than-average pressure in the vicinity of Hawai'i and over the mountainous region of western North America, and lower-than-average pressure south of Alaska and over the southeastern United States.
• Enhances the strength of the mid-latitude jet stream, above-average temperatures over western Canada and western US and below-average temperatures over the southern parts.
• During winter phase means below-average precipitation in Pacific Northwest and eastern US.

PNA Negative Phase

• Associated with a weaker jet stream across the central Pacific Ocean, high-pressure "blocking" of atmospheric flow in the high latitudes of the North Pacific, and a split-flow of the jet stream over the central North Pacific
• Temperature and precipitation departures from normal during negative PNA phases are generally opposite those of the positive phase.
• The positive phase of the PNA pattern tends to be associated with El Niño conditions, and the negative phase tends to be associated with La Niña conditions.
• During the positive phase, temperatures across the U.S. tend to be sharply split between a warm West and a cool East.
• During the negative phase, the West is often cooler than average while the East is warmer than average.