Oscillations-Extended Forecasting Flashcards
Madden-Julian Oscillation (MJO)
An intraseasonal fluctuation or “wave” occurring in the global tropics.
-responsible for the majority of weathervariabilityin
theseregionsandresultsinvariationsinseveralimportant atmosphericandoceanicparameterswhichinclude:
-Lower and upper levelwindspeedanddirection(divergence)
-Cloudinessandrainfall
-Surfacepressure
-Seasurfacetemperature(SST)
TheMJOisanaturallyoccurringcomponentofourcoupledocean‐atmospheresystem.
ThetypicallengthoftheMJOcycleorwaveisapproximately
30‐60days
MJO Characteristics
- eastward propagation of regions of enhanced and suppressed tropical rainfall, primarily over the Indian and Pacific Oceans.
-distinctpatternsoflower‐ andupper‐leveldivergenceand
atmosphericcirculationanomaliesinthetropicsand
subtropics.
-Thesefeaturesextendaroundtheglobeandarenot
confinedtotheeasternhemisphere.Thus,theyprovide
importantinformationregardingtheregionsofascending
anddescendingmotionassociatedwithparticularphases
oftheoscillation.
WhenconvectionisactiveintheIndianOceanand
Indonesia: Upper-Levels
- Anomalous easterlies exit the area of enhanced convection in the upper-levels of the atmosphere associated with anticyclonic gyres straddling the equator in both hemispheres along and to the west of the convection.
- Conversely, cyclonic gyres straddle the equator to the east in areas of suppressed convection in both hemispheres.
WhenconvectionisactiveintheIndianOceanand
Indonesia: Lower-Levels
Anomalouseasterliesareevidenttotheeastofthearea
ofenhancedconvection,associatedwithanticyclonic
gyresstraddlingtheequatorinbothhemispheres.
-Anomalous westerlies are evident within and to the west of the area of enhanced convection, associated with cyclonic gyres straddling the equator in both hemispheres.
Walker Circulation
-Zonal east-west direct circulation in the tropical Pacific
-zonal circulation oriented parallel to latitude lines
-a direct circulation is composed of a circulation where
warm air is rising and cold air is sinking.
-Warm air rises over the west Pacific during a normal walker circulation regime
-cooler air sinks over the cooler water of the east Pacific
-under normal conditions, the west pacific experiences heavy rainfall, whereas the east pacific is dry
Walker Circulation-at the surface
The West and East Pacific are linked by westward directed trade winds.
Walker Circulation-upper troposphere
The West and East Pacific are link by eastward directed winds
When the Walker Circulation is strong:
- low pressure resides over the tropical West Pacific
- high pressure resides over the tropical East Pacific
- the trade winds are strong over the Pacific
- the West Pacific is wet and stormy
- the East Pacific is dry and tranquil
What is an El Nino?
A disruption of the ocean atmosphere system in the tropical Pacific having important consequences for weather around the globe.
- characterized by major warming of waters in the tropical central and eastern Pacific.
- occurs every 2-10 years and is characterized by a major weakening or total reversal of the walker circulation.
- drastic changes in weather patters are experienced all over the world.
What was El Nino first called?
“The Child” bc it occurs or was first noticed along the west coast of South America around Christmas time.
Southern Oscillation Index (SOI)
- SOI=(Tahiti Sea Level Pressure)-(Darwin Sea Level Pressure)
- El Ninos are characterized by a “negative” SOI
- When Tahiti SLP is significantly lower than Darwin, the SOI is negative and an El Nino may be occurring
Oceanic Nino Index (ONI)
- NOAA’s CPC recently changed from using the SOI to the ONI.
- based on SST departures from average in the Nino 3.4 region, and is now the principle measure of monitoring and assessing El Nino.
- based on a 3-month running mean of SST departures in the Nino 3.4 Region.
- ONI used to place current events into a historical perspective.
NOAA Operational Definitions for El Nino and La Nina
- El Nino characterized by a positive ONI greater than or equal to +0.5 C
- La Nina characterized by a negative ONI less than or equal to -0.5 C.
- by historical standards, to be classified as a full-fledged El Nino or La Nina episode, these thresholds must be exceeded for a period of at least 5 consecutive overlapping 3-month seasons.
Walker Circulation during El Nino: When the walker circulation is weak (or reversed)
- El Nino forms
- Great havoc occurs in the world’s weather patterns
- great hardship is experienced by many people and much wildlife across the world.
Walker Circulation during El Nino: When the walker circulation is weak (i.e. El Nino)
- low pressure moves to the tropical Central and East Pacific
- high pressure forms over the tropical West Pacific
- the trade winds weaken or reverse over the Pacific
- The West Pacific becomes dry (drought is common in rainforests)
- The Central and East Pacific become very stormy
El Nino Evolution: The Formative Stages (late summer and fall)
- Walker circulation weakens
- pressure falls in east pacific and rises in west pacific
- SOI index becomes negative
- ONI index becomes positive (>0.5 C)
- Trade winds fail (weaken)
- Equatorial Kelvin Wave moves from West to East Pacific
- Warm SST’s shift eastward as sea level mound moves eastward
- upwelling in east pacific weakens
- convection and precipitation shifts eastward
El Nino Evolution: The Mature Stage (winter and spring)
- Walker Circulation reverses
- low pressure resides in central and east Pacific
- high pressure resides in west pacific
- SOI index becomes strongly negative
- ONI index becomes strongly positive
- Trade winds calm or actually reverse
- Equatorial Kelvin wave moves from West to East Pacific
- warm SST’s over central and east Pacific
- sea level mound resides in East pacific (high sea level along s. american coast)
- upwelling of cold water in East pacific is absent
- Heavy thunderstorms cover central and east pacific
El Nino Evolution: Dissipating Stage (late spring and summer)
- Walker circulation begins to strengthen
- pressure falls in West Pacific and rises in East Pacific
- SOI index becomes positive
- ONI index becomes neutral or slightly negative (s slosh back to the west Pacific
- sea level mound disappears in east Pacific (sea level rises in west pacific)
- upwelling of cold water in east pacific resumes
- heavy thunderstorms move back to west pacific
- dry weather returns to east pacific
La Nina
A major cooling of the Tropical East Pacific Ocean
- generally (but not always) follows a strong El Nino event and is characterized by an overly strong Walker Circulation
- “strongly positive” SOI
- “strongly negative” ONI
North Atlantic Oscillation (NAO)
Often defined as the difference of sea-level pressure (or 500 mb heights) anomalies between 2 regions: one near the Azores and one near Iceland.
NAO index=SLP (Azores)-SLP (Iceland)
-one of the most dominant modes of global climate variability
Hans Egede Saabye
Missionary who first observed NAO while in Greenland during 1770-78
Positive NAO
associated with a net displacement of air from over the Arctic and Icelandic regions towards the subtropical belt near the Azores and the Iberian peninsula
- this strengthens the Icelandic low near the southeast coast of Greenland and also strengthens the subtropical high in the Atlantic, resulting in more intense westerlies over the North Atlantic ocean.
- stronger westerlies bring more warm moist air over the European continent and gives rise to milder maritime winters.
Positive NAO
-warmer temps also observed over the western Atlantic and eastern US
-the stronger Atlantic Subtropical High also strengthens the trade winds over the tropical atlantic.
->African dust is often blown across the tropical atlantic
toward the Caribbean and Florida.
-> increased upwelling of cold water off the NW coast of
Africa help to keep sfc temps cooler over the tropical
eastern atlantic. Dry weather is often enhanced here
as well.
Negative NAO
Associated with a net displacement of air from over the subtropical belt near the Azores and Iberian Peninsula northward to the Arctic and Icelandic Regions.
-this mass displacement:
-> weakens the Icelandic low and forces it to become
much more variable in location
-> weakens the subtropical high in the Atlantic, resulting
in less intense westerlies over the North Atlantic
ocean.
-blocking patterns often develop in the Atlantic jet stream pattern.
Negative NAO
- Arctic high pressure is better developed in the polar regions and often invades into northern and central Europe when significant blocking patterns develop over the Atlantic.
- colder temps observed over western Atlantic and eastern US
- a weaker subtropical high promotes weaker trade winds in the tropical Atlantic.
Causes of the NAO
The NAO may arise from several different influences:
-stochastic interaction between storms, climatological stationary eddies and the time-mean jet stream pattern.
-Long term stratospheric influences on the troposphere:
-> influence of volcanic ash tends to promote +PNA
phase
-> ozone depletion tends to promote +PNA phase
-> greenhouse gas accumulation may tend to promote
+PNA phase
-the intensity and distribution of tropical convection.
-long-term feedback from oceanic circulation patterns and sea ice changes.
Arctic Oscillation (AO)
The dominant pattern of non-seasonal sea-level pressure variations north of 20N, and it is characterized by SLP anomalies of one sign in the Arctic and anomalies of opposite sign centered in the mid-latitudes (between about 37-45N).
AO index=(SLP Mid-lat - SLP Arctic) Regions
Positive AO
Associated with higher pressure in the mid-latitudes and lower pressure near the North Pole.
- Often associated with:
1. fast, strong zonal flow of westerlies across the mid-latitudes
2. mild winter weather in the US and Europe
3. melting arctic sea ice
-Known as the Warm Phase of the AO
Negative AO
Associated with lower pressure in the mid-latitudes and higher pressure near the North Pole.
- Often associated with:
1. significant high-latitude blocking in the jet stream which leads to greater meridional flow and weaker zonal flow of westerlies in the mid-latitudes.
2. severe cold winter weather in the US and Europe
3. Expanding Arctic sea ice
-known as the Cold Phase of the AO
Pacific North American Pattern (PNA)
Associated with a Rossby wave pattern with centers of action over the North Pacific and over North America.
Positive PNA
Characterized by:
- low 500 mb height anomalies over the northern Pacific and eastern North America/western Atlantic
- high 500 mb height anomalies over western North America
Pacific Decadal Oscillation (PDO)
A long-lived El Nino-like pattern of Pacific climate variability
- defined by the North Pacific monthly sea surface temperature variability (poleward of 20N)
- PDO Index=SST(east pacific)-SST(central pacific)
Warm PDO Index
Contain cooler-than-normal waters in the north central Pacific Ocean and warmer-than-normal waters along the west coast of North America.
Cold PDO Index
Contain warmer-than-normal waters in the north central Pacific ocean and cooler-than-normal waters along the west coast of North America.
Two main characteristics distinguishing PDO from El Nino/Southern Oscillation (ENSO)
1) 20th century PDO “events” persisted for 20 to 30 years, while typical ENSO events persisted for 6 to 18 months.
2) the climatic fingerprints of the PDO are most visible in the North Pacific/North American sector, while secondary signatures exists in the tropics–the opposite is true for ENSO.
Effects of the PDO most drastic in….
the Pacific NW. In this region:
- a positive (or warm phase) PDO generally correlates with dry conditions and warmer temps.
- a negative (or cool phase) PDO generally correlates with wet conditions and cooler temperatures.
