Climate Lecture 6- Climate Variability Flashcards
What is weather?
Day to day variation in atmospheric conditions
What is climate?
Long-term average weather of a location as described by variables such as temperature and precipitation
What does climate variability refer to?
Variations in the mean state and other statistics of the climate.
What might climate variability be due to?
Natural internal processes within the climate system or external variations in natural or even anthropogenic forcing.
How is climate variability often measured?
Comparing the observed conditions to the long-term mean conditions - ‘climate normal’
Name small scale phenomena (climate variations)
Wind gusts
Localised thunderstorms
Tornadoes
Name larger scale features (climate variations)
Fronts and storms
Hurricanes
Droughts
Floods
Name timescales climate fluctuations can occur on.
Seasonal
Annual
Decadal
Multi -century
Examples inclue an abnormally cold and snowy winter or a mild winter followed by a severe winter.
What are longer time-scale phenomena associated with?
Changes in the atmospheric/oceanic circulation that occur over a much larger area than where the climate variation is observed.
What are modes of climate variability?
Modes of climate variability are semi-regular fluctuations of global or regional scale climate variables (natural variability, known as modes or oscillations).
- Eg sea surface temperature, rainfall, surface pressure or wind speed
Such fluctuations arise from the interaction of the components of the climate system
(atmosphere, hydrosphere etc) on all time scales
Their phases and states can be monitored using measurements of critical climate
variables (e.g. SST, atmospheric pressure) which are indicative of changing ‘behaviour’.
There are many known climate oscillations or modes of climate variability which
extend over large geographical areas, with some having near global scale impacts
Name 8 modes of climate variability.
NAO- north Atlantic oscillation
ENSO- El nino-southern oscillation (Pacific west of SA)
PDO- pacific decadal oscillation (west coast US)
AMO- Atlantic meridional oscillation (ocean)
AO/NAM- Arctic oscillation/northern annular mode
IOD- Indian ocean dipole
MJO- madden julian oscillation (near Japan)
SAM- southern annular mode (near Antarctica)
What can modes of climate variability produce?
Recurring and persistent atmospheric and ocean circulation features that occur simultaneously over vast geographical regions (e.g. the Pacific Basin)
These may result in climate anomalies (abnormal climate patterns) in different
parts of the world.
Sometimes climate anomalies that occur in different parts of the world are related and can be opposite to each other (hot in one place, but cold in another, and vice versa)
What are teleconnections?
Associations of climate between places separated by large distances are called teleconnections
Teleconnections occur because the climate in two separate locations are related/linked via the atmospheric and ocean circulations
What does NAO refer to?
Refers to variation (seesaw) in pressure between Iceland and the Azores (barometric seesaw)
“Normally” over Iceland: area of low pressure called Icelandic Low
“Normally” over the Azores: Azores High
What does NAO encompass?
Occasionally Icelandic L and Azores H weaken and there is almost a reversal of the pressure patterns
This weakening of the H to L pressure gradient
brings about a reversal of the “normal” climate
patterns over W Europe
NAO has positive or negative phases – most
obvious in the winter
Associated with each phase is a distinct climate
pattern
The NAO accounts for about 1/3 of the Northern Hemisphere inter-annual surface temperature variance over the past 60 winters
What occurs in a NAO positive phase?
Storms move over NW Europe
North wet and south dry
What occurs in a NAO negative phase?
Storms move further south compared to positive NAO
North dry and south wet
Are climate anomalies associated with NAO?
Yes
E.g. During positive phase strong low pressure over greenland and strong subtropical high pressure.
This reverts to weak in negative phase.
What is NAO index?
Differences in pressure between Azores high and Icelandic Low
What is ENSO?
The El Niño/ Southern Oscillation (ENSO) cycle = a global-scale, naturally occurring phenomenon, linked with variations in SST and atmospheric pressure across the Pacific Basin
Important aspect of global inter-annual variability
El Niño (EN) is the ocean component (SST) and
Southern Oscillation (SO) is the atmospheric
component (atmospheric pressure)
The terms El Niño (Christ child/the boy) and La Niña (the girl) represent opposite extremes of the ENSO cycle
Discuss ENSO and periodicity.
El Nino and La Niña episodes typically last
approximately 9-12 months.
Often begin to form during June-August, reach peak strength during December-April, and then decay during May-July of the next year.
Some prolonged episodes have lasted 2 years and even as long as 3-4 years.
While their periodicity can be quite irregular, El Niño and La Niña occur every 3-5 years on average (neutral conditions otherwise)
Discuss equatorial pacific oceanic and atmospheric circulation.
Cool water in Lima, Peru (east) which flows west towards Darwin where it is warm.
Descending branch of walker circulation in Peru (cool, dry and high pressure)
Ascending branch in Darwin (warm, wet and low pressure)
What are features of high pressure systems?
Anticyclones
Wind - winds blow clockwise in high pressure and wind blows along the isobars; wind blows gently when isobars are widely spaced. Due to gentle winds these systems can remain in place for several days.
Dry - there are no fronts in an anticyclone so there is no rain.
Temperature - in summer skies are clear with little cloud and temperatures are high, while in winter, skies are also clear with little cloud, but temperatures are cold and there is a likelihood of frost and fog.
What are features of low pressure systems?
Low-pressure systems bring unstable air, clouds, and precipitation – ranging from light drizzle to heavy storms.
Lower pressure at centre than areas around it
