Lecture 10
el nino and la nina- why we care?
-because they affect world weather climates -they can cause droughts, wildfires, floods -impacts vary depending on where you are but either way, it can cause an enormous amount of damage
Artic Oscillation/AO/NAM
-seasonal pattern and seasonal pressure located in northern hemisphere -The Arctic Oscillation (AO) is a large scale mode of climate variability, also referred to as the Northern Hemisphere annular mode has to also do with winds in the atmodshere. -It's counter part is SAM
transition from el nino and la nina can occur and probability.
quickly but a couple of years ago, a strong el nino passed and emerged a weak la nina. (this doesnt always occur but just be aware that it can occur) right now we are in ending el nino phase. prediction is we will probably see midyear that it will be a neutral el nino. later on tho, it can be anything.
what does el nino need
sea surface temp anomalies and wind anomalies
SAM (Southern Annular Mode)
seasonal pattern and seasonal pressure located in southern hemisphere
Negative and Positive NAO (timescale and most profound variation during?)
the variation between negative and positive NAO is most profound during winter months. there is no time scale for these events. they can occur either monthly, weekly, days, or anytime they want to occur.
el nino occurence- why does it happen
the wind stress is turned off (no more pushing) then trade winds decrease and el nino happens.
a titled boundary between warm and cold air due to
upwelling pushing the cold water up. down welling caues the tilted line to go down.
Jet stream affects
weather -depending on what side of the jet stream you get, you can get both cold and warm weather within one day
sandy jet stream
westward jet stream pushed the storm westward which is an abnormal situation due to the meandering of the jet streams made sandy get caught in the jet steam. the storm ended up going to new jersey and there was a lake weather landfall there.
la nina
when trade winds become stronger, the air is pushed in the opposite direction which leads to a negative condition. this is the cold phase of enso. La Niña, which occurs when the trade winds blow unusually hard and the sea temperature become colder than normal. During La Niña or El Viejo year, winter temperatures are warmer than normal in the Southeast and cooler than normal in the Northwest.
what happens when trade winds weaken?
when trade winds weaken, they end up decreasing their ability to blow water and thus they are not able to push the water which causes the warm water to begin to travel back to where it came from, thus the boundary of the cold water and warm water then tends to be flatter. this then causes sea surface temps to be redistributed and convection occurs elsewhere with the warm water. Convection has a new circulation that reinforces this pattern.
el nino is most prevelant during
winter months. in US the northern states will have milder winters and south will have colder winters in the south. la nina causes warmer winters in the south and cooler in the north.
El Niño and seasonal forecast
A cold, snowy winter of Chicago 2010/2011 • Groundhog Day Blizzard •Possibly the result of "La Niña" -positive phase of NAO also contributed to this anomaly. it also makes things harder to affect what will occur in that region. NAO overwhlemed la nina and caused the region to be warmer.
NAO and AO positive and negative
AO and NAO work in the same direction. the positive states of the AO and NAO are characterized by the existence of a strong polar vortex that helps to constrain cold arctic air further north. The process that accomplishes this is the development of a strong subtropical high-pressure center. The result of the positive AO and NAO is that the warmer air from the southern latitudes is able to extend further north into the US and Europe. Negative AO and NAO: is characterized by weaker atmospheric pressure in both the subtropics and sub-arctic. this produces a weakened polar vortex, which allows breakouts of cold air masses, but there are periodic breakouts of cold that extend southward as the southern jet stream dips toward lower latitudes. this allows cold air to move south in US and Europe.
AMO
Atlantic Multidecadal Oscillation (AMO) is a climate cycle that affects the sea surface temperature (SST) of the North Atlantic Ocean
iod
indian ocean dipole- Sustained changes in the difference between sea surface temperatures of the tropical western and eastern Indian Ocean are known as the Indian Ocean Dipole and affect atmosphere
return period for el nino
is not consistent.
ENSO and Hurricane frequency
it affects sea surface temps- warm tropical air and ocean surface. when there is higher sea surface temps, we would expect higher hurricane formation, but at the same time, the vertical sheer is affected. it affects atmospheric circulation and el nino increases vertical shear so the effects are not immediately predictable because higher vertical shear is prohibitory to hurricane formation. in the pic: the more isnt that much it is just a modest change in an increase.
Positive NAO
jet streams blow more or less straight from west to east with little meandering. cold air masses locked to the north of the hemisphere (polar side). This jet stream does not wiggle much.
how to get anomilies
just subtract the normal condition from the one you have and if it is different, then it is an anomaly.
ENSO and jet stream
la nina is more variant while el nino is not. la nina causes the colder air to travel further to north.
Negative NAO
meandering of jet streams causes cold air mass to go to the south and warm tropical air to go to the north which causes the jet steam to have an exchange in heat. In North america, the Midwest place does experience cold unlike the rest of Europe.
Sandy occurred during
negative El nino phase. which means that the jet stream was highly meandering.
how does el nino happen
occurs from slight reduction in trade winds that, in turn, causes warm water in the eastern equatorial pacific ocean to flow eastward.this change further reduces the trade winds, causing more warm water to move eastward, until an El nino is established. (we still do not know how trade winds weaken).
PDO and AMO are
ocean variability exclusive
PDO
pacific decay oscillation variation of sea surface temperature
decaying oscillation and sea surface temp have a correlation effect on the
atmospheric changes
Weather Makers-Known Natural Oscillations in Climate Patterns
Must Know: ENSO, NAO, AO/NAM, PDO, IOD, AMO, SAM
how do thunderstorms and atmosphere interact?
atmosphere collects moisture and causes thunderstorms. the thunderstorms occur more under warmer sea temps because there is more moisture to collect.
trade winds
a wind blowing steadily toward the equator from the northeast in the northern hemisphere or the southeast in the southern hemisphere, especially at sea. Two belts of trade winds encircle the earth, blowing from the tropical high-pressure belts to the low-pressure zone at the equator. blow westward in low altitudes. it pushes the water with the wind. it creates a water movement and so mass is escaping and in order to be replenished, it needs upwelling. brings cold water up to the surface. the below water has nutrient rich water which attracts fish. upwelling causes the ocean temp to get cooler.
NAO
amplifies the atmospheric signals over the north Atlantic similar to AO but it is different because signal is localized in northern hemisphere.
NAO,SAM,AO
associated with atmospheric changes
ENSO and IOD are associated with
climate variabilities that involve coupling with ocean and atmospheric changes
we can use _______ technique to determine when the probability that these things occur.
correlation
El nino and precipitation
during winter months, el nino causes more precipitation than normal in the south and less than normal in the north. wetter and colder in the south and drier and warmer in the north.
ENSO
el nino southern oscillation -atmosphere and ocean changes
Climate change and natural hazards
• Extreme heat -Arctic sea ice is decreasing --loss of land masses --An ice-free ocean is conducive to coastal erosion because it is now exposed to the surface. --decrease in land mass also due to erosion. -Sea level rise --So far mostly due to expansion of sea water due to the absorption of heat and not the melting of the polar ice for now. some of the heat will be deposited into the ocean which will cause the water to expand and is consistent with the sea level rise. -if we melt what is already floating, sea ice does not change the water level, it remains the same. its the ground ice that melts that will be problematic
El Niño
•"The Christ Child" in Spanish •Originally used by fishermen along the coasts of Peru and Ecuador in the 1800s to describe the warm southward ocean current that appears around Christmas time and lasts a few months •Warm water resulted less fish -- vacation for fishermen -there is a time scale associated with el nino and la nina -the nutrient level in the water is lowered
El Niño (more info)
•But in some years the water is especially warm, and it can last 12-18 months •Today, the term El Niño is only used for the years with exceptionally warm Eastern Tropical Pacific temperatures that last for more than 6 months •During the last 50 years 12 El Niños have occurred, with the strongest on record being the 1982/83, 1997/98, 2015/16 events •Opposite phase (cold anomaly) is termed La Niña -every 3 to seven years we notice an El nino effect. -even tho el nino is localized to tropic regions, it as a ripple effect that allows it cause changes in other places.
North Atlantic Oscillation (NAO) (time it takes up, jet stream location, what it affects- dynamics and temps)
•Internal dynamics of the atmosphere •No particular time scale (days to weeks) certain low level sea surface temp is can be followed by high temp sea surface temps -jet stream condition over north Atlantic center
El Niño-Southern Oscillation (ENSO)
•Natural climate variability of the tropical Pacific •Involves both the ocean (El Niño) and the atmosphere (Southern Oscillation) and their mutual reinforcement •Change in sea surface temperature distribution and wind pattern- when we can first see the changes •3-7 year cycle (quasi-periodic) •Creates world-wide weather anomalies -atmosphere affects the ocean and the ocean affects the atmosphere.
