What is an Extreme Event?

According to the National Oceanic and Atmospheric Administration (NOAA) an extreme event is most often classified as an event that is in the top or bottom 10% of cases for a specific location. Extreme events are classified through both their magnitude and frequency. The magnitude is related to anomalous strength of an event while frequency denotes an abnormal number of events within a given time frame. Climate does not cause extreme events, however, a change in climate can increase the magnitude or frequency of extreme events. For example, an increase in mean global temperature would increase positive extremes as the mean value is increasing over time.

Precipitation Extremes

Precipitation extremes vary from region to region, as the average rainfall is not the same across the globe. These events can be thought of as drought or flood type events, which can cause problems for the environment and society. Droughts can be dangerous, especially when they are prolonged and occur over regions that are already arid and water-stressed, such as the southwestern United States. Heavy precipitation can be dangerous as well as it can lead to flooding and major damage to agriculture or infrastructure over large areas. The study of both extremes will improve our understanding of their underlying causes and allow implementation of better adaptation measures against the most adverse effects from these phenomena.

Pressure Systems

The atmosphere plays a major role contributing to the weather we experience at the Earth’s surface. Barometric pressure is used to understand and portray winds and pressure systems. Wind travels from high to low pressure, which is the basic principle that creates the complex circulation patterns found across the globe. Air flowing out from the high-pressure system is being replaced by sinking air entering the region from aloft. This process leads to warmer, drier conditions on the ground. Convergence of air occurs in the low-pressure region, causing the air to rise. This rising air cools and starts to condense, leading to the creation of clouds along with cooler and wetter weather. When there are strong high-pressure or low-pressure systems in a region, extreme conditions can potentially occur. Read more>

Sea Surface Temperatures

The ocean is also important in determining where extreme weather conditions are most likely to occur. Energy is transferred from the ocean to the atmosphere, which influences air temperatures above the regions of anomalous sea surface temperature and leads to anomalous atmospheric circulation patterns that are often linked to climatic extremes. On timescales of years to multiple decades, climate over land is greatly affected by this process, which can promote extreme events. Read more>

El Niño Southern Oscillation

The El Niño or warm phase of the El Niño-Southern Oscillation phenomenon or ENSO is  associated with anomalously warm sea surface temperatures in the tropical Pacific Ocean, which occurs once every 2-7 years. This phenomenon is caused by emergence of anomalously warm surface water in the usually cold central-eastern tropical Pacific Ocean. The warm water causes the air above to warm, weakening the high-pressure system normally found within the region. This affects global circulation patterns, impacting both precipitation and temperature across the globe. El Niño directly contributes to extreme conditions, such as flooding along the coast of Peru, which occurs almost exclusively during such events. But El Niño can also affect the frequency or intensity of extreme events across the globe by changing the atmospheric circulation, thereby creating so-called ‘teleconnections’. La Niña is the reverse phase of this oscillation and features cooler sea surface temperatures in the eastern tropical Pacific. The cooler waters produce a stronger high-pressure system in the region, strengthening the westerly winds in the Pacific. This phase can also impact the climate over land and for the most part will produce a response that is opposite to the one created by El Niño. Read more>

Monsoons

Monsoons are defined as strong variations in the seasonal precipitation within a tropical region. There are dry and wet seasons aligned with the winter and summer season respectively, linked to the seasonal transition of the Intertropical Convergence Zone (ITCZ), a band of intense precipitation near the equator. Over many monsoon regions the wind also reverses seasonally although this is not universally true. During the wet summer monsoon season the wind blows from the ocean toward the land, transporting moisture onshore over the land. When the wind reverses, drier conditions prevail, characteristic of the dry winter monsoon. Monsoons are not what you would normally think of as an extreme event, as it is a climate state that occurs over many months. However, these conditions promote the possibility for more extreme events as the mean climate is already anomalous, so that there is a greater chance for extreme conditions such as drought or floods. Climate modes of variability like El Niño can also strengthen or weaken precipitation over monsoon regions, creating more extreme conditions under ideal circumstances. Read more>

Floods

Contrasting drought, prolonged high-precipitation events can be just as destructive as drought. These extreme events can cause destructive flooding, damaging private property, infrastructure and agriculture. Mostly extreme rainfall is associated with single weather events, however, on seasonal and annual time scales, frequent periods of increased precipitation can lead to the same result.

As part of the PIRE CREATE Project, scientists are documenting the sensitivity of the South American monsoon to external forcing; Creating a new reconstruction of the El Nino - Southern Oscillation and the Interdecadal Pacific Oscillation and identifying past extreme events and analyze their causes and societal response;

PIRE CREATE project in the news:

A new study says a megadrought worse than anything known from recorded history is very likely in progress in the western United States and northern Mexico, and warming climate is playing a key role. Read the article>