CHAPTER 4 EFFECTS ON THE CLIMATE SYSTEM
Ocean Circulation
 Surface Currents
o Driven by overlying winds such as the great global subtropical anticyclones that
exist in ocean basins centered near the 30° parallel
 Deep Ocean Thermohaline Circulations
o Driven by thermohaline currents that flow in response to temperature and/or
salinity
o Deep water currents are usually found layered with the Antarctic Bottom Water
(AABW) at the bottom, the Antarctic Deep Water (ADW) above the AABW, and
the North Atlantic Deep Water (NADW) above the ADW in the Atlantic
o Deep water currents in the Pacific are also usually found layered with the
Common Water (CW) current and the Pacific Subarctic Water (PSW) at the
bottom and the Antarctic Intermediate Water (AAIW) and the North Pacific
Intermediate Water (NPIW) above them
El Niño/Southern Oscillation Events
 Walker Circulation – refers to the connection between atmospheric pressure centers in the
equatorial Pacific that are associated with the Southern Oscillation, the sea surface
temperatures, and the tropical trade winds that blow from east to west near the surface in the
tropics
 Historical Observations of ENSO
o Indirect data sources such as tree rings, flood frequency, sediment cores, and
coral reef growth suggest that anomalous weather events associated with ENSO
events have been occurring for thousands of years
 El Niño Characteristics
o Coincides with a reversal of the normal Walker Circulation, occurs once every
three to seven years, and usually lasts ten to fourteen months
 La Niña Characteristics
o Trade wind flow along the equatorial Pacific Ocean becomes even stronger than
normal, which increases warm water accumulation in the western equatorial
Pacific and cold water upwelling in the east
 Global Effects
o Higher than normal air pressure and subsiding airflow over tropical eastern South
America, causing many rainforests in the area to experience a drought
o The relatively-high-pressure regime that normally prevails over eastern equatorial
Africa becomes an area of lower atmospheric pressure and the region becomes
anomalously wet
o Other effects elsewhere
 Effects in the United States
o A trough in the northern branch of the polar jet streams can cause colder air to
infiltrate the eastern U.S. all the way to the Gulf coast
o In the northeastern United States, El Niño causes slightly higher temperatures and
slightly drier conditions caused by a net reduction of snowfall
o Other effects elsewhere
 Relationship to Global Warming?
o It is widely assumed that global warming is caused by human emission of
pollutants known as greenhouse gases and that global warming causes more
extreme ENSO events, but increased frequency of ENSO events could be causing
increased global temperatures
Volcanic Activity and Climate
 General Effects
o Volcanic activity contributes to both short and long term climate fluctuations and is
encouraged in certain locations as Earth’s crustal plates move slowly
 Aerosol Indices
o The Dust Veil Index (DVI) and the Volcanic Explosivity Index (VEI) were created to
estimate the amount of aerosols (especially those caused by volcanoes) in the
atmosphere
 Major Volcanic Eruptions
o Mount Pinatubo’s eruption in the Philippines on June 25, 1991, led to surface
global cooling
o In 1815, Tambora’s eruption in Sumbawa, Indonesia, caused the “year without a
summer”
o Krakatau’s eruption in Indonesia in 1883 and is believed to have killed up to
40,000 people
o Laki Fissure erupted in 1783 and was responsible for devastating most of the
human and animal population of Iceland
o The eruption of Mount Toba in Sumatra approximately 71,000 years ago is
believed to be the most catastrophic of all volcanic eruptions, and is believed to
have accelerated the onset of the last glacial advance
Deforestation and Desertification
 Humans play a role in climatic variation through fossil fuel combustion, deforestation, slash
and burn agriculture, and desertification
Cryospheric Changes
 Ice on the Earth’s Surface
o The cryosphere exists in the form of continental ice sheets, semi-permanent alpine
glaciers, seasonal sea ice, and/or seasonal snowpack
 Feedbacks in the Cryosphere
o Feedback systems, whereby changes can either be magnified (positive feedback
system) or dampened (negative feedback system), are important in climatology,
and particularly in the cryosphere
o Addition of snow and ice increases albedo, causing a reduction in temperature,
more extensive snow and ice, and a positive feedback system
o Likewise, melting of snow and ice decreases albedo, raising the temperature,
melting more snow and ice, and a positive feedback system
o Some researchers now estimate that the Greenland ice sheet, the largest in the
northern hemisphere, could lose as much as one-half of its mass over the next
thousand years, leading to an increase in global sea level of about 2.7 m (9 ft)
o If all permanent continental ice were to melt, sea levels would rise about 67 m
(220 ft)