El Niño: Atmospheric-Ocean Interactions and Their Global Impact
The complex interactions between oceanic and atmospheric systems, particularly focusing on El Niño phenomena and tropical cyclones. It highlights how oceans cover 70% of the Earth's surface, affecting climate through energy and water exchange. The text discusses sea surface temperatures, ocean layers, and energy transfer, emphasizing the critical role of ocean currents and wind patterns. El Niño's effects on global weather are explored, showcasing its significance in climate variability, especially in the Pacific region.
El Niño: Atmospheric-Ocean Interactions and Their Global Impact
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Presentation Transcript
Chapter 8 Atmospheric-Ocean Interactions: El Niño and Tropical Cyclones
Figure CO: Chapter 8, Atmosphere-Ocean Interactions © ultimathule/ShutterStock, Inc.
The Oceans • Cover about 70% of the earth’s surface • Cover an even higher percentage in the tropics • Exchange large amounts of energy and water with the atmosphere at the surface • Give water vapor to the atmosphere • Give energy to the atmosphere in the form of latent heating • Exchange energy with the atmosphere by convection and conduction • Advect large amounts of energy from the equator towards the poles
The rate of energy transfer between atmosphere and oceans • Is greater when the temperature difference is greater. • Is in the direction from the higher to the lower temperature. • Is greater when the wind speed is greater • For latent heating, rates are greatest when saturation vapor pressure in the atmosphere is largest (high temperatures).
Energy Gains and Losses • When the oceans give up energy (negative, blue, purple) the atmosphere gains energy from the oceans • When the oceans gain energy (positive, red, yellow) the atmosphere loses energy to the oceans • Oceans supply energy to storms in middle latitudes • Oceans warm in summer and cool in winter
The Sea Surface • Is the part of the ocean that interacts directly with the atmosphere. • Has a temperature called the sea surface temperature (SST) actually measured a few feet below the surface at the intake level of a ship. Cold water is used by ships for air conditioning. • The skin temperature is the temperature right at the surface.
Temperature in the Oceans • Generally there are three layers • The surface zone has the highest temperatures • Sometimes called the well-mixed (by waves and convection) zone • The thermocline is a zone of rapidly decreasing temperature as depth increases • In the deep zone the temperature is slightly above freezing • Salt water freezes at a temperature lower than fresh water
Temperature Profiles for Different Latitudes • The tropics have the steepest thermocline, because sea surface temperatures are greatest there • The middle latitudes have the deepest surface layer • Polar regions have surface temperatures near freezing
Figure 04: the depth and steepness of the thermocline are functions of latitude
Water, salt water, and air • Salt water is denser than fresh water. • Icebergs, made of fresh water, float. • Colder water is only very slightly denser than warmer water. • Pressure in the oceans increases downward by 1 atmosphere about every 35 feet. • Density is nearly constant in the oceans, whatever the depth.
Sea Surface Temperatures • Are highest in the tropics, lowest at the poles • In middle latitudes and subtropics, are higher on east coasts than west coasts • In polar regions, lowest temperatures on east coasts • In tropical regions, higher temperatures on west coasts • Are highest in the equatorial western Pacific and the Indian Ocean • Correspond to warm and cool ocean currents
Figure 05: SST distributions across the globe. (Reproduced from Kara, Wallcraft, Hurlburt, J. Atmos. Oceanic Technol., 20 [2003]: 1616–1632.
Surface Pressure, Surface Wind, and Ocean Currents • Subtropical highs are persistent enough to create persistent anticyclonic wind flow • These winds create gyres of anticyclonic ocean surface currents • Ocean currents are bounded by land
Figure 07: SST satellite image of the Gulf Stream Courtesy of SSEC, University of Wisconsin-Madison
Wind and the Ekman Spiral • Friction between the air and the sea surface forces the air to move • The Coriolis force turns the water to the right (NH) or left (SH) • Moving water influences the layer of water beneath • The entire pattern is called the Ekman spiral • On average, water moves to right (NH) or left (SH) in Ekman transport
Cold Current, West Coast and Upwelling • Ekman transport moves water away from the shore • The water must be replaced • Replacement water comes from below the thermocline in the process called upwelling • Mixing and cold water brings nutrients close to the surface and favors sea life
El Niño • Named “The (boy) child” for the season of most common occurrence. • Is a common but short-lived feature, but occasional episodes last for months or a year or more. The episodes are what we call El Niño today. • El Niño is a phenomenon that affects the entire Pacific Ocean and weather around the globe.
Figure 11: Temperature anomalies Courtesy of University of Washington, Joint Institute for the Study of the Atmosphere and Ocean (JISAO)
Figure 14: El Niño weather Source: NOAA
Characteristics of El Niño • Abnormal warming of the waters off Ecuador and Peru. • Upwelling ceases • Warm waters come from the western Pacific • Trade winds weaken
Figure 15: USA snowfall during El Niño Impacts of El Nino on Snowfall by Angel, Jim, Image courtesy of the Midwestern Regional Climate Center, Illinois State Water Survey
La Niña • Generally, opposite conditions to El Niño • Also described as an enhancement of normal conditions • Abnormal cooling of ocean waters in the eastern Pacific • Upwelling is enhanced • Trade winds are stronger
Figure 17: La Niña weather Source: NOAA
How to get the latest information about El Niño • Google on “ENSO diagnostic discussion” • Choose the first entry, the National Climate Prediction Center • Look at the latest discussion and the weekly update • ENSO is “El Niño Southern Oscillation”
Other Oscillations • The Pacific Decadal Oscillation (PDO) • The North Atlantic Oscillation (NAO) • The Arctic Oscillation
Figure 18: NAO Reproduced from www.ldeo.columbia.edu/NAO, Courtesy of Martin Visbeck
Tropical cyclones: what are they? • Hurricanes in waters of North and Central America • Typhoons in the western Pacific • Cyclones in the Indian Ocean and Southern Hemisphere • All have sustained winds of 65 knots or 74 mph
Tropical cyclones: what are they? • From space, they look like large circular swirls of clouds several hundred km in diameter • The most highly organized and destructive weather systems on Earth • A grouping of a large number of thunderstorms with a circulation about a center of low pressure.
Figure 19: Hurricane Isabel Courtesy of CIMSS/University of Wisconsin-Madison
Figure 20: Andrew damage Courtesy of NOAA
What’s inside a hurricane • At the center of low pressure is the eye, 8 to 80 km across, often almost entirely clear of clouds • Surrounding the eye is an eye wall, a narrow, circular, rotating region of intense thunderstorms and strong upward motion • Spiral bands of thunderstorms and cumulus clouds extend outwards from the eye wall.
Figure 21A: Hurricane Mitch satellite images Courtesy of CIMSS/University of Wisconsin-Madison
Figure 22: Inside eyewall © Chris Sattlberger/Photo Researchers, Inc.
Figure 21B: Hurricane Mitch satellite images Courtesy of CIMSS/UW-Madison
Figure T02: The Most Damaging Tropical Cyclones to Affect the United States 1900–2009
Hurricanes: How do they form? • Atmosphere and ocean interact to fuel a hurricane • Latent heating near the surface when strong winds evaporate large amounts of water • Energy is transferred from the warmer water to the cooler atmosphere • Updrafts in cumulus clouds transport energy upward
Figure 27: Cross section of processes involved in fueling a hurricane
Where do hurricanes form? • Where sea surface temperatures are 80°F or higher • No closer to the equator than 5° latitude: the Coriolis effect is needed for rotation about the cyclone center • Where there is little or no vertical wind shear to tilt the center of the storm • Where a disturbance already is present
Figure 23: Hurricane paths and SST Courtesy of NASA
Figure 25: Swirling winds Adapted from Nese, J. and Grenci, L., A World of Weather: Fundamentals of Meteorology. Kendall/Hunt, 1998,
Figure B01: Hurricane hunter plane Courtesy of UCAR/NSF/NOAA
