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
Figure 01: The energy gains and losses of Earth’s oceans
Figure 02: Energy transport versus latitude
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
Figure 03: Temperature vs depth in ocean
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 : 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 06: Ocean currents.
Figure 07: SST satellite image of the Gulf Stream Courtesy of SSEC, University of Wisconsin-Madison
Figure 08: North Atlantic gyre
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
Figure 09: Ekman spiral
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
Figure 10: Ekman winds
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 12: Trade winds & El Niño, T-storms
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
Figure T01: Global Impacts of Five Major El Niño Events
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 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 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
Figure 26: Hurricane Hugo Courtesy of NOAA
Figure 28: Hurricane relative winds
What is the life cycle of a hurricane? A tropical disturbance is a disorganized clump of thunderstorms, often associated with the trough of a wave in the easterlies A tropical depression is a disturbance (about one in ten) with a weak low-pressure center of about 1010 mb. There is cyclonic rotation of the winds. A tropical depression gets a number, but not yet a name
Figure 29: Tropical disturbances Courtesy of CIMSS/SSEC, University of Wisconsin-Madison
Figure 30: Isabel Montage Courtesy of Gary Wade, CIMSS/NOAA
Figure 33: 2005 hurricane season Data from NOAA Coastal Services Center: q.csc.noaa.gov/hurricanes/viewer.html
What is the life cycle of a hurricane? Some tropical depressions have a central pressure that drops below about 1000 mb, a strengthening horizontal pressure gradient, and strengthening winds. When sustained winds reach 35 knots (39 mph), the depression becomes a tropical storm, and gets a name.
What is the life cycle of a hurricane Roughly half of tropical storms develop further Central pressures drop below 990 mb and winds keep strengthening When sustained winds exceed 65 knots (74 mph) the storm is reclassified as a hurricane A storm may fall from and regain the status of a hurrican
What is the life cycle of a hurricane? Hurricanes weaken and die as they – Pass over land and lose their moisture supply – Pass over cold ocean waters and lose their energy supply – Experience vertical wind shear that tilts the center of the storm The moisture in a former hurricane can cause severe flooding
More about the life cycle of tropical cyclones Some go on to become extratropical cyclones in middle latitudes In the Western Pacific Ocean, an occasional typhoon will go on to become a supertyphoon. These have sustained winds > 150 miles per hour A widening eye usually indicates a weakening of the storm
More about the life cycle of a tropical cyclone On a day to day basis, the intensity of tropical cyclones is influenced by the sea surface temperatures it encounters. The hurricane season of 2005 broke many records for tropical cyclones—27 named storms in the North Atlantic
How hurricanes cause destruction Winds: hurricanes are classified as to wind damage using the Saffir-Simpson scale from 1 (weakest) to 5 (strongest) Hurricanes can contain “mini-swirls” that are tornado-sized with even stronger winds than the main hurricane
How hurricanes cause destruction The storm surge is the process of wind- induced seawater flooding – Causes 90% of hurricane deaths – Worst at high tide (new and full moon) – Worst with strong winds – Worst with lowest sea-level pressure – Worst with low-lying land – Depends on coastal and underwater shape
Figure 34: Aerial damage of Katrina Courtesy of NGS Remote Sensing Division/NOAA
Figure T03: The Saffir–Simpson Scale for Hurricanes
Figure 36: Doppler image of Allison Courtesy of National Weather Service/NOAA
How hurricanes cause destruction Rainwater causes flooding From more deaths were caused by flooding than other types of hazards in hurricanes Massive flooding can occur with even weak tropical cyclones or tropical storms Hurricane rains have some beneficial effects
Figure 37: Photo of Floyd flooding Courtesy of Dave Saville/FEMA News Photo
How hurricanes cause destruction Flooding from tropical storm Allison in June 2001 killed at least 20 people and damaged houses with costs near $5 billion Allison caused rain in every Gulf and Atlantic state from Texas to Maine Allison caused nearly 36 inches of rain in Houston, TX
Figure B03a: Photo of a dust storm from a plane Courtesy of Jason Dunion, NOAA/AOML/Hurricane Research Division
Figure B03b: Satellite Image of a dust storm Courtesy of Earth Observatory/NASA
Figure 38: Tracks of the Galveston hurricane and Katrina