Chapter 8 Atmosphere Ocean Interactions El Ni o and Tropical Cyclones Key Terms Arctic oscillation AO Bermuda High Cape Verde hurricanes Deep zone Easterly waves Ekman spiral Ekman transport El Ni o ENSO Extratropical cyclone Eye Eye wall Gyre Hurricanes La Ni a Mixed layer North Atlantic Oscillation NAO Ocean current Oceanography Pacific Decadal Oscillation PDO Rainbands Recurvature Saffir Simpson scale Southern Oscillation Southern Oscillation Index SOI Storm Surge Supertyphoon Surface zone Thermocline Tropical cyclones Tropical depression Tropical disturbance Tropical storm Typhoon Upwelling Wind shear Summary Based on temperature the vertical structure of the ocean can be classified into three basic layers the surface zone the deep zone and the transition zone thermocline In the bottom layer or the deep zone the temperature is relatively uniform with depth The surface zone is the warmest layer and is warmest in tropical waters Ocean temperature decreases toward the pols and with increasing depth The atmosphere and the ocean interact in a variety of ways through exchanges of heat and moisture and also by the creation surface currents These currents are driven by the winds Some like the Gulf Stream move in well defined boundaries like a river while others are broad and diffuse Surface currents generated by the winds can induce vertical circulations in the oceans This can cause upwelling of nutrient rich waters For example over the eastern tropical Pacific Ocean winds typically rotate counterclockwise around a high pressure system producing along coast winds off eastern Pacific Coast and easterly winds along the equator in the eastern Pacific The combined force of the wind and the Coriolis force produces offshore flow off Peru casing upwelling in which cold deep nutrient rich comes up to the surface This produces cold SSTs off coast along equator in eastern Pacific In the western Pacific on the equator this upwelling does not occur and as a result the SST is higher there than in the east El Ni o and La Ni a are extreme phases of a naturally occurring climate cycle and describe large scale changes in SST across tropical Pacific El Ni o and La Ni a result form interaction between the surface of the ocean and the atmosphere in the tropical Pacific During El Ni o warm SSTs replace cold upwelling waters in the eastern Pacific This shifts thunderstorm activity eastward across the Pacific and ultimately affects weather and climate across the globe In many locations especially in the tropics La Ni a cold episode produces the opposite climate variations from El Ni o and essentially enhances or increases the severity of normal conditions The oceanic patterns of El Ni o and La Ni a are closely related to a Pacific wide seesaw of atmospheric pressure known as the Southern Oscillation Recently similar oscillations have been discovered over the north Pacific and Atlantic Oceans highlighting the importance of the interaction between the ocean and the atmosphere Tropical cyclones are large white whirling storms that obtain their energy from warm ocean waters They stand out on satellite photos because of their circular cloud patterns and in stronger storms a nearly clear eye at the center of the storm The clarity and size of the eye on satellite helps estimate the strength of the cyclone A tropical cyclone begins as a disorganized tropical disturbance A few grow to hurricane or typhoon strength with winds up to 200 mph Most weaken within a week or two They typically move west or northwest and then recurve due to Coriolis forces and the Bermuda high toward the northeast but each storm s path is unique Hurricanes affecting the US often form in the Gulf of Mexico and Caribbean Sea in early summer to late fall with powerful Cape Verde hurricanes often dominating attention in August and September The location of warm ocean waters largely determines the birthplaces and intensities of tropical cyclones Tropical cyclones cause destruction with extremely high winds storm surges of seawater and flooding rainfall Of these storm surge has been the most deadly and devastating as in Hurricane Katrina however in many years flooding rains have killed more US residents than wind or storm surge Each hurricane packs its own unique combination of these weapons
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