ATM OCN 100 Final Exam Study Guide Study Guide for Final Exam Chapter 23 El Nino La Nina and the Southern Oscillation The term El Nino was first used by fishermen to refer to warm ocean waters along the coasts of Ecuador and Peru Fish become scarce when warm water is present local fishing industry experiences a seasonal lull El Nino events are generally accompanied by heavy rains over coastal Peru and Ecuador El Nino is part of a much larger atmospheric phenomenon known as the Southern Oscillation The Southern Oscillation encompasses most of the equatorial Pacific atmosphere Southern Oscillation refers to the east west seesaw in the pressures of the equatorial Pacific Walker found an intriguing correlation between the surface pressures on the eastern and western sides of the tropical Pacific Ocean Southern implies that the seesaw effect is strongest in the tropical Pacific south of the equator Normally surface winds in the tropical Pacific are from east to west as air flows counterclockwise around the subtropical high and toward the low pressure near Australia The Coriolis force is weak near the equator the wind pattern is a manifestation of the pressure gradient force which points from east to west Vertical motions associated with the two major pressure centers in the tropical Pacific are downward above the high in the east and upward above the low in the west These two branches of vertical motion and the westward surface winds force a return flow of air aloft from west to east forming a gigantic closed circulation cell This circulation in the east west and vertical plane is known as the Walker Cell The Walker Cell strengthens and weakens over time with a tropical cycle of its intensity lasting several years As the Walker Cell strengthens so do the intensities of the South Pacific subtropical high the Australian low and the trade winds When it weakens so do the surface pressure centers and the trade winds The strengthening and weakening of the Walker Cell explain the seesaw of pressure between eastern and western parts of the tropical Pacific ENSO describes the complete cycle of events in the ocean and the atmosphere Once every few years the normal pattern of tropical Pacific pressure tends to weaken or break down And these are things that tend to happen Surface pressure systems weaken They decrease in the eastern high pressure area while surface pressure increases in the western low This makes both the high and low weaken This results in a reduction of the cross Pacific pressure gradient Trade winds weaken or stop or even reverse The pressure gradient responsible for the trade winds weakens decreasing wind speeds Water in the eastern Pacific warms Because trade winds are too weak to maintain tilt of the sea level the warm water of the western Pacific flows slowly eastward returning warm surface water to the eastern Pacific and stopping the upwelling along the South American coast Precipitation shifts eastward The area of upward motion in the atmosphere and its associated clouds and precipitation move eastward from the Australia Indonesia region to the central and even the eastern Pacific in response to the reduction of surface pressure and downward motion aloft in the vicinity of the normal subtropical high pressure When these conditions coincide for a sufficient time to allow the average sea surface temperature anomaly to be at least 0 5 C for three consecutive months an El Nino is defined to be occurring The eastward shift of clouds and precipitation causes a major change in the pattern of atmospheric heating which in turn affects the upper level pressures and the large scale atmospheric circulation Consequences ENSO can affect the weather over North America because the altered heating pattern over the Pacific favors a shift in the jetstream and the associated storm tracks Storms that normally move along the Jetstream into the Pacific northwest Washington and Oregon are diverted northward into the Gulf of Alaska Meanwhile a secondary storm track following a stronger than normal subtropical branch of the jetstream extends into California and across the southern United States The stronger then normal subtropical jetstream favors occurrence of East Coast extratropical cyclones Warmer conditions are experienced in many parts of the globe Strengthening of the subtropical jetstream can be viewed as a response to the enhanced heating on the southern tropical side of the jetstream since enhanced heating raises the upper air heights pressures Heating of the air over the eastern Pacific triggers thunderstorms that rise into the upper troposphere The moisture that is carried aloft by these thunderstorms is transported northward into the subtropical jetstream which carries moisture northeastward toward North America This leads to more storms than usual along the California coast Chapter 24 Tropical Cyclones Tropical Cyclones are the most destructive storms on the planet They always originate over tropical oceans but their paths can take them into the middle latitudes and over land Different names Hurricane strong tropical cyclones that occur over the Atlantic and eastern Pacific oceans Typhoons occur over western Pacific Cyclones when they occur over the Indian Ocean Tropical disturbance tropical weather system of apparently organized thunderstorms generally about 250 600 km in diameter originating in the tropics or subtropics that maintains its identity for 24 hours Tropical depression when a cluster of thunderstorms has an identifiable surface pressure drop and a closed wind circulation but its winds do not exceed 34 knots A storm is given a name when wind speeds increase to tropical storm strength which are winds over 34 knots A hurricane is when knots exceed 64 2 3 of all tropical cyclones occur in the Northern Hemisphere This is because ocean temperatures in the Northern Hemisphere are generally warm over broader areas than in the Southern Hemisphere Hurricanes never originate within 5 degrees of the equator and never cross the equator Hurricanes rarely originate north or south in the Southern Hemisphere of about 25 degrees latitude About 80 tropical storms develop annually around the globe 50 70 develop into hurricanes The western Pacific in the Northern Hemisphere produces the largest number of tropical cyclones Hurricanes form over all tropical oceans except the south Atlantic and southeast Pacific Hurricanes almost always form within the belt of trade winds winds from east to