Climate & Earth System Science Introduction to Meteorology & Climate MAPH 10050 Peter Lynch Peter Lynch Meteorology & Climate Centre School of Mathematical Sciences University College Dublin Meteorology & Climate Centre School of Mathematical Sciences University College Dublin MAPH 10050: Introduction to Met & Climate Chapter 08 Lecture Time and Location: Tue-Thu 11:00-11:50 Bldg. Room. Discussion Time and Location: Wed. 2:00-2:50 Textbook: Meteorology: Understanding the Atmosphere, by Ackerman and Knox Atmosphere Ocean Interactions A: Structure of the Ocean NHS Winter Annual-mean energy gains and losses in the oceans (W m 2 ) ENERGY GAINS AND LOSSES energy flow from equator to midlatitudes IN THE OCEANS NHS Summer Note: positive values indicate energy flow out of the ocean 1
OCEAN TEMPERATURE We can divide the ocean into three layers The top 100 meters is called the surface zone, or mixed layer Wind driven waves and currents mix this layer uniform temperature Bottom layer, below about 1000 meters, cold water -1 to 3 ºC Transition zone Thermocline NEXT: Sea Surface Temperature (SST) Global Zonal Mean Temperature SST 2
Gulf Stream Eddies Ekman Drift Upwelling 3
OCEAN CURRENTS Massive pattern of water flow Winds blow steadily over the ocean and friction pushes the ocean surface in the direction of the winds. Ocean currents tend to follow the wind patterns However the coriolis force makes the ocean current move at an angle to the wind direction Ekman spiral and Ekman transport Upwelling Chapter 08 Surface wind stress in tropical Pacific Atmosphere Ocean Interactions B: EL NIÑO AND LA NIÑA Equatorial currents Motion of surface ocean layer in the tropics Tropical Pacific SST 29º 24º 4
Equatorial temperature El Niño, La Niña and ENSO cross-section 29º 24º La Niña Bjerknes feedback Correlation coefficient of annual-mean sealevel pressure with pressure at Darwin. Equatorial sections La Niña ocean atmos El Niño El Niño longitude longitude Darwin Tahiti ENSO Time Series 5
ENSO Spectrum How EL NIÑO Affects Global Weather period (yrs) EL NIÑO AND LA NIÑA El Niño is the periodic warming of the equatorial pacific ocean between South America and the date line. Usually the winds blow from the east - which causes the ocean to flow in a westward direction. Strong upwelling along the Peruvian coast - high nutrient level - large shoals of fish. EL NIÑO AND LA NIÑA In El Niño the winds weaken or reverse- no upwelling - poor fishing. Also changes the weather patterns. La Niña is the counterpoint of El Niño. Winds intensify and the upwelling increases. Chapter 08 Atmosphere Ocean Interactions C: Tropical Depressions Tropical cyclones These storms go by different names in different parts of the world: Typhoons in Southeast Asia and Japan Tropical cyclones in Australia and India Hurricanes in Central America, the Caribbean and the USA. 6
Stages in the formation of a tropical cyclone Individual cumulus cloud, which may be between 1 and 10 km across and may last only a few hours. cumulus clouds grow into deep cumulonimbus clouds. These become grouped into clusters. Such cloud clusters may be many hundreds of kilometres in diameter. Some cloud clusters persist for just a day or two, but a few develop into an organised system. Keys to the growth of a cluster into a full-scale tropical cyclone: Convergence of winds at the surface Outflow of air at higher altitude Rotation of system about its centre. A vertical cross-section shows the organised convection that is essential for such a storm to develop. Cross-section of a tropical cyclone The air aloft warms quickly because condensation within the clouds releases enormous amounts of latent heat. The surrounding air is warmed, making it less dense and hence promoting more vigorous convection. The air aloft warms and more air flows out from the top of the system than converges at the surface The surface pressure at the centre of the system is reduced. The lowest surface pressure is always at this centre, the region known as the eye. SCHEMATIC CIRCULATION IN A HURRICANE Question: When a surface low-pressure area forms, how do the surface winds behave? 7
Question: When a surface low-pressure area forms, how do the surface winds behave? Answer: The winds circulate round the low and blow slightly inwards towards the centre of the low. Surface air moves towards the area of low pressure from all directions. Convergence forces the air to rise. The rising air has already picked up moisture from the ocean. More and more deep clouds form. These produce more heavy rain. The process of condensation releases more latent heat. The warmer the air, the faster it rises, and the lower the pressure at the centre. the whole system continually reinforces itself. Cross-section of a tropical cyclone Several development stages Once a core of low surface pressure has formed and winds are circulating around it, though with sustained wind speeds of less than Beaufort Force 7 (i.e. less than about 60 kph), the system is termed a tropical depression. Most tropical depressions do not intensify further, but some develop sufficiently to become tropical storms. On the Beaufort scale, these may be classed as a gale (Force 8 9) or a storm (Force 10 11). Only if conditions are favourable will a tropical storm evolve into a full-scale tropical cyclone with winds in excess of Force 12 (117 kph). Structure of Tropical Cyclones Structure of a tropical cyclone. Surrounding the eye is the eye wall, where the most vigorous cumulonimbus form. The eye-wall zone produces the strongest winds and the most intense rainfall. 25 cm per hour is typical! The outflow from the tops of the cumulonimbus clouds at altitudes of about 15 km spreads out as a dense shield of cirrus cloud. The diameters of tropical cyclones vary greatly. They are typically 500 800 km across. A few are many times that size: Hurricane Gilbert in 1998 had a diameter of 3500 km. The eye is often only 25 50 km across. 8
Hurricane Mitch October 26, 1998 COMPUTER GENERATED IMAGE OF HURRICANE MITCH OCTOBER 26, 1998. NEAR HONDURAS The surface pressure at the centre of the eye is frequently 950 hpa or less. The record low pressure of 870 hpa is held by the Typhoon Tip, a North Pacific tropical cyclone of 1979. Fortunately, Tip did not make landfall in its intense phase. Necessary Conditions for Formation The ocean surface temperature must be in excess of 27 C in its upper layers. The atmosphere at middle levels of the troposphere must be fairly moist. The instability of the atmospheric environment must be suitable for the development of cumulonimbus clouds. Winds at the surface must be relatively light. Vertical wind shear must be weak: strong winds would simply blow the system apart. 9
Location of Tropical Cyclones Latitudinal Distribution The spiral motion of the accelerating winds round the eye is a consequence of the Coriolis effect. The system must be far enough away from the Equator for this effect to operate. In general, tropical cyclones are born within latitude bands between 5 N and 10 N or between 5 S and 10 S Tropical Cyclone Tracks: North Atlantic, 1980-2005 NAMES, DATES AND PATHS OF TROPICAL STORMS AND HURRICANES IN 2005 Fig. 8-31, p. 237 Fig. 8.29 10
The Saffir-Simpson Simpson Hurricane Wind Scale The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 categorization based on the hurricane's intensity. The scale indicates the type of damage and impacts associated with winds of this intensity. For more information, see http://www.nhc.noaa.gov/ TROPICAL CYCLONES: Final Stage Tropical cyclones diminish when: They move over colder ocean waters. Move onto land. Reach a location when large-scale flow aloft is unfavourable. TROPICAL CYCLONE DAMAGE Wind damage Storm surge Inland flooding 11
Damage from hurricane Andrew, August 1992 NORMAL CONDITIONS STORM SURGE Thank You Thank You 12