Tropical Climate - From El Nino to Sea Breeze Review of last lecture • • • • Primary high and lows Three-cell model. Mechanism for each cell Two characteristics of temperature structure Two characteristics of wind structure. Why does westerly winds prevail in the extratropical troposphere? What cause the jet streams? • What drives the ocean currents? In the case of Ekman spiral, what is the direction of surface current relative to surface wind? Vertical structure and mechanisms Polar Cell (thermal): Driven by heating at 50 degree latitude and cooling at the poles Ferrel Cell (dynamical): Dynamical response to Hadley and polar cells Hadley Cell (thermal): Heating in tropics - forms surface low and upper level high - air converges equatorward at surface, rises, and diverges poleward aloft - descends in the subtropics The tropical climate strongly affects the global climate system 3 stages of research Phenomena: structure, teleconnection, scale-interaction, effects on extreme events (disasters) Mechanism: theory, observation Prediction: GCM, statistical model The TAO buoy array Mean state: Sea surface temperature (SST) Indo-Pacific warm pool Eastern Pacific cold tongue 2 basic regions Mean state: Precipitation Meiyu-Baiyu front Inter-tropical convergence zone (ITCZ) Warm pool South Pacific convergence zone (SPCZ) GPCP Annual Mean Precipitation for 1979-2005 (mm/day) 4 basic regions Mean State: Walker Circulation • An atmospheric circulation cell oriented along the equator with rising motion in the west and sinking motion in the east, which is induced by the thermal contrast between the Indo-Pacific warm pool and eastern Pacific cold tongue • Interacts with underlying Pacific Ocean with stronger upwelling (cooling) in the east Theories of tropical mean climate: Ocean-atmosphere feedback mechanisms SST gradient - trade wind (Bjerknes) feedback SST - Latent Heat Flux feedback SST - Shortwave Flux feedback From Lin (2007) El Nino/Southern Oscillation (ENSO): The dominant interannual variability • El Nino: Very warm sea surface temperature over central and eastern tropical Pacific, which occurs every 3-7 years. The Walker Circulation becomes disrupted during El Niño events, which weakens upwelling in eastern Pacific. • La Nina: the opposite condition to El Nino • Southern Oscillation: The atmospheric oscillation associated with the El NinoLa Nina cycle. • The whole phenomena is now called El Nino/Southern Oscillation (ENSO) ENSO: SST Change Typical ENSO period is 3-7 years, but with significant irregularity The 1997-1998 El Nino event The current La Nina event Existing ENSO theories (not required) (6) Stochastic forcing theory (2) Delayer oscillator theory (1) Slow coupled mode theory (4) Western Pacific oscillator theory (3) Advective-reflective oscillator theory (5) Recharge oscillator theory From Lin (2008) El Niño impacts on Peruvian fishing • Large numbers of fish & marine plants die as upwelling is reduced by weakened Ekman transport from weakened trades • Reduce Peruvian anchovy fish catch The dramatic impacts of ENSO around the globe Flood in Lakeport, California as a result of the 1998 El Nino event Bushfire in Australia as a result of the 1998 El Nino event Disastrous effects of 1982-1983 El Nino: 1.Australia-Drought and devastating brush fires 2.Indonesia, Philippines-Crops fail, starvation follows 3.India, Sri Lanka-Drought,fresh water shortages 4.Tahiti-6 tropical cyclones 5.South America-Fish industry devastated - decrease in nutrients off Peru- fewer fish (anchovy) 6.Across the Pacific-Coral reefs die 7.Colorado River basin-Flooding, mud slides 8.Gulf states-Downpours cause death, property damage 9.Peru, Ecuador-Floods, landslides 10.Southern Africa-Drought, disease, malnutrition The dominant intraseasonal variability: The Madden-Julian Oscillation (MJO) and convectively coupled equatorial waves • In 1966, T. Matsuno conducted a theoretical study of the tropical atmosphere and found the dominant equatorial waves • 30 years later, Takayabu (1994) and Wheeler and Kiladis (1999) confirmed the existence of these waves in the real atmosphere Wavenumber-Frequency Spectrum of Tropical Rainfall Symmetric waves • Madden-Julian Oscillation (MJO): 30-60 day • Kelvin wave • Equatorial Rossby (ER) wave • Westward inertial gravity (WIG) wave Anti-Symmetric waves • Mixed Rossby-Gravity (MRG) wave • Eastward inertial gravity (EIG) wave Real-time monitoring of the MJO and other waves Vertical structure of the MJO and wave-heating feedback mechanisms Vertical heating profile Column-integrated diabatic heating has six major components (Mean state and higherfrequency modes affect the MJO through the nonlinear terms) Global Impacts of the MJO (Lin, J.L., G.N. Kiladis, B.E. Mapes, K.M. Weickmann, K.R. Sperber, W.Y. Lin, M. Wheeler, S.D. Schubert, A. Del Genio, L.J. Donner, S. Emori, J.-F. Gueremy, F. Hourdin, P.J. Rasch, E. Roeckner, and J.F. Scinocca, 2006, Journal of Climate) Impacts of MJO on the number of Atlantic hurricanes MJO westerly phase MJO easterly phase (From Maloney and Hartmann 2001) Monsoons • A seasonal reversal of wind due to seasonal thermal differences between landmasses and large water bodies • Orographic lifting often enhances precipitation totals A 3D view of the Indian monsoon World’s Major Monsoon Systems • Asian (Indian, Western North Pacific, Eastern Asian) • Australian • Western African • North American • South American Diurnal Variation I: Sea and Land Breeze • Daily reversal of winds resulting from differences in thermal properties of land and sea • During the day (night) land (water) surfaces are hotter (cooler) than large water (land) surfaces • Thermal low develops over warmer region - air converges into the low, ascends, and produces clouds Diurnal Variation II: Mountain and Valley Breeze • mountain slopes heat/cool rapidly • creates pressure differences • day: air heats, rises upward • night: air cools, sinks Summary • Mean state: The two basic regions of SST? The four basic regions of precipitation. What is the Walker circulation? • ENSO: What is the El Nino? What is the La Nina? What is the Southern Oscillation? What is the ENSO? • MJO and convectively coupled equatorial waves • Monsoons: What is a monsoon? The 5 major monsoon systems in the world. • Diurnal variation: What are sea and land breezes? What are mountain and valley breezes?