Tropical Circulation in a Global ContextEl Niño Southern Oscillation ENSOENSO - Mode of Variability of Atmosphere-Ocean SystemInternal Climate System OscillationsPowerPoint PresentationHistory of El NiñoSlide 7Slide 8Slide 9Slide 10Slide 11The Walker CirculationSlide 13Slide 14Overview of Atmosphere-Ocean System during El NiñoSlide 16Slide 17The importance of the westerly wind burstsSlide 19Slide 20What can cause the westerly wind bursts?Do we understand how and Why an ENSO episode occurs?Advanced ENSO Theory: the delayed OscillatorSlide 24Kelvin and Rossby waves in the OceanHow does Kelvin and Rossby Waves evolve in the Delayed Oscillator theory?Slide 27Slide 28Slide 29Slide 30This simple scenario is different from ENSO in the sense that it represents oscillations deriving from a single wind impulse, whereas the real ENSO involves a continuous wind-stress forcing throughout the cycle; also the time scale of the ocean oscillations in the present example are much shorter than those of ENSO. These issues are addressed through one other critical element of the theory – the positive feedback, or coupled instability mechanism How to understand that???.Slide 32Slide 33In conclusion:The recent history of El Ninos/La Nina eventsEl Nino and the ocean heat budgetSlide 37The 1997- 98 El NiñoEl Nino global Impacts1997-98 wind anomalies and OLR anomaliesSlide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Connections to Climate ChangeTypical US Temperature, Precipitation and Jet Stream Patterns during El Niño and La Niña WintersHow do we know so much?U.S. Precipitation Departures (mm) and Frequency of Occurrence (%) for El Niño during Dec - FebU.S. Temp. Departures (°C) and Frequency of Occurrence (%) for El Niño during Dec - FebSlide 53Slide 54Tsonis et al. 2003: Periods with large rate of change of global temperature (not the actual temperature) are related to an increase in the frequency of El Nino (opposed for La Nina) El Nino is activated to reverse positive global temperature trends, and La Nina to reverse negative trendsA Little Bit of Rationale… (Tsonis et al. 2003)Tropical Circulation in a Global ContextMean surface pressure and winds during DJFWesterliesWesterliesWesterliesWesterliesSE tradesSE tradesNE tradesNE tradesSE tradesSE tradesNE tradesNE tradesNE tradesNE tradesEasterliesEasterliesEasterliesEasterliesNE tradesNE tradesEl Niño Southern OscillationENSO2ENSO - Mode of Variability of Atmosphere-Ocean System•Climate System has many modes of variability•Some modes are induced by external forcing from Sun (diurnal, seasonal, Milankovitch cycle – relate to changes in orbital parameters of the Earth)•Some modes are internal to the system and result from complex and non-linear interactions between the system’s fluids (atmosphere and ocean) over a variety of time scales3Internal Climate System Oscillations•Climate system has its own internal oscillations independent of external forcing•Modes of internal variability (Oscillations) vary over a variety of time scales•Some of the strongest oscillations are:–20 – 100 days tropical waves (hold your curiosity for a few more classes)–El Nino Southern Oscillation - ENSO–North Atlantic Oscillation NAO–Pacific Decadal Oscillation PDO–Arctic Oscillation AO–Antarctic Oscillation AAO4High signal in high High signal in high latitudes but can affect latitudes but can affect low latitudes toolow latitudes tooWhat is El Niño: Tropical Pacific Sea Surface Temperature Anomalies5History of El Niño•Locally recognized in precolonial times•Sir Gilbert Walker (1924)–While working for the Indian Meteorological Service, he noticed pressure anomalies–Attempted to relate anomalies to the failure of the Indian Monsoon, but failed•H.P. Berlage (1950s): Develops Southern Oscillation Index (SOI)•1972-1973: Major El Niño causes collapse of Peruvian fishing•1983: Major El Niño causes flooding in Southern California•1998: Strongest El Niño of 20th centuryPressure AnomaliesFrom Caviedes, C., 2001, El Niño in HistoryPg. 86Sea Level Pressure at Darwin Negative correlation:Sea Level at S Pacific High7TahitiDarwinThe Southern Oscillation Index - SOI•Calculated as the pressure differences between Tahiti and Darwin (Tahiti-Darwin)Negative SOI is an indicator of El NiñoSea Surface Temperature at equator and 110oW8TD91011The Walker Circulation12Typical Atmospheric CirculationWalker Cell1314Thermocline: Depth at which the rate of decrease of temperature with increase of depth is the largest. It separates the zone between the mixed layer above, much influenced by atmospheric fluxes, and the deep ocean. In the tropics, the thermocline is shallow on average, as in the eastern Pacific (50m), or deeper as in the western part (160-200m).Overview of Atmosphere-Ocean System during El Niño•Increased SST in the Eastern Pacific•Weakened Easterlies•Humboldt Current is weakened–Weakened upwelling•Depression of the thermocline–Decreased nutrient mixing•Shifting rainfall patterns–Drought in Australia–Torrential floods in Peru in December•Not every El Niño is the same–They differ in timing, intensity & extent15Tropical Pacific Sea Surface Temperature Anomalies16El Niño and Water Temperature17The importance of the westerly wind bursts18The question is: What triggers an El Nino and why?The 1997-98 El NiñoChanges in the direction of the tropical easterly = westerly wind burst19El Niño, Easterlies and Westerly Winds20TimeWesterliesWesterliesWesterlyWesterlyAnomaliesAnomaliesWhat can cause the westerly wind bursts?21Several theories try to explain they onset of an El Nino. Some say that an oscillation with period of 30-60 days known as the “Madden-Julian” oscillation could be important if occurring in the right time.Sorry but we will see more on the Madden-Julian oscillation in a few weeks… But let’s examine some of the Advanced ENSO theories to explain the consequences for ENSODo we understand how and Why an ENSO episode occurs?22There are several theories to explain ENSO and none is considered absolute. There are many unknown physical and dynamical processes and interactions among scales that cannot be represented in the state-of-the-art climate models. Here we will explore one famous theory: The Delayed Oscillator The Delayed OscillatorAdvanced ENSO Theory: the delayed Advanced ENSO Theory: the delayed OscillatorOscillator23Imagine a very simplified model in which the Pacific ocean is