I. What is El Nino--Southern Oscillation phenomonologically?
|El Nino refers to the warm phase of a natural mode of swing of the coupled tropical ocean-atmosphere system—the El Nino-Southern Oscillation, or for short, ENSO. The cold phase of ENSO is called La Nina. The atmospheric and upper oceanic conditions of the two phases are schematically shown here. The cold phase is characterized by a more contracted warm-pool, a more westward positioned deep convection, and a stronger zonal SST contrast. The zonal wind in the tropics generally increases its strength with the zonal SST contrast, so the zonal wind is stronger in the cold phase. The stronger wind also result in a larger zonal slope of the equatorial thermocline and more intense equatorial upwelling in the eastern Pacific which in turn augments the zonal SST contrast. The loop that links the zonal SST contrast, the zonal winds, equatorial upwelling is called the Bjerknes feedback. The warm phase is characterized by an eastward extension of the warm-pool. Following the warm water, deep convection is also shifted eastward. The zonal SST contrast is weak or nonexistent as it is observed in the strongest El Nino events. The bottom figure shows the variability of the SST in the east equatorial Pacific associated with ENSO over the last hundred years. The figures shows that El Nino events occurs every 2 to 7 years with varying magnitudes. On average or measured by its maximum magnitude, El Nino events tend to be stronger than La Nina. This asymmetry is particularly obvious in the later period of the record. For example, the 1982-83 El Nino and the 1997-98 El Nino reaches the magnitude of 3 oC while La Nina’s magnitude never exceeded 2 oC.|
II. Why Do We Have El Nino?
|Full Text (pdf)||Abstract|
|Wyrtki, K., 1975: El Nino- The Dynamical Response of the Equatorial Upper Ocean to Atmospheric Forcing||El Niño is the occasional appearance of warm water off the coast of Peru; its presence results in catastrophic consequences in the fishing industry. A new theory for the occurrence of El Niño is presented.It is shown that El Niño is not due to a weakening of the southeast trades over the waters off Peru, but that during the two years preceding El Niño, excessively strong southeast trades are present in the central Pacific. These strong southeast trades intensify the subtropical gyre of the South Pacific, strengthening the South Equatorial Current, and increase the east-west slope of sea level by building up water in the western equatorial Pacific. As soon as the wind stress in the central Pacific relaxes, the accumulated water flows eastward, probably in the form an internal equatorial Kelvin wave. This wave leads to the accumulation of warm water off Ecuador and Peru and to a depression of th usually shallow thermocline. In total, El Nino is the results of the response of the equatorial pacific ocean to atmospheric forcing by trade winds.|
|Wyrtki, K., 1985: Water displacements in the Pacific and the genesis of El Niño cycles. J. Geophys. Res., 90, 7129-7132.||Sea level observations are used to estimate the amounts of warm water exchanged during the 1982- 1983 El Nino event, indicating an eastward flux of about 40 x 106 m 3 s. At the end of El Nino the equatorial Pacific is depleted of warm water which is lost toward higher latitudes. The duration of a complete El Nine cycle is determined by the time required for the slow accumulation of warm water in the western Pacific. The cycle constitutes an energy relaxation of the ocean-atmosphere system|
|Cane, M. A., and S. E. Zebiak, 1985: A theory for El Niño and the Southern Oscillation. Science, 228, 1085-1087.||A coupled atmosphere-ocean model is presented for El Niño and the Southern Oscillation that reproduces its major features, including its recurrence at irregular intervals. The interannual El Niño-Southern Oscillation cycle is maintained by deterministic interactions in the tropical Pacific region. Ocean dynamics alter sea-surface temperature, changing the atmospheric heating; the resulting changes in surface wind alter the ocean dynamics. Annually varying mean conditions largely determine the spatial pattern and temporal evolution of El Niño events.|
|Zebiak, S. E., and M. A. Cane, 1987: A model El Niño-Southern Oscillation. Mon. Wea. Rev., 115, 2262-2278.||A coupled atmosphere-ocean model is developed and used to study the ENSO (El Niñ/Southern Oscillation) phenomenon. With no anomalous external forcing, the coupled model reproduces certain key features of the observed phenomenon. including the recurrence of warm events at irregular intervals with a preference for three to four years. It is shown that the mean sea surface temperature, wind and ocean current fields determine the characteristic spatial structure of ENSO anomalies. The tendency for phase-locking of anomalies is explained in terms of a variation in coupling strength associated with the annual cycle in the mean fields. Sensitivity studies reveal that both the amplitude and the time of scale of the oscillation are sensitive to several parameters that affect the strength of the atmosphere–ocean coupling. Stronger coupling implies larger oscillations with a longer time scale. A critical element of the model oscilliation is the variability in the equatorial heat content of the upper ocean. Equatorial heat content increases prior to warm events and decreases sharply during the events. A theory for this variability and the associated transitions between the non-El Niño and El Niño states is presented. Implications of the model results for the prediction of El Niño events are discussed.|
|Battisti, D. S. 1988: Dynamics and Thermodynamics of a Warming Event in a Coupled Tropical Ocean-Atmosphere System Model. J. Atmos. Sci., 45, 2889-2919.||A simple coupled ocean–atmosphere model, similar to that of Zebiak and Cane, is used to examine the dynamic and thermodynamic processes associated with El Niño/Southern Oscillation (ENSO). The model is run for 300 years. The interannual variability which results is regular, with a period of either 3 or 4 years, quantized by the annual cycle. The amplitude (∼1.5 m s−1 wind and 2°C SST anomalies), period and structure of the interannual variability compare well with observations. The model warm event is initiated in the spring prior to the event peak, and is well described as an instability of the coupled system. During instability growth, the sea surface temperature (SST) anomaly is primarily generated by vertical upwelling processes. The SST anomaly can be approximately described by the expression ∂T/∂t = KTh − α*T, where T is the SST anomaly, t time, h the upper layer thickness (pycnocline) perturbation and α* an effective damping time which includes heat loss to the atmosphere. KT parameterizes vertical upwelling and mixed layer processes.
Oceanic wave dynamics determines the fate of the growing instability. The warming of the SST produces westerly wind anomalies in the equator central Pacific, forcing equatorially trapped Rossby waves that propagate freely to the western boundary. These waves reflect at the western boundary, sending upwelling equatorial Kelvin waves back to the central basin. These cooling Kelvin waves act to terminate instability growth and rapidly plunge the coupled system into a cold regime. The western boundary reflection is necessary for event termination. The system returns from a cold regime via reduced heat flux to the atmosphere and, to a lesser extent, by wave induced processes like that which lead to the warm event termination. The interannual variability is not produced by vacillation between two equilibrium states: a cold and a warm state. The growth rate to either the cold or warm state is too slow for the system to achieve equilibrium, even for a basin the size of the Pacific. The model results indicate that shortly after the initial set of gravest mode Rossby reflections on the western boundary, the instability growth is already being substantially moderated by the equatorial wave processes in the ocean. Thus the system is oscillatory around a single basic state.
Of the Rossby waves produced in the central Pacific by the warm event, only the two gravest mode symmetric modes are important in the reflection process, which produce the Kelvin waves that terminate the warm event. In nature, the actual western boundary for the equatorial Pacific wave guide is very ambiguous. Calculations indicate, however, that efficient reflection of the gravest symmetric Rossby waves from a more realistic boundary than the meridional wall in the model is possible. Finally, if the model is indeed simulating the correct processes controlling ENSO events, the nature of the instability mechanism that leads to growth and the wave-induced termination of the model warm event suggests that, for realistic instability growth rates for the coupled equatorial ocean-atmosphere system, interannual variability analogous to ENSO should not be possible in equatorial basins significantly smaller than the Pacific.
|Suarez, M. J., and P. S. Schopf, 1988: A delayed action oscillator for ENSO. J. Atmos. Sci., 45, 3283-3287.||A simple nonlinear model is proposed for the El Niño/Southern Oscillation (ENSO) phenomenon. Its key feature is the inclusion of oceanic wave transit effects through a negative, delayed feedback. A linear stability analysis and numerical results are presented to show that the period of the oscillation is typically several times the delay. It is argued such an effect can account for the long time scale of ENSO.|
|Jin, F.-F., 1996: Tropical ocean-atmosphere interaction, the pacific cold tongue, and the El Niño Southern Oscillation. Science, 274, 76-78.||ABSTRACT The tropical Pacific basin allows strong feedbacks among the trade winds, equatorial zonal sea surface temperature contrast, and upper ocean heat content. Coupled atmosphere-ocean dynamics produce both the strong Pacific cold tongue climate state and the El Niño-Southern Oscillation phenomenon. A simple paradigm of the tropical climate system is presented, capturing the basic physics of these two important aspects of the tropic Pacific and basic features of the climate states of the Atlantic and Indian ocean basins.|
|Sun, D.-Z., 1997: El Niño: a coupled response to radiative heating? Geophys. Res. Lett., 24, 2031-2034.||The very existence of El Niño— the oscillatory behavior of the tropical Pacific climate—may be due to the warmth of the tropics (relative to the coldness of the high latitudes). This is elucidated by subjecting a mathematical model for the coupled tropical ocean-atmosphere system to a varying radiative heating. The temperature of the deep ocean is kept fixed. In response to an increasing radiative heating, the coupled system first experiences a pitch-fork bifurcation that breaks the zonal symmetry imposed by the solar radiation. The resulting zonal sea surface temperature (SST) gradients increase with increases in the radiative heating. When the zonal SST gradients exceed a critical value, a Hopf bifurcation takes place which brings this system to an oscillatory state, a state that closely resembles the observed tropical Pacific climate. Further increases in the radiative heating result in increases in the magnitude of the oscillation. The results shed new light on the physics of El Niño and suggest that climate change due to anthropogenic forcing may occur through the same dynamic modes that sustain natural variability.|
|Sun, D.-Z. and T. Zhang 2006: A Regulatory Effect of ENSO on the Time-Mean Thermal Stratification of the Equatorial Upper Ocean. Geophys. Res. Lett., Vol. 33, L07710, doi:10.1029/2005GL025296.||To investigate the role of ENSO in regulating the time-mean thermal stratification of the equatorial Pacific, perturbation experiments are conducted in pairs with a coupled model. In one experiment, ENSO is turned off while in the other experiment ENSO is kept on. Perturbations are introduced through either enhancing tropical heating or increasing subtropical cooling. In the absence of ENSO, the time-mean difference between the warm-pool SST (Tw) and the characteristic temperature of the equatorial thermocline (Tc) responds sensitively to either enhanced tropical heating or enhanced subtropical cooling. In the presence of ENSO, such a sensitivity to destabilizing forcing disappears. The lack of sensitivity in the response of Tw-Tc is linked to a stronger ENSO in response to the destabilizing forcing. ENSO in the model acts as a basin-scale heat “mixer” that enables surface heat to be transported to the depths of the equatorial thermocline. The study raises the question whether models with poor simulations of ENSO can give reliable predictions of the response of the time-mean climate to global warming.|
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