Locking of El Nino's Peak Time to the End of the Calendar Year in the Delayed Oscillator Picture of ENSO

El Nino events owe their name to their tendency to be locked to the seasonal cycle. A simple explanation is proposed here for the locking of the peak of ENSO's basin-scale warming to the end of the calendar year. The explanation is based on incorporating a seasonally varying coupled ocean-atmosphere instability strength into the delayed oscillator mechanism for the ENSO cycle. It is shown that the seasonally varying amplification of the Rossby and Kelvin ocean waves by the coupled instability forces the events to peak when this amplification is at its minimum strength, at the end of the calendar year. The mechanism is demonstrated using a simple delayed oscillator model and is further analyzed using the Cane-Zebiak model. Being based on the oversimplified delayed oscillator paradigm of ENSO, the proposed mechanism cannot be expected to fully explain the locking of observed events to the end of the year. However, the wave dynamics perspective it offers to approaching the ENSO phase-locking problem may serve as a first step toward a fuller explanation based on more realistic models and additional data analysis.

[1]  E. Rasmusson,et al.  Variations in Tropical Sea Surface Temperature and Surface Wind Fields Associated with the Southern Oscillation/El Niño , 1982 .

[2]  M. Mcphaden,et al.  The Seasonal Cycle and Its Modulation in the Eastern Tropical Pacific Ocean , 1997 .

[3]  Mechanisms of Seasonal – ENSO Interaction , 1995, ao-sci/9508001.

[4]  Michael Ghil,et al.  El Ni�o on the Devil's Staircase: Annual Subharmonic Steps to Chaos , 1994, Science.

[5]  Mark A. Cane,et al.  A study of self-excited oscillations of the tropical ocean-atmosphere system , 1990 .

[6]  S. Philander,et al.  El Niño Southern Oscillation phenomena , 1983, Nature.

[7]  Bin Wang,et al.  Interactions between the Seasonal Cycle and El Niño-Southern Oscillation in an Intermediate Coupled Ocean-Atmosphere Model , 1995 .

[8]  J. Bjerknes.  ATMOSPHERIC TELECONNECTIONS FROM THE EQUATORIAL PACIFIC1 , 1969 .

[9]  T. Barnett,et al.  Sea Surface Temperature, Surface Wind Divergence, and Convection over Tropical Oceans , 1987, Science.

[10]  Fei-Fei Jin,et al.  An Equatorial Ocean Recharge Paradigm for ENSO. Part I: Conceptual Model , 1997 .

[11]  Geoffrey K. Vallis,et al.  Conceptual models of El Niño and the Southern Oscillation , 1988 .

[12]  B. Goswami,et al.  Predictability of a coupled ocean-atmosphere model , 1991 .

[13]  Max J. Suarez,et al.  A Delayed Action Oscillator for ENSO , 1988 .

[14]  A. Hirst.  Unstable and Damped Equatorial Modes in Simple Coupled Ocean-Atmosphere Models , 1986 .

[15]  E. Sarachik,et al.  Understanding and predicting ENSO , 1995 .

[16]  A. E. Gill.  An Estimation of Sea-Level and Surface-Current Anomalies during the 1972 El Niño and Consequent Thermal Effects , 1983 .

[17]  Eli Tziperman,et al.  Irregularity and Locking to the Seasonal Cycle in an ENSO Prediction Model as Explained by the Quasi-Periodicity Route to Chaos , 1995 .

[18]  J. David Neelin,et al.  Modes of Interannual Tropical Ocean–Atmosphere Interaction—a Unified View. Part II: Analytical Results in the Weak-Coupling Limit , 1993 .

[19]  M. Cane,et al.  On the prediction of ENSO: a study with a low-order Markov model , 1994 .

[20]  Eli Tziperman,et al.  El Ni�o Chaos: Overlapping of Resonances Between the Seasonal Cycle and the Pacific Ocean-Atmosphere Oscillator , 1994, Science.

[21]  Mojib Latif,et al.  A review of ENSO prediction studies , 1994 .

[22]  S. Gadgil,et al.  Ocean–atmosphere coupling over monsoon regions , 1984, Nature.

[23]  Bin Wang,et al.  Interactions between the seasonal cycle and the Southern Oscillation - Frequency entrainment and chaos in a coupled ocean-atmosphere model , 1994 .

[24]  M. Blumenthal.  Predictability of a Coupled Ocean–Atmosphere Model , 1991 .

[25]  D. Battisti,et al.  Dynamics and Thermodynamics of a Warming Event in a Coupled Tropical Atmosphere–Ocean Model , 1988 .

[26]  Dake Chen,et al.  An Improved Procedure for EI Ni�o Forecasting: Implications for Predictability , 1995, Science.

[27]  J. Picaut,et al.  An Advective-Reflective Conceptual Model for the Oscillatory Nature of the ENSO , 1997 .

[28]  John M. Wallace,et al.  Large-scale atmospheric circulation features of warm and cold episodes in the tropical Pacific , 1990 .

[29]  N. Graham,et al.  The El Ni�o Cycle: A Natural Oscillator of the Pacific Ocean—Atmosphere System , 1988, Science.

[30]  United Kingdom..  Global Ocean Surface Temperature Atlas (GOSTA), and Land Surface Atlas , 1991 .