THEORETICAL CLIMATE DYNAMICS GROUP

Coupled Atmosphere-Ocean System

Project Menu Data
Assim.		 Low-Freq.
Variability		 Stats. &
Prediction		 3-D
Ocean		 Physical
Processes		 Dynamical
Systems

Atmosphere-ocean interactions are an important factor in the Earth's climate system. They take place on a wide range of spatial and temporal scales.

In the tropical Pacific, the interaction leads to the well-known El Niño / Southern Oscillation (ENSO) phenomenon, whose behavior exhibits both regular and irregular aspects. We study this highly nonlinear atmosphere-ocean system in terms of observations, theories, and models; estimate the coupling parameters; and carry out experimental forecasts.

For more details of the phenomenon, please see NOAA El Niño Theme page.

ENSO and the seasonal cycle

Fig: devil's stair case Modeling ENSO has a long history (Philander, 1990; Neelin et al., 1994). Introducing seasonal variability into the Neelin-Jin model (Jin and Neelin, 1993; Neelin and Jin, 1993) helps one understand both ENSO's regularity (locking of warm events to boreal winter) and irregularity (occurrence of major warmings every 1--10 years). Rich variability arises from the interaction of the seasonal forcing and an intrinsic ocean-atmosphere instability, present already in annually averaged models. Dynamical systems theory provides powerful tools to help understand the system's governing mechanisms and parameter dependence. In the figure, regimes of subharmonic, frequency-locked, and chaotic solutions are shown in the plane of the coupling parameter and a subsurface flow parameter that determines the model's intrinsic ENSO period. Blank areas represent regions where no interannual signal is present. Color scale represents the frequency ratio of the interannual oscillation to the annual cycle in regimes that are frequency locked; e.g., 0.22 indicates two ENSO cycles every nine years. Chaotic regimes are indicated in grey, regardless of what spectral peaks emerge from their broad spectral background figure (from Jin et al., 1996).


Forecast of Niño-3

In the September 1997 issue of the National Oceanic and Atmospheric Administration's Experimental Long-Lead Forecast Bulletin, A. Saunders M. Ghil D. Neelin used an autoregressive process using singular spectrum analysis (SSA), multi-channel SSA (M-SSA), and the maximum entropy method (MEM). The forecast is up to one year ahead, based on the last 47 years of observed data. for the forecasts of equatorial Pacific sea-surface temperature (SST) anomalies (SSTAs). More detailed information on the forecast method based on single-channel SSA combined with MEM is given by Keppenne and Ghil (1992), while multi-channel SSA (M-SSA: Kimoto et al. 1991; Keppenne and Ghil 1993; Plaut and Vautard 1994) combined with MEM is documented in the March 1995 issue of this Bulletin (see also Jiang et al. 1995). Figure 1 shows area-averaged Nino-3 SSTAs, forecast and observed, since 1990, using the SSA- and MSSA-MEM schemes for a 3-, 6-, 9- and 12-month lead.
Fig1: SST Forecast
The last forecast, for the next 1-4 seasons, using data through July 1997, is shown in Figure 2. In accordance with our former predictions, as early as December 1996 in this Bulletin, the forecast indicates a continuous warm anomaly during the next few seasons. The current warm event is expected to peak in late 1997, and begin to decay in Spring 1998. We base this overall assessment on the quarter-to-quarter tendency of our forecasts, since December 1996, rather than on the latest forecast alone.
Fig2: SST Forecast
The skill in our method is based on the low-frequency, oscillatory components of the system, and so forecasts tend to filter out short-lived from longer-term anomalies. According to the current and immediate-past predictions, the observed sharp rise of SST anomalies over the last few months does not foretell an El Nino of unprecedented magnitude; it is associated, in all likelihood, with a short-lived spike asuperimposed on an ENSO event of typical magnitude.
Figure 3 shows the SSA-MEM forecast for the SOI from August 1997 through January 1998. The SOI is expected to stay well below its mean through the middle of 1998. The anticorrelation between the present SOI and Nino-3 SSTA indicates higher predictability (Ghil and Jiang 1997), thus enhancing our confidence in the forecast.
Fig3: SST Forecast
It is a great pleasure to thank Ning Jiang for her contributions to these forecasts in the past and wish her well in her new endeavors to forecast the even less predictable financial indicators.


Further information is available in the Department of Atmospheric Sciences brochure.



TCD Members:
M. Ghil, A.W. Robertson, A. Saunders, W.M. Weibel.
Collaborator at UCLA:
J.D. Neelin.
References:
Climate Research Committee (E. J. Barron, D. S. Battisti, B. A. Boville, K. Bryan, G. F. Carrier, R. D. Cess, R. E. Davis, M. Ghil, M. M. Hall, T. R. Karl, J. T. Kiehl, D. G. Martinson, C. L. Parkinson, B. Saltzman, R. P. Turco), 1994: Global Ocean-Atmosphere- Land System (GOALS) for Predicting Seasonal-to-Interannual Climate, National Academy Press, Washington, D.C., 103 pp.
Hao, Z., and M. Ghil, 1994: Data assimilation in a simple tropical ocean model with wind-stress errors, J. Phys. Oceanogr., 24, 2111-2128.
Hao, Z., and M. Ghil, 1995: Sequential parameter estimation for a coupled ocean-atmosphere model. In Proc. 2nd WMO Int'l Symp. on Assim. of Obs. in Meteor. & Oceanogr., Tokyo, March 1995, WMO/TD-No. 651, PWPR Report Series No. 5, WMO Geneva, Switzerland, Vol. I, pp. 181-186.
Jiang, N., M. Ghil and D. Neelin, 1995: Forecasts of equatorial Pacific SST anomalies by using an autoregressive process and singular spectrum analysis. Experimental Long-Lead Forecast Bulletin, Vol. 4, No. 1, pp. 24-27, and Vol. 4, No. 2, pp. 35-36, National Meteorological Center, NOAA, U.S. Department of Commerce.
Jiang, N., C. Keppenne, M. Ghil and D. Neelin, 1995: Forecasts for tropical Pacific SST anomalies and the SOI based on singular spectrum analysis combined with the maximum entropy method. Experimental Long-Lead Forecast Bulletin, Vol. 4, No. 3, pp. 38-40. National Meteorological Center, NOAA, U.S. Department of Commerce.
Jiang, N., C. Keppenne, M. Ghil and D. Neelin, 1995: Forecasts of equatorial Pacific SST anomalies based on singular spectrum analysis combined with the maximum entropy method. Experimental Long-Lead Forecast Bulletin, Vol. 4, No. 4, 42-43. National Meteorological Center, NOAA, U.S. Department of Commerce.
Jiang, N., D. Neelin and M. Ghil, 1995: Quasi-quadrennial and quasi-biennial variability in the equatorial Pacific. Clim. Dyn., 12, 101-112.
Jiang, N., M. Ghil and D. Neelin, 1996: Forecasts of Niño 3 SST anomalies and SOI based on singular spectrum analysis combined with the maximum entropy method. Experimental Long-Lead Forecast Bulletin, Vol. 5, Nos. 2-4. National Meteorological Center, NOAA, U.S. Department of Commerce.
Jin, F.-f., and J. D. Neelin, 1993: Modes of interannual tropical ocean-atmosphere interaction - a unified view. Part I: numerical results. J. Atmos. Sci., 50, 3477-3503.
Jin, F.-f., J. D. Neelin and M. Ghil, 1994: El Niño on the Devil's Staircase: Annual subharmonic steps to chaos, Science, 264, 70-72.
Jin, F.-F., J. D. Neelin, and M. Ghil, 1996: El Niño/Southern Oscillation and the annual cycle: Subharmonic frequency-locking and aperiodicity. Physica D, 98, 442-465.
Keppenne, C. L. and M. Ghil, 1992-1995: Forecasts of the Southern Oscillation Index Using Singular Spectrum Analysis and the Maximum Entropy Method. Experimental Long-Lead Forecast Bulletin, Vol. 1, Nos. 1-4, Vol. 2, Nos. 1-4, Vol. 3, Nos. 1-4, and Vol. 4, Nos. 1 & 2, National Meteorological Center, NOAA, U.S. Department of Commerce.
Neelin, J. D. and F.-f. Jin, 1993: Modes of interannual tropical ocean-atmosphere interaction - a unified view. Part II: Analytical results in the weak-coupling limit. J. Atmos. Sci., 50, 3504-3522.
Neelin, J. D., M. Latif, and F.-f. Jin, 1994: Dynamics of coupled ocean-atmosphere models: the tropical problem. Ann. Rev. Fluid Mech., 26, 617-659.
Neelin, J. D., M. Latif, M. A. F. Allaart, M. A. Cane, U. Cubasch, W. L. Gates, P. R. Gent, M. Ghil, C. Gordon, N. C. Lau, C. R. Mechoso, G. A. Meehl, J. M. Oberhuber, S. G. H. Philander, P. S. Schopf, K. R. Sperber, A. Sterl, T. Tokioka, J. Tribbia, and S. E. Zebiak, 1992: Tropical air-sea interaction in general circulation models, Climate Dyn., 7, 73-104.
Philander, S. G. H., 1990: El Niño, La Niña and the Southern Oscillation. Academic Press, San Diego, 293 pp.
Robertson, A. W., C.-C. Ma, C. R. Mechoso, and M. Ghil, 1995a: Simulation of the Tropical-Pacific climate with a coupled ocean-atmosphere general circulation model. Part I: The seasonal cycle, J. Climate, 8, 1178-1198.
Robertson, A. W., C.-C. Ma, M. Ghil, and C. R. Mechoso, 1995b: Simulation of the Tropical-Pacific climate with a coupled ocean-atmosphere general circulation model. Part II: Interannual variability, J. Climate, 8, 1199-1216.
Unal, Y. S., and M. Ghil, 1995: Interannual and interdecadal oscillation patterns in sea level, Climate Dyn., 11, 255-278.



Sept 18, 1997