Engineering topics

Zebiak, Stephen E.

Publications and source records attributed to Zebiak, Stephen E..

5 records

NASA NTRS1997

Initialization and Predictability of a Coupled ENSO Forecast Model

The skill of a coupled ocean-atmosphere model in predicting ENSO has recently been improved using a new initialization procedure in which initial conditions are obtained from the coupled model, nudged toward observations of wind stress. The previous procedure involved direct insertion of wind stress observations, ignoring model feedback from ocean to atmosphere. The success of the new scheme is attributed to its explicit consideration of ocean-atmosphere coupling and the associated reduction of "initialization shock" and random noise. The so-called spring predictability barrier is eliminated, suggesting that such a barrier is not intrinsic to the real climate system. Initial attempts to generalize the nudging procedure to include SST were not successful; possible explanations are offered. In all experiments forecast skill is found to be much higher for the 1980s than for the 1970s and 1990s, suggesting decadal variations in predictability.

Chen, Dake↗

NASA NTRS1996

On the Role of Meridional Winds in a Coupled Model of ENSO

The role of the interannual meridional wind anomalies in a simple coupled tropical Pacific ocean-atmosphere model is investigated. It is found that these anomallies play a key role in maintaining a finite ampltude interannual variability in the coupled simulations.Observed wind stress anomalies are also analyzed, and found similar in certain respects to those of the coupled model in the near-equatorial region.

ENSO wind anomalies Pacific ocean atmosphere↗

NASA NTRS1989

Tropical Pacific climate trends since 1960

Merchant ship observations appear to indicate an increase in the strength of the surface winds in the tropical Pacific and elsewhere in recent decades. Here, trends in tropical Pacific sea surface temperature and sea level, which has repeatedly been shown to be closely related to the winds, are investigated. The results suggest that sea levels since 1960 have been rising oceanwide at about 3.5 cm/decade, while simultaneously tilting about 2 cm/decade higher in the east and lower in the west, and that surface temperatures have been rising about 0.6 C/decade. These results are not consistent with the apparent wind change; rather, they support the contention that the apparent wind changes are an artifact introduced by changes in measurement technique, and suggest that tropical Pacific winds may have actually decreased in strength.

Posmentier, Eric S.↗

NASA NTRS1988

A model of the tropical Pacific sea surface temperature climatology

A model for the climatological mean sea surface temperature (SST) of the tropical Pacific Ocean is developed. The upper ocean response is computed using a time dependent, linear, reduced gravity model, with the addition of a constant depth frictional surface layer. The full three-dimensional temperature equation and a surface heat flux parameterization that requires specification of only wind speed and total cloud cover are used to evaluate the SST. Specification of atmospheric parameters, such as air temperature and humidity, over which the ocean has direct influence, is avoided. The model simulates the major features of the observed tropical Pacific SST. The seasonal evolution of these features is generally captured by the model. Analysis of the results demonstrates the control the ocean has over the surface heat flux from ocean to atmosphere and the crucial role that dynamics play in determining the mean SST in the equatorial Pacific. The sensitivity of the model to perturbations in the surface heat flux, cloud cover specification, diffusivity, and mixed layer depth is discussed.

Seager, Richard↗

NASA NTRS1987

A model El Nino-Southern Oscillation

A coupled atmosphere-ocean model is developed and used to study the ENSO (El Nino/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 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 timescale. A critical element of the model oscillation 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 non-El Nino and El Nino states is presented. Implications of the model results for the prediction of El Nino events are discussed.

Zebiak, Stephen E.↗