Paul S. Schopf

A Delayed Action Oscillator for ENSO

Abstract A simple nonlinear model is proposed for the El Nino/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.

Vacillations in a Coupled Ocean–Atmosphere Model

Abstract Results are presented from a 35-year integration of a coupled ocean-atmosphere model. Both ocean and atmosphere are two-level, nonlinear primitive equations models. The global atmospheric model is forced by a steady, zonally symmetric Newtonian heating. The ocean model is solved in a rectangular tropical basin. Heat fluxes between ocean and atmosphere are linear in air-sea temperature differences, and the interfacial stress is proportional to lower-level atmospheric winds. The coupled models produce ENSO-like variability on time scales of 3 to 5 years. Since there is no external time-dependent forcing, these are self-sustained vacillations of the nonlinear system. It is argued that the energetics of the vacillations is that of unstable coupled modes and that the time scale is crucially dependent on the effects of ocean waves propagating in a closed basin.

The Seasonal Cycle over the Tropical Pacific in Coupled Ocean–Atmosphere General Circulation Models

Abstract The seasonal cycle over the tropical Pacific simulated by 11 coupled ocean–atmosphere general circulation models (GCMs) is examined. Each model consists of a high-resolution ocean GCM of either the tropical Pacific or near-global means coupled to a moderate- or high-resolution atmospheric GCM, without the use of flux correction. The seasonal behavior of sea surface temperature (SST) and eastern Pacific rainfall is presented for each model. The results show that current state-of-the-art coupled GCMs share important successes and troublesome systematic errors. All 11 models are able to simulate the mean zonal gradient in SST at the equator over the central Pacific. The simulated equatorial cold tongue generally tends to be too strong, too narrow, and extend too far west. SSTs are generally too warm in a broad region west of Peru and in a band near 10°S. This is accompanied in some models by a double intertropical convergence zone (ITCZ) straddling the equator over the eastern Pacific, and in others...

Observations of Coupling between Surface Wind Stress and Sea Surface Temperature in the Eastern Tropical Pacific

Satellite measurements of surface wind stress from the QuikSCAT scatterometer and sea surface temperature (SST) from the Tropical Rainfall Measuring Mission Microwave Imager are analyzed for the three-month period 21 July‐20 October 1999 to investigate ocean‐atmosphere coupling in the eastern tropical Pacific. Oceanic tropical instability waves (TIWs) with periods of 20‐40 days and wavelengths of 1000‐2000 km perturb the SST fronts that bracket both sides of the equatorial cold tongue, which is centered near 1 8S to the east of 1308W. These perturbations are characterized by cusp-shaped features that propagate systematically westward on both sides of the equator. The space‐time structures of these SST perturbations are reproduced with remarkable detail in the surface wind stress field. The wind stress divergence is shown to be linearly related to the downwind component of the SST gradient with a response on the south side of the cold tongue that is about twice that on the north side. The wind stress curl is linearly related to the crosswind component of the SST gradient with a response that is approximately half that of the wind stress divergence response to the downwind SST gradient. The perturbed SST and wind stress fields propagate synchronously westward with the TIWs. This close coupling between SST and wind stress supports the Wallace et al. hypothesis that surface winds vary in response to SST modification of atmospheric boundary layer stability.

Decadal Variability in ENSO Predictability and Prediction

Abstract A simple coupled model is used to examine decadal variations in El Nino–Southern Oscillation (ENSO) prediction skill and predictability. Without any external forcing, the coupled model produces regular ENSO-like variability with a 5-yr period. Superimposed on the 5-yr oscillation is a relatively weak decadal amplitude modulation with a 20-yr period. External uncoupled atmospheric “weather noise” that is determined from observations is introduced into the coupled model. Including the weather noise leads to irregularity in the ENSO events, shifts the dominant period to 4 yr, and amplifies the decadal signal. The decadal signal results without any external prescribed changes to the mean climate of the model. Using the coupled simulation with weather noise as initial conditions and for verification, a large ensemble of prediction experiments were made. The forecast skill and predictability were examined and shown to have a strong decadal dependence. During decades when the amplitude of the interannua...

Tropical air-sea interaction in general circulation models

An intercomparison is undertaken of the tropical behavior of 17 coupled ocean-atmosphere models in which at least one component may be termed a general circulation model (GCM). The aim is to provide a taxonomy—a description and rough classification—of behavior across the ensemble of models, focusing on interannual variability. The temporal behavior of the sea surface temperature (SST) field along the equator is presented for each model, SST being chosen as the primary variable for intercomparison due to its crucial role in mediating the coupling and because it is a sensitive indicator of climate drift. A wide variety of possible types of behavior are noted among the models. Models with substantial interannual tropical variability may be roughly classified into cases with propagating SST anomalies and cases in which the SST anomalies develop in place. A number of the models also exhibit significant drift with respect to SST climatology. However, there is not a clear relationship between climate drift and the presence or absence of interannual oscillations. In several cases, the mode of climate drift within the tropical Pacific appears to involve coupled feedback mechanisms similar to those responsible for El Niño variability. Implications for coupled-model development and for climate prediction on seasonal to interannual time scales are discussed. Overall, the results indicate considerable sensitivity of the tropical coupled ocean-atmosphere system and suggest that the simulation of the warm-pool/cold-tongue configuration in the equatorial Pacific represents a challenging test for climate model parameterizations.

On Equatorial Dynamics, Mixed Layer Physics and Sea Surface Temperature

Abstract We describe a new numerical model designed to study the interactions between hydrodynamics and thermodynamics in the upper ocean. The model incorporates both primitive equation dynamics and a parameterization of mixed layer physics. There is a consistent treatment of mixed layer structure for all physical processes. In order to study interplay between dynamics and mixed layer physics in the equatorial ocean, we carried out a series of numerical experiments with simple patterns of wind stress and surface heating. In some cases stratification and/or mixed layer physics were suppressed. On the basis of these experiments we reached the following conclusions: The vertical circulation at the equator is so vigorous that surface heating is essential if stratification is to be maintained for periods longer than a few months. Without stratification to inhibit mixed layer deepening momentum will be mixed uniformly to the main thermocline and the equatorial undercurrent will disappear. Vertical transfers of ...

A Reduced-Gravity Isopycnal Ocean Model: Hindcasts of El Niño

Abstract A global isopycnal ocean model is presented for the study of interannual to interdecadal variability in the global ocean. The model treats the primitive equations on a sphere with a generalized vertical coordinate. This coordinate is designed to represent a turbulent well-mixed surface layer and nearly isopycnal deeper layers. Disappearing isopycnics are treated through the quasi-isopycnal technique, in which the coordinate separates from the isopycnic in order to maintain a minimum layer thickness. A reduced gravity treatment is made, with the deepest interface at a mean depth of 2300 m. Coastal topography is represented, but the reduced gravity treatment precludes the use of variable bottom depth. The model is used for hindcast studies of El Nino during the decade from 1982 through 1991 using a combination of climatological wind forcing and wind anomalies derived from various sources. In order to carry out the hindcast experiments, a technique is developed for constructing a mean climatological...

Kelvin-Wave-Induced Anomalous Advection and the Onset of Surface Warming in El Niño Events

Abstract The initial surface warming of the 1982 El Nino event was of quite different timing and pattern from that associated with most El Nino events; strong anomalous warming occurred first in July along the equator and subsequently along the South American coast. We show here that a simple advective model for tropical ocean surface warming can produce anomalous sea-surface temperature (SST) fields like those found in the first few months of the 1982 El Nino. The model physics assumes that the existing SST field is advected by anomalous currents to produce the anomalous warming, and that the anomalous currents are those induced subsequent to the passage of downwelling Kelvin wave front(s). With the initial SST field taken to be that of July 1982, the anomalous eastward currents of the model lead to a satisfactory prediction of the evolution of anomalous SST for several months. Numerical experiments with a fully nonlinear and thermally active ocean model support the physical relevance of the more idealiz...

A Simple Mechanism for ENSO Residuals and Asymmetry

Abstract A simple mechanism is offered that accounts for a change in the long-term (decadal scale) mean of ocean temperatures as the El Nino–Southern Oscillation (ENSO) amplitude changes. It is intended as an illustration of a kinematic effect of oscillating a nonlinear temperature profile with finite-amplitude excursions that will cause the Eulerian time mean temperature to rise (fall) where the curvature of the temperature is positive (negative) as the amplitude of the oscillations increases. This mechanism is found to be able to mimic observed changes in the mean sea surface temperatures in the Pacific between the 1920s, 1960s, and 1990s due to the changing ENSO amplitude. The effects alter both the calculated mean surface temperatures and the time mean temperatures at depth. It also results in a skewness of the temperature distribution that shares many properties with the observed SST. In this model, the time-local gradients of temperature never change if referenced to a single isotherm (i.e., the Lag...