arXiv subjects

Gerrit Burgers

Publications and source records attributed to Gerrit Burgers.

5 recordsLinked to original sources

arXiv2006

Predictability of El Niño as a Nonlinear Stochastic Limit Cycle

The El Niño phenomenon, synonymously El Niño-Southern Oscillation (ENSO), is an anomalous climatic oscillation in the Equatorial Pacific that occurs once every 3-8 years. It affects the earth's climate on a global scale. Whether it is a cyclic or a sporadic event, or whether its apparently random behaviour can be explained by stochastic dynamics have remained matters of debate. Herein ENSO is viewed, unconventionally, as a two-dimensional dynamical system on a desktop. The main features of ENSO: irregularity, interannual variability, and the asymmetry between El Niño and La Niña are captured, simply, comprehensibly, quickly and cheaply, in a nonlinear stochastic limit cycle paradigm. Its predictability for ENSO compares remarkably well with that of the best state-of-the-art complex models from the European and American meteorological centres. Additionally, for the first time, by analyzing subsurface Equatorial Pacific data since 1960, this model finds that long-term variations are not caused by ENSO itself, but by external sources.

nlin.CD↗

arXiv1998

On the El-Nino Teleconnection to Spring Precipitation in Europe

In a statistical analysis of more than a century of data we find a strong connection between strong warm El Nino winter events and high spring precipitation in a band from Southern England eastwards into Asia. This relationship is an extension of the connection mentioned by Kiladis and Diaz (1989), and much stronger than the winter season teleconnection that has been the subject of other studies. Linear correlation coefficients between DJF NINO3 indices and MAM precipitation are higher than r=0.3 for individual stations, and as high as r=0.49 for an index of precipitation anomalies around 50N from 5W to 35E. The lagged correlation suggests that south-east Asian surface temperature anomalies may act as intermediate variables.

physics.ao-ph↗

arXiv1998

Tracking down the ENSO delayed oscillator with an adjoint OGCM

The adjoint of an ocean general circulation model is used as a tool for investigating the causes of changes in ENSO SST indices. We identify adjoint Kelvin and Rossby waves in the sensitivities to sea level and wind stress at earlier times, which can be traced back for more than a year through western and weak eastern boundary reflections. Depending on the thermocline depth the first and second baroclinic modes are excited. The sensitivities to the heat flux and SST are local and decay in about a month. The sensitivities to the fluxes are converted into the influence of SST using the adjoint of a statistical atmosphere model. Focusing on SST perturbations in the index region itself, we recover, up to a scale factor, the delayed oscillator concept.

physics.ao-ph↗

arXiv1997

The El Nino Stochastic Oscillator

Anomalies during an El Nino are dominated by a single, irregularly oscillating, mode. Equatorial dynamics has been linked to delayed-oscillator models of this mode. Usually, the El Nino mode is regarded as an unstable mode of the coupled atmosphere system and the irregularity is attributed to noise and possibly chaos. Here a variation on the delayed oscillator is explored. In this stochastic-oscillator view, El Nino is a stable mode excited by noise. It is shown that the autocorrelation function of the observed NINO3.4 index is that of a stochastic oscillator, within the measurement uncertainty. Decadal variations as would occur in a stochastic oscillator are shown to be comparable to those observed, only the increase in the long-term mean around 1980 is rather large. The observed dependence of the seasonal cycle on the variance and the correlation is so large that it can not be attributed to the natural variability of a stationary stochastic oscillator. So the El Niño stochastic-oscillator parameters must depend on the season. A forecast model based on the stochastic oscillator with a variance that depends on the season has a skill that approaches that of more comprehensive statistical models: over the period 1982-1993, the anomaly correlation is 0.65 for two-season lead forecasts.

physics.ao-ph↗

arXiv1996

Comment on Enhanced TKE Dissipation under Breaking Waves

It is noted that the results of recent experiments on the enhancement of turbulent kinetic energy (TKE) dissipation below surface waves can be stated as follows. TKE dissipation is enhanced by a factor $15 H_{ws}/z$ at depths $0.5 H_{ws} < z < 20 H_{ws}$ with respect to the wall-layer result $ε= u_{*w}^3/κz$, where $u_{*w}$ is the friction velocity in water and $H_{ws}$ is the significant wind-sea wave height. For open ocean conditions, this reduces in most cases to an enhancement factor $10^6 u_{*w}^2/gz \approx U_{10}^2/gz$.

physics.ao-ph↗