Abstract <p>The description of nonlinear characteristics of sea surface waves by models based on the Stokes expansion is discussed. The limitations of existing models are noted, which do not allow describing the variety of waveforms present on the sea surface. The necessity of introducing an additional parameter is justified. A modified second-order nonlinear model is proposed that allows obtaining negative values of skewness, which are observed in field experiments but are not described by existing models. The proposed model also allows obtaining horizontal asymmetry of waves.</p>