The cosmological fluid equations are used to study the nonlinear mode coupling of density fluctuations .
We find that for realistic cosmological spectra there is a significant contribution to the nonlinear evolution on scales of interest to large-scale structure from the long-wave part of the initial spectrum .
A consequence of this mode coupling is that at high redshift , z , the nonlinear scale [ defined by \sigma ( z ) = 1 ] can be significantly larger than a linear extrapolation would indicate .
For the standard CDM spectrum with a \sigma _ { 8 } = 1 normalization the mass corresponding to the nonlinear scale at z = 20 , 10 , 5 is about 100 , 10 , 3 times ( respectively ) larger than the linearly extrapolated value .
We also investigate the possibility of divergent contributions to the density field from long-wave modes if the spectral index of the power spectrum n < -1 .
Using an approximate non-perturbative approach we find that for n > -3 the divergent contribution appears only in the phase .
This can be related to the large-scale bulk velocity , and clarifies previous results from N-body simulations .