One- and two-parameter families of flows in $R^3$ near an Andronov-Hopf
bifurcation (AHB) are investigated in this work. We identify conditions on the
global vector field, which yield a rich family of multimodal orbits passing
close to a weakly unstable saddle-focus and perform a detailed asymptotic
analysis of the trajectories in the vicinity of the saddle-focus. Our analysis
covers both cases of sub- and supercritical AHB. For the supercritical case, we
find that the periodic orbits born from the AHB are bimodal when viewed in the
frame of coordinates generated by the linearization about the bifurcating
equilibrium. If the AHB is subcritical, it is accompanied by the appearance of
multimodal orbits, which consist of long series of nearly harmonic oscillations
separated by large amplitude spikes. We analyze the dependence of the
interspike intervals (which can be extremely long) on the control parameters.
In particular, we show that the interspike intervals grow logarithmically as
the boundary between regions of sub- and supercritical AHB is approached in the
parameter space. We also identify a window of complex and possibly chaotic
oscillations near the boundary between the regions of sub- and supercritical
AHB and explain the mechanism generating these oscillations. This work is
motivated by the numerical results for a finite-dimensional approximation of a
free boundary problem modeling solid fuel combustion