The analytical expression for the spectral intensity of chirped Gaussian pulsed beam diffracted by a circular aperture propagating through the oceanic turbulence is derived based on the extended Huygens-Fresnel principle, and the spectral behaviors are analyzed in detail for both cases with aperture and without one. The results obtained show that there is a great difference between the spectra with aperture and without one, spectral switch occurs according to the size of aperture or pulse duration, which can be applied for information encoding. It is also found that the aperture radius and the pulse duration are important controllable parameters which can be used in the practice for spectral diversity technique and information encoding, and the corresponding spectral changes occur according to the propagation distance of pulsed beam as a result of competition between source and turbulence correlation. The results of this work can have potential applications in underwater laser communication and imaging, in conditions where spectral diversity technique and information encoding are used.