Voila!! It's bubble motion~
If you're familiar with 3DS Max...
yes, I was inspired by the bubble motion feature of its particle system.
Here's a list of related parameters for you to play with:
(you can toy with them in the example source file)
bubbleMotion: Boolean - whether to use bubble motion
bubbleAmplitude: Number - the amplitude of bubble motion
bubbleAmplitudeVar: Number - amplitude variation
bubbleFrequency: Number - the frequency of bubble motion
bubbleFrequencyVar: Number - frequency variation