I have been searching through some of the literature, as well as this forum - but I have not found a clear explanation on how exactly the B-modes in the CMB polarization pattern are caused by gravitational waves. I have seen Wayne Hu's pages ( see for instance http://background.uchicago.edu/~whu/intermediate/Polarization/polar5.html ) and the Berkeley cosmology pages ( http://cosmology.berkeley.edu/~yuki/CMBpol/CMBpol.htm ). I do think I understand the mechanism behind the generation of E-modes from density variations, vorticity and gravitational waves, but the B-modes and their 'handedness' are confusing. How would local quadrupolar patterns of effective temperature generate polarizations that have a 45-degree angle with respect to them? I have attached a diagram I made of what happens in the case of density variations, from which (irrotational) E-modes are generated. Is there a similar picture for B-modes?