Having comfortably won the home series over the summer, the tourists trail Australia 2-0 Down Under following a 218-run defeat at the Adelaide Oval on Monday.
"When we win we quick to take a lot of plaudits. So when we lose, we gotta accept the criticism & I'm totally cool with that! #wewillbeback," Pietersen wrote on Twitter.
Australia can reclaim the Ashes and end a run of three straight series losses to England with another win in the third Test, which gets underway at the WACA in Perth on December 13.