You could use the soft-edged brush to paint in the center hole to expose your destination, but I prefer the Ellipse Marquee Tool. Still in the Layer Mask, use the tool to make an ellipse in the center of your warp (you'll see how to best fit it to match the warp's waves), then feather it (Select - Feather) about 8 - 15 pixels (varies based on the resolution of your images - experiment!), and with your foreground set to Black, press Alt-Delete to fill the selection with black, exposing the center hole of your warp. You'll end up with something like the following: