What is causing this color banding?

[Ellis5E19]

I'm not sure why this is happening. I am painting with a default soft round brush with pen pressure opacity. Canvas size is 2500px by 1000px, 72ppi. The image is set to 16 bit channel. I'm pretty sure I've done the most I can. There's even banding when I use the gradient tool. It could be possible I'm just ignorant and this amount of banding is normal (which would stink). Any help is appriciated 

[Barbara Ash]

Adding noise to the gradient can help with banding. Also, the more contrast there is between the colors in the gradient, the less banding. Looks like you are going from dark blue to black, colors that are close in value.