I'm going to diverge from the others and call this a software issue. Lower back upright, arms dead straight, hands not even reaching the hoods.
I caught a friend riding like that once and asked her why. "I'm copying you!", she said. Oh.

I always suffered from low back pain, especially after tough races or endless climbs. I found that my back would hunch right up when trying to find more power, even if I was in the drops. My lower back was almost vertical, just like yours.
In recent years I've been focussing more on form. Flattening out the back and trying to draw my navel towards the saddle. Sticking the butt out. The pelvic tilt requires more hamstring flexibility, but stretching and yoga help with that. Comfort on the saddle is critical too. No guy is going want to tilt forward if the 'boys' are going to get mashed. Look at saddle angle and shorts. I find that good bibs can help me flatten my back right out due to their effective management of aforementioned items.

Once you get that organized, and you find you are riding with deeply flexed arms, then you can start looking seriously at optimizing stem length and position.