Someone called?I would have thought that when it's colder your car will have better performance as its getting cold air through the intake.
One for the prof I believe.
OK... I'm a petrol head, not diesel head, but here's my view...
I petrol cars, the colder the air, the more dense, which boosts compression by a few percent. Cold air frequently has a high moisture content (at least here in the UK) which is incompressible, also boosting compression.
However, in a diesel, I would expect the following:
Diesels run much higher compression ratios (15 to 25:1), so the additional air density should make a difference, but the extra damp and cold will affect the diesel timing (something that can't be controlled precisely) and almost certainly alleviate this.
Diesel fuel also needs to be injected a ludicrously high pressures >1,000 bar. at lower temperatures, the fuel starts to become more viscous and I expect injector performance will decrease and acheivable rail pressures to be reduced. This I expect will lead to the reduced performance you are seeing.
...just an educated guess though!