Odd Behavior

I have a cat about 10-12 years old that, since our 20 year old cat died last November, has started meowing constantly (especially when he wants water and to be fed) and has started jumping on the kitchen table, when he never used to do so.

I should note our cat that died started to yowl in a similar way because she became deaf. Is this is a late type of mourning or old age? Any ideas appreciated.

It could be grief, but because of your cat's age, I would encourage you to have his thyroid levels checked. Hyperthyroidism often causes yowling in older cats.

It could very well be grief. Perhaps he's looking for his buddy, and is distressed that he can't find him.
I would, however, take your kitty to the vet and have them do a full senior panel to rule out any physiological problems.

I agree, grief plays a huge factor in cats psychology. When my friend's long-time cat died, Rosey had owned her for most of her adult life, and when my friend got married, Rosey shared her with Pumpkin (her hubby's cat). When Rosey died last year , Pumpkin all of a sudden looked "haunted" -- he would be standing by himself in a corner, then all of a sudden JUMP! He was sad for a few months afterwards as well. My friend was CONVINCED he saw Rosey's ghost.

My dad lived with my grandmother and his cat till she passed away several years ago. Dad's cat Terrance was moping around the house. He didn't get along with grandma, but he sure did notice when she wasn't there.

That said, the yowling could be in response to his grief. Since he is 10 y/o though, you should take him for a senior as others have suggested.