La Niña is characterized by unusually cold ocean temperatures in the Equatorial Pacific, compared to El Niño, which is characterized by unusually warm ocean temperatures in the Equatorial Pacific.
From my amateur point of view, it sounds like the Pacific Ocean temperature is constantly either in, or moving toward, a warm-water high (El Niño) or a cold-water low (La Niña)...which makes me wonder what all of the fuss is about. I mean, isn't that, like, nature? (Joe?)
It's hard to characterize the effects of either pattern as a whole, but here's the upshot for NYC: in North America, La Niña doesn't directly affect us, but El Niño tends to make our weather wetter and cooler in the Winter. Maybe that's what makes it confusing--El Niño is a warming pattern, yet, while the rest of the world warms, we manage to keep cooling off.
Start scheduling your February phototherapy treatments now, people!