What is La Nina, which could lead to dip in temperature in North India in Jan-Feb?

La Nina, due to which temperature in North India is expected to fall to 3°C in January and February, is a phenomenon that is characterised by unusually cold ocean temperatures in the Equatorial Pacific. It forms when equatorial trade winds strengthen to bring colder, deep water up from the bottom of the sea, leading to below-normal temperatures in northern hemisphere.