La Nina occurs after an El Nino event and is part of the changing pattern of the ocean's temperature as it relates to atmospheric conditions. It occurs when the surface water temperature in the Eastern Pacific Ocean is cooler than normal and this is then accompanied by stronger than usual westbound trade winds. It produces unusual weather patterns around the world. One effect of a La Nina is warmer weather in the Southeastern United States. It can affect the Hurricanes that occur in the Atlantic Ocean.
A process known as upwelling is responsible for bring deeper cold water to the ocean surface. When this water gets to the surface it triggers the La Nina event. The ocean conveyor belt which is the direction that the ocean currents follow helps to distribute thermal energy within the ocean as well as to the atmosphere. This in turn affects climate and weather patterns globally.
During a La Nina event, the air pressure over the western Pacific ocean is lower than normal. Lower air pressure means more moisture in the atmosphere and an increased amount of precipitation. It could lead to flooding however. For example, there was a devastating flood in Queensland Australia tied to the 2010 La Nina that occurred. In other parts of the world La Nina can produce higher than normal air pressure resulting in drier than normal conditions.