La Nina in progress

Continuing a trend that began earlier in the year, La Niña conditions strengthened through the summer of 2010, evidenced by a streak of cool water across the equatorial Pacific Ocean.
Acquired by the Ocean Surface Topography Mission/Jason-2 satellite, this map shows a 10-day average of sea-surface height centered on September 6, 2010. Because water expands with rising temperatures, satellites can use sea-surface height as a proxy for temperature. Areas where the water surface is higher (and therefore warmer) than average are shades of red-brown, and areas where the water surface is lower (cooler) than average are blue. Normal conditions appear in white.
The El Niño weakens the westward trade winds that normally blow over the eastern tropical Pacific Ocean. Those winds keep eastern Pacific waters cool and concentrate warm waters in the western Pacific. A weakening of trade winds enables warm waters to gradually spread eastward, heating up the central Pacific. La Niña typically follows El Niño, and causes essentially the opposite conditions. La Niña strengthens the trade winds, spreading cool water from the South American coast to the central Pacific. This see-saw pattern of El Niño and La Niña can drive large-scale weather changes, especially in the tropics.
This map reveals a broad swath of cool water stretching from South America to New Guinea. The ocean is not, however, uniformly cool. Pockets of warm water are mixed with the cool, particularly in the western Pacific. Warmer waters in this region can lead to increased rainfall, and La Niña conditions may have played a role in the devastating floods in Pakistan during the Northern Hemisphere summer of 2010.
Over the eastern Pacific Ocean, cooler waters lead to less moisture along the coasts of North and South America. So as more rain pounds some parts of the globe, La Niña conditions can deepen drought in others. By the time La Niña conditions intensified in September 2010, the southwestern United States had experienced more than 10 years of mostly dry conditions. A release from NASA’s Jet Propulsion Laboratory reported that the American Southwest might experience not only drier conditions, but also intensified wildfires under strong La Niña conditions.
OSTM/Jason-2 is a joint oceanographic mission of NASA and the Centre National d’Études Spatiales (CNES), France’s space agency.