The European Centre for Medium-Range Weather Forecasts (ECMWF) provides a great resource for following daily developments in the equatorial Pacific: http://www.ecmwf.int/products/forecasts/d/charts/ocean/.
I’ve made a video using images produced by the ECMWF to show how La Ninas may evolve. The images being cycled through are zonal sections of temperature anomaly in the equatorial Pacific. The images are weekly, beginning in February 2007 and ending in January 2008.
The video illustrates that at least the 2007/2008 La Nina was driven by periodic jolts of anomalous cold water that originate between 50 and 150 meters down from the ocean surface, and between 160 and 100 degrees West. This cold water takes a path upward and Eastward until it reaches the surface. The cold water has the most powerful effect when it is first forming, causing cooling on the surface far from the coast. As the cold water takes its upward and Eastward path, it weakens, and the cold surface anomaly begins to disappear. The best illustration of this begins at 53 seconds. The origin of this cold water is not the Humboldt Current pumping exceptionally cold water periodically from the Southern Ocean for the current follows too close to the coast. Instead, it is likely that the transfer of warm water back-and-forth between the East and West Pacific noted by Bob Tisdale here is responsible for the cold jolts.
Below is a Hovmoller Diagram of Sea Level in the equatorial Pacific. The Hovmoller extends over a 12-month period, covering the 2007/2008 La Nina. As you can see, the La Nina was not a continually growing event, but rather an event sustained and strengthened by the periodic intensification that would occur every couple of months. It is apparent that the rising regions of anomalous cold water shown in the video coincide with the intensified periods during the recent La Nina.
Here is one more video, with the same format as the previous video, except using daily data over the past week.
A very large amount of cold water is approaching the surface, so we may see a resurgance of La Nina conditions over the next week or two. However, no new pockets of cold water appear to be forming, and unless that changes, the La Nina conditions in the Pacific may soon fade.