Over the years I’ve seen a statement similar to the one made by MarkR in the SkepticalScience post The 2012 State of the Climate is easily misunderstood (my boldface):
Global surface temperatures were the 8th or 9th highest recorded, partly because the first two months were cool-ish thanks to a La Nina in the Pacific, where cooler waters sit on the top of the ocean and suck up heat from the atmosphere.
The error in MarkR’s statement, which has been repeated many times before, may stem from the assumption that La Niñas are the opposite of El Niños. That is, it is well known that El Niño events release enormous amounts of heat from the ocean to the atmosphere. I assume the flawed logic is that La Niña events must then remove heat from the atmosphere.
The vast majority of heat released from the ocean to the atmosphere, however, occurs through evaporation.
It’s likely those assuming that La Niñas “suck up heat from the atmosphere” are thinking only in terms of “sensible heat flux”.
SENSIBLE VERSUS LATENT HEAT
Sensible heat flux is the flow of heat from the ocean surface to the atmosphere associated only with changes in temperature; that is, it relates to the change in temperature, without a change in phase or state.
The latent heat flux is the flow of heat from the ocean surface to the atmosphere that is associated with evaporation (a change in phase).
SENSIBLE AND LATENT HEAT FLUX FROM THE EQUATORIAL AND TROPICAL PACIFIC IN PERSPECTIVE
El Niño and La Niña events are focused on the central and eastern equatorial Pacific. Figure 1 presents a time-series graph of sensible heat flux at the surface of the equatorial Pacific. It runs from January 1979 to September 2013. It’s for the NINO3.4 region, which is bordered by the coordinates of 5S-5N, 170W-120W. The illustration here shows where the NINO3.4 region is situated along the equatorial Pacific. (The sea surface temperature anomalies of the NINO3.4 region are a commonly used index for the timing, strength, and duration of El Niño and La Niña events.) The sensible heat flux values are from the NCEP/DOE Reanalysis-2. Note: always keep in mind that a reanalysis is not observations-based data; a reanalysis is the output of a computer model that includes observations-based data as inputs.
As illustrated, the sensible heat flux is primarily positive (the average is +2.7 watts/meter^2 for the period of January 1979 to September 2013), meaning the heat is flowing from ocean to atmosphere. Occasionally, there have been short periods where the sensible heat flux is negative, according to the reanalysis, and they occurred often during La Niñas, but they are not limited to La Niña events.
Now let’s compare the sensible heat flux and the latent heat flux at the surface of the NINO3.4 region. See Figure 2. For the period of January 1979 to September 2013, the average latent heat flux at the surface is about 118 watts/meter^2, or about 42 times greater than the sensible heat flux, according to the reanalysis. And regardless of the state of the tropical Pacific (El Niño, La Niña or ENSO-neutral), there is always a substantial positive latent heat flux along the eastern and central equatorial Pacific, meaning evaporation is always taking place in the NINO3.4 region of the equatorial Pacific…and it always greatly outweighs the sensible heat flux.
El Niño and La Niña events are focused on the central and eastern equatorial Pacific, but they directly impact the entire tropical Pacific. So, as a reference, Figure 3 compares sensible heat flux and the latent heat flux at the surface of the tropical Pacific (24S-24N, 120E-80W). Again, the latent heat flux dwarfs the sensible heat flux at all times. The average latent heat flux from the surface of the tropical Pacific is about 138 watts/m^2, while the average sensible heat flux is only about 9 watts/m^2.
Bottom line: the equatorial Pacific and tropical Pacific are always releasing heat to the atmosphere, even during La Niña events. Or, in other words, a La Niña does not suck heat from the atmosphere.
The NCEP/DOE Reanlysis-2 outputs are available through the NOAA NOMADS website here.