[Vision2020] AGU: Developing La Nina Cools the Planet in September

[Paul Rumelhart]

This reminds me of a topic I've wanted to bring up for discussion.  From 
my understanding, La Nina events are caused by an upwelling of colder 
waters in the equatorial Pacific that then go on to have large impacts 
on weather globally.  Here is a description from an FAQ at the NOAA 
(http://www.elnino.noaa.gov/lanina_new_faq.html):

(begin quote)

# What causes La Niña?*
# Typically, a La Niña is preceded by a buildup of cooler-than-normal 
subsurface waters in the tropical Pacific. Eastward-moving atmospheric 
and oceanic waves help bring the cold water to the surface through a 
complex series of events still being studied. In time, the easterly 
trade winds strengthen, cold upwelling off Peru and Ecuador intensifies, 
and sea-surface temperatures (SSTs) drop below normal. During the 1988- 
89 La Niña, SSTs fell to as much as 4 degrees C (7 degrees F) below 
normal. Both La Niña and El Niño tend to peak during the Northern 
Hemisphere winter.

(end quote)

So, the colder-than-normal sea surface temperatures are caused by 
deeper, colder water being brought to the surface.  This cools down the 
sea surface temperatures and affects the trade winds and general 
circulation patterns.  It also, apparently, has the effect of cooling 
down the average global lower atmospheric temperatures.  But where does 
this heat go?

Presumably, the heat is transferred to the colder water that has been 
brought to the surface, leaving the overall heat content of the oceans 
slightly higher than it was before.  Conversely, the warmer-than-normal 
sea surface temperatures that raise the global atmospheric surface 
temperatures during an El Nino event would have the effect of cooling 
the overall heat content of the oceans to be slightly lower than it 
was.  Since the heat capacity of the oceans far exceeds that of the 
atmosphere, the Earth itself presumably cools slightly.

So we're left with the odd case that the warmest years occur when the 
overall heat content of the Earth decreases, and the cooler years (at 
least relatively) occur when the overall heat content increases.

This leads me to wonder if a globally-averaged surface temperature is 
really the best metric for determining if the globe is warming or cooling.

Any thoughts?  I don't know what the best way to measure this is, though 
I suspect it would involve satellite measurements of the overall energy 
gained from and lost to space.  There is also the added factor of what 
cloud cover does in each case as this affects albedo.

Just something I've been kicking around trying to get a handle on.

Paul

On 10/23/2011 07:01 PM, Ted Moffett wrote:
> http://blogs.agu.org/wildwildscience/2011/10/17/developing-la-nina-cools-the-planet-in-september/
>
> http://blogs.agu.org/wildwildscience/files/2011/10/201109sigclim.gif
>
> 17 October 2011
>
> Developing La Nina Cools The Planet in September
>
> Dan Satterfield
>
> The developing La Nina in the Pacific has brought the global
> temperature in September down slightly, but it was still the 8th
> warmest on record. NASA does a separate calculation and ranks Sep.
> 2011 as 9th warmest. When a La Nina develops the world’s largest ocean
> sees noticeably cooler surface waters.  This chills the air enough to
> bring the global temperatures down, while El Nino does  just the
> opposite. The super El Nino in 1998 still ranks as one of the hottest
> years recorded on the planet since reliable temp. records began.
>
> Keep in mind that this is a natural fluctuation on top of the steadily
> increasing forcing from greenhouse gases, so in spite of the La Nina
> cooling the planet some, this past September was still ranked as the
> 8th warmest on record. Some more on the global temps from NCDC here:
>
> The January – September worldwide land surface temperature was 0.80°C
> (1.44°F) above the 20th century average — the 7th warmest such period
> on record. The margin of error is +/- 0.20°C (0.36°F). The global
> ocean surface temperature for the year to date was 0.41°C (0.74°F)
> above the 20th century average and was the 12th warmest such period on
> record. The margin of error is +/- 0.04°C (0.07°F).
>
> The combined global land and ocean average surface temperature for
> September 2011 was the eighth warmest on record at 15.53°C (59.95°F),
> which is (0.53°C) 0.95°F above the 20th century average of 15.0°C
> (59.0°F). The margin of error associated with this temperature is +/-
> 0.11°C (0.20°F).
> Separately, the global land surface temperature was 0.87°C (1.57°F)
> above the 20th century average of 12.0°C (53.6°F), making this the
> fourth warmest September on record. The margin of error is +/- 0.24°C
> (0.43°F).
> ------------------------------------------
> Vision2020 Post: Ted Moffett
>
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