Cold Equator Model
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[edit] General
Some models known as Cold Equator Models develop unrealistic climates by having cold spots near the Equator.
Here are some pictures:
http://cpdn.tuxie.org/uk_nick/Emma_boinc_24kf.png
http://cpdn.tuxie.org/uk_nick/Emma_temps_24kf.png
The usual place for this effect is in the Pacific off Ecuador. However as shown in the pictures above, other small spots can develop as well.
[edit] Why?
Thanks to William Ingram from the Met Office/ Oxford University for this explanation..
The short answer is that if unrealistic amounts of heat go into an ocean grid-point of the model, it would move upwards & be spread out ("hot air rises") while unrealistic cooling can be trapped.
In more detail:
In the real world, the tropical ocean along some continental coasts & on the Equator in the Pacific is cooled at the surface by upwelling of colder water. Since the 1st experiment (see strategy page) of climateprediction.net does not use a full ocean model, the ocean heating calculated in the first, calibration phase and kept fixed afterwards will represent this process by a cooling (in the calibration phase, the model calculates what flux of heat in/ out of the ocean would be necessary to keep the model at a given, fixed, temperature - in the 2nd and 3rd phases the ocean temperature is allowed to vary, but the flux of heat which was calculated in the first phase is applied) But there is a destabilizing process (a "positive feedback"): the colder the sea surface, the colder the air above it, the less it mixes up & away, the more the moisture from the surface builds up and forms low cloud, the more sunlight is reflected away, & the colder the sea surface. This occurs to an extent in the real world & probably all the climateprediction.net models.
But some models have much too little low cloud to start with, & so have excessive ocean fluxes of heat out of the ocean in the calibration phase to compensate for the excessive solar heating - if at some time when running with interactive sea surface temperatures (2nd & 3rd phases) they then develop a lot of low cloud at such a grid-point, the combined effect of the low cloud and the prescribed cooling may be to cool so strongly that the "positive feedback" prevents it ever warming up again (In the real world, or with a full ocean model, the cooling due to upwelling would be reduced as the surface temperature approached the temperature of the deeper water being upwelled - a stabilizing process or "negative feedback". But this cannot occur in the model set-up we use, because we prescribe a constant cooling, for simplicity and to maintain conservation of energy, a fundamental law of physics.) Sometimes just one grid-point is affected & the rest of the model runs on sensibly, but sometimes so large an area is affected that the model's global-mean temperature falls to absurd levels.
[edit] How frequent are they?
UK_Nick calculated about 12% from a fairly small sample (something like 10 out of 84 models). The figure really depends on how major an effect there has to be before the model is counted as having a cold equator. The nature paper rejected some 43% of models for failing to meet quality control tests, however it is known that some of these 43% were rejected because the control Phase temperature which should stay more or less constant just drifted too much without there being a cold equator effect.
[edit] Do certain Parameters cause these effect?
Some Parameters make them more likely. However it is impossible to predict precisely whether cold equator effects will develop with a particular Parameter set. It is thought that Parameter sets can make models more susceptible to the effect but whether it develops depends on whether suitable weather conditions occur. For many models it is only a matter of time, others may get though a 15 year Phase without the effect. The parameters that make the effect more likely are those that tend to cause more variability in the weather. Therefore it is not suprising that some of the parameter combinations that can cause high sensitivity models tend to be quite similar to the parameter combinations that cause these cold equators.
[edit] Does this mean that some parameter combinations that cause these models can be ruled out as unrealistic when the transient coupled models are started?
No. It is believed that the effect is due to the limitations of a Slab Ocean so it is quite possible that the parameter combination will lead to a good Coupled Model. Therefore the parameters combinations have not been ruled out.
