SST

SST

Difference between the SST in observations and (top) 2° run and (bottom) 0.5° run of CCSM4 for 1990-1999

Another way we can test climate models is to look at the spatial distribution of bias in the mean model fields. These plots show the bias in simulated sea surface temperatures from a fully coupled model, the Community Climate System Model compared to the observed SST climatology. The top plot is for a coupled system where the atmospheric model is run at a nominally 2 resolution, about two hundred kilometers. The bottom plot is for the same system, but we have increased the atmospheric model resolution to 0.5, including the winds forcing the ocean model. The dynamics and physical parameterizations remain unchanged.

Where do we see the largest reduction in SST bias when resolution is improved? (Choose the best answer.)

The correct answer is (d) West coasts of continents.

The large warm biases that existed off the west coasts of North America, South America, and Africa dramatically decreased when the resolution was increased.

Why?

Higher horizontal resolution increased temperature gradients, which increased the strength of winds in these regions. This increased upwelling of cold water to the surface, reducing the warm temperature bias.

Note the large cold bias in the North Atlantic. It's still there even in the 0.5 model.

Why?

Because it appears that this bias isn't sensitive to the resolution of the atmospheric model, it is likely due to ocean processes. Preliminary experiments have shown that a much higher resolution ocean model would allow the ocean model to simulate a turnaround of the Gulf Stream as it approaches Greenland. With this turn, the Gulf Stream warms these regions, eliminating the cold bias.