ESSAY: Model interpretation of climate signals: an application to the Asian monsoon climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 294

Table 9.1 Possible experimental designs for climate sensitivity experiments.
The letters C and O denote climatology and observation respectively.
A check mark indicates an interactive component.

Atmosphere Ocean Land

Exp-I (control) [ C [

Exp-II [ O [

Exp-III [ O C

Exp-IV [ C C

Exp-V [ [ [

forcings external to the climate system, e.g. annual cycle of solar radiation.
Here, deviations from the climatology are due either to internal dynamics
within the atmosphere or its interaction with the land surface. In the control,
the model can be integrated for typically 50 simulated years to obtain a stable
climatology. In Exp-II, the model is run under identical conditions as in the
control, except that the SST is obtained from actual observations that cover a
50-year period, which include several major El Ni˜os. The anomalies in
Exp-II will then be computed with respect to the control. The impacts of
El Ni˜o SST forcings on global climate can be estimated from the anomaly
fields of rainfall, temperature, and wind, and from comparison with the actual
observations. It is possible that some of the regional impacts over land may be
due to land processes feedback induced by the SST. To estimate the effect of
land-atmosphere interactions, results from Exp-III and Exp-I need to be
analyzed. If the interest is in isolating the natural variability generated by
land-atmosphere interactions alone, without SST anomaly forcings, Exp-IV
should be compared to Exp-I. Finally, comparing the fully interactive run
Exp-V with Exp-II will provide insight on the role of coupled ocean-
atmosphere processes in producing the model climate anomalies.
An example of a set of experiments to show the effect of ocean forcings vs.
land-atmosphere interactions on the generation of the Southern Oscillation
(SO) is shown in Figure 9.3 (Lau and Bua 1998 ). The SO (see Section 2.8 )is
known to have strong impacts on the Asian monsoon climate anomalies.
Comparing Exp-II (ALO, in Figure 9.3a ) with Exp-I (AL in Figure 9.3c ), it
can be seen that the eastern portion of the see-saw is missing in the latter when
the anomalous SST forcing is withheld. This suggests that the SO arises
primarily from anomalous SST forcing. The similarity in the SO in Exp-III
(AO in Figure 9.3b ) with Figure 9.3a implies that land-atmosphere interac-
tion is not important in generating the SO, but may have some impact on the
signal at higher latitudes. Finally, the negative anomalies over the extratro-
pical North Pacific and the North Atlantic in Exp-III (Figure 9.3c ) and