Predictability and Diagnosis of Low-Frequency Climate Processes in the Pacific Page: 3 of 13
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these issues in a detailed examination of the simulations of PCM and HadCM3. In our model-observed
comparisons we sampled the models at the same locations as the gridded yearly observed data, a key step
that makes possible a direct comparison of the warming of the model oceans and the real oceans. When
this is done correctly, we find that in the top 100 m of the water column the ocean warming is well
separated from natural variability, both internal and that arising from solar and volcanic fluctuations.
Between 125 and 200 m the agreement between the model and observed warming is not statistically
significant, a finding that should be studied further, and suggests the importance of correctly accounting
for thermocline variability in the comparison. The agreement between model and observed warming then
increases below 200 m and remains significant down to 600 m. Analysis of the model's heat budget
indicate that the warming is driven by an increase in net surface heat flux that reaches 0.7 W/m2 by the
1990s; the downward longwave flux increases by 3.7 W/m2, which is not fully compensated by an
increase in the upward longwave flux of 2.2 W/m2. Latent and net solar heat flux each decrease by about
0.6 W/m2. An interesting finding was that the changes in the individual longwave components are
distinguishable from the pre-industrial mean by the 1920s, but due to cancellation of components,
changes in the net surface heat flux do not become well separated from zero until the 1960s. Changes in
ocean advection also play an important role in local warming, depending on the location. The observed
sampling of the ocean temperature is highly variable in space and time, but sufficient to detect the
anthropogenic warming signal in all basins, at least in the surface layers, by the 1980s. We also examined
whether the likely biased seasonal sampling (i.e., more ocean observations in local summer than in local
winter) is enough to affect our conclusions, and found that it was not. In AchutaRao et al. (2006), the
variability of ocean heat uptake in 22 different coupled climate models in the IPCC database was
compared, rounding out our detailed analysis of two models. Again, special attention was paid to the issue
of correctly accounting for the effect of large year-to-year changes in the actual sampling of the oceans,
and how this can be reasonably compared to model results. One interesting result from this analysis was
the importance of including observed volcanic forcing over the historical era; models that lacked
historical volcanic forcing tended, as a group, to produce too much ocean warming.
One implication of the ocean warming that has received considerable attention is its possible effect on
hurricane development. Previous work had identified links between changes in ocean surface temperature
and the intensity of hurricanes. We contributed to this work in the study of Santer et al. 2006, which
examined the causes of the increase in SST in the North Atlantic and Pacific tropical cyclogenesis
regions. The observed SST increases in these regions range from 0.32 C to 0.67 C over the 20th century.
Results from the 22 climate models examined suggest that century-timescale SST changes of this
magnitude cannot be explained solely by unforced variability of the climate system. Model simulations of
natural internal variability indicated that there is an 84% chance that external forcing, including
anthropogenic gasses combined with aerosols, explains at least 67% of the observed SST increases in the
two tropical cyclogenesis regions. Model simulations of the 20th century are, in fact, generally capable of
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Miller, Dr. Arthur J. Predictability and Diagnosis of Low-Frequency Climate Processes in the Pacific, report, October 15, 2008; United States. (https://digital.library.unt.edu/ark:/67531/metadc845056/m1/3/: accessed April 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.