Supplementary material to “Reconciling Differing Views of Tropical Pacific Climate Change”

20 April 2010

Pedro DiNezio, Cooperative Institute for Marine and Atmospheric Studies, University of Miami, Miami, Florida

Amy Clement, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida

Gabriel A. Vecchi, Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, Princeton, New Jersey

Citation:

DiNezio, P., A. Clement, and G. A. Vecchi (2010), Reconciling differing views of tropical Pacific climate change, Eos Trans. AGU, 91(16), 141–142. [Full Article (pdf)]

Climate models

Global climate models (GCMs) simulate the variations of, and interactions between, various elements of the climate system (ocean, atmosphere, cryosphere and land) forced by radiative agents, such as anthropogenic gases and aerosols. The GW experiments analyzed here were performed for the IPCC AR4 by 21 GCMs coordinated by the Coupled Model Intercomparison Project (CMIP3). The GW projections for the 21^st Century are derived from the SRESA1B experiment, which considers medium-range greenhouse emissions close to CO2-doubling by 2100. Only one run from each model is used to estimate the multi-model response. In each model, the GW response is computed from the least-squares trend between years 2000 and 2100 of the SRESA1B experiment. An additional experiment considering anthropogenic and natural forcing for the 20^th Century (20C3M) is used to compare with observations. In this case, the change in sea surface temperature (SST) and sea level pressure (SLP) is computed from years 1990 to 1999 from the 20C3M experiment and years 2000 to 2004 from the SRESA1B experiment to complete the period with reliable observations (1900-2004). For each individual model, the El Niño response in any given variable is a composite of anomalies simulated in the 20C3M experiment during months when NINO3 SST anomalies exceed 0.5 K. The same criterion is used for computing composites from observations. The multi-model El Niño response is computed as the multi-model mean of each model composite. The multimodel-mean El Niño exhibits thermocline changes that are realistic compared with a composite of observed El Niño events during the 1980-2006 period. Furthermore, the multimodel-mean El Niño exhibits precipitation anomalies with a spatial pattern that is realistic compared with a composite of observed El Niño events during the 1980-2006 period, albeit with weaker amplitude. The observed precipitation anomalies are obtained from the GCPC version 2 analysis (Adler et al. 2003). In all cases the multi-model climate response is considered robust when 15 out of 21 of models simulate changes of a given sign.

Observations

Three global reconstructions of observed SST are used: ERSST2, ERSST3 [Smith et al., 2008], and HadISST [Rayner et al., 2003] to compute the change in the east-west SST gradient during the 1900-2004 period. The east-west change is computed as the difference in least-squares trends between an 'East' region (5°S–5°N, 140°W–80°W) minus a 'West' region (5°S–5°N, 140°E–170°W). The HadSLP2 dataset of sea level pressure (SLP) [Allan and Ansell, 2006] is used to compute the changes in the SOI index during the same period. The change in SOI index is computed from the difference in least-squares trends of SLP between a “Tahiti” region (150°W–90°W, 10°S–10°N) minus a “Darwin” region (100°E–180°, 10°S–10°N). The ERSLP2 dataset is not considered here because it extends up to 1997 and its quality is greatly reduced before 1910, resulting in a much shorter record to compute trends.

Observations vs. 20^th Century Model Projections

GCM projections for the 20^th Century indicate that changes in SLP gradient do not need to coincide with changes in the SST gradient. In fact, various SLP datasets suggest a slowing down of the Walker circulation [Vecchi et al., 2006; Zhang and Song, 2006; Bunge and Clarke, 2009], while the observed trends in SST estimated from the different reconstructions do not agree in the sign of the gradient changes for the 20th century, even during the satellite era (Vecchi et al., 2008). However, according to the climate models, any of the SST reconstructions could be physically consistent with the observed changes in SLP (Figure S1). For instance, the HadISST dataset shows a strengthened SST gradient and a weakened SLP gradient during the 20^th Century (Figure S1) in apparent contradiction according to the ENSO analogy. However, this response is physically consistent with the changes simulated by the MIROC v3.2 (hires) model, and cannot be ruled out as real.

References

Adler, R. F., and Coauthors (2003), The Version-2 Global Precipitation Climatology Project (GPCP) Monthly Precipitation Analysis (1979–Present). J. Hydrometeor., 4, 1147–1167.

Allan, R., and T. Ansell (2006), A New Globally Complete Monthly Historical Gridded Mean Sea Level Pressure Dataset (HadSLP2): 1850–2004. J. Climate, 19, 5816–5842.

Bunge, L., and A. J. Clarke (2009), A verified estimation of the El Niño index NINO3.4 since 1877, J. Climate, 22(14), 3979–3992.

Rayner, N. A., and Coauthors (2003), Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. J. Geophys. Res., 108(D14), 4407.

Smith, T. M., R. W. Reynolds, T. C. Peterson, and J. Lawrimore (2008), Improvements to NOAA's Historical Merged Land-Ocean Surface Temperature Analysis (1880–2006). J. Climate, 21, 2283–2296.

Vecchi, G. A., B. J. Soden, A. T. Wittenberg, I. M. Held, A. Leetmaa, and M. J. Harrison (2006), Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing, Nature, 441, 73–76.

Vecchi, G. A., A. Clement, and B. J. Soden (2008), Examining the tropical Pacific's response to global warming, EOS, Trans. Amer. Geophys. Union, 89, 8183.

Zhang, M., and H. Song (2006), Evidence of deceleration of atmospheric vertical overturning circulation over the tropical Pacific, Geophys. Res. Lett., 33, L12701.

Change in Sea Surface Temperature

Fig. S1 — Change in sea surface temperature (SST) and sea level pressure (SLP) gradient along the equator in observational datasets (blue) and model projections (red) for the 1900 to 2004 period. The SST gradient is computed as the difference between an 'East' (5°S–5°N, 140°W–80°W) and a 'West' region (5°S–5°N, 140°E–170°W). The SLP gradient is a measure of the strength of the Walker circulation and is computed as the area-average SLP change between a “Tahiti” region (150°W–90°W, 10°S–10°N) minus a “Darwin” region (100°E–180°, 10°S–10°N). The model projections correspond to the 20C3M and SRESA1B experiments used in the IPCC AR4.