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ArticleNorth American climate in CMIP5 experiments. Part II: evaluation of historical simulations of intraseasonal to decadal variability(American Meteorological Society, 2013-12-01) Sheffield, Justin ; Camargo, Suzana J. ; Fu, Rong ; Hu, Qi ; Jiang, Xianan ; Johnson, Nathaniel ; Karnauskas, Kristopher B. ; Kim, Seon Tae ; Kinter, Jim ; Kumar, Sanjiv ; Langenbrunner, Baird ; Maloney, Eric ; Mariotti, Annarita ; Meyerson, Joyce E. ; Neelin, J. David ; Nigam, Sumant ; Pan, Zaitao ; Ruiz-Barradas, Alfredo ; Seager, Richard ; Serra, Yolande L. ; Sun, De-Zheng ; Wang, Chunzai ; Xie, Shang-Ping ; Yu, Jin-Yi ; Zhang, Tao ; Zhao, MingThis is the second part of a three-part paper on North American climate in phase 5 of the Coupled Model Intercomparison Project (CMIP5) that evaluates the twentieth-century simulations of intraseasonal to multidecadal variability and teleconnections with North American climate. Overall, the multimodel ensemble does reasonably well at reproducing observed variability in several aspects, but it does less well at capturing observed teleconnections, with implications for future projections examined in part three of this paper. In terms of intraseasonal variability, almost half of the models examined can reproduce observed variability in the eastern Pacific and most models capture the midsummer drought over Central America. The multimodel mean replicates the density of traveling tropical synoptic-scale disturbances but with large spread among the models. On the other hand, the coarse resolution of the models means that tropical cyclone frequencies are underpredicted in the Atlantic and eastern North Pacific. The frequency and mean amplitude of ENSO are generally well reproduced, although teleconnections with North American climate are widely varying among models and only a few models can reproduce the east and central Pacific types of ENSO and connections with U.S. winter temperatures. The models capture the spatial pattern of Pacific decadal oscillation (PDO) variability and its influence on continental temperature and West Coast precipitation but less well for the wintertime precipitation. The spatial representation of the Atlantic multidecadal oscillation (AMO) is reasonable, but the magnitude of SST anomalies and teleconnections are poorly reproduced. Multidecadal trends such as the warming hole over the central–southeastern United States and precipitation increases are not replicated by the models, suggesting that observed changes are linked to natural variability.
ArticleA simple mechanism for the climatological midsummer drought along the Pacific coast of Central America(Centro de Ciencias de la Atmósfera, UNAM, 2013-04) Karnauskas, Kristopher B. ; Seager, Richard ; Giannini, A. ; Busalacchi, Antonio J.The global distribution, seasonal evolution, and underlying mechanisms for the climatological midsummer drought (MSD) are investigated using a suite of relatively high spatial and temporal resolution station observations and reanalysis data with particular focus on the Pacific coast of Central America and southern Mexico. Although the MSD of Central America stands out in terms of spatial scale and coherence, it is neither unique to the Greater Caribbean Region (GCR) nor necessarily the strongest MSD on Earth based on an objective analysis of several global precipitation data sets. A mechanism for the MSD is proposed that relates the latitudinal dependence of the two climatological precipitation maxima to the biannual crossing of the solar declination (SD), driving two peaks in convective instability and hence rainfall. In addition to this underlying local mechanism, a number of remote processes tend to peak during the apex of the MSD, including the North American monsoon, the Caribbean low-level jet, and the North Atlantic subtropical high, which may also act to suppress rainfall along the Pacific coast of Central America and generate interannual variability in the strength or timing of the MSD. However, our findings challenge the existing paradigm that the MSD owes its existence to a precipitation-suppressing mechanism. Rather, aided by the analysis of higher-temporal resolution precipitation records and considering variations in latitude, we suggest the MSD is essentially the result of one precipitation-enhancing mechanism occurring twice.