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The authors use singular spectrum analysis to investigate the relative magnitude of decadal to multidecadal (D2M) variability in annual and seasonal precipitation anomalies across North America. In most places, decadal (10–20 yr) or multidecadal (20–50 yr) variability makes up less than 10% of the total variance in either annual or seasonal precipitation, with interannual variability or secular trends having much greater importance. Decadal variability is most prominent (contributing 25%–30% of the total variance) in Minnesota and northern California during winter, and the central Rocky Mountains in autumn. Eastern Québec is the only major region where precipitation exhibits significant variance in the multidecadal band. Precipitation across much of Canada exhibits significant variance at extremely low frequencies (greater than 50 yr), but variability at these time scales cannot be separated from secular trends because of the limited length of instrumental climate records. Decadal signals in the discharge of the Sacramento River and, to a lesser degree, the Colorado River are coherent and in phase with similar signals in regional precipitation. Prominent D2M signals do not resemble the low-frequency components of major climate modes such as ENSO or the PDO, which suggests that this behavior is not a product of a simple linear translation of a single climate forcing. [ABSTRACT FROM AUTHOR]

Annual area burned (AAB) variability in northwestern North America was inferred from 38 treering width chronologies widely distributed across boreal regions and spanning the past 300 years and the minimum 1833-1998 interval. AAB estimates accounted for up to 61% of the variance in AAB observed from 1959 to 1998, and were verified using a split sample calibration-verification scheme. Spatial correlation maps of gridded temperature and precipitation data provided an indication of the reliability of the reconstruction to approximate fire-conducive climate variability beyond the period of calibration. Singular spectrum analysis and analysis of variance suggested that AAB has significantly changed during the course of the past 150 years toward increasing variance. Recent 1959-1998 decadal changes in AAB of northwestern North America fitted well within an oscillatory mode centred on 26.7 years and accounting for 21.1% of the variance in the reconstruction. As in previous studies, the current findings suggest that AAB is correlated to seasonal land/ocean temperature variability and that future warming could lead to greater AAB. However, the study has the weakness of not accounting for complex interactions between climate and ecosystem processes and thus its results should be interpreted with caution. We suggest that a calibration model conducted on multiple types of fire proxies (tree rings, charcoal data, fire scars and stand-establishment records) could be relevant for addressing these weaknesses. [ABSTRACT FROM AUTHOR]

This study investigates the capability of the dynamic downscaling method (DDM) in a North American regional climate study using the Eta/Simplified Simple Biosphere (SSiB) Regional Climate Model (RCM). The main objective is to understand whether the Eta/SSiB RCM is capable of simulating North American regional climate features, mainly precipitation, at different scales under imposed boundary conditions. The summer of 1998 was selected for this study and the summers of 1993 and 1995 were used to confirm the 1998 results. The observed precipitation, NCEP–NCAR Global Reanalysis (NNGR), and North American Regional Reanalysis (NARR) were used for evaluation of the model’s simulations and/or as lateral boundary conditions (LBCs). A spectral analysis was applied to quantitatively examine the RCM’s downscaling ability at different scales. The simulations indicated that choice of domain size, LBCs, and grid spacing were crucial for the DDM. Several tests with different domain sizes indicated that the model in the North American climate simulation was particularly sensitive to its southern boundary position because of the importance of moisture transport by the southerly low-level jet (LLJ) in summer precipitation. Among these tests, only the RCM with 32-km resolution and NNGR LBC or with 80-km resolution and NARR LBC, in conjunction with appropriate domain sizes, was able to properly simulate precipitation and other atmospheric variables—especially humidity over the southeastern United States—during all three summer months—and produce a better spectral power distribution than that associated with the imposed LBC (for the 32-km case) and retain spectral power for large wavelengths (for the 80-km case). The analysis suggests that there might be strong atmospheric components of high-frequency variability over the Gulf of Mexico and the southeastern United States. [ABSTRACT FROM AUTHOR]