N.A. Randazzo, A.M. Michalak and A.R. Desai
The forests of northeastern North America serve as an important carbon sink, absorbing a large amount of carbon dioxide from the atmosphere. However, uncertainty remains about how the carbon exchange of these ecosystems responds to meteorological events and climate conditions. We found that forests in the temperate-boreal transition zone of the Great Lakes region absorb significantly less carbon dioxide during times of strong storm activity along the storm track associated with the Jet Stream during the growing season. In addition to the direct response of these forests to this storm system, a general dampening of the seasonal cycle of net carbon uptake was seen during summers with a high frequency of these storms during the study period, leading to an overall reduction in summertime uptake during those years.
Figure: This study focused on a storm system centered around the upper Midwest and consistently associated with a mid-latitude cyclone resulting from an interaction between the Great Plains Low Level Jet and the North American Jet Stream. Triangles denote the locations of the forested study sites. (a) The map of average daily precipitation [mm] shows enhanced precipitation over and to the northwest of the Great Lakes, with trails of precipitation from the southerly and northwesterly directions. (b) This system occurs primarily during the summer. (c) The average vector winds associated with this system reveal the mechanism behind this precipitation system, with a strong Great Plains Low Level Jet, strong northwesterly circulation associated with the North American Jet Stream, and a cyclone in the region of enhanced precipitation (which is also the region where these two circulation systems meet). (d) The anomaly of average vector winds associated with this system from June to August compared to all other days in this time of year highlights the circulation anomaly.
While substantial attention has been paid to the effects of both global climate oscillations and local meteorological conditions on the interannual variability of ecosystem carbon exchange, the relationship between the interannual variability of synoptic meteorology and ecosystem carbon exchange has not been well studied. Here we use a clustering algorithm to identify a summertime cyclonic precipitation system northwest of the Great Lakes to determine (a) the association at a daily scale between the occurrence of this system and the local meteorology and net ecosystem exchange at three Great Lakes region forested eddy covariance sites and (b) the association between the seasonal prevalence of this system and the summertime net ecosystem exchange of these sites. We find that temperature, in addition to precipitation and cloud cover, is an important explanatory factor for the suppression of net ecosystem productivity that occurs during these cyclonic events in this region. In addition, the prevalence of this cyclonic system can explain a significant proportion of the interannual variability in summertime forest ecosystem exchange in this region. This explanatory power is not due to a simple accumulation of low‐productivity days that cooccur with this meteorological event, but rather a broader association between the frequency of these events and several aspects of prevailing seasonal conditions. This work demonstrates the usefulness of conceptualizing meteorology in terms of synoptic systems for explaining the interannual variability of regional carbon fluxes.