North American monsoon variability from paleoclimate era to climate change
projection: a multiple dataset perspective
Carlos Mauricio Carrillo Cruz, ATMO Doctoral Candidate, December 2014
In the Southwest United States, the North America monsoon (NAM) is the main driver of severe
weather. How the monsoon has behaved in the past and how it will change in the future is a
question of major importance for natural resource management and infrastructure planning. In
this dissertation, I present the results of three studies that have investigated the North
American monsoon variability and change from the perspective of paleoclimate records, future
climate change projections, and retrospective simulation of the low-frequency variability with
the longest reanalysis.
In the first study, we took advantage of the first systematic monsoon-sensitive network of treering chronologies to analyze the variability of the NAM during the past four centuries. We
present evidence of the ability of tree-ring chronologies to characterize the dominant modes of
NAM climate variability in the high- and low-frequency regime. In the high-frequency regime,
the monsoon tree-ring network is able to reproduce the interannual variability of cool and warm
season precipitation, in a manner similar to the period of the instrumental record. In the lowfrequency regime, earlywood and latewood adjusted chronologies reveal low-frequency climate
variability in the 50-100 year band. This variability seems to be part of a much larger cycle of
natural climate variability and probably the most related to the occurrence of megadroughts.
In the second study, we consider natural climate variability identified in the previous study as
an objective approach to address whether high modes of variability can be used to assess the
uncertainty on climate change projections in the NAM region. The proposed objective approach
is based on the associated NAM teleconnections, or quasi-stationary Rossby wave trains, as
physical mechanism of the response to the El Niño Southern Oscillation – Pacific Decadal
Variability (ENSO-PDV) variability. We provide information that reveals the weakness of using
ensemble mean as an approach to evaluate the NAM precipitation change. Reproducing higher
modes of variability related to the NAM rainfall regime is a challenge for IPCC modes as our
results support. Only one realization is able to reproduce a proper rainfall regime in a seasonal
(annual cycle) and interannual (ENSO) scale in both the 20th and 21st centuries.
In the third study, we expand on the centennial drought signal found with tree-ring
chronologies by exploring the significance of this signal in a modeling framework at continental
scale. We have found convenient to dynamically downscale (DD) the version 2 of the
Twentieth-Century Reanalysis (20CR). The evidence presented suggests that the drought
centennial signal is part of the climate system not only in the Southwest, but also part of the
large-scale circulation. We have found that this low-frequency mode is comparable with
observational records and tree-ring chronologies. Therefore, it supports out hypothesis of a lowfrequency mode that might be responsible for the mega droughts in the last four centuries,
independently evaluated with the tree-ring monsoon-sensitive network.