Date of Graduation

7-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Environmental Dynamics (PhD)

Degree Level

Graduate

Department

Environmental Dynamics

Advisor/Mentor

Feng, Song

Committee Member

Stahle, David W.

Second Committee Member

Tipton, John R.

Keywords

seasonal droughts; pluvials; moisture variations; climate variations; Paleoclimate Data Assimilation (PDA); seasonal moisture reconstruction; seasonal tree rings

Abstract

The seasonal tree-ring chronologies that recorded discrete moisture signal for warm and cool season have been used to reconstruct the North America Seasonal Precipitation Atlas (NASPA) by the point-by-point regression method, and the results indicated that they can fairly reproduce the seasonal precipitation variability in the past. Compared with reconstructions using only paleoclimate proxy data, the paleoclimate data assimilation (PDA) considers both proxy data and climate model output, so the PDA reconstructions are consistent with both the climate signals reflected by the proxy data and the physical mechanisms represented by climate models. Based on the tree rings with discrete seasonal signals, this doctoral research reconstructed the physically and dynamically consistent seasonal moisture over North America (NASM) in the last millennium using PDA method. Additionally, the global SST and 500 hPa geopotential height were also reconstructed using PDA. The validation of various instrumental data shows that the reconstruction skill of NASM is slightly better than the proxy-based reconstruction over most areas except the Western United States. NASM is subsequently used to investigate the seasonal, spatial, and temporal variations of major drought and pluvial events in North America as well as their causal mechanisms. The spatial and temporal characteristic of moisture variations and hydroclimatic extremes are notably different in warm and cool season, indicating that the seasonality is an important component of hydroclimatic variability behind dryness and wetness regimes in North America. Seasonal precipitation variability in North America during the last millennium was mainly affected by El Niño/Southern Oscillation (ENSO). In addition to ENSO, the North Atlantic Oscillation (NAO) and North Pacific SST also played important roles on the warm and cool season precipitation, respectively. The PDA method was also adopted to reconstruct warm season moisture in Asia (WSMA). The inclusion of Chinese historical documentary proxies, which mainly represent warm season precipitation, can improve the reconstruction skill compared with the previous PDA-based moisture studies, especially in East and North China. The nature of PDA also ensures that the WSMA can successfully reproduce persistent droughts and corresponding atmospheric and oceanic conditions. Furthermore, the fidelity of PDA method beyond the instrumental era evaluated by pseudoproxy experiments indicated that reconstruction skill varies in time and space due to the uneven distribution of available moisture proxies.

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