Sullivan, Richard Marshall (2022-07). The North Atlantic Subtropical High Modulates Extreme Holocene Hydroclimate Variability in the Intra-American Seas. Doctoral Dissertation.
Thesis
The North Atlantic Subtropical High (NASH) is a dominant driver of regional hydroclimate in the Intra-American Seas (IAS). During the boreal summer, the NASH expands westward along steep land-sea temperature gradients. This suppresses convection, increases divergence, deflects precipitable moisture, and influences the trajectory of Atlantic tropical cyclones. The maximum seasonal extent of the NASH is responsible for a recurrent mid-summer drought in the IAS and Caribbean region, while meridional displacements of the NASH western edge create a dipole that drives hydroclimate variability in the eastern IAS and southeast United States. However, little is known about how the NASH mediates regional IAS hydroclimate on centennial and millennial timescales. To address this deficiency, we developed a ~6,000-years long rainfall record from the northeast Bahamas and reconstructed Common Era tropical cyclone variability over the northern Yucatan peninsula. Rainfall over the northern Bahamas is driven by NASH variability, and Yucatan tropical cyclone susceptibility is influenced by the NASH expansion and related climate modes (such as the Caribbean Low-Level Jet). These interpretations are supported by simulations from the Community Earth System Model. These simulations improve our understanding of NASH behavior on timescales congruous with these reconstructions and suggest an important role for Atlantic and Pacific climate modes. Furthermore, model simulations and a compilation of regional hydroclimate reconstructions reveal that a NASH-driven dipole dominates northern and southern Caribbean rainfall on centennial timescales. This work also shows that NASH expansion increases the susceptibility of the Yucatan Peninsula to hurricanes that form in the tropical west Atlantic. These results bring middle and late Holocene Caribbean hydroclimate volatility into sharper focus while providing important context for the ocean-atmosphere processes that modulate changes to regional climate.
ETD Chair
van Hengstum, Pete Interim Associate Vice President for Research & Associate Dean of Graduate Studies
