Earth and Planetary Sciences ETDs


John Hurley

Publication Date



I completed atmospheric transport modeling studies into distributions of subtropical water vapor, to identify the mechanisms by which humidity varies over distinct temporal scales: inter-annual time-scales associated with the El Nino – Southern Oscillation, over the next century in response to global warming, and over periods of about 2 to 7 days. The latter study on 2 to 7 day variability is a modeling component of a field campaign from October – November, 2008, involving continuous in-situ measurement of water vapor stable isotope compositions at the Mauna Loa Observatory, Hawaii. These studies were conducted from a last saturation perspective, considering humidity and water vapor isotope composition of sub-saturated air to have effectively been set by the conditions at which the air parcels last encountered saturation. During El Nino northern winter, the free troposphere over the subtropical north Pacific is both drier and warmer than during La Nina. For instance, during El Nino (La Nina) northern winter, 57% (49%) of the air at 20˚N and 633 hPa over the north Pacific was last saturated poleward of 20˚N and above 500 hPa. ENSO humidity variability can be explained in terms of changes in the location of last saturation, and not by changes in the temperature field. Modeled specific humidity at the subtropical relative humidity minimum is projected to increase by about 0.2-0.3 g/kg, by the end of this century, in the simulations completed for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Most of this projected increase can be attributed to last saturation within what will be a warmer free troposphere. The increase in specific humidity is partially offset by a poleward and upward shift of baroclinic instability and saturation patterns that effectively yield drier air to the RH minimum via isentropic transport. Two to 7 day variability of nighttime water vapor δD values at the National Oceanic and Atmospheric Administration's Mauna Loa Observatory, Hawaii, range from -365° to -137°. Last saturation of air at Mauna Loa Observatory occurs primarily in the extra-tropical middle-upper troposphere, along mid-latitude baroclinic zones, and secondarily near Hawaii within mesoscale convective systems. Two to 7 day periods of lower (higher) water vapor δD values at MLO correspond to extra-tropical (near-Hawaii) last saturation.

Degree Name

Earth and Planetary Sciences

Level of Degree


Department Name

Department of Earth and Planetary Sciences

First Advisor

Galewsky, Joseph

First Committee Member (Chair)

Sharp, Zachary

Second Committee Member

Gutzler, David

Third Committee Member

Fawcett, Peter

Fourth Committee Member

Kenneth, Minschwaner



Document Type