The Vertical Structure of Tropospheric Water Vapor: Comparing Radiative and Ocean-Driven Climate Changes

Changes in column-integrated water vapor (Q) in response to increased CO2 and ocean heat uptake (OHU) are investigated in slab-ocean aquaplanet simulations. The simulations span a wide range of warming and moistening patterns due to the spatial structures of the imposed OHU. Fractional changes in Q per degree of surface warming range from 0% to 20% K-1 locally and from 3.6% to 11% K-1 globally. A new diagnostic technique decomposes these changes into relative humidity (RH), surface temperature, and lapse rate contributions. Single-column calculations demonstrate substantial departures from apparent (surface temperature based) Clausius-Clapeyron (CC) scaling due to lapse rates changes; a moist-adiabatic column with fixed, uniform RH exceeds the CC rate by 2.5% K-1. The RH contribution is very small in most simulations. The various Q scalings are thus all consistent CC, but result from different patterns of polar amplification and lapse rate change. Lapse rates are sensitive to location and magnitude of OHU, with implications for Q under transient climate change. CO2 with subpolar (tropical) OHU results in higher (lower) Q scalings than CO2 alone. The weakest Q scaling (and largest RH effects) is found for increased poleward ocean heat transport, which causes strongly polar-amplified warming and near-zero tropical temperature change. Despite weak RH changes and fidelity to the CC relation, Q is expected to vary widely on different time scales in nature due to sensitivity of lapse rates to OHU along with the nonlinearity of the diagnostics.