Research ArticleCLIMATOLOGY

The lightness of water vapor helps to stabilize tropical climate

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Science Advances  06 May 2020:
Vol. 6, no. 19, eaba1951
DOI: 10.1126/sciadv.aba1951
  • Fig. 1 Proposed mechanism of the vapor buoyancy feedback.

    (A) The atmosphere is organized into moist and dry regions, with the dry region being responsible for most of the atmosphere’s OLR. In an atmosphere with vapor buoyancy, the WBG necessitates a warmer dry region than if there were no vapor buoyancy effect, which increases OLR. (B) The expected dependence of OLR on column water vapor according to our hypothesis. We expect there to be little difference in the OLR originating from the moist regions, but the vapor buoyancy effect will yield greater OLR in the dry region. We expect this OLR difference to be greater in warmer climates. This constitutes a negative climate feedback.

  • Fig. 2 Simulated water vapor fields and OLR in different climates.

    (A, B, D, E, G, and H) Precipitable water over time, showing convective self-aggregation. During the course of a simulation, the atmosphere organizes into distinct moist and dry regions despite the uniform boundary conditions. Deep convection occurs almost exclusively in the moist regions, while clear skies and low humidity characterize the dry region. (C, F, and I) A 10-day moving average of clear-sky OLR. Once a distinct dry region develops in each simulation, the atmosphere with vapor buoyancy (V.B.) exhibits greater radiative cooling.

  • Fig. 3 Temperature and virtual temperature fields as organized by column PW.

    Red contours correspond to absolute temperature (isotherms), and black contours correspond to virtual temperature (i.e., density) for the pair of simulations with (A and B) SST = 290 K, (C and D) SST = 300 K, and (E and F) SST = 310 K. The temperature profile is nearly identical in the moist region of either atmosphere. The virtual temperature gradient is weak in the free troposphere, warming the dry columns when the vapor buoyancy effect is turned on.

  • Fig. 4 Simulated difference in OLR.

    (A) Relationship between clear-sky OLR and the percentile rank of PW, with and without the vapor buoyancy effect. There are similar values for OLR in the moist region, but the with-vapor-buoyancy atmosphere emits greater OLR in the dry region, consistent with the prediction in Fig. 1B. (B) Increase in OLR from adding the vapor buoyancy effect.

  • Fig. 5 Feedback analysis.

    (A) Increase in clear-sky OLR from adding the vapor buoyancy effect. Because radiative cooling increases with SST, this constitutes a negative climate feedback. (B) Domain-averaged ΔOLR decomposed into components due to the linear effects of both temperature and water vapor. The temperature contribution is split into free troposphere, boundary layer, and above-tropopause components. The boundary layer top was assumed at 900 hPa. Tropopause was identified as the lowest level where domain- and time-averaged radiative heating is approximately zero in the simulation with vapor buoyancy. The nonlinear component is calculated as the residual of the ΔOLR from the simulations. The free-troposphere temperature component contains the proposed vapor buoyancy radiative effect. (C) The marks represent the radiative effect, which is calculated by multiplying the temperature radiative kernel by the ΔT profiles. The curves represent exponential fits to the marks. (D) The total feedback parameter λt, calculated on the basis of the exponential fits, shows the strength of the vapor buoyancy feedback.

  • Fig. 6 Temperature difference due to vapor buoyancy.

    (A, C, and E) ∆T is the additional temperature due to the vapor buoyancy effect as calculated by subtracting the temperature profiles of the simulations with and without the vapor buoyancy effect. That is, we subtract the left-hand panels of Fig. 3 from the right-hand panels. (B, D, and F) ∆T as calculated from a single simulation using Eq. 4. The white lines denote the 900-hPa level we use for the boundary layer top. We do not show values of ∆T above the tropopause, where WBG no longer applies.

Supplementary Materials

  • Supplementary Materials

    The lightness of water vapor helps to stabilize tropical climate

    Seth D. Seidel and Da Yang

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