4 Key Points: 5 • The SST contrast increases with warming, primarily because the clear-sky green-6 house effect feedback is stronger in the warm region. 7 • As the climate warms, the integrated cooling rate of the atmosphere increases by 8 moving upward into lower pressures and increasing in strength, giving a more top-9 heavy cooling profile. 10 • The more top-heavy cooling rate profile results in increased cloud ice as the cli-11 mate warms. Abstract 13 Warming experiments with a uniformly insolated, non-rotating climate model with a slab 14 ocean are conducted by increasing the solar irradiance. As the climate warms, the sur-15 face temperature contrast between the warm, rising and cooler, subsiding regions increases, 16 mostly as a result of the stronger greenhouse effect in the warm region. The convective 17 heating rate becomes more top-heavy in warmed climates, producing more cloud ice, prin-18 cipally because the radiative cooling rate moves to lower pressures and increases. To pro-19 duce this more top-heavy convective heating, precipitation shifts from the convective to 20 the stratiform parameterization. The net cloud radiative effect becomes more negative 21 in the warm region as the climate warms. At temperatures above about 310K surface 22 temperature contrast begins to decline, and the climate becomes more sensitive. The re-23 duction in SST contrast above 310K again appears to be initiated by clear-sky radiative 24 processes, although cloud processes in both the rising and subsiding regions contribute. 25 The response of clear-sky outgoing longwave to surface warming begins to accelerate in 26 the region of rising motion and decline in the region of subsidence, driving the SST con-27 trast to smaller values. One-dimensional simulations are used to isolate the most rele-28 vant physics. 29 Plain Language Summary 30 A global model of a non-rotating Earth with an ocean that stores heat but does 31 not transport it is run to equilibrium with different values of globally uniform solar heat-32 ing. Despite the complete uniformity of the system, it still develops regions of warm sea 33 surface temperature where rain and rising motion occur, and regions with downward, 34 subsiding air motion where rainfall does not occur. These contrasts look very similar to 35 what is observed in the present-day tropics. As the climate is warmed from current tem-36 peratures toward warmer temperatures, the warm regions warm faster, mostly because 37 the rising regions contain more water vapor. The clouds rise to higher altitudes in the 38 warmer climates, and produce more cloud ice. These changes are shown to arise from 39 well-understood physical processes that are expected to operate in nature. 40

The Quasi-Biennial Oscillation (QBO) dominates the interannual variability in the tropical lower stratosphere and is characterized by the descent of alternating easterly and westerly zonal winds. The QBO impact on tropical clouds and convection has received great attention in recent years due to its implications for weather and climate. In this study, a 15-year record of high vertical resolution observations from CALIPSO are used to document the QBO impact on equatorial (10°S-10°N) clouds. Observations from radio occultations, the CERES instrument, and the ERA5 reanalysis are also used to document the QBO impact on temperature, radiative energy budget, and zonal wind. It is shown that the QBO impact on zonal mean equatorial cloud fraction has a strong seasonality. The strongest cloud fraction response to the QBO occurs in boreal spring and early summer extending down to ~12 km and results in a significant longwave cloud radiative effect anomaly. The seasonality of the cloud fraction changes is synchronized with those of temperature and zonal wind in the tropical upper troposphere.

ERA5 reanalysis with hourly time steps and 30 km horizontal resolution resolves a substantially larger fraction of the gravity wave spectrum than its predecessors. Based on a novel representation of the two-sided zonal wavenumber-frequency spectrum, we show evidence of gravity wave signatures with phase speeds centered around ±35 m/s in a suite of atmospheric fields. Cross-spectrum analysis reveals (i) a substantial upward flux of geopotential for both eastward and westward propagating waves, (ii) an upward flux of westerly momentum in eastward propagating waves and easterly momentum in westward propagating waves, and (iii) anticyclonic rotation of the wind vector with time—all characteristics of vertically propagating gravity and inertio-gravity waves. That two-sided meridional wavenumber-frequency spectra computed along individual meridians and then zonally averaged exhibit characteristics similar to the spectra computed on latitude circles indicates that these waves propagate in all directions. The three-dimensional structure of these waves is also documented in composites of the temperature field relative to grid-resolved, wave-induced downwelling events at individual reference grid points along the equator. It is shown that the waves radiate outward and upward relative to the respective reference grid points, and their amplitude decreases rapidly with time. Within the broad continuum of gravity wave phase speeds there are preferred values around ±49 m/s and ±23 m/s , the former associated with the first baroclinic mode in which the vertical velocity perturbations are of the same sign throughout the depth of the troposphere, and the latter with the second mode in which they are of opposing polarity in the lower and upper troposphere.