Toward the achievement of reliable global kilometer-scale (k-scale) climate simulations, we improve the Nonhydrostatic ICosaherdral Atmospheric Model (NICAM) by focusing on moist physical processes. A goal of the model improvement is to establish a configuration that can simulate realistic fields seamlessly from the daily-scale variability to the climatological statistics. Referring to the two representative configurations of the present NICAM, of which each has been used for climate-scale and sub-seasonal-scale experiments, we try to find the appropriate partitioning of fast/local and slow/global-scale circulations. In a series of sensitivity experiments at 14-km horizontal mesh, (1) the tuning of terminal velocities of rain, snow, and cloud ice, (2) the implementation of turbulent diffusion by the Leonard term, and (3) enhanced vertical resolution are tested. These tests yield reasonable convection triggering and convection-induced tropospheric moistening, and result in better performance than in previous NICAM climate simulations. In the mean state, double Intertropical Convergence Zone bias disappears, and the zonal contrast of equatorial precipitation, top-of-atmosphere radiation balance, vertical temperature profile, and position/strength of subtropical jet are dramatically better reproduced. Variability such as equatorial waves and the Madden–Julian oscillation (MJO) is spontaneously realized with appropriate spectral power balance, and the Asian summer monsoon, boreal-summer MJO, and tropical cyclone (TC) activities are more realistically simulated especially around the western Pacific. Meanwhile, biases still exist in the representation of low-cloud fraction, TC intensity, and precipitation diurnal cycle, suggesting that both finer spatial resolutions and the further model development are warranted.

In the zonal direction, the downward branch of the Walker circulation above the Indian Ocean is only 20 degrees wide, whereas the Pacific counterpart is 90 degrees wide. This zonal sharpness is notable because atmospheric disturbances smaller than the planetary scale, such as the Asian Summer Monsoon, can interact with the planetary-scale Walker circulation through this branch. As a moist circulation, this zonal sharpness is imprinted on a unique zonal discontinuity of the tropical rain belt above Northeast Africa. Therefore, in this study, we refer to this narrow downward branch as the “Wall”, investigate its climatology and interannual variability, and aim at determining its reason for existence. The strongest season of the lower tropospheric Wall in boreal summer is sustained by horizontal cold advection associated with the Asian Summer Monsoon. Two weak phases of the Wall correspond to two rainy seasons at the Eastern Horn of Africa, which are not reproduced well by state-of-the-art global climate models. As for interannual variability, one standard deviation change of a strength of the downward motion at the Wall is associated with about one degree of sea surface temperature variation in the tropical Pacific, and the regression and correlation coefficients are highest in boreal autumn. Nevertheless, total variance is explained more by local sea surface temperature. Experiments using a convection-permitting atmospheric model show that vertical mixing forced by mountain waves in East Africa are necessary for sustaining the Wall. After flattening the East African topography, zonal discontinuity of the tropical rain belt disappears.

Sea surface temperatures (SSTs) of the Gulf Stream and the Kuroshio are shown to be synchronized for the decadal time scale. This synchronization, which we refer to as the Boundary Current Synchronization (BCS), is associated with meridional migrations of the atmospheric jet stream. The singular value decomposition (SVD) between SST and zonal wind shows that, within the context of known climate modes, BCS can be understood as the covariability shared by the Pacific Decadal Oscillation and the Northern Annular Mode. Nevertheless, because the SVD time series exhibit high correlations with the zonal-mean meridional SST difference between the subtropics and the midlatitudes, BCS can be understood more simply as an oceanic annular mode. Air temperature regressed on the BCS index exhibits a similar spatial pattern to temperature observed in July 2018.

Two cloud-resolving models (SCALE and VVM) take different pathways toward convective self-aggregation (CSA) in the radiative-convective equilibrium simulations, although they have similar domain-averaged properties. Analyses in the moisture space show that radiative cooling in the dry area mainly drives CSA in SCALE, while subsidence triggered by the convection in the moist region dominates in VVM. The change in the convective structures is found on the isentropic diagram in VVM, but this transition is unclear in SCALE. The object-based analysis provides that the convective systems with larger sizes are rare in SCALE. In contrast, large-size convective systems frequently develop in the moist region as CSA evolves in VVM. The large-size systems can efficiently drive the circulation between the dry and moist areas. The different pathways to CSA are associated with the transition of convective structures, which provides a new insight to understand CSA among cloud-resolving models based on the perspective of the mechanism.

Two cloud-resolving models (SCALE and VVM) take different pathways toward convective self-aggregation under a radiative-convective equilibrium condition, although the model setups are the same. The physical processes driving the development of self-aggregation in SCALE and VVM, respectively, correspond to the mechanisms for self-aggregation in cold and warm climate states discussed in a previous study. Analyses in the moisture space show that radiative cooling in the dry area mainly drives the aggregation in SCALE, while subsidence induced by the convection in the moist region dominates in VVM. The transition of the convective circulation from convective scale to mesoscale is found on the isentropic diagram in VVM, but this transition is unclear in SCALE. The convective clouds with larger sizes are rare in SCALE. The frequent occurrence of larger convective clouds efficiently drives self-aggregation from the moist area in VVM. These results provide a new insight to understand convective self-aggregation among models.

A possibly important dynamical process for the Madden–Julian oscillation (MJO) convective initiation is proposed. An MJO event during the “CINDY2011” field campaign is triggered by eastward-moving lower-tropospheric mixed Rossby-gravity (MRG) wave packets, and its leading precursor is predominance of upper-tropospheric MRGs in the Indian Ocean (IO). Simple three-dimensional model experiments reveal that the upper-tropospheric MRGs in the IO are amplified particularly in the western IO (WIO) by their westward advection and wave accumulation due to the upper-level convergence in mean easterlies of the Walker circulation. The model also predicts downward dispersion of the amplified upper-tropospheric MRGs and resultant lower-tropospheric MRG wave packet formation. This MRG evolution consistently explains the MJO initiation process during CINDY2011, which is further verified by ray tracing for MRGs. Upper-tropospheric circumnavigating Kelvin waves assist the proposed mechanism by promoting MRG-wave accumulation (advection) in their westerly (easterly) phases via enhanced zonal convergence and weakened easterlies (enhanced easterlies).

The eastward movement speed of Madden–Julian Oscillation (MJO) events simulated in a 30-year simulation on a global cloud resolving model, nonhydrostatic icosahedral atmospheric model (NICAM), following the atmospheric model intercomparison project (AMIP) protocol, but with a slab ocean, was analyzed and compared with the observation. The simulation reproduced the observed tendency of the MJO to decelerate when they are embedded within stronger Walker circulation, intensified by background sea surface temperature states with larger zonal gradients between the warmer western Pacific and the cooler Indian ocean and eastern Pacific. However, the simulated MJO events displayed a slow bias and occurred disproportionately during El Niño events. These biases were associated with an overestimation of the western Walker circulation cell strength, which was partially counteracted during El Niño events. Our results highlight the importance of accurately reproducing the mean atmospheric circulation for the realistic reproduction of MJO in long term simulations.

The nature of convective organization remains elusive, despite its importance for understanding the role of clouds in climate systems. This study reports a new type of large-scale structure formed by the self-organization of deep moist atmospheric convection in radiative-convective equilibrium. To understand the natural behavior of convection unaffected by the computational domain, we conducted cloud-resolving simulations by systematically increasing the horizontal domain size to approximately 25,000 km. We found that if the domain side length exceeded 5,000 km, the domain-averaged thermodynamic fields and the horizontal characteristic length converged in quasi-equilibrium; the cloud aggregation area exhibited a mesh-like pattern, analogous to the shallow convective organizations despite their different scales. Its characteristic length is estimated to be approximately 3,000–4,000 km. The results suggest that this length scale is related to the upper-limit size of mesoscale convective systems or the scale of supercloud clusters in a real tropical atmosphere.

Climatological features regarding the sharp downward branch (SDB) of the Walker circulation above the Indian Ocean are comprehensively investigated. Compared to the Pacific downward branch, SDB has two distinctive features: two-peak seasonality and deep subsidence extension. The two weak phases of SDB in boreal spring and fall correspond well to the two rainy seasons at the Eastern Horn of Africa, which is not reproduced well by state-of-the-art global climate models. Unlike the Pacific counterpart, the annual-mean subsidence of SDB extends to the surface, and is supported by horizontal cold advection associated with the Asian Summer Monsoon. Two experiments using a convection-permitting atmospheric general circulation model show that mountains in East Africa, particularly the Ethiopian Highlands, is necessary for the existence of SDB. The dry and clear climate in the Northeast Africa, which is imprinted as a discontinuity of the Intertropical Convergence Zone, is sustained by the East African topography.