This work describes the implementation and evaluation of the Slab Ocean Model component of the Energy Exascale Earth System Model version 2 (E3SMv2-SOM), and its application to understanding the climate sensitivity to ocean heat transport (OHT) strength and CO$_{2}$ forcing. E3SMv2-SOM reproduces the baseline climate and Equilibrium Climate Sensitivity (ECS) of the E3SMv2 fully coupled experiments, reasonably well, with a pattern correlation close to 1 and global mean bias that is less than 1$\%$ of the fully coupled surface temperature, precipitation and sea ice extent and volume. Similar to other model behaviour, the ECS estimated from the SOM (4.5$^\circ$C) is greater than the estimate from fully coupled model (4.0$^\circ$C; from 150 years regression). The E3SMv2 baseline climate is also very sensitive to the strength of the OHT from which the prescribed ocean heat convergence (OHC) for the SOM is derived, with a surface temperature difference of about 4.0$^\circ$C between high- and low-OHT SOM experiments. The surface temperature response in the high/low-OHT experiments occur through a positive/negative Shortwave cloud radiative effect, caused by a decrease/increase in marine low-level clouds over subpolar regions. This surface temperature sensitivity to prescribed OHCs is particularly large in the Southern hemisphere and is associated with an overcompensation of between prescribed OHC/OHT by atmosphere heat transports. This large sensitivity indicates stronger low-level cloud feedbacks in E3SM. The SOMâ\euro™s ECS estimate is also sensitive to the baseline climate it is initialized from, with an ECS difference of 0.5$^\circ$C between the high- and low- OHT CO$_2$ increase experiments.

A rare record-breaking extreme rainfall event, the highest amount recorded since 1866, hit Indonesia’s capital, Jakarta, in early January 2020. The torrential rainfall was mainly caused by an active cross-equatorial northerly surge (CENS) that occurred concurrently with equatorial waves and Madden-Julian oscillation (MJO). A strong and persistent low-level northerly wind and moisture transport induced by CENS created favorable atmospheric conditions for the formation of deep convection and heavy rainfall over Jakarta. The concurrent occurrences of convectively active phases of equatorial waves (mainly Kelvin, TD-type, and eastward propagating inertia-gravity waves) and MJO during the event further supported the development of heavy rainfall by increasing low-level moisture flux convergence, whereas equatorial Rossby waves contributed indirectly to the increased moisture transport by amplifying cross-equatorial meridional flows toward Jakarta. Together, these large-scale dynamical forcing factors provided a conducive convective environment for the development of mesoscale convective systems and, hence, extreme rainfall over the region.

The present study examines the characteristics of the MJO events represented in the Energy Exascale Earth System Model version 1 (E3SMv1), DOE’s new Earth system model. The coupled E3SMv1 realistically simulates the eastward propagation of precipitation and Moist Static Energy (MSE) anomalies associated with the MJO. As in observation, horizontal moisture advection and longwave radiative feedback are found to be the dominant processes in E3SMv1 that lead to the eastward movement and maintenance of the MSE anomalies, respectively. Modulation of the diurnal cycle of precipitation in the Maritime Continent region by the MJO is also well represented in the model despite systematic biases in the magnitude and phase of the diurnal cycle. On the midlatitude impact of the MJO, E3SMv1 reasonably captures the pattern of the MJO teleconnection across the North Pacific and North America, with improvement in the performance in a high-resolution version, despite the magnitude being a bit weaker than the observed feature. About interannual variability of the MJO, the El Niño-Southern Oscillation (ENSO) modulation of the zonal extent of MJO’s eastward propagation, as well as associated changes in the mean state moisture gradient in the tropical west Pacific, is well reproduced in the model. However, MJO in E3SMv1 exhibits no sensitivity to the Quasi-Biennial Oscillation (QBO), with the MJO propagation characteristics being almost identical between easterly QBO and westerly QBO years. Processes that have been suggested as critical to MJO simulation are also examined by utilizing recently developed process-oriented diagnostics.