Coupling with ocean mixed layer leads to intraseasonal variability in tropical deep convection: Evidence from cloud‐resolving simulations

Abstract

The effect of coupling a slab ocean mixed layer to atmospheric convection is examined in cloud‐resolving model (CRM) simulations in vertically sheared and unsheared environments without Coriolis force, with the large‐scale circulation parameterized using the Weak Temperature Gradient (WTG) approximation. Surface fluxes of heat and moisture as well as radiative fluxes are fully interactive, and the vertical profile of domain‐averaged horizontal wind is strongly relaxed toward specified profiles with vertical shear that varies from one simulation to the next. Vertical wind shear is found to play a critical role in the simulated behavior. There exists a threshold value of the shear strength above which the coupled system develops regular oscillations between deep convection and dry nonprecipitating states, similar to those found earlier in a much more idealized model which did not consider wind shear. The threshold value of the vertical shear found here varies with the depth of the ocean mixed layer. The time scale of the spontaneously generated oscillations also varies with mixed layer depth, from 10 days with a 1 m deep mixed layer to 50 days with a 10 m deep mixed layer. The results suggest the importance of the interplay between convection organized by vertical wind shear, radiative feedbacks, large‐scale dynamics, and ocean mixed layer heat storage in real intraseasonal oscillations.

The Impact of the QBO on MJO Convection in Cloud-Resolving Simulations

“…Examining the transition behavior of w in such states suggests that the dry equilibrium originates from an ascent-descent couplet in the middle troposphere (e.g., day 10 in Fig. 3d; Anber et al 2017). This couplet grows and descends because of interaction between radiation and temperature anomalies under WTG (Mapes and Zuidema 1996;Emanuel et al 2014).…”

Response of the Madden–Julian Oscillation and Tropical Atmosphere to Changes in Oceanic Mixed Layer Depth over the Indian Ocean

“…Numerous studies suggest that the inability of climate models to simulate a realistic MJO may in part be due to deficiencies in modeling the dynamics of oceanic mixed layer (e.g., DeMott et al 2015;Ling et al 2017;Anber et al 2017;Maloney and Sobel 2004). In particular, climate models have difficulty in simulating a realistic mixed layer depth (MLD).…”