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Abstract Previous observational and modeling studies have suggested that moisture plays a dominant role in Madden–Julian oscillation (MJO) evolution. Using a realistic MJO simulation by incorporating the role of mesoscale stratiform heating in the Zhang–McFarlane deep convection scheme in the National Center for Atmospheric Research Community Atmosphere Model, version 5.3 (NCAR CAM5.3), this study investigates the factors responsible for the improved MJO simulation by examining moisture variations during different MJO phases. The results of column moist static energy (MSE) and moisture budgets show that during the suppressed phases of MJO, vertical advection acts to increase MSE anomalies for the development of deep convection while radiative heating and surface heat flux decrease MSE. The opposite holds true at the MJO mature phase. However, their roles largely cancel each other, leaving horizontal advection to play a major role in the low-level MSE increase during the suppressed phase of the MJO and MSE decrease after the MJO mature phase. A further analysis combining moisture and temperature budget equations is performed to demonstrate the effects of vertical advection and cloud processes within the column at each level. The vertical profiles of column-confined moisture tendency show that large-scale vertical advection induced by latent heat release and evaporation within shallow convective clouds is also important to the lower-tropospheric moistening during suppressed phases. This confirms the role of shallow convection in low-level moistening ahead of MJO deep convection. Radiative heating is vital across all MJO phases, and its warming effects keep the column humidity anomaly maintained in mature phases. None of these features are reproduced by the standard CAM5.3.
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A Cold Lid on a Warm Ocean: Indian Ocean Surface Rain Layers and Their Feedbacks to the Atmosphere
Abstract Ocean surface rain layers (RLs) form when relatively colder, fresher, less dense rain water stably stratifies the upper ocean. RLs cool sea surface temperature (SST) by confining surface evaporative cooling to a thin near‐surface layer, and generate sharp SST gradients between the cool RL and the surrounding ocean. In this study, ocean‐atmosphere coupled simulations of the November 2011 Madden‐Julian Oscillation (MJO) event are conducted with and without RLs to evaluate two pathways for RLs to influence the atmosphere. The first, termed the “SST gradient effect,” arises from the hydrostatic adjustment of the boundary layer to RL‐enhanced SST gradients. The second, termed the “SST effect,” arises from RL‐induced SST reductions impeding the development of deep atmospheric convection. RLs are found to sharpen SST gradients throughout the MJO suppressed and suppressed‐to‐enhanced convection transition phases, but their effect on convection is only detected during the MJO suppressed phase when RL‐induced SST gradients enhance low‐level convergence/divergence and broaden the atmospheric vertical velocity probability distribution below 5 km. The SST effect is more evident than the SST gradient effect during the MJO transition phase, as RLs reduce domain average SST by 0.03 K and narrow vertical velocity distribution, thus delaying onset of deep convection. A delayed SST effect is also identified, wherein frequent RLs during the MJO transition phase isolate accumulated subsurface ocean heat from the atmosphere. The arrival of strong winds at the onset of the MJO active phase erodes RLs and releases subsurface ocean heat to the atmosphere, supporting the development of deep convection.
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- Award ID(s):
- 1924659
- PAR ID:
- 10560306
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 129
- Issue:
- 4
- ISSN:
- 2169-897X
- Subject(s) / Keyword(s):
- surface fluxes convection rain freshening
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2023JD039272
