The role of tilted heating in initiation and maintenance of the MJO

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Description

There is mixed evidence surrounding the role of tilted heating in perpetuating a Madden-Julian Oscillation (MJO) in the Indian and Western Pacific Oceans. When tilt is observed, it shows low-level convergence, upward motion, and a positive moisture anomaly on the east side of the MJO convective center. This fosters new convection and enables an eastward propagation of the signal. On the west side, the tilted structure shows low-level divergence, downward motion, and negative moisture anomalies. In these cases, it is believed that shallow convection ahead of the MJO’s convection center (low-level tilt) preconditions the lower atmosphere with the heat and moisture needed to sustain the MJO, while stratiform heating behind the MJO signal (upper-level tilt) is helpful in maintaining the deep convection.

Implementing a technique we developed to add heating distributions to version 4 of the Community Climate System Model (CCSM4), we showed in our previous work that correctly simulating the horizontal distribution of tropical heating is critical for accurate modeling of the MJO. In this current work, we employ the same technique to look at tilted heating distributions in CCSM4, both idealized and observed. We use idealized distributions of heating that are tilted to mimic the east-west progression of shallow convective, deep convective, and stratiform clouds (observed to pass during particular MJO phases). We isolate the low-level tilt and the upper-level tilt and individually analyze the importance of each in initiating and maintaining a strong MJO signal. In addition, International Satellite Cloud Climatology Project (ISCCP) data are analyzed for tilted heating distributions and is also used to force additional CCSM4 runs.

For the idealized cases, the heating input is shifted among latitude and longitude points in a manner that is consistent with the eight phases of the MJO. CCSM4 is then run for 15 years, and the resulting MJO that is produced by all distributions is compared to that of control runs done with little or no horizontally varying heating. We perform sensitivity studies on the strength and location of heating, as well as the angle to which the tilting occurs. Preliminary results show that the addition of the low-level tilt is more important than the upper level tilt in the overall life cycle of the MJO. We also show that the most robust MJO occurs when the tilting angle between the stratiform and mid-levlevel convection is steeper than that of the tilting angle between the shallow and mid-level convection.