92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Monday, 23 January 2012
Moisture Cycle of the Madden-Julian Oscillation: An Analysis of TRMM and SP-CAM Data
Hall E (New Orleans Convention Center )
Jennifer A. Gahtan, Univ. of Miami/RSMAS, Coral Gables, FL; and K. H. Straub and D. A. Randall

The Madden-Julian Oscillation (MJO) is an eastward moving oscillation of deep convection and large-scale circulation occurring in the equatorial Indian and western/central Pacific Oceans with a timescale of about 30-90 days. CSU's superparameterized Community Atmosphere Model version 3.0 (SP-CAM) has a fairly realistic MJO (Benedict and Randall 2009); therefore it is of interest to compare its moisture cycle to that in the observed MJO. Understanding the moisture cycle of the MJO may be key in understanding the MJO itself: for example, in the “Discharge-Recharge Mechanism” of the MJO (Blade and Hartmann 1993), moisture builds up locally until it reaches a tipping point, causing large-scale convection which allows the air to dry through horizontal and vertical advection. The timescale of this cycle determines the timescale of the MJO.

In this study, the SP-CAM's MJO moisture cycle is compared to that of the observed MJO. Observations include precipitable water and precipitation from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), and specific humidity and zonal wind data from the NCEP-DOE Reanalysis 2 dataset.

In general, the SP-CAM's moisture cycle is more variable than the observed MJO moisture cycle. During the active phase of the MJO, SP-CAM has more individual peaks in precipitable water and a quicker recharge phase than the observations, which show a slower, smoother progression. Plots of TMI precipitable water vs. precipitation averaged from 5N to 10S show a clockwise progression with time, implying higher precipitable water during the recharge phase. This trend is less clear when averaged over 15N to 15S, and not as evident in the SP-CAM for either latitudinal average. Additionally, cross-sections of specific humidity anomalies show that the SP-CAM accumulation of moisture occurs earlier in the MJO cycle than in observations, with more simultaneity at all levels, and for a longer period of time. In some cases, SP-CAM also has negative specific humidity anomalies at lower levels during the MJO, which is not consistent with observations.

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