A22E-05
Nature of Year-to-Year MJO Variability: SST-Forced vs. Internally Driven?

Tuesday, 15 December 2015: 11:20
3008 (Moscone West)
Daehyun Kim, University of Washington Seattle Campus, Assistant Professor, Seattle, WA, United States, JinHo Yoon, Pacific Northwest National Laboratory, Richland, WA, United States, Hyemi Kim, Stony Brook University, Stony Brook, NY, United States, Changhyun Yoo, Ewha Womans University, Seoul, South Korea, Sungsu Park, Seoul National University, Seoul, South Korea and Yoo-Geun Ham, Chonnam National University, Gwangju, South Korea
Abstract:
How much of the year-to-year variability of the seasonal Madden-Julian oscillation (MJO) activity is forced by the seasonal sea-surface temperature (SST) anomalies? We address this question by analyzing ensemble simulations made with an atmospheric general circulation model (AGCM); the Community Atmospheric Model version 5 with a newly developed unified convection scheme which can robustly simulate the tropical MJO activity.

The AMIP-type simulation, in which the observed SST is prescribed as the boundary condition, is repeated 10 times by starting the integrations with randomly perturbed initial conditions. The seasonal MJO activity level is measured as a variance during the boreal winter (November-April) of the MJO-filtered outgoing longwave radiation (OLR) anomalies. The SST-forced portion is estimated as the year-to-year MJO variability common to all ensemble members, while the inter-ensemble spread is considered as the internally driven component.

We show that the partitioning between the SST-forced and the internally driven components varies with geographical location. Over the tropical Pacific, the SST-forced component explains more than 50% of the total year-to-year variability, with the percentage reaching about 70% over the east Pacific. The SST anomalies associated with the El-Niño southern oscillation is identified as a primary driver of the SST-forced part over the tropical Pacific. In contrast, the SST-driven component over the Indian Ocean, which is associated with the basin-wide warming/cooling over the Indian Ocean, explains about 30% only. The remaining internally driven portion is related to the timing and duration of individual MJO events on which the internal atmospheric dynamics seem to have strong influence.

Our results suggest that revealing the dynamical mechanisms that determines the timing and duration of MJO events is a key to enhance understanding of the internally driven component.

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