A Study of CINDY/DYNAMO MJO Suppressed Phase

Abstract

The diurnal variability and the environmental conditions that support the moisture resurgence of MJO events observed during the Cooperative Indian Ocean Experiment on Intraseasonal Variability (CINDY)/DYNAMO campaign in October–December 2011 are investigated using in situ observations and the cloud-resolving fully air–ocean–wave Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS). Spectral density and wavelet analysis of the total precipitable water (TPW) constructed from the DYNAMO soundings and TRMM satellite precipitation reveal a deep layer of vapor resurgence during the observed Wheeler and Hendon real-time multivariate MJO index phases 5–8 (MJO suppressed phase), which include diurnal, quasi-2-, quasi-3–4-, quasi-6–8-, and quasi-16-day oscillations. A similar oscillatory pattern is found in the DYNAMO moorings sea surface temperature analysis, suggesting a tightly coupled atmosphere and ocean system during these periods. COAMPS hindcast focused on the 12–16 November 2011 event suggests that both the diurnal sea surface temperature (SST) pumping and horizontal and vertical moisture transport associated with the westward propagating mixed Rossby–Gravity (MRG) waves play an essential role in the moisture resurgence during this period. Idealized COAMPS simulations of MRG waves are used to estimate the MRG and diurnal SST contributions to the overall moisture increase. These idealized MRG sensitivity experiments showed the TPW increase varies from 9% to 13% with the largest changes occurring in the simulations that included a diurnal SST variation of 2.5°C as observed.

Ocean Response to CINDY/DYNAMO MJOs in Air-Sea-Coupled COAMPS

“…2 suggests active MJO phases in three events: MJO1 from 15 Oct. to 2 Nov. MJO2 from 23 Nov to 6 Dec, and MJO3 from 15 Dec to Dec 31. The same three events can be seen in rainfall from the Tropical Rainfall Measuring Mission (TRMM), which traditionally has been used to help define MJO onset and the duration of the DYNAMO MJO events by several authors (e.g., Shinoda et al 2013b;Matthews et al 2014;Chen et al 2015). In Fig.…”

“…In a detailed analysis and comparison between COAMPS SST and surface fluxes and observations from the R/V Revelle from 12-16 Nov, during the suppressed phase of the MJO (Chen et al 2015), COAMPS had a positive SST bias of 0.01°C, but RMS errors of 0.46°C. This suggests that the overall model SST bias is small, but the instantaneous SST differences between the model and the buoy are at times significant.…”

Multiple and spin off initiation of atmospheric convectively coupled Kelvin waves

“…These case studies suggest that both the upper ocean temperature diurnal variability and ocean surface flux variability are important factors for formation of convection and precipitation, especially when they are organized in multiple CCKWs. Such link between atmospheric convection and upper ocean diurnal variability was noted previously (Chen et al 2015; Ruppert and Johnson 2015) and it will be further elaborated using composites of multiple CCKW initiation cases and a modeling study.…”

“…In the variable source simulations, the geopotential height anomaly, the wind anomaly and the wind convergence are stronger than in control simulation with the constant source. This link between the diurnally varying heat source and Kelvin waves and a relationship between the diurnal SST variability and development of atmospheric convection during the MJO suppressed phase, noticed by Chen et al (2015) and Ruppert and Johnson (2015), indicates that our simplified simulations are qualitatively agreeing with observations with the caveat that some of the incoming diurnal shortwave signal can be partitioned to directly heat the atmosphere and influence convection development (Ruppert 2016;Ruppert and Johnson 2016). Furthermore, shallow water model solutions suggest that the link between short-term variability of a source and a structure of Kelvin wave response is truly nonlinear and exemplifies interactions across many temporal and spatial scales.…”

“…Subsequent modeling work (Ruppert 2016;Ruppert and Johnson 2016) emphasizes importance of the direct impact of the shortwave radiation on convection development and suggests that diurnal oceanic warm layers and the resulting diurnal cycle of SST is not the complete picture. Model studies suggest that in the absence of other forcing, the diurnal oscillation of precipitation responds to the diurnal variability of SST (Chen et al 2015;McLay et al 2012). These results provide motivation to use the diurnally varying heat source in the idealized numerical simulations to study initiation of equatorial waves (which we discuss in the Appendix).…”