Detangling Convective Oscillations at ARM Tropical Western Pacific Site: Manus

Wang, Y., Department of Geography


Radiation Processes

Cloud Properties

Wang Y, C Long, J Mather, and X Liu. 2010. "Convective signals from surface measurements at ARM Tropical Western Pacific site: Manus." Climate Dynamics, 36(3-4), 10.1007/s00382-009-0736-z.


Figure 1: (A) The time series includes the clear-sky shortwave (SW) flux (blue) and the all-sky SW flux (black) over Manus. The green line indicates a 60-day running mean. (B) Wavelet power (WP) of CRF (color shading) with two red lines that indicate the 30- and 80-day periods. (C) Spectrum of CRF (black, left Y-axis) and global WP (blue, right Y-axis). (D) Scale-averaged WP (left Y-axis without labels and units) for periods of <30 days (green), 30–80 days (red), and >80 days (blue).


Figure 2: The diurnal composite of the Manus SW fraction sky cover (FSC, unitless). Color shading indicates the “Hovmueller” diagram for diurnal cycles over the seven lags (Y-axis), while the overlaying black curve indicates the phase of diurnal cycle (X-axis). Attached to the right are two plots for the diurnal mean and amplitude (AMP) of Manus SW FSC over the seven lags (Y-axis). Also attached above is the plot for the diurnal cycles of 21 MJO mean (red) and boreal winter climatology (blue).


Figure 1: (A) The time series includes the clear-sky shortwave (SW) flux (blue) and the all-sky SW flux (black) over Manus. The green line indicates a 60-day running mean. (B) Wavelet power (WP) of CRF (color shading) with two red lines that indicate the 30- and 80-day periods. (C) Spectrum of CRF (black, left Y-axis) and global WP (blue, right Y-axis). (D) Scale-averaged WP (left Y-axis without labels and units) for periods of <30 days (green), 30–80 days (red), and >80 days (blue).

Figure 2: The diurnal composite of the Manus SW fraction sky cover (FSC, unitless). Color shading indicates the “Hovmueller” diagram for diurnal cycles over the seven lags (Y-axis), while the overlaying black curve indicates the phase of diurnal cycle (X-axis). Attached to the right are two plots for the diurnal mean and amplitude (AMP) of Manus SW FSC over the seven lags (Y-axis). Also attached above is the plot for the diurnal cycles of 21 MJO mean (red) and boreal winter climatology (blue).

Observed tropical convections are typically organized as intraseasonal oscillations (about 30-80 day periods) throughout the equatorial zone, the so-called Madden-Julian Oscillation (MJO). The most common feature of the MJO is that it weakens over the Maritime Continent and grows back to about full strength towards the Tropical Western Pacific (TWP), and then propagates to the east with diminishing impacts on atmospheric circulations and precipitation. A study by Dr. Yi Wang and his colleagues, using long-term high-resolution surface radiation measurements and retrievals over the ARM TWP site at Manus, has detangled the secret of the MJO.

The MJO is the main mode of tropical intraseasonal variability and involves convective anomalies and wave activities in tropical regions. It was first discovered by Madden and Julian (1971, 1972). Over the past decades, many observational and modeling studies have contributed to our knowledge of the life cycle of the MJO and its interactions with interannual and other timescale climate variability, such as diurnal cycles. The MJO has significant impacts on Indian, Australian, and Asian monsoons, and on the genesis of typhoons in the western Pacific and hurricanes in the tropical Atlantic. It is therefore critical to understand the life cycle of the MJO and interactions between the MJO and other climate variability. The current generation of climate models has been well known for their inability to simulate the life cycle of the MJO, in particular, passing the Maritime Continent.

Using the unique location in the TWP and long-term high-resolution continuous surface observations at ARM Manus site from 1996 to 2006, our preliminary analyses indicate that the surface radiation measurements and retrievals (e.g., shortwave cloud radiative forcing, shortwave fractional sky cover) indeed detect the MJO convective signals (Fig. 1). Those convective signals are consistent with satellite retrievals of outgoing longwave radiation and precipitation. Furthermore, we composited 21 MJO events that passed ARM Manus site in boreal winter from 1996 to 2006 and detangled the interactions between the MJO and diurnal cycles of cloudiness (Fig. 2). We found that during the convective phase of the MJO, the diurnal mean cloudiness is at maximum, but the diurnal amplitude is at minimum. We also detected that the diurnal phases had not changed throughout the passage of the MJO. We suggest that the diurnal phases are mainly controlled by the predominating solar heating, which peaks in the afternoon. To extend our study, we plan to investigate the vertical distributions of clouds, radiative heatings, and other properties using routine observations and retrievals available at ARM TWP site: Manus. Furthermore, a field campaign has been funded for boreal winter 2011-2012 over Manus Island: ARM MJO Investigation Experiment (AMIE). Using the observations collected from AMIE, we plan to use regional and global climate models to study those issues related to the inability for current generation climate models to simulate the whole MJO life cycle.