Observations spanning 2004-2012 from two Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) buoys along the equator in the Indian Ocean are used in conjunction with Tropical Rainfall Measuring Mission (TRMM) to assess the relative importance of surface latent heat fluxes to intraseasonal convection. This work is motivated by previous observational and modeling studies that have suggested the importance of wind-induced surface fluxes to the dynamics of the Madden-Julian Oscillation (MJO). Intraseasonal variability is isolated in two ways: 1) 20-100-day bandpass filtering and 2) using the global real time multivariate MJO index. Linear regression shows latent heat flux anomalies to be between 4 % and 8 % of precipitation anomalies when the two variables are compared using similar Wm-2 energy units. From a moist static energy budget viewpoint, these results confirm the potential of wind-induced latent heat fluxes to aid destabilization of MJO convection. Results derived from using both simple intraseasonal filtering and global MJO indices indicate that precipitation leads latent heat flux on the order of a few days, indicating surface fluxes may be more important for maintenance of deep MJO convection in addition to helping set the MJO’s propagation speed. Sensitivity tests using smoothed wind speed or thermodynamics (i.e., air temperature, relative humidity, and sea surface temperature) to compute latent heat flux show wind speed variability explains most of the latent heat flux variability on intraseasonal timescales. A similar conclusion is found via linearization of the latent heat flux formula. Additional analysis shows mesoscale and synoptic scale wind variability have negligible impact on intraseasonal latent heat flux anomalies.
A satellite-based method of moisture and thermal budget analysis is examined in comparison with sounding array observations from Cooperative Indian Ocean experiment on Intraseasonal variability in the Year of 2011 (CINDY2011)/Dynamics of the Madden-Julian Oscillation (MJO) (DYNAMO)/Atmospheric Radiation Measurements (ARM) MJO Investigation Experiment (AMIE) and from the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE). Overall, the satellite analysis is found to quantitatively reproduce the statistical behaviors of large-scale mean vertical motion, moisture convergence, and moist static energy (MSE) convergence as observed from the sounding arrays. However, individual convective events generally do not delineate a systematic evolutionary track but are heavily spread around the ensemble mean of moisture and MSE convergences in composite space. Next, the convective events are broken down into “developing”, “off-centered”, and “passing-by” classes using geostationary infrared measurements in an attempt to sort irrelevant samples that are not representative of convective dynamics. All the three composite classes show qualitatively similar evolutions except for the amplitude of variability, with genuine developing events being greatest in amplitude and passing-by disturbances being weakest. The spread among individual events is substantially reduced when the convective events immune to strong synoptic-scale influences are isolated and the contribution of horizontal advection is excluded from MSE convergence.
The cumulus convection activity in the tropical oceanic regions is strongly regulated by the large-scale environmental atmosphere, while at the same time, cumulus convection will influence the large-scale atmosphere. It is thus recognized that the spatiotemporal variability of moisture content plays an important role in determining such multiscale interaction processes relevant to tropical cumulus convection. This study investigates the relationship between cumulus convection and environmental moisture in the tropical Indian Ocean by conducting convection-resolving simulations through the nesting capability with which the innermost domain has the 100 m grid resolution. We examine the cases observed from October to November 2011 during the Cooperative Indian Ocean Experiment on Intraseasonal Variability in the Year 2011/Dynamics of the Madden-Julian oscillation (MJO) field experiment. Based on the favorable performance of the outermost domain simulations in reproducing eastward propagating signals over the Indian Ocean, the data obtained from the 100 m mesh simulations are examined. It is shown that the cloud cover whose tops exceed a middle level sharply increases with the increase in precipitable water vapor over about 55 mm. The increase in relative humidity in a lower layer results in the increase in cloud cover at a level above the humid layer. From the convection-resolving simulations, the existence of updraft cores that are less diluted with the environment is demonstrated. It is considered that cloud-core parcels are less susceptible to the negative effects of dilution with the environment and survive to penetrate to upper levels, which contributes to the moistening of the environmental atmosphere. The existence of updraft cores plays a key role in the inter-relationship between cumulus convection and its environment. The effects from cumulus clouds on their environment are regarded as a preconditioning influence for the convective initiation of MJO.
In this study, we investigated the equatorially antisymmetric features in the initiation processes of the Madden-Julian oscillation (MJO) event in late October during the cooperative Indian Ocean experiment on intraseasonal variability in the year 2011. This MJO event developed when the thermal equator was drastically shifted from the Northern Hemisphere to the equator from September to October as the Mascarene High over the southern Indian Ocean decayed and shifted southward. The large-scale fields of sea level pressure, temperature, and moisture around the MJO exhibited equatorially antisymmetric features. According to their different features in terms of surface convergence, temperature, and moisture, MJO convection over the Indian Ocean after its onset consisted of four distinct convective components: the southern intertropical convergence zone between 10°S and 0° along the meridional sea surface temperature (SST) gradients (s-ITCZ), the northern-ITCZ at the southern edge of the high SST above 29°C over the Bay of Bengal, the vortex disturbance over the Arabian Sea (VDAS) in association with the zonal SST gradients, and the westward-propagating diurnal convection originating from Sumatra. In particular, the double-ITCZ and VDAS were characterized by a steady, low-level convergence zone along the surface potential temperature gradients forced by the SST. Before the onset of the MJO convection, the double-ITCZ was characterized by cross-equatorial vertical circulation that was baroclinically tilted northward, and s-ITCZ convection was inhibited owing to the strong Mascarene High. After the onset, single, larger-scale upward motion was barotropically formed over the equator because of the equatorward shift of the double-ITCZ. Such changes in the equatorially antisymmetric meridional circulation are relevant for the organization of the MJO convection.
This study analyzes radiosonde observations and other datasets to examine variability in the moist static energy (MSE) budget over the eastern Maritime Continent during the CINDY2011/DYNAMO field campaign of October 2011-March 2012. During this period, five events bearing key characteristics of the Madden-Julian oscillation (MJO) are identified. Our analysis focuses on both these events and longer-term seasonal evolution. On the seasonal time scale, the characteristics of column-integrated MSE budget are different between periods before and after the Australasian summer monsoon onset in early December. Both the net source term (the sum of surface turbulent fluxes and radiative heating) and the net advection term (the sum of horizontal and vertical advection of MSE) have small magnitudes before the onset. After the onset, the source term becomes large and positive, while the advection term becomes large and negative. On the intraseasonal scale, both the source and advection terms fluctuate as the MJO events come and go. The surface fluxes and radiative heating contribute to the maintenance of the amplitude of column-integrated MSE anomaly and thus to the intensity of MJO. The vertical advection term, along with horizontal advection term, seems to contribute to the phase progression and eastward propagation of MJO, mainly because of lower-tropospheric descent after the precipitation and MSE maxima, presumably associated with rain re-evaporation. This study also examines how the MSE budget would be different if key components of the budget were parameterized by two assumptions used in recent idealized models of MJO: (1) the column-integrated radiative heating anomaly is considered proportional to the column water vapor anomaly and (2) the normalized gross moist stability is considered constant. We find that the former tends to speed up the phase progression of MJO, while the latter tends to slow it down.
We investigated the role of Sumatra Island convection over the maritime continent during the preconditioning stage of the Madden-Julian Oscillation (MJO) using intensive observations of CINDY2011/DYNAMO and HARIMAU2011. CINDY2011/DYNAMO and HARIMAU2011 were conducted over the Indian Ocean from October 2011 to January 2012 and Sumatra Island, Indonesia in December 2011. Both observation datasets covered the preconditioning stage of the MJO in December 2011. We found that convection was activated over the Sumatra Island with diurnal cycle associated with the moist air mass, which originated from a tropical depression generated in the South China Sea. Then, two-day period disturbances that propagated westward to the central Indian Ocean were coupled with the diurnal cycle of convection over the Sumatra Island. The structure of the two-day period disturbances was consistent with that of westward propagating inertio-gravity waves. When the westward propagating disturbances arrived over the central Indian Ocean, low-level moisture advection was excited. Moistening process was promoted in Gan Island over the central Indian Ocean, which had a two-day period. After the favorable condition of large-scale convection was established, the MJO was activated in the central Indian Ocean. The two-day period westward disturbances were organized when large-scale moisture convergence became positive in Sumatra Island and continued until a strong low-level westerly wind of the active phase of the MJO was formed.
The ensemble hindcast initialized during 12-16 October 2011 is performed using a global cloud-system-resolving model (CSRM) with a horizontal mesh size of approximately 14 km. The ensemble size is five and the duration of each simulation is 60 days. When sea surface temperature (SST) with a realistic time evolution is prescribed, not only the first but also the second Madden-Julian oscillation (MJO) event observed during the CINDY2011/DYNAMO period emerges in the ensemble mean, although the signal of the second MJO is unsatisfactory in each member. This result leads to a hypothesis that the second MJO is significantly constrained by the prescribed seasonal change of SST. The analyses of the observational data indicate that an MJO favorable environment, in which SST of the southeastern Maritime Continent is higher than that of the Indian Ocean, is established in late November to early December. Humidity in the lower troposphere increases substantially in the southeastern Maritime Continent during the period. A pair of sensitivity tests using a global CSRM clearly shows that the eastward migration of convection during the second MJO is at least partly caused by the climatological seasonal change of SST. The results of this study indicates that it is inappropriate to treat the climatological seasonal change as the background of the MJO during this season, because its timescale is short enough to be comparable with the intraseasonal timescale of the MJO. We provide a perspective that a certain type of the MJO can be regarded as a transition process, responding to the eastward shift of the region of large-scale positive buoyancy production following the warmer SST.
The relation among sea surface temperature (SST) cooling in the southeastern Indian Ocean (SEIO), oceanic Rossby waves, and the seasonal onset of the Madden-Julian oscillation (MJO) is examined for the period 1993-2012. A westward propagation of the annual downwelling Rossby waves occurs in the south Indian Ocean for most of the years. However, their amplitude and phase speed vary interannually. Positive SST anomalies (SSTA) migrate concurrently with the Rossby waves but are followed by a wide-spread cold SST area in the SEIO from boreal summer to fall. Although SEIO cooling tends to persist for a longer period until November during positive Indian Ocean Dipole (IOD) and/or El Niño years, it occurs irrespective of the IOD. Convection related to the MJO events during boreal winter propagates from the Indian Ocean to the Pacific only after SEIO cooling is terminated. A correlation analysis indicates that negative SSTA during SEIO cooling are confined to the Southern Hemisphere, but their influence on convection reaches north of the equator via the excitation of local circulations over the eastern Indian Ocean and the tropical western Pacific. The resulting southerly surface wind anomalies may advect dry air from the south of the equator to the north and suppress atmospheric convection around the equator. Thus, SEIO cooling tends to prevent intraseasonal convection from propagating eastward to the Pacific. By briefly analyzing the process of SEIO cooling, the SST variability in the SEIO from boreal summer to fall is found to correlate well with zonal advection and surface heat flux. Zonal advection, in turn, is connected with the strength of westward currents associated with the Rossby waves. An understanding of SEIO upper-ocean processes can contribute to predict the seasonal onset of an MJO sequence.
The response of the ocean to three Madden-Julian Oscillation (MJO) events during the fall of 2011 is simulated by the Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) in a fully coupled mode with high resolution in the atmosphere and ocean. The model simulates the cooler sea surface temperature (SST) and disappearance of the diurnal cycle in SST during the active phase of the MJO and it compares well with the observed SST from Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) buoys. The most striking direct response to the westerly zonal wind stress associated with the onset of the MJO is a rapidly accelerating Yoshida jet in the ocean mixed layer with equatorial zonal currents exceeding 1 m s-1. These jets are found in the model as well as the RAMA buoy observations. In the model, the Yoshida jet is superimposed on the seasonal Wyrtki jet that has a subsurface local maximum between 50 and 150 m. A shear layer separates the subsurface seasonal jet and the surface jet forced by the MJO. The sea surface elevation response and upper ocean heat content show wind generated westward propagating Rossby waves symmetric around the equator and an associated eastward propagating equatorial Kelvin wave response. After the third MJO event, the Yoshida jet spans most of the equatorial Indian Ocean. Upon reaching the Indonesian coast, the associated equatorial Kelvin wave reflects and generates additional westward propagating equatorial Rossby waves. The volume transport associated with these waves causes the westward advection of low-salinity north and south of the equator, impacting the tropical ocean circulation.