ABSTRACT

The Madden–Julian oscillation (MJO) dominates tropical weather on intraseasonal 30–90-day time scales, yet mechanisms for its generation, maintenance, and propagation remain unclear. Although surface moist static energy (MSE) flux is greatest under strong winds in the convective phase, sea surface temperature (SST) warms by ~0.3°C in the clear nonconvective phase of the MJO. Winds converging into the hydrostatic low pressure under warm air over the warm SST increase the vertically integrated MSE. We estimate column-integrated MSE convergence using a model of mixed layer (ML) winds balancing friction, planetary rotation, and hydrostatic pressure gradients. Small (0.3 K) SST anomalies associated with the MJO drive 7 W m⁻² net column MSE convergence averaged over the equatorial Indian Ocean ahead of MJO deep convection. The MSE convergence is in the right phase to contribute to MJO generation and propagation. It is on the order of the total MSE tendency previously assessed from reanalysis, and greater than surface heat flux anomalies driven by intraseasonal SST fluctuations.

Abstract

During the 2011 special observing period of the Dynamics of the Madden–Julian Oscillation (DYNAMO) field experiment, two sounding arrays were established over the central Indian Ocean, one north and one south of the equator, referred to here as the NSA and SSA, respectively. Three-hourly soundings from these arrays augmented by observations of radiation and rainfall are used to investigate the diurnal cycle of ITCZ convection during the MJO suppressed phase. During the first half of October, when convection was suppressed over the NSA but prominent over the SSA, the circulation over the sounding arrays could be characterized as a local Hadley cell. Strong rising motion was present within the ITCZ extending across the SSA with compensating subsidence over the NSA. A prominent diurnal pulsing of this cell was observed, impacting conditions on both sides of the equator, with the cell running strongest in the early morning hours (0500–0800 LT) and notably weakening later in the day (1700–2000 LT). The declining daytime subsidence over the NSA may have assisted the moistening of the low to midtroposphere there during the pre-onset stage of the MJO. Apparent heating Q ₁ within the ITCZ exhibited a diurnal evolution from early morning bottom-heavy profiles to weaker daytime top-heavy profiles, indicating a progression from convective to stratiform precipitation. Making use of the weak temperature gradient approximation, results suggest that both horizontal radiative heating gradients and direct cloud radiative forcing have an important influence on diurnal variations of vertical motion and convection within the ITCZ.

Abstract

Two Madden–Julian oscillation (MJO) episodes observed during the 2011 Atmospheric Radiation Measurement Program MJO Investigation Experiment (AMIE)/DYNAMO field campaign are simulated using a regional model with various cumulus parameterizations, a regional cloud-permitting model, and a global variable-resolution model with a high-resolution region centered over the tropical Indian Ocean. Model biases in relationships relevant to existing instability theories of MJO are examined and their relative contributions to the overall model errors are quantified using a linear statistical model. The model simulations capture the observed approximately log-linear relationship between moisture saturation fraction and precipitation, but precipitation associated with the given saturation fraction is overestimated especially at low saturation fraction values. This bias is a major contributor to the excessive precipitation during the suppressed phase of MJO. After accounting for this bias using a linear statistical model, the spatial and temporal structures of the model-simulated MJO episodes are much improved, and what remains of the biases is strongly correlated with biases in saturation fraction. The excess precipitation bias during the suppressed phase of the MJO episodes is accompanied by excessive column-integrated radiative forcing and surface evaporation. A large portion of the bias in evaporation is related to biases in wind speed, which are correlated with those of precipitation. These findings suggest that the precipitation bias sustains itself at least partly by cloud radiative feedbacks and convection–surface wind interactions.

Abstract

Lower-tropospheric (1000–700 hPa) moistening processes of the two Madden–Julian oscillations (MJOs) over the Indian Ocean during Dynamics of the MJO (DYNAMO)/Cooperative Indian Ocean Experiment on Intraseasonal Variability in Year 2011 (CINDY) are investigated by using soundings, operational assimilation, and satellite data. A scale-separated moisture budget is calculated at the sounding site by using time-decomposed wind and moisture fields. Each budget term is projected onto the intraseasonal moisture anomaly and its time tendency change. The projections and the corresponding temporal correlations are analyzed together with the temporal evolution of the budget terms to identify the dominant moistening process responsible for the MJO evolution. Results indicate that broad-scale advection by low-frequency and MJO flow and moisture fields are dominant moisture sources, while the residual of the moisture budget (−Q ₂) is a dominant sink contributing to the tendency term (propagation) and intraseasonal moisture anomaly (growth and decay). Dividing their life cycles into four phases (suppressed, cloud developing, convective, and decaying phases), the two MJOs exhibit different budget balances in the premoistening stage from the suppressed phase to the cloud-developing phase when low-frequency vertical motion is downward in MJO1 but upward in MJO2. The corresponding drying and moistening are balanced by negative Q ₂ (reevaporation in nonraining cloud) in MJO1 and positive Q ₂ in MJO2. The result implies that low-frequency flow (>60 days) can affect the initiation of MJOs. The premoistening in the lower troposphere by boundary layer moisture convergence leading the deep convection is observed but only in the cloud-developing phase to convective phase of the MJOs. Nonlinear moisture advection by synoptic disturbances always acts as a diffusive term. It is the dominant moisture source in the suppress phase of the two MJOs.

Abstract

A multination joint field campaign called the Dynamics of MJO/Cooperative Indian Ocean Experiment on Intraseasonal Variability in Year 2011 (DYNAMO/CINDY2011) took place in the equatorial Indian Ocean (IO) in late 2011. During the campaign period, two strong MJO events occurred from the middle of October to the middle of December (referred to as MJO I and MJO II, respectively). Both the events were initiated over the western equatorial Indian Ocean (WIO) around 50°–60°E. Using multiple observational data products (ERA-Interim, the ECMWF final analysis, and NASA MERRA), the authors unveil specific processes that triggered the MJO convection in the WIO. It is found that, 10 days prior to MJO I initiation, a marked large-scale ascending motion anomaly appeared in the lower troposphere over the WIO. The cause of this intraseasonal vertical motion anomaly was attributed to anomalous warm advection by a cyclonic gyre anomaly over the northern IO. The MJO II initiation was preceded by a low-level specific humidity anomaly. This lower-tropospheric moistening was attributed to the advection of mean moisture by anomalous easterlies over the equatorial IO. The contrast of anomalous precursor winds at the equator (westerly versus easterly) implies different triggering mechanisms for the MJO I and II events. It was found that upper-tropospheric circumnavigating signals did not contribute the initiation of both the MJO events. The EOF-based real-time multivariate MJO (RMM) indices should not be used to determine MJO initiation time and location because they are primarily used to capture large zonal scale and eastward-propagating signals, not localized features.

Abstract

This study investigates the October and November MJO events observed during the Cooperative Indian Ocean Experiment on Intraseasonal Variability in the Year 2011 (CINDY)/Dynamics of the MJO (DYNAMO) field campaign through cloud-permitting numerical simulations. The simulations are compared to multiple observational datasets. The control simulation at 9-km horizontal grid spacing captures the slow eastward progression of both the October and November MJO events in surface precipitation, outgoing longwave radiation, zonal wind, humidity, and large-scale vertical motion. The vertical motion shows weak ascent in the leading edge of the MJO envelope, followed by deep ascent during the peak precipitation stage and trailed by a broad second baroclinic mode structure with ascent in the upper troposphere and descent in the lower troposphere. Both the simulation and the observations also show slow northward propagation components and tropical cyclone–like vortices after the passage of the MJO active phase. Comparison with synthesized observations from the northern sounding array shows that the model simulates the passage of the two MJO events over the sounding array region well. Sensitivity experiments to SST indicate that daily SST plays an important role for the November MJO event, but much less so for the October event.

Analysis of the moist static energy (MSE) budget shows that both advection and diabatic processes (i.e., surface fluxes and radiation) contribute to the development of the positive MSE anomaly in the active phase, but their contributions differ by how much they lead the precipitation peak. In comparison to the observational datasets used here, the model simulation may have a stronger surface flux feedback and a weaker radiative feedback. The normalized gross moist stability in the simulations shows an increase from near-zero values to ~0.8 during the active phase, similar to what is found in the observational datasets.

Abstract

Dynamics of the Madden–Julian Oscillation (DYNAMO) and Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) observations and reanalysis-based surface flux products are used to test theories of atmosphere–ocean interaction that explain the Madden–Julian oscillation (MJO). Negative intraseasonal outgoing longwave radiation, indicating deep convective clouds, is in phase with increased surface wind stress, decreased solar heating, and increased surface turbulent heat flux—mostly evaporation—from the ocean to the atmosphere. Net heat flux cools the upper ocean in the convective phase. Sea surface temperature (SST) warms during the suppressed phase, reaching a maximum before the onset of MJO convection. The timing of convection, surface flux, and SST is consistent from the central Indian Ocean (70°E) to the western Pacific Ocean (160°E).

Mean surface evaporation observed in TOGA COARE and DYNAMO (110 W m⁻²) accounts for about half of the moisture supply for the mean precipitation (210 W m⁻² for DYNAMO). Precipitation maxima are an order of magnitude larger than evaporation anomalies, requiring moisture convergence in the mean, and on intraseasonal and daily time scales. Column-integrated moisture increases 2 cm before the convectively active phase over the Research Vessel (R/V) Roger Revelle in DYNAMO, in accordance with MJO moisture recharge theory. Local surface evaporation does not significantly recharge the column water budget before convection. As suggested in moisture mode theories, evaporation increases the moist static energy of the column during convection. Rather than simply discharging moisture from the column, the strongest daily precipitation anomalies in the convectively active phase accompany the increasing column moisture.

Abstract

A surface diurnal warm layer is diagnosed from Seaglider observations and develops on half of the days in the Cooperative Indian Ocean Experiment on Intraseasonal Variability/Dynamics of the Madden–Julian Oscillation (CINDY/DYNAMO) Indian Ocean experiment. The diurnal warm layer occurs on days of high solar radiation flux (>80 W m⁻²) and low wind speed (<6 m s⁻¹) and preferentially in the inactive stage of the Madden–Julian oscillation. Its diurnal harmonic has an exponential vertical structure with a depth scale of 4–5 m (dependent on chlorophyll concentration), consistent with forcing by absorption of solar radiation. The effective sea surface temperature (SST) anomaly due to the diurnal warm layer often reaches 0.8°C in the afternoon, with a daily mean of 0.2°C, rectifying the diurnal cycle onto longer time scales. This SST anomaly drives an anomalous flux of 4 W m⁻² that cools the ocean. Alternatively, in a climate model where this process is unresolved, this represents an erroneous flux that warms the ocean. A simple model predicts a diurnal warm layer to occur on 30%–50% of days across the tropical warm pool. On the remaining days, with low solar radiation and high wind speeds, a residual diurnal cycle is observed by the Seaglider, with a diurnal harmonic of temperature that decreases linearly with depth. As wind speed increases, this already weak temperature gradient decreases further, tending toward isothermal conditions.

Abstract

This study quantifies, from a systematic set of regional ocean–atmosphere coupled model simulations employing various coupling intervals, the effect of subdaily sea surface temperature (SST) variability on the onset and intensity of Madden–Julian oscillation (MJO) convection in the Indian Ocean. The primary effect of diurnal SST variation (dSST) is to raise time-mean SST and latent heat flux (LH) prior to deep convection. Diurnal SST variation also strengthens the diurnal moistening of the troposphere by collocating the diurnal peak in LH with those of SST. Both effects enhance the convection such that the total precipitation amount scales quasi-linearly with preconvection dSST and time-mean SST. A column-integrated moist static energy (MSE) budget analysis confirms the critical role of diurnal SST variability in the buildup of column MSE and the strength of MJO convection via stronger time-mean LH and diurnal moistening. Two complementary atmosphere-only simulations further elucidate the role of SST conditions in the predictive skill of MJO. The atmospheric model forced with the persistent initial SST, lacking enhanced preconvection warming and moistening, produces a weaker and delayed convection than the diurnally coupled run. The atmospheric model with prescribed daily-mean SST from the coupled run, while eliminating the delayed peak, continues to exhibit weaker convection due to the lack of strong moistening on a diurnal basis. The fact that time-evolving SST with a diurnal cycle strongly influences the onset and intensity of MJO convection is consistent with previous studies that identified an improved representation of diurnal SST as a potential source of MJO predictability.