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Eric D. Maloney

Abstract

The NCAR CCM3.6 with microphysics of clouds with relaxed Arakawa–Schubert convection produces an intraseasonal oscillation that is highly dependent on lower-tropospheric moistening by surface convergence. Model intraseasonal convection is most highly correlated with surface convergence at zero lag, causing enhanced convection to be associated with 850-mb easterly anomalies, where surface convergence is strongest. The tendency for surface convergence to maximize within 850-mb easterly anomalies is consistent with meridional frictional convergence into equatorial surface pressure troughs associated with planetary-scale tropical wave circulations. Anomalous vertical advection associated with meridional surface convergence influences model convection by moistening the lower troposphere. Observed Madden–Julian oscillation (MJO) convection and lower-tropospheric specific humidity are also significantly correlated with surface convergence, although correlations are weaker than in the model, and convergence leads convection anomalies. Observed MJO enhanced convection tends to fall closer to the point of maximum convergence in the 850-mb equatorial zonal wind anomaly field. Although surface convergence appears important for both observed and model intraseasonal convection, the significant differences between observed and modeled intraseasonal variability suggest that interactions between convection and the large-scale circulation in the model are not completely realistic.

The wind-induced surface heat exchange (WISHE) mechanism cannot explain the preference for model intraseasonal enhanced convection to coincide with 850-mb easterly anomalies. When the effects of WISHE are removed by fixing the surface wind speed in the calculation of surface latent heat fluxes, the phase relationship between model intraseasonal wind and convection anomalies does not change. Removing WISHE may produce a more robust model intraseasonal oscillation, however. Model intraseasonal oscillation circulation features are better defined, and spectral power in the MJO band is more prominent when WISHE is removed.

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Eric D. Maloney

Abstract

The intraseasonal moist static energy (MSE) budget is analyzed in a climate model that produces realistic eastward-propagating tropical intraseasonal wind and precipitation variability. Consistent with the recharge–discharge paradigm for tropical intraseasonal variability, a buildup of column-integrated MSE occurs within low-level easterly anomalies in advance of intraseasonal precipitation, and a discharge of MSE occurs during and after precipitation when westerly anomalies occur. The strongest MSE anomalies peak in the lower troposphere and are, primarily, regulated by specific humidity anomalies.

The leading terms in the column-integrated intraseasonal MSE budget are horizontal advection and surface latent heat flux, where latent heat flux is dominated by the wind-driven component. Horizontal advection causes recharge (discharge) of MSE within regions of anomalous equatorial lower-tropospheric easterly (westerly) anomalies, with the meridional component of the moisture advection dominating the MSE budget near 850 hPa. Latent heat flux anomalies oppose the MSE tendency due to horizontal advection, making the recharge and discharge of column MSE more gradual than if horizontal advection were acting alone. This relationship has consequences for the time scale of intraseasonal variability in the model.

Eddies dominate intraseasonal meridional moisture advection in the model. During periods of low-level intraseasonal easterly anomalies, eddy kinetic energy (EKE) is anomalously low due to a suppression of tropical synoptic-scale disturbances and other variability on time scales shorter than 20 days. Anomalous moistening of the equatorial lower troposphere occurs during intraseasonal easterly periods through suppression of eddy moisture advection between the equator and poleward latitudes. During intraseasonal westerly periods, EKE is enhanced, leading to anomalous drying of the equatorial lower troposphere through meridional advection. Given the importance of meridional moisture advection and wind-induced latent heat flux to the intraseasonal MSE budget, these findings suggest that to simulate realistic intraseasonal variability, climate models must have realistic basic-state distributions of lower-tropospheric zonal wind and specific humidity.

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Eric D. Maloney and Jeffrey Shaman

Abstract

Intraseasonal variability of boreal summer rainfall and winds in tropical West Africa and the east Atlantic is examined using daily Tropical Rainfall Measuring Mission (TRMM) precipitation and the NCEP–NCAR reanalysis during 1998–2006. Intraseasonal precipitation variability is dominated by two significant spectral peaks at time scales near 15 and 50 days, accompanied by corresponding peaks in eddy kinetic energy (EKE) and eddy enstrophy. Regional precipitation variability on 30–90-day time scales is significantly correlated (+0.6) with a global Madden–Julian oscillation time series based on equatorial zonal winds, supporting the results of A. J. Matthews. The overall amplitude of the 30–90-day West African monsoon precipitation variability during a given summer, however, does not appear to be strongly regulated by interannual variability in MJO amplitude.

Composite analysis and complex empirical orthogonal function analysis shows that 30–90-day precipitation anomalies are generally zonally elongated, grow and decay in place, and have maximum amplitude near the Gulf of Guinea and in the Atlantic ITCZ. Composite 30–90-day enhanced precipitation events are accompanied by a significant suppression of eastern North Atlantic trade winds. Suppressed 30–90-day precipitation events are associated with an enhancement of the Atlantic trade winds. Enhanced (suppressed) EKE occurs just to the north of the east Atlantic ITCZ during positive (negative) 30–90-day precipitation events, with the maximum EKE magnitude lagging precipitation events by about 5 days.

East Atlantic tropical cyclone activity is significantly modulated on intraseasonal time scales. The number of tropical cyclones that occur in the Atlantic’s main development region to the east of 60°W is suppressed about 5–10 days before maxima in a regional intraseasonal precipitation time series, and enhanced about 5–10 days after time series maxima. An analysis of east Atlantic tropical cyclone activity based on an equatorial MJO index produces similar results. Consistent with the results of K. C. Mo, variations in vertical shear may help explain this modulation of tropical cyclone activity.

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Eric D. Maloney and Chidong Zhang

Abstract

This chapter reviews Professor Michio Yanai’s contributions to the discovery and science of the Madden–Julian oscillation (MJO). Professor Yanai’s work on equatorial waves played an inspirational role in the MJO discovery by Roland Madden and Paul Julian. Professor Yanai also made direct and important contributions to MJO research. These research contributions include work on the vertically integrated moist static energy budget, cumulus momentum transport, eddy available potential energy and eddy kinetic energy budgets, and tropical–extratropical interactions. Finally, Professor Yanai left a legacy through his students, who continue to push the bounds of MJO research.

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Dennis L. Hartmann and Eric D. Maloney

Abstract

A stochastic barotropic model linearized about the 850-mb flow is used to investigate the relationship between wind variations associated with the Madden–Julian oscillation (MJO) and eddy kinetic energy variations in the Tropics. Such a model is successful in predicting the observed location of eddy kinetic energy maxima during the westerly phase of the MJO and the suppression of eddy activity during the easterly phase of the MJO. The concentration of eddy energy during the westerly phase results from the strong east–west and north–south gradients of the large-scale wind fields. The model shows that barotropic wave propagation and wave mean–flow interaction tend to concentrate small-scale Rossby wave energy in regions of convergence, which may be an important mechanism for organizing convection into tropical cyclones. The structure and barotropic energetics of the wave activity are similar to those observed, but the modeled eddies are smaller in scale and do not move westward as do the observed eddies. The eddies that dominate the observed correlations are heavily modified by convection, but barotropic processes can explain the localization of eddy energy by the MJO that is observed.

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Eric D. Maloney and Dennis L. Hartmann

Abstract

The National Center for Atmospheric Research (NCAR) Community Climate Model, version 3.6 (CCM3) simulation of tropical intraseasonal variability in zonal winds and precipitation can be improved by implementing the microphysics of cloud with relaxed Arakawa–Schubert (McRAS) convection scheme of Sud and Walker. The default CCM3 convection scheme of Zhang and McFarlane produces intraseasonal variability in both zonal winds and precipitation that is much lower than is observed. The convection scheme of Hack produces high tropical intraseasonal zonal wind variability but no coherent convective variability at intraseasonal timescales and low wavenumbers. The McRAS convection scheme produces realistic variability in tropical intraseasonal zonal winds and improved intraseasonal variability in tropical precipitation, although the variability in precipitation is somewhat less than is observed. Intraseasonal variability in CCM3 with the McRAS scheme is highly sensitive to the parameterization of convective precipitation evaporation in unsaturated environmental air and unsaturated downdrafts. Removing these effects greatly reduces intraseasonal variability in the model. Convective evaporation processes in McRAS affect intraseasonal variability mainly through their time-mean effects and not through their variations. Convective rain evaporation and unsaturated downdrafts improve the modeled specific humidity and temperature climates of the Tropics and increase convection on the equator. Intraseasonal variability in CCM3 with McRAS is not improved by increasing the boundary layer relative humidity threshold for initiation of convection, contrary to the results of Wang and Schlesinger. In fact, intraseasonal variability is reduced for higher thresholds. The largest intraseasonal moisture variations during a model Madden–Julian oscillation life cycle occur above the boundary layer, and humidity variations within the boundary layer are small.

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Ghassan J. Alaka and Eric D. Maloney
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Eric D. Maloney and Jeffrey T. Kiehl

Abstract

Coupling the NCAR Community Climate Model version 3.6 (CCM3.6) with relaxed Arakawa–Schubert convection to a slab ocean model (SOM) improves the simulation of eastern Pacific convection during a composite June–November intraseasonal oscillation (ISO) life cycle. Intraseasonal oscillations in the SOM simulation produce convective variability over the tropical northeastern Pacific that is similar to that produced by the observed Madden–Julian oscillation (MJO). A composite ISO life cycle in the SOM simulation exhibits stronger, more coherent, and more widespread eastern Pacific warm pool convective anomalies than in a control simulation using climatological SSTs. Competing convective forcings over land and ocean make eastern Pacific low-level circulation anomalies more complex in the SOM simulation than in the observed MJO.

Off-equatorial eastern Pacific SST variations of more than 0.6°C are associated with the June–November SOM simulation ISO. These variations are similar to those observed with the MJO. No significant equatorial east Pacific SST anomalies occur in the model, supporting the contention that observed MJO SST anomalies on the equator are caused by ocean dynamics. Positive off-equatorial SOM simulation SST anomalies are nearly in phase with enhanced precipitation during significant MJO events, whereas observed SST anomalies lead enhanced precipitation by just under 10 days. Latent heat flux and surface shortwave radiation anomalies are the dominant terms in controlling east Pacific intraseasonal SST in the SOM simulation, as in observations. Positive latent heat flux and shortwave radiation anomalies (positive defined as downward into the ocean) lead enhanced SST by about 10 days during significant ISO events in the SOM simulation.

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Eric D. Maloney and Michael J. Dickinson

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The tropical intraseasonal oscillation (ISO) causes variations in the large-scale flow over the western North Pacific during June–August that strongly impact the energetics of tropical depression (TD)-type disturbances. An energetics analysis is conducted with NCEP–NCAR reanalysis data during June–August of 1979–2001. Composite TD-type disturbance perturbation kinetic energy (PKE) is significantly higher during ISO 850-hPa westerly periods than during ISO 850-hPa easterly periods. ISO westerly periods are associated with enhanced barotropic conversion and enhanced perturbation available potential energy (PAPE) to PKE conversion. ISO easterly periods are characterized by diminished TD-type PKE, negligible barotropic conversion, and weakened PAPE to PKE conversions, as compared to composite TD-type disturbances during ISO westerly periods and the entire June–August record. Barotropic conversion accounts for a larger fraction of the PKE generation during ISO westerly periods than during the entire June–August record, and vertically averaged barotropic conversion during ISO westerly periods is 3–4 times that during ISO easterly periods. Barotropic conversion during ISO westerly periods maximizes in the lower troposphere, coincident with the maximum in TD-type disturbance kinetic energy. PAPE to PKE conversion maximizes in the upper troposphere, where it is redistributed to the lower-troposphere and tropopause levels, and horizontally, by the perturbation geopotential flux. PAPE is primarily generated through convective heating associated with the TD-type disturbances and is converted to PKE through the negative correlation of pressure velocity and temperature.

The effect of western Pacific ISO flow variations on the energy budgets of TD-type disturbances may help explain the ISO-related modulation of tropical cyclones observed by Liebmann et al. Energetic TD-type disturbances during ISO westerly periods may provide suitable seed disturbances from which tropical cyclones may form.

June–August TD-type disturbance structure and energetics (unstratified by ISO phase) were compared to the results of Lau and Lau, who used a different analysis product, lower-resolution dataset, and shorter data record. TD-type disturbance structure and energetics are consistent with those shown in Lau and Lau. The largest deviation in the present analysis from that of Lau and Lau is the strong destruction of PKE found at 150 hPa, a level not resolved in their study. Although the sign of the 150-hPa signal is consistent with southwest–northeast-tilted TD-type disturbances interacting with strongly sheared easterly flow aloft, the nonlinear nature of the energy budget calculations may also amplify the effects of unrelated variability.

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Eric D. Maloney and Dennis L. Hartmann

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A composite life cycle of the Madden–Julian oscillation (MJO) is constructed using an index based on the first two EOFs of the bandpass-filtered (20–80 days) 850-mb zonal wind averaged from 5°N to 5°S every 2.5° around the equator. Precipitation, 1000-mb convergence, 850-mb wind, and 200-mb wind are composited for the period 1979–95. Water vapor integrated from the surface to 300 mb is composited for the period 1988–92.

Frictional moisture convergence at the equator is shown to play an important role in the life cycle of the Madden–Julian oscillation (MJO). Regions of boundary layer convergence foster growth of positive water vapor anomalies to the east of convection. This convergence coincides with 850-mb easterly wind anomalies, as is consistent with Kelvin wave dynamics. Drying of the atmosphere occurs rapidly after the passage of convection with the onset of 850-mb westerly perturbations. Possible mechanisms for this drying include boundary layer divergence and subsidence or horizontal advection from the west or extratropics associated with Rossby wave circulations. Frictional convergence in front of convection helps to slowly moisten the atmosphere to a state that is again favorable for convection. This moistening may set the timescale for the reinitiation of convection in the Indian and west Pacific Oceans after strong drying and provides a mechanism for slow eastward propagation. A significant correlation exists between surface convergence and column water vapor anomalies in the west Pacific and Indian Oceans. Weaker correlations exist between 850-mb convergence and water vapor anomalies. Over the west Pacific, surface convergence leads positive water vapor anomalies, while 850-mb convergence lags positive water vapor anomalies.

Northern Hemisphere summer (May–October) composites show that the phases of the MJO coincide with“active” and “break” periods of the Indian summer monsoon at intraseasonal timescales. The northward propagation of precipitation across India during the summer monsoon is associated with northward and westward movement of Rossby wave features trailing the main center of equatorial convection associated with the MJO.

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