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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
Steven K. Esbensen

Abstract

Tropical intraseasonal variability in the eastern North Pacific during June–September of 2000–03 is analyzed using satellite and buoy observations. Quick Scatterometer ocean vector winds and the Tropical Rainfall Measuring Mission (TRMM) precipitation indicate that periods of anomalous surface westerly flow over the east Pacific warm pool during a summertime intraseasonal oscillation (ISO) life cycle are generally associated with an enhancement of convection to the east of 120°W. An exception is a narrow band of suppressed precipitation along 8°N that is associated with negative column-integrated precipitable water anomalies and anticyclonic vorticity anomalies. Periods of surface easterly anomalies are generally associated with suppressed convection to the east of 120°W. Summertime wind jets in the Gulfs of Tehuantepec and Papagayo exhibit heightened activity during periods of ISO easterly anomalies and suppressed convection. Strong variations in east Pacific warm pool wind speed occur in association with the summertime ISO. Anomalous ISO westerly flow is generally accompanied by enhanced wind speed to the east of 120°W, while anomalous easterly flow is associated with suppressed wind speed. Intraseasonal vector wind anomalies added to the climatological flow account for the bulk of the wind speed enhancement in the warm pool during the westerly phase, while the easterly phase shows strong contributions to the negative wind speed anomaly from both intraseasonal vector wind anomalies and suppressed synoptic-scale eddy activity. An analysis using Tropical Atmosphere Ocean buoys and TRMM precipitation suggests that wind–evaporation feedback is important for supporting summertime intraseasonal convection over the east Pacific warm pool. A statistically significant correlation of 0.6 between intraseasonal latent heat flux and precipitation occurs at the 12°N, 95°W buoy. Correlations between precipitation and latent heat flux at the 10°N, 95°W and 8°N, 95°W buoys are positive (0.4), but not statistically significant. Intraseasonal latent heat flux anomalies at all buoys are primarily wind induced. Consistent with the suppressed convection there during the ISO westerly phase, a negative but not statistically significant correlation (−0.3) occurs between precipitation and latent heat flux at the 8°N, 110°W buoy.

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Stephanie A. Henderson
and
Eric D. Maloney

Abstract

A real-time statistical model based on the work of Leroy and Wheeler is developed via multiple logistic regression to predict weekly tropical cyclone activity over the Atlantic and east Pacific basins. The predictors used in the model include a climatology of tropical cyclone genesis for each ocean basin, an El Niño–Southern Oscillation (ENSO) index, and two indices representing the propagating Madden–Julian oscillation (MJO). The Atlantic model also includes a predictor representing the variability of sea surface temperature (SST) in the Main Development Region (MDR). These predictors are suggested as useful for the prediction of tropical cyclogenesis based on previous work in the literature and are further confirmed in this study using basic statistics. Univariate logistic regression models are generated for each predictor in each region to ensure the choice of prediction scheme. Using all predictors, cross-validated hindcasts are developed out to a seven-week forecast lead. A formal stepwise predictor selection procedure is implemented to select the predictors used in each region at each forecast lead.

Brier skill scores and reliability diagrams are used to assess the skill and dependability of the models. Results show an increase in model skill over the time-varying climatology at predicting tropical cyclogenesis by the inclusion of the MJO out to a three-week forecast lead for the east Pacific and a two-week forecast lead for the Atlantic. The importance of ENSO and MDR SST for Atlantic genesis prediction is highlighted, and the uncertain effects of ENSO on east Pacific tropical cyclogenesis are revisited.

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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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Adam V. Rydbeck
and
Eric D. Maloney

Abstract

Processes associated with the local amplification of easterly waves (EWs) in the east Pacific warm pool are explored. Developing EWs favor convection in the southwest and northeast quadrants of the disturbance. In nascent EWs, convection favors the southwest quadrant. As the EW life cycle progresses, convection in the northeast quadrant becomes increasingly prominent and southwest quadrant convection wanes. The EW moisture budget reveals that anomalous meridional winds acting on the mean meridional moisture gradient of the ITCZ produce moisture anomalies supportive of convection in the southwest quadrant early in the EW life cycle. As EWs mature, moisture anomalies on the poleward side of the EW begin to grow and are supported by the advection of anomalous moisture by the mean zonal wind.

In the southwest and northeast portions of the wave, where convection anomalies are favored, lower-tropospheric vorticity is generated locally through vertical stretching that supports a horizontal tilt of the wave from the southwest to the northeast. EWs with such tilts are then able to draw energy via barotropic conversion from the background cyclonic zonal wind shear present in the east Pacific. Convection anomalies associated with EWs vary strongly with changes in the background intraseasonal state. EWs during westerly and neutral intraseasonal periods are associated with robust convection anomalies. Easterly intraseasonal periods are, at times, associated with very weak EW convection anomalies because of weaker moisture and diluted CAPE variations.

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Eric D. Maloney
and
Dennis L. Hartmann

Abstract

Low-level barotropic dynamics may help to explain the modulation of eastern and western North Pacific tropical cyclones by the Madden–Julian oscillation (MJO) during Northern Hemisphere summer. The MJO is characterized by alternating periods of westerly and easterly 850-mb zonal wind anomalies across the tropical Pacific Ocean. When MJO 850-mb wind anomalies are westerly, small-scale, slow-moving eddies grow through barotropic eddy kinetic energy (EKE) conversion from the mean flow. These growing eddies, together with strong surface convergence, 850-mb cyclonic shear, and high mean sea surface temperatures, create a favorable environment for tropical cyclone formation. Periods of strong MJO easterlies over the Pacific are characterized by lesser EKE and negligible eddy growth by barotropic conversion.

The term − u2 u /∂x is a leading contributor to low-level barotropic EKE conversion during MJO westerly periods across the Pacific, indicating the importance of zonal variations in the westerly jet for producing concentrations of eddy energy. This mechanism can be described as wave accumulation associated with variations of the low-level zonal flow. The conversion term − uυ u /∂y contributes a smaller portion of the total conversion over the eastern Pacific, but is of comparable importance to − u2 u /∂x during westerly MJO events in the western Pacific.

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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
Michael J. Dickinson

Abstract

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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Justin W. Whitaker
and
Eric D. Maloney

Abstract

The east Pacific warm pool exhibits basic-state variability associated with the Madden–Julian oscillation (MJO) and Caribbean low-level jet (CLLJ), which affects the development of easterly waves (EWs). This study compares and contrasts composite changes in the background environment, eddy kinetic energy (EKE) budgets, and EW tracks during MJO and CLLJ events. While previous studies have shown that the MJO influences jet activity in the east Pacific, the influence of the MJO and CLLJ on the east Pacific and EWs is not synonymous. The CLLJ is a stronger modulator of the ITCZ than the MJO, while the MJO has a more expansive influence on the northeastern portion of the basin. Anomalous low-level westerly MJO and CLLJ periods are associated with favorable conditions for EW development paralleling the Central American coast, contrary to previous findings about the relationship of the CLLJ to EWs. Easterly MJO and CLLJ periods support enhanced ITCZ EW development, although the CLLJ is a greater modulator of EW tracks in this region, which is likely associated with stronger moisture and convection variations and their subsequent influence on the EKE budget. ITCZ EW growth during easterly MJO periods is more reliant on barotropic conversion than during strong CLLJ periods, when eddy available potential energy (EAPE)-to-EKE conversion associated with ITCZ convection is more important. Thus, the influence of these phenomena on east Pacific EWs should be considered distinct.

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