Duane E. Waliser

The Influence of Coupled Sea Surface Temperatures on the Madden–Julian Oscillation: A Model Perturbation Experiment

Abstract In this study, the authors compare the Madden–Julian oscillation (MJO) variability in the Goddard Laboratory for Atmospheres atmospheric general circulation model for two different sea surface temperature (SST) boundary conditions. In the “control” simulation, the model employs specified annual cycle SSTs. In the “coupled” simulation, the model employs the same annual cycle SSTs but in addition is coupled to a slab ocean mixed layer that provides prognostic SST anomalies equatorward of 24°. The results show that the simplified interactive SST facilitates a better simulation with respect to a number of general model shortcomings associated with the MJO that were recently documented by Slingo et al. in an Atmospheric Model Intercomparison Project study. These improvements include 1) increased variability associated with the MJO, 2) a tendency for the timescales of the modeled intraseasonal variability to more closely match and consolidate around the timescales found in the observations, 3) a reduce...

Application of MJO Simulation Diagnostics to Climate Models

The ability of eight climate models to simulate the Madden‐Julian oscillation (MJO) is examined using diagnostics developed by the U.S. Climate Variability and Predictability (CLIVAR) MJO Working Group. Although the MJO signal has been extracted throughout the annual cycle, this study focuses on the boreal winter (November‐April) behavior. Initially, maps of the mean state and variance and equatorial space‐time spectra of 850-hPa zonal wind and precipitation are compared with observations. Models best represent the intraseasonal space‐time spectral peak in the zonal wind compared to that of precipitation. Using the phase‐ space representation of the multivariate principal components (PCs), the life cycle properties of the simulated MJOs are extracted, including the ability to represent how the MJO evolves from a given subphase and the associated decay time scales. On average, the MJO decay (e-folding) time scale for all models is shorter (;20‐ 29 days) than observations (;31 days). All models are able to produce a leading pair of multivariate principal components that represents eastward propagation of intraseasonal wind and precipitation anomalies, although the fraction of the variance is smaller than observed for all models. In some cases, the dominant time scale of these PCs is outside of the 30‐80-day band. Several key variables associated with the model’s MJO are investigated, including the surface latent heat flux, boundary layer (925 hPa) moisture convergence, and the vertical structure of moisture. Low-level moisture convergence ahead (east) of convection is associated with eastward propagation in most of the models. A few models are also able to simulate the gradual moistening of the lower troposphere that precedes observed MJO convection, as well as the observed geographical difference in the vertical structure of moisture associated with the MJO. The dependence of rainfall on lower tropospheric relative humidity and the fraction of rainfall that is stratiform are also discussed, including implications these diagnostics have for MJO simulation. Based on having the most realistic intraseasonal multivariate empirical orthogonal functions, principal component power spectra, equatorial eastward propagating outgoing longwave radiation (OLR), latent heat flux, low-level moisture convergence signals, and vertical structure of moisture over the Eastern Hemisphere, the superparameterized Community Atmosphere Model (SPCAM) and the ECHAM4/ Ocean Isopycnal Model (OPYC) show the best skill at representing the MJO.

Potential Predictability of the Madden–Julian Oscillation

The objective of this study is to estimate the limit of dynamical predictability of the Madden–Julian oscillation (MJO). Ensembles of “twin” predictability experiments were carried out with the NASA Goddard Laboratory for the Atmospheres (GLA) atmospheric general circulation model (AGCM) using specified annual cycle SSTs. Initial conditions were taken from a 10-yr control simulation during periods of strong MJO activity identified via extended empirical orthogonal function (EOF) analysis of 30–90-day bandpassed tropical rainfall. From this analysis, 15 cases were chosen when the MJO convective center was located over the Indian Ocean, Maritime Continent, western Pacific Ocean, and central Pacific Ocean, respectively, making 60 MJO cases in total. In addition, 15 cases were selected that exhibited very little to no MJO activity. Two different sets of small random perturbations were added to these 75 initial states. Simulations were then performed for 90 days from each of these 150 perturbed initial conditi...

MJO Simulation Diagnostics

The Madden–Julian oscillation (MJO) interacts with and influences a wide range of weather and climate phenomena (e.g., monsoons, ENSO, tropical storms, midlatitude weather), and represents an important, and as yet unexploited, source of predictability at the subseasonal time scale. Despite the important role of the MJO in climate and weather systems, current global circulation models (GCMs) exhibit considerable shortcomings in representing this phenomenon. These shortcomings have been documented in a number of multimodel comparison studies over the last decade. However, diagnosis of model performance has been challenging, and model progress has been difficult to track, because of the lack of a coherent and standardized set of MJO diagnostics. One of the chief objectives of the U.S. Climate Variability and Predictability (CLIVAR) MJO Working Group is the development of observation-based diagnostics for objectively evaluating global model simulations of the MJO in a consistent framework. Motivation for this activity is reviewed, and the intent and justification for a set of diagnostics is provided, along with specification for their calculation, and illustrations of their application. The diagnostics range from relatively simple analyses of variance and correlation to more sophisticated space–time spectral and empirical orthogonal function analyses. These diagnostic techniques are used to detect MJO signals, to construct composite life cycles, to identify associations of MJO activity with the mean state, and to describe interannual variability of the MJO.

Air-sea interaction

Air—sea interaction associated with tropical intraseasonal variability (ISV) and, particularly, the Madden—Julian Oscillation (MJO) is of interest for three reasons. First, variations of the air—sea fluxes of heat and moisture may be fundamental to mechanisms of tropical ISV. For instance, air—sea interaction may promote the slow eastward propagation of the MJO and its northward propagation in the Indian summer monsoon. Besides playing a critical role for the interplay between convection and dynamics, surface fluxes of heat, moisture, and momentum drive sea surface temperature (SST) perturbations that may feedback to the surface fluxes and ultimately to the atmospheric dynamics, thus, for instance, contributing to the growth of the MJO. Second, the episodic variations of surface momentum, heat, and freshwater fluxes driven by atmospheric ISV may play a role in the maintenance and low-frequency variability of the warm pool in the tropical Indian and Pacific Oceans. For example, the MJO induces transports in the equatorial west Pacific that act in the mean to remove about the same amount of heat from the warm pool as is provided by the mean surface heat flux (Ralph et al., 1997). From the opposite perspective of the ocean driving the atmosphere, interannual variations of SST in the warm pool may also drive interannual variations in MJO activity, which may bear on the ability to predict seasonal variations of MJO activity. Third, the MJO forces surface currents that drive SST variations at the eastern edge of the warm pool (e.g., Kessler et al., 1995). Kelvin waves are also efficiently excited by the MJO (e.g., Hendon et al., 1998), which radiate into the eastern Pacific where they can perturb the SST (e.g., Giese and Harrison, 1991; Zhang, 2001; McPhaden, 2002). These intraseasonal SST variations may lead to a rectified coupled-response, which plays a role in the evolution of the El Nino Southern Oscillation (ENSO) (e.g., Bergman et al, 2001; Zhang and Gottschalck, 2002).

Vertical Moist Thermodynamic Structure and Spatial–Temporal Evolution of the MJO in AIRS Observations

Abstract The atmospheric moisture and temperature profiles from the Atmospheric Infrared Sounder (AIRS)/Advanced Microwave Sounding Unit on the NASA Aqua mission, in combination with the precipitation from the Tropical Rainfall Measuring Mission (TRMM), are employed to study the vertical moist thermodynamic structure and spatial–temporal evolution of the Madden–Julian oscillation (MJO). The AIRS data indicate that, in the Indian Ocean and western Pacific, the temperature anomaly exhibits a trimodal vertical structure: a warm (cold) anomaly in the free troposphere (800–250 hPa) and a cold (warm) anomaly near the tropopause (above 250 hPa) and in the lower troposphere (below 800 hPa) associated with enhanced (suppressed) convection. The AIRS moisture anomaly also shows markedly different vertical structures as a function of longitude and the strength of convection anomaly. Most significantly, the AIRS data demonstrate that, over the Indian Ocean and western Pacific, the enhanced (suppressed) convection is g...

Convective cloud systems and warm-pool sea surface temperatures : coupled interactions and self-regulation

Questions regarding the upper limits on tropical sea surface temperatures and the processes determining those limits have recently come under renewed interest and debate. We present results from an analysis of the relationship between observed sea surface temperature (SST) and organized deep convection in the tropics that has produced new and important findings relevant to this issue. First, the analysis reveals that the highest observed tropical SSTs are generally associated with diminished convection. Second, the maximum convective activity occurs, on average, at an SST of about 29.5°C. Third, at SSTs of about 29°C and greater, intense deep convection is associated with ocean surface cooling of approximately 0.1°C per month, while suppressed deep convection is associated with a similar degree of ocean surface warming. These three findings, together with results from simplified model analyses, emphasize the importance of the cooling mechanisms associated with deep convection in determining the observed upper limits on tropical SST. Implications of the observed relationship between deep convection and SST on the temporal correlations between these fields is discussed, as is the convective cloud systems relative influence on the solar and evaporative heat flux components of the surface energy budget.

A Framework for Assessing Operational Madden–Julian Oscillation Forecasts: A CLIVAR MJO Working Group Project

Abstract The U.S. Climate Variability and Predictability (CLIVAR) MJO Working Group (MJOWG) has taken steps to promote the adoption of a uniform diagnostic and set of skill metrics for analyzing and assessing dynamical forecasts of the MJO. Here we describe the framework and initial implementation of the approach using real-time forecast data from multiple operational numerical weather prediction (NWP) centers. The objectives of this activity are to provide a means to i) quantitatively compare skill of MJO forecasts across operational centers, ii) measure gains in forecast skill over time by a given center and the community as a whole, and iii) facilitate the development of a multimodel forecast of the MJO. The MJO diagnostic is based on extensive deliberations among the MJOWG in conjunction with input from a number of operational centers and makes use of the MJO index of Wheeler and Hendon. This forecast activity has been endorsed by the Working Group on Numerical Experimentation (WGNE), the internationa...

Comparison of the Highly Reflective Cloud and Outgoing Longwave Radiation Datasets for Use in Estimating Tropical Deep Convection

Abstract Currently, there are two long-term satellite-derived datasets most are frequently used as indices for tropical deep convection. These are the Outgoing Longwave Radiation (OLR) and Highly Reflective Cloud (HRC) datasets. Although both of these datasets have demonstrated their value, no direct comparison of these datasets has been conducted, to determine how well they agree when used to estimate tropical convection, nor has there been much work toward comparing these long-record datasets with more recently developed convection datasets. This information is vital since the inhomogeneous sampling of the in situ rainfall record makes it inadequate for many studies concerning tropical convection and the more modern datasets have not achieved a climatologically useful record length for all studies. The goal of this paper is to compare these two datasets in order to quantify their strengths and weaknesses. This information will provide guidance in choosing the most appropriate dataset(s) for subsequent s...

A Framework for Assessing Operational Madden–Julian Oscillation Forecasts

Abstract The U.S. Climate Variability and Predictability (CLIVAR) MJO Working Group (MJOWG) has taken steps to promote the adoption of a uniform diagnostic and set of skill metrics for analyzing and assessing dynamical forecasts of the MJO. Here we describe the framework and initial implementation of the approach using real-time forecast data from multiple operational numerical weather prediction (NWP) centers. The objectives of this activity are to provide a means to i) quantitatively compare skill of MJO forecasts across operational centers, ii) measure gains in forecast skill over time by a given center and the community as a whole, and iii) facilitate the development of a multimodel forecast of the MJO. The MJO diagnostic is based on extensive deliberations among the MJOWG in conjunction with input from a number of operational centers and makes use of the MJO index of Wheeler and Hendon. This forecast activity has been endorsed by the Working Group on Numerical Experimentation (WGNE), the internationa...