Klaus M. Weickmann

Tropical Intraseasonal Variability in 14 IPCC AR4 Climate Models Part I: Convective Signals

Abstract This study evaluates the tropical intraseasonal variability, especially the fidelity of Madden–Julian oscillation (MJO) simulations, in 14 coupled general circulation models (GCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Eight years of daily precipitation from each model’s twentieth-century climate simulation are analyzed and compared with daily satellite-retrieved precipitation. Space–time spectral analysis is used to obtain the variance and phase speed of dominant convectively coupled equatorial waves, including the MJO, Kelvin, equatorial Rossby (ER), mixed Rossby–gravity (MRG), and eastward inertio–gravity (EIG) and westward inertio–gravity (WIG) waves. The variance and propagation of the MJO, defined as the eastward wavenumbers 1–6, 30–70-day mode, are examined in detail. The results show that current state-of-the-art GCMs still have significant problems and display a wide range of skill in simulating the tropical intraseasonal va...

30–60 Day Atmospheric Oscillations: Composite Life Cycles of Convection and Circulation Anomalies

Abstract Life cycles of the 30–60 day atmospheric oscillation were examined by compositing 30–60 day filtered NMC global wind analyses (250 mb and 850 mb) and outgoing longwave radiation (OLR) for the years 1979–84. Separate composite life cycles were constructed for the May–October and November–April seasons using empirical orthogonal function analysis of the large-scale divergent wind field (250 mb velocity potential) to define the oscillations phase. Monte Carlo simulations were used to assess the statistical significance of the composite OLR and vector wind fields. Large-scale (wavenumber one) tropical divergent wind features propagate eastward around the globe throughout the seasonal cycle. The spatial relationships between these propagating circulation features and OLR are shown using sequences of composite maps. Good agreement exists between areas of upper-air divergence and areas of convection inferred from the OLR satellite data. Convection anomalies are smaller over tropical Africa and South Am...

Forcing of intraseasonal Kelvin waves in the equatorial Pacific

Ten-year time series of sea surface temperature (SST), 20°C depth, and zonal winds measured by moored buoys across the equatorial Pacific are used to define the intraseasonal (30- to 90-day period) Kelvin waves. The Kelvin waves are observed to be forced west of the date line and propagate at a speed of 2.4 m s−1, with high zonal coherence over at least 10,000 km. They form a major component of thermocline depth variability in the east-central Pacific. The intraseasonal-band variance has a low-frequency modulation both at the annual and interannual frequencies; higher amplitudes are observed in boreal fall/winter and during the onset phase of El Nino warm events. The oceanic intraseasonal variability and its low-frequency modulation are coherent with atmospheric intraseasonal variations (the Madden-Julian Oscillation (MJO)), which are known to propagate eastward into the Pacific from the Indian Ocean as part of a planetary-scale signal. The life cycle of an individual or series of MJOs is determined by a combination of factors including tropical SSTs over the warm pool regions of the Indian and Pacific Oceans and interaction with the planetary-scale atmospheric circulation. Thus the intraseasonal Kelvin waves should be taken as an aspect of a global phenomenon, not simply internal to the Pacific. The oceanic intraseasonal variability peaks at periods near 60–75 days, while the corresponding atmospheric variations have somewhat higher frequencies (35- to 60-day periods). We show that this period offset is potentially related to the zonal fetch of the wind compared to the frequency-dependent zonal wavelength of the Kelvin wave response. A simple model is formulated that suggests an ocean-atmosphere coupling by which zonal advection of SST feeds back to the atmosphere; the model duplicates the steplike advance of warm water and westerly winds across the Pacific at the onset of the El Nino of 1991–1992. The key dynamics of the model is that the atmosphere responds rapidly to the state of the ocean, but the oceans response to the atmosphere is lagged because it is an integral over the entire wind forcing history felt by the wave. This results in a nonlinear ordinary differential equation that allows a net nonzero lowfrequency ocean signal to develop from zero-mean sinusoidal forcing at intraseasonal frequencies.

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.

Circulation Anomalies Associated with Tropical Convection during Northern Winter

Abstract Lagged cross correlations between outgoing longwave radiation (OLR) and National Meteorological Center global analyses are utilized to isolate the preferred upper-level and surface circulation anomalies associated with tropical convection during northern winter. Three intraseasonal time scales are studied: 30–70, 14–30, and 6–14 days. In the 30–70-day band, the upper-level circulation signals are zonally elongated, with zonal wavenumbers 0–2 dominant. Higher-frequency signals are dominated by zonal wavenumbers 5 and 6. In the 14–30-day band, convection over the eastern hemisphere is associated with upper-level anticyclones in the subtropics and appears to be linked in some cases to midlatitude wave trains. The strongest signals are for convection over Africa, Australia, and the eastern Indian Ocean. Only weak signals are seen for convection over Indonesia. In these regions of upper-level easterlies, OLR anomalies peak prior to the maximum anomalies in wind, suggesting forcing of the circulation b...

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.

Intraseasonal (30–60 Day) Fluctuations of Outgoing Longwave Radiation and 250 mb Streamfunction during Northern Winter

Abstract Ten years of outgoing longwave radiation (OLR) and 250 mb circulation data are used in a statistical study which concentrates on 28–72 day fluctuations during Northern Hemisphere winter. The results of spectral and cross-spectral analyses show that 28–72 day planetary-scale oscillations of OLR and 250 mb circulation are statistically significant features of the entire 10-year dataset. The strongest OLR fluctuations at 28–72 day periods are located from the equator to 15°S and extend from about 60 to 160°E and in the vicinity of the South Pacific Convergence Zone (SPCZ). The streamfunction variance shows significant 28–72 day fluctuations over the subtropics of both hemispheres and over the extratropical North Atlantic. The OLR anomalies propagate from west to east between 60 and 160°E at about 5 m s−1. There are statistically significant relationships between the regions of (inferred) equatorial cloudiness and planetary-scale circulation features. Fluctuations in the windfield near the exit regio...

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...

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...

Real-Time Monitoring and Prediction of Modes of Coherent Synoptic to Intraseasonal Tropical Variability

Abstract A technique of near-real-time monitoring and prediction of various modes of coherent synoptic to intraseasonal zonally propagating tropical variability is developed. It involves Fourier filtering of a daily updated global dataset for the specific zonal wavenumbers and frequencies of each of the phenomena of interest. The filtered fields obtained for times before the end of the dataset may be used for monitoring, while the filtered fields obtained for times after the end point may be used as a forecast. Tests of the technique, using satellite-observed outgoing longwave radiation (OLR) data, reveal its skill for monitoring. For prediction, it demonstrates good skill for the Madden–Julian oscillation (MJO), and detectable skill for other convectively coupled equatorial modes, although the decaying amplitude of the predictions with time is a characteristic that users need to be aware of. The skill for the MJO OLR field appears to be equally as good as that obtained by the recent empirical MJO forecas...