Paul E. Roundy

The Role of Tropical Waves in Tropical Cyclogenesis

Abstract This paper analyzes relationships between tropical wave activity and tropical cyclogenesis in all of the earth’s major tropical cyclone basins. Twenty-nine years of outgoing longwave radiation data and global reanalysis winds are filtered and analyzed to determine statistical relationships between wave activity in each basin and the corresponding cyclogenesis. Composite analyses relative to the storm genesis locations show the structures of the waves and their preferred phase relationships with genesis. Five wave types are examined in this study, including mixed Rossby–gravity waves, tropical-depression-type or easterly waves, equatorial Rossby waves, Kelvin waves, and the Madden–Julian oscillation. The latter is not one of the classical tropical wave types, but is a wavelike phenomenon known to have a strong impact on tropical cyclogenesis. Tropical cyclone formation is strongly related to enhanced activity in all of the wave filter bands except for the Kelvin band. In each basin the structure o...

A Climatology of Waves in the Equatorial Region

Abstract Propagating anomalies of moisture and moist deep convection in the Tropics are organized into a variety of large-scale modes. These include (but are not limited to) the so-called intraseasonal oscillations, convectively coupled waves similar to those predicted by shallow water theory on the equatorial beta plane, and tropical-depression-type disturbances. Along with the annual and diurnal cycles, these modes act and interact to control much of the variance of tropical convection. Analyses of 10 yr of outgoing longwave radiation (OLR) and precipitable water (PW) data are carried out to develop comparative climatologies of these wavelike modes. The analysis relaxes the commonly used cross-equatorial symmetry constraints, which allows study of the portions of the wavelike processes that are asymmetric across the equator. Mean background states are found for OLR and for PW as functions of day of the year. Examination of anomalies together with the background reveals much about how the waves are affec...

Large-Scale Atmospheric and Oceanic Conditions during the 2011–12 DYNAMO Field Campaign

AbstractAn international field campaign, Dynamics of the Madden Julian Oscillation (DYNAMO), took place in the Indian Ocean during October 2011–March 2012 to collect observations for the Madden–Julian oscillation (MJO), especially its convective initiation processes. The large-scale atmospheric and oceanic conditions during the campaign are documented here. The ENSO and the Indian Ocean dipole (IOD) states, the monthly mean monsoon circulation and its associated precipitation, humidity, vertical and meridional/zonal overturning cells, and ocean surface currents are discussed. The evolution of MJO events is described using various fields and indices that have been used to subdivide the campaign into three periods. These periods were 1) 17 September–8 December 2011 (period 1), which featured two robust MJO events that circumnavigated the global tropics with a period of less than 45 days; 2) 9 December 2011–31 January 2012, which contained less coherent activity (period 2); and 3) 1 February–12 April 2012, a...

Analysis of Convectively Coupled Kelvin Waves in the Indian Ocean MJO

Abstract The active convective phase of the Madden–Julian oscillation (hereafter active MJO) comprises enhanced moist deep convection on its own temporal and spatial scales as well as increased variance in convection associated with higher-frequency modes. Synoptic-scale cloud superclusters apparently associated with convectively coupled Kelvin waves occur within the active convective envelopes of most MJO events. These convectively coupled Kelvin waves also occur during the suppressed convective phase of the MJO (hereafter suppressed MJO). This observational study presents an analysis of outgoing longwave radiation and reanalysis data to determine how these waves behave differently as they propagate through the active and suppressed MJO. Time indices of the MJO and Kelvin waves are derived for over the equatorial Indian Ocean. Dates of local extrema in these indices are used to composite data to discern how the waves and associated circulations behave on average; then, further composites are made based o...

Observed Relationships between Oceanic Kelvin Waves and Atmospheric Forcing

Abstract The Madden–Julian oscillation (MJO) has been implicated as a major source of the wind stress variability that generates basin-scale Kelvin waves in the equatorial Pacific. One source of debate concerning this relationship is the apparent difference in the frequencies of the two processes. This work utilizes data from the Tropical Atmosphere Ocean (TAO) array of moored buoys along with outgoing longwave radiation data to show by means of a multiple linear regression model and case studies that the frequency discrepancy is due to a systematic decrease in the phase speeds of the Kelvin waves and an increase in the period of the waves toward the east as conditions adjust toward El Nino. Among the potential contributing factors to this phase speed decrease is an apparent air–sea interaction that enhances the wind forcing of some of the Kelvin waves, allowing them to continue to amplify because the propagating wind stress anomaly decelerates to the speed of the developing Kelvin wave instead of the sig...

Contributions of Convectively Coupled Equatorial Rossby Waves and Kelvin Waves to the Real-Time Multivariate MJO Indices

Abstract The real-time multivariate (RMM) Madden–Julian oscillation (MJO) indices have been widely applied to diagnose and track the progression of the MJO. Although it has been well demonstrated that the MJO contributes to the leading signals in these indices, the RMM indices vary erratically from day to day. These variations are associated with noise in the outgoing longwave radiation (OLR) and wind data used to generate the indices. This note demonstrates that some of this “noise” evolves systematically and is associated with other types of propagating modes that project onto the RMM eigenmodes. OLR and zonal wind data are filtered in the wavenumber–frequency domain for the MJO, convectively coupled equatorial Rossby (ER) waves, and convectively coupled Kelvin waves. The filtered data are then projected onto the RMM modes. An example phase space associated with these projections is presented. Linear regression is then applied to isolate the wave signals from random variations in the same bands of the w...

Modulation of the global atmospheric circulation by combined activity in the Madden-Julian oscillation and the El Niño-Southern Oscillation during boreal winter.

Abstract Composite global patterns associated with the El Nino–Southern Oscillation (ENSO) and the Madden–Julian oscillation (MJO) are frequently applied to help make predictions of weather around the globe at lead times beyond a few days. However, ENSO modulates the background states through which the MJO and its global response patterns propagate. This paper explores the possibility that nonlinear variations confound the combined use of composites based on the MJO and ENSO separately. Results indicate that when both modes are active at the same time, the associated patterns in the global flow are poorly represented by simple linear combinations of composites based on the MJO and ENSO individually. Composites calculated by averaging data over periods when both modes are present at the same time more effectively describe the associated weather patterns. Results reveal that the high-latitude response to the MJO varies with ENSO over all longitudes, but especially across the North Pacific Rim, North America...

The Madden–Julian Oscillation’s Influence on African Easterly Waves and Downstream Tropical Cyclogenesis

AbstractThe influence of the Madden–Julian oscillation (MJO) over tropical Africa and Atlantic is explored during the Northern Hemisphere summer months. The MJO is assessed by using real-time multivariate MJO (RMM) indices. These indices divide the active convective signal of the MJO into 8 phases. Convection associated with the MJO is enhanced over tropical Africa during RMM phases 8, 1, and 2. Convection becomes suppressed over tropical Africa during the subsequent RMM phases (phases 3–7). African convective signals are associated with westward-propagating equatorial Rossby waves.The MJO modulates African easterly wave (AEW) activity. AEW activity is locally enhanced during RMM phases 1–3 and suppressed during RMM phases 6–8. Enhanced AEW activity occurs during periods of enhanced convection over tropical Africa, consistent with stronger or more frequent triggering of AEWs as well as more growth associated with latent heat release. Enhanced AEW activity occurs during the low-level westerly wind phase of...

Observed Structure of Convectively Coupled Waves as a Function of Equivalent Depth: Kelvin Waves and the Madden–Julian Oscillation

AbstractThe view that convectively coupled Kelvin waves and the Madden–Julian oscillation (MJO) are distinct modes is tested by regressing data from the Climate Forecast System Reanalysis against satellite outgoing longwave radiation data filtered for particular zonal wavenumbers and frequencies by wavelet analysis. Results confirm that nearly dry Kelvin waves have horizontal structures consistent with their equatorial beta-plane shallow-water-theory counterparts, with westerly winds collocated with the lower-tropospheric ridge, while the MJO and signals along Kelvin wave dispersion curves at low shallow-water-model equivalent depths are characterized by geopotential troughs extending westward from the region of lower-tropospheric easterly wind anomalies through the region of lower-tropospheric westerly winds collocated with deep convection. Results show that as equivalent depth decreases from that of the dry waves (concomitant with intensification of the associated convection), the ridge in the westerlie...

Variability of Intraseasonal Kelvin Waves in the Equatorial Pacific Ocean

Abstract Previous observational work has demonstrated that the phase speed of oceanic equatorial Kelvin waves forced by the Madden–Julian oscillation (MJO) appears to vary substantially. Processes that are responsible for systematic changes in the phase speed of these waves are examined using an ocean general circulation model. The model was integrated for 26 yr with daily wind stress derived from the NCEP–NCAR reanalysis. The model is able to reproduce observed systematic changes of Kelvin wave phase speed reasonably well, providing a tool for the analysis of their dynamics. The relative importance of the upper ocean background state and atmospheric forcing for phase speed changes is determined based on a series of model experiments with various surface forcings. Systematic changes in phase speed are evident in all model experiments that have different slowly varying basic states, showing that variations of the upper ocean background state are not the primary cause of the changes. The model experiments t...