Katherine H. Straub

Zonal and Vertical Structure of the Madden–Julian Oscillation

Abstract A statistical study of the three-dimensional structure of the Madden–Julian oscillation (MJO) is carried out by projecting dynamical fields from reanalysis and radiosonde data onto space–time filtered outgoing longwave radiation (OLR) data. MJO convection is generally preceded by low-level convergence and upward motion in the lower troposphere, while subsidence, cooling, and drying prevail aloft. This leads to moistening of the boundary layer and the development of shallow convection, followed by a gradual and then more rapid lofting of moisture into the middle troposphere at the onset of deep convection. After the passage of the heaviest rainfall, a westerly wind burst region is accompanied by stratiform precipitation, where lower tropospheric subsidence and drying coincide with continuing upper tropospheric upward motion. The evolution of the heating field leads to a temperature structure that favors the growth of the MJO. The analysis also reveals distinct differences in the vertical structure...

Observations of a Convectively Coupled Kelvin Wave in the Eastern Pacific ITCZ

Abstract A case study of a convectively coupled Kelvin wave in the eastern Pacific intertropical convergence zone (ITCZ) is presented, as observed during the 1997 Pan American Climate Studies (PACS) Tropical Eastern Pacific Process Study (TEPPS). The large-scale convective envelope associated with this disturbance, with a zonal scale of approximately 1000–2000 km, propagates eastward at 15 m s−1 along the mean convective axis of the ITCZ. This envelope consists of many smaller-scale, westward-moving convective elements, with zonal scales on the order of 100–500 km. As the convectively coupled Kelvin wave disturbance propagates eastward, it exerts a strong control on local convection. Radar and vertical profiler data collected aboard the NOAA R/V Ronald H. Brown during the wave passage show that convection deepens rapidly as the Kelvin wave approaches from the west, progressing from isolated, shallow cumuli to organized deep convective features within just 12 h. Initially, rainfall in the vicinity of the s...

The Observed Structure of Convectively Coupled Kelvin Waves: Comparison with Simple Models of Coupled Wave Instability

Observations of the horizontal and vertical structure of convectively coupled Kelvin waves are presented and are compared with the predicted structures of moist Kelvin (or gravity) waves in three simple models of coupled wave instability: wave‐conditional instability of the second kind (CISK), wind-induced surface heat exchange (WISHE), and stratiform instability. The observations are based on a linear regression analysis of multiple years of ECMWF reanalysis and station radiosonde data. Results suggest that both the wave-CISK and stratiform instability theories successfully predict many important features of observed moist Kelvin waves, but that unrealistic aspects of these models limit their ability to provide comprehensive explanations for the dynamics of these waves. It is suggested that an essential component of any theory for moist Kelvin waves is the second baroclinic mode heat source associated with stratiform precipitation.

A Comparison of OLR and Circulation-Based Indices for Tracking the MJO

AbstractTwo univariate indices of the Madden–Julian oscillation (MJO) based on outgoing longwave radiation (OLR) are developed to track the convective component of the MJO while taking into account the seasonal cycle. These are compared with the all-season Real-time Multivariate MJO (RMM) index of Wheeler and Hendon derived from a multivariate EOF of circulation and OLR. The gross features of the OLR and circulation of composite MJOs are similar regardless of the index, although RMM is characterized by stronger circulation. Diversity in the amplitude and phase of individual MJO events between the indices is much more evident; this is demonstrated using examples from the Dynamics of the Madden–Julian Oscillation (DYNAMO) field campaign and the Year of Tropical Convection (YOTC) virtual campaign. The use of different indices can lead to quite disparate conclusions concerning MJO timing and strength, and even as to whether or not an MJO has occurred. A disadvantage of using daily OLR as an EOF basis is that ...

MJO Initiation in the Real-Time Multivariate MJO Index

AbstractMadden–Julian oscillation (MJO) initiation in the real-time multivariate MJO (RMM) index is explored through an analysis of observed case studies and composite events. Specific examples illustrate that both the dates of MJO initiation and the existence of the MJO itself can vary substantially among several well-known MJO indices, depending on whether the focus is on convection or circulation. Composites of “primary” MJO initiation events in which the RMM index rapidly increases in amplitude from a non-MJO state to an MJO state are presented and are supplemented by two case studies from the 1985/86 winter season. Results illustrate that, for primary MJO initiation events in the Indian Ocean (RMM phase 1), slowly eastward-propagating 850-hPa (200 hPa) easterly (westerly) anomalies over the Indian Ocean precede the amplification of the RMM index by at least 10 days, while suppressed convection over the western Pacific Ocean precedes the amplification by 5 days. These “local” Eastern Hemispheric prede...

Interactions between the boreal summer intraseasonal oscillation and higher-frequency tropical wave activity

Abstract Interactions between the convection and circulation fields of the boreal summer intraseasonal oscillation (ISO) and two types of higher-frequency tropical wave activity are examined through a statistical analysis of 22 yr of data. During the convectively active phase of the ISO, westward-propagating mixed Rossby–gravity (MRG)–tropical depression (TD)-type wave activity is enhanced within the low-frequency ISO convective envelope, and is strongly correlated with low-frequency 850-hPa westerly anomalies. At the same time, eastward-propagating convectively coupled Kelvin wave activity is enhanced well to the east of the active ISO convection, in the central Pacific. A case study of an ISO event during July–September 1987 illustrates these statistically derived relationships. The enhanced phase of the ISO is shown to consist primarily of westward-propagating higher-frequency variability, including seven named tropical cyclones in the western Pacific, two of which project onto MRG–TD-type modes as the...

A Model for Convectively Coupled Tropical Waves: Nonlinearity, Rotation, and Comparison with Observations

Abstract Recent observational analysis of both individual realizations and statistical ensembles identifies moist convectively coupled Kelvin waves in the Tropics with supercluster envelopes of convection. This observational analysis elucidates several key features of these waves including their propagation speed of roughly 15 m s−1 and many aspects of their dynamical structure. This structure includes anomalously cold temperatures in the lower troposphere and warm temperatures in the upper troposphere (below 250 hPa) within and sometimes leading the heating region and strong updrafts in the wave, and an upward and westward tilting structure with height below roughly 250 hPa. Other key features in the wave are that anomalous increases in convective available potential energy (CAPE) and surface precipitation lead the wave while the trailing part of the supercluster is dominated by stratiform precipitation. The main result in this paper is the development of a simple model convective parameterization with n...

Extratropical Forcing of Convectively Coupled Kelvin Waves during Austral Winter

Abstract Observations are presented that link extratropical Rossby wave disturbances excited in the Southern Hemisphere subtropical jet to the initiation of convectively coupled Kelvin waves in the Pacific intertropical convergence zone (ITCZ) during austral winter. A baroclinic, zonal wavenumber 6, eastward-propagating Rossby wave train in the subtropical jet turns northeastward in the vicinity of Australia, inducing upper tropospheric divergence and vertical motion fields that spread equatorward and induce cloudiness anomalies in the Tropics. Lower tropospheric pressure surges excited from the extratropics also induce Kelvin wave–like geopotential height and temperature anomalies at the surface, providing additional lower tropospheric convergence and vertical motion forcing. The tropical outgoing longwave radiation (OLR) and circulation fields propagate eastward in tandem with the extratropical Rossby wave train at approximately 17 m s–1. Kelvin wave activity in the central Pacific ITCZ thus appears to ...

An Analysis of Convectively Coupled Kelvin Waves in 20 WCRP CMIP3 Global Coupled Climate Models

Abstract Output from 20 coupled global climate models is analyzed to determine whether convectively coupled Kelvin waves exist in the models, and, if so, how their horizontal and vertical structures compare to observations. Model data are obtained from the World Climate Research Program’s (WCRP’s) Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel dataset. Ten of the 20 models contain spectral peaks in precipitation in the Kelvin wave band, and, of these 10, only 5 contain wave activity distributions and three-dimensional wave structures that resemble the observations. Thus, the majority (75%) of the global climate models surveyed do not accurately represent convectively coupled Kelvin waves, one of the primary sources of submonthly zonally propagating variability in the tropics. The primary feature common to the five successful models is the convective parameterization. Three of the five models use the Tiedtke–Nordeng convective scheme, while the other two utilize the Pan and Randall scheme...

Correction to “Convectively coupled equatorial waves”

] In the paper “Convectively coupled equatorial waves”by G. N. Kiladis et al. (Reviews of Geophysics, 47, RG2003,doi:10.1029/2008RG000266, 2009), we have discovered anerror in the units specified for the geopotential contours inFigures 7, 9, 13, 15, and 17. The contours represent thegeopotential, not the geopotential height as stated in thecaptions. The units for the geopotential are m