Kazuyoshi Kikuchi

Diurnal Precipitation Regimes in the Global Tropics

Abstract Diurnal variations of the global tropical precipitation are documented by using two complementary Tropical Rainfall Measuring Mission (TRMM) datasets (3B42 and 3G68) for 1998–2006 in an attempt to provide a unified view of the diurnal cycle and a metric for evaluating numerical model performance. The 3B42 data have better spatial coverage; the 3G68 data offer more accurate diurnal phase information. The first and second empirical orthogonal function (EOF) modes represent the diurnal cycle and account for 89% of the total variance in 3B42. The third and fourth EOF modes, which account for 10% of the total variance, represent the semidiurnal cycle. Both datasets yield consistent spatial structures and temporal evolution, but they have different advantages: the patterns derived from 3B42 exhibit less noise, while 3G68 yields an arguably more accurate diurnal phase. The diurnal phase derived from 3G68 systematically leads 3B42 by about 3 h. Three tropical diurnal cycle regimes (oceanic, continental, ...

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 ...

Global Perspective of the Quasi-Biweekly Oscillation*

Abstract The quasi-biweekly oscillation (QBW: here defined as a 12–20-day oscillation) is one of the major systems that affect tropical and subtropical weather and seasonal mean climate. However, knowledge is limited concerning its temporal and spatial structures and dynamics, particularly in a global perspective. To advance understanding of the QBW, its life cycle is documented using a tracking method and extended EOF analysis. Both methods yield consistent results. The analyses reveal a wide variety of QBW activity in terms of initiation, movement, development, and dissipation. The convective anomalies associated with the QBW are predominant in the latitude bands between 10° and 30° in both hemispheres. The QBW modes tend to occur regionally and be associated with monsoons. Three boreal summer modes are identified in the Asia–Pacific, Central America, and subtropical South Pacific regions. Five austral summer modes are identified in the Australia–southwest Pacific, South Africa–Indian Ocean, South Ameri...

Spatiotemporal Wavelet Transform and the Multiscale Behavior of the Madden–Julian Oscillation*

Abstract Meteorological and geophysical phenomena involve multiple-scale processes. Here the spatiotemporal wavelet transform (STWT) is applied to detect significant, nonstationary, wave propagation signals from a time–space domain. One of the major advantages of the STWT is the capability to localize the wave properties in both space and time, which facilitates the study of interactions among multiple-scale disturbances by providing relevant information about energy concentration at a given time and space. The global wavelet spectrum (scalogram) of the STWT, which gives an integrated view of the spectrum as wavenumber and frequency, provides a lucid picture of the spectral power distribution that is consistent with the result obtained from the Fourier-based space–time power spectrum. The STWT has also the capability of reconstruction and thus can be used as a spatiotemporal wave filter. The STWT analysis is applied to analyze the multiscale structure of the Madden–Julian oscillation (MJO) studied by Naka...

Distinctive Roles of Air-Sea Coupling on Different MJO Events: A New Perspective Revealed from the DYNAMO/CINDY Field Campaign*

AbstractPrevious observational analysis and modeling studies indicate that air–sea coupling plays an essential role in improving MJO simulations and extending MJO forecasting skills. However, whether the SST feedback plays an indispensable role for the existence of the MJO remains controversial, and the precise physical processes through which the SST feedback may lead to better MJO simulations and forecasts remain elusive.The DYNAMO/Cooperative Indian Ocean Experiment on Intraseasonal Variability in the Year 2011 (CINDY) field campaign recently completed over the Indian Ocean reveals a new perspective and provides better data to improve understanding of the MJO. It is found that among the five MJO events that occurred during the DYNAMO/CINDY field campaign, only two MJO events (the November and March ones) have robust SST anomalies associated with them. For the other three MJO events (the October, December, and January ones), no coherent SST anomalies are observed. This observational scenario suggests th...

Moistening Processes before the Convective Initiation of Madden–Julian Oscillation Events during the CINDY2011/DYNAMO Period

AbstractConvective initiation processes in the Madden–Julian oscillation (MJO) events that occurred during the Cooperative Indian Ocean Experiment on Intraseasonal Variability in the Year 2011 (CINDY2011)/Dynamics of the Madden–Julian Oscillation (DYNAMO) intensive observation period (IOP) were investigated. Two episodes that were initiated in mid-October (MJO1) and mid-November (MJO2) 2011 were analyzed using European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis and satellite data. Moisture budgets in the equatorial Indian Ocean (IO) domain (10°S–10°N, 60°–90°E) were analyzed in detail by separating each variable into basic-state (>80 day), intraseasonal (20–80 day), and high-frequency (<20 day) variations. The quality of the ECMWF reanalysis was also evaluated against the sounding data collected during the field campaign.In both MJO events, the increase in precipitable water started 8–9 days prior to the convective initiation. Moisture advection decomposition revealed that advection of b...

Tropical Intraseasonal Variability in the MRI-20km60L AGCM*

This study documents the detailed characteristics of the tropical intraseasonal variability (TISV) in the MRI-20km60L AGCM that uses a variant of the Arakawa‐Schubert cumulus parameterization. Mean states, power spectra, propagation features, leading EOF modes, horizontal and vertical structures, and seasonality associated with the TISV are analyzed. Results show that the model reproduces the mean states in winds realistically and in convection comparable to that of the observations. However, the simulated TISV is less realistic. It shows low amplitudes in convection and low-level winds in the 30‐60-day band. Filtered anomalies have standing structures. Power spectra and lag correlation of the signals do not propagate dominantly either in the eastward direction during boreal winter or in the northward direction during boreal summer. A combined EOF (CEOF) analysis shows that winds and convection have a loose coupling that cannot sustain the simulated TISV as realistically as that observed. In the composited mature phase of the simulated MJO, the low-level convergence does not lead convection clearly so that the moisture anomalies do not tilt westward in the vertical, indicating that the low-level convergence does not favor the eastward propagation. The less realistic TISV suggests that the representation of cumulus convection needs to be improved in this model.

CINDY2011/DYNAMO Madden-Julian oscillation successfully reproduced in global cloud/cloud-system resolving simulations despite weak tropical wavelet power

The role of tropical atmospheric waves in the propagation mechanism of the Madden-Julian oscillation (MJO), a huge eastward-propagating atmospheric pulse that dominates intraseasonal variation of the tropics and affects the entire globe, has been long discussed but remains unclear. An MJO event observed in a major field campaign is reproduced using a front-running global cloud/cloud-system resolving model with 3.5 km, 7 km, and 14 km meshes. The eastward-migration speed of the MJO convective envelope in the 3.5 km and 14 km simulations agree well with observation, despite weak Kelvin wave signal power calculated by applying a combined Fourier-wavelet transform method. Our results suggest that the eastward propagation of this MJO event was principally controlled by an MJO-scale energy balance, and not by dynamical interaction of embedded tropical waves. The eastward propagation is delayed in the 7 km simulation, which features the highest surface latent heat flux to the west of the convective envelope center. This latent heat flux appears to be caused by prolonged existence of westward-migrating Rossby wave-like cyclonic disturbances near the equator; the embedded waves may not be part of the essential mechanism for the MJO eastward propagation, but can affect it by altering the energy balance.

Influences of the MJO on the space-time organization of tropical convection: MJO AND ORGANIZED TROPICAL CONVECTION

The fact that the Madden-Julian Oscillation (MJO) is characterized by large scale patterns of enhanced tropical rainfall has been widely recognized for decades. However, the precise nature of any two-way feedback between the MJO and the properties of smaller scale organization that makes up its convective envelope is not well understood. Satellite estimates of brightness temperature are used here as a proxy for tropical rainfall and a variety of diagnostics are applied to determine the degree to which tropical convection is affected either locally or globally by the MJO. To address the multi-scale nature of tropical convective organization, the approach ranges from space-time spectral analysis to an object-tracking algorithm. In addition to the intensity and distribution of global tropical rainfall, the relationship between the MJO and other tropical processes such as convectively coupled equatorial waves, mesoscale convective systems and the diurnal cycle of tropical convection are also analyzed. The main findings of this paper are that, aside from the well known increase in rainfall activity across scales within the MJO convective envelope, the MJO does not favor any particular scale or type of organization , and there is no clear signature of the MJO in terms of the globally integrated distribution of brightness temperature or rainfall.