scholarly journals Identifying Convectively Coupled Equatorial Waves Using Theoretical Wave Eigenvectors

2016 ◽  
Vol 144 (6) ◽  
pp. 2235-2264 ◽  
Author(s):  
H. Reed Ogrosky ◽  
Samuel N. Stechmann
Keyword(s):  
Longwave Radiation ◽  
Warm Pool ◽  
Reanalysis Data ◽  
Space Time ◽  
Boreal Summer ◽  
Equatorial Waves ◽  
Convectively Coupled Equatorial Waves ◽  
The Pacific ◽  
Potential Applications ◽  
Good Agreement

Abstract Convectively coupled equatorial waves (CCEWs) are often identified by space–time filtering techniques that make use of the eigenvalues of linear shallow water theory. Here, instead, a method is presented for identifying CCEWs by projection onto the eigenvectors of the theory. This method does not use space–time filtering; instead, wave signals corresponding to the first baroclinic Kelvin, Rossby, and mixed Rossby–gravity (MRG) waves are constructed from reanalysis data by a series of projections onto (i) vertical and meridional modes and (ii) the wave eigenvectors. In accordance with the theory, only dry variables, that is, winds and geopotential height, are used; no proxy for convection is used. Using lag–lead regression, composites of the structures associated with each eigenvector signal during boreal summer are shown to contain all the features of the theory as well as some additional features seen in previous observational studies, such as vertical tilts. In addition, these composites exhibit propagation in good agreement with the theory in certain regions of the tropics: over the eastern Pacific ITCZ for the Kelvin and MRG composites and over the Pacific warm pool for the Rossby composite. In these respective regions, the Kelvin eigenvector signal is also in good agreement with space–time-filtered outgoing longwave radiation (OLR), and the Rossby and MRG eigenvector signals are in reasonable agreement with space–time-filtered OLR; it is shown that the eigenvector projections used here contribute to this agreement. Finally, a space–time-filtered version of the eigenvector projection is briefly discussed, as are potential applications of the method.

scholarly journals Space-time variability of hydrological drought and wetness in Iran using NCEP/NCAR and GPCC datasets

2010 ◽  
Vol 14 (10) ◽  
pp. 1919-1930 ◽  
Author(s):  
T. Raziei ◽  
I. Bordi ◽  
L. S. Pereira ◽  
A. Sutera
Keyword(s):  
Time Series ◽  
Singular Spectrum Analysis ◽  
Standardized Precipitation Index ◽  
Principal Component ◽  
Reanalysis Data ◽  
Space Time ◽  
Hydrological Drought ◽  
Time Variability ◽  
Climate Conditions ◽  
Good Agreement

Abstract. Space-time variability of hydrological drought and wetness over Iran is investigated using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis and the Global Precipitation Climatology Centre (GPCC) dataset for the common period 1948–2007. The aim is to complement previous studies on the detection of long-term trends in drought/wetness time series and on the applicability of reanalysis data for drought monitoring in Iran. Climate conditions of the area are assessed through the Standardized Precipitation Index (SPI) on 24-month time scale, while Principal Component Analysis (PCA) and Varimax rotation are used for investigating drought/wetness variability, and drought regionalization, respectively. Singular Spectrum Analysis (SSA) is applied to the time series of interest to extract the leading nonlinear components and compare them with linear fittings. Differences in drought and wetness area coverage resulting from the two datasets are discussed also in relation to the change occurred in recent years. NCEP/NCAR and GPCC are in good agreement in identifying four sub-regions as principal spatial modes of drought variability. However, the climate variability in each area is not univocally represented by the two datasets: a good agreement is found for south-eastern and north-western regions, while noticeable discrepancies occur for central and Caspian sea regions. A comparison with NCEP Reanalysis II for the period 1979–2007, seems to exclude that the discrepancies are merely due to the introduction of satellite data into the reanalysis assimilation scheme.

A Modified Multivariate Madden–Julian Oscillation Index Using Velocity Potential

2013 ◽  
Vol 141 (12) ◽  
pp. 4197-4210 ◽  
Author(s):  
Michael J. Ventrice ◽  
Matthew C. Wheeler ◽  
Harry H. Hendon ◽  
Carl J. Schreck ◽  
Chris D. Thorncroft ◽  
...  
Keyword(s):  
Zonal Wind ◽  
Velocity Potential ◽  
Longwave Radiation ◽  
Warm Pool ◽  
Boreal Summer ◽  
Madden Julian Oscillation ◽  
Systematic Analysis ◽  
Planetary Scale ◽  
Daily Data ◽  
The Indian Ocean

Abstract A new Madden–Julian oscillation (MJO) index is developed from a combined empirical orthogonal function (EOF) analysis of meridionally averaged 200-hPa velocity potential (VP200), 200-hPa zonal wind (U200), and 850-hPa zonal wind (U850). Like the Wheeler–Hendon Real-time Multivariate MJO (RMM) index, which was developed in the same way except using outgoing longwave radiation (OLR) data instead of VP200, daily data are projected onto the leading pair of EOFs to produce the two-component index. This new index is called the velocity potential MJO (VPM) indices and its properties are quantitatively compared to RMM. Compared to the RMM index, the VPM index detects larger-amplitude MJO-associated signals during boreal summer. This includes a slightly stronger and more coherent modulation of Atlantic tropical cyclones. This result is attributed to the fact that velocity potential preferentially emphasizes the planetary-scale aspects of the divergent circulation, thereby spreading the convectively driven component of the MJO’s signal across the entire globe. VP200 thus deemphasizes the convective signal of the MJO over the Indian Ocean warm pool, where the OLR variability associated with the MJO is concentrated, and enhances the signal over the relatively drier longitudes of the equatorial Pacific and Atlantic. This work provides a useful framework for systematic analysis of the strengths and weaknesses of different MJO indices.

scholarly journals Identifying the MJO, Equatorial Waves, and Their Impacts Using 32 Years of HIRS Upper-Tropospheric Water Vapor

2013 ◽  
Vol 26 (4) ◽  
pp. 1418-1431 ◽  
Author(s):  
Carl J. Schreck ◽  
Lei Shi ◽  
James P. Kossin ◽  
John J. Bates
Keyword(s):  
Water Vapor ◽  
Rossby Waves ◽  
North Pacific Ocean ◽  
Low Frequency ◽  
Longwave Radiation ◽  
Equatorial Waves ◽  
Climate Data ◽  
Convectively Coupled Equatorial Waves ◽  
Intersatellite Calibration ◽  
The Tropics

Abstract The Madden–Julian oscillation (MJO) and convectively coupled equatorial waves are the dominant modes of synoptic-to-subseasonal variability in the tropics. These systems have frequently been examined with proxies for convection such as outgoing longwave radiation (OLR). However, upper-tropospheric water vapor (UTWV) gives a more complete picture of tropical circulations because it is more sensitive to the drying and warming associated with subsidence. Previous studies examined tropical variability using relatively short (3–7 yr) UTWV datasets. Intersatellite calibration of data from the High Resolution Infrared Radiation Sounder (HIRS) has recently produced a homogeneous 32-yr climate data record of UTWV for 200–500 hPa. This study explores the utility of HIRS UTWV for identifying the MJO and equatorial waves. Spectral analysis shows that the MJO and equatorial waves stand out above the low-frequency background in UTWV, similar to previous findings with OLR. The fraction of variance associated with the MJO and equatorial Rossby waves is actually greater in UTWV than in OLR. Kelvin waves, on the other hand, are overshadowed in UTWV by horizontal advection from extratropical Rossby waves. For the MJO, UTWV identifies subsidence drying in the subtropics, poleward of the convection. These dry anomalies are associated with the MJO’s subtropical Rossby gyres. MJO events with dry anomalies over the central North Pacific Ocean also amplify the 200-hPa flow pattern over North America 7 days later. These events cannot be identified using equatorial OLR alone, which demonstrates that UTWV is a useful supplement for identifying the MJO, equatorial waves.

scholarly journals Horizontal and Vertical Structure of Easterly Waves in the Pacific ITCZ

2008 ◽  
Vol 65 (4) ◽  
pp. 1266-1284 ◽  
Author(s):  
Yolande L. Serra ◽  
George N. Kiladis ◽  
Meghan F. Cronin
Keyword(s):  
Deep Convection ◽  
Longwave Radiation ◽  
Wave Structure ◽  
Boreal Summer ◽  
Convective Heating ◽  
Central Pacific ◽  
Horizontal Structure ◽  
Easterly Waves ◽  
The Pacific ◽  
The Waves

Abstract Outgoing longwave radiation (OLR) and low-level wind fields in the Atlantic and Pacific intertropical convergence zone (ITCZ) are dominated by variability on synoptic time scales primarily associated with easterly waves during boreal summer and fall. This study uses spectral filtering of observed OLR data to capture the convective variability coupled to Pacific easterly waves. Filtered OLR is then used as an independent variable to isolate easterly wave structure in wind, temperature, and humidity fields from open-ocean buoys, radiosondes, and gridded reanalysis products. The analysis shows that while some Pacific easterly waves originate in the Atlantic, most of the waves appear to form and strengthen within the Pacific. Pacific easterly waves have wavelengths of 4200–5900 km, westward phase speeds of 11.3–13.6 m s−1, and maximum meridional wind anomalies at about 600 hPa. A warm, moist boundary layer is observed ahead of the waves, with moisture lofted quickly through the troposphere by deep convection, followed by a cold, dry signal behind the wave. The waves are accompanied by substantial cloud forcing and surface latent heat flux fluctuations in buoy observations. In the central Pacific the horizontal structure of the waves appears as meridionally oriented inverted troughs, while in the east Pacific the waves are oriented southwest–northeast. Both are tilted slightly eastward with height. Although these tilts are consistent with adiabatic barotropic and baroclinic conversions to eddy energy, energetics calculations imply that Pacific easterly waves are driven primarily by convective heating. This differs from African easterly waves, where the barotropic and baroclinic conversions dominate.

scholarly journals Wavelet Analysis of the Convectively Coupled Equatorial Waves in the Wavenumber–Frequency Domain

2009 ◽  
Vol 66 (1) ◽  
pp. 209-212 ◽  
Author(s):  
Martin L. M. Wong
Keyword(s):  
Wavelet Analysis ◽  
Frequency Domain ◽  
Power Spectra ◽  
Longwave Radiation ◽  
Equatorial Region ◽  
Equatorial Waves ◽  
Tropical Depression ◽  
Radiation Data ◽  
Convectively Coupled Equatorial Waves ◽  
Zonal Wavenumber

Abstract Wavelet analysis is performed on 31 yr (1975–2007, except 1978 and 1979) of daily outgoing longwave radiation data in the global equatorial region (15°S–15°N). Power spectra in the zonal wavenumber–frequency domain are obtained. With different scales and bandwidths than in previous Fourier-based analysis, peaks of variances that are associated with the various convectively coupled waves are found. Further, possibly because of the ability to resolve shorter waves that have limited zonal extent, significant variances are also found at tropical depression–type scales. However, waves of zonal wavenumber zero cannot be explicitly analyzed.

scholarly journals Space-time variability of hydrological drought and wetness in Iran using NCEP/NCAR and GPCC datasets

2010 ◽  
Vol 7 (3) ◽  
pp. 3249-3279 ◽  
Author(s):  
T. Raziei ◽  
I. Bordi ◽  
L. S. Pereira ◽  
A. Sutera
Keyword(s):  
Time Series ◽  
Singular Spectrum Analysis ◽  
Standardized Precipitation Index ◽  
Principal Component ◽  
Reanalysis Data ◽  
Space Time ◽  
Climatic Conditions ◽  
Hydrological Drought ◽  
Time Variability ◽  
Good Agreement

Abstract. Space-time variability of hydrological drought and wetness over Iran is investigated using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis and the Global Precipitation Climatology Centre (GPCC) dataset for the common period 1948–2007. The aim is to complement previous studies on the detection of long-term trends in drought/wetness time series and on the applicability of reanalysis data for drought monitoring in Iran. Climatic conditions of the area are assessed through the Standardized Precipitation Index (SPI) on 24-month time scale, while Principal Component Analysis (PCA) and Varimax rotation are used for investigating drought/wetness variability, and drought regionalization, respectively. Singular Spectrum Analysis (SSA) is applied to the time series of interest to extract the leading nonlinear components and compare them with linear fittings. Differences in drought and wetness area coverage resulting from the two datasets are discussed also in relation to the change occurred in recent years. NCEP/NCAR and GPCC are in good agreement in identifying four sub-regions as principal spatial modes of drought variability. However, the climate variability in each area is not univocally represented by the two datasets: a good agreement is found for south-eastern and north-western regions, while noticeable discrepancies occur for central and Caspian sea regions. A comparison with NCEP Reanalysis II for the period 1979–2007, seems to exclude that the discrepancies are merely due to the introduction of satellite data into the reanalysis assimilation scheme.

scholarly journals Modulation of Seasonal Precipitation over the Tropical Western/Central Pacific by Convectively Coupled Mixed Rossby–Gravity Waves

2013 ◽  
Vol 70 (2) ◽  
pp. 600-606 ◽  
Author(s):  
Takeshi Horinouchi
Keyword(s):  
Gravity Waves ◽  
Sea Surface ◽  
Boreal Summer ◽  
Seasonal Precipitation ◽  
Equatorial Waves ◽  
Central Pacific ◽  
Central Pacific Ocean ◽  
Convectively Coupled Equatorial Waves ◽  
Similar Relation ◽  
The Relationship

Abstract The relationship between the interannual variations of the activity of convectively coupled equatorial waves and seasonal mean precipitation in the tropical western to central Pacific Ocean is investigated. It is found that the convectively coupled mixed Rossby–gravity (MRG) waves are highly and negatively correlated with the seasonal precipitation near the equator in boreal summer. It is suggested that the MRG waves, which have convection centers off the equator, suppress the equatorial precipitation. The relation is insignificant in the other seasons, when the interannual variation of sea surface temperature near the equator is greater than in boreal summer. Also, a similar relation is not found in the eastern Pacific in any season.

scholarly journals Differences between Faster versus Slower Components of Convectively Coupled Equatorial Waves

2013 ◽  
Vol 71 (1) ◽  
pp. 98-111 ◽  
Author(s):  
Kazuaki Yasunaga ◽  
Brian Mapes
Keyword(s):  
Gravity Waves ◽  
Tropical Rainfall Measuring Mission ◽  
Space Time ◽  
Equatorial Waves ◽  
Convectively Coupled Equatorial Waves ◽  
Spectral Signal ◽  
Equatorial Wave ◽  
Significant Difference ◽  
Signal Peak ◽  
Inertia Gravity Waves

Abstract This paper describes an analysis of multiyear satellite datasets that subdivide two halves (faster and slower) of the space–time spectral signal peaks corresponding to convectively coupled equatorial waves such as Kelvin and inertia–gravity waves [n = 0 eastward inertia–gravity wave (EIGn0 wave), and n = 1 and n = 2 westward inertia–gravity waves (WIGn1 and WIGn2 waves, respectively)]. The faster (slower) component of an equatorial wave is defined as that which has a spectral signal peak in the regions with deeper (shallower) equivalent depths. The data obtained from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (TRMM-PR) are composited around space–time-filtered equatorial-belt data from the TRMM-3B42 rainfall product to separately estimate the convective and stratiform rainfall modulations. Results indicate that the faster components of WIGn1 and WIGn2 waves modulate convective rain relatively more (and stratiform rain relatively less) than their slower counterparts. For Kelvin and EIGn0 waves, however, there is no significant difference in the rainfall modulation between their faster and slower components. A space–time cospectral analysis of the satellite-retrieved rainfall and moisture shows that in the spectral regions corresponding to WIGn1 and WIGn2 waves, precipitation is significantly correlated with low-level moisture but not with midlevel moisture. In contrast, significant coherence between rainfall and moisture at these levels is found in the spectral regions corresponding to the Kelvin and EIGn0 waves. These results may bear on different convection–wave coupling mechanisms for these “divergent” waves (stratiform instability versus moisture–stratiform instability).

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