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 Development of a One-Month-Long, Westward-Propagating Subtropical Low in Boreal Summer

2018 ◽  
Vol 146 (1) ◽  
pp. 231-242 ◽  
Author(s):  
John Molinari ◽  
David Vollaro
Keyword(s):  
Wave Breaking ◽  
Northeast Asia ◽  
Wave Structure ◽  
Boreal Summer ◽  
Central Pacific ◽  
The Pacific ◽  
Monsoon Gyre ◽  
Jet Streak ◽  
Q Vector ◽  
Equatorial Rossby Wave

Abstract A strong MJO event produced an upper-tropospheric jet streak in northeast Asia and repeated wave breaking in the jet exit region along 150°E during July 1988. A midlatitude low moved equatorward and intensified in the presence of bandpass-filtered (15–100 day) Q vector forcing for upward motion associated with the wave breaking. This forced ascent helped to moisten the atmosphere enough to increase the column water vapor to above 55 mm. This value was sufficiently large to support a self-sustaining low even after the upper forcing weakened. The horizontal scale of the Q vector forcing was about 1500 km, consistent with the scale of most favorable convective response to quasigeostrophic forcing in the subtropics described by Nie and Sobel. The low lasted one month as it moved southwestward, then westward, while remaining north of 20°N. Maximum precipitation along the track of the low exceeded 700 mm, with an anomaly more than 400 mm. A climatology of long-lasting lows was carried out for the monsoon gyre cases studied previously. During El Niño, long-lasting lows often began near the equator in the central Pacific, and were likely to have a mixed Rossby–gravity wave or equatorial Rossby wave structure. It is speculated that the quasi-biweekly mode, the submonthly oscillation, the 20–25-day mode, and the Pacific–Japan pattern are each variations on this kind of event. During La Niña, long-lasting lows that originated in midlatitudes were more common. It is argued that these lows from midlatitudes represent a unique disturbance type in boreal summer.

scholarly journals Implication of tropical lower stratospheric cooling in recent trends in tropical circulation and deep convective activity

2019 ◽  
Vol 19 (4) ◽  
pp. 2655-2669 ◽  
Author(s):  
Kunihiko Kodera ◽  
Nawo Eguchi ◽  
Rei Ueyama ◽  
Yuhji Kuroda ◽  
Chiaki Kobayashi ◽  
...  
Keyword(s):  
Deep Convection ◽  
Equatorial Indian Ocean ◽  
Boreal Summer ◽  
Convective Activity ◽  
Tropical Circulation ◽  
Surface Cooling ◽  
Central Pacific ◽  
Tropical Tropopause Layer ◽  
Tropical Tropopause ◽  
Stratospheric Cooling

Abstract. Large changes in tropical circulation from the mid-to-late 1990s to the present, in particular changes related to the summer monsoon and cooling of the sea surface in the equatorial eastern Pacific, are noted. The cause of such recent decadal variations in the tropics was studied using a meteorological reanalysis dataset. Cooling of the equatorial southeastern Pacific Ocean occurred in association with enhanced cross-equatorial southerlies that were associated with a strengthening of the deep ascending branch of the boreal summer Hadley circulation over the continental sector connected to stratospheric circulation. From boreal summer to winter, the anomalous convective activity center moves southward following the seasonal march to the equatorial Indian Ocean–Maritime Continent region, which strengthens the surface easterlies over the equatorial central Pacific. Accordingly, ocean surface cooling extends over the equatorial central Pacific. We suggest that the fundamental cause of the recent decadal change in the tropical troposphere and the ocean is a poleward shift of convective activity that resulted from a strengthening of extreme deep convection penetrating into the tropical tropopause layer, particularly over the African and Asian continents and adjacent oceans. We conjecture that the increase in extreme deep convection is produced by a combination of land surface warming due to increased CO2 and a reduction of static stability in the tropical tropopause layer due to tropical stratospheric cooling.

Reexamination of the Wave Activity Envelope Convective Scheme in Theoretical Modeling of MJO

2017 ◽  
Vol 30 (3) ◽  
pp. 1127-1138 ◽  
Author(s):  
Guosen Chen ◽  
Bin Wang
Keyword(s):  
General Model ◽  
Wave Structure ◽  
Theoretical Models ◽  
Wave Activity ◽  
Convective Heating ◽  
Model Framework ◽  
Horizontal Structure ◽  
Skeleton Model ◽  
Low Level ◽  
Neutral Mode

Abstract The skeleton model is one of the theoretical models for understanding the essence of the Madden–Julian oscillation (MJO). The heating parameterization scheme in the skeleton model assumes that precipitation tendency is in phase and proportional to the low-level moisture anomaly. The authors show that the observed MJO precipitation tendency is not in phase with the low-level moisture anomaly. The consequence of the wave activity envelope (WAE) scheme is reexamined by using a general MJO theoretical framework in which trio-interaction among convective heating, moisture, and wave–boundary layer (BL) dynamics are included and various simplified convective schemes can be accommodated. Without the BL dynamics, the general model framework can be reduced to the original skeleton model. The authors show that the original skeleton model yields a neutral mode that exhibits a “quadrupole” horizontal structure and a quadrature relationship between precipitation and low-level moisture; both are inconsistent with observations. With the BL dynamics and damping included, the model can produce a growing mode with improved horizontal structure and precipitation–moisture relationship, but deficiencies remain because of the WAE scheme. The authors further demonstrate that the general model with the simplified Betts–Miller scheme and BL dynamics can produce a realistic horizontal structure (coupled Kelvin–Rossby wave structure) and precipitation–moisture relationship (i.e., the BL moisture convergence leads precipitation, and column-integrated moisture coincides with precipitation).

Simulations of the West African Monsoon with a Superparameterized Climate Model. Part II: African Easterly Waves

2014 ◽  
Vol 27 (22) ◽  
pp. 8323-8341 ◽  
Author(s):  
Rachel R. McCrary ◽  
David A. Randall ◽  
Cristiana Stan
Keyword(s):  
General Circulation ◽  
Climate Model ◽  
Deep Convection ◽  
West African Monsoon ◽  
General Circulation Models ◽  
African Easterly Waves ◽  
Climate System Model ◽  
Easterly Waves ◽  
Cloud Resolving Model ◽  
The Waves

Abstract The relationship between African easterly waves and convection is examined in two coupled general circulation models: the Community Climate System Model (CCSM) and the “superparameterized” CCSM (SP-CCSM). In the CCSM, the easterly waves are much weaker than observed. In the SP-CCSM, a two-dimensional cloud-resolving model replaces the conventional cloud parameterizations of CCSM. Results show that this allows for the simulation of easterly waves with realistic horizontal and vertical structures, although the model exaggerates the intensity of easterly wave activity over West Africa. The simulated waves of SP-CCSM are generated in East Africa and propagate westward at similar (although slightly slower) phase speeds to observations. The vertical structure of the waves resembles the first baroclinic mode. The coupling of the waves with convection is realistic. Evidence is provided herein that the diabatic heating associated with deep convection provides energy to the waves simulated in SP-CCSM. In contrast, horizontal and vertical structures of the weak waves in CCSM are unrealistic, and the simulated convection is decoupled from the circulation.

Analysis of Convectively Coupled Kelvin Waves in the Indian Ocean MJO

2008 ◽  
Vol 65 (4) ◽  
pp. 1342-1359 ◽  
Author(s):  
Paul E. Roundy
Keyword(s):  
Indian Ocean ◽  
Spatial Scales ◽  
Deep Convection ◽  
Equatorial Indian Ocean ◽  
Longwave Radiation ◽  
Kelvin Waves ◽  
Synoptic Scale ◽  
Left Behind ◽  
Geographical Regions ◽  
The Waves

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 on subsets of this list of dates that are consistent with the two MJO phases. Results show that the MJO phase modulates the intensity of moist deep convection associated with the Kelvin waves, the evolution of the vertical structure of cloudiness linked to Kelvin waves, and patterns of upper-level outflow from convection coupled to Kelvin waves. Composites reveal that synoptic-scale circulations associated with the release of latent heat in convection coupled to Kelvin waves amplify and are left behind the waves in preferred geographical regions. The MJO modulates the amplitudes of these circulations and the locations where they get left behind the waves. Previous results have suggested a sharp distinction between the phase speeds of the MJO (4–8 m s−1) and of convectively coupled Kelvin waves (specifically 17 m s−1). In contrast, the present work suggests that convectively coupled Kelvin waves have a broad range of characteristic phase speeds, extending from 10 to 17 m s−1, depending on both the region of the world and the phase of the MJO through which they propagate.

The Potential Vorticity Structure and Dynamics of African Easterly Waves

2020 ◽  
Vol 77 (3) ◽  
pp. 871-890 ◽  
Author(s):  
James O. H. Russell ◽  
Anantha Aiyyer
Keyword(s):  
Potential Vorticity ◽  
Deep Convection ◽  
Wave Structure ◽  
Wave Model ◽  
Tropical Cyclogenesis ◽  
African Easterly Waves ◽  
Moist Convection ◽  
Easterly Waves ◽  
Stratiform Clouds ◽  
Diabatic Forcing

Abstract The dynamics of African easterly waves (AEWs) are investigated from the perspective of potential vorticity (PV) using data from global reanalysis projects. To a leading order, AEW evolution is governed by four processes: advection of the wave-scale PV by background flow, advection of background PV by the AEW, diabatic forcing due to wave-scale moist convection, and coupling between the wave and background diabatic forcing. Moist convection contributes significantly to the growth of AEWs in the midtroposphere, and to both growth and propagation of AEWs near the surface. The former is associated with stratiform clouds while the latter with deep convection. Moist convection helps maintain a more upright AEW PV column against the background shear, which makes the wave structure conducive for tropical cyclogenesis. It is also argued that—contrary to the hypothesis in some prior studies—the canonical diabatic Rossby wave model is likely not applicable to AEWs.

Analysis of Convection and Its Association with African Easterly Waves

2006 ◽  
Vol 19 (20) ◽  
pp. 5405-5421 ◽  
Author(s):  
Ademe Mekonnen ◽  
Chris D. Thorncroft ◽  
Anantha R. Aiyyer
Keyword(s):  
West Africa ◽  
Time Scale ◽  
Deep Convection ◽  
Subsequent Development ◽  
Eastern Africa ◽  
Boreal Summer ◽  
Easterly Waves ◽  
Day Range ◽  
Wave Development ◽  
Dynamic Measures

Abstract The association between convection and African easterly wave (AEW) activity over tropical Africa and the tropical Atlantic during the boreal summer is examined using satellite brightness temperature (TB) and ECMWF reanalysis datasets. Spectral analysis using 18 yr of TB data shows significant variance in the 2–6-day range across most of the region. Within the regions of deep convection, this time scale accounts for about 25%–35% of the total variance. The 2–6-day convective variance has similar amplitudes over western and eastern Africa, while dynamic measures of AEW activity show stronger amplitudes in the west. This study suggests that weak AEW activity in the east is consistent with initial wave development there and indicates that convection triggered on the western side of the mountains over central and eastern Africa, near Darfur (western Sudan) and Ethiopia, has a role in initiating AEWs westward. The subsequent development and growth of AEWs in West Africa is associated with stronger coherence with convection there. Results show large year-to-year variability in convection at the 2–6-day time scale, which tends to vary consistently with the mean convection and dynamical measures of AEW activity over West Africa and the Atlantic, but not over central and eastern Africa. The Darfur region is particularly important for providing convective precursors that propagate westward and trigger AEWs downstream. During wet years, convection over eastern Africa (western Ethiopian highlands) can be a significant source of AEW initiation. In addition to being important for precursors of AEWs, the Darfur region is also a source of convection that propagates eastward toward Ethiopia.

scholarly journals The Interaction between Two Separate Propagations of Rossby Waves

2007 ◽  
Vol 135 (10) ◽  
pp. 3521-3540 ◽  
Author(s):  
Yafei Wang ◽  
Koji Yamazaki ◽  
Yasushi Fujiyoshi
Keyword(s):  
Caspian Sea ◽  
Wave Train ◽  
Deep Convection ◽  
Longwave Radiation ◽  
Stationary Wave ◽  
The Philippines ◽  
Boreal Summer ◽  
The Caspian Sea ◽  
Wave Ray ◽  
Zonal Wavenumber

Abstract This study deals with two teleconnection patterns and the subsequent wave train propagations during an East Asian summer. Diagnostic results are as follows: 1) a stationary wave ray with zonal wavenumber 5 approximates the arc path linking the correlation centers originating from the Caspian Sea via Lake Baikal to the sea off the southeast coast of Japan (i.e., the OKJ arc path as a focus area) in a pentad correlation map between 500-hPa geopotential height (Z500) and outgoing longwave radiation (OLR) at 30°N, 150°E in June 1979–98. Ray tracing shows that it took 8–10 days for this stationary wave to propagate from an initial position around the Caspian Sea to the focus area, which roughly coincides with the observed case in July 1998. 2) A wave train pattern (P-Ja) observed in the boreal summer propagated along the arc line in the same way as the normal poleward Rossby wave train originating from the Philippines across the North Pacific (P-J), but with a phase shift northeastward of about 90°. 3) Further correlation analyses showed that the P-J-like waves belong mainly to intraseasonal propagating ones while OKJ waves belong mainly to intraseasonal stationary ones. 4) Propagation of the newly observed wave train pattern (P-Ja) occurred following another wave train along the OKJ arc path in mid-July 1998. Both northeastward and southeastward wave propagations merged off the east coast of Japan. 5) The northeastward-propagating wave train observed in mid-July 1998 was triggered by the southeastward-propagating (OKJ) wave train that produced a deep cyclonic circulation and a strong convective activity in the focus area. The link of wave forcing and deep convection was made solely because of a strong upper-level divergence in the focus area.

Equatorial Convectively Coupled Waves in a Simple Multicloud Model

2008 ◽  
Vol 65 (11) ◽  
pp. 3376-3397 ◽  
Author(s):  
Boualem Khouider ◽  
Andrew J. Majda
Keyword(s):  
Water Waves ◽  
Deep Convection ◽  
Low Frequency ◽  
Phase Speed ◽  
Longwave Radiation ◽  
Wave Structure ◽  
Parabolic Cylinder ◽  
Horizontal Wave ◽  
Equatorial Belt ◽  
Galerkin Truncation

Abstract Linear stability results for the multicloud model recently developed by the authors on an equatorial beta plane are presented here. The linearized equations, about a realistic radiative–convective equilibrium (RCE) are projected in the meridional direction via a Galerkin truncation procedure based on the parabolic cylinder functions. In a suitable parameter regime, the multicloud model exhibits convectively coupled Kelvin, M = 0 eastward (Yanai), and M = 1 westward inertia–gravity waves, unstable at the synoptic scales in agreement with the outgoing longwave radiation (OLR) spectral peaks observed by Wheeler and Kiladis. The horizontal wave structure and vertical wavenumber of the unstable waves qualitatively match those of the rotating equatorial shallow water waves but with a reduced phase speed, as in the observations. More importantly, they exhibit the same self-similar front-to-rear vertical tilt in the zonal winds, temperature, and heating fields as observed by Kiladis and colleagues. Similar to the case without rotation (from earlier work) a wave life cycle is identified, once again demonstrating the crucial role, played by congestus clouds and moisture, of preconditioning and moistening prior to deep convection and of triggering and maintaining the instability. When the troposphere is excessively dry, the convective wave instability fades out and an instability of low-frequency modes moving in both eastward and westward directions takes place. The eigenstructure of the low-frequency modes projects heavily on the congestus and moisture components and exhibits a quadruple vortex configuration reminiscent of Rossby waves with strong meridional convergence of warm and moist air toward the equatorial belt, suggesting a moistening and preconditioning role resembling the congestus standing mode seen in the case without rotation.

scholarly journals The Impact of Kelvin Wave Activity during Dry and Wet African Summer Rainfall Years

Atmosphere ◽  
2020 ◽  
Vol 11 (6) ◽  
pp. 568
Author(s):  
Ademe Mekonnen ◽  
Carl J. Schreck ◽  
Bantwale D. Enyew
Keyword(s):  
North Africa ◽  
Deep Convection ◽  
Summer Rainfall ◽  
Moisture Flux ◽  
Zonal Flow ◽  
Reanalysis Data ◽  
Boreal Summer ◽  
Kelvin Wave ◽  
Easterly Waves ◽  
The Impact

This study highlights the influence of convectively coupled Kelvin wave (KW) activity on deep convection and African easterly waves (AEWs) over North Africa during dry and wet boreal summer rainfall years. Composite analysis based on 25 years of rainfall, satellite observed cold cloud temperature, and reanalysis data sets show that KWs are more frequent and stronger in dry Central African years compared with wet years. Deep convection associated with KWs is slightly more amplified in dry years compared with wet years. Further, KW activity over North Africa strengthens the lower level zonal flow and deepens the zonal moisture flux in dry years compared with wet years. Results also show that enhanced KW convection is in phase with above-average AEW variance in dry years. However, enhanced KW convection is out-of-phase with average AEW activity in wet years. In general, this study suggests that KW passage over Africa enhances convective activity and more strongly modulates the monsoon flow and moisture flux during the dry years than wet years.

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