Modulation of the Convectively Coupled Kelvin Waves over South America and the Tropical Atlantic Ocean in Association with the Madden–Julian Oscillation

2014 ◽  
Vol 71 (4) ◽  
pp. 1371-1388 ◽  
Author(s):  
Yanjuan Guo ◽  
Xianan Jiang ◽  
Duane E. Waliser
Keyword(s):  
South America ◽  
Wind Shear ◽  
Phase 1 ◽  
Vertical Wind Shear ◽  
Western Indian Ocean ◽  
Specific Humidity ◽  
Tropical Atlantic ◽  
Madden Julian Oscillation ◽  
Kelvin Wave ◽  
Study Region

Abstract In this study, evidence of the strong modulation of the convectively coupled Kelvin wave (CCKW) activity by the Madden–Julian oscillation (MJO) is presented, with a particular focus over the South America and tropical Atlantic region. The MJO impacts on CCKWs over this region, as noted in anomalous fields of rainfall as well as vertical profiles of wind, moisture, and temperature, are primarily through the modulation of Kelvin wave amplitude, with secondary effects on vertical structure, and little impact on wavenumber. CCKW activity is enhanced during MJO phases 8, 1, and 2 and damped during MJO phases 4, 5, and 6. Further analyses reveal that the strong modulation of the MJO on the CCKW activity could be largely through two factors: namely, the vertical zonal wind shear and the lower- to middle-tropospheric specific humidity. The CCKW activity tends to be enhanced during MJO phases when the easterly vertical wind shear and positive low- to midtroposphere moisture anomalies are present and vice versa. These two physical processes associated with the MJO are found to have positively (negatively) reinforcing influences on the CCKW activity in phase 1 (4 and 5), while counteracting influences in phases 2, 3, 6, 7, and 8, leading to the observed MJO cycle of the CCKW activity anomalies in the study region. The results presented in this study may have important implications for extended-range prediction of tropical wave activity and might suggest possible roles of the upstream CCKWs in the initiation of the MJO in the western Indian Ocean.

scholarly journals Projections of the Tropical Atlantic Vertical Wind Shear and Its Relationship with ENSO in SP-CCSM4

2014 ◽  
Vol 27 (22) ◽  
pp. 8342-8356 ◽  
Author(s):  
Xiaojie Zhu ◽  
Li Xu ◽  
Cristiana Stan
Keyword(s):  
Wind Shear ◽  
Linear Trend ◽  
Vertical Wind Shear ◽  
Vertical Motion ◽  
Walker Circulation ◽  
Vertical Wind ◽  
Tropical Atlantic ◽  
Convective Activity ◽  
Equatorial Atlantic ◽  
Climate System Model

Abstract The vertical wind shear over the tropical Atlantic Ocean and its relationship with ENSO are analyzed in the superparameterized Community Climate System Model, version 4 (SP-CCSM4) and in the conventional CCSM4. The climatology of vertical wind shear over the tropical Atlantic and the ENSO–shear relationship are well simulated in the control runs of SP-CCSM4 and CCSM4. However, because of different representations of cloud processes, in a warmer climate such as the representative concentration pathway 8.5 (RCP8.5) scenario, SP-CCSM4 projects increased mean westerlies at 200 hPa during July through October (JASO), whereas CCSM4 projects decreased mean westerlies at 200 hPa over the equatorial Atlantic. The different changes in the upper-level wind further contribute to different projection of JASO mean vertical wind shear over the equatorial Atlantic. In the RCP8.5 scenario, when excluding the linear trend, projection of the ENSO–shear relationships by SP-CCSM4 retains similar features as in the observed current climate, whereas the ENSO–shear relationship projected by CCSM4 indicates an increase in the vertical wind shear dominating the tropical Atlantic during El Niño events. The difference in projection of ENSO–shear relationship is, to a certain extent, related to the different response of the tropical Atlantic SST to ENSO. Analysis of the climate change projection of Walker circulation, cloud cover, and convective activity illustrates that superparameterization simulates a stronger suppression of African convection than the conventional parameterization of moist processes. The weak convective activity diminishes the divergent wind associated with the vertical motion, which contributes to increased westerlies projected in SP-CCSM4.

scholarly journals Testing Vertical Wind Shear and Nonlinear MJO/ENSO Interactions as Predictors for Subseasonal Atlantic Tropical Cyclone Forecasts

2022 ◽  
Author(s):  
Matthew A. Janiga
Keyword(s):  
Wave Breaking ◽  
Wind Shear ◽  
Southern Oscillation ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Lead Times ◽  
Madden Julian Oscillation ◽  
Rossby Wave Breaking ◽  
Non Linear ◽  
Linear Interactions

Abstract Hansen et al. (2020) found patterns of vertical wind shear, relative humidity (RH) and non-linear interactions between the Madden-Julian Oscillation and El Niño-Southern Oscillation that impact subseasonal Atlantic TC activity. We test whether these patterns can be used to improve subseasonal predictions. To do this we build a statistical-dynamical hybrid model using Navy-ESPC reforecasts as a part of the SUBX project. By adding and removing Navy-ESPC reforecasted values of predictors from a logistic regression model, we assess the contribution of skill from each predictor. We find that Atlantic SSTs and the MJO are the most important factors governing subseasonal Atlantic TC activity. RH contributes little to subseasonal TC predictions, however, shear predictors improve forecast skill at 5-10 day lead times, before forecast shear errors become too large. Non-linear MJO/ENSO interactions did not improve skill compared to separate linear considerations of these factors but did improve the reliability of predictions for high-probability active TC periods. Both non-linear MJO/ENSO interactions and the subseasonal shear signal appear linked to PV streamer activity. This study suggests that correcting model shear biases and improving representation of Rossby wave-breaking is the most efficient way to improve subseasonal Atlantic TC forecasts.

The Influence of Amazon Rainfall on the Atlantic ITCZ through Convectively Coupled Kelvin Waves

2007 ◽  
Vol 20 (7) ◽  
pp. 1188-1201 ◽  
Author(s):  
Hui Wang ◽  
Rong Fu
Keyword(s):  
South America ◽  
Surface Wind ◽  
Phase Speed ◽  
Kelvin Waves ◽  
Tropical Atlantic ◽  
Kelvin Wave ◽  
Reanalysis Dataset ◽  
Large Variability ◽  
Boreal Spring ◽  
Atlantic Itcz

Abstract Using outgoing longwave radiation (OLR) and Tropical Rainfall Measuring Mission (TRMM) daily rain-rate data, systematic changes in intensity and location of the Atlantic intertropical convergence zone (ITCZ) were detected along the equator during boreal spring. It is found that the changes in convection over the tropical Atlantic may be induced by deep convection in equatorial South America. Lagged regression analyses demonstrate that the anomalies of convection developed over the land propagate eastward across the Atlantic and then into Africa. The eastward-propagating disturbances appear to be convectively coupled Kelvin waves with a period of 6–7.5 days and a phase speed of around 15 m s−1. These waves modulate the intensity and location of the convection in the tropical Atlantic and result in a zonal variation of the Atlantic ITCZ on synoptic time scales. The convectively coupled Kelvin wave has substantial signals in both the lower and upper troposphere. Both a reanalysis dataset and the Quick Scatterometer (QuikSCAT) ocean surface wind are used to characterize the Kelvin wave. This study suggests that synoptic-scale variation of the Atlantic ITCZ may be linked to precipitation anomalies in South America through the convectively coupled Kelvin wave. The results imply that the changes of Amazon convection could contribute to the large variability of the tropical Atlantic ITCZ observed during boreal spring.

scholarly journals Climate Changes of Atlantic Tropical Cyclone Formation Derived from Twentieth-Century Reanalysis

2013 ◽  
Vol 26 (22) ◽  
pp. 8995-9005 ◽  
Author(s):  
Ruifang Wang ◽  
Liguang Wu
Keyword(s):  
Global Warming ◽  
North Atlantic ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
The United States ◽  
Tropical Atlantic ◽  
The North ◽  
The Caribbean ◽  
The North Atlantic ◽  
Twentieth Century Reanalysis

Abstract Whereas some studies linked the enhanced tropical cyclone (TC) formation in the North Atlantic basin to the ongoing global warming, other studies attributed it to the warm phase of the Atlantic multidecadal oscillation (AMO). Using the National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory (ESRL) Twentieth Century Reanalysis (20CR) dataset, the present study reveals the distinctive spatial patterns associated with the influences of the AMO and global warming on TC formation in the North Atlantic basin. Two leading empirical orthogonal function (EOF) patterns are identified in the climate change of TC formation on time scales longer than interannual. The first pattern is associated with the AMO and its spatial pattern shows the basin-scale enhancement of TC formation during the AMO positive phase. The second pattern is associated with global warming, showing enhanced TC formation in the east tropical Atlantic (5°–20°N, 15°–40°W) and reduced TC formation from the southeast coast of the United States extending southward to the Caribbean Sea. In the warm AMO phase, the basinwide decrease in vertical wind shear and increases in midlevel relative humidity and maximum potential intensity (MPI) favor the basinwide enhancement of TC formation. Global warming suppresses TC formation from the southeast coast of the United States extending southward to the Caribbean Sea through enhancing vertical wind shear and reducing midlevel relative humidity and MPI. The enhanced TC formation in the east tropical Atlantic is due mainly to a local increase in MPI or sea surface temperature (SST), leading to a close relationship between the Atlantic SST and TC activity over the past decades.

scholarly journals Impacts of Saharan Dust on Atlantic Regional Climate and Implications for Tropical Cyclones

2018 ◽  
Vol 31 (18) ◽  
pp. 7621-7644 ◽  
Author(s):  
Bowen Pan ◽  
Yuan Wang ◽  
Jiaxi Hu ◽  
Yun Lin ◽  
Jen-Shan Hsieh ◽  
...  
Keyword(s):  
Tropical Cyclones ◽  
Wind Shear ◽  
Large Scale ◽  
Hydrological Cycle ◽  
Regional Climate ◽  
Vertical Wind Shear ◽  
Saharan Dust ◽  
Specific Humidity ◽  
Shortwave Radiation ◽  
Cloud Formation

The radiative and microphysical properties of Saharan dust are believed to impact the Atlantic regional climate and tropical cyclones (TCs), but the detailed mechanism remains uncertain. In this study, atmosphere-only simulations are performed from 2002 to 2006 using the Community Atmospheric Model, version 5.1, with and without dust emission from the Sahara Desert. The Saharan dust exhibits noticeable impacts on the regional longwave and shortwave radiation, cloud formation, and the convective systems over West Africa and the tropical Atlantic. The African easterly jet and West African monsoon are modulated by dust, leading to northward shifts of the intertropical convergence zone and the TC genesis region. The dust events induce positive midlevel moisture and entropy deficit anomalies, enhancing the TC genesis. On the other hand, the increased vertical wind shear and decreased low-level vorticity and potential intensity by dust inhibit TC formation in the genesis region. The ventilation index shows a decrease in the intensification region and an increase in the genesis region by dust, corresponding to favorable and unfavorable TC activities, respectively. The comparison of nondust scenarios in 2005 and 2006 shows more favorable TC conditions in 2005 characterized by higher specific humidity and potential intensity, but lower ventilation index, wind shear, and entropy deficit. Those are attributable to the observed warmer sea surface temperature (SST) in 2005, in which dust effects can be embedded. Our results imply significant dust perturbations on the radiative budget, hydrological cycle, and large-scale environments relevant to TC activity over the Atlantic.

Convective-storm environments in subtropical South America from high-frequency soundings during RELAMPAGO-CACTI

2021 ◽  
Author(s):  
Russ S. Schumacher ◽  
Deanna A. Hence ◽  
Stephen W. Nesbitt ◽  
Robert J. Trapp ◽  
Karen A. Kosiba ◽  
...  
Keyword(s):  
South America ◽  
High Frequency ◽  
Wind Shear ◽  
Field Experiments ◽  
Deep Convection ◽  
Convective Available Potential Energy ◽  
Vertical Wind Shear ◽  
Mobile Platforms ◽  
Convective Storms ◽  
Low Level

AbstractDuring the RELAMPAGO-CACTI field experiments in 2018-19, an unprecedented number of balloon-borne soundings were collected in Argentina. Radiosondes were launched from both fixed and mobile platforms, yielding 2712 soundings during the period 15 October 2018-30 April 2019. Approximately 20% of these soundings were collected by highly mobile platforms, strategically positioned for each intensive observing period, and launching approximately once per hour. The combination of fixed and mobile soundings capture both the overall conditions characterizing the RELAMPAGO-CACTI campaign, as well as the detailed evolution of environments supporting the initiation and upscale growth of deep convective storms, including some that produced hazardous hail and heavy rainfall. Episodes of frequent convection were characterized by sufficient quantities of moisture and instability for deep convection, along with deep-layer vertical wind shear supportive of organized or rotating storms. Eleven soundings showed most-unstable convective available potential energy (MUCAPE) exceeding 6000 J kg−1, comparable to the extreme instability observed in other parts of the world with intense deep convection. Parameters used to diagnose severe-storm potential showed that conditions were often favorable for supercells and severe hail, but not for tornadoes, primarily owing to insufficient low-level wind shear. High-frequency soundings also revealed the structure and evolution of the boundary layer leading up to convection initiation, convectively generated cold pools, the South American Low-Level Jet (SALLJ), and elevated nocturnal convection. This sounding dataset will enable improved understanding and prediction of convective storms and their surroundings in subtropical South America, as well as comparisons with other heavily studied regions such as the central United States that have not previously been possible.

scholarly journals Observations of the temporal variability in aerosol properties and their relationships to meteorology in the summer monsoonal South China Sea/East Sea: the scale-dependent role of monsoonal flows, the Madden–Julian Oscillation, tropical cyclones, squall lines and cold pools

2015 ◽  
Vol 15 (4) ◽  
pp. 1745-1768 ◽  
Author(s):  
J. S. Reid ◽  
N. D. Lagrosas ◽  
H. H. Jonsson ◽  
E. A. Reid ◽  
W. R. Sessions ◽  
...  
Keyword(s):  
Boundary Layer ◽  
South China Sea ◽  
Wind Shear ◽  
Large Scale ◽  
Aerosol Particles ◽  
Vertical Wind Shear ◽  
Sea Salt ◽  
Madden Julian Oscillation ◽  
Cold Pools ◽  
Squall Lines

Abstract. In a joint NRL/Manila Observatory mission, as part of the Seven SouthEast Asian Studies program (7-SEAS), a 2-week, late September 2011 research cruise in the northern Palawan archipelago was undertaken to observe the nature of southwest monsoonal aerosol particles in the South China Sea/East Sea (SCS/ES) and Sulu Sea region. Previous analyses suggested this region as a receptor for biomass burning from Borneo and Sumatra for boundary layer air entering the monsoonal trough. Anthropogenic pollution and biofuel emissions are also ubiquitous, as is heavy shipping traffic. Here, we provide an overview of the regional environment during the cruise, a time series of key aerosol and meteorological parameters, and their interrelationships. Overall, this cruise provides a narrative of the processes that control regional aerosol loadings and their possible feedbacks with clouds and precipitation. While 2011 was a moderate El Niño–Southern Oscillation (ENSO) La Niña year, higher burning activity and lower precipitation was more typical of neutral conditions. The large-scale aerosol environment was modulated by the Madden–Julian Oscillation (MJO) and its associated tropical cyclone (TC) activity in a manner consistent with the conceptual analysis performed by Reid et al. (2012). Advancement of the MJO from phase 3 to 6 with accompanying cyclogenesis during the cruise period strengthened flow patterns in the SCS/ES that modulated aerosol life cycle. TC inflow arms of significant convection sometimes span from Sumatra to Luzon, resulting in very low particle concentrations (minimum condensation nuclei CN < 150 cm−3, non-sea-salt PM2.5 < 1 μg m−3). However, elevated carbon monoxide levels were occasionally observed suggesting passage of polluted air masses whose aerosol particles had been rained out. Conversely, two drier periods occurred with higher aerosol particle concentrations originating from Borneo and Southern Sumatra (CN > 3000 cm−3 and non-sea-salt PM2.5 10–25 μg m−3). These cases corresponded with two different mechanisms of convection suppression: lower free-tropospheric dry-air intrusion from the Indian Ocean, and large-scale TC-induced subsidence. Veering vertical wind shear also resulted in aerosol transport into this region being mainly in the marine boundary layer (MBL), although lower free troposphere transport was possible on the western sides of Sumatra and Borneo. At the hourly time scale, particle concentrations were observed to be modulated by integer factors through convection and associated cold pools. Geostationary satellite observations suggest that convection often takes the form of squall lines, which are bowed up to 500 km across the monsoonal flow and 50 km wide. These squall lines, initiated by cold pools from large thunderstorms and likely sustained by a veering vertical wind shear and aforementioned mid-troposphere dry layers, propagated over 1500 km across the entirety of the SCS/ES, effectively cutting large swaths of MBL aerosol particles out of the region. Our conclusion is that while large-scale flow patterns are very important in modulating convection, and hence in allowing long-range transport of smoke and pollution, more short-lived phenomena can modulate cloud condensation nuclei (CCN) concentrations in the region, resulting in pockets of clean and polluted MBL air. This will no doubt complicate large scale comparisons of aerosol–cloud interaction.

Climatology of Vertical Wind Shear over the Tropical Atlantic

2006 ◽  
Vol 19 (12) ◽  
pp. 2969-2983 ◽  
Author(s):  
Anantha R. Aiyyer ◽  
Chris Thorncroft
Keyword(s):  
Wind Shear ◽  
Large Scale ◽  
Southern Oscillation ◽  
Vertical Wind Shear ◽  
Vertical Wind ◽  
Vertical Shear ◽  
Spatiotemporal Variability ◽  
Tropical Atlantic ◽  
Highly Correlated ◽  
Sahel Precipitation

Abstract The spatiotemporal variability of the 200–850-hPa vertical wind shear over the tropical Atlantic is examined for a period of 46 yr. This work extends and updates past studies by considering a longer data record as well as a tropospheric-deep measure of vertical wind shear. Composite fields are constructed to illustrate the spatial pattern of the large-scale circulation associated with the mean and extreme cases of vertical shear within the tropical Atlantic. The contemporaneous relationship of vertical shear with El Niño–Southern Oscillation (ENSO) and Sahel precipitation are also examined. While the ENSO–shear correlation appears to have slightly strengthened during the past decade, the Sahel–shear correlation has become significantly degraded. A combined empirical orthogonal function (EOF) analysis of the zonal and meridional components of the vertical shear reveals interannual and multidecadal modes. The leading EOF exhibits mainly interannual variability and is highly correlated with ENSO. The second EOF is associated with a multidecadal temporal evolution and is correlated with Sahel precipitation. Both EOFs correlate at the same level with tropical cyclones in the main development region of the tropical Atlantic.

scholarly journals Multiple-Platform and Multiple-Doppler Radar Observations of a Supercell Thunderstorm in South America during RELAMPAGO

2020 ◽  
Vol 148 (8) ◽  
pp. 3225-3241
Author(s):  
Robert J. Trapp ◽  
Karen A. Kosiba ◽  
James N. Marquis ◽  
Matthew R. Kumjian ◽  
Stephen W. Nesbitt ◽  
...  
Keyword(s):  
South America ◽  
Residence Time ◽  
Complex Terrain ◽  
Wind Shear ◽  
Additional Data ◽  
Doppler Radar ◽  
Vertical Wind Shear ◽  
Long Duration ◽  
Multiple Platform ◽  
Supercell Thunderstorm

Abstract On 10 November 2018, during the RELAMPAGO field campaign in Argentina, South America, a thunderstorm with supercell characteristics was observed by an array of mobile observing instruments, including three Doppler on Wheels radars. In contrast to the archetypal supercell described in the Glossary of Meteorology, the updraft rotation in this storm was rather short lived (~25 min), causing some initial doubt as to whether this indeed was a supercell. However, retrieved 3D winds from dual-Doppler radar scans were used to document a high spatial correspondence between midlevel vertical velocity and vertical vorticity in this storm, thus providing evidence to support the supercell categorization. Additional data collected within the RELAMPAGO domain revealed other storms with this behavior, which appears to be attributable in part to effects of the local terrain. Specifically, the IOP4 supercell and other short-duration supercell cases presented had storm motions that were nearly perpendicular to the long axis of the Sierras de Córdoba Mountains; a long-duration supercell case, on the other hand, had a storm motion nearly parallel to these mountains. Sounding observations as well as model simulations indicate that a mountain-perpendicular storm motion results in a relatively short storm residence time within the narrow zone of terrain-enhanced vertical wind shear. Such a motion and short residence time would limit the upward tilting, by the left-moving supercell updraft, of the storm-relative, antistreamwise horizontal vorticity associated with anabatic flow near complex terrain.

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