Northward Propagation Mechanisms of the Boreal Summer Intraseasonal Oscillation in the ERA-Interim and SP-CCSM

2013 ◽  
Vol 26 (6) ◽  
pp. 1973-1992 ◽  
Author(s):  
Charlotte A. DeMott ◽  
Cristiana Stan ◽  
David A. Randall
Keyword(s):  
Boundary Layer ◽  
Arabian Sea ◽  
Intraseasonal Oscillation ◽  
Lower Troposphere ◽  
Boreal Summer ◽  
Climate System Model ◽  
Boreal Summer Intraseasonal Oscillation ◽  
Moisture Advection ◽  
Northward Propagation ◽  
Barotropic Vorticity

Abstract Mechanisms for the northward propagation (NP) of the boreal summer intraseasonal oscillation (BSISO) and associated Asian summer monsoon (ASM) are investigated using data from the interim ECMWF Re-Analysis (ERA-Interim, herein called ERAI) and the superparameterized Community Climate System Model (SP-CCSM). Analyzed mechanisms are 1) destabilization of the lower troposphere by sea surface temperature anomalies, 2) boundary layer moisture advection, and boundary layer convergence associated with 3) SST gradients and 4) barotropic vorticity anomalies. Mechanism indices are regressed onto filtered OLR anomaly time series to study their relationships to the intraseasonal oscillation (ISO) and to equatorial Rossby (ER) waves. Northward propagation in ERAI and SP-CCSM is promoted by several mechanisms, but is dominated by boundary layer moisture advection and the barotropic vorticity effect. SST-linked mechanisms are of secondary importance but are nonnegligible. The magnitudes of NP mechanisms vary from the Indian Ocean to the west Pacific Ocean, implying that NP is accomplished by different mechanisms across the study area. SP-CCSM correctly simulates observed NP mechanisms over most of the ASM domain except in the Arabian Sea during the early stages of the monsoon life cycle. Reduced NP in the Arabian Sea arises from weaker-than-observed easterly shear, reducing the effectiveness of the barotropic vorticity mechanism. The ability of SP-CCSM to correctly simulate NP mechanisms in other regions results from the model’s ability to simulate reasonable mean wind and moisture fields, a realistic spectrum of variability, and the capability of convection to respond to boundary layer changes induced by large-scale NP mechanisms.

scholarly journals Impacts of Ocean Surface on the Northward Propagation of the Boreal Summer Intraseasonal Oscillation in the NCEP Climate Forecast System

2009 ◽  
Vol 22 (24) ◽  
pp. 6561-6576 ◽  
Author(s):  
Wanqiu Wang ◽  
Mingyue Chen ◽  
Arun Kumar
Keyword(s):  
Latent Heat ◽  
Intraseasonal Oscillation ◽  
Ocean Surface ◽  
Boreal Summer ◽  
Climate Forecast ◽  
Forecast System ◽  
Climate Forecast System ◽  
Boreal Summer Intraseasonal Oscillation ◽  
Northward Propagation ◽  
Sst Anomalies

Abstract Impacts of the ocean surface on the representation of the northward-propagating boreal summer intraseasonal oscillation (NPBSISO) over the Indian monsoon region are analyzed using the National Centers for Environmental Prediction (NCEP) coupled atmosphere–ocean Climate Forecast System (CFS) and its atmospheric component, the NCEP Global Forecast System (GFS). Analyses are based on forecasts of five strong NPBSISO events during June–September 2005–07. The inclusion of an interactive ocean in the model is found to be necessary to maintain the observed NPBSISO. The atmosphere-only GFS is capable of maintaining the convection that propagates from the equator to 12°N with reasonable amplitude within the first 15 days, after which the anomalies become very weak, suggesting that the atmospheric internal dynamics alone are not sufficient to sustain the anomalies to propagate to higher latitudes. Forecasts of the NPBSISO in the CFS are more realistic, with the amplitude of precipitation and 850-mb zonal wind anomalies comparable to that in observations for the entire 30-day target period, but with slower northward propagation compared to that observed. Further, the phase relationship between precipitation, sea surface temperature (SST), and surface latent heat fluxes associated with the NPBSISO in the CFS is similar to that in the observations, with positive precipitation anomalies following warm SST anomalies, which are further led by positive anomalies of the surface latent heat and solar radiation fluxes into the ocean. Additional experiments with the atmosphere-only GFS are performed to examine the impacts of uncertainties in SSTs. It is found that intraseasonal SST anomalies 2–3 times as large as that of the observational bulk SST analysis of Reynolds et al. are needed for the GFS to produce realistic northward propagation of the NPBSISO with reasonable amplitude and to capture the observed phase lag between SST and precipitation. The analysis of the forecasts and the experiments suggests that a realistic representation of the observed propagation of the oscillation by the NCEP model requires not only an interactive ocean but also an intraseasonal SST variability stronger than that of the bulk SST analysis.

scholarly journals Mechanisms of Northward-Propagating Intraseasonal Oscillation over the South China Sea during the Pre-Monsoon Period

2019 ◽  
Vol 32 (11) ◽  
pp. 3297-3311 ◽  
Author(s):  
Bin Zheng ◽  
Yanyan Huang
Keyword(s):  
South China Sea ◽  
South China ◽  
Intraseasonal Oscillation ◽  
The South China Sea ◽  
Boreal Summer ◽  
The South ◽  
China Sea ◽  
Northward Propagation ◽  
The Mean ◽  
Barotropic Vorticity

Abstract In the present study, the spatiotemporal structures of the northward-propagating intraseasonal oscillation (ISO) over the South China Sea (SCS) in the premonsoon period are analyzed by using the TropFlux air–sea flux and the JRA-55 reanalysis datasets. It is found that the SCS ISO is significant in the premonsoon season with a strong component of the northward propagation and that the mean state is different from that of summertime. Moreover, there are similar structures to those of a boreal summer ISO event except for the perturbation vorticity with no obvious phase leading. An internal atmospheric dynamics mechanism is proposed to understand the cause of the northward propagation of the ISO during the premonsoon period based on the spatial and temporal structures of the ISOs. The key process associated with this mechanism is the barotropic vorticity advection by the mean barotropic southerly winds, and the main barotropic vorticity around the convection center can be induced by the vertical advection of the mean vorticity. Low-level moisture convergence caused by anomalous flow is a supplementary mechanism to drive the ISOs northward during the premonsoon period, particularly over the northern SCS. In this mechanism, the SST-induced wind anomalies play a more important role than the convection-induced wind anomalies. The summer monsoon circulation has not built up during the premonsoon period, and thus the vertical wind shear effect and the barotropic vorticity effect associated with the meridional advection of baroclinic vorticity are not essential to cause the northward propagation of the ISOs over the SCS.

scholarly journals Structures and Mechanisms of 20–60-Day Intraseasonal Oscillation of the Observed Rainfall in Vietnam

2019 ◽  
Vol 32 (16) ◽  
pp. 5191-5212
Author(s):  
Nguyen Minh Truong ◽  
Bui Minh Tuan
Keyword(s):  
Boundary Layer ◽  
Cross Sections ◽  
Large Scale ◽  
Intraseasonal Oscillation ◽  
Moisture Flux ◽  
Boreal Summer ◽  
South Vietnam ◽  
Central Vietnam ◽  
Northward Propagation ◽  
North Vietnam

Abstract The present study explores the characteristics of the 20–60-day intraseasonal oscillation (ISO) in the 29-yr observed rainfall in north Vietnam (NVN), central Vietnam (CVN), and south Vietnam (SVN) in rainy seasons. Composite analyses reveal that the 20–60-day ISO in NVN accompanies dual vortices straddling Taiwan, which alternately favor and suppress convection extending from the northern Philippines to NVN. The wet phase in CVN coincides with convergence of northerly and easterly winds over the region. The large-scale pattern governing the 20–60-day ISO in SVN resembles the characteristics of the boreal summer ISO (BSISO). Conditionally unstable anomalies are observed within anomalous anticyclones where the moisture flux diverges out during the dry phase in NVN and SVN, and vice versa. Such anomalies prevent the existence of the anticyclones and finally replace them with anomalous cyclones to start the wet phase. The unstable anomalies could result from descending motion that increases the boundary layer temperature due to adiabatic compression of air. Conversely, boundary layer cooling due to evaporation of rain and interception of solar radiation by clouds produces stable anomalies. The unstable anomalies, moisture flux convergence, and vertical motions shift northward from the convection maximum, leading to the northward propagation of the BSISO convection. The 20–60-day ISO in CVN is not governed by local instability. Vertical cross sections indicate that the ISO in SVN possesses a westward-tilting structure, which is not observed in the NVN and CVN case.

scholarly journals Modulations of the Diurnal Cycle of Coastal Rainfall over South China Caused by the Boreal Summer Intraseasonal Oscillation

2019 ◽  
Vol 32 (7) ◽  
pp. 2089-2108 ◽  
Author(s):  
Xingchao Chen ◽  
Fuqing Zhang ◽  
James H. Ruppert
Keyword(s):  
South China ◽  
Diurnal Cycle ◽  
Phase 1 ◽  
Intraseasonal Oscillation ◽  
Active Phase ◽  
Boreal Summer ◽  
Boreal Summer Intraseasonal Oscillation ◽  
Moisture Advection ◽  
Rainfall Anomalies ◽  
Offshore Region

AbstractThe influence of the boreal summer intraseasonal oscillation (BSISO) on the diurnal cycle of coastal rainfall over south China during the mei-yu (heavy rainfall) season is investigated using the OLR-based Madden–Julian oscillation index (OMI), satellite rainfall data, and atmospheric reanalysis. Results show that the mei-yu season coastal rainfall is enhanced during the BSISO phase 1 (convectively active phase over the western Indian Ocean), with 25% greater rainfall than the climatological regional mean. Rainfall is suppressed during the BSISO phases 4 and 5 (convectively active phase in the Bay of Bengal and South China Sea), with negative rainfall anomalies of 39% and 46%, respectively. During phase 1, the rainfall enhancement is mostly over the inland region during the afternoon, while there is little diurnal variability of the rainfall anomaly offshore. During phases 4 and 5, the rainfall suppression is considerably stronger over the offshore region in the morning, whereas stronger rainfall suppression occurs inland during the afternoon. In phase 8, positive rainfall anomalies are found over the offshore region with a peak from the morning to the early afternoon, whereas negative rainfall anomalies are found over the inland region with the strongest suppression in the late afternoon. Analysis of phase composites and horizontal moisture advection shows that the diurnal variation of rainfall anomalies over the south China coastal area during different BSISO phases can be interpreted as the interaction between the large-scale anomalous moisture advection and the local land and sea breeze circulations.

Cloud and radiative heating profiles associated with the boreal summer intraseasonal oscillation

2017 ◽  
Vol 50 (5-6) ◽  
pp. 1485-1494 ◽  
Author(s):  
Jinwon Kim ◽  
Duane E. Waliser ◽  
Gregory V. Cesana ◽  
Xianan Jiang ◽  
Tristan L’Ecuyer ◽  
...  
Keyword(s):  
Intraseasonal Oscillation ◽  
Radiative Heating ◽  
Boreal Summer ◽  
Boreal Summer Intraseasonal Oscillation ◽  
Heating Profiles
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