scholarly journals Initiation of Boreal Summer Intraseasonal Oscillation: Dynamic Contribution by Potential Vorticity

2012 ◽  
Vol 140 (6) ◽  
pp. 1748-1760 ◽  
Author(s):  
Kyong-Hwan Seo ◽  
Eun-Ji Song
Keyword(s):  
Potential Vorticity ◽  
Intraseasonal Oscillation ◽  
Vertical Wind Shear ◽  
Boreal Summer ◽  
Dynamical Process ◽  
Initiation Mechanism ◽  
Middle Troposphere ◽  
Boreal Summer Intraseasonal Oscillation ◽  
Low Level ◽  
The Tropics

Abstract Potential vorticity (PV) thinking conceptually connects the upper-level (upper troposphere in the extratropics and middle troposphere for the tropics) dynamical process to the lower-level process. Here, the initiation mechanism of the boreal summer intraseasonal oscillation (BSISO) in the tropics is investigated using PV thinking. The authors demonstrate that the midtropospheric PV anomaly produces a dynamical environment favorable for the BSISO initiation. Under seasonal easterly vertical wind shear, the PV anomaly enhances low-level convergence and upward motion at its western edge. Tropical PV forcing in the middle troposphere produces balanced mass and circulation fields that spread horizontally and vertically so that its effect can reach even the lowest troposphere. The downward influence of the midtropospheric PV forcing is one of the key aspects of the PV thinking. Direct piecewise PV inversions confirm that the anomalous lower-level zonal wind and its convergence necessary for the initiation of BSISO convection do not arise solely from the response to the lower-level PV forcing but from the summed contribution by PV forcing at all levels. About 50% of the low-level circulation variations result from PV forcing from 700 to 450 hPa, with the largest contribution from the 600–650-hPa PV anomalies for the convection initiation region over the western Indian Ocean. The current study is compared with and incorporated into the thermodynamic recharge process and the frictional moisture flux convergence mechanism for the BSISO initiation. This study is the first qualitative application of the PV thinking approach that reveals the BSISO dynamics.

Impact of the Boreal Summer Intraseasonal Oscillation on the Diurnal Cycle of Precipitation near and over the Island of Luzon

2020 ◽  
Vol 148 (5) ◽  
pp. 1805-1827
Author(s):  
Kyle Chudler ◽  
Weixin Xu ◽  
Steven A. Rutledge
Keyword(s):  
Intraseasonal Variability ◽  
Intraseasonal Oscillation ◽  
Vertical Wind Shear ◽  
The Philippines ◽  
Heat Fluxes ◽  
Boreal Summer ◽  
Diurnal Variability ◽  
Boreal Summer Intraseasonal Oscillation ◽  
Precipitation Estimates ◽  
Monsoon Winds

Abstract During the boreal summer, satellite-based precipitation estimates indicate a distinct maximum in rainfall off the west coast of the island of Luzon in the Philippines. Also occurring during the summer months is the boreal summer intraseasonal oscillation (BSISO), a main driver of intraseasonal variability in the region. This study investigates the diurnal variability of convective intensity, morphology, and precipitation coverage offshore and over the island of Luzon. The results are then composited by BSISO activity. Results of this study indicate that offshore precipitation is markedly increased during active BSISO phases, when strong low-level southwesterly monsoon winds bring increased moisture and enhanced convergence upwind of the island’s high terrain. A key finding of this work is the existence of an afternoon maximum in convection over Luzon even during active BSISO phases, when solar heating and instability are apparently reduced due to enhanced cloud cover. This result is important, as previous studies have shown in other areas of the tropics afternoon convection over landmasses is a key component to offshore precipitation. Although offshore precipitation is maximized in the evening hours during active phases, results indicate that precipitation frequently occurs over the ocean around the clock (both as organized systems and isolated, shallow showers), possibly owing to an increase in sensible and latent heat fluxes, vertical wind shear, and convergence of the monsoon flow with land features.

scholarly journals Impact of the Extended Boreal Summer Intraseasonal Oscillation on Western North Pacific Tropical Cloud Cluster Genesis Productivity

2018 ◽  
Vol 31 (22) ◽  
pp. 9175-9191 ◽  
Author(s):  
Haikun Zhao ◽  
Shaohua Chen ◽  
Philip J. Klotzbach ◽  
G. B. Raga
Keyword(s):  
Relative Humidity ◽  
North Pacific ◽  
Western North Pacific ◽  
Large Scale ◽  
Intraseasonal Oscillation ◽  
Boreal Summer ◽  
Synoptic Scale ◽  
Boreal Summer Intraseasonal Oscillation ◽  
Low Level ◽  
The Impact

Tropical cloud clusters (TCCs) are traditionally viewed as precursors of tropical cyclone (TC) genesis. Most studies have focused on the impact of the extended boreal summer intraseasonal oscillation (ISO) on TC activity over the western North Pacific (WNP), while the modulation of the ISO on WNP TCC genesis productivity (TCCGP), that is, the ratio of TC to TCC counts, has been investigated much less frequently. This study suggests that the extended boreal summer ISO modulates WNP TCCGP, with higher (lower) TCCGP during convectively active (inactive) ISO phases. Changes in TCCGP are found to be closely associated with changes of large-scale environmental factors. During the convectively active ISO phase, significantly increased TCCGP is associated with strengthened low-level cyclonic circulation anomalies and increased midlevel relative humidity anomalies over the WNP basin. The genesis potential index (GPI) contains several large-scale environmental variables demonstrated to relate to TCs and TCCs. The GPI can adequately depict the ISO modulation of WNP TCCGP through its alterations of large-scale parameters. Low-level vorticity makes the largest contribution to the change of TCCGP with a secondary contribution from midlevel relative humidity. Interestingly, the nonlinear GPI terms make comparable contributions, which can be partly explained by the synoptic-scale wave activity associated with the ISO mode. Stronger (weaker) 3–8-day synoptic-scale wave train intensity and increased (decreased) low-level eddy kinetic energy are found to be associated with the enhanced (weakened) monsoon circulation over the WNP basin during convectively active (inactive) ISO phases.

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

scholarly journals Impacts of atmospheric and oceanic initial conditions on boreal summer intraseasonal oscillation forecast in the BCC model

2020 ◽  
Vol 142 (1-2) ◽  
pp. 393-406
Author(s):  
Zhongkai Bo ◽  
Xiangwen Liu ◽  
Weizong Gu ◽  
Anning Huang ◽  
Yongjie Fang ◽  
...  
Keyword(s):  
Indian Ocean ◽  
North Pacific ◽  
Western North Pacific ◽  
Initial Conditions ◽  
Intraseasonal Oscillation ◽  
Tropical Indian Ocean ◽  
Boreal Summer ◽  
Model Errors ◽  
Maritime Continent ◽  
Boreal Summer Intraseasonal Oscillation

Abstract In this paper, we evaluate the capability of the Beijing Climate Center Climate System Model (BCC-CSM) in simulating and forecasting the boreal summer intraseasonal oscillation (BSISO), using its simulation and sub-seasonal to seasonal (S2S) hindcast results. Results show that the model can generally simulate the spatial structure of the BSISO, but give relatively weaker strength, shorter period, and faster transition of BSISO phases when compared with the observations. This partially limits the model’s capability in forecasting the BSISO, with a useful skill of only 9 days. Two sets of hindcast experiments with improved atmospheric and atmosphere/ocean initial conditions (referred to as EXP1 and EXP2, respectively) are conducted to improve the BSISO forecast. The BSISO forecast skill is increased by 2 days with the optimization of atmospheric initial conditions only (EXP1), and is further increased by 1 day with the optimization of both atmospheric and oceanic initial conditions (EXP2). These changes lead to a final skill of 12 days, which is comparable to the skills of most models participated in the S2S Prediction Project. In EXP1 and EXP2, the BSISO forecast skills are improved for most initial phases, especially phases 1 and 2, denoting a better description for BSISO propagation from the tropical Indian Ocean to the western North Pacific. However, the skill is considerably low and insensitive to initial conditions for initial phase 6 and target phase 3, corresponding to the BSISO convection’s active-to-break transition over the western North Pacific and BSISO convection’s break-to-active transition over the tropical Indian Ocean and Maritime Continent. This prediction barrier also exists in many forecast models of the S2S Prediction Project. Our hindcast experiments with different initial conditions indicate that the remarkable model errors over the Maritime Continent and subtropical western North Pacific may largely account for the prediction barrier.

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