Mechanism for northward propagation of boreal summer intraseasonal oscillation: Convective momentum transport

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.

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The role of shallow convection in promoting the northward propagation of boreal summer intraseasonal oscillation

Theoretical and Applied Climatology ◽

10.1007/s00704-017-2064-2 ◽

2017 ◽

Vol 131 (3-4) ◽

pp. 1387-1395 ◽

Cited By ~ 2

Author(s):

Fei Liu ◽

Jiuwei Zhao ◽

Xiouhua Fu ◽

Gang Huang

Keyword(s):

Intraseasonal Oscillation ◽

Boreal Summer ◽

Shallow Convection ◽

Boreal Summer Intraseasonal Oscillation ◽

Northward Propagation

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

Journal of Climate ◽

10.1175/jcli-d-12-00191.1 ◽

2013 ◽

Vol 26 (6) ◽

pp. 1973-1992 ◽

Cited By ~ 59

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.

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Roles of Barotropic Convective Momentum Transport in the Intraseasonal Oscillation*

Journal of Climate ◽

10.1175/jcli-d-14-00575.1 ◽

2015 ◽

Vol 28 (12) ◽

pp. 4908-4920 ◽

Cited By ~ 15

Author(s):

Fei Liu ◽

Bin Wang ◽

In-Sik Kang

Keyword(s):

General Circulation ◽

Intraseasonal Oscillation ◽

Vertical Wind Shear ◽

General Circulation Models ◽

Basic Mechanism ◽

Momentum Transport ◽

Ekman Pumping ◽

The North ◽

Northward Propagation ◽

Convective Momentum Transport

Abstract Both observational data analysis and model simulations suggest that convective momentum transport (CMT) by cumulus convection may play a significant role in the intraseasonal oscillations (ISO) by redistributing atmospheric momentum vertically through fast convective mixing process. The authors present a simple theoretical model for the ISO by parameterizing the cumulus momentum transport process in which the CMT tends to produce barotropic wind anomalies that will affect the frictional planetary boundary layer (PBL). In the model with equatorial easterly vertical wind shear (VWS), it is found that the barotropic CMT tends to select most unstable planetary-scale waves because CMT suppresses the equatorial Ekman pumping of short waves, which reduces the shortwave instability from the PBL moisture convergence and accelerates the shortwave propagation. The model with subtropical easterly VWS has behavior that can be qualitatively different from the model with equatorial easterly VWS and has robust northward propagation. The basic mechanism of this northward propagation is that the CMT accelerates the barotropic cyclonic wind to the north of ISO, which will enhance the precipitation by PBL Ekman pumping and favor the northward propagation. The simulated northward propagation is sensitive to the strength and location of the seasonal-mean easterly VWS. These results suggest that accurate simulation of the climatological-mean state is critical for reproducing the realistic ISO in general circulation models.

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

Climate Dynamics ◽

10.1007/s00382-017-3700-3 ◽

2017 ◽

Vol 50 (5-6) ◽

pp. 1485-1494 ◽

Cited By ~ 4

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

Monthly Weather Review ◽

10.1175/mwr-d-11-00105.1 ◽

2012 ◽

Vol 140 (6) ◽

pp. 1748-1760 ◽

Cited By ~ 10

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.

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