Asymmetric Boreal Summer Intraseasonal Oscillation Events Over the Western North Pacific and Their Impacts on East Asian Precipitation

The boreal summer intraseasonal oscillation (BSISO) is the most prominent tropical subseasonal signature especially over the western North Pacific (WNP). Due to restrictions of methodology in extracting BSISO with band-pass filtering or EOF decomposition, most of the previous studies ignored the asymmetry of BSISO. This study reexamines the BSISO events over WNP and their impacts on the East Asian precipitation. With a hierarchical cluster analysis, the BSISO events over WNP during the summers of 1985-2010 are classified into two categories, the long-period (30-60 day) and short-period (10-20 day) events. The long-period BSISO events manifest as a northward propagating mode with a significant phase asymmetry characterized by a fast development, but a slow decay of the intraseasonal convection. The fast development tends to cause a rapid reversal of the atmospheric anomalies over WNP from an anomalous anticyclone induced by the preceding slow convection suppression to an anomalous cyclone, leading to a fast northeastward retreat of the preceding enhanced western North Pacific subtropical high. Accordingly, the middle and lower reaches of Yangtze River valley experience a rapid reversal from the increased precipitation to the decreased, while the precipitation in coastal South China keeps decreased. The short-period BSISO events which are symmetric in phase act as a northwestward propagating mode, mainly affecting East Asian precipitation in an oblique belt extending from southwest China to southern Japan and southern Korean Peninsula. Therefore, the two types of the BSISO events especially the asymmetric long-period BSISO events over WNP and their impacts on the East Asian precipitation revealed in this study would provide a new potential for subseasonal-to-seasonal forecast of the East Asian summer monsoon precipitation.

A unified moisture mode theory for the Madden Julian Oscillation and the Boreal Summer Intraseasonal Oscillation

The Madden Julian Oscillation (MJO) and the Boreal Summer Intraseasonal Oscillation (BSISO) are fundamental modes of variability in the tropical atmosphere on the intraseasonal time scale. A linear model, using a moist shallow water equation set on an equatorial beta plane, is developed to provide a unified treatment of the two modes and to understand their growth and propagation over the Indian Ocean. Moisture is assumed to increase linearly with longitude and to decrease quadratically with latitude. Solutions are obtained through linear stability analysis, considering the gravest (n = 1) meridional mode with nonzero meridional velocity. Anomalies in zonal moisture advection and surface fluxes are both proportional to those in zonal wind, but of opposite sign. With observation-based estimates for both effects, the zonal advection dominates, and drives the planetary-scale instability. With a sufficiently small meridional moisture gradient, the horizontal structure exhibits oscillations with latitude and a northwest-southeast horizontal tilt in the northern hemisphere, qualitatively resembling the observed BSISO. As the meridional moisture gradient increases, the horizontal tilt decreases and the spatial pattern transforms toward the “swallowtail” structure associated with the MJO, with cyclonic gyres in both hemispheres straddling the equatorial precipitation maximum. These results suggest that the magnitude of the meridional moisture gradient shapes the horizontal structures, leading to the transformation from the BSISO-like tilted horizontal structure to the MJO-like neutral wave structure as the meridional moisture gradient changes with the seasons. The existence and behavior of these intraseasonal modes can be understood as a consequence of phase speed matching between the equatorial mode with zero meridional velocity (analogous to the dry Kelvin wave) and a local off-equatorial component that is characterized by considering an otherwise similar system on an f-plane.

Assessment of the Ability of CMIP6 GCMS to Simulate the Boreal Summer Intraseasonal Oscillation Over Southeast Asia

The boreal summer intraseasonal oscillation (BSISO) plays an important role in the intraseasonal variability of a wide range of weather and climate phenomena across the region modulated by the Asian summer monsoon system. This study evaluates the strengths and weaknesses of 19 Coupled Model Intercomparison Project Phase 6 (CMIP6) models to reproduce the basic characteristics of BSISO. The models' rainfall and largescale climates are evaluated against GPCP and ERA5 reanalysis datasets. All models exhibit intraseasonal variance of 30–60-day bandpass-filtered rainfall and convection anomalies but with diverse magnitude when compared with observations. The CMIP6 models capture the structure of the eastward/northward propagating BSISO at wavenumbers 1 and 2 but struggle with the intensity and location of the convection signal. Nevertheless, the models show a good ability to simulate the power spectrum and coherence squared of the principal components of the combined empirical orthogonal function (CEOF) and can capture the distinction between the CEOF modes and red noise. Also, the result shows that some CMIP6 models can capture the coherent intraseasonal propagating features of the BSISO as indicated by the Hovmöller diagram. The contribution of latent static energy to the relationship between the moist static energy and intraseasonal rainfall over Southeast Asia is also simulated by the selected models, albeit the signals are weak. Taking together, some of the CMIP6 models can represent the summertime climate and intraseasonal variability over the study region, and can also simulate the propagating features of BSISO, but biases still exist.

Atmospheric Rivers over the Indo-Pacific and its Associations with Boreal Summer Intraseasonal Oscillation

Recent works have revealed that the wintertime atmospheric river (AR) activity is closely related to the 30–60-day tropical intraseasonal variability, yet it remains unclear whether summertime AR activity is also significantly influenced by the intraseasonal variability, often referred to as the boreal summer intraseasonal oscillation (BSISO). Diagnosing the 40-year (1979–2018) ERA5 reanalysis dataset, the present study examines the climatological features of ARs over the Indo-Pacific region during June to October and its associations with the BSISO. Results suggest that the western North Pacific Subtropical High (WNPSH) provides a favorable circulation background for the summertime AR activity, which conveys the moisture from the tropics to midlatitude North Pacific along its periphery. Our analysis reveals that the BSISO has substantial impacts on the occurrence and distribution of ARs. More ARs are found over the western North Pacific (WNP) when the BSISO convective envelope propagates northward to the subtropical regions, while fewer ARs can be seen when convection is suppressed there. Specifically, in phases 7–8, the active BSISO convection over the Philippine Sea induces a low-pressure anomaly and the corresponding anomalous cyclonic circulation, leading to the enhanced poleward moisture transport and more frequent AR activity over the WNP. Moreover, the WNP ARs tend to be longer and have larger sizes during these two phases. It is also found that more frequent occurrence of tropical cyclones in phases 7–8 can significantly enhance the moisture transport and AR occurrence over the WNP.

The representation of the boreal summer intraseasonal oscillation in a global convection-permitting simulation

<p>The Boreal Summer Intraseasonal Oscillation (BSISO) is a major mode of intraseasonal variability in the Indian summer monsoon. The characteristic pattern includes northward/north-eastward propagating anomalies of convection and circulation over the Indian longitudes, and concurrent eastward propagating anomalies that move through the tropics from the equatorial Indian ocean. In the Indian monsoon region, the BSISO interacts with other processes to affect the rainfall variability on a range of spatial and temporal scales. Convection-permitting simulations are known to improve the representation of some of these smaller-scale processes, but until recently, it has not been feasible to use convection-permitting simulations to model the entire BSISO because of the temporal and spatial scales on which it occurs. Here we assess how well a global multi-year convection-permitting simulation with a coarse grid-spacing of ~10km at the equator models the BSISO. Using Empirical Orthogonal Function (EOF) analysis, we show that overall, the convection-permitting simulation does not give a substantially better representation of the BSISO, when compared with a simulation which parametrises convection. In the observations, the first two EOF eigenvectors and their Principal Component (PC) time series describe the BSISO. The characteristic northwest-to-southeast slope of the observed EOF 1 and 2 patterns is not captured in the parametrised simulation but is better captured in the convection-permitting simulation. However, the convection-permitting simulation does not capture the observed relationship between the PC1 and PC2 time series that describe the strength and phase of the BSISO. The observed pattern is of a fairly constant phase difference between the PC1 and PC2 time series, but in the convection-permitting simulation, there are periods of both negative and positive phase differences. Our results demonstrate that the BSISO is very sensitive to the representation of convection and future higher resolution runs will provide useful routes for understanding scale interactions in the BSISO.</p>