Usability of the low frequency mode in southwest monsoon circulation for long range prediction of rainfall activity over central India

pro pagation have been ctudicd on the hasi, of upper air data of a few sta tions,The frequency of occurrence Ill' significant periodicity in th is mode i.. rchuively high for Visakhapatnam andMadras. 1 here appears a large inter-annual variability of the maximum amplitudes of rhe filtered series with nospecial preference to any latitudinal bell. Northward propagation of this mode also slums large inn.....-annunlvarisbility. In some ~ears the propaga tion " as totally absent. The phusc changes in the filtered ser ies o fVisakbapnma rn match ed with the cha nges in weekly ra infall activity over central India and thi.. may, pcrhups.be used to foreshadow the activ ity of the monsoon over central India ,

The Intraseasonal Fluctuation of Indian Summer Monsoon Rainfall and its Relation with Monsoon Intraseasonal Oscillation (MISO) and Madden Julian Oscillation (MJO)

The intra-seasonal fluctuations of Indian summer monsoon rainfall (ISMR) are mainly controlled by northward propagating Monsoon Intra-seasonal Oscillation (MISO) and eastward propagating Madden Julian Oscillation (MJO). In the current study, we examine the relationship between the intra-seasonal fluctuations (active and break spells) of ISMR with the phase propagation and amplitude of MISO and MJO. We notice that active spells generally occur during MISO phase 2–5 (MJO phase 3–6), and break spells mainly occur during MISO phase 6–8 (MJO phase 6–8 and 1). The association of active/break spells with MISO phases is more prominent than with MJO phases. We show the phase composite of unfiltered and regression based reconstructed rainfall for eight MISO and MJO phases, and the same is consistent with the earlier findings. We notice that the reconstructed field shows a systematic and well-organised northward propagation compared to the unfiltered field. Phase composite also indicates that there is a lead-lag relationship between MISO and MJO phases. MISO phase composite shows more robust northward propagation than the MJO phase composite. MISO reconstructed rainfall explained more percentage variance than MJO reconstructed rainfall with reference to 20–90 days filtered rainfall. It is found that long active (> 7 days) predominantly occurs when either MISO or MJO, or both of them are active, and the associated signal is somewhere in between phase 2–5. A long break occurs when either one or both of them are feeble, or even though associated signals are strong, they are primarily located in phases 1, 6, 7 and 8.

On the Weakening of Northward Propagation of Intraseasonal Oscillations During Positive Indian Ocean Dipole Events.

The northward propagation of intraseasonal oscillations (ISO) is one of the major modes of variability in the tropics during boreal summer, associated with active and break spells of monsoon rainfall over the Indian region, and modulate the Indian summer monsoon rainfall (ISMR). The northward march starts close to the equator over warm waters of the Indian Ocean and continues till the foothills of the Himalayas. The northward propagations tend to be weaker during positive Indian Ocean Dipole (pIOD) years. We have used the "moisture mode" framework to understand the processes responsible for the weakening of northward propagations during IOD years. Our analyses show that moistening caused by the horizontal advection was the major contributor for the northward propagations during negative IOD (nIOD) years, and its amplitude is much smaller during pIOD years. The reduction in the zonal advection during pIOD is responsible for the weakening of northward propagations. Also, the mean structure of entropy between 925hpa – 500hpa levels remained similar over most of the monsoon region across the contrasting IOD years. The reason for weaker northward propagations can be attributed to the weaker zonal wind perturbations at intraseasonal timescales. The weaker zonal wind perturbations during ISO events in pIOD years owing to cooler sea surface temperatures (SST) in the South-East Equatorial Indian Ocean (SEIO) and warmer West Equatorial Indian Ocean (WEIO) and South-East Arabian Sea (SEAS) is proposed to be the possible reason for the weakening of northward propagations during pIOD years.

Isolating non-subduction-driven tectonic processes in Cascadia

Several tectonic processes combine to produce the crustal deformation observed across the Cascadia margin: (1) Cascadia subduction, (2) the northward propagation of the Mendocino Triple Junction (MTJ), (3) the translation of the Sierra Nevada–Great Valley (SNGV) block along the Eastern California Shear Zone–Walker Lane and, (3) extension in the northwestern Basin and Range, east of the Cascade Arc. The superposition of deformation associated with these processes produces the present-day GPS velocity field. North of ~ 45° N observed crustal displacements are consistent with inter-seismic subduction coupling. South of ~ 45° N, NNW-directed crustal shortening produced by the Mendocino crustal conveyor (MCC) and deformation associated with SNGV-block motion overprint the NE-directed Cascadia subduction coupling signal. Embedded in this overall pattern of crustal deformation is the rigid translation of the Klamath terrane, bounded on its north and west by localized zones of deformation. Since the MCC and SNGV processes migrate northward, their impact on the crustal deformation in southern Cascadia is a relatively recent phenomenon, since ~ 2 –3 Ma.

Benefits of stochastic parameterizations in subseasonal to seasonal (S2S) forecasts

<p>Recently, there has been much interest in issuing subseasonal to seasonal (S2S) forecasts, although their skill is often debated. In addition to large systematic errors, ensemble systems are often overconfident, i.e. have incorrect information about the uncertainty of a particular forecast. Stochastic parameterization schemes are used routinely to remedy the problem of overconfidence, but also have the potential to reduce systematic model errors. </p><p>Here, we study the impact of adding a stochastic parameterization scheme in coupled simulations with the climate model CESM.  Physical processes associated with S2S-predictability, like the Madden-Julian  Oscillation (MJO) and Northern Hemispheric blocking are analyzed. In the simulations with a stochastic parameterization scheme, the northward propagation of the MJO is captured better, leading to an improved MJO lifecycle. The impact on other atmospheric fields like precipitation and winds will be discussed. </p>

Is 2020 super Meiyu a result of changing annual cycle of East Asian monsoon?

<p>2020 was exceedingly difficult for humans. As the world was experienced surge waves of COVID19, East Asia was also facing a one in a century, record-breaking flood,  as the result of a super 47-day Meiyu/Baiu stage of East Asian summer monsoon. As East Asian monsoons (EAM) follow a yearly cyclical pattern, we wonder which stage(s) were collateral damages of the extended Meiyu. Was it an early termination of the anomalous dry Spring, or was it a delayed northward propagation of the rain belt, i.e. late Mid-summer? The hypothesis stems from our recent finding (Dai et al., 2020) that the duration of the Spring stage is informative for the onset of Meiyu, while the duration of Meiyu is negatively correlated with that of Mid-summer, i.e., the longer the Meiyu, the shorter the Mid-summer. To verify this, we first positioned the 2020 pre-Meiyu, Meiyu, Mid-summer stages in the 40-year climatology annual cycle (Dai et al., 2020). Although neither the onset nor the termination was beyond the 40-year variance, Meiyu indeed hastened to arrive but postponed its departure. Rain belt stalled over the Yangtze river basin and southern Japan since mid-June; until the end of July, a planetary-scale anomalous high pressure band was in place encompassing the Arabian sea and north Pacific. It hindered the South Asian monsoonal flow to the South China Sea, curbing the northward propagation of the rain belt with assistance by both southeast-ward shift of South Asian High and lower level high pressure system persistent over the northern China. With these observations, we put forward a framework of ocean-atmosphere coupled mechanisms that traces back to the summer in 2019, and reveal the climate teleconnection and circulation systems that pave the road to the 2020 super Meiyu. With this study, we address the question of whether the 2020 super Meiyu was a “black swan” or a manifestation of ongoing systematic changes of the EAM annual cycle?</p><p> </p><p> </p><p> </p><p>Reference</p><p>Dai, L., Cheng, T. F., & Lu, M. (2020). Define East Asian monsoon annual cycle via a self‐organizing map‐based approach. Geophysical Research Letters, 47. e2020GL089542. https://doi.org/10.1029/2020GL089542</p>

Causes of Interdecadal Increase in the Intraseasonal Rainfall Variability over Southern China around the Early 1990s

The southern China (SC) summer rainfall exhibits prominent intraseasonal variability, which exhibits a significant increase in the early 1990s with the turning point at 1993. The SC intraseasonal rainfall events could be divided into three categories according to different propagations, including the southward-propagating (SP) events, the northwestward-propagating (NWP) events, and the northward-propagating (NP) events. This study explores the causes of the observed interdecadal increase in the intraseasonal rainfall variability over SC by comparing the SC intraseasonal rainfall events of each category between the former decadal period (P1) and the later decadal period (P2). The result indicates that such interdecadal change is due to the more frequent NP events coming from the South China Sea (SCS). Based on the moisture and vorticity budget analysis, it is revealed that the summer mean southerly wind in the middle to lower troposphere is the dominant factor of the northward propagation over the SCS, as it could induce positive meridional moisture and vorticity advection anomalies ahead of the convection. A marked interdecadal enhancement of the summer mean southerly wind over the SCS is the cause of more frequent occurrence of NP events over SC, as it provides more favorable conditions for the northward propagation. The change of the atmospheric instability over the SCS where the NP convection perturbation originates was also investigated, but no significant change was found.