Low-frequency Intraseasonal Oscillations Research Articles

Two Types of Mid-High-Latitude Low-Frequency Intraseasonal Oscillations near the Ural Mountains during Boreal Summer

AbstractTwo types of mid-high-latitude low-frequency intraseasonal oscillations (LF-ISOs), featuring eastward and westward propagation, have been identified over the Eurasian continent in the past 37 summers (1982–2018). The eastward and westward propagating modes commonly have a dominant periodicity of 30–50 days near the Ural Mountains (UM) but have different origins and evolutions. The eastward propagating LF-ISO initiates over eastern North America, migrates northeastward across northeastern North America–western North Atlantic, central North Atlantic, western Europe, and the UM, then propagates southeastward to northwestern and eastern China, which is the Atlantic-Eurasian continental mode. In contrast, the westward propagating mode is quasi-circumpolar, initiating over the East Siberian Sea and moving southwestward across the UM and northern Europe and eventually reaching Greenland and the Canadian Arctic Archipelago. These two mid-high-latitude LF-ISOs are accompanied by significant tropical intraseasonal variations with evident tropical–extratropical interactions. Meanwhile, these two LF-ISOs have different decadal preferences before and after 2000, which are ascribed to the decadal change of both intraseasonal efficient kinetic energy obtained from the mean flow over their genesis region and their background flow associated with the North Atlantic Oscillation/Arctic Oscillation decadal change. This study deepens the understanding of subseasonal variations for mid-high latitudes and subseasonal prediction sources for low-latitude regions.

On the variations in the frequency of 25\u201370-day intraseasonal oscillations in Central Africa using wavelet-based indices

Atmospheric variability at the intraseasonal timescale remains of great concern in tropical Africa because of the vulnerability of the population to variations in the distribution and amount of rainfall within a season. Then, the parameterization of the processes that induce the intraseasonal variability of the rainfall is still a challenge for the sub-seasonal-to-seasonal forecast in the tropics. In the study of intraseasonal oscillations (ISOs) in Central Africa, almost all of the authors focused only on the amplitude of the oscillations, even though the frequency is also very important because it also undergoes strong spatiotemporal variations. The novelty of this study is that we applied wavelet transform on the 2.5° × 2.5° daily Outgoing Long-wave Radiation (OLR) to extract the frequency (period) of intraseasonal oscillations (ISO) and then study its spatiotemporal variations over Central Africa (CA) within the period 1981–2015 (35 years). By the algorithm used, we obtained a dataset of daily ISO Period Indices (ISOPI) within the study period, with the same dimensions as the original OLR datasets. The analyses showed that the mean ISOPI globally fluctuates between 32 and 52 days, but undergoes strong day-to-day variations. The ISO frequency is highly seasonal, with high ISOPI (low frequency) during December–February and June–August, and short low ISOPI (high frequency) during March–May and September–November. The composites of OLR and 850 hpa zonal winds revealed that the low-frequency ISOs (LFISOs) are predominant in Eastern Central Africa and around the Cameroon Volcanic Line, while the long-frequency events (HFISOs) are mostly found in Western Central Africa, especially around the Congo basin. The plots of yearly mean ISOPI showed that the ISO period exhibits strong interannual variations with years of very high ISOPI such as 1983, 1985, 1987, 1989, 1999, 2002 and 2009, and years of lower ISOPI as 1988, 1994, 1995. Finally, it was proved in this study that there is an enhancement of rainfall during LFISOs, especially in northern hemisphere, while HFISOs are generally associated with normal or suppressed rainfall regime.

On the study of seasonal patterns of 25–70-day intraseasonal oscillations in Central Africa

With its strong influence on weather and climate, the Madden–Julian Oscillation (MJO) plays a crucial role in the tropical climate system. However, one of the greatest challenges of climate modeling remains definition of suitable MJO-related metrics for the efficient use in climate models. This requires a good documentation of the MJO variation patterns throughout the year and from tear to another. As contribution to the definition of these MJO-related metrics in General Circulation Models (GCMs), this paper addresses the seasonal patterns of 25–70-day Intraseasonal Oscillations (ISO) in CA using the 2.5° × 2.5° daily Outgoing Longwave Radiation (OLR), for the period 1981–2015 (35 years). One novelty in this study is the use of a Discrete Wavelet Transform (DWT) model to compute ISO amplitude and frequency indices and then investigate their variations from one season to another. The analyses showed that the ISO characteristics (amplitude, frequency and propagation speed) strongly vary from season to another. The ISO intensity (ISOI) is extremely high during December–February (DJF) and March–May (MAM) and lower during June–August (JJA) and September–November (SON) seasons. As for the ISO period (ISOP), the low-frequency ISO (high ISOP) are observed during DJF and JJA and the high frequency (low ISOP) during MAM and SON seasons. However, for the four seasons, the period is predominantly contained between 40 and 50 days, suggesting the dominance of MJO. The investigation of propagation characteristics for the selected Active Intraseasonal Events (AIEs) found the eastward propagation to be the dominant feature in all seasons but the propagation speed is lower for DJF and MAM, when compared with JJA and SON. At global scale, we found that the incursions of the moisture fluxes from Atlantic and Indian oceans are responsible for the convection and subsequently the rainfall enhancement over CA at intraseasonal timescales. We also accessed the impact of ISO on rainfall using and index called the Impact Rate (IR). The plots of spatial distribution of IR during the four seasons showed that the influence of ISO on rainfall varies from one season to another, with some zones of very high IR and other zones of lower IR. Moreover, the zone of high IR propagates northwards from one season to another.

The fidelity of a regional coupled model in capturing the relationship between intraseasonal variability and the onset/demise of the Indian summer monsoon

Onset and demise of the Indian summer monsoon (ISM) and intraseasonal variability (ISV) embedded within the ISM are dominant climatological phenomena observed over the Indian region. In this study, a quantitative relationship between these two phenomena is assessed and the performance of a regional coupled model in simulating the relationship in the ISM is examined. An objective definition of the local onset and demise of the ISM is used based on more than a century-long India Meteorological Department rain-gauge observation and 10-year-long model simulated precipitation. Two distinct modes of intraseasonal oscillations (ISOs), northwestward propagating 10–20-day periodic high-frequency ISO and northward moving 20–70-day periodic low-frequency ISOs are extracted using Multichannel Singular Spectrum Analysis (MSSA). It is found that nearly 60% of the local onset of the ISM occurs during the positive developing phases of the ISO; whereas, a similar fraction of local demise occurs during positive decaying phases of the ISO across almost entire India. Essentially, the phase-locking of two ISOs creates the most conducive environment for onset or demise of the ISM to occur. In this study we found that the model not only captures the overall structure of the ISO modes, but also simulates the observed distribution of the onset and demise dates of ISM in different ISO phases. The diagnostics described here, with the ability of the model to produce the observed phenomena, provide an essential tool to improve our understanding of the monsoon system and its predictability.

Increased sporadic extremes decrease the intraseasonal variability in the Indian summer monsoon rainfall

The Indian summer monsoon (ISM) shows quasi-rhythmic intraseasonal oscillations (ISO) manifested as alternate ‘active’ phases of copious rainfall and quiescent phases of ‘break’. Within these periodic phases, the daily rainfall shows large variability and exhibits spatiotemporally sporadic extreme rainfall events. The recent decades have witnessed a significant increase in the number of these extreme rainfall events, especially in the quiescent phases. This increase is accompanied by a decreasing trend in the mean monsoon rainfall and a weakening variance of its low-frequency ISO (LF-ISO) cycle. However, any physical link between this apparent paradox of increased extreme rainfall events and weakened slower-time-scale components is not yet reported. Here, using observations and numerical model simulations, we show that the occurrence of extreme rainfall events, primarily in the break phase of an LF-ISO cycle, reduce the intensity of the following active phase by stabilizing the atmosphere. We found that extreme events in a monsoon break leads to a reduction in the vertical shear of zonal winds and an increase in the static stability of the atmosphere in the following break-to-active transition and active phases. These conditions oppose the initiation and development of an active phase and lessen its intensity. This reduces the LF-ISO intensity and mean ISM rainfall.

Vertical variation of low-frequency oscillations in UHF radar derived winds at a tropical Indian station

[1] Signatures of the low-frequency intraseasonal oscillations and their vertical variation in the lower troposphere are studied using multilevel wind observations made at a tropical Indian station, Pune (18°32′N, 73°51′E), from wind profiler/UHF radar during the period June 2003 to May 2006. Spectral analysis using fast Fourier transform showed the presence of oscillations (30–40 days and 50–70 days) in daily mean zonal winds and revealed their evolution in height-period structures. The 30–40 days oscillation mode has the largest amplitude compared to the other modes in zonal and meridional winds. All oscillation modes have much less amplitude in the vertical wind component. The high temporal (3 hourly) zonal and meridional wind observations obtained during the southwest monsoon period (June to September, JJAS) during the years 2003, 2004, and 2005 showed a prominent regime of oscillation, namely, 30–40 days and a moderate signature of 10–20 day mode. In all three years it is seen that these oscillations are not occurring as a single-frequency sinusoidal oscillation but are seen as broadband spectrum. The amplitude of these wave modes showed significant vertical variation. The vertical structures of radar-observed intraseasonal oscillations (30–40 days mode) and Tropical Rainfall Measuring Mission derived mean precipitation intensity during the monsoon period (JJAS) during the years 2003–2005 showed a reasonable comparison. The results presented in this study show significant intraseasonal and interannual variability in lower tropospheric winds, which are closely related to the performance of the southwest monsoon.

Brief review of some CLIVAR-related studies in China

The Climate Variability and Predictability (CLIVAR) program is one of the sub-programs of the World Climate Research Program (WCRP). In this paper, CLIVAR related research in China (2003–2006) is briefly reviewed, including four major components, namely, low-frequency intraseasonal oscillations, interannual variability, decadal variations in East Asia, and global warming simulations.

The role of low-frequency intraseasonal oscillations in the anomalous Indian summer monsoon rainfall of 2002

We analyze the dynamical features and responsible factors of the low-frequency intraseasonal time scales which influenced the nature of onset, intensity and duration of active/break phases and withdrawal of the monsoon during the anomalous Indian summer monsoon of 2002 — the most severe drought recorded in recent times. During that season, persistent warm sea surface temperature anomalies over the equatorial Indian Ocean played a significant role in modulating the strength of the monsoon Hadley circulation. This in turn affected the onset and intense break spells especially the long break during the peak monsoon month of July. Strong low-frequency intraseasonal modulations with significant impact on the onset and active/break phases occurred in 2002 which were manifested as a good association between low-frequency intraseasonal oscillations and the onset and active/break spells. Further, SST anomalies over the equatorial Indo-Pacific region on low-frequency intraseasonal time scales were found to affect the equatorial eastward and thereby off-equatorial northward propagations of enhanced convection over the Indian region. These propagations in turn modulated the active/break cycle deciding the consequent severity of the 2002 drought.

Low frequency intraseasonal oscillations in Indian Rainfall and outgoing longwave radiation

. In this study the interannual variations of the period and intensity of 30-40 days oscillations in monsoon rainfall rare examined by utilising daily rainfall data of 290 stations for a period of 80 years. Their relationship with the overall performance of the monsoon and ENSO phenomenon is also examined. Potential of these oscillations of about 40 days period in extended range forecasting of rainfall is examined by computing the cross-correlations between the rainfall and the OLR over certain key regions and the rainfall distribution over the whole country in subsequent few pentads. Finally concurrent correlations between the S-day OLR and rainfall are also studied.

A PC-Based Interactive Imaging System Designed for INSAT Data Analysis and Monsoon Studies

A PC-based interactive image processing system has been developed for aiding the analysis of Indian Geosynchronous Satellite (INSAT) data for Asian monsoon studies. In view of its diminutive stature, the system has been given the name “MIDGET,” for a Micro-based Image Display and Graphics Enhancement Tool. Various analysis procedures involving INSAT data and other monsoon datasets are described in conjunction with the MIDGET system. These include the use of the system for monitoring monsoon evolution and the behavior of organized tropical storms, analysis of low-frequency intraseasonal oscillations, diagnostic studies of cloudiness, retrieval of monsoon precipitation and its relationship to satellite cloudiness, and statistical prediction of monsoon cloud bands associated with low-frequency intraseasonal fluctuations of monsoon rainfall. The system is designed as a workstation terminal in a supercomputer environment. The basic virtue of this system is that it is an inexpensive approach for generating a high-resolution video time-lapse of large satellite datasets in an interactive environment familiar to PC users. The motivation for developing this system derives from a new source of Indian satellite data that has been made available to United States scientists through archive holdings at the National Center for Atmospheric Research.