Somali Low-level Jet Research Articles

Recent trends and variability of temperature and atmospheric water vapor over South Asia

This study examines the recent trends and variability of temperature and atmospheric water vapor over South Asia during the summer monsoon season from 1970 to 2022. We employ gridded reanalysis and observational datasets to assess the trends and variability in surface temperature and further investigate the associated physical processes. Our results show that the surface temperature over South Asia has increased by 0.5 °C per decade. Notably, we find a statistically significant major transition in the surface temperature series in 1995, such that 1970–1994 (1995–2022) is associated with anomalous cooling (anomalous warming). Since 1995, not only have the temporal variations in regional surface temperature, atmospheric water vapor, and clear-sky downward longwave radiation been synchronous at interannual scales, but their largest spatial variations have been located mainly over the same region. A diagnosis of the atmospheric moisture budget reveals that the observed changes in water vapor convergence over South Asia are dominated by changes in mean circulation dynamics, while the change in thermodynamics is relatively small. The enhancement of the Somali low-level jet may contribute to increased water vapor in South Asia by modulating low-level circulations.

Investigating the unprecedented summer 2022 penetration of the Indian monsoon to Iran and evaluation of global and regional model forecasts

The Indian Summer Monsoon (ISM) significantly impacts the climate of the Asian continent. During the summer of 2022, the penetration of monsoonal waves towards higher latitudes led to severe and unprecedented floods in various parts of Iran, Pakistan, and southern Afghanistan. In this study, we utilized meteorological data from weather stations, satellite remote sensing, reanalysis data, and teleconnection indices to investigate the penetration of monsoonal waves at higher latitudes in Iran. We also employed outputs from two global models, the Global Forecast System (GFS) and Climate Forecast System (CFS), and the Weather Research and Forecasting Model (WRF) regional model, to examine their forecasts of heavy monsoon rains. Our analysis of teleconnection indices revealed that La Niña, combined with a negative or neutral Dipole Mode Index (DMI) and a positive Indian Monsoon Index (IMI), intensified monsoon-related rainfall in the region. The low-pressure system over India weakened, while the system over central Iran strengthened. Additionally, we observed a meridional rotation of the Somali low-level jet. Generally, southern to southwestern Iran, as well as central and eastern regions, receive moisture from the Arabian Sea due to southerly and easterly winds from water surfaces. Comparing forecasts with 2–7 days lead times and extended 10–15 days from the CFS and GFS global models demonstrated that neither of models accurately predicted the observed range of rainfall over Iran in the extended period. However, the WRF regional model predictions were significantly better. We also discovered that the 48-hour forecast from the WRF model outperformed other forecasts for this case study.

Moisture source identification for precipitation associated with tropical cyclone development over the Indian Ocean: a Lagrangian approach

In this study, we investigated the moisture sources for precipitation through a Lagrangian approach during the genesis, intensification, and dissipation phases of all tropical cyclones (TCs) that occurred over the two hemispheric sub-basins of the Indian Ocean (IO) from 1980 to 2018. In the North IO (NIO), TCs formed and reached their maximum intensity on both sides of the Indian Peninsula, to the east in the Bay of Bengal (BoB), and to the west in the Arabian Sea (AS). The oceanic areas where TCs occurred were their main moisture sources for precipitation associated with TCs. Additionally, for TCs over the BoB, continental sources from the Ganges River basin and the South China Sea also played a notable role; for TCs over the AS, the Somali Low-Level jet (along the African coast in a northerly direction) also acted as an essential moisture transport. In the South IO (SIO), the western, central, and eastern basins were identified as the preferred areas for the genesis and development of TCs. During TC activity, the central IO and the Wharton and Perth basins mostly supplied atmospheric moisture. The Mascarene High circulation was the main moisture transport mechanism for the precipitation of TCs formed in the SIO basin. In both basins, during their intensification process, TCs gained more moisture (even more intensely when reaching the hurricane category) than during the genesis or dissipation stages. Additionally, the modulation during monsoonal seasons of the moisture contribution to the TCs was more noticeable over the NIO basin than for the SIO. Overall, the moisture uptake for precipitation from the sources for TCs occurred slightly faster in the NIO basin than in the SIO basin.

Tracing the Origin of the South Asian Summer Monsoon Precipitation and Its Variability Using a Novel Lagrangian Framework

AbstractThe water sources and their variability responsible for the South Asian summer monsoon precipitation were analyzed using Lagrangian atmospheric water-mass trajectories. The results indicated that evaporated waters from the central and south Indian Ocean are the major contributors to the South Asian summer monsoon rainfall, followed by the contribution from the local recycling (precipitated water that evapotranspirated from the South Asian landmass), the Arabian Sea, remote sources, and the Bay of Bengal. It was also found that although the direct contribution originating from the Bay of Bengal is small, it still provides a pathway for the atmospheric water that comes from other regions. This pathway is hence only crossing over the Bay of Bengal. The outcomes further revealed that the evaporated waters originating from the central and south Indian Ocean are responsible for the net precipitation over the coastal regions of the Ganges–Brahmaputra–Meghna Delta, northeast India, Myanmar, the foothills of the Himalayas, and central-east India. Evaporated waters from the Arabian Sea are mainly contributing to the rainfall over the western coast and west-central India. Summer monsoon precipitation due to the local recycling is primarily restricted to the Indo-Gangetic plain. No recycled precipitation was observed over the mountain chain along the west coast of India (Western Ghats). The month-to-month precipitation variation over South Asia was analyzed to be linked with the Somali low-level jet variability. The interannual variability of the South Asian summer monsoon precipitation was found to be mainly controlled by the atmospheric waters that were sourced and traveled from the central and south Indian Ocean.

Bifurcations and catastrophes in a nonlinear dynamical model of the western Pacific subtropical high ridge line index and its evolution mechanism

Despite much previous effort, the establishment of an accurate model of the western Pacific subtropical high (WPSH) and analysis of its chaotic behavior has proved to be difficult. Based on a phase-space technique, a nonlinear dynamical model of the WPSH ridge line and summer monsoon factors is constructed here from 50 years of data. Using a genetic algorithm, model inversion and parameter optimization are performed. The Lyapunov spectrum, phase portraits, time history, and Poincare surface of section of the model are analyzed and an initial-value sensitivity test is performed, showing that the model and data have similar phase portraits and that the model is robust. Based on equilibrium stability criteria, four types of equilibria of the model are analyzed. Bifurcations and catastrophes of the equilibria are studied and related to the physical mechanism and actual weather phenomena. The results show that the onset and enhancement of the Somali low-level jet and the latent heat flux of the Indian monsoon are among the most important reasons for the appearance and maintenance of the double-ridge phenomenon. Violent breakout and enhancement of the Mascarene cold high will cause the WPSH to jump northward, resulting in the “empty plum” phenomenon. In the context of bifurcation and catastrophe in the dynamical system, the influence of the factors considered here on the WPSH has theoretical and practical significance. This work also opens the way to new lines of research on the interaction between the WPSH and the summer monsoon system.

Sensitivity of Horn of Africa Rainfall to Regional Sea Surface Temperature Forcing

The Abdus Salam International Center for Theoretical Physics (ICTP) version 4.4 Regional Climate Model (RegCM4) is used to investigate the rainfall response to cooler/warmer sea surface temperature anomaly (SSTA) forcing in the Indian and Atlantic Oceans. The effect of SSTA forcing in a specific ocean basin is identified by ensemble, averaging 10 individual simulations in which a constant or linearly zonally varying SSTA is prescribed in individual basins while specifying the 1971–2000 monthly varying climatological sea surface temperature (SST) across the remaining model domain. The nonlinear rainfall response to SSTA amplitude also is investigated by separately specifying +1K, +2K, and +4K SSTA forcing in the Atlantic and Indian Oceans. The simulation results show that warm SSTs over the entire Indian Ocean produce drier conditions across the larger Blue Nile catchment, whereas warming ≥ +2K generates large positive rainfall anomalies exceeding 10 mm·day−1 over drought prone regions of Northeastern Ethiopia. However, the June–September rainy season tends to be wetter (drier) when the SST warming (cooling) is limited to either the Northern or Southern Indian Ocean. Wet rainy seasons generally are characterized by deepening of the monsoon trough, east of 40°E, intensification of the Mascarene high, strengthening of the Somali low level jet and the tropical easterly jet, enhanced zonal and meridional vertically integrated moisture fluxes, and steeply vertically decreasing moist static energy. The opposite conditions hold for dry monsoon seasons.

Reconstruction and Forecast Experiments of a Statistical–Dynamical Model of the Western Pacific Subtropical High and East Asian Summer Monsoon Factors

Abstract Abnormal activity of the western Pacific subtropical high (WPSH) may result in extreme weather events in East Asia. However, because the relationship between the WPSH and other components of the East Asian summer monsoon (EASM) system is unknown, it is still difficult to forecast such abnormal activity. The delay-relevant method is used to study 2010 data for abnormal weather and it is concluded that the Indian monsoon latent heat flux, the Somali low-level jet, and the Tibetan high activity index can significantly affect anomalies in the WPSH in the EASM system. By combining genetic algorithms and statistical–dynamical reconstruction theory, a nonlinear statistical–dynamical model of the WPSH and these three influencing factors was objectively reconstructed from actual 2010 data and a dynamically extended forecasting experiment was carried out. To further test the forecasting performance of the reconstructed model, further experiments using data from nine abnormal WPSH years and eight normal WPSH years were performed for comparison. All the results suggest that the forecasts of the subtropical high area index, the Indian monsoon latent heat flux, the Somali low-level jet, and the Tibetan high activity index all have good performance in the short and medium terms (<25 days). Not only is the forecasting trend accurate, but the mean absolute percentage error is ≤9%. This work suggests new areas of research into the association between the WPSH and EASM systems and provides a new method for the prediction of the WPSH area index.

Modulation of Daily Precipitation over East Africa by the Madden–Julian Oscillation*

Abstract Spatiotemporal variability in East African precipitation affects the livelihood of tens of millions of people. From the perspective of floods, flash droughts, and agriculture, variability on intraseasonal time scales is a critical component of total variability. The principal objective of this study is to explore subseasonal impacts of the Madden–Julian oscillation (MJO) on tropospheric circulations affecting East Africa (EA) during the long (March–May) and short (October–December) rains and associated variability in precipitation. Analyses are performed for 1979–2012 for dynamics and 1998–2012 for precipitation. Consistent with previous studies, significant MJO influence is found on wet and dry spells during the long and short rains. This influence, however, is found to vary within each season. Specifically, indices of MJO convection at 70°–80°E and 120°W are strongly associated with precipitation variability across much of EA in the early (March) and late (May) long rainy season and in the middle and late (November–December) short rainy season. In the early short rains (October) a different pattern emerges, in which MJO strength at 120°E (10°W) is associated with dry (wet) spells in coastal EA but not the interior. In April the MJO influence on precipitation is obscured but can be diagnosed in lead time associations. This diversity of influences reflects a diversity of mechanisms of MJO influence, including dynamic and thermodynamic mechanisms tied to large-scale atmospheric circulations and localized dynamics associated with MJO modulation of the Somali low-level jet. These differences are relevant to problems of subseasonal weather forecasts and climate projections for EA.

Influence of Madden-Julian Oscillation on water budget transported by the Somali low-level jet and the associated Indian summer monsoon rainfall

[1] Recent studies suggest that there is a strong linkage between the moisture uptake over the equatorial area of the Somali low level jet (SLLJ) and the rainfall variability over most of continental India. Additionally, the Madden-Julian Oscillation (MJO) strongly modulates the intraseasonal variability of the Indian summer monsoon rainfall, since the northward propagation of the boreal summer MJO is closely associated with the active and break phases of monsoon rainfall. But a question remains open: is there a relationship between the moisture transported by the SLLJ and the MJO evolution? In this paper, a Lagrangian approach is used to track the evaporation minus precipitation (E − P) evolution along trajectories of particles initially situated over the equatorial region of SLLJ. The impact of the MJO on the water budget transport of the SLLJ is examined by making composites of the obtained (E-P) fields for the different MJO phases. The spatial structures of the boreal summer intraseasonal oscillation are revealed in our results, which strongly suggest that the main responsible for the rainfall variability associated to the MJO in these regions are the changes in the moisture advected by the SLLJ. In order to assess the MJO-SLLJ interaction, an analysis of the total-column mass and the total-column specific humidity transported by the SLLJ during the MJO life cycle is performed. While a systematic difference between air mass advected to India during active and break phases of MJO is not detected, changes in the moisture of particles are found, with wet (dry) anomalies over enhanced (suppressed) convection region. This result implicitly leads to assume air-sea interaction processes.

Biot-Savart law and the formation mechanism of Somali low-level jet

Firstly, we investigate the impact of cross-equatorial Somali low-level jet on the atmospheric circulation in the east of Tibet Plateau using lattice Boltzmann model simulation. Secondly, we study the relationship between thermal conditions on the bottom boundary and the formation of Somali jet based on Biot-Savart law using the data from National Centres for Environmental Prediction (NCEP). As the radiation from the Sun gradually moves from the southern meridian, the temperature on the ground surface of Somali Peninsular and Arabic Peninsular gradually increases. During the same period the surface temperature of the Northern Indian Ocean increases much slower. It is shown that this increase in the temperature difference between the land and sea is inductive to the formation and development of Rayleigh-Benard convection and leads to the increasing relative vorticity strength between positive and negative vertical vortices over the land and sea. According to Biot-Savart law, the increase in vorticity strength will induce correspondingly a large horizontal velocity. The pair of positive and negative vorticity fields over the two Peninsulars and the sea surface is effective in forming and maintaining this current. This mechanism is referred to as "Somali suction pump". It draws air continually from the Southern hemisphere and releases it at the coastal area of Somali.

Influence of the Mascarene high and Australian high on the summer monsoon in East Asia: Ensemble simulation

By using a nine-level atmospheric general circulation model developed at the Institute of Atmospheric Physics (IAP 9L AGCM), two sets of numerical experiments are carried out to investigate the influence of the Mascarene high (MH) and Australian high (AH) over the southern subtropics upon the East Asian summer monsoon circulation and summer precipitation in East Asia. The use of ensemble statistics is adopted to reduce the simulation errors. The result shows that with the intensification of MH, the Somali low-level jet is significantly enhanced together with the summer monsoon circulation in the tropical Asia and western Pacific region. Furthermore, the anticyclonic anomaly in the tropical western Pacific to the east of the Philippines may induce a weak East-Asia-Pacific teleconnection pattern. In the meantime, geopotential height in the Tropics is enhanced while it is reduced over most regions of mid-high latitudes, thus the northwestern Pacific subtropical high at 500 hPa extends southwestward, resulting in more rainfall in southern China and less rainfall in northern China. A similar anomaly pattern of the atmospheric circulation systems is found in the experiment of the intensification of AH. On the other hand, because the cross-equatorial currents associated with AH are much weaker than the Somali jet, the anomaly magnitude caused by the intensification of AH is generally weak, and the influence of AH on summer rainfall in China seems to be localized in southern China. Comparison between the two sets of experiments indicates that MH plays a crucial role in the interactions of general atmospheric circulation between the two hemispheres.

A Case Study of a Fluctuation of the Somali Jet During the Indian Summer Monsoon

Abstract A fluctuation in the intensity of the Somali low-level jet flowing along the East African coast during the Indian summer monsoon is studied. It is shown that this variation of the intensity of cross-equatorial flow in western Indian Ocean results from the interaction between extratropical perturbations moving eastward at midlatitudes of the Southern Hemisphere and southeast trade winds of the monsoon circulation over the southern equatorial Indian Ocean.

The Burst of the 1978 Indian Summer Monsoon as Seen from METEOSAT

Abstract Low-level wind fields over the western Indian Ocean west of 65°E have been determined from the analysis of METEOSAT images from 10 to 22 May 1978 (one field every two days). The data show the abrupt change in the low-level airflow circulation which takes place over the western Indian Ocean during the burst of the monsoon. The burst of the monsoon along the African coast is characterized by the sudden establishment of the Somali low-level jet and associated strong cross-equatorial flow. This modification which augurs the beginning of the monsoon season over India is closely related to the first rainfalls over the southern part of the western coast of India which occur with a lag of 3–4 days.

Low-level air flow over the western Indian Ocean as seen from METEOSAT

DURING the northern summer, the low-level air circulation over the Indian Ocean is characterised by cross-equatorial flow from the Southern to the Northern Hemisphere. The south-east trade winds of the Southern Hemisphere are sucked in by the zone of continental low pressure located over central India and are mainly deflected by the Coriolis force after crossing the Equator; they give rise to the south-west Indian monsoon and associated precipitation during the summer months1. The cross-equatorial air flow is not uniform at all longitudes: it is weak over the eastern Indian Ocean and particularly strong along the East African coast, where it is concentrated into a low-level jet (maximum intensity 30 ms−1 at between 1 and 3 km) flowing to the Somalia coast from the northern tip of Madagascar2. We now report the first results from a study of the low-level airflow circulation over the western Indian Ocean and more particularly the Somali low-level jet stream. Successive images taken by the European geostationary satellite METEOSAT were used to determine wind vectors.