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The usability gaps between climate information producers and users have always been an issue in climate services. This study aims to tackle the gap for rice farmers in Bangladesh by exploring the potential value of tailored agronomic monsoon onset definitions. Summer aman rice is primarily cultivated under rainfed conditions, and farmers rely largely on monsoon rainfall and its onset for crop establishment. However, farmers’ perception of the arrival of sufficient rains does not necessarily coincide with meteorological definitions of monsoon onset. Therefore, localized agronomic definitions of monsoon onset need to be developed and evaluated to advance in the targeted actionable climate forecast. We analyzed historical daily rainfall from four locations across a north-south gradient in Bangladesh and defined dynamic definitions of monsoon onset based on a set of local parameters. The agronomic onset definition was evaluated in terms of attainable yields simulated by a rice simulation model compared to results obtained using conventional meteorological onset parameters defined by the amount of rainfall received and static onset dates. Our results show that average simulated yields increase up to 7 – 9% and probabilities of getting lower yields are reduced when the year-to-year varying dynamic onset is used over the two drier locations under fully rainfed conditions. It is mainly due to earlier transplanting dates, avoiding the impact of drought experienced with early monsoon demise. However, no yield increases are observed over the two wetter locations. This study shows the potential benefits of generating “localized and translated” climate predictions.

During certain years, a synoptic scale vortex called the monsoon onset vortex (MOV) forms within the northward advancing zone of precipitating convection over the Arabian Sea. The MOV does not form each year and the reason is unclear. Since the Madden‐Julian Oscillation (MJO) is known to modulate convection and tropical cyclones in the tropics, we examined its role in the formation of the MOV. While the convective and transition phases of the MJO do not always lead to MOV formation, the suppressed phase of the MJO hinders the formation of the MOV more consistently. This asymmetric relationship between the MJO and MOV can be partially explained by the modulation of the large‐scale environment, measured by a tropical cyclone genesis index. It also suggests that the Arabian Sea is generally near a critical state that is favorable for MOV formation during the monsoon onset period. Plain Language Summary The monsoon onset vortex (MOV) is a cyclonic vortex, which forms in the Arabian Sea in some years during the onset of the Indian summer monsoon. It often intensifies into a tropical cyclone. The MJO is an eastward‐moving band of clouds and rainfall near the equatorial regions, having a cycle of 30–60 days. The MJO enhances the formation of tropical depressions and tropical cyclones worldwide. This study shows that the wet phase of the MJO is neither a necessary nor a sufficient condition for the MOV to form over the Arabian Sea. Additionally, the peak dry phase of the MJO is least likely to witness the formation of a MOV. Key Points The monsoon onset vortex (MOV)'s response to the Madden‐Julian Oscillation (MJO) phases is asymmetric A convectively active MJO is neither a necessary nor a sufficient condition for the formation of the MOV The genesis potential index is a useful metric for studying MOV formation

Understanding the variability and mechanisms of monsoon onset is extremely prominent for water management and rain-fed agriculture. Previous studies have shown a significant interdecadal advance in Asian summer monsoon (ASM) onset after the late-1990s and attributed it to the sea surface temperature anomalies (SSTA) in the tropical Pacific. However, the westward-propagating mechanisms revealed by previous studies (Walker circulation, equatorial Rossby wave response) are gradually decaying westward, which cannot explain the observational facts of stronger low-level winds over the Arabian Sea than the South China Sea. Based on longer datasets and multiple methods, this study reveals the influences of Pacific SST on the interdecadal changes of ASM onset through two eastward-propagating mechanisms: the equatorial Kelvin wave response to the SSTA in the equatorial central Pacific, and the extratropical Rossby wave train associated with SSTA in the subtropical North Pacific. These two eastward-propagating mechanisms mainly modulate the ASM onset via altering the meridional temperature gradient, which is more evident over the Arabian Sea and is more consistent with the observations. Special attention has been paid to the generation and maintenance of the extratropical Rossby wave train, which is less understood compared to the other mechanisms. This Rossby wave train can be excited by the upper-level divergence associated with the warm SSTA in the subtropical North Pacific. In addition, it can effectively gain available potential energy and kinetic energy from the basic flow, and exhibits strong positive interactions with the synoptic-scale eddies. This Rossby wave train is a newly recognized mechanism by which the extratropical Pacific SSTA influences the tropical ASM.

The interannual variation of the South China Sea (SCS) summer monsoon onset (SMO) may bring extreme weather and climate disasters in East Asia. However, its skillful forecast still remains challenging. This study investigates the intraseasonal ocean‐atmosphere interaction that affects the SCSSMO through diagnostic analysis and numerical experiments. It reveals that the cold sea surface temperature in the Southern SCS during winter (referred as cold tongue, CT) is the key pathway controlling the propagation of the 30–60 days intraseasonal oscillation (ISO) convective system from the Bay of Bengal (BOB) to the SCS. The CT variations affect the interannual variation of the SCSSMO. Specifically, the strong (weak) CT after the peak of La Niña (El Niño) years suppresses (enhances) the propagating ISO from the BOB to the SCS, resulting in a delayed (advanced) SCSSMO. This finding offers the new scientific insights for improving the forecasting of the SCSSMO. Plain Language Summary The occurrence of a strong cold tongue (CT) in winter, commonly detected in the southern South China Sea (SCS) during La Niña decay years, plays a significant role in delaying the SCS summer monsoon onset (SCSSMO). It adversely affects the northeastward progression of the 30‐60‐day convective Intraseasonal Oscillation (ISO) from the equatorial Indian Ocean to the southern SCS. This interference leads to the delayed onset of the SCSSMO. It highlights the crucial influence of the CT sea surface temperatures (SST) on the linkage between Bay of Bengal and SCS at the intraseasonal scale, thus significantly impacting the timing of the SCSSMO. Key Points The winter cold tongue in South China Sea (SCS) is a crucial oceanic precursor for the interannual variation of the SCS summer monsoon onset (SCSSMO) Strong cold tongue in the SCS can suppress the propagation of the 30‐60‐day intraseasonal oscillation into the SCS Strong SCS cold tongue during La Niña years tends to delay the SCSSMO

Monsoon onset marks an abrupt seasonal transition from a dry to a moist atmosphere, but physical processes associated with the monsoon onset over India and the Arabian Sea (AS) are not fully understood. In this study, a northward propagating convective phase of intraseasonal oscillations (ISOs), associated with low-level cyclonic circulation, is identified as a crucial factor in initiating the monsoon onset. The northward propagation is sustained by a positive moist static energy (MSE) tendency to the north and a simultaneous negative tendency to the south of the convective center. Results from the MSE budget diagnosis indicate that the MSE tendency dipole is attributed to horizontal moisture advection. Under a wetter (dryer) background environment over southeastern (northwestern) AS, the intraseasonal cyclonic circulation enhances (reduces) the MSE to its north (south). In addition, the northward propagation is controlled by a meridional asymmetry of background convective instability (BCI). During the pre-onset stage, the accumulation of background low-level moisture over the northern AS due to meridional moisture transport by cross-equatorial flow enhances local BCI. A more unstable background environment over the AS, compared to the equatorial western Indian Ocean (EWIO), facilitates the northward propagation of ISOs. ENSO exerts a marked impact on the monsoon onset through the modulation of the meridional asymmetry of BCI. During post-La Nina Springs, both the enhanced meridional SST gradient over EWIO and the stronger cross-equatorial low-level flow over the AS help trigger the northward-propagating ISOs and thus lead to an earlier monsoon onset.

In this study, we demonstrate a new mechanism, on how the warm phase of El Niño and Southern Oscillation (ENSO) delays the Indian Summer Monsoon onset through coupled ocean-atmospheric processes. Strong basin-wide warming is prominent over North Indian Ocean (NIO) during the El Niño years. The warming is intense over the South West Indian Ocean (SWIO) due to the westward propagation of the equatorial Rossby waves from the Pacific Ocean. It increases the convective activities over the southern tropical Indian Ocean (0–10° S), adjacent to the SWIO region. The warming over the SWIO and the NIO strengthens the divergent wind from the Indian Ocean to the sub-tropics via a wind-evaporation-SST feedback mechanism, which causes more upper-level convergence within 30° N latitudes. Besides, a warmer Indian Ocean enhances the upper-level diabatic heating over the southern Arabian Peninsula and Eastern Indian landmass. These factors strengthen but shift the local Hadley circulation over the Indian Ocean more southward, with an ascending branch centered over the SWIO region. The shifting of the local Hadley circulation during the El Niño years causes the Sub Tropical Jet (STJ) to migrate more southward and centered more over the Indian subcontinent. This southward movement of the STJ over the Indian subcontinent in response to the El Niño condition inhibits the establishment and propagation of the tropical easterly jet during the monsoon season, which subsequently hinders the monsoon circulation, thus delays its onset.

Previous studies suggest that interannual variability of surface air temperature (SAT) over India peaks in June, a month later than its climatology, which is due to the significant year-to-year variability of abrupt summer monsoon onset. However, April is the hottest month over Indochina Peninsula (ICP), and SAT variability is much larger in April than summer monsoon season which starts in mid-May. The first empirical orthogonal function (EOF) mode of daily-mean SAT evolution over ICP from 1 April to 20 May on the interannual timescale captures 49.6% of total variance, and tends to occur during post-ENSO spring with significant intraseasonal modulations. During post-El Niño April-May, persistent positive SAT anomalies over ICP are associated with anomalous north Indian Ocean warming and tropical Indo-northwestern (TNW) Pacific cooling, which induce a low-level anomalous anticyclonic circulation (AAC) over the TNW Pacific region through coupled ocean-atmosphere feedbacks. The easterly wind anomalies on the southern flank of the AAC induce anomalous warming over ICP by delaying the onset of local monsoonal rainfall. On the intraseasonal timescale, the onset process of ICP summer monsoon is modulated by both 10-20-day and 30-80-day oscillations. At the positive phase of EOF1, the dry spells of both 30-80-day (phase 3 & 4) and 10-20-day (phase 1) are much stronger than the climatology, with consistent easterly wind anomalies over ICP as part of the TNW Pacific AAC, contributing to 9 days late of ICP summer monsoon onset. These results provide important implications for the prediction of both SAT and summer monsoon onset over ICP.

The present study examines atmosphere–ocean interaction before MOK using various observational data sets during the last 35 years (1982–2016). The analyses suggest a new mechanism for the evolution of MOK involving the coupling of radiation, sea surface temperature (SST), wind, evaporation, SST gradient, wind stress, total precipitable water (TPW), and convection processes. During the pre-monsoon period, Arabian Sea (ArS) starts warming and reaches a maximum just four pentads before MOK. Hence, a meridional SST gradient develops that pulls air from the colder southern Indian Ocean, which leads to enhanced wind speed and thus wind stress towards the warmer region. This is followed by the cooling of SST over southwestern ArS due to the wind-induced evaporation and upwelling along Somali coast in response to the enhanced wind stress curl. The low-level anticyclonic circulation over north ArS also aids this Somali coast upwelling, which builds up a zonal SST gradient between East and West ArS. In response, surface wind speed enhances further. The north-eastward directed gradient drives the low-level wind north-eastward towards the eastern ArS. The increase in evaporation enhances moisture content of the air which is carried by low-level wind and accumulates in a column of high TPW near the Kerala coast and adjoining south-east ArS. That moist air column fosters rainfall and thus monsoon onset over Kerala. In short, ocean warming by short-wave radiation during pre-monsoon sets the background, then the ocean surface provides the necessary energy to the atmosphere in the form of latent heat flux in response to wind driven by the developing SST gradient. Significant inter-annual correlations are found between wind stress, TPW and precipitation over the North Indian Ocean during this period, supporting the interpretation detailed above.

Using surface and balloon-sounding measurements, satellite retrievals, and ERA5 reanalysis during 2011–20, this study compares the precipitation and related wind dynamics, moisture and heat features in different areas of the South China Sea (SCS) before and after SCS summer monsoon onset (SCSSMO). The rainy sea around Dongsha (hereafter simply referred to as Dongsha) near the north coast, and the rainless sea around Xisha (hereafter simply referred to as Xisha) in the western SCS, are selected as two typical research subregions. It is found that Dongsha, rather than Xisha, has an earlier and greater increase in precipitation after SCSSMO under the combined effect of strong low-level southwesterly winds, coastal terrain blocking and lifting, and northern cold air. When the 950-hPa southwesterly winds enhance and advance northward, accompanied by strengthened moisture flux, there is a strong convergence of wind and moisture in Dongsha due to a sudden deceleration and rear-end collision of wind by coastal terrain blocking. Moist and warm advection over Dongsha enhances early and deepens up to 200 hPa in association with the strengthened upward motion after SCSSMO, thereby providing ample moisture and heat to form strong precipitation. However, when the 950-hPa southwesterly winds weaken and retreat southward, Xisha is located in a wind-break area where strong convergence and upward motion centers move in. The vertical moistening and heating by advection in Xisha enhance later and appear far weaker compared to that in Dongsha, consistent with later and weaker precipitation.

The summer monsoon onset in the northern Indian Ocean is crucial for the densely populated South Asia, as it ends the pre‐monsoon heatwave and kicks off the rainy agricultural season. Climatologically, this onset occurs in two distinct phases: first in the Bay of Bengal (BoB), followed by the Arabian Sea (AS). However, the possible mechanistic linkage between these phases remains unclear. Based on observational analysis, this study investigates how the BoB monsoon onset preconditions the subsequent AS monsoon onset, with particular focus on the pivotal trigger of the AS monsoon—the intraseasonal oscillation (ISO‐AS). We demonstrate that the BoB monsoon onset establishes an easterly vertical wind shear across the northern Indian Ocean. The shear environment interacts with the ascending motion of the ISO‐AS, which develops approximately 1 month later, enhancing cyclonic vorticity north of the convection center of the ISO‐AS. This process, in turn, promotes moisture convergence in the boundary layer and facilitates the northward propagation of the ISO‐AS, ultimately triggering the summer monsoon onset in the AS. By elucidating the stepwise nature of monsoon onset in the northern Indian Ocean, this work offers valuable insights for improving predictions of the Asian summer monsoon. The earlier BoB monsoon onset establishes an easterly vertical wind shear across the northern Indian Ocean. This shear environment interacts with the ascending motion of the ISO‐AS approximately 1 month later, enhancing cyclonic vorticity north of the convection center of the ISO‐AS. This process, in turn, promotes moisture convergence in the boundary layer and facilitates the northward propagation of the ISO‐AS, ultimately triggering the summer monsoon onset in the AS.