Unraveling the discrepancies between Eulerian and Lagrangian moisture tracking models in monsoon- and westerly-dominated basins of the Tibetan Plateau
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Abstract. Eulerian and Lagrangian numerical moisture tracking models, which are primarily used to quantify moisture contributions from global sources to specific regions, play a crucial role in hydrology and (paleo)climatology studies on the Tibetan Plateau (TP). Despite their widespread applications in the TP region, potential discrepancies in their moisture tracking results and their underlying causes remain unexplored. In this study, we compare the most widely used Eulerian and Lagrangian moisture tracking models over the TP, i.e., WAM2layers (the Water Accounting Model – 2 layers) and FLEXPART-WaterSip (the FLEXible PARTicle dispersion model coupled with the “WaterSip” moisture source diagnostic method), specifically focusing on a basin governed by the Indian summer monsoon (Yarlung Zangbo River basin, YB) and a westerly-dominated basin (upper Tarim River basin, UTB). Compared to the bias-corrected FLEXPART-WaterSip, WAM2layers generally estimates higher moisture contributions from westerly-dominated and distant sources but lower contributions from local recycling and nearby sources downwind of the westerlies. These differences become smaller with higher spatial and temporal resolutions of forcing data in WAM2layers. A notable advantage of WAM2layers over FLEXPART-WaterSip is its closer alignment of estimated moisture sources with actual evaporation, particularly in source regions with complex land–sea distributions. However, the evaporation biases in FLEXPART-WaterSip can be partly corrected through calibration with actual surface fluxes. For moisture tracking over the TP, we recommend using high-resolution forcing datasets, prioritizing temporal resolution over spatial resolution for WAM2layers, while for FLEXPART-WaterSip, we suggest applying bias corrections to optimize the filtering of precipitation particles and adjust evaporation estimates.Keywords:
Tracking (education)
Westerlies
We offer a new perspective on a relationship between sea surface temperature (SST) over the windward region of the Philippines and rainfall in the western Philippines during the Asian summer monsoon season, which has been known as the negative correlation, using observational daily SST, rainfall, and atmospheric circulation datasets. This study focuses on the local SST effect rather than the remote effect. A warmer local SST results in greater rainfall over the western Philippines under similar monsoon westerlies conditions, particularly during moderate and relatively stronger monsoon regimes. This result is obtained after selecting only the moderate or relatively stronger monsoon days, because the positive effect of SST on rainfall is masked by the apparent negative correlation between SST and rainfall. The warmer SSTs being associated with less rainfall correspond to weaker cooling by weaker monsoon westerlies and the cooler SSTs being associated with more rainfall correspond to stronger cooling by stronger monsoon westerlies. The cooler SSTs are the result of stronger monsoon cooling and are not the cause of the greater rainfall, which is the apparent statistical relationship. This also implies that the monsoon westerly is the primary driver of the variation in rainfall in this region. We conclude that the local SST makes a positive contribution toward rainfall, although it does not primarily control rainfall. This conclusion can be applicable to coastal regions where, climatologically, rainfall is controlled by winds from the ocean.
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The paper presents the results of a study of the daily precipitation, temperature at 6 km and winds between 6 and 9 km a.s.l. Over the Everest area during the months of May and June for the period 1952 to 1962. The indications are that there is no well marked 'monsoon lull' or a 'monsoon onset' over the Everest. The transition from the strong and steady westerlies between 6 and 9 km a.s.1. of ,the pre-monsoon season to the steady easterlies of the monsoon season occurs over a period highly variable in duration and can occasionally extend over the whole of May and June. Winds over Everest during this period can be moderate or weak westerlies or weak easterlies. With weak westerlies precipitation can occur practically every day and in most of the years it is not possible to distinguish the onset of the monsoon rains as distinct from the’pre-monsoon thundershower activity'. The temperature of the strong and steady westerlies at 6 km a.s.l. over Everest increases gradually from -10oC in early May to about 0°C by the time the variable winds set in. From a consideration of the weather features that were obtained by the successful and uJ18uccessful expeditions to the Mount Everest since 1952 it is considered more profitable for future expeditions to get to the high reaches of the Everest by early May instead of late May.
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Westerlies
Atmospheric Circulation
East Asian Monsoon
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Monsoon droughts over the Indian subcontinent emanate from failures in the seasonal (June–September) monsoon rains. While prolonged dry‐spells (“monsoon‐breaks”) pervade on sub‐seasonal/intra‐seasonal time‐scales, the underlying causes for these long‐lasting anomalies remain elusive. Based on analyses of a suite of observed data sets, we report an ocean‐atmosphere dynamical coupling on intra‐seasonal time‐scales, in the tropical Indian Ocean, which is pivotal in forcing extended monsoon‐breaks and causing droughts over the subcontinent. This coupling involves a feedback between the monsoonal flow and thermocline depth in the Equatorial Eastern Indian Ocean (EEIO), in which an anomaly of the summer monsoon circulation induces downwelling and maintains a higher‐than‐normal heat‐content. The near‐equatorial anomalies induce strong and sustained suppression of monsoon rainfall over the subcontinent. It is concluded that the intra‐seasonal evolution of the ocean‐monsoon coupled system is a vital key to unlocking the dynamics of monsoon droughts.
Indian subcontinent
Downwelling
Forcing (mathematics)
Anomaly (physics)
Tropical monsoon climate
East Asian Monsoon
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Circulation (fluid dynamics)
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Utilising the MONEX (Monsoon Experiment) data 1973, the wind field over the Arabian Sea in the lower and middle troposphere during two epochs, one of active and one of weak monsoon over the Indian Peninsula and the central parts of the country, was analysed and the results are presented in this paper. The study shows that the low level westerlies attain a mean maximum velocity of 50 to 55 kt over the west central Arabian Sea both in active and weak monsoon, the level of the maximum wind being at about 900 mb in active monsoon and 950 mb in weak monsoon. But in east Arabian Sea, the westerlies weaken considerably during weak monsoon. The slope and other features associated with the low level westerly maximum are also discussed.
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Peninsula
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It is hypothesized that the South China Sea (SCS) monsoon and the Indian monsoon intensify each other in the boreal summer. The Indian monsoon westerlies strengthen the SCS monsoon through the WE (Wind- Evaporation feedback) mechanism, but weaken it through the reduction in SCS SST. The SCS monsoon may have a positive feedback to enhance the Indian monsoon westerlies through the Walker circulation. Further question is how the convective activity over the Western North Pacific (WNP) interacts the Indian monsoon westerlies. The WNP SST may affect the Indian monsoon through the WNP precipitation. In this study, we investigate the relationships among the Indian monsoon, SCS monsoon and WNP activities from various observational data. The regression coefficient analyses are made. The data are NCEP reanalysis data, the CMAP precipitation data and ICOADS SST data averaging over June-July-August (JJA) for 21 years (1982-2002). First, the precipitation anomalies are regressed to the Indian monsoon westerly anomaly. The result shows that the Indian monsoon westerlies are positively correlated with the SCS precipitation. It indicates that the surface heating exchange over the SCS region is largely induced by the Indian monsoon westerlies and/or the SCS monsoon enhances the Indian monsoon westerlies. Another map of the regression of the 850hPa wind field anomalies to the WNP precipitation anomaly also shows that the Indian monsoon westerlies are positively correlated with the WNP precipitation. Completing this result, we confirm that the WNP precipitation and WNP SST have a positive correlation. Thus, the strengthening of the Indian monsoon westerlies and SCS westerlies may be also induced by the precipitation increase on the WNP region through the stronger east-west circulation associated with the warmer WNP SST. From the above results, it is found that the Indian monsoon westerlies are strongly correlated with the SCS monsoon activity and the WNP precipitation. Probably, the SCS monsoon is affected by the Indian monsoon through the WE, and affects the Indian monsoon through the diabatic heating. Further link is found to the precipitation over the WNP. To confirm the mechanism of correlation, the model study is underway.
Westerlies
East Asian Monsoon
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