Some aspects of the monsoon circulation and monsoon rainfall

Summary The south Asian summer monsoon from June to September accounts for the greater part of the annual rainfall over most of India and southeast Asia. The evolution of the summer and winter monsoon circulations over India is examined on the basis of the surface and upper air data of stations across India. The salient features of the seasonal reversals of temperature and pressure gradients and winds and the seasonal and synoptic fluctuations of atmospheric humidity are discussed. The space-time variations of rainfall are considered with the help of climatic pentad rainfall charts and diagrams. The rainfall of several north and central Indian stations shows a minimum around mid-August and a maximum around mid-February which seem to be connected with the extreme summer and winter positions of the ITCZ and the associated north-south shifts in the seasonal circulation patterns. Attention is drawn to the characteristic features of the monsoon rainfall that emerge from a study of daily and hourly rainfall of selected stations. Diurnal variations of temperature, pressure, wind and rainfall over the monsoon belt are briefly treated.     

Mid-tropospheric cyclones of the summer monsoon

Mid-tropospheric cyclones are often observed on daily and monthly mean maps over southern Asia during the summer southwest monsson season. Although they are important activators of monsoon rains over certain regions, only a few observational studies are available, and even fewer theoretical and numerical studies have been performed. This article attempts to summarize the present observational knowledge about mid-tropospheric cyclones and reviews current ideas on the formation, maintenance and dissipation of these systems. A recent numerical simulation of a mid-tropospheric cyclone is described. In general, latent heat release plays the dominant role in the dynamics of the cyclone. Much is still to be learned about mid-tropospheric cyclones and several promising areas of future research are suggested.     

Hemispheric simulation of the Asian summer monsoon

A three-level, -plane, filtered model is used to simulate the Northern Hemisphere summer monsoon. A time-averaged initial state, devoid of sub-planetary scale waves, is integrated through 30 days on a 5° latitude-longitude grid. Day 25 through day 30 integrations are then repeated on a 2.5° grid. The planetary-scale waves are forced by time-independent, spatially varying diabatic heating. Energy is extracted via internal and surface frictional processes. Orography is excluded to simplify synoptic-scale energy sources.During integration the model energy first increases, but stabilizes near day 10. Subsequent flow patterns closely resemble the hemisphere summer monsoon. Climatological features remain quasi-stationary. At 200 mb high pressure dominates the land area, large-scale troughs are found over the Atlantic and Pacific Oceans, the easterly jet forms south of Asia, and subtropical jets develop in the westerlies. At 800 mb subtropical highs dominate the oceans and the monsoon trough develops over the Asian land mass. The planetary scales at all levels develop a realistic cellular structure from the passage of transient synoptic-scale features, e.g., a baroclinic cyclone track develops near 55°N and westward propagating waves form in the easterlies.Barotropic redistribution of kinetic energy is examined over a low-latitude zonal strip using a Fourier wave-space. In contrast to higher latitudes where the zonal flow and both longer and shorter waves are fed by barotropic energy redistribution from the baroclinically unstable wavelengths, the low-latitude waves have a planetary-scale kinetic energy source. Wave numbers 1 and 2 maintain both the zonal flow and all shorter scales via barotropic transfers. Transient and standing wave processes are examined individually and in combination.Wave energy accumulates at wave numbers 7 and 8 at 200 mb and at wave number 11 in the lower troposphere. The 800-mb waves are thermally indirect and in the mean they give energy to the zonal flow. These characteristics agree with atmospheric observation. The energy source for these waves is the three wave barotropic transfer. The implications of examining barotropic processes in a Fourier wave-space, vice the more common approach of separating the flow into a mean plus a deviation are discussed.     

Some remarks on the european monsoon

Summary 1. The so called sigularities want all reality and are of no value in monsoon investigations. — 2. In this section a short summary ofChromow's monsoon studies is given (Fig. 1). — 3. The monsoon in Western Europe manifests itself in impulses during some days alternating with the general west circulation. Monthly means of the wind direction give the resultant direction of these two independent wind systems and do by no means elucidate the true behaviour of the wind.Chromow's method, the appliance of monthly means of pressure gradients, is also insufficient. The only reliable method of researching the monsoon in moderate regions is the investigation of the separate wind octants. A month is too long to reveal the rather short impulses and therefore calculating decade means is recommended. The monsoon depends upon the direction and the velocity of the wind. Calculating the product of the frequency of the direction (in %) and the wind velocity is in practice a sufficient approximation. I have called this product the «relative wind vector». — 4. Whereas generally the months of January and July are accepted as the central monsoon months, in the Netherlands and Germany these months appear to be November–December and May–June (Table 1, 2; Fig. 2, 3, 5, 6, 7), respectively with S and NE winds. The summer monsoon seems to back from E to N and perhaps even to NW. This backing may be caused by the form of the european continent. — 5. A research into the maximal development of the monsoon impulses shows that they equal or even surpass the general western ciruclation concerning both their number (Table 1, right-hand side) and their intensity (Table 3).Willett's opinion that the normal geographical distribution of air mass source regions in the spring and the autumn are intermediate between those of the months of Janaury and July cannot be maintained for West Europe. — 6. A research by means ofConrad's monsoon index also shows that the full monsoon months in W-Europe are November–December and May–June (Table 4). The application of his method to the relative wind vector at Den Helder and Maastricht confirms this result (Table 5). — All results arrived at show the activity of the monsoon phenomenon in a good deal of West Europe.

---

Zusammenfassung 1. Die Singularitäten entbehren jede Realität und sind wertlos für Monsununtersuchungen. — 2. In diesem Paragraph findet man eine Zusammenfassung vonChromow's Anschauungen (Fig. 1). — 3 Der europäische Monsun äussert sich in Stössen während einiger Tage, abwechselnd mit der allgemeinen Westzirkulation. Montasmittel der Windrichtung geben nur die resultierende Richtung dieser zwei unabhängigen Windsystemen und zeigen in keinem Fall die wahren Windverhältnisse. Die einzige Methode ist die Untersuchung der einzelnen Windoktanten. Eine Monat, ist zu lang um die ziemlich kurzen Stösse zu zeigen. Ein gutes Mass ist das Produkt der Richtungsverteilung in Prozenten mit der Geschwindigkeit, der «relative Windvektor» genannt. — 4. In den Niederlanden und Deutschland sind nicht die allgemein angenommenen Monate Januar und Juli die Zentralmonate des Monsuns, sondern November–Dezember und Mai–Juni (Tab. 1, 2.; Fig. 2, 3, 5, 6, 7), beziehungsweise mit S- und NE-Winden. Der NE-Monsun scheint zurückzudrehen von E nach N, vielleicht selbst, nach NW. Diese Eigenschaft kann verursacht werden von der Form des europäischen Kontinents. — 5. Die Wichtigkeit der Monsunstösse ist gleich, der Westzirkulation oder selbst stärker als diese, sowohl was ihre Zahl (Tab. 1, rechts) als ihre Intensität (Tab. 3) betrifft.Willett's Behauptung, dass die normale geographische Verteilung der Luftmassen im Frühling und im Herbst die mittleren Eigenschaften der Verteilung im Januar und im Juli besitzt kann für Westeuropa nicht aufrecht gehalten werden. — 6.Conrad's Monsunindex zeigt auch dass die vollen Monsunmonate in Westeuropa die oben genannten sind (Tab. 4). Die Anwendung seiner Methode auf dem relativen Windvektor bestätigt dieses Resultat (Tab. 5). Alle gefundenen Resultate zeigen die Aktivität des Monsunphänomens in Westeuropa.

---

Résumé 1. Les singularités manquent toute réalité et n'ont pas de valeur pour l'étude des moussons dans les régions modérées. — 2. Un résumé court des études deChromow est donné (Fig. 1). — 3. Le mousson européen se manifeste par des impulsions pendant quelques jours alternant avec la circulation atmosphérique générale. Les moyennes mensuelles de la direction du vent ne donnent que la direction résultante mais ne démontrent rien concernant les particularités réelles du vent. La méthode deChromow, l'application des gradients mensuels de la pression, est aussi insuffisante. En outre la durée d'une mois est trop longue pour révéler ces impulsions courtes. Il faut préférer les décades. Pour conclure il faut tenir compte de la vitesse du vent aussi. J'ai calculé le produit de la fréquence (en pourcentage) et la vitesse, le «vecteur relatif du vent» 4. Tandis que généralement les mois de Janvier et de Juin sont acceptées comme les mois centrales du mousson, dans les Pays Bas et en Allemagne ces mois sont Novembre–Décembre et Mai–Juin (Tabl. 1, 2; Fig. 2, 3, 5, 6, 7) avec resp. le mousson de S et le mousson de NE. Le mousson de NE semble changer de l'E à N et peutêtre même à NW. Ce recul du vent peut être causé par la configuration du continent européen. — 5. Une recherche des impulsions du mousson démontre qu'elles égalent, même surpassent la circulation générale aussi bien concernant leur nombre (Tabl. 1, à droite) que leur intensité (Tabl. 3). L'opinion deWillett que les masses d'air du printemps et de l'automne sont intermédiaires entre celles de Janvier et de Juin n'est pas acceptable pour l'Europe occidentale. — 6. Une recherche au moyen de l'index deConrad montre aussi que les mois essentielles du mousson européen sont les mois mentionnées (Tabl. 4). L'application de la méthode deConrad sur le vecteur du vent relatif confirme ce résultat (Tabl. 5). — Tous les résultats gagnés montrent l'activité du mousson dans une grande partie de l'Europe occidentale.

     

Break in the Indian summer monsoon

Summary The changes in circulation patterns over Eurasia during break monsoon condition over India are studied in comparison to the active monsoon condition. Break monsoon condition seems to set in over the India Sub-Continent in association with eastward movement of middle latitude westerly trough at 500 mb, having large amplitude extending into west Pakistan and northern India. Simultaneously the subtropical anticyclonic ridge over Arabia protrudes into central and Peninsular India. The high latitude blocking high over East Siberia retrogrades and the East Asiatic trough deepens and moves eastwards. The west Pacific subtropical ridge recedes eastward from the China continent. During this period the monsoon trough shifts, from its normal position over Gangetic plains, northwards to the foot of the Himalayas and the monsoon westerlies in the lower troposphere extends right upto the rim of the Tibetan Plateau. The sub-tropical ridge line in the upper troposphere shifts northwards during break and lies approximately above the lower monsoon trough. This seems to provide an effective process of removing ascending air in the lower monsoon trough causing exceptionally heavy rainfall over Assam and along the foot of the Himalayas.     

Indian summer monsoon and El Nino

The associations between strong to moderate El Nino events and the all-India and subdivisional summer monsoon rainfall is examined for the period 1871 to 1978. The significance of the association is assessed by applying the Chi-square test to the contingency table. The analysis indicates that during 22 El Nino years the Indian monsoon rainfall was mostly below normal over most parts of the country. However, the association between El Nino and deficient rainfall or drought is statistically significant over the subdivisions west of longitude 80°E and north of 12°N. During the five strong El Nino years—1877, 1899, 1911, 1918, and 1972—many areas of India suffered large rainfall deficiencies and severe droughts. There are four moderate El Nino years—1887, 1914, 1953, and 1976—when the suffering was marginal. The relationship between El Nino and the Indian monsoon rainfall is expected to be useful in forecasting large-scale anomalies in the monsoon over India.     

African monsoon variability during the previous interglacial maximum

Little is known about centennial- to millennial-scale climate variability during interglacial times, other than the Holocene. We here present high-resolution evidence from anoxic (unbioturbated) sediments in the eastern Mediterranean Sea that demonstrates a sustained ∼800-yr climate disturbance in the monsoonal latitudes during the Eemian interglacial maximum (∼125 ka BP). Results imply that before and after this event, the Intertropical Convergence Zone (ITCZ) penetrated sufficiently beyond the central Saharan watershed (∼21°N) during the summer monsoon to fuel flooding into the Mediterranean along the wider North African margin, through fossil river/wadi systems that to date have been considered only within a Holocene context. Relaxation in the ITCZ penetration during the intra-Eemian event curtailed this flux, but flow from the Nile - with its vast catchment area - was not affected. Previous work suggests a concomitant Eurasian cooling event, with intensified impact of the higher-latitude climate on the Mediterranean basin. The combined signals are very similar to those described for the Holocene cooling event around 8 ka BP. The apparent type of concurrent changes in the monsoon and higher-latitude climate may reflect a fundamental mechanism for variability in the transfer of energy (latent heat) between the tropics and higher latitudes.     

Numerical weather prediction relevant to the monsoon problem

In this short paper we have identified some of the modelling groups that have the capability of simulating or carrying out short range numerical weather prediction over the monsoon belt. We have next outlined some of the important and desirable ingredients for a multilevel primitive equation model over the tropics, with most of the emphasis on the present version of Florida State University's Tropical Prediction Model. Finally, we present briefly some important results based on the present version of our prediction models that relate to the NWP efforts over the monsoon belt. Here we have identified the importance of mountains, convection, the radiative heating balance of the earth's surface, and the planetary boundary layer over the Arabian Sea.     

Baroclinic energetics and zonal plane distribution of monsoon disturbances

The adiabatic, quasi-geostraphic, 25-layer, numerical, linear model with Ekman boundary layer friction is utilised to perform the baroclinic stability analysis of the mean monsoon zonal wind profile. It is shown thec _i is a function of the resultant wavenumber alone. This relation is able to explain the effects of the lateral walls on the unstable waves.The energetics and zonal plane distribution of the short and long preferred viscous waves are computed. The upward motion of the short wave together with the warm (cold) core lies to the west of the surface trough position above (below) 850 mb. Further, it is shown that the main source of kinetic energy for the wave lies in the middle layer (850–700 mb) which is transported to the lower and upper layers. Computed is found to be in good agreement with observed values.     

Seasonal variability of the complementary relationship in the Asian monsoon region

The complementary relationship (CR) between potential evaporation (LE_p) and actual evaporation (LE) is widely used to explain the evaporation paradox and to estimate LE, in which wet environment evaporation (LE_w) is usually calculated using the Priestley–Taylor equation. However, in many studies on the CR, it has been found that the Priestley–Taylor parameter α is not a constant. Through seasonal variation of α for estimating LE_w in the CR, this paper analyses its seasonal variability. Based on flux observation data at two flux experiment sites (Kogma in Thailand and Weishan in China) in the Asian monsoon region, seasonal variability of the CR is detected, i.e. the α value is larger in winter than in summer. This seasonal variability might be caused by seasonal variability in the transport of water vapor and sensible heat between oceans and continent. The monsoon increases air humidity and lowers air temperature in summer, which leads to a decrease in α; it increases atmospheric air temperature and vapor content in winter, increasing α. Nevertheless, during May–September, α has a range of 1.06–1.16 at the Kogma site and 1.00–1.36 at the Weishan site, which is approximate to the typical range 1.1–1.4. Copyright © 2012 John Wiley & Sons, Ltd.     

Late quaternary monsoon patterns on the Loess Plateau of China

China's Loess Plateau was formed under special conditions. The tectonic movement, topographical characteristics, and monsoon patterns combined to create a favourable environment for the accumulation of thick loessic deposits. The Loess Plateau itself is part of the ‘Monsoon Triangle’ of China, a region very susceptible to climatic changes. Throughout the Upper Pleistocene the palaeoenvironment on the Loess Plateau alternated from steppe, to deciduous forest and coniferous forest, in response to shifts in the atmospheric circulation. Three monsoon patterns appear to be indicated: (1) a full glacial monsoon pattern (18000–15000 yr BP) which induced a cold and dry climate favouring loess accumulation in steppe conditions; (2) an interglacial monsoon pattern (last interglacial and Holocene) in which a warm humid climate prevailed with deciduous forests, leaving palaeosols interbedded within the loess sequence; and (3) a transitional or interstadial monsoon pattern (50 000–23 000 yr BP) in which the climate was cold and humid in the Loess Plateau, encouraging the development of coniferous forest.     

Some aspects of the life history,structure and movement of monsoon depressions

Summary Monsoon depression is one of the most important synoptic scale disturbances on the quasi-stationary planetary scale monsoon trough over the Indian region during the summer monsoon season (June to September). Salient features of the climatology of the depressions with regard to frequency of cyclogenesis, life expectancy, horizontal scale and tracks are discussed. Rainfall aspects of the depressions are discussed in some detail and the role of local, dynamical and sub-synoptic scale factors are brought out. Work done on the life history such as formation, intensification and maintenance of depressions has been reviewed based on synoptical and theoretical approaches. Structure of the depression based on composited, synoptical and dynamical studies is discussed. Wind circulation, thermal and moisture patterns, vertical motion field, vorticity budget etc., of a recent case study are brought out in some detail. The problem of movement of the depression against the low level basic westerly wind is briefly discussed and the results of several numerical and climatological prediction models are presented.     

Application of Fourier series in managing the seasonality of convective and monsoon rainfall

ABSTRACT

This study assesses the performance of Fourier series in representing seasonal variations of the tropical rainfall process in Malaysia. Fourier series are incorporated into a spatial-temporal stochastic model in an attempt to make the model parsimonious and, at the same time, capture the annual variation of rainfall distribution. In view of Malaysia’s main rainfall regime, the model is individually fitted for two regions with distinctive rainfall profiles: one being an urban area receiving rainfall from convective activities whilst the other receives rainfall from monsoonal activities. Since both regions are susceptible to floods, the study focuses on the rainfall process at fine resolution. Fourier series equations are developed to represent the model’s parameters to describe their annual periodicity. The number of significant harmonics for each parameter is determined by inspecting the cumulative fraction of total variance explained by the significant harmonics. Results reveal that the number of significant harmonics assigned for the parameters is slightly higher in the region with monsoonal rains. The overall simulation results show that the proposed model is capable of generating tropical rainfall series from convective and monsoonal activities.

Editor D. Koutsoyiannis Associate editor K. Hamed     

Orographic effects on the southwest monsoon: A review

An overview of the problem of orographic effects on the southwest monsoon using the contributions of all the available analytical and numerical models is attempted. A quasi-geostrophic model is applied to deduce the effect of the topographic complex on the Indian peninsula. This model suggests that the southward bending of the low-level isobars on the peninsula can be ascribed to the topographically-induced southward velocity. This southward velocity triggers a Rossby wave to the east of the peninsula which is manifested as a trough on the southern Bay of Bengal.     

The Red Sea outflow regulated by the Indian monsoon

To investigate why the Red Sea water overflows less in summer and more in winter, we have developed a locally high-resolution global OGCM with transposed poles in the Arabian peninsula and India. Based on a series of sensitivity experiments with different sets of idealized atmospheric forcing, the present study shows that the summer cessation of the strait outflow is remotely induced by the monsoonal wind over the Indian Ocean, in particular that over the western Arabian Sea. During the southwest monsoon (May–September), thermocline in the Gulf of Aden shoals as a result of coastal Ekman upwelling induced by the predominantly northeastward wind in the Gulf of Aden and the Arabian Sea. Because this shoaling is maximum during the southwest summer monsoon, the Red Sea water is blocked at the Bab el Mandeb Strait by upwelling of the intermediate water of the Gulf of Aden in late summer. The simulation also shows the three-dimensional evolution of the Red Sea water tongue at the mid-depths in the Gulf of Aden. While the tongue meanders, the discharged Red Sea outflow water (RSOW) (incoming Indian Ocean intermediate water (IOIW)) is always characterized by anticyclonic (cyclonic) vorticity, as suggested from the potential vorticity difference.     

Application of a stochastic model to the frequency and duration of monsoon rainfall

The paper deals with an application of a stochastic model to the frequency and duration of precipitation events. With the aid of the model, the magnitudes ofmth highest rainfall amount in 24 hours' duration with 97.5% probability are obtained for various climatic regimes over a tropical monsoon region. There is good agreement between them-day minimum rainfall estimated through the model and the observed value. The model satisfactorily explains the frequency of the extreme rainfall event.     

Spectrum analysis relevant to Indian monsoon

Recent investigations on Indian monsoon by the method of spectrum analysis are reviewed from the viewpoint of various aspects of its application. The assessment is also made on their results and discussions. Through these processes, it emerged that there exist at least two kinds of quasi-periodic variations in the summer monsoonal fluctuation over India and adjacent regions. One appears as a spectral peak around the 5-day period and another is around the 15-day period.The former one is conspicuous along the monsoon trough region in northern India. The results of cross-spectrum analysis show that it reflects the passage of westward-moving, synoptic-scale disturbances called monsoon lows. The latter seems to be related to the large-scale variation of monsoon activity itself, though it is also likely to have some associations with the cyclogenesis of monsoon depressions. The recent application of spectral method to this large-scale interrelation is also discussed along with the results of some non-spectral studies and the remaining problems are pointed out.     

Precipitable water and dew point temperature relationship during the Indian summer monsoon

Two years of contrasting monsoon rainfall over the Indian subcontinent are studied with reference to (a) total precipitable water and precipitation efficiencies during the respective years and (b) the correlation between the dew point temperature at a particular level and the total precipitable water. It is found that the maximum correlation occurs between the dew point temperature at the 850 mb level and the total precipitable water at an individual station. The precipitation efficiencies are less during the year of bad monsoon. A linear regression equation is attempted between the total precipitable water and dew point temperature.     

A hypothesis on onset,advance and withdrawal of the Indian summer monsoon

The paper defines the intertropical convergence zone. (ITCZ) in the Indian monsoon region during the northern summer, identifies it with the northern boundary of the advancing monsoon and suggests that its seasonal movement can serve as an indicator of onset, advance and withdrawal of the monsoon. Evidence suggesting the movement of the ITCZ which is associated with the equatorial trough of low pressure is indirectly furnished by an analysis of the isallobaric or height-tendency field which reveals a distinct gradient towards the north/south during period of advance/withdrawal of the monsoon. A comparative study of the dates of onset of monsoon during two successive years appears to suggest that some of the problems encountered in using rainfall as the sole criterion for determining the onset and advance of the monsoon may be over-come by using the ITCZ concept as proposed in the present paper. Attention is drawn to the effects of synoptic-scale disturbances on the normal dates of onset, advance and withdrawal of the monsoon.