Southwest Monsoon Winds Research Articles

Late Pleistocene variability of Arabian Sea summer monsoon winds and continental aridity: Eolian records from the lithogenic component of deep‐sea sediments

The modern Indian Ocean summer monsoon is driven by differential heating between the Asian continent and the Indian Ocean to the south. This differential heating produces a strong pressure gradient which drives southwest monsoon winds during June, July, and August. Satellite and meteorological observations, aerosol measurements, sediment trap studies, and mineralogical studies indicate an atmospheric mode of transport for modern lithogenic sediments in the northwest Arabian Sea. Analyses of lithogenic grain size and mass accumulation rate (MAR) records from the Owen Ridge indicate that eolian transport has been the primary mode of transport for the past 370 kyr. Visual inspection shows that the MAR record is positively correlated with global ice volume as indicated by the marine δ18O record. In contrast, the grain‐size record varies at a much higher frequency, showing little correlation to either the MAR or the δ18O records. Spectral analyses confirm these relationships, indicating that the lithogenic grain‐size and MAR records are coherent only over the precession band whereby the grain size leads the MAR by 124° (∼8 kyr). We conclude that an eolian transport mechanism is the only mechanism that allows for this phase difference and at the same time is supported by comparison of the grain size and MAR with independent eolian records. We use lithogenic grain size as a paleoclimatic indicator of summer monsoon wind strength and lithogenic MAR as a paleoclimatic indicator of source‐area aridity. These interpretations are supported by comparison of the lithogenic records to independent indicators of wind strength (Globigerina bulloides upwelling record) and aridity (a loess record from central China). Such comparisons indicate high coherence and zero phase relationships. Our work supports the findings of previous studies which have documented the link between monsoon strength and the Earth's axial precession cycles. Both the lithogenic MAR and the grain‐size records have high coherency with precessional insolation. Maximum lithogenic MAR (source‐area aridity) is in phase with δ18O (global ice volume) and leads maximum precessional insolation by 88° (∼6 kyr). We attribute this lead to the influence of glacial conditions on the aridity, and therefore the deflation potential, of the source areas. Maximum lithogenic grain size (summer monsoon wind strength) lags maximum precession by 148° (∼9 kyr). We attribute this lag both to the influence of global and/or local ice volume and to the availability of latent heat from the southern hemisphere Indian Ocean, the two of which combine to determine the strength of the Indian Ocean monsoon.

Development of Near-Surface Flow Pattern and Water Mass Distribution in the Somali Basin in Response to the Southwest Monsoon of 1979

Abstract Near-surface observations of temperature, salinity and current are used to describe the seasonal reversal of the Somali Current during 1979, in response to the onset of the southwest monsoon winds. During April, prior to the reversal of the winds north of the equator, the northward flowing East African Coastal Current (EACC) and the southward flowing Somali Current (SC) converged near the equator. The EACC was characterized by surface waters with salinities less than 35.1%, and the SC by salinities greater than 35.3%. The winds reversed north of the equator during the first week of May, and the boundary current intruded in the form of an anticyclonic gyre to 2.5°N. Most of the low-salinity water was recirculated back south of the equator by the offshore limb of the gyre. It did not flow continuously at the surface into the eastward equatorial jet, which was present farther offshore during May and June. That current was fed by high-salinity water from the region to the north of the low-latitude gy...

Notes on the Somali Current System during the Southwest Monsoon

Four hydrographic sections by the vessels Ob, Vityaz, and Atlantis 2 off the Somali coast during or just after the southwest monsoon season are discussed. Each section indicates a strong relative geostrophic current with a northward or northeastward component just off the coast (Somali current) and, to the right of this current, a current in the opposite direction. It is suggested that the currents are part of an anticyclonic circulation that may be formed by early June and that persists at least until late October after the cessation of the southwest monsoon winds.

The sediments of the northern Indian Ocean

The solid phases from surface sediments, atmospheric dusts, and rivers of the Indian Ocean environment have been analyzed for their clay minerals and quartz. Such data have been used to delimit the transport paths and sources of the detrital minerals in the oceanic deposits. Diagnostic in distinguishing fluvial and eolian inputs to the northern Indian Ocean is a combination of the clay mineral assemblages and of their geographic distributions. River borne solids are the primary components of the Bay of Bengal deposits. The eastern part receives its continental input through the Ganges-Brahmaputra river system, while drainage of the Indian Peninsula by rivers introduces solids to the western part. The former materials are characterized by high illite and chlorite in the clay mineral assemblages; the latter by montmorillonite. The winds over the Bay bear distinctive dust burdens based upon their directions. However, their contributions to the sediments are insignificant. The eastern sector of the Arabian Sea receives major contributions of continental debris from the rivers and the high montmorillonite levels clearly indicate a source in the Indian Peninsula. The rest of the Sea appears to receive most of its land-derived materials from the north, perhaps the desert regions of northern India and West Pakistan, and they are wind-borne. These materials are also transported to the equatorial regions of the Indian Ocean. A gradient in attapulgite, just north of the equator, may indicate an eolian contribution to the Arabian Sea from the African continent. The halogenated hydrocarbon pesticides were assayed in the southwest monsoon winds and enter the Bay of Bengal at levels of a half ton per month, an amount comparable to those introduced by other wind and river systems to the marine environment.

A SHORT DRY PERIOD OF MID-SUMMER IN JAPAN

1. The abundant summer rainfall in the eastern part of Asia including Japan has been commonly explained by the southwesterly monsoon winds carrying with much moisture from the Pacific Ocean. However, this problem is not so simple because the rainy season in summer has a threefold structure instead of a single continuous period. Except the northern region, there exists in Japan the double or triple maxima of monthly amount of precipitation. Fig. 1. shows the three regimes of precipitation, A, B and C, classified as follows A: Pacific type with double maxima in summer. A1 Primary maximum in. September and the secondary one in June. A2 Inverted type of A1, primary maximum in June and the secondary one in September. B: Japan Sea type with triple maxima. Of them, the primary maximum appears in winter and the other two in summer (June and September). C: Northern type with single maximum in September. In both regions of type A and B, two maxima of summer appear in September and June separated by the July-August minimum forming a short dry period which is very important not only from the geographical but also from the economic points of view. 2. The dry period above-mentioned is considered to be a result of the special type of pressure pattern developing in the special type of pressure patterns developing in the mid-summer over the north Pacific Ocean. After the Bai-u front along which the cloudy and rainy weather persists for a long spell parse: away to the north, the north Pacific anticyclone extends westward to the Japanese Islands and its western part is called the Ogasawara High. Even in a normal year, the main part of Japan, especially its western half, is located underneath a huge anticyclone (Fig, 2.) and fine weather lasts for many days with scarce rainfall, bright sunshine and strong evaporation. Because of the prevalence and westward expansion of the Ogasawara High, northward movement of cyclones and typhoons from the southwestern seas are checked to turn their paths to the westward direction along the outer periphery of the high pressure area and sc the frequencies of cyclones become very scarce (Fig. 3.). This strengthens further the occurrence of clear sky and scanty rain during this period. In connection with this subject, summer rain in the Far East is very important for the theory of non -soon, and it has been commonly explained by the monsoon winds from the Pacific Ocean laden with abundant moisture. However, this is inconsistent with the threefold structure of summer rainy season in Japan. Recent upper air observations afford another interpretation for this subject. That is, water vapor for summer precipitation in the Far East is originally transfered by the subtropical airmass from the equatorial region inflowing as southwesterly winds along the southern coast of Asiatic continent. This airmass is very moist with high equivalent potential temperature and connective instability. With the westward expansion of the Ogasawara High, tropical easterly with less moisture becomes strong and interrupts northward invasions of the southwesterly winds. As a result, fine weather persists for a long period. In such a way, the monsoon does really exist if it means only the seasonal alternation of winds with opposite directions. Hence; summer monsoon in East Asia is not exactly the same as that with abundant rainfall in India and other regions of southeastern Asia. 3. Furthermore, in summer season, the temperature becomes much higher with less rainfall because of intense solar heating unhindered and unweakened by the thick layer of clouds. A striking negative correlation is found between the amount of monthly rainfall and average monthly temperature in summer. Numerical figures for Fig. 4. give an amount of August rainfall in 1939 expressed as a percentage of normal year and those for Fig. 5, the August temperature deviation in 1939 from the normal.