Monsoon

About: Monsoon is a research topic. Over the lifetime, 16087 publications have been published within this topic receiving 599888 citations.

Abrupt shift of the ITCZ over West Africa and intra‐seasonal variability

Abstract: The onset of the monsoon system over West Africa is linked to the northward migration of the Inter-Tropical Convergence Zone (ITCZ) during the northern spring and summer. By using daily gridded rainfall data and NCEP/NCAR wind reanalyses over the period 1968–1990, we show that this migration is characterised by an abrupt latitudinal shift of the ITCZ in late June from a quasi-stationary location at 5N in May–June to another quasi-stationary location at 10N in July–August. A composite analysis based on the shift dates shows that this northward shift is associated with the occurrence of a westward-travelling monsoon depression pattern over the Sahel with characteristic periodicities of 20–40 days.

Intrusion of the North Pacific waters into the South China Sea

Abstract: Water mass distribution was studied by analyzing historical hydrographic data in the South China Sea. Despite considerable modification of characteristics as a result of mixing, waters of both salinity maximum and minimum of the North Pacific origin were traced on the density surfaces around 25.0 and 26.73 σθ, respectively. In the salinity maximum layer, property distribution suggests an intrusion into the South China Sea all year-round through the Luzon Strait. The seasonal variation of the intrusion contains a pronounced semiannual signal, with greater strength in winter and summer than in spring and fall. From spring to fall, the intrusion water from the Pacific is narrowly confined in the continental slope south of China; only in winter, when the northeast monsoon becomes fully developed, can it spread in the southern South China Sea. In the salinity minimum layer, water enters the South China Sea only in spring, when the intrusion in the salinity maximum layer is weakest. A combined use of the “island rule” with climatological data suggests a mean Luzon Strait transport of the order 4 Sv (1 Sv = 106 m3 s−1).

South Asian Summer Monsoon and Its Relationship with ENSO in the IPCC AR4 Simulations

Abstract: In this paper we use the extensive integrations produced for the IPCC Fourth Assessment Report (AR4) to examine the relationship between ENSO and the monsoon at interannual and decadal timescales. We begin with an analysis of the monsoon simulation in the 20th century integrations. Six of the 18 models were found to have a reasonably realistic representation of monsoon precipitation climatology. For each of these six models SST and anomalous precipitation evolution along the equatorial Pacific during El Nino events display considerable differences when compared to observations. Out of these six models only four (GFDL{_}CM{_}2.0, GFDL{_}CM{_}2.1, MRI, and MPI{_}ECHAM5) exhibit a robust ENSO-monsoon contemporaneous teleconnection, including the known inverse relationship between ENSO and rainfall variations over India. Lagged correlations between the all-India rainfall (AIR) index and Nino3.4 SST reveal that three models represent the timing of the teleconnection, including the spring predictability barrier which is manifested as the transition from positive to negative correlations prior to the monsoon onset. Furthermore, only one of these three models (GFDL{_}CM{_}2.1) captures the observed phase lag with the strongest anticorrelation of SST peaking 2-3 months after the summer monsoon, which is partially attributable to the intensity of simulated El Nino itself. We find thatmore » the models that best capture the ENSO-monsoon teleconnection are those that correctly simulate the timing and location of SST and diabatic heating anomalies in the equatorial Pacific, and the associated changes to the equatorial Walker Circulation during El Nino events. The strength of the AIR-Nino3.4 SST correlation in the model runs waxes and wanes to some degree on decadal timescales. The overall magnitude and timescale for this decadal modulation in most of the models is similar to that seen in observations. However, there is little consistency in the phase among the realizations, suggesting a lack of predictability of the decadal modulation of the monsoon-ENSO relationship. The analysis was repeated for each of the four models using results from integrations in which the atmospheric CO{sub 2} concentration was raised to twice pre-industrial values. From these ''best'' models in the double CO{sub 2} simulations there are increases in both the mean monsoon rainfall over the Indian sub-continent (by 5-25%) and in its interannual variability (5-10%). We find for each model that the ENSO-monsoon correlation in the global warming runs is very similar to that in the 20th century runs, suggesting that the ENSO-monsoon connection will not weaken as global climate warms. This result, though plausible, needs to be taken with some caution because of the diversity in the simulation of ENSO variability in the coupled models we have analyzed. The implication of the present results for monsoon prediction are discussed.« less

On the Origin of the Bolivian High and Related Circulation Features of the South

Abstract: The climatological structure in the upper-tropospheric summertime circulation over South America is diagnosed using a GCM (with and without South American topography), a linear model, and observational data. Emphasis is placed on understanding the origin of observed features such as the Bolivian high and the accompanying ‘‘Nordeste low’’ to the east. Results from the linear model indicate that these two features are generated in response to precipitation over the Amazon basin, central Andes, and South Atlantic convergence zone, with African precipitation also playing a crucial role in the formation of the Nordeste low. The direct mechanical and sensible heating effects of the Andes are minimal, acting only to induce a weak lee trough in midlatitudes and a shallow monsoonal circulation over the central Andes. In the GCM, the effects of the Andes include a strengthening of the Bolivian high and northward shift of the Nordeste low, primarily through changes in the precipitation field. The position of the Bolivian high is primarily determined by Amazonian precipitation and is little affected by the removal of the Andes. Strong subsidence to the west of the high is found to be important for the maintenance of the high’s warm core, while large-scale convective overshooting to the east is responsible for a layer of cold air above the high.

Oceanography of the northern Arabian Sea

Abstract: The euphotic zone of the northern Arabian Sea ranges from 20 to 60 m with an average depth of about 40 m. Surface temperatures range from 22.5 to 28.5°C, increasing from north to south. At 1000 m, the temperature is about 15°C lower than at the surface. Maximum difference in temperature is found during the SW monsoon. Salinity also decreases from north to south. High salinity in the northern region is probably due to the excess of evaporation over precipitation and runoff, and to high salinity water coming from the Persian Gulf. There is little seasonal effect on the salinity, but there are large differences in the dissolved oxygen at the surface during the SW monsoon. There are two oxygen minima, the first between 100 and 400 m and the second between 800 and 1500 m. The formation of the first oxygen minimum is probably due to high organic production in the euphotic zone, sinking of a large amount of organic matter, the lack of horizontal advection due to the land-locked nature of the sea, and the presence of high salinity water in the upper layers. High oxygen at intermediate depths and the second oxygen minimum in the range 800 to 1500 m probably occur as a result of physical processes peculiar to this part of the Arabian Sea. The flow pattern consists of several eddies and meanders. Inorganic phosphorus is high in the surface layer and still higher at greater depths. Nitrate-nitrogen is low at the surface and increases with depth. Ratios between apparent oxygen utilization (AOU), and changes in carbon, silicon, nitrogen, and phosphorus by atoms were 280:108:40:16:1. Using this relationship it was possible to estimate nitrate reduction at the intermediate depth range and the residence time of water in the oxygen-deficient layer. The rate of primary production in the northern Arabian Sea was 835 mg C m−2day−1 or 530 × 106 tonnes of carbon each year. The average chlorophyll in the euphotic zone was also high. About 25% of the total photosynthetic production of the Arabian Sea occurs in the northern part in about 8% of the area. Zooplankton biomass was also several times that in the rest of the Arabian Sea, and was largely confined to the upper 200 m. Benthic biomass, on the other hand, was lower than in the other regions of the Indian Ocean.