Monsoon

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

Orbital Asian summer monsoon dynamics revealed using an isotope-enabled global climate model

Abstract: The Asian summer monsoon dynamics at the orbital scale are a subject of considerable debate. The validity of Asian speleothem delta O-18 records as a proxy for summer monsoon intensity is questioned together with the ultimate forcing and timing of the monsoon. Here, using the results of a 150,000-year transient simulation including water isotopes, we demonstrate that Asian speleothem d18O records are not a valid proxy for summer monsoon intensity only at the orbital timescale. Rather, our results show that these records reflect annual variations in hydrologic processes and circulation regime over a large part of the Indo-Asian region. Our results support the role of internal forcing, such as sea surface temperature in the equatorial Pacific, to modulate the timing of monsoon precipitation recorded in paleo-proxies inside the Asian region.

A possible mechanism of the Asian summer monsoon-ENSO coupling

Abstract: A significant coupling of the Asian summer monsoon and ENSO was examined using the NCEP/NCAR reanalysis for the period 1973-1995. Results show that a monsoon index, which is defined as meridional gradient of summertime upper-tropospheric thickness (200-500 hPa) anomalies across 20°N over the Indian subcontinent, is highly correlated with Nino-3 SST anomalies in the preceding spring. This is strongly suggestive of the presence of the indirect impact of anomalous SST forcing associated with ENSO on the Asian summer monsoon. Due to attenuated Walker circulation in response to a warm episode, convection is suppressed over the northern tropical Indian Ocean and the maritime continent from the preceding winter to spring. The suppressed tropical convection in the preceding spring generates anomalous cyclonic circulation to the west of the Tibetan Plateau as a result of the Rossby-type response to convective heating off the equator. The convection-induced anomalous cyclonic circulation accompanied by large-scale ascending atmospheric motion contributes substantially to increased rainfall and greater soil moisture, thus resulting in decreased land-surface temperature over central Asia to the northwest of the Indian subcontinent. On the other hand, warm SST anomalies are initially introduced over the tropical Indian Ocean in late spring prior to the onset of the monsoon due to the changes in the surface heat flux and/or dynamic response of the ocean to wind forcing, in intimately association with pronounced in situ low-level northeasterly wind anomalies and less cloud cover. Both these different physical processes in the land and ocean areas are crucially responsible for reduced land-ocean thermal contrast (or reduced meridional tropospheric temperature gradient), eventually bringing about the weakening of the Asian summer monsoon. The reverse situation is quite true for strong monsoon years. Once the summer monsoon becomes weak (strong) at its early stage due to these processes, the initially induced warm (cool) SST anomalies over the tropical Indian Ocean are further intensified. The mechanism proposed here is valid during the period from the late 1970s to the early 1990s when weak and strong monsoon years are categorized. During that period, the unusual Nino-3 SST anomalies tend to persist from the preceding winter until summer, hence serving as a bridge between the ENSO prevailing in the preceding winter and anomalous summer monsoon. However, regardless of when the monsoon-ENSO coupling is prominent, both the springtime outgoing longwave radiation and low-level wind anomalies dominating over the tropical Indian Ocean, associated with anomalous Walker circulation, are still crucial factors in terms of the potential predictability of the Asian summer monsoon.

On the Relationship between Western Maritime Continent Monsoon Rainfall and ENSO during Northern Winter

Abstract: Several studies have reported that Indonesian rainfall is poorly correlated with El Nino–Southern Oscillation (ENSO) events during the northern winter wet monsoon season. This work studies the relationship between the Nino-3 (5°S–5°N, 150°–90°W) sea surface temperature (SST) and the Maritime Continent monsoon rainfall during 1979–2002. The study indicates that the correlations are mostly negative except in the vicinity of Sumatra and Malay Peninsula (SMP, including the western sections of Java and Borneo), where the correlations range from zero to weakly positive. The monsoon rainfall during ENSO events is influenced by a pair of anomalous Walker cells and a low-level closed circulation centered near the Philippines. East of SMP, the rainfall is negatively correlated with Nino-3 SST. The anomalous low-level wind over the Indian Ocean west of SMP causes rainfall to also be correlated negatively with Nino-3 SST, but rainfall over SMP is sheltered from this effect because of the high mountains along...

Spatio‐temporal characteristics of the recent rainfall recovery in West Africa

Abstract: Using daily (monthly) rainfall data from 167 (254) stations across West Africa with at least 80% data availability for the 31-year period 1980–2010 and the gridded African Rainfall Climatology Version 2 (ARC2) for the period 1983–2010, linear trends in yearly and monthly rainfall totals were investigated. Measures of the Expert Team on Climate Change Detection and Indices (ETCCDI) and two rainy season onset and retreat definitions were employed to assess the corresponding trends in frequency and intensity of daily rainfall and changes to monsoon season length. A rotated Empirical Orthogonal Function analysis yielded two homogeneous rainfall regions, the Sahel and Guinea Coast, in terms of interannual to decadal rainfall variability, and this led to analysis of station data and Standardised Precipitation Index for the two regions. Results show that the majority of stations in the Sahel between the West Coast and 15°E shows a statistically significant positive rainfall trend for annual totals. The August–October period exhibits the largest rainfall recovery in the Sahel and the date of the retreat of the rainy season significantly moved later into the year by 2 days decade−1. The recovery is reflected both in more rainy days associated with longer wet spell duration and more extreme rainfall events. Trends along the Guinea Coast are weak and non-significant except for extreme rainfall related indices. This missing significance is partly related to the hiatus in rainfall increase in the 1990s, but also to the larger interannual rainfall variability. However, the tendency towards a more intense second rainy season suggests a later withdrawal of rains from the West African subcontinent. ARC2 trends are broadly consistent where ground calibration was undertaken, but are dubious for Nigeria and Ghana, and especially for the Guinea, Jos and Cameroon Line highlands due to missing gauge data.