The Mechanism Of The Influence Of Subtropical Indian Ocean Dipole Like Pattern On Summer Precipitation Over Low Latitude Highlands In China

In China, Low-latitude highlands (LLH) is located in southwest China, the geographic location of this area is unique, with the Bay of Bengal (BOB) to its southwest, South China Sea (SCS) to its southeast, and Tibetan Plateau to its northwest. The Indian and East Asian summer monsoons is in the area and contributes to substantial temporal and spatial climate variability, which is apart from other area in China. Droughts and floods are widespread and frequent in the LLH. These droughts had significant impacts on economic growth and the daily lives of millions of people. It is a great practical significance for disaster prevention, to study the influence factors of summer precipitation in LLH, and improve the ability of short-term climate prediction. Base on precipitation data over LLH, MERRA reanalysis data, The Met Office Hadley Centre’s sea surface temperature (SST) dataset, and 20th’s reanalysis data from NOAA, a comprehensive study is performed focus on the influence mechanism of Indian Ocean SST anomaly on summer precipitation over LLH, with the application of composite analysis, empirical orthogonal function (EOF), singular value decomposition (SVD), linear regression and numerical modeling. The physical mechanism that the subtropical Indian Ocean dipole like (SIOD-like) sea surface temperature (SST) abnormal pattern impacted on precipitation over LLH, is systematically investigated. Main conclusions are summarized as follows:(1) Summer (June-August) is the main flood season in LLH, and monthly rainfall is more than 150 mm. The distribution of summer mean precipitation appears more (less) than the domain averagein the southwest and Southeast (middle and northwest) LLH. The leading mode of the summer mean precipitation over LLH is the consistent change type, which shows a weak reducing trend. The secondary mode is the east west seesawtype, which has a significant oscillation period of 2～5 years in the period from 1985 to 2005.The composite analysis of consistent change type indicates that the southwest wind anomlies is an important factor to the precipitation anomaly over LLH in summer. When the southwest(southeast) wind anomalies appear over the southwesternLLH, suggesting that the summer mean southwesterlies over LLH are strengthened (weakened), the strengthened (weakened) southwesterlies will carry more (less) water vapor into LLH, resulting in more(less) precipitation anomaly over LLH.(2) The singular value decomposition between summer 925hPa horizontal wind, Indian Ocean SST, and precipitation over LLH distribution showed that there is a significant relationship between SIOD-like SST abnormal pattern and precipitation anomalies over LLH in summer. When the subtropical southwest India Ocean SST is below (above) normal, the southeast Indian Ocean SST is above (below) normal, then more (less) precipitation are observed over LLH in summer. Further correlation analysis between SST and the summer regional mean precipitation over LLH was found that the SIOD-like pattern is mainly external thermal forcing factor influenced summer precipitation over LLH, which have more effect on summer precipitation over LLH, than other types of the air-sea interaction (such as ENSO).(3) The singular value decomposition between summer 925hPa horizontal wind and Indian Ocean SST joint field anomaly and precipitation over LLH first distribution revealed that the zonal wind anomaly is an important factorfor the summer precipitation abnormal over LLH. During positive (negative) SIOD-like summers, that is, SST anomalies below(above) normal in the southeastern Indian Ocean, and above(below) normal in the southwestern Indian Ocean, anticyclonic(cyclonic) anomalies are excited over the western tropical Indian Ocean. The associated lower-tropospheric convergence may cause divergence (convergence) over the tropical Indian Ocean and convergence (divergence) over the Arabian Sea. That convergence (divergence) may induce easterly (westerly) anomalies of the divergent (convergence) wind component off the eastern coast of the BOB. The divergence (convergence) over the tropical Indian Ocean may enhance (suppress) the interhemispheric vertical circulation and generate descending (rising) motion over the same region and cyclonic (anticyclonic) anomalies over the Indian peninsula. The combined effect of the divergent and rotational wind anomalies and interhemispheric vertical circulation facilitates easterly (westerly) anomalies and weakens (strengthen) the climatological water vapor flux from the northern BOB to the LLH. Therefore, anomalous water vapor divergence (convergence) and less (more) precipitation are observed over the LLH.(4) The summer SIOD-like pattern can be traced to preceding seasons, and maintained until summer especially in positive SIOD-like years. The SST-wind-evaporation (WES) feedback mechanism could account for the maintenance of the SIOD-like pattern, and oceanic advection may also contribute.(5)The numerical simulation results fromthe atmospheric circulation model showed that the SST abnormal pattern that subtropical southwest India ocean is relatively warm (cold), and southeast India ocean is simultaneously cold (warm), can indeed force more (less) precipitation over LLH in summer. Numerical model is better to verify the response of summer precipitation over LLH to the SIOD-like SST pattern. The ocean-atmosphere coupled model better simulate the basic characteristics of SIOD-like.The numerical results are consistent with the diagnostic analysis conclusions of chapter fourth.The results show that the numerical model successfully reproduced the physical mechanism, which of SIOD-like effected on summer precipitation Anomalies over LLH, and also reproduced the maintaining mechanism of summer SIOD-like pattern, which can be traced to preceding seasons.