Under the background of vigorously promoting the construction of ecological civilization, it is of great reference value to study the applicability of vegetation indices in vegetation monitoring in Hainan Island. Based on the NDVI (Normalized Differnce Vegetation Index), EVI (Enhanced Vegetation Index), DVI (Difference Vegetation Index) data extracted from MODIS (Moderate-Resolution Imaging Spectroradiometer), monthly mean temperature and precipitation data of 18 national meteorological observation stations from 2001 to 2020 and land cover data in 2015 and 2017 of Hainan Island, the applicability of three vegetation indices NDVI, EVI and DVI in vegetation monitoring of Hainan Island was studied by using the univariate linear fitting, root mean square error analysis and correlation analysis method. The results are as follows: (1) Among the three vegetation indices, EVI has the best fitting effect on the vegetation coverage area extracted from the land cover data of Hainan Island, the root mean square error accounts for 9.57% of the actual mean. (2) EVI can best reflect the seasonal variation characteristics of vegetation in Hainan Island, it began to increase slowly in February, slow decline after reaching its peak in August, and fell to the lowest in February next year; EVI has the widest range, which can best reflect the vegetation coverage with different thicknesses, and can better overcome the NDVI saturation problem in high vegetation area; EVI has the greatest correlation with climate factors and can best reflect the characteristics of vegetation's response to climate. (3) The fitting effect of three vegetation indices on the vegetation coverage area of Hainan Island was comprehensively evaluated, and the ability to characterize the seasonal variation and density of vegetation was evaluated. The EVI was determined to be the most suitable vegetation index for characterizing the vegetation characteristics of Hainan Island.

Based on the daily maximum temperature from 18 national meteorological stations in the Hainan Island, NCEP/NCAR daily and monthly reanalysis data and NOAA SST data, the characteristics and cause of abnormally high temperature in the Hainan Island in spring 2019 were analyzed. The results are as follows: (1) The extremely high temperature days were more, maximum temperature was higher, and high temperature coverage in the Hainan Island was wider in spring 2019. (2) The west Pacific subtropical high (WPSH) was the dominant circulation system affecting high temperature in spring in the Hainan Island. The area of WPSH was larger, the intensity of WPSH was stronger and its position was to the west in spring 2019. Hainan Island was controlled by the stronger sinking motion of the WPSH main body. (3) In spring 2019, the westerly jet at middle and high latitudes over East Asia was stronger, the tropical convective activities over the south China Sea and the Philippines were inhibited, which were beneficial for strengthening, westward extending of WPSH and enhancing of local subsidence adiabatic warming. Meanwhile, combined with low-level warm advection transport, it was conducive to the local temperature rising, which further led to the development of high temperature. (4) From autumn 2018 to spring 2019, an El Niño event had formed in the tropical middle-east Pacific, and it was more prominent in autumn and winter. The tropical Indian Ocean was being warmer constantly, which was more prominent in spring. The distribution of SST anomaly was beneficial for strengthening and westward extending of WPSH, and then WPSH controlled the whole area of the Hainan Island in spring, forming extremely high temperature.