| dc.description.abstract | The typhoon needs to obtain the energy for growth from the ocean through the atmospheric process. Therefore, the typhoon can continue to develop only when the atmospheric and ocean conditions are favorable. Favorable ocean conditions require warm sea water. The higher sea water temperature, the typhoon can extract more energy from the ocean, so the typhoon has a better chance to develop better. However, when a typhoon passes over the surface of the sea, the sea surface wind will cause Sea Surface Temperature cooling(SST Cooling). Generally speaking, when a typhoon has a larger size, slower moving speed and colder seawater subsurface conditions, the seawater will be strongly cooled, and the extent of the seawater cooling is proportional to the size of the typhoon. Since the cooling of sea water reduces the temperature difference between the ocean and the atmosphere, the energy that the sea water can transport to the typhoon is affected, and a negative feedback mechanism is formed for the typhoon, thereby reducing the energy extracted by the typhoon and affecting the intensity of the typhoon. Therefore, from the perspective of air-sea energy transmission, the size of the typhoon plays a very important key factor in the strengthening process of the typhoon.
This study uses the wind field data reanalyzed by NCEP CFSR (National Centers for Environmental Prediction Climate Forecast System Reanalysis) to calculate the size of typhoons in the Northwest Pacific from 1980 to 2018 for a total of 39 years. The distance between the typhoon center and the 17m/s wind of the typhoon outer core (R17) is used as the typhoon size. By using two regression equations and distinguishing three situations, that is, the CFSR intensity is greater than or equal to 17 m/s, the CFSR intensity is less than 17 m/s, and the CFSR size is negative after correction, and then the typhoon size is calculated. the size of all typhoon cases (16,658 cases) from 1980 to 2018 was finally calculated, and a 39-year climatology of typhoon size was established, then use the climatology of typhoon size for this study.
From the statistics of all typhoon cases from 1980 to 2018, the average size of the Northwest Pacific Ocean is 2.03 degrees. On the monthly average size, there has been a seasonal change in size, the smallest in February, the largest in September and October; on the annual average size, there has been a downward trend. The annual average size is higher correlation with stronger ENSO events, the correlation coefficient is 0.68. The trend detection of extreme size cases found that the number of extreme size cases showed a significant decrease, and the P-value of the trend detection reached 0.005. The trend detection of the typhoon annual average size of different intensities found that Cat.1、Cat.4、Cat.5 showed a significant decrease in annual average size. The trend detection of the typhoon annual average size of different translation speeds found that only the annual average size with a translation speed of 12m/s showed a significant decrease, the P-value of the trend detection was 0.018. The trend detection of the typhoon annual average size of different latitudes found that the annual average size of the latitude 30~35 degrees showed a significant decrease, and the P-value of the trend detection was 0.003. | en_US |