Climate and Agriculture on the Hainan Island, South China
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Geographical Review of Japan Vol. 57 (Ser. B), No. 2, 166-182, 1984 Climate and Agriculture on the Hainan Island, South China: a Preliminary Study Masatoshi M. YOSHINO* Climate and agriculture on the Hainan Island is described based on the literature published in the recent years and on the author's study trip in January and February, 1984. First, the synoptic climatological background such as the frontal zones, cloud distribution, stream lines at the 850mb level, typhoon frequencies and fog distribution are described. The southwesterly winds and north easterly winds prevail at the surface level to the 850mb level and the typhoon plays an important role. Secondly, distributions of air temperature, rainfall, and durations of rainy and dry seasons are presented using the climatological records. Climatic division by Ha Daizhang is introduced to show the eight regions. Thirdly, agriculture in the Hainan Island is reviewed with special reference to climatic conditions. Rubber, rice and tea cultivations are dealt with in detail. In particular, the five areas of rubber cultivation were described in terms of cold and wind damages. The upper limit of rubber cultiva tion can be 300-350m a. s. l. in the northern part of the island, but 500m in the southern part. Of special interest is the recent development of hybridized rice cultivation. They grow in winter, ear and flower normally in middle and later March and mature in middle and later April in the southern part of the island. Then they are brought to the home regions in the other part of China and sown there in the normal growing season. Fourthly, typhoon damage and cold damage are described and damage classes are introduced. Especially, early rice and late rice cultivation and cold damages on rubber trees are dealt with, because they are most striking in the development of tropical crop agriculture on the Hainan Island. Lastly, a perspective is given for further development. •@ are striking. I.Introduction The arable land area is 449•~103ha and tropical crop fields, such as rubber plantations, The Hainan Island, located on 18ß10•L-20ß occupy 227•~103ha. Lands for forestry and 10•LN and 108ß37•L-110ß03•LE, has an area of animal husbandry are 660•~103ha. Popula 33,907.7km2. It is the second-largest island tion is 5.4million, of which 1.6million is non next to Taiwan in China. The climate of the agricultural. The agricultural production Hainan Island is characterized by high tem amounts to about 500million U.•@S. dollars, perature throughout the year, plenty rainfall Main crops are rubber, rice, coconuts, oil palm, in summer and autumn, frequent typhoon pepper, coffee, coco, agave, fruits, etc. The visits, and clear difference between dry and economic tree species numbers about 2,900 and wet seasons. The terrain relief is intermediate, bird species 344. There are many wild species that is, low lands and hilly lands prevail of plants, such as wild rice and wild tea. broadly. The area lower than 500m a.•@s.•@l. In the present preliminary study, some occupies 74.6% of the island, surrounding the aspects of the agroclimate on the island will higher mountainous areas. Soils are reddish be discussed and the results recently published in color and develop deeply. mainly in China will be summarized, together Rivers are short, but the available water with some findings obtained by the author discharge is 297•~108m3/year. The seasonal during the study trip made in January and change and interannual variation of discharge February, 1984. * Institute of Geoscience, University of Tsukuba. Sakura-mura, Niihari-gun, Ibaraki-ken 305, Japan. Climate and Agriculture on the Hainan Island 167 II. Climate of the Hainan Island 30ßN to 20ßN in January as shown in Figure 1 (YosHINO,1969, 1971). In the east of this 1.Synoptic climatological background frontal zone in South China steadiness is relatively high and about 75% over the Hainan In January, northeasterly winds prevail as Island. However, the northerly air flow from a winter monsoon. This winter monsoon, the main cold airmass located over the inner however, flows out originally from North China Asian continent occurs frequently toward the as a NW winter monsoon, changing to the N Hainan Island almost along 110ßE south of winter monsoon on the East China Sea, and to 35ßN. The details of this air flow should be the NE-ENE winter monsoon along the South studied synoptically, case by case, because China coast and over the South China Sea. A cold advection causes cold damages in the quasi-stationary polar frontal zone results in island, as will be mentioned later. From glum weather for many days in winter months winter to spring, the cold air from inland (called yin chun chun in Chinese) in South China causes fogs and drizzle when it flows China and in the northern part and central over the relatively warm sea surface. Such mountain regions of the Hainan Island. The a weather in spring is called monyu in so-called Kummin Polar Frontal Zone runs Chinese and has been called crachan in the from NNW to SSE along the 105ßE line from foreign literature (HE, 1980). The wind does not reach the southern part of the island, however, and the people enjoy the sun. The cloudiness distribution in the region surrounding the Hainan Island is given in Figure 2. Computing the divergence fields based on the data at the grid points at each 2.5ß•~2.5ß intersection, SADLER (1977) presented the tropospheric circulation over the global tropics. Generally, the regions of deep con vective cloudiness are associated with diver gence and those of little cloudiness or low level stratus with convergence. Figure 2, extracted from these figures, makes clear the strong con vergence in South Asia to East Asia through South China in January. North of 30ßN, there is an observable divergence (2•~10-5 sec-1). The maximum cloudiness axis running in the meridional direction along 108ßE and another maximum axis from SW to NE along a line from Taiwan to Kyushu are striking in January and correspond to the frontal zones shown in Figure 1. These conditions are reflected in the local weather, i.e., the foggy, drizzle weather in the northern and eastern parts of the Hainan Island in January. As shown in Figure 3, streamlines over the South China Sea at the 850mb level are from NE to SW in January. However, over the continental South China, they are complicated, in the same month, indicating stationary frontal activities there. Figure 1. Positions of polar frontal zones In April, streamlines over the Hainan Island and ITCZ in January and July. are southwesterly, although they are originally 168 MM. YOSHINO Figure 2. Divergence (10-5 sec-1) at the 300mb level (positive is divergence) and the mean cloudiness (oktas) in January and July. Compiled from the figures by SADLER(1977). easterlies over the Philippines and the southern Island in the summer monsoon months. part of the South China Sea. Contrary to the conditions in January, the The summer monsoon flow prevails markedly Hainan Island is placed in an area of divergence over the Hainan Island. According to Figures in July, which means a region of deep con 1 and 3, the southwesterly monsoonal flows vective clouds, as shown in Figure 2. In from the Indochina Peninsula and the South October, as shown in Figure 3, the streamlines China Sea prevails over South China, the at the 850mb level become ENE over the Yangtze River region, and the East China Sea Hainan Island, indicating the winter mon and reaches the Kyushu Island, Japan. It soonal circulation dominating South China. prevails over the Hainan Island and its eastern A marked difference between October and sea region, where the ITCZ is either absent or January is that there is no obvious frontal rarely present. Steadiness of the southwester zone in South China in October. lies in July is 50-75% over the Hainan In Figure 4, the occurrence frequency of Island, but it is more than 75% over the Gulf typhoons and mean paths are shown. These of Tonkin (YOSHINO,1969, 1971). This im fi gures were compiled from the figures based plies the stronger wind conditions with more on the statistics for 1949-1969 (Central desiccating effect on the western coast of the Meteorological Bureau, 1979). Typhoon fre Climate and Agriculture on the Hainan Island 169 Figure 3. Mean streamlines at the 850mb level over Southeast Asia. quencies were computed for each 2.5ß•~2.5ß Mostly there are 3-5 fog days in each month, quadrangle. The figures show that the north but in extreme months they are more than 10 ern part of the island is visited more in June, days. The sea surface temperature is lower July, August and November+December, but than 25•Ž and the value [monthly maximum the southern part is in September and October. water vapor (aM) minus the saturation water Sea fogs are striking in the zonal sea region vapor for the sea surface temperature (am)] is with 100-200km width along the coast. Figure plus in these months. 5 shows the distribution of the number of days Typhoons hit the island from June to October with fogs over the sea along the South China generally. Their paths are mainly from ESE, coast from December to May (WANG,1983). i.e., from the direction of the Luzon Island of There are practically no fog days in the other the Philippines toward the north of the Hainan months in this region. Of much interest is the Island or toward the Leizhou Peninsula. As relatively high occurrence of sea fogs in the will be mentioned later, therefore, typhoons region extending from the northern part of cause frequent damage to the eastern part of the Hainan Island to Hong Kong, which is the Hainan Island.