Indian Journal of Geo Marine Sciences Vol. 47 (04), April, 2018, pp. 804-811

Chlorophyll variations over coastal area of China due to typhoon Rananim

Gui Feng & M. V. Subrahmanyam*

Department of Marine Science and Technology, Zhejiang Ocean University, Zhoushan, Zhejiang, China 316022

*[E.Mail mvsm.au@gmail.com]

Received 12 August 2016; revised 12 September 2016

Typhoon winds cause a disturbance over sea surface water in the right side of typhoon where divergence occurred, which leads to upwelling and chlorophyll maximum found after typhoon landfall. Ekman transport at the surface was computed during the typhoon period. Upwelling can be observed through lower SST and Ekman transport at the surface over the coast, and chlorophyll maximum found after typhoon landfall. This study also compared MODIS and SeaWiFS satellite data and also the chlorophyll maximum area. The chlorophyll area decreased 3% and 5.9% while typhoon passing and after landfall area increased 13% and 76% in SeaWiFS and MODIS data respectively.

[Key words: chlorophyll, SST, upwelling, Ekman transport, MODIS, SeaWiFS]

Introduction Due to its unique and complex geographical

environment, China becomes a country which has a high frequency of natural disasters and severe influence over coastal area. Since typhoon causes severe damage, meteorologists and oceanographers have studied on the cause and influence of typhoon for a long time. Typhoon will cause strong air-sea interaction1,2&3, which has an obvious influence on cooling of sea surface temperature (SST). SST cooling can be highly related to the ocean’s surface condition4,5,6,7,8,9,10&11. Previous studies indicated that, intensity of typhoon is very sensitive to the level of SST12,13&14. Warm waters are the energy source of typhoon and prevailing strong wind can cause a disturbance on surface water resulting in driving surface water apart, creating zones of upwelling.

Upwelling events have been largely studied along eastern boundary coastal systems all over the world. Coastal upwelling is a result of horizontal divergence near the sea surface and manifests itself by the presence at surface of relatively cold water. When the process occurs over a continental shelf, it can have both economic and climatological effects15. Upwelling brings subsurface cold nutrient-rich water to the surface layer is one of the reason for lower SST and the passage of typhoon can also results in chlorophyll enhancement9,16,17,18,19,20,21,22,23&24. Particulate organic carbon flux in subsurface waters, suspended matter concentration, re-suspension and terrestrial

runoff, entrainment of riverine-mixing, Integrated Primary Production are also affected by typhoons when passing and landfall25,26,27,28&29. It is well known that, ocean phytoplankton production (primate production) plays a considerable role in the ecosystem. Primary production can be indexed by chlorophyll concentration. The spatial and temporal variation patterns of Chlorophyll were quite diverse; however detailed information on Chlorophyll variations in the subsurface was lacking25. Typhoon can impact the ocean surface leading to improve the productivity of a certain area of the sea. Typhoon Rananim, which recorded as the strongest typhoon that landed Zhejiang since 199730, was chosen as a case study to identify typhoon induced SST and chlorophyll changes over coastal region. Present article consist typhoon relation to SST cooling and chlorophyll concentration. The dynamical processes happening over the coastal area by calculating Ekman transport and compared with the higher Chlorophyll area. We also compared the chlorophyll concentration between two satellites data (MODIS and SeaWiFS) and the escalation of maximum Chlorophyll area over the coastal region after typhoon passage. Materials and Methods

To understand the effect of typhoon Rananim on ocean surface, the study area is delimitated to 10-35°N; 110-140°E and the study period is set between 5th and 13th August, 2004. The following data

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is used for analysis: Oceanographic research need more accurate SST provides great advances. Recently Satellite microwave radiometers are capable of retrieving SST accurately31. Daily SST data used for this study has been acquired from Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) sensor on NASA's Aqua satellite. Data corresponding to one pass present numerous shadow areas, therefore, to increase the coverage, composites of both pass are considered, however there are still missing values in every image. QuikSCAT winds have been reported to identify tropical depressions prior to their detection by other observations32&33. Wind component (u and v) data include both on ascending and descending passes has been obtained from QuickSCAT. The composite of both passes data has been extrapolated to one degree for better visualisation and to ensure the variations during typhoon. The most typical optical sensors used in the SeaWiFS sensor and Moderate Resolution Imaging Spectroradiometer (MODIS) sensor are used for Chlorophyll survey with Global Algorithms34,35&36. The Weekly chlorophyll concentration data has been obtained from MODIS Aqua and SeaWiFS used for this analysis and also compare the chlorophyll data especially after the typhoon landfall. TRMM/TMI rainfall data used to confirm the water discharge from the river stream to coastal waters which can relate to the chlorophyll boom.

Typhoon Rananim began with a tropical disturbance on northwest Pacific on 5th August 2004. It turned to a tropical low pressure in the morning of 8th August (decrease in pressure to 990 mb and wind speed of 45 knots). Under the influence of southwest wind flow, it moved to northeast and enhanced by the rather good condition of the atmosphere environment. Finally, the tropical cyclone formed n the evening as what we called Rananim. On the next day, Rananim turned to north direction and enhanced to be a strong tropical cyclone. On 9th August, Rananim turned to northwest direction and came near Taiwan (pressure decreased to 980 mb and wind speed increased to 55 knots). Rananim became strong enough to be a typhoon. On 10th August, the pressure decreased to 965 mb and wind speed increased to 70 knots. On 11th August, pressure decreased to 950 mb with 80 knots winds speed and the strength of typhoon Rananim reached a peak. On the evening of 12th August, with the peak intensity, Rananim landed at Zhejiang province with heavy rainfall and strong winds. After

landing Zhejiang, Rananim lowed rapidly and turned to northeast. Results

Figure 1, depicts the variation of SST (contours) from AMSR-E SST and wind (vectors) from QuickScat during the typhoon Rananim period (5-13 August, 2004). Over the study area there are gaps in data due to the satellite passes. Over the Northwest Pacific, SST is indicating warmer leading to the formations of low pressure on 6th August, then intensified on 8th August with decrease in pressure of 10 mb when compared with the previous day and the wind speed increased to 45 knots. On 9th August, low pressure system became typhoon (10 mb decrease in pressure and 10 knots increase in wind speed). Typhoon following warm temperature, however after typhoon passing temperature decreased of 1°C and cooler temperatures persisting. On 10th August Rananim intensified (15 mb decrease in pressure with 15 knots increase in wind speed when compared with the previous day) and moving towards China coast. Along the typhoon track, there are obvious low temperatures found from 10th August. Typhoon is moving over warmer surfaces with higher intensity of winds, upwelling is happening which is leading to cold wake and lower SST patches can be observed after typhoon passes. Low temperature area (of 28.9°C) formed immediately when typhoon passed. Moreover, the temperature remains low for a couple of days. On 11th August peak intensity (950 mb pressure with 80 knots wind speed) of the typhoon is observed and cool wake (decrease in temperature of 1.15°C) observed after passing. Peak intensity persisted on 12th August with 3°C cool wake is visible. Rananim landfall on 12th August with strong winds, however cooler waters persisted on 13th August after landfall also. The areas are obtaining lower temperature waters due to entrainment or upwelling. It takes few days for the temperature to recover to normal SST.

SeaWiFS and MODIS Aqua-Weekly Ocean Colour data of Chlorophyll concentration has chosen for the study area and study period of 4th to 20th August 2004. Weekly chlorophyll figures depict before typhoon, during typhoon and after typhoon landfall are in the figure 3. From the three weekly figures it is clearly observe that chlorophyll concentrations are higher along the shore except during the typhoon for both. Figure 3a and d reveals the chlorophyll concentration

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before typhoon, indicating higher concentration is resulted from the previous typhoon. When typhoon landed on 12thAugust 2004 over Zhejiang province figures 3b and e, chlorophyll concentration decreased significantly. However, figures 3c and f reveals the chlorophyll boom after typhoon landfall. From the figures, the detailed process ensues for chlorophyll concentration will enhanced after the passage of typhoon. Factors influencing chlorophyll enhancement is given in further sections. Typhoon Rananim landed in Zhejiang province on the night of

12thAugust, however a low chlorophyll concentration is obvious during the typhoon due to strong winds entrain the sub-surface water. Chlorophyll concentration enriched after typhoon passed, which can be observed in the figure 3 c and f.

The temporal and spatial resolutions of SeaWiFS, and MODIS are very different. The temporal and spatial resolution of space borne sensors degrades further passing from raw to processed imagery. In this study, weekly data were used both for SeaWiFS and MODIS, compared chlorophyll and ensures the best

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Acknowledgements This work was supported by Science and technology

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