The salinity of the Arctic Ocean is influenced by the fresh water input from sea ice melting, river discharge and precipitation, and its variation could provide a useful reference for the study of changes in Arctic environment. Since the SMOS satellite launched in 2009, there have been three microwave salinity satellites (Aquarius, SMOS and SMAP satellites) that could provide us sea surface salinity (SSS) data from space. In this study, we compared the SSS products derived from these three salinity satellites: the Aquarius satellite data have a limited spatial resolution and a short time series of less than 5 years, the SMOS SSS product for the Arctic from Barcelona Expert Center provide a 7-year record of the Arctic SSS in 2011-2017, the SMAPSSS data is more sensitive in low salinity regions. With the data from the SMAP and SMOS satellites, we presented a general view on summertime SSS variation during 2011-2017 in the Arctic area. The SSS maps showed that the SSS mainly varied in the plume area and were stable in the Arctic open sea. Satellite-derived salinity could detect the extremely low SSS area caused by sea ice melting and continental runoff. We extracted the ice-melt water area by combing the microwave salinity satellite data and the ocean color satellite data, which could exclude the continental fresh water input. The result showed consistence with sea ice concentration and former literature. The distribution of the ice-melt water could be of great significance for the study of Arctic sea ice anomaly and relative biogeochemical changes.
Two unusual phytoplankton bloom events were identified in the southwestern Bay of Bengal from MODIS-derived chlorophyll-a concentration data collected between 2003 and 2015. The occurrence of the unusual phytoplankton bloom in December 2005 (called Bloom 1 in this study) has been reported in the literature to be triggered by multiple forcings, including strong cyclonic eddy, frequent typhoons, and strong wind-induced mixing. Interestingly, the other unusual phytoplankton bloom (called Bloom 2 in this study) was identified in almost the same location in December 2013. Further, it is the strongest bloom during our study period with large area of high Chl-a > 1.0 mg/m3 and shared some similar features with Bloom 1, such as wide coverage and long duration. At the same time, there were also frequent typhoons and a cyclonic eddy. The possible causes of Bloom 2 were examined using time series of multi-satellite datasets, including sea surface height anomalies (SSHA), sea surface temperature (SST), together with Argo profile data. We found that the cyclonic eddy might be not yet the dominant factor for Bloom 2 as the eddy was much weaker than that of Bloom 1. Specially, SST in December 2013 was lowest among all the December from 2003 to 2015. That is, the stratification is weakest. Therefore, the weak stratification can be broken easily by mixing induced by typhoons and cyclonic eddies and finally result in the strong bloom. This comparative studies could provide us some insight in understanding the role of eddies and tropical cyclones in phytoplankton dynamics in the Bay of Bengal.
The Bay of Bengal (BOB) is a semi-enclosed marginal sea in the northeastern part of the Indian Ocean. The South China Sea (SCS) is almost an enclosed marginal sea and is part of the northwestern Pacific Ocean. Both of them are tropical marginal seas, and have similar hydrological properties, such as high surface temperature, stable thermocline, deep euphotic zone, etc. Moreover, they are all greatly affected by East Asian monsoon and typhoons. However, there are also several significant differences between them. The current circulation structures in the South China Sea are more complex with significant season variations. A large amount of fresh water through river inputs is one of the remarkable hydrological characteristics in the Bay of Bengal. In addition, the Bay of Bengal has a large volume of precipitation. Therefore, it is naturally interesting to investigate the different response of the marine ecological properties represented by chlorophyll concentration to climate change. The Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) and Moderate Resolution Imaging Spectroradiometer (MODIS) mission have provided a successive, long-term observations of global ocean color from space. In this study, we investigate the Chl-a trends in the BOB and SCS during the SeaWiFS and MODIS observation periods (1998-2010 and 2003-2016), respectively. In 1998-2010, Chl-a increased in the western central basin of BOB, while it decreased in almost all other parts. During 2003-2016, Chl-a significantly increased in the western part of BOB, and reduced in the southern part. In the SCS, Chl-a in almost the whole region increased during the SeaWiFS observation period. In 2003-2016, the major rising trends appeared in China and Vietnam coast, the Beibu Gulf, and the Gulf of Thailand. The Chl-a of the Straits of Malacca and the Karimata Straits showed downward trends. The rise in temperature might be the main cause of the Chl-a decrease in the BOB. The deepened MLD and/or decreasing SST may contribute to the increasing Chl-a of the BOB and SCS. Remarkably, coastal Chl-a has continued to rise over the past 19 years. The freshwater input might have significant effect on it.
The Persian Gulf and the Gulf of Oman locate at the northwest of the Arabian Sea, with the total area more than 50,0000 km2. The Persian Gulf is a semi-enclosed subtropical sea with high water temperature, extremely high salinity, and an average depth of 50 meters. By the Strait of Hormuz, the Persian Gulf is connected to the Gulf of Oman which is significantly affected by the monsoonal winds and by water exchange between the Arabian Sea and the Persian Gulf. Algal blooms occurred frequently in the Persian Gulf and the Gulf of Oman, and some of them are harmful algal blooms which may lead to massive fish death and thereby serious economic loss. Due to the widely spatial coverage and temporal variation, it is difficult to monitoring the dynamic of the algal bloom based on in situ measurement. In this study, we used the remote sensing data from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua satellite to investigate a massive algal bloom event in the Persian Gulf and the Gulf of Oman during 2008-2009. The time series of MODIS-derived chlorophyll concentration (Chl-a) indicated that the bloom event with high Chl-a concentration (~60 percent higher than corresponding climatological data) appeared to lasting more than 8 months from autumn of 2008 to spring of 2009. In addition, the bloom was widespread from the Persian Gulf to the Gulf of Oman and neighboring open ocean. The MODIS-derived net primary production (NPP) collected from MODIS showed the same trend with Chl-a. Multiple forces including upwelling, dust deposition was taken into account to elucidate the mechanisms for the long-lasting algal bloom. The time series chlorophyll concentration of the Persian Gulf emerges a significant seasonal pattern with maximum concentrations seen during the winter time and lowest during the summer. It also indicated slight disturbances occurred in June (May/July) and December (November/ January) in some years. The sea surface temperature and water transparency in the Persian Gulf increased with the rates of 0.3% (<0.01) and 3.02% (p<0.01) during 2003-2014, respectively. Chl-a and NPP declined with the rates of 1.61% (p=0.06) and 1.09% (p=0.08), respectively. However, there are no significant changes of the bloom initiation, termination and duration time among years over 2003-2014.