Evaluation of Freshwater Flux over the Ocean

Huge amounts of evaporation and precipitation exist over the ocean and play an important role in the global climate system. Therefore, evaluation of freshwater flux is critical for understanding the mechanism of global climate change. Recently we can use three kinds of global freshwater flux products such as in situ data, satellite data and reanalysis data. Although various studies were carried out using those data, it is difficult to determine which data is most accurate because we don’t know the true value for global freshwater flux. One of important thing is to clarify the differences among those data and the cause of the differences. For this purpose we compare various freshwater flux products over the ocean in this study. We use GSSTF2, HOAPS3, and J-OFURO2 evaporation products and GPCP2, CMAP and HOAPS3 precipitation products as satellite-based products. OAflux evaporation product that is hybrid data is also used here. On the other hand, we use ERA40, NRA1, and NRA2. We assume a reference freshwater flux product to be J-OFURO2-GPCP2. Temporal and spatial resolutions are one month and 1 °x1° degree, respectively. The analysis period is 1988-2000. Reanalysis products generally tend to overestimate freshwater flux from the ocean to the atmosphere in the mid-latitudes and underestimate in the low-latitudes. We estimated integrated freshwater flux over the ocean. If we use reanalysis products, the integrated value is considerably large compared with that of the reference data. Moreover, if we use a reanalysis product for precipitation and another product for evaporation, we obtain a negative freshwater flux that means freshwater flux from the ocean to the atmosphere. We investigated the trend of integrated freshwater flux. For reference data integrated freshwater flux shows positive trend that means freshwater flux from the ocean to the atmosphere increases as time goes on. On the other hand, if we use reanalysis products, integrated freshwater flux shows negative trend that means freshwater flux from the atmosphere to the ocean.