Roles of initial ocean states on predicting the 2002/03 central Pacific El Nino

The 2002/03 El Ni？o event, a new type of El Ni？o with maximum warm anomaly occurring in the central equatorial Pacific, is known as central-Pacific（CP） El Ni？o. In this study, on the basis of an El Ni？o prediction system, roles of the initial ocean surface and subsurface states on predicting the 2002/03 CP El Ni？o event are investigated to determine conditions favorable for predicting El Ni？o growth and are isolated in three sets of hindcast experiments. The hindcast is initialized through assimilation of only the sea surface temperature（SST）observations to optimize the initial surface condition（Assim_SST）, only the sea level（SL） data to update the initial subsurface state（Assim_SL）, or both the SST and SL data（Assim_SST＋SL）. Results highlight that the hindcasts with three different initial states all can successfully predict the 2002/03 El Ni？o event one year in advance and that the Assim_SST＋SL hindcast performs best. A comparison between the various sets of hindcast results further demonstrates that successful prediction is significantly affected by both of the initial surface and subsurface conditions, but in different developing phases of the 2002/03 El Ni？o event. The accurate initial surface state can easier trigger the prediction of the 2002/03 El Ni？o, whereas a more reasonable initial subsurface state can contribute to improving the prediction in the growth of the warm event.

Wave height estimation using the singular peaks in the sea echoes of high frequency radar

The popular methods to estimate wave height with high-frequency（HF） radar depend on the integration over the second-order spectral region and thus may come under from even not strong external interference. To improve the accuracy and increase the valid detection range of the wave height measurement, particularly by the smallaperture radar, it is turned to singular peaks which often exceed the power of other frequency components. The power of three kinds of singular peaks, i.e., those around ±1,±√2 and ±1√2 times the Bragg frequency, are retrieved from a one-month-long radar data set collected by an ocean state monitoring and analyzing radar,model S（OSMAR-S）, and in situ buoy records are used to make some comparisons. The power response to a wave height is found to be described with a new model quite well, by which obvious improvement on the wave height estimation is achieved. With the buoy measurements as reference, a correlation coefficient is increased to 0.90 and a root mean square error（RMSE） is decreased to 0.35 m at the range of 7.5 km compared with the results by the second-order method. The further analysis of the fitting performance across range suggests that the peak has the best fit and maintains a good performance as far as 40 km. The correlation coefficient is 0.78 and the RMSE is 0.62 m at 40 km. These results show the effectiveness of the new empirical method, which opens a new way for the wave height estimation with the HF radar.

Exploring seasonal acidification in the Yellow Sea

The Yellow Sea on the western continental margin of the North Pacific Ocean is of major ecological and economic importance. Four field surveys were conducted during May and November 2012, August 2015, and January 2016, investigating seasonal variations in dissolved oxygen and carbonate system parameters of this marginal sea. Results showed that the Yellow Sea cold water mass accumulated respiration-induced CO_2 in subsurface and bottom waters in summer and autumn, leading to acidified seawaters with critical carbonate saturation states of aragonite(Ω_(arag)) of less than 1.5. These seriously acidified seawaters occupied one third of surveyed areas in summer and autumn, likely affecting local calcified organisms and benthic communities. In a future scenario for the 2050 s, in which the atmospheric CO_2 mole fraction increases by 100 μmol mol-1, half of the Yellow Sea benthos would be seasonally covered by acidified seawater having a critical Ω_(arag) of less than 1.5. The corresponding bottom-water p H_T would be around 7.85 in summer, and 7.80 in autumn. Of the China seas, the Yellow Sea cold water mass represents one of the ecosystems most vulnerable to ocean acidification.