Information related to mars system.
In the petitions, it should be specified the exact timesteps you need. For example, with tas the mars request should ask for daily averages (ie. 12-36,…) and with psl it should ask for instant daily values (ie. 12, 24,…). Alternatively, it can be ask as 'step=all', and it will get whatever steps there are for that variable.
More information about the different content in: http://s2s.cma.cn/index