Given the following passage  "The French Marines and naval infantry intended for the invasion of northern Germany were dispatched to reinforce the French Army of Châlons and fell into captivity at Sedan along with Napoleon III. A shortage of officers, following the capture of most of the professional French army at the Siege of Metz and at the Battle of Sedan, led naval officers to be sent from their ships to command hastily assembled reservists of the Garde Mobile. As the autumn storms of the North Sea forced the return of more of the French ships, the blockade of the north German ports diminished and in September 1870 the French navy abandoned the blockade for the winter. The rest of the navy retired to ports along the English Channel and remained in port for the rest of the war.",  answer the following question. Note that the answer is present within the text.  Question: The autumn storms had what effect on the North Sea by 1870?
Answer:
French navy abandoned the blockade for the winter