The seasonal change to cold air is the trigger for the date of the rain, but the decade long average ocean temperature is the engine that drives the quantity. (wattsupwiththat.com)
According to NOAA, the global average ocean temperature for the first half of the year is 1.42 °F (0.79 °C) above the 20th century average, the largest such departure in 137 years of records. (scientificamerican.com)
September, for example, had the highest global average ocean temperatures on record, for any month, since 1880, according to NOAA, with a global average temperature of 61.1 degrees Fahrenheit. (mashable.com)