December Pacific SST gave an uncertain spring North Atlantic forecast gain in a 1981-2025 ERSSTv5 test

We tested whether December sea surface temperature in the Nino 3.4 region adds spring forecast information for the tropical North Atlantic. We used NOAA ERSSTv5 and 45 March-May targets from 1981 to 2025. Each fit used earlier target years only. Adding Pacific temperature to local December temperature and a linear year term reduced mean squared error by 24.6%. RMSE fell from 0.444 to 0.385 C. The conditional 95% five-year block interval for error reduction was -13.6% to 47.6%. Rolling 30-year fits and a local November control gave positive point gains with intervals that included zero. The known Pacific-Atlantic link thus gave an uncertain added forecast benefit in this test. One revised SST product, a short test record, model choices and conditional intervals limit inference. This result does not establish an operational gain or a physical cause. Code, aggregate inputs, every forecast, source hashes, checks and a figure are included.

ENSO · North Atlantic · seasonal forecast · persistence · uncertainty