Building a Climatology of Extratropical Hurricane Force Lows in the North Atlantic and North Pacific Oceans

ABSTRACT:

The primary responsibility of the Ocean Prediction Center (OPC) of the National Centers for Environmental Prediction (NCEP) is the issuance of marine wind warnings and forecasts for maritime users for the protection of life and property and economic opportunity. The OPC is responsible for large portions of the North Atlantic and North Pacific oceans which contain vital shipping and passenger routes. The most extreme storms, extratropical lows of hurricane force (HF) strength, pose a major risk to vessels on these routes and, therefore, are the most critical forecasts produced by the OPC. A climatology of these HF lows could be a valuable tool for assessing risk, building awareness and gaining expertise.

For operational marine forecasters, adequate ground truth has always been difficult to acquire given the size of the oceans and relative lack of persistent observations. The availability of satellites with scatterometer technology has helped to fill observational gaps. With the operational scatterometers from the three European MetOp satellites, OPC forecasters can distinguish between the three warning categories of gale, storm, and hurricane force and observe the wind fields twice a day. Forecasters at the Ocean Prediction Center have been able to classify storms as hurricane force, or having winds greater than 65 knots, with greater certainty and build a database of events going back to at least 2005.

Over the 14 years of the climatology, from July 2004 to June of 2019, the North Atlantic has been the more active basin. The North Atlantic averages 45 HF lows per season (July to June) and was observed to have a peak from December to February with an average of 9 HF lows per month. The North Pacific averages 37 HF lows per season, with a shorter peak of December and January, and 7 and 8 HF lows per month, respectively. Using all of the HF low point data, heat maps were created to show the relative density of points over the basins.

These heat maps showed the importance of the warm water currents of the Gulf Stream (Atlantic) and Kuroshio (Pacific) to extratropical low formation and strengthening, and the strong influence of Greenland on strengthening winds. A heat map was created for each month containing all HF lows occurring in that month during the 14 year period. These maps show clearly how the storm track shifts through the winter, along with the peak seasons of each basin. While the Pacific may be slightly less active, HF lows in the Pacific may pose a greater risk to mariners. By overlaying major shipping routes onto the heat map of all HF lows it shows that Pacific routes are more often crossing the path of HF storms.