A Warm Ocean Leads to a Big Year for Northern Hemisphere Tropical Cyclones

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A whopping 165% of normal accumulated cyclone energy (ACE) has been observed in 2018 through October 11 across the ocean basins of the northern hemisphere. The East Pacific is a leading contributor (247% of normal) plus an over-achieving North Atlantic basin (130% of normal). A leading contributor to the well above normal activity is a warmer than normal ocean.

Big Year for Northern Hemisphere Tropical Cyclones

North Atlantic Overachieves

Fig. 1: Accumulated Cyclone Index for each northern hemisphere ocean basin and total ACE for the northern hemisphere through October 11, 2018 is indicated. Data is provided by the Colorado State University Tropical Meteorology Project.

Discussion: A whopping 165% of normal accumulated cyclone energy (ACE) has been observed in 2018 through October 11 across the ocean basins of the northern hemisphere. The ACE index is calculated by squaring the maximum sustained surface wind of the tropical system every 6 hours and totaling that value for a given storm and for each individual storm for a season. ACE index more accurately measures the intensity of a given season versus just the number of storms and hurricanes. Hurricane expert Phil Klotzbach of the Colorado State University Tropical Cyclone Project provides real-time data used in this report. You can find the data at the link below…

http://tropical.atmos.colostate.edu/Realtime/

In 2018 the most active basin has been the northeast North Pacific (east of the Dateline) where an incredible 247% of normal ACE has been observed. In the northeast North Pacific a total of 19 tropical cyclones of which 12 have become hurricanes and 9 intense hurricanes are values which are above normal climatology. However, the number of days in which a hurricane and intense hurricane have been present are stunning…64.5 hurricane days (27 is normal) and 32.75 intense hurricane days (8.1 is normal). These northeast North Pacific values are the leading contributor to the well above normal northern hemisphere activity.

The North Atlantic has over-achieved this year. Seasonal forecasts indicated near to below normal activity anticipating an inhibiting El Nino arrival. But El Nino is delayed until after the tropical cyclone season which allowed the North Atlantic tropics to flourish producing 14 tropical cyclones of which 7 became hurricanes with 2 intense hurricanes (both smashing into the U.S.). The North Atlantic ACE index is 130% of normal. Seasonal forecasts were in the 50% range.

The northwest North Pacific (east of the Dateline) produced 25 tropical cyclones of which 13 became typhoons and 8 intense typhoons. Each value is slightly above climatology. Once again the number of days the typhoons and intense typhoons compared to normal drove the buoyant seasonal ACE index for this basin.

The north Indian Ocean was 247% more active than normal.

The above normal tropical cyclone activity is caused primarily due to the above normal ocean surface temperatures and upper ocean heat content. The most dramatic warmth is located in the central and eastern tropical Pacific increasing recently as an El Nino episode is organizing. The strongest warming in the tropical Pacific Ocean has been toward the Dateline providing an explanation for record number of tropical cyclone events in the Hawaiian Islands in 2018.

Entering the tropical cyclone season the tropical North Atlantic was unusually cool. But dramatic warming evolved well into the tropical cyclone season allowing the North Atlantic to produce an above normal seasonal ACE index.

The middle latitudes of the North Pacific and North Atlantic were much warmer than normal allowing storms to be stronger than normal north of the tropics. Through August 2018 the global oceans were 5th warmest in the historical record. Ranking in the top 5 has been common in this decade.

Fig. 2: The global ocean temperature anomalies for the past 90 days in which the bulk of northern hemisphere tropical cyclone activity occurred is provided by NCDC/PSD. Note the strongest warmth in the most active basin…the North Pacific east of the Dateline.

Fig. 3: Current global ocean temperature anomalies identify the warming trend well into the tropical cyclone season fueling above normal tropical cyclone activity for each basin in the northern hemisphere.