An average Tropical Storm Season for 2021 in the North Pacific?

In a year of record weather breaking feats across the world mainly linked to climate change, there maybe some good news in that the busy North Pacific tropical storm season is expected to remain average. Now that is not to say that this region hasn’t had its share of notable systems this season already and that there won’t be more to come. Here is a summary of the mid-season:

Northwest Pacific

The Northwest Pacific alone has already had 15 named storms so far. The most notable systems so far have been Super Typhoon Surigae, which was the most intense Tropical Cyclone to form in the Northern Hemisphere before 1st May, and the more recent Super Typhoon Chanthu, which has impacted on both the East China Sea and Japan Sea region.

Figure 1: North Western Pacific Hurricane Tracking Chart as of Sept 15th, 2021. Source: Japan Aerospace Exploration Agency’s Earth Observation Agency

Northeast Pacific

The Northeast Pacific has also had its share of storms this season with a total of 15 named storms so far with Major Hurricanes Felicia and Linda, both of which moved west across the eastern Pacific away from land.

Figure 2: North Eastern Pacific Hurricane Tracking Chart as of Sept 15th, 2021. Source: Japan Aerospace Exploration Agency’s Earth Observation Agency

As the Pacific is the largest ocean basin and with the peak of tropical cyclone season upon us, many seafarers and entities with a stake in the ocean travels have got their eyes on the tropics and how it could impact their way of life.

The Predicting Mechanisms

When trying to predict how active a tropical season will be, it is important to look at the different climatological patterns and compare their current state and the forecast state to the climate norm over an extended period. Due to it being the most recognizable observation, the change in sea surface temperatures, most specifically in the tropical Eastern Pacific, is one of the most widely known ways of predicting how active a tropical season will be. 

This climatological tool is known as the El-Nino Southern Oscillation (ENSO) which categorizes the periodic fluctuation of the sea surface temperature into 3 different phases: El Nino, La Nina, and Neutral. According to the United States Climate Prediction Center (CPC) we have been in a Neutral phase for the majority of the tropical cyclone season thus far. With that we can compare the Northwest Pacific and the Northeast Pacific season thus far to seasons of similar ENSO-neutral phases.

Figure 3. ENSO phasing since 1950 in respect to global temperature changes (HadCrut)

As indicated in Figure 3 there have been 8 years since 2000 with ENSO-neutral characteristics within the range of Β±0.5Β°C of average SST’s: 2001-2, 2004-5, 2006, 2009 and 2013-14. Taking a closer look at the two regions you can see that 2021 follows with the average number of named storms in the Northwest Pacific of about 15 and above the average of about 11 in the Northeast Pacific through Sept 15th of each year shown in Figure 4, noting outliers in the NW Pacific of 2004 (above average) and the NE Pacific of 2002 (below average). This could be a good indicator that the Pacific Tropical Season is on par to shape up to be an average season in the two regions of the basin. 

Figure 4. Left: # of occurrences for each ENSO-neutral NW Pacific Typhoon season (2000-2021 to 15 Sept only, and Right: # of occurrences for each ENSO-neutral NE Pacific Hurricane season (2000-2021 to 15 Sept only)

With that being said we must now factor in the fact that the CPC is also predicting ENSO to return to La Nina phase towards the end of the season with a 70-80% chance of return by the Northern Hemisphere winter as seen by forecasted models shown in Figure 5.

Figure 5: Model Predictions for ENSO from Aug 2021. Source: United States Climate Prediction Center

If this forecast is valid and ENSO returns to the La-Nina phase, this could decrease the number of systems that could form in the Eastern Pacific. This could also limit the tracks of the Western Pacific systems with formations more likely closer to land in the Western Pacific which could lead to weaker systems. 

The Forecast

Based on the recently updated outlook from the Tropical Storm Risk on Aug 5th shown in Figure 6, the Northwest Pacific is expected to have a near to below average season with about 25 named storms, which is just below the 56 year norm and on par with the 30 year and 10 year norm. When comparing these predictions to the season thus far and accounting for the ENSO outlook, 2021 so far seems to fall in line with being an average season for the Northwest Pacific. 

Figure 6. NW Pacific Tropical Storm Risk Forecast Comparison, Source: Tropical Storm Risk

Looking at Figure 7 below, the Northeast Pacific was forecast to be a below average or average season by CPC in May with only a 20% chance of being above average. Comparing this to the season thus far and accounting for the CPC’s forecast to return to La Nina it seems very likely that 2021 will stay near the average.Β To see more on the Atlantic hurricane season see out latest blog here.

Figure 7. NCEP Climate prediction Center outlook Left: Eastern Pacific and Right: Probabilities for each type in the Eastern Pacific (issued May 2021)

With the Pacific basin accounting for a significant amount of the commercial shipping density especially in the busy South China Sea and East China Sea region, tropical forecasting is especially important due to systems that can greatly disrupt the normal shipping operations. Tropical storm prediction is complex and requires very close monitoring 24/7 by experts when optimising ship routes. So the prospect of an average season in terms of the number of tropical cyclones should give a small sigh of relief to seafarers and Marine Routers and Forecasters alike as we all work together to keep crews, vessels, and cargo safe.

Stay tuned for further updates.

Stay connected and safe.

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