Trans-Pacific hurricanes

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As I write this in late August, the Northern Hemisphere hurricane seasons are well underway. So far, the Atlantic season has been fairly quiet with only five systems, two of which became hurricanes for only brief periods. The eastern Pacific season has been more active with 15 systems so far, six of which have become hurricanes. I want to focus on two of these systems because they are especially interesting.

The North Pacific is divided into three regions for the purposes of tracking tropical cyclones. Systems in the eastern Pacific are tracked by the National Hurricane Center in Miami, those in the central Pacific are tracked by the Central Pacific Hurricane Center based in Honolulu, and those in the western Pacific are tracked by the Japan Meteorological Agency in Tokyo and by the Joint Typhoon Warning center based in Pearl Harbor for the U.S. military. The areas of responsibility are shown in Figure 1.

Figure 1: Map showing the breakdown of tropical cyclone responsibilities in the Pacific. Regional Specialized Meteorological Center (RSMC) Miami is the National Hurricane Center, RSMC Honolulu is the Central Pacific Hurricane Center and RSMC Tokyo is the Japan Meteorological Agency. The western Pacific is also served by the Joint Typhoon Warning Center.

Let’s start with a quick refresher on Pacific hurricane climatology. In the eastern Pacific region during the summer and autumn, tropical cyclones often form in the waters west of Central America and southern Mexico, and track generally toward the west or northwest. Often these systems will strengthen rapidly and can sometimes evolve from a nascent tropical depression to a major hurricane in less than two days. As these systems move farther west or northwest though, they will generally weaken as they encounter colder sea surface temperatures provided by the California current, which flows from the California coastal waters south into tropical latitudes. Occasionally a system will turn more to the north or northeast before reaching the colder water and make landfall along the Pacific coast of Mexico, but the majority of eastern Pacific tropical cyclones never make landfall — though they obviously impact maritime concerns operations.

The central Pacific region is the least active of the three regions, and in some years no tropical cyclones impact this region at all. This is because the water temperatures tend to be a bit lower in this region, and upper-level wind shear tends to be a bit stronger than in other parts of the Pacific Basin; both of these factors work against tropical cyclone development and strengthening. Tropical cyclones can form in this region, but they can also track into the region from the eastern Pacific if the conditions (sea surface temperature, wind shear) are favorable, such that they do not dissipate before reaching the area. Despite its location in tropical latitudes, Hawaii is not a hurricane-prone location.

In the western Pacific, sea surface temperatures are warmer and wind shear is typically weaker, so tropical cyclones are quite common in this region. In fact, tropical cyclones can form at any time of year in this region, though they are most frequent during the summer and fall. Occasionally a system that forms in the central Pacific region will track across the international date line and move into the western Pacific region, but the large majority of systems that impact the western Pacific region are ones that originate there.

Figure 2: Graphical Tropical Weather Outlook for the eastern Pacific issued at 0500 PDT July 31, 2018.

Now on to the two interesting systems I mentioned above. On July 31 of this year, a tropical depression formed in the eastern Pacific. Figure 2 is the graphical tropical weather outlook issued at 5 a.m. on that day, and the red area shows the system that would be designated a tropical depression six hours later and upgraded to Tropical Storm Hector later that afternoon. Hector tracked generally west for the next several days and reached hurricane strength as it neared 125° W early in the morning of Aug. 2. It strengthened rapidly during that day and approached major hurricane status (Category 3) in the afternoon. As it continued west, it weakened through the night of Aug. 2 but then rapidly strengthened again on Aug. 3, attaining major hurricane status by evening. It was a powerful Category 4 hurricane on the evening of Aug. 5 as it crossed 140° W and moved from the eastern Pacific region into the central Pacific region.

Figure 3: Graphical depiction of the forecast for Hurricane Hector issued at 1700 HST Aug. 6, 2018.

Hector continued moving generally west through the central Pacific region for the next several days with only a slight northward drift. This track kept the system far enough south of the Hawaiian Islands that impacts there were minimal. Tropical storm warnings were issued for the big island on the afternoon of Aug. 7, and the system passed well south of this island on Aug. 8. Hector peaked in intensity just below Category 5 status on the afternoon of Aug. 6 (Figure 3), then slowly weakened but remained a major hurricane for the next several days. Figure 4 is a satellite image from the afternoon of Aug. 9 showing Hector moving west of Hawaii’s longitude. After this time, the system turned more to the northwest and began to slowly weaken. It dropped below major hurricane strength on Aug. 11, and was downgraded to a tropical storm on the afternoon of Aug. 12, shortly before reaching the international date line.

Figure 4: Satellite image from 0000 GMT Aug. 10, 2018 (2PM HST Aug. 9), showing Hurricane Hector south-southwest of Hawaii. At this time, Hector was a Category 3 hurricane.

Tropical Storm Hector crossed the date line early in the morning of Aug. 13, moving into the Eastern Hemisphere and into the western Pacific region. Its weakening continued as the system moved farther northwest over colder waters and into increasing upper-level wind shear. The final advisory on this system was issued by the Joint Typhoon Warning Center late on Aug. 14 (Figure 5).

Figure 5: Final graphical advisory on Hector issued at 0300 UTC on Aug. 15, 2018, by the Joint Typhoon Warning Center.

Hector is of interest because it is very unusual for a system to be tracked through all three of the Pacific tropical cyclone regions. One of the factors influencing the track of Pacific hurricanes is El Nino. During an El Nino episode, tropical cyclones are more likely in the central Pacific region because the sea surface temperatures are higher and wind shear is lower. The situation this season does not qualify as an El Nino yet because the ocean temperatures are not warm enough, though they are above normal (Figure 6), which has not been the case for the past couple of years. The outlook is for a bona fide El Nino pattern to develop either later this year or early next year.

Figure 6: Sea surface temperature anomalies issued on Aug. 20, 2018. The yellow colors between the equator and 20° N indicate sea surface temperatures up to 1° Celsius above normal.

A short time later, yet another tropical cyclone moved from the eastern Pacific region into the central Pacific region. Lane formed just a bit farther southwest than Hector did, but much like Hector it became a hurricane within two days of formation. Its track was more west-northwest than Hector’s, and it crossed into the central Pacific region on Aug. 18 as a Category 4 hurricane. Once into the central Pacific region, it briefly attained Category 5 status on the evening of Aug. 21. It was around this time that a turn toward the northwest began posing a serious threat to the Hawaiian Islands. Slow weakening occurred through the next couple of days, but the system remained a major hurricane until very early on Aug. 24, at which time it was producing damaging winds and prodigious amounts of rain over much of the Hawaiian Island chain. Figure 7 is a close-up satellite view of Lane on Aug. 22 when it was a Category 4 hurricane nearly due south of the Big Island of Hawaii. Lane continued to weaken through Aug. 24 and was downgraded to a tropical storm during the afternoon, continuing to weaken through Aug. 25. However, it also moved very slowly during this time, remaining near the Hawaiian Islands and continuing to produce extremely heavy rains with historic flooding, along with strong and gusty winds. Lane is the most serious hurricane to impact Hawaii since Iniki in 1992.

Figure 7: Satellite image from 1900 UTC Aug. 22, 2018 (9AM HST), showing Hurricane Lane due south of the Big Island of Hawaii. At this time, Lane was a strong Category 4 hurricane, just below Category 5 strength.

Again, to sum up, Hector was very unusual because it tracked across all three Pacific tropical cyclone regions, and Lane was unusual because of its serious impact on Hawaii, which, as noted above, is not particularly prone to hurricanes. One final thought: Hector and Lane are the only tropical cyclones to occur in the central Pacific region so far this season, as none have formed in the region.

By Ocean Navigator