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OMINATO

TROPICAL CYCLONES AFFECTING OMINATO

Tropical Cyclone Climatology at Ominato

For the purpose of this study, any tropical storm or typhoon approaching within 180 nmi of Ominato is considered to represent a threat to the port. Table V-45 contains a descriptive history of all 46 tropical storms and typhoons passing within 180 nmi of Ominato during the 48-year period 1945-1992. Unless otherwise indicated, all of the tropical cyclone statistics used in this report are based on the data set used to compile Table V-45.

Tropical cyclones which affect Japan generally form in the area bounded by 5ºN to 30ºN and 120ºE to 165ºE. The latitudinal boundaries shift poleward during the summer months and then equatorward in winter in response to the seasonal changes of the synoptic environment.

Considering the entire western North Pacific basin, about two-thirds of the tropical cyclones reach at least typhoon intensity at some point in their life cycle. Although there is a positive correlation between the maximum storm intensity and the eventual intensity of the storm at CPA to Ominato, the relationship is very weak. Much depends on the track of the storm before it reaches Ominato.

Tropical cyclones are nurtured by a warm marine environment. In this basin maximum storm intensity typically occurs between 20ºN and 25ºN, where sea-surface temperatures average near 84ºF (29ºC) during the month of August. However, after recurvature into the westerlies and the association with a colder environment, tropical cyclones lose their tropical characteristics. In this situation, the size of the circulation usually expands, the speed of the maximum wind decreases, the translational (forward) speed of motion increases and the distribution of winds, rainfall and temperature becomes increasingly asymmetric.

Although tropical cyclones have occurred during all months in the genesis area described in the preceding paragraphs, the primary season for Ominato is during August and September. As shown in Table V-46, tropical storms have passed within 180 nmi of Ominato as early as June and as late as December, but none has passed during November or during the five-month period of January through May. Few typhoon-strength (64 kt) storms penetrate the 180 nmi threat radius around Ominato. Only 11% (5 of 46) of the total number of storms occurring during the 48-year period of record have been of typhoon strength when at their closest point of approach (CPA) to Ominato. The typhoon strength storms all occurred during September.

Table V-46 also shows the motion history of the 46 tropical storms and typhoons which passed within 180 nmi of Ominato during the 48-year period 1945-1992. The table shows that the average storm speed at CPA is 35 kt. The reason for the relatively rapid movement lies in the location of Ominato. As shown in Appendix A, most tropical cyclones that pass close to Ominato have already recurved and are moving northeastward under the influence of upper level westerlies. Since such storms commonly accelerate after recurvature, many of the storms entering Ominato's 180 nmi threat radius are in the acceleration phase, and rapid movement is common. While the average movement for all storms at CPA to Ominato is 042º/35 kt, storms occurring during September have a 38 kt average speed. As shown in Table V-45, it is not uncommon for September and October storms to exceed 50 kt. The fastest, tropical storm Tip in October 1979, was moving at a speed of over 73 kt when at CPA to Ominato.

During the 48-year period from 1945 through 1992 there were 46 tropical storms and typhoons that met the 180 nmi threat criterion for Ominato. Figure V-187 shows the monthly distribution of the 46 storms by 7-day periods. The period of peak activity extends from early August through late September.

Figure V-188 depicts the annual distribution of tropical storms and typhoons passing within 180 nmi of Ominato during the 48-year period 1945-1992. Although the occurrence of 46 storms in 48 years suggests an average of nearly one storm per year passing within 180 nmi of Ominato, there were many years when this event did not occur and, likewise, many years when there were multiple occurrences. For example, the figure shows that 29 tropical cyclones entered the 180 nmi threat radius around Ominato during the 19-year period 1949 through 1967, an average of approximately 1.5 per year. Only one year during the 19-year period had no storm activity recorded. However, the 21-year period 1968 through 1988 had only 9 storms enter the 180 nmi threat radius, an average of only 0.4 per year. No tropical cyclone entered the 180 nmi threat radius of Ominato during two-thirds (14 of 21) of the years in the latter period.

Figure V-189 depicts, on an 8-point compass, the octants from which the 46 tropical cyclones in the data set approached Ominato. Fifty-nine percent (27 of 46) of the storms approached Ominato from the southwest octant, with the remainder almost evenly split between the west and south octants. It should be noted that the approach direction is determined at CPA, and may not represent the initial approach direction of the tropical cyclone toward Ominato.

Because of climatological considerations, there are preferred areas of the western North Pacific basin from which tropical cyclones eventually affect Ominato. However, there are some tropical cyclones, which, even through they traverse these preferred areas, do not affect Ominato. Also, as might be expected, there are seasonal shifts to these preferred areas.

Figures V-190 and V-l91 address the probability of tropical cyclones affecting Ominato. Using a grid system, a tabulation was made of the total number of tropical cyclones passing through a given grid area regardless of whether they eventually passed within 180 nmi of Ominato. A further tabulation was made of those storms which did eventually pass within that distance from Ominato. After smoothing, the two tabulations were converted into probabilities and contours were drawn to connect points of equal probability.

The solid lines on the figures represent a "percent threat" for any tropical cyclone location within the depicted area. The heavy, dashed lines represent the approximate time in days for a system to reach Ominato. For example, in Figure V-190, during the months of July and August a tropical cyclone located at 30ºN 135ºE has an approximate 25% probability of passing within 180 nmi of Ominato and would reach Ominato in about 1-1/2 to 2 days.

A comparison of Figures V-190 and V-l91 shows that the threat axes for the two time periods are similar, with only minor differences distinguishing one from the other. North of 35ºN, the primary axes are oriented southwestward from Ominato along the west coast of Honshu to approximately 35ºN. Both axes extend southward between 35ºN and 30ºN and then turn southeastward south of 30ºN.

Because of Ominato's northerly location, essentially all of the storms that enter its 180 nmi threat radius have recurved and their movement is being influenced by upper level westerly winds. Consequently, they are moving in a general north to northeasterly direction. To pass within 180 nmi of Ominato, most storms cross southern Honshu between 131ºE and 140ºE and approach Ominato through the eastern Sea of Japan. A few pass through the Tsushima (Korea) Strait or across southern Korea before entering the Sea of Japan. North of 35ºN, some travel northward over land, paralleling the Japanese Alps along the length of Honshu.

Wind and Topographical Effects

The Port of Ominato has no wind measuring equipment. The Mutsu establishment of the Japan Atomic Energy Research Institute has a "wind gauge" on one of its wharfs in Ashizaki Bay, but its exact location is uncertain. The gauge is reportedly located on the end of Shimokita Wharf, but available charts do not identify wharfs and piers by name. The availability of the wind gauge readings for use by ships entering the port is not known.

The location of the port with respect to surrounding topography would make winds fairly representative of general wind flow when winds are from the east clockwise through south. However, Kamabuse Mountain, a conical 2,881 ft (878 m) volcanic peak located about 3 nmi west of the mouth of Tanabu River (Figures V-185 and V-186), may enhance northeasterly or southwesterly winds at the port. Northwesterly winds will likely reach the port as westerly or northerly and be weakened somewhat by the generally rugged terrain northwest of the port. Regardless of direction, however, ships in the port are unprotected from wind flow, and will feel the full effects of the force of the wind.

Local Weather Conditions

A total of 10 tropical storms or typhoons passed within 180 nmi of Ominato during the 11-year period when observational data are available, 1982-1992. The data contained in Table V-47 have been selected from observations recorded at Ominato during the passage of the tropical cyclones listed in the table. The listed winds represent the strongest wind recorded during the passage of each storm. No observational data are available for Ominato prior to 1982.

The latitude and longitude of the observation sites are listed on the record of observations as 41º14'N 141º08'E on some and 41º14'N 141º09'E on others. The sites are identified as station identifier 47516 (Ominato Japan Air Self Defense Force (JASDF)), and station identifier 47517, (Ominato). Either location places the observation site at or very close to a small airport located just south of Ashizaki Bay (Figure V-185).

Figures V-192 and V-193 show tracks and track segments of tropical cyclones considered to have a high probability of having produced sustained winds 22 kt and 34 kt over water near Ominato during the 49-year period 1945-1993. It should be noted that the two figures are derived from theoretical calculations rather than actual wind observations. The calculated figures are presented because real-time observational data are available only for the relatively short 11-year period 1982-1992. Figures V-192 and V-193 are based on: (1) storm intensity (maximum wind near center), (2) distance of the storm center from Ominato, (3) bearing of the storm from Ominato, (4) translational speed of the storm, and (5) frictional characteristics of the terrain between the storm and Ominato.

Wave Motion

The location of the Port of Ominato in the extreme northeast part of Mutsu Bay limits the threat of hazardous wave motion at the port. The protective spit of land defining the east side of Ashizaki Bay effectively protects any vessel anchored or moored in the bay from any significant wave motion. Ships moored near the mouth of the Tanabu River may feel some refracted wave motion if strong south to west-southwesterly winds are blowing across Mutsu Bay. The wave motion would likely be parallel to the longitudinal axis of the moored ships. It is also possible that waves reflected from the north side of the river could reach the berths. The berths on the south side of the river are well protected from all other directions.

The only portions of the port liable to feel the full effects of south to west-southwesterly wave motion are the berth on the north side of the Tanabu River near its mouth, and the exposed area of the bay lying between Ashizaki Bay and the mouth of the Tanabu River. There is a 24 nmi long fetch south of the port and a 32 nmi fetch to the southwest. Given those fetch limitations, a sustained wind from south clockwise through west-southwest of minimum gale force (34 kt) can theoretically raise waves of about 6 to 8 ft at the port. Because of the fetch limitations, waves greater than 8 ft are improbable, even with storm strength (47 kt) winds.

Storm Surge

Storm surge may be visualized as a raised dome of water, moving with the storm, and centered a few miles to the right of its path. The dome height is related to local pressure (i.e., a barometric effect dependent on the intensity of the storm) and to wind stress on the water caused by local winds. Other significant contributing factors are storm speed, direction of approach, bottom topography, and coincidence with astronomical tide.

The worst combination of circumstances (Harris, 1963, and Pore and Barrientos, 1976) would include:

(1) An intense storm approaching perpendicular to the coast with the harbor within 30 nmi to the right of the storm's track.

(2) Broad, shallow, slowly shoaling bathymetry.

(3) Coincidence with high astronomical tide.

The location of Ominato on Mutsu Bay effectively precludes any threat of storm surge at the port. In fact, no location on Mutsu Bay appears to be at significant risk from storm surge. The semi-enclosed configuration of relatively small Mutsu Bay, the northward facing bay entrance, and the bay's location on the north end of Honshu where rapid storm movement is common, all suggest that Ominato has a negligible storm surge threat. A local Mutsu City official could recall no past occurrences of storm surge at the port.

Source: http://www.nrlmry.navy.mil/port_studies/thh-nc/japan/ominato/text/sect5.htm