Ports.com

OTARU

TROPICAL CYCLONES AFFECTING OTARU

Tropical Cyclone Climatology at Otaru

For the purpose of this study, any tropical storm or typhoon approaching within 180 nmi of Otaru is considered to represent a threat to the port. Table V-39 contains a descriptive history of all tropical storms and typhoons passing within 180 nmi of Otaru during the 48-year period 1945-1992. Unless otherwise indicated, all of the tropical cyclone statistics utilized in this report for storms passing within 180 nmi of Otaru are based on the data set used to compile Table V-39.

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

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 closest point of approach (CPA) to Otaru, the relationship is very weak. Much depends on the track of the storm before it reaches Otaru.

Tropical cyclones are nurtured by a warm marine environment. In this basin maximum storm intensity typically occurs between 20°N and 25°N, where sea-surface temperatures average near 84°F (29°C) during the month of August. 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 Otaru is during August and September. As shown in Table V-40, tropical storms have passed within 180 nmi of Otaru as early as June, but none has passed during the months of October through May. It can readily be seen that few typhoon-strength storms penetrate the 180 nmi threat radius around Otaru. Approximately 12% (4 of 33) of the total number of storms occurring during the 48-year period of record have been of typhoon strength when at their CPA to Otaru. Those storms occurred during September.

Table V-40 also shows the motion history of the 33 tropical storms and typhoons which passed within 180 nmi of Otaru during the period 1945-1992. The table indicates that the average storm speed at CPA is 34 kt. The reason for the relatively rapid movement lies in the location of Otaru. As shown in Appendix A, most tropical cyclones that pass close to Otaru have already recurved and are moving northeastward under the influence of upper level westerlies. Since many of the storms affecting Otaru are in the acceleration phase, rapid movement is common. While the average movement for all storms is 040°/34 kt, storms occurring during September have a 38 kt average speed. As shown in Table V-39, it is not uncommon for September storm speeds to exceed 50 kt.

During the 48-year period from 1945 through 1992 there were 33 tropical storms and typhoons that met the 180 nmi threat criterion for Otaru. Figure V-161 shows the monthly distribution of the 33 storms by 7-day periods. the period of peak activity extends from early August through late September.

Figure V-162 depicts the anual distribution of tropical storms and typhoons passing within 180 nmi of Otaru during the 48-year period 1945-1992. The designation as a tropical storm or typhoon is based on the intensity of the storm at the time of CPA to Otaru. It is interesting to note that while 21 storms were recorded during the 14-year period 1952-1965, only one storm was recorded during the 16-year period 1973-1988.

Figure V-163 depicts, on an 8-point compass, the octants from which the 33 tropical cyclones in the data set approached Otaru. Over 97% (32 of 33) of the storms approached Otaru from the south, southwest or west octants. The approach direction is determined at CPA, and may not represent the initial approach direction of the tropical cyclone toward Otaru.

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

Figures V-164 and V-165 address the probability of tropical cyclones affecting Otaru. 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 Otaru. A further tabulation was made of those storms which did eventually pass within that distance from Otaru. After smoothing, these 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 Otaru. For example, in Figure V-164, during the months of July and August a tropical cyclone located at 30°N 145°E has an approximate 5% probability of passing within 180 nmi of Otaru and would reach Otaru in approximately 1-1/2 to 2 days.

A comparison of Figures V-164 and V-165 indicates that there is little difference in threat axes according to time of year. Because of Otaru'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 Otaru, the majority of storms cross the Japanese island of Honshu between 131°E and 140°E and travel northward along the west coast of northern Honshu in the eastern Sea of Japan. A few pass through the Tsushima (Korea) Strait or across Korea and transit the central or eastern Sea of Japan before entering Otaru's threat radius.

Wind and Topographical Effects

The observation station for the Port of Otaru is located approximately 98 ft (30 m) above mean sea level. It is located on a small hill about 515 yd (470 m) west of the west end of Central Wharf (Figure V-159). Local port authorities consider the observed winds to be representative of conditions in the harbor.

Local Weather Conditions

The data contained in Table V-41 have been selected from observations recorded at Otaru during the passage of the tropical cyclones listed in the table. A total of five tropical storms or typhoons passed within 180 nmi of Otaru during the period 1989 through 1992, the years for which surface observations for Otaru are available. The winds listed in Table V-41 are the maximum winds recorded at Otaru during storm passage.

The two strongest winds listed, west 16 kt for tropical cyclone Roger in 1989 and west 18 kt for tropical cyclone Mirielle in 1991, do not appear to be representative of typical storm circulations. Given the position of each storm relative to the location of Otaru, a more representative direction at Otaru for each storm would be northerly. It is possible that the winds are a result of the pressure gradient west of each storm and not of the primary storm circulation. It should be noted that the available observations did not record any occurrences of winds 22 kt during the passage of the storms.

Local harbor authorities state that they believe the greatest wind threat to the port is posed by extratropical cyclones rather than those of tropical origin. Strong post-frontal pressure gradients behind transient extratropical systems bring strong northwest to northeast winds to the port, posing the greatest hazard to vessels in the harbor.

Figures V-166 and V-167 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 Otaru during the 49-year period 1945-1993. It should be noted that the two figures are derived from theoretical calculations because real-time observational data are available only for the 4-year period 1989-1992. Figures V-166 and V-167 are based on: (1) storm intensity (maximum wind near center), (2) distance of the storm center from Otaru, (3) bearing of the storm from Otaru, (4) translational speed of the storm, and (5) frictional characteristics of the terrain between the storm and Otaru.

The beginning and end points of the arrows in Figures V-166B and V-167B show the positions of the tropical cyclone centers when sustained winds 22 kt and 34 kt would, theoretically, have begun and ended over water near Otaru.

Wave Motion

The Port of Otaru is exposed and vulnerable to wave motion when winds are from northwest clockwise through northeast. Local authorities state that the maximum wave height experienced in the outer harbor near the entrance to the inner harbor is 10 ft (3 m), and is considered to be a rare occurrence. Most wave energy is effectively blocked by the extensive breakwater system. However, higher waves break over the tops of the 6.5 ft (2 m) high breakwaters thus creating wave motion of unspecified height in the inner harbor.

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 Otaru on north-facing Ishikari Bay and the generally rapid movement of tropical cyclones as they approach Otaru would minimize any storm surge effect associated with tropical cyclone passage. Local harbor authorities were unable to recall any recent storm surge event at the port. They did, however, cite a storm surge of 5.6 ft (1.7 m) that occurred during "Showa 26th year" (1951). Since Table V-39 lists no tropical cyclone passing within 180 nmi of Otaru in 1951, it is possible the surge was related to the passage of a strong extratropical storm circulation.

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