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The 1975 Central Pacific Tropical Cyclone Season
The first known documented case history of a mid-Pacific upper tropospheric low which developed into a surface system that rapidly attained tropical character north of 30 latitude constitutes the only item on record which might be ascribed to tropical cyclone activity during 1975 in the Central North Pacific.
Other than that particular occurrence there were no tropical cyclones reported in this area. A sharp decrease in tropical cyclone activity in the Western North Pacific was seen in 1975.
No detailed study has yet been conducted to ascertain possible causes for such inactivity in the Central North Pacific, however, two environmental anomalies were noted. The first was the slightly depressed sea surface temperature that prevailed over this area during most of the season. Secondly, a prevailing westerly upper atmospheric flow extended unusually far to the south. The resulting vertical shear tended to inhibit tropical cyclone development.
Atypical monsoonal trough positioning in the Western North Pacific during this season may also have had an indirect effect on the Central North Pacific activity as well as inhibiting that of the Western North Pacific. During most of July the monsoonal trough was located along 5N, south of its climatological position between 10N and 15N. The trough was masked late in the month by Tropical Storm MAMIE and was reestablished near the normal summertime latitudes as Super Typhoon NINA formed in the Philippine Sea. Throughout September the trough was again south of the expected long term mean position. Early October saw the return of the trough to its normal latitude, but in mid month it migrated south again, where it remained throughout November.
Donald R. Cochran, a meteorologist assigned to the Satellite Field Services Station, NESS, NOAA, Honolulu International Airport, observed and documented the case history of the unique cyclonic activity which occurred in the Central North Pacific during this year. A near total excerpt of his excellent report which appeared in the June 1976 issue of Monthly Weather Review is included forthwith.
"Unusual Tropical Development from a Mid-Pacific Cold Low"
"Tropical storms in Hawaiian waters (between 140 and 180W are rare. Nontropical vortices deserving of gale warnings are more frequent. The characteristics of such storms, called by various names, have been well described by Gray (1968), Simpson (1952, 1973), Hebert (1973), Spiegler (1972), Sadler (1967), and Ramage (1961). Hebert and Poteat (1975) (HP) describe a system for classifying both the type (tropical or not) and intensity of these vortices by using satellite photographs. The HP system was specifically designed to complement the well- known Dvorak (1975) technique for classifying tropical storms. The present case provides an initial opportunity to verify the compatibility of the two techniques in the Eastern Pacific. Hereafter, HP's terminology will be used. If the cyclone never achieves tropical nature it is called subtropical. Subtropical storms after evolving into tropical storms are said to become extratropical as they degenerate or are absorbed into baroclinic systems.
"2. Sequence of events
"Figure 9 shows the track of a remarkable storm. It began as a cold low and became a probable hurricane--near 40 north latitude. In the SMS-2 picture taken at 0048 GMT 31 August the parent cold low was seen some 1500 km northeast of Honolulu. Both satellite and aircraft reports indicated that the system was a typical upper low which normally migrates westward along the mid-Pacific trough. Also evident in the picture some 600 km south- southwest of the first was a second vortex. This circulation, which had been drifting north- westward in the stratocumulus field, was the Hurricane ILSA which had traversed the Eastern North Pacific to 20.3N 129.9W before being downgraded.
"The ILSA remnants, carried as a 1014 mb low on the Honolulu Weather Service Forecast Office surface analysis, were defined by cumulus swirls, well removed from the upper cold low's convection farther north. At this time the Honolulu Satellite Field Services Station (SFSS) classified the cold low's maximum winds at 25-30 knots (ST1.5) using the HP system. During the period 06-16 GMT 1 September, movie loops of SMS-2 infrared (IR) imagery received by the SFSS indicated that the vorticity associated with the former ILSA was absorbed by the rapidly developing cold low. By 00 GMT September 2, the low had assumed the characteristic double-banded configuration of a well-developed vortex. A 35-40 knot wind estimate was required by the HP system. Continued growth occurred so that by the time the first visible spectrum imagery was received on the morning of September 2, convection near the circulation center had increased substantially and had become more isolated. These two-key characteristics foretold an incipient tropical storm.
"Early confirmation was supplied by the morning NOAA 4 pass, which revealed a tightly wound system with a growing central dense overcast (CDO). By 00 GMT September 3, the system was clearly a tropical cyclone warranting 40-knot intensity estimate (T3.0) on the Dvorak scale. Several satellite analysts using the NOAA 3 photo with 1 km resolution obtained a 0614 GMT estimated intensity of T3.5 or 55-knots, and waited anxiously for reports from nearby ships. Unfortunately, throughout the evening no useful ship reports arrived even after the IR views during the late night hours hinted at the formation of an eye. All remaining uncertainty dissolved, however, in the rays of the morning sun. The 2018 GMT 4 view could not be mistaken. Analysts gaped at the classic round eye--a probable hurricane-and at 40 north!
"Using the approximate one-half degree of latitude distance of the center of the eye from the nearest edge of the CDO, along with the technique's other components, a DVORAK wind estimate of 65 knots or greater was warranted. Unfortunately, confirmation from ship data was still lacking. At 1800 GMT 3 September the highest ship wind reported was only 35 knots, some 130 km south-southeast of the apparent center. Further, a SMS-2 4 km resolution, visible spectrum 2218 GMT 3 September 1975, indicated that near 00 GMT 4 September the storm was becoming involved in the adjacent frontal clouds. Finally, perhaps fortuitously, support for the satellite data arrived. At 00 GMT 4 September a ship, which apparently (and no doubt inadvertently) passed near the eye of this tight storm, reported. Heading west and located some 90 km WSW of the satellite-fixed storm center, this ship's barometer read 1003.2 mb at 00 GMT 4 September with a 3-hourly tendency of +13.5 mb, suggesting a pressure under 990 mb near 2100 GMT. The ship further reported northwest winds at 45 knots and seas of 4 m with swells of 7.5 m. Using Holliday's (1969) well-known graphical composite of the various hurricane wind/pressure equations, a speed of about 65 knots is obtained from this 990 mb pressure. Additional ship reports at 0600 GMT continued to confirm the storm's fury. A ship about 200 km SSE reported the highest ship winds during the sequence; 1900, 55 knots. Seas were reported at only 2.5 m but with 8 s, 8.5 m swells from 180. On into the evening this small but dangerous storm tracked rapidly northeastward. Before dawn she had become extratropical, enmeshed in the extensive frontal cloudiness and strong south-westerlies ahead of the upper trough.
"3. Summary and conclusion
"NESS satellite meteorologists, using the improved hemispheric photographic coverage from SMS 1 and 2, tracked an African wave off the west coast of Africa on 6 August 1975. This wave traversed the Atlantic, crossed Central America, blossomed into Hurricane ILSA in the east Pacific, and was absorbed into the upper cold low north of Hawaii on 1 September 1975. Soon transformed into the subject hurricane, the system then recurved northeastward and was easily tracked to a point north of Montana where only then it became untraceable. Central Pacific Hurricane Center records for 25 years show few tropical storm tracks extending north of 25N and none north of 35N. This sequence apparently represents the first observed case of a hurricane forming north of the Hawaiian Islands. Very likely there have been other cases in the past, before the advent of night and day geosynchronous satellite observation.
"Dr. Neil Frank, Director of the National Hurricane Center, has stated (1975) that the ultimate measure of an analysis or forecast technique is its ability to handle the "rare" event, e.g., the explosively deepening hurricane. The Hebert-Poteat and Dvorak systems seem to have handled a similar rarity quite well in this case. Despite cautions from Hebert and Poteat--that theirs is a North Atlantic system which might not apply in the Pacific--this initial test seems to suggest that the world's weather systems may not be as different as, for example, its peoples. Planetary atmospheric coherence should become even more demonstrable when the planned globe-girdling network of geosynchronous satellites becomes operational."