Marine Resources Assessment for the Marianas Operating ... - SPREP

AUGUST 2005 FINAL REPORT
65 m/sec (JTWC 1998). Super typhoons have sustained surface winds with speeds greater than 65
m/sec from 1960 to 2001, there were on average 2.7 to 3.5 typhoons per year in the northwestern Pacific
Ocean (JTWC 2005). Typhoons have occurred on Guam in every month of the year (Paulay personal
communication).
Storm surge, winds, salt stress, and heavy rainfall generated by tropical cyclones can cause a number of
damages to marine and terrestrial resources (Schlappa 2004). The storm surge (difference between the
mean tide level and the tide level during the tropical cyclone) and excessive rainfalls caused by tropical
cyclones can cause flooding, a change in the nearshore salinity, the erosion and sedimentation of marine
resources, destruction of shoreline structures, and terrestrial and marine habitat destruction. Strong winds
and salt stress can cause the defoliation and uprooting of trees which in turn will cause a pulse of debris
and nutrients affecting both terrestrial and marine resources (Schlappa 2004). Typhoons have impacted
algal and coral communities of the Mariana Islands (Randall and Eldredge 1977; Paulay 2003). In waters
shallower than 30 m, windward exposed fore reefs of the Mariana Islands rarely include fragile growth
forms (including tabular growth forms) because of the recurrent typhoon wave damage (Paulay 2003;
Schlappa 2004). Acropora as a genus is abundant in this depth zone (Paulay personal communication).
2.1.2 El Niño/Southern Oscillation, La Niña
The ENSO is the result of interannual swings in sea level pressures in the tropical Pacific between the
eastern and western hemispheres (Conlan and Service 2000). ENSO events typically last 6 to 18 months,
and can initiate large shifts in the global atmospheric circulation. El Niño occurs when unusually high
atmospheric pressure develops over the western tropical Pacific and Indian Oceans and low sea level
pressures develop in the southeastern Pacific (Trenberth 1997; Conlan and Service 2000). El Niño
means The Little Boy or Christ child in Spanish, and was originally defined by fisherman off the western
coast of South America with the onset of unusually warm waters occurring near the beginning of the year.
This name was used for the tendency of the phenomenon to arrive around Christmas. During El Niño
conditions, the trade winds weaken in the central and west Pacific which impedes the east to west
surface water transport and the upwelling of cold water along South America and causes the SST to
increase across the mid to eastern Pacific (Donguy et al. 1982). In the western equatorial Pacific, SST is
lower than in non-El Niño years (Kubota 1987) and rainfall patterns shift eastward across the Pacific as
the strength of the tradewinds weakens, resulting in increased (sometimes extreme) rainfall in the
southern U.S. and Peru and drought conditions in the west Pacific (Conlan and Service 2000).
La Niña and El Niño are opposite phases of the ENSO cycle (NOAA 2005a). La Niña is a condition in
which the tradewinds strengthen and push the warmer surface waters back to the western tropics. Under
these conditions, the thermocline in the western Pacific deepens and becomes shallower in the eastern
Pacific resulting in abnormally cold SST along the equatorial Pacific. Often with La Niña, the climatic
effects are the opposite of those encountered during an El Niño warming event (e.g., higher SST in the
western equatorial Pacific, high production along Pacific upwelling coasts, and heavy rainfall in Australia
and Indonesia) (NOAA 2005a).
The study area experiences considerable changes during El Niño or La Niña events. While the average
annual rainfall in Guam does not appear to be affected during an El Niño event (93 to 100% of average
conditions), the Northern Mariana Islands experience substantial differences in annual rainfall. During an
El Niño, the Northern Mariana Islands experience conditions in which only 84 to 88% of average seasonal
rains fall in the dry season and the beginning of the rainy season (January to September), and rainfall
exceeds the average values during the rainy season (104% of historical averages) (Pacific ENSO
Applications Center 1995). In addition, there is a general weakening of the Hadley circulation (in which
warm air rises from the equator and travels to the north and south, sinking at 30°). This weakening
reduces the strength of the high pressure system located over the western equatorial Pacific and the
overall SST in the region increases (Kubota 1987). Further, typhoons in the western Pacific basin are
more frequent during warm ENSO periods although their tracks are oriented northwest and away from the
study area (Saunders et al. 2000; Elsner and Liu 2003).
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