2.2.1 The northeast trade winds prevail throughout
the year, but westerly blows can be expected during the winter. The
average velocity of the winds is 12 knots, with monthly averages of
16 knots in December and 9.5 knots in August. Gales have been experienced
in July and September. Occasional heavy showers of short duration
also occur, cutting visibility to about 2 miles (Coast Pilot 7, 38th
ed., 2006).
2.2.2 Tropical storms and hurricanes are a potential,
but infrequent, threat to the shallow coral reef community structure
of the NWHI. They can generate extreme wave energy events that can
damage the coral and are the primary natural force in altering and
shaping coral reef community structures (Dollar 1982; Dollar and Grigg
2004). Since 1979, two hurricanes (category 2) have passed near the
NWHI. The most recent significant tropical storm was Hurricane Nele
which passed near Gardner Pinnacles in 1985 (Friedlander et
al. 2005).
2.2.3 Pacific Decadal Oscillation (PDO) events
and the El Nino/La Nina phenomenon (ENSO) are two other meteorological
factors that occur in the area of the NWHI. PDO events have been described
as long-lived El Nino-like patterns of Pacific climate variability.
They appear to persist for 20 to 30 years, compared to the 6 to 18
months for an El Niņo event. The effects of the PDO are strongest
in the North Pacific, while secondary signatures exist in the tropics.
PDO sea level pressure anomalies vary with low pressures over the
North Pacific and high pressure over the subtropical Pacific. These
pressure patterns cause enhanced counter-clockwise wind stress over
the North Pacific. With regard to the ENSO, while scientists do not
fully understand how one is triggered, the initial detection occurs
by a rise in atmospheric pressure in the western Pacific and a drop
in pressure in the eastern Pacific (Garrison 1999). This causes trade
winds to shift direction, which subsequently causes warm water in
the western Pacific to flow across the Pacific basin. This mass of
warm water has a number of effects on climate and ocean conditions.
For example, it can cause trade wind speeds to drop, which can cause
an increase in sea surface temperature (Hoeke et al. 2004).
Light winds are likely the cause of recent coral bleaching in the
NWHI. Increased water temperatures stress the coral, which causes
it to expel the symbiotic zooxanthellae. If water temperature does
not decrease and zooxanthellae do not return to the coral tissue,
the coral will die.