December 14, 2006

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Season's greetings to El Ni?o 2006-2007

Left: Normally, strong trade winds blow from the east along the equator, pushing warm water into the Pacific Ocean. The thermocline layer of water is the area of transition between the warmer surface waters and the colder water of the bottom. Right: An El Nino condition results from weakened trade winds in the western Pacific Ocean near Indonesia, allowing piled-up warm water to flow toward South America. From http://kids.earth.nasa.gov/archive/nino/intro.html

As the season's holidays rapidly approach, many keiki (children) in the islands are eagerly awaiting the arrival of Jolly Saint Nick. However, it is also a good opportunity to check in with another recurrent wintertime visitor, El Ni?o.

The term "El Ni?o", Spanish for "the Christ Child," was originally used by fisherman, who noticed a warm ocean current that appeared along the western coast of Peru and Ecuador around Christmas time each year. Over time, however, El Ni?o has come to refer to the increasingly familiar, larger scale, climatically significant warm events. Historically, Los Ni?os have occurred at irregular intervals of 2-7 years and have usually lasted one to two years.

El Ni?o is defined as a sustained increase of at least 0.5 degrees Celsius in sea-surface temperature across the central tropical Pacific Ocean. La Ni?a (a reverse El Ni?o) brings an analogous cooling. In fact, El Ni?o, also referred to as an ENSO (El Ni?o-Southern Oscillation) warm event, is a combined ocean-atmosphere pattern that includes other anomalies. Among these are an increase in sea-surface heights in the eastern pacific and an easterly shift in clouds that are generally situated over the western Pacific.

The atmospheric signature of El Ni?o is the Southern Oscillation (SO). It refers to the see-saw in atmospheric pressure at sea level between the tropical western and eastern hemispheres. During a "normal" year, there are high pressures over the southeastern tropical Pacific (Tahiti) and low pressures over the western tropical Pacific and Indian Ocean (Darwin, Australia). Since the easterly trade winds flow from regions of high to low pressure, they depend on a generous pressure difference in order to blow steadily. During an El Ni?o event, barometers fall over Tahiti and rise over Darwin. The reduction in the pressure gradient is associated with weaker than normal trade winds.

Normally, the east-to-west trade winds pile up warm solar-heated ocean water in the western Pacific, while nutrient-rich cold water from deep in the ocean rises to the surface along the South American Coast. When the usual trade winds slacken, the warm water slowly moves back eastward, like water sloshing in a giant bathtub. This blocks the rising cold water, contributing to a rise in sea-surface temperature.

Many island residents are familiar with the change in the distribution of rainfall over the tropical Pacific as rain follows the warm water eastward. This can result in wetter-than-average conditions in places like coastal Ecuador, northern Peru and California and drought in Indonesia, Australia, and islands in the central and western Pacific, including Hawai`i. The effects of El Ni?o are not all bad, however; for instance, a reduction in Atlantic hurricanes typically accompany El Ni?o.

An El Ni?o event has far reaching effects on global weather. The warm ocean water supplies moisture and energy for huge thunderstorms which, in turn, feed moisture and energy into the upper atmosphere's jet stream. This "river" of wind above 20,000 feet or so helps determine the movement and strength of storms. During El Ni?o, since the warmest ocean water will have moved to the east, moisture and energy will move into the jet stream at a different point than usual. This can change the flow of the upper-air winds, much as moving a boulder in a stream of water will change the flow of water. Changes in the jet stream will affect weather far from the local anomalies in the Pacific.

Currently, most of the modeled forecasts indicate that El Ni?o conditions are underway and are expected to peak between December 2006 and February 2007, followed by weakening during March-May 2007.

According to one method for tracking El Ni?o strength (NOAA's Multivariate ENSO Index, MEI), although the current event had a slow start, its October-November values ranked fifth highest in strength as compared to all events since 1950.

Here in Hawai'i we have already experienced the weaker-than-average trade winds. For communities close to Kilauea Volcano, these conditions can bring periods of intense volcanic air pollution. For example, on November 26, a maximum one-hour average of 1.7 ppm SO₂ was measured at Hawaii Volcanoes National Park Visitor Center, exceeding U.S. and international health standards.

On Hawai'i Island, appropriate seasonal behavior for El Ni?o includes paying attention to water use, and, for east Hawaii residents, planning activities to minimize exposure to volcanic pollution when trade wind are weak or absent.

Activity update

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater. Lava is fed through the PKK lava tube from its source on the southwest flank of Pu`u `O`o to the ocean. About 1 kilometer south of Pu`u `O`o, the Campout flow branches off from the PKK tube. The PKK and Campout tubes feed two widely separated ocean entries, at East Lae`apuki and East Ka`ili`ili, respectively. Both entries are located inside Hawaii Volcanoes National Park.

In the past week, intermittent breakouts from the Campout tube have occurred on the slope of Pulama pali and on the coastal plain.

Access to the sea cliff near the ocean entries is closed, due to significant hazards. The surrounding area, however, is open. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

Only one earthquake beneath Hawai`I Island was reported felt within the past week. A magnitude-2.8 earthquake at 8:55 a.m. H.s.t. on Friday, December 8 was located 24 km (15 miles) west of Kawaihae at a depth of 23 km (14 miles). This location is in the area where aftershocks of the October 15 earthquakes have occurred.

Mauna Loa is not erupting. During the past week, earthquake activity remained low beneath the volcano's summit (two earthquakes were located). Extension of distances between locations spanning the summit, indicating inflation, continues at slow rates.

Visit our Web site (hvo.wr.usgs.gov) for daily Kilauea eruption updates and nearly real-time Hawai`i earthquake information.