MAR 110: Lecture 23 Outline – ENSO / Little Ice Age 1

MAR110 LECTURE #23

El Nino/Southern Oscillation

& Little Ice Age

--------------------------------------------------------------------------------------- ENSO is a Two Component Process

The 3-7 year El Nino/Southern Oscillation A Coupled Atmosphere-Ocean Interaction (?)

MAR 110: Lecture 23 Outline – ENSO / Little Ice Age

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ENSO events are Global These large variations from the norm in sea surface temperature and the accompanying changes in the winds and rain patterns can greatly affect the weather across the globe. (NG 3-99)

MAR 110: Lecture 23 Outline – ENSO / Little Ice Age

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ENSO Global Consequences: During the El Nino phase of ENSO there is tendency for (Left) more flooding in Chili, Peru, and India/Bangladesh; and (Right) drought in the western Paciifc – e,g, Australia. (??, LEiO)

MAR 110: Lecture 23 Outline – ENSO / Little Ice Age

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ENSO shifts jet stream patterns over the continental US - sometimes inducing “jet stream “ trajectories that guide more snow storms to New England.(??)

MAR 110: Lecture 23 Outline – ENSO / Little Ice Age

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ENSO Dynamics (above) The “normal” phase of ENSO is characterized by strong tropical easterly (“trade”) winds and a concentration of warm surface waters on the western side of the ocean; (below) The “El Nino” phase of ENSO is characterized by weak or reversed tropical easterly (“trade”) winds and concentration of warm surface waters on the eastern side of the ocean; (??)

Tropical Winds The trade wins normally keep the warm equatorial surface water pool on the west of the ocean basins. When the trade winds weaken enough this pool of warm water is the one that surges eastward during an El Nino event. (LEiO)

MAR 110: Lecture 23 Outline – ENSO / Little Ice Age

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Southern Oscillation Index The strength of the trade winds varies with time which is why the mound of warm water in the equatorial Pacific does not always stay in place. A weakening in the trade winds can cause an El Nino while a strengthening causes a La Nina. The SOI is the difference in atmospheric pressure between Darwin and Tahiti which drives the trade winds. (UWaC)

Historical SOI The highly variable SOI between1850 and 2005. (NH)

MAR 110: Lecture 23 Outline – ENSO / Little Ice Age

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During the ”normal” ENSO phase, the tropical warm water mass in the western Pacific ocean generates convective activitiy, while upwelling is strong in the coastal eastern Paciifc. (??)

ENSO Phases (left) During “normal” ENSO conditions, the warm tropical surface waters reside in the western side of the Pacific, where they support strong convection that helps to maintain a strong atmospheric Walker cell circulation and associated strong westward trade winds. (right) During an El Nino event, the weakened trade winds allow the warm surface water move eastward across the Pacific. The associated vertical convection tends to further weaken and in some cases reverse the westward trade winds – thus allowing further eastward movement of the warm surface water. When the warmer surface waters are in the eastern Pacific, they counteract the normal upwelling tendencies. (??)

MAR 110: Lecture 23 Outline – ENSO / Little Ice Age

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The satellite sea surface temperature image shows the effects of the 1997-98 El Nino event in which warm surface waters flowed from the western into the eastern Pacific due to weakened Trade Winds. (UWaC)

The El Nino/La Nina Dance (NG 3-99)

MAR 110: Lecture 23 Outline – ENSO / Little Ice Age

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Eastern Equatorial Pacific Currents During “normal” times upwelling supports fisheries productivity. However, the warm surface water during an El Nino acts as a cap over the waters along the coast of Peru, preventing the upwelling of denser and colder nutrient rich water that normally occurs. With the upwelling stopped there is much less food for the fish and the birds that feed on them, greatly impacting the economy of the region. (ItO)

El Nino Impact:Eastern Equatorial Pacific With the upwelling stopped there is much less food for the fish and the birds that feed on them, greatly impacting the economy of the region. (ItO)

MAR 110: Lecture 23 Outline – ENSO / Little Ice Age

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ENSO Characteristics

Measuring El Nino-Southern Oscillation (ENSO) A number of moorings along the equatorial pacific measure sea surface temperature and are used to predict/track the movement of warm water that characterizes an El Nino. (??, LEiO)

MAR 110: Lecture 23 Outline – ENSO / Little Ice Age 11