// lecture 18 Flashcards
Bjerknes Feedback
1) warm temps. in west and cold temps. in east mean it rains more in west over warm water.
2) more rain in the west drives upward motion there and downward motion in east easterlies at equator.
3) easterly winds on equator drive ocean water toward west,piling up warm water there and pulling up cold water in east.
4) downwelling (warm)in west and upwelling (cool) in east gives a strong temp. gradient with warm in west and cold in east.
- and back to 1) to complete feedback loop. if you warm it in the east, a feedback is initiated that can cause El Nino to happen.
normal conditions
thermocline slopes, theremocline reaches surface in East Pacific, and more rain in far western equatorial Pacific.
La Nina conditions
thermocline slope increased, lots of cold water in east, cold tongue extended. also warm SST in far west Pacifics and more rain in west Pacific, less in East.
El Nino conditions
thermocline slope reduced, warm SST in central and east Pacific, and more rain in central and eastern equatorial Pacific.
El Nino rain changes
rain shifts with the warmer water. for ex, the coast of peru (normally desert-like) becomes very rainy and tropical plants grow. peaks around christmas.
How El Nino affects the extra-tropics in NH winter (NDJFM)
heating of tropical atmosphere by convection over warmest SST anomalies drives a Rossby Wave that propagates along a great circle rouute.
El Nino impacts on NH winter
- drought in India/Australia, floods in S. America.
- even PNW weather is affected.
- La Nina impacts are opposite to this.
- it’s currently slightly positive.
El Nini impacts - North America NH winter
- warm in coastal Canada and Alaska
- wet in California
- wet and cold in the gulf states
Much warmer water in the
Pacific during El Nino. Enough to raise global temp. by 0.1 - 0.2 C.
Sir Gilbert Walker
noted slow variations in the surface pressure at Darwin and Tahiti that were anti-correlated and of large scale.
Southern Oscillation Pressure Index
Tahiti minus Darwin sea level pressure.
- when Tahiti pressure is low and Darwin pressure is high, that is the El Nino condition.
- low pressure goes with high precipitation and high SST anomlaies.
current SST anomalies
cold most places along equator east of dateline in pacific ~weak La Nina.
Oceanic Nino Index (ONI)
based on SST departures from average in the Nino 3.4 region, and is a principal measure for monitoring, assessing, and predicting ENSO. Defined as the 3-mnoth running-mean SST departures in the Nino 3.4 region.
El Nino characterized by a
positive ONI greater than or equal to +0.5 C. These thresholds must be exceeded for a period of at least 4 consecutive overlapping 3-month seasons.
La Nina characterized by a
negative ONI less than or equal to -0.5 C. These thresholds must be exceeded for a period of at least 4 consecutive overlapping 3-month seasons.
