1 Marine Science 320 El Niño Southern Oscillation 1 Equatorial upwelling chlorophyll from SeaWiFS during June-August 1998

El Niño-Southern Oscillation (ENSO) NOAA PMEL El Niño Theme Page Southern Oscillation (atmosphere) and El Niño (ocean) processes were identified separately, and subsequently linked Walker: noted ‘see-saw’ connection in barometer data from Tahiti and Darwin and coined the term Southern Oscillation Jacob Bjerknes connected SST, winds and SLP with atmosphere/ocean dynamics Sir Gilbert Walker failure of monsoon rains (and famine) in 1899 claimed Asian monsoon linked to drought in Africa and Australia, and mild winter in Canada Widely criticized, theory dismissed (no dynamical explanation) Never succeeded in predicting monsoon failures 2

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Definition of the Southern Oscillation Index [ P diff - P diffav ] SOI = SD(P diff ) P diff = (Tahiti MSLP) - (Darwin MSLP) monthly averaged P diffav = long term (years) average of P diff for that month SD(P diff ) = long term standard deviation of P diff Sometimes SOI is given multiplied by 10 in which case it ranges from about –35 to about +35 (i.e. +/- 3.5 std. dev.) When SOI is negative: p.g. is weaker than usual corresponds to El Nino (or ENSO warm event) 5 Oct 2015

6 Equatorial upwelling chlorophyll from Seawifs during June-August 1998

7 Chlorophyll from Seawifs during Sep-Nov 1997

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easterlies westerlies Trades weaken in Feb/Mar Upwelling slows and water warms 9 Strong Trade winds Cold upwelled water

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Positive anomaly means winds are more westerly (toward the east) than average December

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15 Normal conditionsEl Nino conditions

El Nino - warming first occurs subsurface in the central Pacific because the thermocline is being displaced downward. 16

Warming due to thermocline displacement occurs all across. It is shallower in the east because the thermocline itself is shallower 17

Strong subsurface warming 18

SST now increases, apparently starting from the east, but largely due to what is happening subsurface 19

Loss of heat from the west has cooled the WPWP at depth Warming travels up the N.Amer west coast and spreads out into the Pacific – very slowly 20

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El Nino contracts but waters are still cool to the west. The WPWP must reset itself slowly … 23

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Top right: Two-month mean rainfall rate (mm/mon) for Jan/Feb Heavy rainfall appears over the South Pacific Convergence Zone (SPCZ), South Indian Ocean, and the South America. Bottom: Difference in the Jan/Feb mean rainfall 1999 minus During ENSO warm event (El Niño) in 1998 central Pacific rainfall is anomalously high, and west Pacific rainfall is anomalously low. Negative means much less rain in 1999 than 1998 Positive is more rain in 1999 than ENSO conditions

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32 Deep water Assessment and Reporting of Tsunamis (DART) mooring. ADCP

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35 Consider what would happen next if any of the following occurred: 1.The Trade winds weakened 2.Central Pacific SST warmed 3.Rainfall increased in the central Pacific

36 Consider what would happen next if any of the following occurred: 1.The Trade winds weakened … * equatorial upwelling would slow down * SST would rise in the central Pacific * atmospheric convection would occur more quickly * the region of precipitation would move to the east closing the Walker cell sooner … weakening the Trade winds in the west A positive feedback that amplifies initial the Trade wind weakening

37 Consider what would happen next if any of the following occurred: 2. Central Pacific SST warmed … * atmospheric convection would occur more toward the east * the region of precipitation would move to the east closing the Walker cell sooner * weakening the Trade winds * equatorial upwelling would slow down * SST would rise in the central Pacific

38 Consider what would happen next if any of the following occurred: 3. Rainfall increased in the central Pacific … * the Walker cell would close more toward the east * weakening the Trade winds * equatorial upwelling would slow down * SST would rise in the central Pacific * atmospheric convection and rainfall would occur more toward the central than western Pacific

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There is some evidence that a trigger for ENSO warm events might be westerly wind bursts in the western equatorial Pacific. A wind burst such as this sets in train wave motions that are characteristic of the equatorial region. The westerly wind burst causes: 1. converging Ekman transports (off equator) that increase sea level 2. and depress the thermocline 3. eastward geostrophic flow converges to the east 4. and diverges to the west 5. the pattern moves eastward Note that the same happens for an easterly wind burst: the equatorial Kelvin waves only go east so the anomalous pattern cannot easily reset itself even if the WWB is followed by easterly wind anomalies. The equatorial Kelvin wave speed is about 2.5 m/s (roughly ~200 km/day) The observed speed is about 10 – 20% faster than this due to advection by the EUC 40 Explain Kelvin wave dynamics and westerly wind anomaly

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Present conditions: NOAA ENSO Advisory: Format size 75% 44 SOI

During September, sea surface temperature (SST) anomalies were well above average across the central and eastern Pacific Ocean (Fig. 1) NOAA ENSO Advisory: 45 EL NIÑO/SOUTHERN OSCILLATION ` DIAGNOSTIC DISCUSSION issued by CLIMATE PREDICTION CENTER/NCEP 8 October 2015 ENSO Alert System Status: El Niño Advisory Synopsis: 95% chance El Niño will continue through NH winter, gradually weakening through spring Fig 1. Average sea surface temperature (SST) anomalies ( o C) for the week centered on 30 September Anomalies are computed with respect to the base period weekly means.

The Niño indices generally increased, although the far western Niño-4 index was nearly unchanged (Fig. 2). Next: upper ocean heat content … Fig 2. Time series of area-averaged sea surface temperature (SST) anomalies ( o C) in the Niño regions. Anomalies are departures from the base period pentad means. EL NIÑO/SOUTHERN OSCILLATION ` DIAGNOSTIC DISCUSSION issued by CLIMATE PREDICTION CENTER/NCEP 8 October 2015 ENSO Alert System Status: El Niño Advisory Synopsis: 95% chance El Niño will continue through NH winter, gradually weakening through spring 2016.

There are positive subsurface temperature anomalies in the central and eastern Pacific (Fig. 3) with largest departures above 6 o C (Fig. 4). There were significant low-level westerly wind anomalies and upper-level easterly wind anomalies persisting from the western to the east-central tropical Pacific. Fig 4. Depth-longitude section of equatorial Pacific upper-ocean (0-300m) temperature anomalies ( o C) centered on the pentad of 30 September The anomalies are averaged between 5 o N-5 o S. Anomalies are departures from the base period pentad means. Fig 3. Area-averaged upper-ocean heat content anomaly ( o C) in the equatorial Pacific (5 o N-5 o S, 180 o -100 o W). The heat content anomaly is computed as the departure from the base period pentad means. EL NIÑO/SOUTHERN OSCILLATION ` DIAGNOSTIC DISCUSSION issued by CLIMATE PREDICTION CENTER/NCEP 8 October 2015 ENSO Alert System Status: El Niño Advisory Synopsis: 95% chance El Niño will continue through NH winter, gradually weakening through spring 2016.

The traditional and equatorial Southern Oscillation Index (SOI) values became more negative (stronger), consistent with enhanced convection over the central and eastern equatorial Pacific and suppressed convection over Indonesia Fig 5. Average outgoing longwave radiation (OLR) anomalies (W/m 2 ) for the four-week period 5-30 September OLR anomalies are computed as departures from the base period pentad means EL NIÑO/SOUTHERN OSCILLATION ` DIAGNOSTIC DISCUSSION issued by CLIMATE PREDICTION CENTER/NCEP 8 October 2015 ENSO Alert System Status: El Niño Advisory Synopsis: 95% chance El Niño will continue through NH winter, gradually weakening through spring 2016.

Fig 6. Forecasts of sea surface temperature (SST) anomalies for the Nino 3.4 region (5 o N-5 o S, 120W o -170 o W). Figure updated 15 September 2015 All models surveyed predict El Niño to continue into the Northern Hemisphere spring 2016, and all multi-model averages predict a peak in late fall/early winter (Fig. 6). Forecasts unanimously favor a strong El Niño, with peak 3-month SST departures in the Niño 3.4 region near or exceeding +2.0 o C Outlooks favor below- average temperatures and above-median precipitation across the southern United States, and above-average temperatures and below-median precipitation over the northern tier of the United States.

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(c) (d) (a) (b) (g) (f) (e ) temperature anomaly (h) SOI chlorophyll Equatorial thermocline