A Model El Nino – A Model El Nino – Southern OscillationSouthern Oscillation

Stephen E. Zebiak and Mark A. CraneStephen E. Zebiak and Mark A. CraneMonthly Weather ReviewMonthly Weather Review

Volume 115Volume 115October 1987October 1987

Presentation by Andrew CondonPresentation by Andrew CondonRosenstiel School of Marine and Atmospheric Rosenstiel School of Marine and Atmospheric

ScienceScienceUniversity of MiamiUniversity of Miami

11-21-0511-21-05

Talking PointsTalking Points

► IntroductionIntroduction► Model DescriptionModel Description

- Atmosphere- Atmosphere- Ocean- Ocean- Coupling- Coupling

► Results (Standard Case)Results (Standard Case)► Model SensitivitiesModel Sensitivities

- Atmospheric Parameterizations- Atmospheric Parameterizations- Oceanic Parameterizations- Oceanic Parameterizations

► Influences of the Annual CycleInfluences of the Annual Cycle► Elements of the Model OscillationElements of the Model Oscillation► SummarySummary

IntroductionIntroduction► El Niño: A phenomenon in the equatorial Pacific Ocean El Niño: A phenomenon in the equatorial Pacific Ocean

characterized by a positive sea surface temperature characterized by a positive sea surface temperature departure from normal (for the 1971-2000 base period) in departure from normal (for the 1971-2000 base period) in the Niño 3.4 region (120the Niño 3.4 region (120°°W-170W-170°°W, 5W, 5°°N-5N-5°°S) greater than S) greater than or equal in magnitude to 0.5or equal in magnitude to 0.5°°C, averaged over three C, averaged over three consecutive months. consecutive months.

► The Southern Oscillation: an oscillation in the surface The Southern Oscillation: an oscillation in the surface pressure (atmospheric mass) between the southeastern pressure (atmospheric mass) between the southeastern tropical Pacific and the Australian-Indonesian regions. When tropical Pacific and the Australian-Indonesian regions. When the waters of the eastern Pacific are abnormally warm (an the waters of the eastern Pacific are abnormally warm (an El Niño event) sea level pressure drops in the eastern El Niño event) sea level pressure drops in the eastern Pacific and rises in the west. The reduction in the pressure Pacific and rises in the west. The reduction in the pressure gradient is accompanied by a weakening of the low-latitude gradient is accompanied by a weakening of the low-latitude easterly trades. easterly trades.

► Major El Niño events since 1950: 1957-58, 1965, 1968-69, Major El Niño events since 1950: 1957-58, 1965, 1968-69, 1972-73, 1976-77, 1982-83, 1986-87, 1991-92, 1994-95, 1972-73, 1976-77, 1982-83, 1986-87, 1991-92, 1994-95, and 1997-98. and 1997-98.

Image Source: http://www.pmel.noaa.gov/tao/proj_over/diagrams/index.html

IntroductionIntroduction► Following the 1982-83 El Nino a large scientific effort was Following the 1982-83 El Nino a large scientific effort was

undertaken to better understand this eventundertaken to better understand this event

http://www.cdc.noaa.gov/map/clim/sst_olr/old_sst/sst_8283_anim.shtml► Key findings from these studies include: the importance in Key findings from these studies include: the importance in

tropical Pacific SST anomalies in producing observed tropical Pacific SST anomalies in producing observed atmospheric anomalies during ENSO and the observed Pacific atmospheric anomalies during ENSO and the observed Pacific SST and sea level anomalies during ENSO result primarily SST and sea level anomalies during ENSO result primarily from the influence of surface wind stress anomaliesfrom the influence of surface wind stress anomalies

► Most studies prior to this had focused on one aspect or the Most studies prior to this had focused on one aspect or the other (i.e. atmosphere or ocean) but few dealt with the other (i.e. atmosphere or ocean) but few dealt with the coupled system. Those that did were highly parameterized coupled system. Those that did were highly parameterized and did a poor job of coupling the systemand did a poor job of coupling the system

Model DescriptionModel Description

► AtmosphereAtmosphere- Circulation is forced by a Circulation is forced by a

heating anomaly that heating anomaly that depends partly on local depends partly on local heating associated with SST heating associated with SST anomalies and partly on low-anomalies and partly on low-level moisture convergence level moisture convergence (found in obs and GCM studies (found in obs and GCM studies to be important, not usually to be important, not usually incorporated in models)incorporated in models)

- Nonlinear convergence - Nonlinear convergence feedback is incorporated feedback is incorporated using an iterative procedure using an iterative procedure in which the heating at each in which the heating at each iteration depends on the iteration depends on the convergence field from the convergence field from the previous iterationprevious iteration

► OceanOcean- Rectangular basin extending Rectangular basin extending

from 124from 124°E to 80°W and °E to 80°W and from 29°N to 29°S.from 29°N to 29°S.

- Frictional layer to account for Frictional layer to account for wind driven currentswind driven currents

- Surface currents are Surface currents are generated by spinning the generated by spinning the model up with monthly model up with monthly climatological mean windsclimatological mean winds

- There is 3D temperature - There is 3D temperature advection by the specified advection by the specified mean current and an mean current and an anomalous current produced anomalous current produced in modelin model

- Heat flux anomaly is - Heat flux anomaly is proportional to SST anomaly, proportional to SST anomaly, always acting to adjust to always acting to adjust to the climatological mean the climatological mean statestate

Model DescriptionModel DescriptionCouplingCoupling

► Ocean component is forced by surface wind Ocean component is forced by surface wind stress anomaliesstress anomalies

► Allow time dependence only in moisture Allow time dependence only in moisture convergence component of heatingconvergence component of heating

► Heating related directly to SST gives a wind Heating related directly to SST gives a wind response in equilibrium with the SST field on response in equilibrium with the SST field on a time scale of 10 daysa time scale of 10 days

► Heating due to internal moisture Heating due to internal moisture convergence operates in a time stepping convergence operates in a time stepping sense and forces a wind field adjustment on sense and forces a wind field adjustment on a time scale of 1 montha time scale of 1 month

ResultsResults► 90 year model run 90 year model run

initiated with a westerly initiated with a westerly wind anomaly in the wind anomaly in the region 145region 145°E to 170°W °E to 170°W for a period of four for a period of four monthsmonths

► Quasi-regular Quasi-regular oscillations with a oscillations with a favored period of 3-4 favored period of 3-4 yearsyears

► Peak in June or around Peak in June or around the end of the year with the end of the year with a total duration of a total duration of between 14 and 18 between 14 and 18 monthsmonths

► Peak first in the east Peak first in the east and later in the central and later in the central regionregion

ResultsResults

► Observations from Observations from Rasmusson and Rasmusson and Carpenter from 1921-Carpenter from 1921-76.76.

► Irregular oscillations Irregular oscillations with a preference for a with a preference for a 3-4 year period3-4 year period

► Major events have a Major events have a duration of somewhat duration of somewhat longer than a yearlonger than a year

► Model was unable to Model was unable to reproduce initial reproduce initial coastal warming which coastal warming which is usually observed is usually observed prior to the major prior to the major central Pacific warmingcentral Pacific warming

ResultsResults► Time series of area Time series of area

averaged wind averaged wind anomaliesanomalies

► During major warm During major warm events the two indices events the two indices vary in a similar vary in a similar fashion indicating a fashion indicating a large scale coherent large scale coherent wind forcingwind forcing

► Western Pacific zonal Western Pacific zonal wind anomalies are wind anomalies are weaker than observed weaker than observed and switch from and switch from westerly to easterly westerly to easterly later than observedlater than observed

ResultsResults

► Following slides Following slides showcase a major showcase a major warm event in three warm event in three month intervals month intervals beginning in the beginning in the end of year 30 and end of year 30 and going to year 32going to year 32

► December of year December of year 30, no appreciable 30, no appreciable anomalies in either anomalies in either SST or windsSST or winds

March (31)March (31)

► A region of warm A region of warm SST has developed SST has developed in the equatorial in the equatorial zone east of 170zone east of 170°W °W with a max near with a max near 130°W130°W

► Small westerly Small westerly wind anomalies wind anomalies near 130°W to near 130°W to 160°W160°W

June (31)June (31)

► Warm event is well Warm event is well underwayunderway

► SST anomalies exceeding SST anomalies exceeding 11°C in the eastern °C in the eastern equatorial Pacific equatorial Pacific

► Sizeable western wind Sizeable western wind anomalies in the central anomalies in the central PacificPacific

► Model warming tends to Model warming tends to occur uniformly rather occur uniformly rather than initially at the coast than initially at the coast

September (31)September (31)

► Expansion and amplification Expansion and amplification of SST and wind anomalies of SST and wind anomalies in the fall of an ENSO yearin the fall of an ENSO year

► Warm anomalies as far Warm anomalies as far westward as 160westward as 160°E and °E and eastern Pacific anomalies eastern Pacific anomalies exceed 2°Cexceed 2°C

► Large westerly wind Large westerly wind anomalies cover the whole anomalies cover the whole equatorial central Pacificequatorial central Pacific

► A region of small negative A region of small negative SST anomaly and easterly SST anomaly and easterly wind anomaly has wind anomaly has developed in the Western developed in the Western PacificPacific

December (31)December (31)

► Peak temperature Peak temperature anomalies occur with a anomalies occur with a max at the coast and max at the coast and another one near 140another one near 140°W°W

► SST anomaly has SST anomaly has expanded meridionallyexpanded meridionally

► Westerly wind Westerly wind anomalies of about 2 m anomalies of about 2 m ss-1-1 in the central Pacific in the central Pacific

► Easterly anomalies in Easterly anomalies in the western Pacific in the western Pacific in obs, not shown in the obs, not shown in the modelmodel

March (32)March (32)

► Temperature anomalies Temperature anomalies begin to decrease, begin to decrease, especially at the east especially at the east coastcoast

► Single max in the Single max in the eastern central Pacificeastern central Pacific

► Large westerly wind Large westerly wind anomalies in the anomalies in the central Pacificcentral Pacific

► Increasing easterly Increasing easterly anomalies farther to anomalies farther to the eastthe east

June (32)June (32)► Eastern ocean is still Eastern ocean is still

warm, but temperatures warm, but temperatures are decreasing rapidlyare decreasing rapidly

► Westerly wind anomalies Westerly wind anomalies have decreased and have decreased and receded westwardreceded westward

► Stronger easterlies are Stronger easterlies are evident in the eastevident in the east

► Composites show cold Composites show cold SST anomalies and SST anomalies and poleward wind poleward wind anomalies in the eastern anomalies in the eastern ocean at this timeocean at this time

September (32)September (32)► Dramatic change in winds Dramatic change in winds

and SST amounting to a and SST amounting to a rapid termination of the rapid termination of the warm eventwarm event

► Equatorial eastern and Equatorial eastern and central ocean is coldcentral ocean is cold

► Winds are primarily Winds are primarily meridional and directed meridional and directed polewardpoleward

► Agrees well with Agrees well with composites for this time composites for this time periodperiod

► All major events evolve in All major events evolve in a similar fashion. Some a similar fashion. Some smaller amplitude smaller amplitude anomalies develop anomalies develop differently and do not differently and do not conform to this scenarioconform to this scenario

ResultsResults► Model thermocline depth anomaly Model thermocline depth anomaly

along the equator between years 30 along the equator between years 30 and 45and 45

► Measure of heat content of the upper Measure of heat content of the upper oceanocean

► Major warm episodes (31 and 41) are Major warm episodes (31 and 41) are characterized by anomalously high characterized by anomalously high heat content in the east and low heat heat content in the east and low heat content in the westcontent in the west

► Periods preceding major warm Periods preceding major warm events are characterized by above-events are characterized by above-normal heat content at all longitudesnormal heat content at all longitudes

► Periods immediately following warm Periods immediately following warm events show a deficit of heat contentevents show a deficit of heat content

► Rise in equatorial heat content Rise in equatorial heat content precedes the development of precedes the development of equatorial westerlies and positive equatorial westerlies and positive SST anomalies in the eastern ocean; SST anomalies in the eastern ocean; that is it precedes the ENSO eventthat is it precedes the ENSO event

ResultsResults► Strong and sustained westerly wind Strong and sustained westerly wind

anomalies in the central Pacific anomalies in the central Pacific during warm phaseduring warm phase

► Occurs in phase with the Occurs in phase with the anomalously high heat contents in anomalously high heat contents in the eastern ocean and low heat the eastern ocean and low heat content in the western oceancontent in the western ocean

► At times (38 and 44) westerly At times (38 and 44) westerly anomalies appear in a fashion anomalies appear in a fashion similar to the onset of ENSO, but similar to the onset of ENSO, but the presence of strong easterly the presence of strong easterly anomalies in the eastern Pacific anomalies in the eastern Pacific contrast themcontrast them

► The easterly anomaly grows with The easterly anomaly grows with the westerly anomaly and the westerly anomaly and development ceases shortly development ceases shortly thereafterthereafter

► Terminated events tend to start Terminated events tend to start early in the year (Jan-March), may early in the year (Jan-March), may be influenced by annual cyclebe influenced by annual cycle

ENSO summaryENSO summary► Large scale pattern of equatorial westerly wind anomalies in Large scale pattern of equatorial westerly wind anomalies in

the central Pacific and equatorial SST anomalies that extend the central Pacific and equatorial SST anomalies that extend across most of the basin and decrease in amplitude from east across most of the basin and decrease in amplitude from east to westto west

► The climatologically mean state includes easterly trade winds The climatologically mean state includes easterly trade winds blowing across the eastern and central oceanblowing across the eastern and central ocean

► Easterly stress induces an equatorial upwelling and sets up a Easterly stress induces an equatorial upwelling and sets up a zonal tilt to the thermoclinezonal tilt to the thermocline

► Cold sub-thermocline water is far removed from the surface in Cold sub-thermocline water is far removed from the surface in the west and very near the surface in the eastthe west and very near the surface in the east

► Sets up a sub-surface temperature anomaly that is largest in Sets up a sub-surface temperature anomaly that is largest in the east and smaller toward the westthe east and smaller toward the west

► Atmospheric response is equatorial westerly wind anomalies Atmospheric response is equatorial westerly wind anomalies that span nearly the entire region of SST anomaliesthat span nearly the entire region of SST anomalies

► Westerly wind deepens the eastern ocean thermocline, Westerly wind deepens the eastern ocean thermocline, suppresses equatorial upwelling, and sets up eastern current suppresses equatorial upwelling, and sets up eastern current anomalies which reinforce the temperature anomaly patternanomalies which reinforce the temperature anomaly pattern

Model SensitivitiesModel SensitivitiesAtmospheric ParameterizationAtmospheric Parameterization

► 25-year run based on initial 25-year run based on initial conditions present at the start conditions present at the start of year 31, only considering of year 31, only considering NINO3 SST indexNINO3 SST index

► Coefficient of heating term Coefficient of heating term proportional toproportional to SST anomalies SST anomalies increased 10%increased 10%

► Results in a large increase in Results in a large increase in amplitude of events, but amplitude of events, but similar periodsimilar period

► Coefficient of heating Coefficient of heating proportional to moisture proportional to moisture convergence is increased by convergence is increased by 7%7%

► Net impact on the larger scale Net impact on the larger scale structure is minimalstructure is minimal

Model SensitivitiesModel SensitivitiesOceanic ParameterizationsOceanic Parameterizations

► Cases 1-4 both the amplitude and Cases 1-4 both the amplitude and time scale of the oscillations time scale of the oscillations increaseincrease

► Reduced thermal dissipation Reduced thermal dissipation allows larger SST anomaliesallows larger SST anomalies

► Larger drag coefficient produces Larger drag coefficient produces greater wind stress forcing greater wind stress forcing resulting in larger anomaliesresulting in larger anomalies

► Stronger mean upwelling yields Stronger mean upwelling yields larger SST anomaly in response to larger SST anomaly in response to thermocline displacementsthermocline displacements

► Smaller equivalent depth produces Smaller equivalent depth produces larger thermocline variations and larger thermocline variations and larger subsurface temperature larger subsurface temperature anomaliesanomalies

► 5 decreases the coupling strength, 5 decreases the coupling strength, resulting in smaller oscillations resulting in smaller oscillations with a shorter periodwith a shorter period

► 6 indicates low sensitivity6 indicates low sensitivity► A sizeable decrease in background A sizeable decrease in background

ocean dissipation produces no ocean dissipation produces no change in the solution, however change in the solution, however other experiments with larger other experiments with larger dissipation result in reduced dissipation result in reduced amplitudeamplitude

Influences of the Annual Influences of the Annual CycleCycle

► Warm events tend to amplify sharply Warm events tend to amplify sharply during the northern summer, reach during the northern summer, reach peak amplitude around the end of the peak amplitude around the end of the year and diminish the following yearyear and diminish the following year

► To examine annual cycle influence in To examine annual cycle influence in the model, experiments were the model, experiments were performed using initial conditions performed using initial conditions from Jan of year 31 and the annual from Jan of year 31 and the annual cycle was turned off at a given cycle was turned off at a given subsequent month by holding the subsequent month by holding the mean fields fixed from then onmean fields fixed from then on

► April (0) shows the growth of warm April (0) shows the growth of warm event is retarded considerably event is retarded considerably (amplitude increases more slowly, (amplitude increases more slowly, reaches a max later and decreases reaches a max later and decreases sharply)sharply)

► August (0) shows the amplitude August (0) shows the amplitude continues to rise for many months, continues to rise for many months, peaks later and larger than with peaks later and larger than with annual cycleannual cycle

► December (0) shows a steady decline December (0) shows a steady decline into the middle of year 1into the middle of year 1

► July (+1) shows growth of negative July (+1) shows growth of negative anomalies continues longer and leads anomalies continues longer and leads to larger anomalies during year 2+to larger anomalies during year 2+

Annual CycleAnnual Cycle► Summer period is most favorable for Summer period is most favorable for

rapid growth of both positive and rapid growth of both positive and negative anomalies (mean winds, negative anomalies (mean winds, mean upwelling and mean SST mean upwelling and mean SST gradient are all large)gradient are all large)

► Spring period is least favorable for Spring period is least favorable for anomaly growth (weak trade winds anomaly growth (weak trade winds and associated upwelling and SST and associated upwelling and SST gradient)gradient)

► Small anomalies present during the Small anomalies present during the spring of an ENSO year amplify spring of an ENSO year amplify rapidly during the summer and fallrapidly during the summer and fall

► During winter the coupling strength During winter the coupling strength decreases significantly and becomes decreases significantly and becomes insufficient to maintain the large insufficient to maintain the large anomaliesanomalies

► Hesitation period in the spring Hesitation period in the spring followed by an increase in coupling followed by an increase in coupling strength and demise of event in the strength and demise of event in the summersummer

► Second experiment in which cycle Second experiment in which cycle turned off July (0) and continues for turned off July (0) and continues for 25 years shows the solution settles 25 years shows the solution settles into periodic oscillationinto periodic oscillation

Elements of the model Elements of the model oscillationoscillation

► The subsurface temperature is made The subsurface temperature is made more or less sensitive to changes in the more or less sensitive to changes in the area-averaged heat contentarea-averaged heat content

► When made completely insensitive to When made completely insensitive to changes in heat content the ENSO changes in heat content the ENSO oscillation stops and is replaced by an oscillation stops and is replaced by an annual oscillationannual oscillation

► When exactly half of the actual heat When exactly half of the actual heat content fluctuation is allowed an content fluctuation is allowed an oscillation with a period of 5-6 years oscillation with a period of 5-6 years developsdevelops

► When the effect is enhanced by a factor When the effect is enhanced by a factor of 3 the period is 1-2 yearsof 3 the period is 1-2 years

► The oscillatory character of the coupled The oscillatory character of the coupled system depends on the effect of system depends on the effect of variations in net equatorial heat contentvariations in net equatorial heat content

► The 3-4 year period preference reflects The 3-4 year period preference reflects a time delay between dynamical a time delay between dynamical changes in the eastern ocean and changes in the eastern ocean and associated large-scale fluctuations in associated large-scale fluctuations in equatorial wind stressequatorial wind stress

SummarySummary► Without anomalous external forcing the model produces recurring Without anomalous external forcing the model produces recurring

warm events that are irregular in both amplitude and spacing, but warm events that are irregular in both amplitude and spacing, but favor a 3-4 year periodfavor a 3-4 year period

► Largest growth occurs during the northern summer and fall and Largest growth occurs during the northern summer and fall and termination during the following spring and summertermination during the following spring and summer

► Signature of model warm events include equatorial westerly wind Signature of model warm events include equatorial westerly wind anomalies in the central Pacific and large SST anomalies in the anomalies in the central Pacific and large SST anomalies in the eastern Pacificeastern Pacific

► All parameter changes that amounted to increasing (decreasing) All parameter changes that amounted to increasing (decreasing) the strength of the atmosphere-ocean coupling tended to produce the strength of the atmosphere-ocean coupling tended to produce larger (smaller) amplitudes and longer (shorter) periodslarger (smaller) amplitudes and longer (shorter) periods

► There is a buildup in heat content prior to the onset of a warm There is a buildup in heat content prior to the onset of a warm episode and a rapid decrease during the course of the eventepisode and a rapid decrease during the course of the event

► The variations in heat content is due to a phase lag between wind The variations in heat content is due to a phase lag between wind stress and thermocline motions, this is why interannual stress and thermocline motions, this is why interannual oscillations occur in the coupled systemoscillations occur in the coupled system

► Above normal heat content is a necessary precondition for the Above normal heat content is a necessary precondition for the onset of a warm episode.onset of a warm episode.

► All mechanisms essential to the ENSO cycle are contained in the All mechanisms essential to the ENSO cycle are contained in the Pacific region and they are largely controlled by deterministic Pacific region and they are largely controlled by deterministic processes in the tropical Pacific atmosphere-ocean systemprocesses in the tropical Pacific atmosphere-ocean system

Additional ReferencesAdditional References

►http://www.nws.noaa.gov/ost/climate/Shttp://www.nws.noaa.gov/ost/climate/STIP/ElNinoDef.htmTIP/ElNinoDef.htm

►http://ess.geology.ufl.edu/usra_esse/elhttp://ess.geology.ufl.edu/usra_esse/el_nino.html_nino.html