A Model El Nino – Southern Oscillation Stephen E. Zebiak and Mark A. Crane Monthly Weather Review Volume 115 October 1987 Presentation by Andrew Condon Rosenstiel School of Marine and Atmospheric Science University of Miami

Talking Points ► Introduction ► Model Description - Atmosphere - Ocean - Coupling ► Results (Standard Case) ► Model Sensitivities - Atmospheric Parameterizations - Oceanic Parameterizations ► Influences of the Annual Cycle ► Elements of the Model Oscillation ► Summary

Introduction ► El Niño: A phenomenon in the equatorial Pacific Ocean characterized by a positive sea surface temperature departure from normal (for the base period) in the Niño 3.4 region (120°W-170°W, 5°N-5°S) greater than or equal in magnitude to 0.5°C, averaged over three consecutive months. ► The Southern Oscillation: an oscillation in the surface pressure (atmospheric mass) between the southeastern tropical Pacific and the Australian-Indonesian regions. When the waters of the eastern Pacific are abnormally warm (an El Niño event) sea level pressure drops in the eastern Pacific and rises in the west. The reduction in the pressure gradient is accompanied by a weakening of the low- latitude easterly trades. ► Major El Niño events since 1950: , 1965, , , , , , , , and Image Source: ov/tao/proj_over/diagra ms/index.html ov/tao/proj_over/diagra ms/index.html

Introduction ► Following the El Nino a large scientific effort was undertaken to better understand this event ► Key findings from these studies include: the importance in tropical Pacific SST anomalies in producing observed atmospheric anomalies during ENSO and the observed Pacific SST and sea level anomalies during ENSO result primarily from the influence of surface wind stress anomalies ► Most studies prior to this had focused on one aspect or the other (i.e. atmosphere or ocean) but few dealt with the coupled system. Those that did were highly parameterized and did a poor job of coupling the system

Model Description ► Atmosphere - Circulation is forced by a heating anomaly that depends partly on local heating associated with SST anomalies and partly on low-level moisture convergence (found in obs and GCM studies to be important, not usually incorporated in models) - Nonlinear convergence feedback is incorporated using an iterative procedure in which the heating at each iteration depends on the convergence field from the previous iteration ► Ocean - Rectangular basin extending from 124°E to 80°W and from 29°N to 29°S. - Frictional layer to account for wind driven currents - Surface currents are generated by spinning the model up with monthly climatological mean winds - There is 3D temperature advection by the specified mean current and an anomalous current produced in model - Heat flux anomaly is proportional to SST anomaly, always acting to adjust to the climatological mean state

Model Description Coupling ► Ocean component is forced by surface wind stress anomalies ► Allow time dependence only in moisture convergence component of heating ► Heating related directly to SST gives a wind response in equilibrium with the SST field on a time scale of 10 days ► Heating due to internal moisture convergence operates in a time stepping sense and forces a wind field adjustment on a time scale of 1 month

Results ► 90 year model run initiated with a westerly wind anomaly in the region 145°E to 170°W for a period of four months ► Quasi-regular oscillations with a favored period of 3-4 years ► Peak in June or around the end of the year with a total duration of between 14 and 18 months ► Peak first in the east and later in the central region

Results ► Observations from Rasmusson and Carpenter from ► Irregular oscillations with a preference for a 3-4 year period ► Major events have a duration of somewhat longer than a year ► Model was unable to reproduce initial coastal warming which is usually observed prior to the major central Pacific warming

Results ► Time series of area averaged wind anomalies ► During major warm events the two indices vary in a similar fashion indicating a large scale coherent wind forcing ► Western Pacific zonal wind anomalies are weaker than observed and switch from westerly to easterly later than observed

Results ► Following slides showcase a major warm event in three month intervals beginning in the end of year 30 and going to year 32 ► December of year 30, no appreciable anomalies in either SST or winds

March (31) ► A region of warm SST has developed in the equatorial zone east of 170°W with a max near 130°W ► Small westerly wind anomalies near 130°W to 160°W

June (31) ► Warm event is well underway ► SST anomalies exceeding 1°C in the eastern equatorial Pacific ► Sizeable western wind anomalies in the central Pacific ► Model warming tends to occur uniformly rather than initially at the coast

September (31) ► Expansion and amplification of SST and wind anomalies in the fall of an ENSO year ► Warm anomalies as far westward as 160°E and eastern Pacific anomalies exceed 2°C ► Large westerly wind anomalies cover the whole equatorial central Pacific ► A region of small negative SST anomaly and easterly wind anomaly has developed in the Western Pacific

December (31) ► Peak temperature anomalies occur with a max at the coast and another one near 140°W ► SST anomaly has expanded meridionally ► Westerly wind anomalies of about 2 m s -1 in the central Pacific ► Easterly anomalies in the western Pacific in obs, not shown in the model

March (32) ► Temperature anomalies begin to decrease, especially at the east coast ► Single max in the eastern central Pacific ► Large westerly wind anomalies in the central Pacific ► Increasing easterly anomalies farther to the east

June (32) ► Eastern ocean is still warm, but temperatures are decreasing rapidly ► Westerly wind anomalies have decreased and receded westward ► Stronger easterlies are evident in the east ► Composites show cold SST anomalies and poleward wind anomalies in the eastern ocean at this time

September (32) ► Dramatic change in winds and SST amounting to a rapid termination of the warm event ► Equatorial eastern and central ocean is cold ► Winds are primarily meridional and directed poleward ► Agrees well with composites for this time period ► All major events evolve in a similar fashion. Some smaller amplitude anomalies develop differently and do not conform to this scenario

Results ► Model thermocline depth anomaly along the equator between years 30 and 45 ► Measure of heat content of the upper ocean ► Major warm episodes (31 and 41) are characterized by anomalously high heat content in the east and low heat content in the west ► Periods preceding major warm events are characterized by above-normal heat content at all longitudes ► Periods immediately following warm events show a deficit of heat content ► Rise in equatorial heat content precedes the development of equatorial westerlies and positive SST anomalies in the eastern ocean; that is it precedes the ENSO event

Results ► Strong and sustained westerly wind anomalies in the central Pacific during warm phase ► Occurs in phase with the anomalously high heat contents in the eastern ocean and low heat content in the western ocean ► At times (38 and 44) westerly anomalies appear in a fashion similar to the onset of ENSO, but the presence of strong easterly anomalies in the eastern Pacific contrast them ► The easterly anomaly grows with the westerly anomaly and development ceases shortly thereafter ► Terminated events tend to start early in the year (Jan-March), may be influenced by annual cycle

ENSO summary ► Large scale pattern of equatorial westerly wind anomalies in the central Pacific and equatorial SST anomalies that extend across most of the basin and decrease in amplitude from east to west ► The climatologically mean state includes easterly trade winds blowing across the eastern and central ocean ► Easterly stress induces an equatorial upwelling and sets up a zonal tilt to the thermocline ► Cold sub-thermocline water is far removed from the surface in the west and very near the surface in the east ► Sets up a sub-surface temperature anomaly that is largest in the east and smaller toward the west ► Atmospheric response is equatorial westerly wind anomalies that span nearly the entire region of SST anomalies ► Westerly wind deepens the eastern ocean thermocline, suppresses equatorial upwelling, and sets up eastern current anomalies which reinforce the temperature anomaly pattern

Model Sensitivities Atmospheric Parameterization ► 25-year run based on initial conditions present at the start of year 31, only considering NINO3 SST index ► Coefficient of heating term proportional to SST anomalies increased 10% ► Results in a large increase in amplitude of events, but similar period ► Coefficient of heating proportional to moisture convergence is increased by 7% ► Net impact on the larger scale structure is minimal

Model Sensitivities Oceanic Parameterizations ► Cases 1-4 both the amplitude and time scale of the oscillations increase ► Reduced thermal dissipation allows larger SST anomalies ► Larger drag coefficient produces greater wind stress forcing resulting in larger anomalies ► Stronger mean upwelling yields larger SST anomaly in response to thermocline displacements ► Smaller equivalent depth produces larger thermocline variations and larger subsurface temperature anomalies ► 5 decreases the coupling strength, resulting in smaller oscillations with a shorter period ► 6 indicates low sensitivity ► A sizeable decrease in background ocean dissipation produces no change in the solution, however other experiments with larger dissipation result in reduced amplitude

Influences of the Annual Cycle ► Warm events tend to amplify sharply during the northern summer, reach peak amplitude around the end of the year and diminish the following year ► To examine annual cycle influence in the model, experiments were performed using initial conditions from Jan of year 31 and the annual cycle was turned off at a given subsequent month by holding the mean fields fixed from then on ► April (0) shows the growth of warm event is retarded considerably (amplitude increases more slowly, reaches a max later and decreases sharply) ► August (0) shows the amplitude continues to rise for many months, peaks later and larger than with annual cycle ► December (0) shows a steady decline into the middle of year 1 ► July (+1) shows growth of negative anomalies continues longer and leads to larger anomalies during year 2+

Annual Cycle ► Summer period is most favorable for rapid growth of both positive and negative anomalies (mean winds, mean upwelling and mean SST gradient are all large) ► Spring period is least favorable for anomaly growth (weak trade winds and associated upwelling and SST gradient) ► Small anomalies present during the spring of an ENSO year amplify rapidly during the summer and fall ► During winter the coupling strength decreases significantly and becomes insufficient to maintain the large anomalies ► Hesitation period in the spring followed by an increase in coupling strength and demise of event in the summer ► Second experiment in which cycle turned off July (0) and continues for 25 years shows the solution settles into periodic oscillation

Elements of the model oscillation ► The subsurface temperature is made more or less sensitive to changes in the area- averaged heat content ► When made completely insensitive to changes in heat content the ENSO oscillation stops and is replaced by an annual oscillation ► When exactly half of the actual heat content fluctuation is allowed an oscillation with a period of 5-6 years develops ► When the effect is enhanced by a factor of 3 the period is 1-2 years ► The oscillatory character of the coupled system depends on the effect of variations in net equatorial heat content ► The 3-4 year period preference reflects a time delay between dynamical changes in the eastern ocean and associated large- scale fluctuations in equatorial wind stress

Summary ► Without anomalous external forcing the model produces recurring warm events that are irregular in both amplitude and spacing, but favor a 3-4 year period ► Largest growth occurs during the northern summer and fall and termination during the following spring and summer ► Signature of model warm events include equatorial westerly wind anomalies in the central Pacific and large SST anomalies in the eastern Pacific ► All parameter changes that amounted to increasing (decreasing) the strength of the atmosphere-ocean coupling tended to produce larger (smaller) amplitudes and longer (shorter) periods ► There is a buildup in heat content prior to the onset of a warm episode and a rapid decrease during the course of the event ► The variations in heat content is due to a phase lag between wind stress and thermocline motions, this is why interannual oscillations occur in the coupled system ► Above normal heat content is a necessary precondition for the onset of a warm episode. ► All mechanisms essential to the ENSO cycle are contained in the Pacific region and they are largely controlled by deterministic processes in the tropical Pacific atmosphere-ocean system

Additional References ► ElNinoDef.htm ElNinoDef.htm ElNinoDef.htm ► o.html o.html o.html