scholarly journals Predictability Loss in an Intermediate ENSO Model due to Initial Error and Atmospheric Noise*

2006 ◽  
Vol 19 (15) ◽  
pp. 3572-3588 ◽  
Author(s):  
Alicia R. Karspeck ◽  
Alexey Kaplan ◽  
Mark A. Cane
Keyword(s):  
Wind Stress ◽  
Zonal Wind ◽  
Initial Conditions ◽  
Sea Surface ◽  
Thermocline Depth ◽  
Error Growth ◽  
Initial Error ◽  
Ensemble Spread ◽  
Observational Error ◽  
Zonal Wind Stress

Abstract The seasonal and interannual predictability of ENSO variability in a version of the Zebiak–Cane coupled model is examined in a perturbation experiment. Instead of assuming that the model is “perfect,” it is assumed that a set of optimal initial conditions exists for the model. These states, obtained through a nonlinear minimization of the misfit between model trajectories and the observations, initiate model forecasts that correlate well with the observations. Realistic estimates of the observational error magnitudes and covariance structures of sea surface temperatures, zonal wind stress, and thermocline depth are used to generate ensembles of perturbations around these optimal initial states, and the error growth is examined. The error growth in response to subseasonal stochastic wind forcing is presented for comparison. In general, from 1975 to 2002, the large-scale uncertainty in initial conditions leads to larger error growth than continuous stochastic forcing of the zonal wind stress fields. Forecast ensemble spread is shown to depend most on the calendar month at the end of the forecast rather than the initialization month, with the seasons of greatest spread corresponding to the seasons of greatest anomaly variance. It is also demonstrated that during years with negative (and rapidly decaying) Niño-3 SST anomalies (such as the time period following an El Niño event), there is a suppression of error growth. In years with large warm ENSO events, the ensemble spread is no larger than in moderately warm years. As a result, periods with high ENSO variance have greater potential prediction utility. In the realistic range of observational error, the ensemble spread has more sensitivity to the initial error in the thermocline depth than to the sea surface temperature or wind stress errors. The thermocline depth uncertainty is the principal reason why initial condition uncertainties are more important than wind noise for ensemble spread.

An Analysis of ENSO Prediction Skill in the CFS Retrospective Forecasts

2009 ◽  
Vol 22 (7) ◽  
pp. 1801-1818 ◽  
Author(s):  
Renguang Wu ◽  
Ben P. Kirtman ◽  
Huug van den Dool
Keyword(s):  
Wind Stress ◽  
Zonal Wind ◽  
Equatorial Pacific ◽  
Prediction Skill ◽  
Thermocline Depth ◽  
Zonal Wind Stress ◽  
Boreal Spring ◽  
Eastern Equatorial Pacific ◽  
Western Equatorial Pacific ◽  
Sst Anomalies

Abstract The present study documents the so-called spring prediction and persistence barriers in association with El Niño–Southern Oscillation (ENSO) in the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) retrospective forecasts. It is found that the spring prediction and persistence barriers in the eastern equatorial Pacific sea surface temperature (SST) are preceded by a boreal winter barrier in the western equatorial Pacific zonal wind stress. The time of the persistence barrier is closely related to the time of the ENSO phase transition, but may differ from the time of the lowest variance. The seasonal change of the signal-to-noise ratio cannot explain the persistence barrier. While the noise may lead to a drop of skill around boreal spring in the western equatorial Pacific zonal wind stress, its impacts on the skill of eastern equatorial Pacific SST is small. The equatorial Pacific zonal winds display an excessive response to ENSO-related SST anomalies, which leads to a longer persistence in the equatorial Pacific thermocline depth anomalies and a delayed transition of the eastern equatorial Pacific SST anomalies. This provides an interpretation for the prediction skill drop in boreal spring in the eastern equatorial Pacific SST. The results suggest that improving the atmospheric model wind response to SST anomalies may reduce the spring prediction barrier.

scholarly journals Seasonal Propagation of Sea Level along the Equator in the Atlantic

2009 ◽  
Vol 39 (4) ◽  
pp. 1069-1074 ◽  
Author(s):  
Lucia Bunge ◽  
Allan J. Clarke
Keyword(s):  
Wind Stress ◽  
Zonal Wind ◽  
Function Analysis ◽  
Sea Surface Height ◽  
Sea Surface ◽  
Water Volume ◽  
Kelvin Wave ◽  
Equatorial Atlantic ◽  
Wind Stress Curl ◽  
Zonal Wind Stress

Abstract In the equatorial Atlantic the sea surface height (SSH) anomaly field is dominated by an annual signal propagating eastward. This signal has been previously interpreted in terms of propagating waves. In this article it is argued that this propagating signal is not a free equatorial Kelvin wave because the phase velocity observed is too small compared to first, second, or third baroclinic mode Kelvin waves, and is not the result of an equatorial forced wave because the zonal wind stress does not show a similar propagation. Rather, it is suggested that the eastward propagation in SSH is due to the sum of two independent modes of variability: one mainly driven by the wind stress curl off the equator, and the other driven by the zonal wind stress along the equator. These two modes are uncorrelated in time and space and therefore can be conveniently separated by an empirical orthogonal function analysis of the equatorial Atlantic sea surface height. The first mode explains 74% of the variance, is one-signed in longitude, and is interpreted as the variability of the warm water volume above the thermocline. The second mode explains 24% of the variance, consists of an east–west tilt along the equator, and is driven by variations of the zonal equatorial wind stress.

scholarly journals Reassessing Conceptual Models of ENSO

2015 ◽  
Vol 28 (23) ◽  
pp. 9121-9142 ◽  
Author(s):  
Felicity S. Graham ◽  
Jaclyn N. Brown ◽  
Andrew T. Wittenberg ◽  
Neil J. Holbrook
Keyword(s):  
Wind Stress ◽  
Zonal Wind ◽  
Conceptual Models ◽  
Southern Oscillation ◽  
Oscillator Model ◽  
Sustained Oscillation ◽  
Complex Nature ◽  
Enso Cycle ◽  
Thermocline Depth ◽  
Zonal Wind Stress

Abstract The complex nature of the El Niño–Southern Oscillation (ENSO) is often simplified through the use of conceptual models, each of which offers a different perspective on the ocean–atmosphere feedbacks underpinning the ENSO cycle. One theory, the unified oscillator, combines a variety of conceptual frameworks in the form of a coupled system of delay differential equations. The system produces a self-sustained oscillation on interannual time scales. While the unified oscillator is assumed to provide a more complete conceptual framework of ENSO behaviors than the models it incorporates, its formulation and performance have not been systematically assessed. This paper investigates the accuracy of the unified oscillator through its ability to replicate the ENSO cycle modeled by flux-forced output from the Australian Community Climate and Earth-System Simulator Ocean Model (ACCESS-OM). The anomalous sea surface temperature equation reproduces the main features of the corresponding tendency modeled by ACCESS-OM reasonably well. However, the remaining equations for the thermocline depth anomaly and zonal wind stress anomalies are unable to accurately replicate the corresponding tendencies in ACCESS-OM. Modifications to the unified oscillator, including a diagnostic form of the zonal wind stress anomaly equations, improve its ability to emulate simulated ENSO tendencies. Despite these improvements, the unified oscillator model is less adept than the delayed oscillator model it incorporates in capturing ENSO behavior in ACCESS-OM, bringing into question its usefulness as a unifying ENSO framework.

scholarly journals On the fast response of the Southern Ocean to changes in the zonal wind

Ocean Science ◽  
2007 ◽  
Vol 3 (3) ◽  
pp. 417-427 ◽  
Author(s):  
D. J. Webb ◽  
B. A. de Cuevas
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
Zonal Wind ◽  
Surface Wind ◽  
Drake Passage ◽  
Fast Response ◽  
Momentum Balance ◽  
Zonal Wind Stress ◽  
Model Studies ◽  
Barotropic Flow

Abstract. Model studies of the Southern Ocean, reported here, show that the Antarctic Circumpolar Current responds within two days to changes in the zonal wind stress at the latitudes of Drake Passage. Further investigation shows that the response is primarily barotropic and that, as one might expect, it is controlled by topography. Analysis of the results show that the changes in the barotropic flow are sufficient to transfer the changed surface wind stress to the underlying topography and that during this initial phase baroclinic processes are not involved. The model results also show that the Deacon Cell responds to changes in the wind stress on the same rapid time scale. It is shown that the changes in the Deacon Cell can also be explained by the change in the barotropic velocity field, an increase in the zonal wind stress producing an increased northward flow in shallow regions and southward flow where the ocean is deep. This new explanation is unexpected as previously the Deacon Cell has been thought of as a baroclinic feature of the ocean. The results imply that where baroclinic processes do appear to be involved in either the zonal momentum balance of the Southern Ocean or the formation of the Deacon Cell, they are part of the long term baroclinic response of the ocean's density field to the changes in the barotropic flow.

scholarly journals Using Initialized Hindcasts to Assess Simulations of 1970–2009 Equatorial Pacific SST, Zonal Wind Stress, and Surface Flux Trends

2014 ◽  
Vol 27 (19) ◽  
pp. 7385-7393 ◽  
Author(s):  
Amy Solomon
Keyword(s):  
Time Scales ◽  
Wind Stress ◽  
Zonal Wind ◽  
Surface Flux ◽  
Warming Trend ◽  
Warm Pool ◽  
Equatorial Pacific ◽  
External Forcing ◽  
Cold Tongue ◽  
Zonal Wind Stress

Abstract Initialized decadal hindcasts are used to assess simulations of 1970–2009 equatorial Pacific SST, zonal wind stress, and surface flux trends. Initialized hindcasts are useful to assess how well the models simulate observed trends, as well as how simulations of observed trends (due primarily to natural variability) differ from ensemble-mean forecasted trends (due to the response to an increase in external forcing). All models forecast a statistically significant warming trend in both the warm-pool and cold-tongue regions. However, while the warm-pool warming trend is within the observed estimates, the cold-tongue warming trend is an order of magnitude larger than an ENSO residual estimated using SST instrumental reconstructions. Multimodel ensemble means formed using forecasts 6–10 years from initialization with 40 ensemble members do not produce an unambiguous zonal SST gradient response to an increase in external forcing. Systematic biases are identified in forecasts of surface fluxes. For example, in the warm-pool region all year-1 forecasts produce SST trends similar to observations but ocean mixed layer and net surface heat flux trends with an opposite sign to air–sea datasets. In addition, year-1 forecasts produce positive shortwave feedbacks on decadal time scales, whereas 6–10-yr forecasts produce negative or statistically insignificant shortwave flux feedbacks on decadal time scales, suggesting sensitivity to circulations forced by the initialized ocean state. In the cold-tongue region initialized ensembles forecast positive net radiative flux trends even though shortwave flux trends are negative (i.e., for increasing cloudiness). This is inconsistent with air–sea datasets, which uniformly show that the net surface radiative flux feedback is a damping of the underlying SSTs.

scholarly journals Causes of Strengthening and Weakening of ENSO Amplitude under Global Warming in Four CMIP5 Models*

2015 ◽  
Vol 28 (8) ◽  
pp. 3250-3274 ◽  
Author(s):  
Lin Chen ◽  
Tim Li ◽  
Yongqiang Yu
Keyword(s):  
Global Warming ◽  
Wind Stress ◽  
Zonal Wind ◽  
Southern Oscillation ◽  
Heat Budget ◽  
Meridional Overturning Circulation ◽  
Cmip5 Models ◽  
Zonal Wind Stress ◽  
Equatorial Thermocline ◽  
Change In Mean

Abstract The mechanisms for El Niño–Southern Oscillation (ENSO) amplitude change under global warming are investigated through quantitative assessment of air–sea feedback processes in present-day and future climate simulations of four models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5). Two models (MPI-ESM-MR and MRI-CGCM3) project strengthened ENSO amplitude, whereas the other two models (CCSM4 and FGOALS-g2) project weakened ENSO amplitude. A mixed layer heat budget diagnosis shows that the major cause of the projected ENSO amplitude difference between the two groups is attributed to the changes of the thermocline and zonal advective feedbacks. A weaker (stronger) equatorial thermocline response to a unit anomalous zonal wind stress forcing in the Niño-4 region is found in CCSM4 and FGOALS-g2 (MPI-ESM-MR and MRI-CGCM3). The cause of the different response arises from the change in the meridional scale of ENSO. A narrower (wider) meridional width of sea surface temperature (SST) and zonal wind stress anomalies causes a strengthening (weakening) of the equatorial thermocline response and thus stronger Bjerknes and zonal advective feedbacks, as the subsurface temperature and zonal current anomalies depend on the thermocline response; consequently, the ENSO amplitude increases (decreases). The change of ENSO meridional width is caused by the change in mean meridional overturning circulation in the equatorial Pacific Ocean, which depends on change of mean wind stress and SST warming patterns under global warming.

Tropical Pacific and Atlantic Climate Variability in CCSM3

2006 ◽  
Vol 19 (11) ◽  
pp. 2451-2481 ◽  
Author(s):  
Clara Deser ◽  
Antonietta Capotondi ◽  
R. Saravanan ◽  
Adam S. Phillips
Keyword(s):  
Climate Variability ◽  
Surface Temperature ◽  
Spatial Pattern ◽  
Sea Surface Temperature ◽  
Wind Stress ◽  
El Nino ◽  
Coupled Model ◽  
Sea Surface ◽  
Thermocline Depth ◽  
Tropical Atlantic

Abstract Simulations of the El Niño–Southern Oscillation (ENSO) phenomenon and tropical Atlantic climate variability in the newest version of the Community Climate System Model [version 3 (CCSM3)] are examined in comparison with observations and previous versions of the model. The analyses are based upon multicentury control integrations of CCSM3 at two different horizontal resolutions (T42 and T85) under present-day CO2 concentrations. Complementary uncoupled integrations with the atmosphere and ocean component models forced by observed time-varying boundary conditions allow an assessment of the impact of air–sea coupling upon the simulated characteristics of ENSO and tropical Atlantic variability. The amplitude and zonal extent of equatorial Pacific sea surface temperature variability associated with ENSO is well simulated in CCSM3 at both resolutions and represents an improvement relative to previous versions of the model. However, the period of ENSO remains too short (2–2.5 yr in CCSM3 compared to 2.5–8 yr in observations), and the sea surface temperature, wind stress, precipitation, and thermocline depth responses are too narrowly confined about the equator. The latter shortcoming is partially overcome in the atmosphere-only and ocean-only simulations, indicating that coupling between the two model components is a contributing cause. The relationships among sea surface temperature, thermocline depth, and zonal wind stress anomalies are consistent with the delayed/recharge oscillator paradigms for ENSO. We speculate that the overly narrow meridional scale of CCSM3's ENSO simulation may contribute to its excessively high frequency. The amplitude and spatial pattern of the extratropical atmospheric circulation response to ENSO is generally well simulated in the T85 version of CCSM3, with realistic impacts upon surface air temperature and precipitation; the simulation is not as good at T42. CCSM3's simulation of interannual climate variability in the tropical Atlantic sector, including variability intrinsic to the basin and that associated with the remote influence of ENSO, exhibits similarities and differences with observations. Specifically, the observed counterpart of El Niño in the equatorial Atlantic is absent from the coupled model at both horizontal resolutions (as it was in earlier versions of the coupled model), but there are realistic (although weaker than observed) SST anomalies in the northern and southern tropical Atlantic that affect the position of the local intertropical convergence zone, and the remote influence of ENSO is similar in strength to observations, although the spatial pattern is somewhat different.

scholarly journals The Effect of Wind Stress Anomalies and Location in Driving Pacific Subtropical Cells and Tropical Climate

2019 ◽  
Vol 32 (5) ◽  
pp. 1641-1660 ◽  
Author(s):  
Giorgio Graffino ◽  
Riccardo Farneti ◽  
Fred Kucharski ◽  
Franco Molteni
Keyword(s):  
Wind Stress ◽  
Zonal Wind ◽  
Tropical Climate ◽  
Global Ocean ◽  
Subsurface Water ◽  
Zonal Wind Stress ◽  
Wind Stress Anomaly ◽  
Equatorial Thermocline ◽  
The Tropics ◽  
Sst Anomalies

Abstract The importance of subtropical and extratropical zonal wind stress anomalies on Pacific subtropical cell (STC) strength is assessed through several idealized and realistic numerical experiments with a global ocean model. Different zonal wind stress anomalies are employed, and their intensity is strengthened or weakened with respect to the climatological value throughout a suite of simulations. Subtropical strengthened (weakened) zonal wind stress anomalies result in increased (decreased) STC meridional mass and energy transport. When upwelling of subsurface water into the tropics is intensified (reduced), a distinct cold (warm) anomaly appears in the equatorial thermocline and up to the surface, resulting in significant tropical sea surface temperature (SST) anomalies. The use of realistic wind stress anomalies also suggests a potential impact of midlatitude atmospheric modes of variability on tropical climate through STC dynamics. The remotely driven response is compared with a set of simulations where an equatorial zonal wind stress anomaly is imposed. A dynamically distinct response is achieved, whereby the equatorial thermocline adjusts to the wind stress anomaly, resulting in significant equatorial SST anomalies as in the remotely forced simulations but with no role for STCs. Significant anomalies in Indonesian Throughflow transport are generated only when equatorial wind stress anomalies are applied, leading to remarkable heat content anomalies in the Indian Ocean. Equatorial wind stress anomalies do not involve modifications of STC transport but could set up the appropriate initial conditions for a tropical–extratropical teleconnection involving Hadley cells, exciting an STC anomalous transport, which ultimately feeds back on the tropics.

scholarly journals Assessing the Twenty-First-Century Shift in ENSO Variability in Terms of the Bjerknes Stability Index*

2014 ◽  
Vol 27 (7) ◽  
pp. 2577-2587 ◽  
Author(s):  
Joke F. Lübbecke ◽  
Michael J. McPhaden
Keyword(s):  
Wind Stress ◽  
Zonal Wind ◽  
Stability Index ◽  
Equatorial Pacific ◽  
Central Pacific ◽  
Western Basin ◽  
Ocean Reanalysis ◽  
Zonal Wind Stress ◽  
Bj Index ◽  
Sst Anomalies

Abstract A decadal change in the character of ENSO was observed around year 2000 toward weaker-amplitude, higher-frequency events with an increased occurrence of central Pacific El Niños. Here these changes are assessed in terms of the Bjerknes stability index (BJ index), which is a measure of the growth rate of ENSO-related SST anomalies. The individual terms of the index are calculated from ocean reanalysis products separately for the time periods 1980–99 and 2000–10. The spread between the products is large, but they show a robust weakening of the thermocline feedback due to a reduced thermocline slope response to anomalous zonal wind stress as well as a weakened wind stress response to eastern equatorial Pacific SST anomalies. These changes are consistent with changes in the background state of the tropical Pacific: cooler mean SST in the eastern and central equatorial Pacific results in reduced convection there together with a westward shift in the ascending branch of the Walker circulation. This shift leads to a weakening in the relationship between eastern Pacific SST and longitudinally averaged equatorial zonal wind stress. Also, despite a steeper mean thermocline slope in the more recent period, the thermocline slope response to wind stress anomalies weakened due to a smaller zonal wind fetch that results from ENSO-related wind anomalies being more confined to the western basin. As a result, the total BJ index is more negative, corresponding to a more strongly damped system in the past decade compared to the 1980s and 1990s.

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