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.

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 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 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 Wind Stress Variations and Interannual Sea Surface Temperature Anomalies in the Eastern Equatorial Pacific

2006 ◽  
Vol 19 (2) ◽  
pp. 226-241 ◽  
Author(s):  
Xuebin Zhang ◽  
Michael J. McPhaden
Keyword(s):  
Wind Stress ◽  
Zonal Wind ◽  
El Niño ◽  
El Nino ◽  
Equatorial Pacific ◽  
Eastern Pacific ◽  
Zonal Wind Stress ◽  
Eastern Equatorial Pacific ◽  
Equatorial Wave ◽  
Stress Variations

Abstract Vertical advection of temperature is the primary mechanism by which El Niño–Southern Oscillation (ENSO) time-scale sea surface temperature (SST) anomalies are generated in the eastern equatorial Pacific. Variations in vertical advection are mediated primarily by remote wind-forced thermocline displacements, which control the temperature of water upwelled to the surface. However, during some ENSO events, large wind stress variations occur in the eastern Pacific that in principle should affect local upwelling rates, the depth of the thermocline, and SST. In this study, the impact of these wind stress variations on the eastern equatorial Pacific is addressed using multiple linear regression analysis and a linear equatorial wave model. The regression analysis indicates that a zonal wind stress anomaly of 0.01 N m−2 leads to approximately a 1°C SST anomaly over the Niño-3 region (5°N–5°S, 90°–150°W) due to changes in local upwelling rates. Wind stress variations of this magnitude occurred in the eastern Pacific during the 1982/83 and 1997/98 El Niños, accounting for about 1/3 of the maximum SST anomaly during these events. The linear equatorial wave model also indicates that depending on the period in question, zonal wind stress variations in the eastern Pacific can work either with or against remote wind stress forcing from the central and western Pacific to determine the thermocline depth in the eastern Pacific. Thus, zonal wind stress variations in the eastern Pacific contribute to the generation of interannual SST anomalies through both changes in local upwelling rates and changes in thermocline depth. Positive feedbacks between the ocean and atmosphere in the eastern Pacific are shown to influence the evolution of the surface wind field, especially during strong El Niño events, emphasizing the coupled nature of variability in the region. Implications of these results for understanding the character of event-to-event differences in El Niño and La Niña are discussed.

On the Impact of Local Feedbacks in the Central Pacific on the ENSO Cycle

2003 ◽  
Vol 16 (14) ◽  
pp. 2396-2407 ◽  
Author(s):  
Gerrit Burgers ◽  
Geert Jan van Oldenborgh
Keyword(s):  
Linear Model ◽  
Wind Stress ◽  
Equatorial Pacific ◽  
Eastern Pacific ◽  
Enso Cycle ◽  
Local Wind ◽  
Central Pacific ◽  
Thermocline Feedback ◽  
Sst Anomaly ◽  
Sst Anomalies

Abstract While sea surface temperature (SST) anomalies in the eastern equatorial Pacific are dominated by the thermocline feedback, in the central equatorial Pacific local wind effects, such as zonal advection, are important as well. El Niño–Southern Oscillation (ENSO) simulations with a linear model improve markedly if these effects are included as a local wind stress feedback on SST. An atmosphere model that reacts both to eastern and central Pacific SST anomalies is needed for producing a realistic ENSO cycle. First, simulations are studied of a linear 1.5-layer reduced-gravity ocean model and a linear SST anomaly equation, forced by observed monthly wind stress. If only the thermocline feedback is present in the SST equation, SST can be simulated well in the eastern Pacific, but, contrary to observations, central Pacific SST is out of phase with the eastern Pacific. If a wind stress feedback is added in the SST equation, as a term proportional to the zonal wind stress, correlations between observed and simulated SST are above 0.8 in both the central and eastern Pacific, and the correlation between the Niño-3 (5°S–5°N, 90°–150°W) and Niño-4 (5°S–5°N, 150°W–160°E) indexes is close to the observed value of 0.75. Next, a statistical atmosphere is added to the ocean module that is based on a regression of observed wind stress to the observed Niño-3 and Niño-4 indexes. The coupled system is driven by noise that is inferred from the residues of the fit and has a red component. The observed Niño-3–Niño-4 index correlation can be reproduced only with a wind stress feedback in the central Pacific. Also, the level of SST variability rises and the ENSO period increases to more realistic values. The interplay between the local wind stress and the thermocline feedbacks, therefore, is an important factor in the structure of ENSO in the coupled linear model. In the eastern Pacific, the thermocline feedback dominates SST anomalies; in the central Pacific, the local wind stress feedback. Due to the local wind stress feedback, the ENSO wind stress response excites SST anomalies in the central Pacific, extending the ENSO SST anomaly pattern well into the central Pacific. In turn, these central Pacific SST anomalies give rise to wind stress anomalies that are situated more westward than the response to eastern Pacific SST anomalies. As a result, the ENSO amplitude is enhanced and the ENSO period increased. Also, central Pacific SST anomalies are not completely determined by eastern Pacific SST anomalies and they persist longer.

scholarly journals A Role for the Equatorial Undercurrent in the Ocean Dynamical Thermostat

2018 ◽  
Vol 31 (16) ◽  
pp. 6245-6261 ◽  
Author(s):  
S. Coats ◽  
K. B. Karnauskas
Keyword(s):  
Twentieth Century ◽  
Wind Stress ◽  
Zonal Wind ◽  
General Circulation ◽  
General Circulation Models ◽  
Equatorial Pacific ◽  
Systematic Bias ◽  
Equatorial Undercurrent ◽  
Zonal Wind Stress ◽  
Sst Gradient

Reconstructions of sea surface temperature (SST) based on instrumental observations suggest that the equatorial Pacific zonal SST gradient has increased over the twentieth century. While this increase is suggestive of the ocean dynamical thermostat mechanism of Clement et al., observations of a concurrent weakening of the zonal atmospheric (Walker) circulation are not. Here we show, using heat and momentum budget calculations on an ocean reanalysis dataset, that a seasonal weakening of the zonal atmospheric circulation is in fact consistent with cooling in the eastern equatorial Pacific (EEP) and thus an increase in the zonal SST gradient. This cooling is driven by a strengthening Equatorial Undercurrent (EUC) in response to decreased upper-ocean westward momentum associated with weakening equatorial zonal wind stress. This process can help to reconcile the seemingly contradictory twentieth-century trends in the tropical Pacific atmosphere and ocean. Moreover, it is shown that coupled general circulation models (CGCMs) do not correctly simulate this process; we identify a systematic bias in the relationship between changes in equatorial surface zonal wind stress in the EEP and EUC strength that may help to explain why observations and CGCMs have opposing trends in the zonal SST gradient over the twentieth century.

The Termination of the 1997–98 El Niño. Part II: Mechanisms of Atmospheric Change

2006 ◽  
Vol 19 (12) ◽  
pp. 2647-2664 ◽  
Author(s):  
Gabriel A. Vecchi
Keyword(s):  
Wind Stress ◽  
Zonal Wind ◽  
El Niño ◽  
El Nino ◽  
Equatorial Pacific ◽  
Atmospheric Response ◽  
Zonal Wind Stress ◽  
El Niño Event ◽  
Southward Shift ◽  
Extreme El Niño

Abstract The mechanisms that drove zonal wind stress (τx) changes in the near-equatorial Pacific at the end of the extreme 1997–98 El Niño event are explored using a global atmospheric general circulation model. The analysis focuses on three features of the τx evolution between October 1997 and May 1998 that were fundamental in driving the oceanic changes at the end of this El Niño event: (i) the southward shift of near-date-line surface zonal wind stress (τx) anomalies beginning November 1997, (ii) the disappearance of the easterly τx from the eastern equatorial Pacific (EEqP) in February 1998, and (iii) the reappearance of easterly τx in the EEqP in May 1998. It is shown that these wind changes represent the deterministic response of the atmosphere to the observed sea surface temperature (SST) field, resulting from changes in the meridional structure of atmospheric convective anomalies in response to the seasonally phase-locked meridional movement of the warmest SST. The southward shift of the near-date-line τx anomalies at the end of this El Niño event was controlled by the seasonal movement of the warmest SST south of the equator, which—both directly and through its influence on the atmospheric response to changes in SST anomaly—brought the convective anomalies from being centered about the equator to being centered south of the equator. The disappearance (reappearance) of easterly EEqP τx has only been evident in extreme El Niño events and has been associated with the development (northward retreat) of an equatorial intertropical convergence zone (ITCZ). The disappearance/return of EEqP easterly τx arises in the AGCM as the deterministic response to changes in the SST field, tied principally to the changes in climatological SST (given time-invariant extreme El Niño SSTA) and not to changes in the underlying SSTA field. The disappearance (return) of EEqP easterly τx in late boreal winter (late boreal spring) is a characteristic atmospheric response to idealized extreme El Niño SST anomalies; this suggests that the distinctive termination of the 1997–98 El Niño event is that to be expected for extreme El Niño events.

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 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.

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