Basin-wide zonal wind stress and ocean thermal variations in the equatorial Pacific Ocean

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.

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

Journal of Climate ◽

10.1175/jcli-d-13-00438.1 ◽

2014 ◽

Vol 27 (7) ◽

pp. 2577-2587 ◽

Cited By ~ 69

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

Journal of Climate ◽

10.1175/jcli3618.1 ◽

2006 ◽

Vol 19 (2) ◽

pp. 226-241 ◽

Cited By ~ 36

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.

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Interdecadal Sea Surface Temperature Variability in the Equatorial Pacific Ocean. Part I: The Role of Off-Equatorial Wind Stresses and Oceanic Rossby Waves

Journal of Climate ◽

10.1175/jcli4145.1 ◽

2007 ◽

Vol 20 (11) ◽

pp. 2643-2658 ◽

Cited By ~ 30

Author(s):

Shayne McGregor ◽

Neil J. Holbrook ◽

Scott B. Power

Keyword(s):

Pacific Ocean ◽

Wind Stress ◽

Rossby Waves ◽

Wave Reflection ◽

Equatorial Pacific ◽

Sea Surface ◽

Thermocline Depth ◽

Equatorial Pacific Ocean ◽

Equatorial Thermocline ◽

The Western Pacific

Abstract The Australian Bureau of Meteorology Research Centre CGCM and a linear first baroclinic-mode ocean shallow-water model (SWM) are used to investigate ocean dynamic forcing mechanisms of the equatorial Pacific Ocean interdecadal sea surface temperature (SST) variability. An EOF analysis of the 13-yr low-pass Butterworth-filtered SST anomalies from a century-time-scale CGCM simulation reveals an SST anomaly spatial pattern and time variability consistent with the interdecadal Pacific oscillation. Results from an SWM simulation forced with wind stresses from the CGCM simulation are shown to compare well with the CGCM, and as such the SWM is then used to investigate the roles of “uncoupled” equatorial wind stress forcing, off-equatorial wind stress forcing (OffEqWF), and Rossby wave reflection at the western Pacific Ocean boundary, on the decadal equatorial thermocline depth anomalies. Equatorial Pacific wind stresses are shown to explain a large proportion of the overall variance in the equatorial thermocline depth anomalies. However, OffEqWF beyond 12.5° latitude produces an interdecadal signature in the Niño-4 (Niño-3) region that explains approximately 10% (1.5%) of the filtered control simulation variance. Rossby wave reflection at the western Pacific boundary is shown to underpin the OffEqWF contribution to these equatorial anomalies. The implications of this result for the predictability of the decadal variations of thermocline depth are investigated with results showing that OffEqWF generates an equatorial response in the Niño-3 region up to 3 yr after the wind stress forcing is switched off. Further, a statistically significant correlation is found between thermocline depth anomalies in the off-equatorial zone and the Niño-3 region, with the Niño-3 region lagging by approximately 2 yr. The authors conclude that there is potential predictability of the OffEqWF equatorial thermocline depth anomalies with lead times of up to 3 yr when taking into account the amplitudes and locations of off-equatorial region Rossby waves.

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An Analysis of ENSO Prediction Skill in the CFS Retrospective Forecasts

Journal of Climate ◽

10.1175/2008jcli2565.1 ◽

2009 ◽

Vol 22 (7) ◽

pp. 1801-1818 ◽

Cited By ~ 22

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.

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Interdecadal Sea Surface Temperature Variability in the Equatorial Pacific Ocean. Part II: The Role of Equatorial/Off-Equatorial Wind Stresses in a Hybrid Coupled Model

Journal of Climate ◽

10.1175/2008jcli2057.1 ◽

2008 ◽

Vol 21 (17) ◽

pp. 4242-4256 ◽

Cited By ~ 10

Author(s):

Shayne McGregor ◽

Neil J. Holbrook ◽

Scott B. Power

Keyword(s):

Pacific Ocean ◽

Wind Stress ◽

Rossby Waves ◽

Coupled Model ◽

Equatorial Region ◽

Equatorial Pacific ◽

Research Centre ◽

Equatorial Pacific Ocean ◽

Hybrid Coupled Model

Abstract Many modeling studies have been carried out to investigate the role of oceanic Rossby waves linking the off-equatorial and equatorial Pacific Ocean. Although the equatorial ocean response to off-equatorial wind stress forcing alone tends to be relatively small, it is clear that off-equatorial oceanic Rossby waves affect equatorial Pacific Ocean variability on interannual through to interdecadal time scales. In the present study, a hybrid coupled model (HCM) of the equatorial Pacific (between 12.5°S and 12.5°N) was developed and is used to estimate the magnitude of equatorial region variability arising from off-equatorial (poleward of 12.5° latitude) wind stress forcing. The HCM utilizes a reduced-gravity ocean shallow-water model and a statistical atmosphere derived from monthly output from a 100-yr Australian Bureau of Meteorology Research Centre (now the Centre for Australian Weather and Climate Research) coupled general circulation model integration. The equatorial region wind stress forcing is found to dominate both the interannual and interdecadal SST variability. The equatorial response to off-equatorial wind stress forcing alone is insufficient to initiate an atmospheric feedback that significantly amplifies the original equatorial region variability. Consequently, the predictability of equatorial region SST anomalies (SSTAs) could be limited to ∼1 yr (the maximum time it takes an oceanic Rossby wave to cross the Pacific Ocean basin in the equatorial region). However, the results also suggest that the addition of off-equatorial wind stress forcing to the HCM leads to variations in equatorial Pacific background SSTA of up to almost one standard deviation. This off-equatorially forced portion of the equatorial SSTA could prove critical for thresholds of El Niño–Southern Oscillation (ENSO) because they can constructively interfere with equatorially forced SSTA of the same sign to produce significant equatorial region ENSO anomalies.

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A Role for the Equatorial Undercurrent in the Ocean Dynamical Thermostat

Journal of Climate ◽

10.1175/jcli-d-17-0513.1 ◽

2018 ◽

Vol 31 (16) ◽

pp. 6245-6261 ◽

Cited By ~ 14

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.

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

Journal of Climate ◽

10.1175/jcli3780.1 ◽

2006 ◽

Vol 19 (12) ◽

pp. 2647-2664 ◽

Cited By ~ 55

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.

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