Equatorial Zonal Wind Stress Research Articles

Impact of equatorial wind stress on Ekman transport during the mature phase of the Indian Ocean Dipole

This paper investigates the impact of equatorial wind stress on the equatorial Ekman transport during the Indian Ocean Dipole (IOD) mature phase. The results show that the equatorial zonal wind stress directly drives the meridional motion of seawater at the upper levels. In normal years, the zonal wind stress south of the equator is easterly and that north of the equator is westerly, which contributes to southward Ekman transport at the upper levels to form the climatological Indian Ocean shallow meridional overturning circulation. During the years of positive IOD events, abnormal easterly winds near the equator bring southward Ekman transport south of the equator while they bring northward Ekman transport north of the equator. This causes the seawater to move away from the equator and hence induces upwelling near the equator, which forms a pair of small circulation cells that are symmetric about the equator at the upper levels (approximately 100 m deep). The abnormal circulation cell south (north) of the equator strengthens (weakens) the southward (southward) motion south (north) of the equator. During years with negative IOD events, the opposite occurs. In addition, during the mature period of IOD, the remote sea surface temperature anomaly (SSTA) such as El Niño–Southern Oscillation (ENSO) may exert some influence on Ekman transport anomaly near the equator during the mature period of IOD.

Seasonal and long-term sea-level variations and their forcing factors in the northern Bay of Bengal: A statistical analysis of temperature, salinity, wind stress curl, and regional climate index data

In the northern Bay of Bengal, mechanisms of seasonal sea-level variation have not previously been examined, and the understanding of longer-term inter-annual sea-level variation is also not concrete. These parameters are addressed in this study utilizing available tide gauge and satellite altimetry data. The contribution of steric sea level to seasonal and longer-term inter-annual sea-level variations is quantified, and statistical analysis is performed to determine the correlations of various atmospheric and oceanic factors with sea level. This study suggests that the trend of sea-level rise in this bay (4 ± 1.33 mm/year) is higher than the global average (3.32 ± 0.46 mm/year) for the studied period 1993 to 2018. The rate of sea-level rise is higher along the coast than in the offshore area and the highest in the central part of the coast. Sea level shows a strong seasonal variation: sea level is the lowest in the winter but the highest in autumn. The contribution from the thermosteric sea level is higher to the observed sea level from winter to early summer, whereas contributions from the halosteric sea level and wind stress curl are higher during autumn. Long-term variations in sea level show strong positive correlations with thermosteric sea level, indicating that temperature is a major local controlling factor for sea-level change. In addition to local factors, long-term sea level also varies by remote forcing (equatorial zonal wind stress), which explains approximately 36 % of the sea-level variation in this bay. Sea level is low during the combined events of positive Indian Ocean dipole (IOD) and El Niño, whereas the sea level is high during the combined events of negative IOD and La Niña. This study provides an improved understanding of seasonal and longer-term inter-annual variations of sea level and the necessary groundworks for a dedicated model study to further quantify all the components of the sea-level budget in the study areas.

Changes in the equatorial mode of the Tropical Atlantic in terms of the Bjerknes Feedback Index

One of the main modes of sea surface temperature variability in the Tropical Atlantic is the Atlantic Equatorial Mode or Atlantic Nino. The region of largest interannual variability, where the Atlantic Cold Tongue forms, is also a region of consistent biases in climate models. In this study, we investigate the interannual variability of the Tropical Atlantic and its changes in the recent decades in terms of the Bjerknes Feedback Index ( $$\hbox {I}_{BJ}$$ ) in a set of seven ocean reanalyses for the periods 1980–1999 and 2000–2010. Differences were observed among the reanalyses regarding their representation of each term of the Bjerknes Feedback, particularly regarding zonal surface currents, leading to differences in dynamical damping and the zonal advective feedback. However, a consistent negative index (damped) is observed in all reanalysis, with the thermocline feedback being the dominant positive term and thermal damping the dominant negative term. Warming trends are observed in sea surface temperatures in the cold tongue region in all reanalyses, as well as a decrease in interannual variability. These in turn are associated with a weakening in the Bjerknes Feedback in the early XXI century, indicated by a stronger and negative index. This results from a stronger thermal damping and weaker thermocline feedback, associated with a weaker response of equatorial zonal thermocline slope to equatorial zonal wind stress. Despite the spread among the reanalysis, the results obtained are consistent with previous studies.

ENSO and Pacific Decadal Variability in the Community Earth System Model Version 2

AbstractThis study presents a description of the El Niño–Southern Oscillation (ENSO) and Pacific Decadal Variability (PDV) in a multicentury preindustrial simulation of the Community Earth System Model Version 2 (CESM2). The model simulates several aspects of ENSO relatively well, including dominant timescale, tropical and extratropical precursors, composite evolution of El Niño and La Niña events, and ENSO teleconnections. The good model representation of ENSO spectral characteristics is consistent with the spatial pattern of the anomalous equatorial zonal wind stress in the model, which results in the correct adjustment timescale of the equatorial thermocline according to the delayed/recharge oscillator paradigms, as also reflected in the realistic time evolution of the equatorial Warm Water Volume. PDV in the model exhibits a pattern that is very similar to the observed, with realistic tropical and South Pacific signatures which were much weaker in some of the CESM2 predecessor models. The tropical component of PDV also shows an association with ENSO decadal modulation which is similar to that found in observations. However, the ENSO amplitude is about 30% larger than observed in the preindustrial CESM2 simulation, and even larger in the historical ensemble, perhaps as a result of anthropogenic influences. In contrast to observations, the largest variability is found in the central Pacific rather than in the eastern Pacific, a discrepancy that somewhat hinders the model's ability to represent a full diversity in El Niño spatial patterns and appears to be associated with an unrealistic confinement of the precipitation anomalies to the western Pacific.

Origin of Weakened Interannual Sea Surface Temperature Variability in the Southeastern Tropical Atlantic Ocean

AbstractObservations and reanalysis products are used to investigate the substantial weakening in the southeastern tropical Atlantic sea surface temperature (SST) variability since 2000. Relative to 1982–1999, the March‐April‐May SST variability in the Angola‐Benguela area (ABA) has decreased by more than 30%. Both equatorial remote forcing and local forcing are known to play an important role in driving SST variability in the ABA. Compared to 1982–1999, since 2000, equatorial remote forcing had less influence on ABA SSTs, whereas local forcing has become more important. In particular, the robust correlation that existed between the equatorial zonal wind stress and the ABA SSTs has substantially weakened, suggesting less influence of Kelvin waves on ABA SSTs. Moreover, the strong correlation linking the South Atlantic Anticyclone and the ABA SSTs has reduced. Finally, multidecadal surface warming of the ABA could also have played a role in the weakening of the interannual SST variability.

Interannual variability in the barrier layer and forcing mechanism in the eastern equatorial Indian Ocean and Bay of Bengal

Interannual variability (IAV) in the barrier layer thickness (BLT) and forcing mechanisms in the eastern equatorial Indian Ocean (EEIO) and Bay of Bengal (BoB) are examined using monthly Argo data sets during 2002–2017. The BLT during November–January (NDJ) in the EEIO shows strong IAV, which is associated with the Indian Ocean dipole mode (IOD), with the IOD leading the BLT by two months. During the negative IOD phase, the westerly wind anomalies driving the downwelling Kelvin waves increase the isothermal layer depth (ILD). Moreover, the variability in the mixed layer depth (MLD) is complex. Affected by the Wyrtki jet, the MLD presents negative anomalies west of 85°E and strong positive anomalies between 85°E and 93°E. Therefore, the BLT shows positive anomalies except between 86°E and 92°E in the EEIO. Additionally, the IAV in the BLT during December–February (DJF) in the BoB is also investigated. In the eastern and northeastern BoB, the IAV in the BLT is remotely forced by equatorial zonal wind stress anomalies associated with the El Nino-Southern Oscillation (ENSO). In the western BoB, the regional surface wind forcing-related ENSO modulates the BLT variations.

Atmospheric Convection Dominates Genesis of ENSO Asymmetry

AbstractObserved El Niño–Southern Oscillation (ENSO) events exhibit distinct amplitude and evolution asymmetries due to nonlinear interactions between sea surface temperature (SST), equatorial zonal wind stress, and thermocline depth anomalies. Establishment of atmospheric convection leads to a nonlinear response of zonal winds to SST, but its relative importance to nonlinear oceanic feedbacks remains unclear. Using a recharge oscillator model modified to incorporate various nonlinearities estimated from observations, we show that the nonlinear wind response to SST alone can induce a nonlinear response of the thermocline to winds and that of SST to the thermocline, as winds affect the growth of SST and damping of thermocline anomalies through the recharge/discharge process. Oceanic nonlinear feedbacks enhance the simulated ENSO asymmetries only when the nonlinear atmospheric feedback is present. Our result suggests that the observed strengthening of ENSO variability in the past century is attributable to an increase in the nonlinear atmospheric feedback.

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

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.

Statistical predictability of Ni\xf1o indices for two types of ENSO

The El Niño-Southern Oscillation (ENSO) has been shown to manifest as primarily two types, the eastern Pacific (EP) type and central Pacific (CP) type, in terms of the zonal positions of the sea surface temperature (SST) anomalies. This study focuses on examining the predictability of the two types of ENSO by developing statistical models for their corresponding Niño indices, which have their own distinct key precursors. The results show that the statistical predictability of the Niño indices representing the two types of ENSO primarily originates from the preceding variations in the equatorial Pacific upper-ocean heat content and the surface zonal wind stress, which intrinsically reflect the zonally uniform and contrasted thermocline patterns, respectively. The traditional Niño3 and Niño4 indices are more predictive than the Niño indices of the EP and CP ENSO types; however, all the indices are subject to predictability barriers with different timings and intensities, which might be weakened by introducing additional external precursors. The EP ENSO indices have overall higher skills than the CP indices, in which the statistical model has much higher skill scores than persistence forecast for the EP ones while it does less for the CP ones. We demonstrate that the precursors outside the tropical Pacific, e.g., the Indian Ocean Dipole, North Pacific oscillation, North American dipole, and Southern Hemispheric SST modes, except the northern tropical Atlantic SST, as suggested in previous studies, only make limited contributions to improving the prediction skills of the two ENSO types at specific initial months and leads compared to a benchmark model built using the equatorial Pacific heat content and zonal wind stress indices. This is primarily because these precursors have already transferred most of their signals into the variation of the two indices in the benchmark model. We further show that conditionally adding the northern tropical Atlantic SST precursor to the benchmark could provide considerable additional prediction skill scores for both types of ENSO and weaken the intensity of the ENSO predictability barriers that occur during boreal spring‒summer.

The dominant role of the atmospheric component of coupled models in ENSO amplitude simulations

To explore the relative contributions of the atmospheric and oceanic components of coupled models to ENSO amplitude simulations, we innovatively “assembled” four coupled models and performed analyses on their ENSO simulations. Specifically, the atmospheric and oceanic components of two commonly used coupled models are cross-coupled to construct four parent models. Based on the simulated ENSO amplitude, the four parent models are classified into two groups: Grid-point Atmospheric Model of IAP LASG Version 2 (GAMIL2)-based models whose ENSO amplitudes are comparable to (although slightly weaker than) observations, and Community Atmosphere Model Version 4.0 (CAM4)-based models whose ENSO amplitudes reach up to twice those of observations. The BJ-index analysis reveals that the atmospheric components modulate ENSO amplitude by affecting the atmospheric thermodynamic (TD) feedback and the oceanic thermocline (TH) feedback. The TD feedback biases in the CAM4-based models are attributable to an overly negative low-cloud fraction feedback and low-cloud liquid water feedback in the Nino-3 region. The underestimated TH feedback in the GAMIL2-based models is due to an underestimated mean upwelling ( $$\bar {w}$$ ), while the seemingly accurate TH term in the CAM4-based models is the result of compensation by an overestimated regression of zonal tilt of the thermocline on the equatorial zonal wind stress ( $${\beta _h}$$ ) and an underestimated $$\bar {w}$$ . Furthermore, $${\beta _h}$$ dominates TH differences in the two atmospheric groups, and is mainly associated with the normalized wind stress anomaly over the Nino-4 region and the vertical ocean subsurface temperature structure.

A Role for the Equatorial Undercurrent in the Ocean Dynamical Thermostat

AbstractReconstructions 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.

Role of atmosphere-ocean interactions in supermodeling the tropical Pacific climate.

The supermodel strategy interactively combines several models to outperform the individual models comprising it. A key advantage of the approach is that nonlinear improvements can be achieved, in contrast to the linear weighted combination of individual unconnected models. This property is found in a climate supermodel constructed by coupling two versions of an atmospheric model differing only in their convection scheme to a single ocean model. The ocean model receives a weighted combination of the momentum and heat fluxes. Optimal weights can produce a supermodel with a basic state similar to observations: a single Intertropical Convergence zone (ITCZ), with a western Pacific warm pool and an equatorial cold tongue. This is in stark contrast to the erroneous double ITCZ pattern simulated by both of the two stand-alone coupled models. By varying weights, we develop a conceptual scheme to explain how combining the momentum fluxes of the two different atmospheric models affects equatorial upwelling and surface wind feedback so as to give a realistic basic state in the tropical Pacific. In particular, we propose a mechanism based on the competing influences of equatorial zonal wind and off-equatorial wind stress curl in driving equatorial upwelling in the coupled models. Our results show how nonlinear ocean-atmosphere interaction is essential in combining these two effects to build different sea surface temperature structures, some of which are realistic. They also provide some insight into observed and modelled tropical Pacific climate.

Feedback process responsible for intermodel diversity of ENSO variability

AbstractThe origin of the intermodel diversity of the El Niño–Southern Oscillation (ENSO) variability is investigated by applying a singular value decomposition (SVD) analysis between the intermodel tropical Pacific sea surface temperature anomalies (SSTA) variance and the intermodel ENSO stability index (BJ index). The first SVD mode features an ENSO‐like pattern for the intermodel SSTA variance (74% of total variance) and the dominant thermocline feedback (TH) for the BJ index (51%). Intermodel TH is mainly modified by the intermodel sensitivity of the zonal thermocline gradient response to zonal winds over the equatorial Pacific (βh), and the intermodel βh is correlated higher with the intermodel off‐equatorial wind stress curl anomalies than the equatorial zonal wind stress anomalies. Finally, the intermodel off‐equatorial wind stress curl is associated with the meridional shape and intensity of ENSO‐related wind patterns, which may cause a model‐to‐model difference in ENSO variability by influencing the off‐equatorial oceanic Rossby wave response.

Do regions outside the tropical Pacific influence ENSO through atmospheric teleconnections?

This paper aims at identifying oceanic regions outside the tropical Pacific, which may influence the El Nino Southern Oscillation (ENSO) through interannual modulation of equatorial Pacific winds. An Atmospheric General Circulation Model (AGCM) 7-members ensemble experiment forced by climatological sea surface temperature (hereafter, SST) in the tropical Pacific Ocean and observed interannually varying SST elsewhere produces ensemble-mean equatorial zonal wind stress interannual anomalies (ZWSA) over the equatorial Pacific. These ZWSA are largest during boreal winter in the western Pacific, and induce a ~0.5 °C response in the central Pacific during the following spring in a simple ocean model, that weakly but significantly correlates with the following ENSO peak amplitude. When correlated with global SST, the residual western equatorial Pacific ZWSA yield SST patterns that are reminiscent of ENSO teleconnections in the Indian, North and South Pacific, and Atlantic Oceans. We further design 20-members ensemble sensitivity experiments forced by typical SST patterns of the main climate modes for each of these regions, in order to identify regions that influence equatorial Pacific ZWSA most. In our experiments, only the Indian Ocean Basin-wide SST warming in late boreal winter produces a statistically significant ZWSA in the western equatorial Pacific, resulting in a weak but significant ~0.35 °C SST response in the central Pacific (i.e. ~35 % of the observed standard deviation) during the following spring, the season when the Bjerkness coupled feedback is particularly efficient. This paper hence agrees with previous studies, which suggest that ENSO-induced basin-wide SST signals in the Indian Ocean may contribute to the phase transition of ENSO. Our results suggest that studies exploring external influences on ENSO should adopt a global approach rather than focus on a specific region. Designing coupled model simulations would also allow investigating air–sea interactions-mediated teleconnection mechanisms, which we can’t reproduce in our forced AGCM framework.

Process-based analysis of climate model ENSO simulations: Intermodel consistency and compensating errors

Systematic and compensating errors can lead to degraded predictive skill in climate models. Such errors may be identified by comparing different models in an analysis of individual physical processes. We examine model simulations of El Nino–Southern Oscillation (ENSO) in five Coupled Model Intercomparison Project (CMIP) models, using transfer functions to analyze nine processes critical to ENSO's dynamics. The input and output of these processes are identified and analyzed, some of which are motivated by the recharge oscillator theory. Several errors and compensating errors are identified. The east-west slope of the equatorial thermocline is found to respond to the central equatorial Pacific zonal wind stress as a damped driven harmonic oscillator in all models. This result is shown to be inconsistent with two different formulations of the recharge oscillator. East Pacific sea surface temperature (SST) responds consistently to changes in the thermocline depth in the eastern Pacific in the five CMIP models examined here. However, at time scales greater than 2 years, this consistent model response disagrees with observations, showing that the SST leads thermocline depth at long time scales. Compensating errors are present in the response of meridional transport of water away from the equator to SST: two different models show different response of the transport to off-equatorial wind curl and wind curl response to East Pacific SST. However, these two models show the same response of meridional transport to East Pacific SST. Identification of errors in specific physical processes can hopefully lead to model improvement by focusing model development efforts on these processes.

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

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.

Observed intraseasonal thermocline variability in the Bay of Bengal

The time series of temperature data obtained from moored buoys deployed at 8°N, 12°N, and 15°N along 90°E in the Bay of Bengal (BoB) shows a persistent intraseasonal variability on 30-120 day time scale in three distinct periods 30-70 day, near 90 day, and near 120 day in the thermocline region. The standard deviation of moored buoy temperature data shows that half of the variability in the thermocline region is contributed from the 30-120 day variability. The relative contribution of local Ekman pumping velocity and remote wind forcing from equatorial Indian Ocean (EIO) to the intraseasonal thermocline variability in the BoB is examined using satellite-derived sea surface height anomaly (SSHA), wind and depths of 23° isotherm (D23, proxy for thermocline depth) derived from moored buoys temperature data. The analysis shows that large amplitude intraseasonal oscillations of thermocline - particularly the near 90 day and 120 day variability - could not be explained by local Ekman pumping velocity alone. The SSHA, D23, and wind fields reveal that the first and second baroclinic mode Kelvin and Rossby waves, which are generated remotely by winds from the EIO and eastern BoB, can significantly influence the thermocline variability in the BoB. The near 90 day and 120 day thermocline variability is driven primarily by the variability of equatorial zonal wind stress. While the 30-70 day thermocline variability is affected most by interior Ekman pumping over the Bay, it also appears to be influenced by zonal wind stress in the EIO and alongshore wind stress in the eastern BoB.

Marine Downscaling of a Future Climate Scenario for Australian Boundary Currents

Abstract Ocean boundary currents are poorly represented in existing coupled climate models, partly because of their insufficient resolution to resolve narrow jets. Therefore, there is limited confidence in the simulated response of boundary currents to climate change by climate models. To address this issue, the eddy-resolving Ocean Forecasting Australia Model (OFAM) was used, forced with bias-corrected output in the 2060s under the Special Report on Emissions Scenarios (SRES) A1B from the CSIRO Mark version 3.5 (Mk3.5) climate model, to provide downscaled regional ocean projections. CSIRO Mk3.5 captures a number of robust changes that are common to most climate models that are consistent with observed changes, including the weakening of the equatorial Pacific zonal wind stress and the strengthening of the wind stress curl in the Southern Pacific, important for driving the boundary currents around Australia. The 1990s climate is downscaled using air–sea fluxes from the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40). The current speed, seasonality, and volume transports of the Australian boundary currents show much greater fidelity to the observations in the downscaled model. Between the 1990s and the 2060s, the downscaling with the OFAM simulates a 15% reduction in the Leeuwin Current (LC) transport, a 20% decrease in the Indonesian Throughflow (ITF) transport, a 12% increase in the East Australian Current (EAC) core transport, and a 35% increase in the EAC extension. The projected changes by the downscaling model are consistent with observed trends over the past several decades and with changes in wind-driven circulation derived from Sverdrup dynamics. Although the direction of change projected from downscaling is usually in agreement with CSIRO Mk3.5, there are important regional details and differences that will impact the response of ecosystems to climate change.

ENSO and Pacific Decadal Variability in the Community Climate System Model Version 4

Abstract This study presents an overview of the El Niño–Southern Oscillation (ENSO) phenomenon and Pacific decadal variability (PDV) simulated in a multicentury preindustrial control integration of the NCAR Community Climate System Model version 4 (CCSM4) at nominal 1° latitude–longitude resolution. Several aspects of ENSO are improved in CCSM4 compared to its predecessor CCSM3, including the lengthened period (3–6 yr), the larger range of amplitude and frequency of events, and the longer duration of La Niña compared to El Niño. However, the overall magnitude of ENSO in CCSM4 is overestimated by ~30%. The simulated ENSO exhibits characteristics consistent with the delayed/recharge oscillator paradigm, including correspondence between the lengthened period and increased latitudinal width of the anomalous equatorial zonal wind stress. Global seasonal atmospheric teleconnections with accompanying impacts on precipitation and temperature are generally well simulated, although the wintertime deepening of the Aleutian low erroneously persists into spring. The vertical structure of the upper-ocean temperature response to ENSO in the north and south Pacific displays a realistic seasonal evolution, with notable asymmetries between warm and cold events. The model shows evidence of atmospheric circulation precursors over the North Pacific associated with the “seasonal footprinting mechanism,” similar to observations. Simulated PDV exhibits a significant spectral peak around 15 yr, with generally realistic spatial pattern and magnitude. However, PDV linkages between the tropics and extratropics are weaker than observed.

The first coupled historical forecasting project (CHFP1)

A set of retrospective multi‐seasonal ensemble predictions based on a coupled global atmosphere‐ocean model is described. These predictions, designated as the first coupled Historical Forecasting Project or CHFP1, are produced with the climate model CGCM3.1 of the Canadian Centre for Climate Modelling and Analysis using a very simple initialization procedure in which model sea surface temperatures (SSTs) are nudged toward the observed values during a multi‐year period preceding the beginning of a forecast. This procedure, in addition to constraining initial SSTs to be close to observations, initializes equatorial Pacific zonal wind stress and thermocline depth with some skill. The ability of the subsequent forecasts to predict the evolution of SSTs, particularly in the equatorial Pacific, and surface air temperatures globally and in Canada is assessed. The results are compared with those of the second Historical Forecasting Project or HFP2, which uses a two‐tier strategy in which model SSTs are externally specified. Skill of CHFP1 forecasts, though generally modest, exceeds those of HFP2 in some cases, despite the larger multi‐model ensemble used in HFP2. CHFP1 represents an initial step in development directed toward a coupled operational seasonal prediction system for Canada.