ENSO-related Sea Surface Temperature Anomalies Research Articles

Distinct Impacts of ENSO on Haze Pollution in the Beijing–Tianjin–Hebei Region between Early and Late Winters

Abstract The Beijing–Tianjin–Hebei (BTH) region has encountered increasingly severe and frequent haze pollution during recent decades. This study reveals that El Niño–Southern Oscillation (ENSO) has distinctive impacts on interannual variations of haze pollution over BTH in early and late winters. The impact of ENSO on the haze pollution over the BTH is strong in early winter, but weak in late winter. In early winter, ENSO-related sea surface temperature anomalies generate double-cell Walker circulation anomalies, with upward motion anomalies over the tropical central-eastern Pacific and tropical Indian Ocean, and downward motion anomalies over the tropical western Pacific. The ascending motion and enhanced atmospheric heating anomalies over the tropical Indian Ocean trigger atmospheric teleconnection propagating from the north Indian Ocean to East Asia, and result in the generation of an anticyclonic anomaly over Northeast Asia. The associated southerly anomalies to the west side lead to more serious haze pollution via reducing surface wind speed and increasing low-level humidity and the thermal inversion. The strong contribution of the Indian Ocean heating anomalies to the formation of the anticyclonic anomaly over Northeast Asia in early winter can be confirmed by atmospheric model numerical experiments. In late winter, vertical motion and precipitation anomalies are weak over the tropical Indian Ocean related to ENSO. As such, ENSO cannot induce a clear anticyclonic anomaly over Northeast Asia via atmospheric teleconnection, and thus has a weak impact on the haze pollution over BTH. Further analysis shows that stronger ENSO-induced atmospheric heating anomalies over the tropical Indian Ocean in early winter are partially due to higher mean SST and precipitation there. Significance Statement There exist large discrepancies regarding the contribution of El Niño–Southern Oscillation (ENSO) events to the wintertime haze pollution over North China. Several studies have indicated that ENSO has a weak impact on the haze pollution over North China. However, some studies have argued that ENSO events can exert impacts on the occurrence of haze pollution over North China. In this study, we present evidence to demonstrate that ENSO has distinctive impacts on interannual variations of the haze pollution over the Beijing–Tianjin–Hebei (BTH) region in North China in early and late winters. Specifically, ENSO has a strong impact on the haze pollution over BTH in early winter, whereas the impact of ENSO on the haze pollution over BTH is fairly weak in late winter. Results of this study could reconcile the discrepancy of previous studies about the impact of ENSO on the haze pollution over North China.

Remote Effects of IOD and ENSO on Motivating the Atmospheric Pattern Favorable for Snowfall Over the Tibetan Plateau in Early Winter

The interannual variation of snowfall over the Tibetan Plateau (TP) in early winter (November–December) and its related atmospheric attribution are clarified. Meanwhile, the influence of tropical sea surface temperatures (SSTs) on TP snowfall is investigated by diagnostic analyses and Community Atmosphere Model (CAM5) simulations. The leading mode of TP snowfall in early winter features a spatially uniform pattern with remarkable interannual variability. It is found that the Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO) are main external forcing factors for TP snowfall. Positive IOD with positive ENSO and positive IOD with neutral ENSO cases both have remote impact on motivating Southern Eurasia (SEA) pattern, which can induce an anomalous cyclone around the TP. The corresponding anomalous ascending motion and cold air in the mid-upper troposphere provide the dynamical and thermal conditions for heavy snowfall. The low-level southwesterly winds are enhanced over the Arabian Sea and Bay of Bengal, bringing abundant water vapor into the TP for excessive snowfall. Furthermore, CAM5 simulation experiments forced by IOD- and ENSO-related SST anomalies are performed to verify their combined and independent effects on TP snowfall in early winter. It is confirmed that either positive IOD or El Niño has certain impacts on motivating circulation anomalies favorable for snowfall over the TP. However, IOD plays a leading role in producing the excessive snowfall-related atmospheric conditions, and there is an asymmetric influence of ENSO and IOD on the TP snowfall.

El Niño–Southern Oscillation signal in a new East Antarctic ice core, Mount Brown South

Abstract. Paleoclimate archives, such as high-resolution ice core records, provide a means to investigate past climate variability. Until recently, the Law Dome (Dome Summit South site) ice core record remained one of few millennial-length high-resolution coastal records in East Antarctica. A new ice core drilled in 2017/2018 at Mount Brown South, approximately 1000 km west of Law Dome, provides an additional high-resolution record that will likely span the last millennium in the Indian Ocean sector of East Antarctica. Here, we compare snow accumulation rates and sea salt concentrations in the upper portion (∼ 20 m) of three Mount Brown South ice cores and an updated Law Dome record over the period 1975–2016. Annual sea salt concentrations from the Mount Brown South site record preserve a stronger signal for the El Niño–Southern Oscillation (ENSO; austral winter and spring, r = 0.533, p < 0.001, Multivariate El Niño Index) compared to a previously defined Law Dome record of summer sea salt concentrations (November–February, r = 0.398, p = 0.010, Southern Oscillation Index). The Mount Brown South site record and Law Dome record preserve inverse signals for the ENSO, possibly due to longitudinal variability in meridional transport in the southern Indian Ocean, although further analysis is needed to confirm this. We suggest that ENSO-related sea surface temperature anomalies in the equatorial Pacific drive atmospheric teleconnections in the southern mid-latitudes. These anomalies are associated with a weakening (strengthening) of regional westerly winds to the north of Mount Brown South that correspond to years of low (high) sea salt deposition at Mount Brown South during La Niña (El Niño) events. The extended Mount Brown South annual sea salt record (when complete) may offer a new proxy record for reconstructions of the ENSO over the recent millennium, along with improved understanding of regional atmospheric variability in the southern Indian Ocean, in addition to that derived from Law Dome.

Impact of Annual Cycle on ENSO Variability and Predictability

AbstractLow-order linear inverse models (LIMs) have been shown to be competitive with comprehensive coupled atmosphere–ocean models at reproducing many aspects of tropical oceanic variability and predictability. This paper presents an extended cyclostationary linear inverse model (CS-LIM) that includes the annual cycles of the background state and stochastic forcing of tropical sea surface temperature (SST) and sea surface height (SSH) anomalies. Compared to a traditional stationary LIM that ignores such annual cycles, the CS-LIM is better at representing the seasonal modulation of ENSO-related SST anomalies and their phase locking to the annual cycle. Its deterministic as well as probabilistic hindcast skill is comparable to the skill of the North American Multimodel Ensemble (NMME) of comprehensive global coupled models. The explicit inclusion of annual-cycle effects in the CS-LIM improves the forecast skill of both SST and SSH anomalies through SST–SSH coupling. The impact on the SSH skill is particularly marked at longer forecast lead times over the western Pacific and in the vicinity of the Pacific North Equatorial Countercurrent (NECC), consistent with westward propagating oceanic Rossby waves that reflect off the western boundaries as eastward propagating Kelvin waves and influence El Niño development in the region. The higher CS-LIM skill is thus associated with the improved representation of both ENSO phase-locking and Pacific NECC variations. These improvements result from explicitly accounting for not only the annual cycle of the background state, but also that of the stochastic forcing.

Enhanced mid-to-late winter predictability of the storm track variability in the North Pacific as a contrast with the North Atlantic

The storm tracks are a major driver of regional extreme weather events. Using the daily output of reanalysis and a latest generation ensemble seasonal forecasting system, this study examines the interannual variability and predictability of the boreal winter storm tracks in the North Pacific and North Atlantic. In both basins, the leading mode of storm track variability describes a latitudinal shifting of the climatological storm tracks. The shifting mode is closely connected with the extratropical large-scale teleconnection patterns (i.e. Pacific-North America teleconnection and North Atlantic Oscillation).The main predictability source for the shifting mode of the North Pacific storm tracks are the ENSO-related sea surface temperature anomalies. Assessment of the seasonal prediction skill further shows that the shifting mode of the North Pacific storm tracks is in general better predicted than that of the North Atlantic storm tracks likely due to stronger ENSO effects.Our analyses also find that, through the modulations of ENSO and the subtropical jet, the shifting mode of the North Pacific storm tracks exhibit a mid-to-late winter predictability enhancement. During El Niño phases, the North Pacific subtropical jet shifts equatorward and becomes strongest in mid-to-late winter, which dominates the upper-level flow and guides the storm track most equatorward. We argue that the intensification and equatorward shift of the North Pacific subtropical jet in mid-to-late winter of El Niño years provide the main reason for the increased mid-to-late winter predictability for the storm tracks. The results imply that good representation of the background subtropical jet in models is important for winter climate prediction of storm tracks.

One-Month-Lead Predictability of Asian Summer Monsoon Indices Based on the Zonal Winds by the APCC Multimodel Ensemble

The seasonal predictability of Asian summer monsoon indices characterizing horizontal and vertical zonal wind shear is investigated using 1-month-lead hindcast datasets from seven coupled global circulation models (CGCMs) participating in the operational multimodel ensemble (MME) seasonal prediction system of the Asia–Pacific Economic Cooperation Climate Center (APCC) for the 1983–2010 period. The summer monsoon indices analyzed in this study represent the South Asian summer monsoon (SASM), western North Pacific summer monsoon (WNPSM), and the newly defined northeastern Asian summer monsoon (NEASM). For the WNPSM and NEASM indices, we also analyze the prediction skill of the index components separately. The study demonstrates that the operational APCC MME system reliably predicts most of the summer monsoon indices and their components, with correlation coefficients exceeding the 99% confidence level. Analysis of the ocean sources of the prediction skill of the indices reveals that the strong relationships of most of the monsoon indices and their components with sea surface temperature (SST) are not confined to the equatorial Pacific but rather are dispersed throughout the World Ocean, with the leading role played by the north Indian Ocean SST anomalies. This conclusion is supported by the analysis of correlations between the monsoon indices and the tropical SST indices. The correlations between the SST anomalies and all the summer monsoon indices in the MME predictions are stronger than those in the observations. However, overestimation of the role of the ENSO-related SST anomalies in the seasonal model hindcasts results in some predictability deterioration of the SASM and NEASM indices.

El Niño–Southern Oscillation–like variability in a late Miocene Caribbean coral

Reconstructions of Pliocene sea-surface temperature (SST) gradients and thermocline depths suggest that the zonal temperature gradient of the tropical Pacific was distinct from the modern. However, the nature of any El Nino–Southern Oscillation (ENSO) variability superimposed on this mean state is difficult to determine. We developed monthly resolved multidecadal stable isotopic time series from an extremely well preserved central Caribbean coral dating to the Miocene-Pliocene transition, prior to closure of the Central American Seaway (CAS). Paleoceanographic modeling suggests that the flow of water associated with El Nino and La Nina events through the CAS allowed Caribbean corals to record the ENSO-related SST anomalies. Spectral analysis of coral oxygen isotope ratios reveals periodicities similar to modern ENSO signatures, suggesting that ENSO-like variability characterized the Miocene-Pliocene transition.

Impact of Two Distinct Teleconnection Patterns Induced by Western Central Pacific SST Anomalies on Korean Temperature Variability during the Early Boreal Summer

Abstract This study investigates the impact of western central Pacific sea surface temperatures (SSTs) on the temperature variability over the Korea Peninsula during the early boreal summer season. The authors found a significant positive correlation between June temperature anomalies across the Korea Peninsula and a horseshoe pattern of ENSO-related SST anomalies in the western central Pacific during May. While this SST pattern is relatively stationary throughout the boreal summer, the SST-driven atmospheric teleconnection exhibits dramatic subseasonal changes. During May, positive precipitation anomalies caused by SST warming in the northern central Pacific (NCP; 20°–30°N, 160°E–160°W) induce a wave train extending in a northwest–southeast direction. During June, while this wave train pattern is weakened, a Pacific–Japan (PJ) pattern is formed by positive precipitation anomalies over the off-equatorial western Pacific (off-WP). Even though dramatic subseasonal changes exist in the teleconnection patterns, both patterns contribute to temperature warming over Korea. It is shown that the subseasonal change in the atmospheric teleconnection is caused by changes in climatological precipitation. During May, significant climatological precipitation related to the pre-mei-yu and pre-baiu fronts occurs over the NCP. These wet background conditions provide a favorable environment for anomalous convection over the NCP but not over the off-equatorial western Pacific. On the other hand, during June, convective activities over the off-WP result in the formation of a PJ pattern, enhanced by an increase in climatological precipitation associated with the activation of the western North Pacific (WNP) monsoon. These results indicate that climatological conditions play an important role in the formation of atmospheric teleconnection linking Pacific and East Asian climates.

The Annual-Cycle Modulation of Meridional Asymmetry in ENSO’s Atmospheric Response and Its Dependence on ENSO Zonal Structure

Abstract Previous studies documented that a distinct southward shift of central Pacific low-level wind anomalies occurring during the ENSO decaying phase is caused by an interaction between the western Pacific annual cycle and El Niño–Southern Oscillation (ENSO) variability. The present study finds that the meridional movement of the central Pacific wind anomalies appears only during traditional eastern Pacific El Niño (EP El Niño) events rather than in central Pacific El Niño (CP El Niño) events in which sea surface temperature (SST) anomalies are confined to the central Pacific. The zonal structure of ENSO-related SST anomalies therefore has an important effect on meridional asymmetry in the associated atmospheric response and its modulation by the annual cycle. In contrast to EP El Niño events, the SST anomalies of CP El Niño events extend farther west toward the warm pool region with its climatological warm SSTs. In the warm pool region, relatively small SST anomalies are thus able to excite convection anomalies on both sides of the equator, even with a meridionally asymmetric SST background state. Therefore, almost meridionally symmetric precipitation and wind anomalies are observed over the central Pacific during the decaying phase of CP El Niño events. The SST anomaly pattern of La Niña events is similar to CP El Niño events with a reversed sign. Accordingly, no distinct southward displacement of the atmospheric response occurs over the central Pacific during the La Niña decaying phase. These results have important implications for ENSO climate impacts over East Asia, since the anomalous low-level anticyclone over the western North Pacific is an integral part of the annual cycle–modulated ENSO response.

The impact of South Pacific extratropical forcing on ENSO and comparisons with the North Pacific

Previous studies suggest that North Pacific extratropical atmospheric variability influences ENSO via the seasonal footprinting mechanism (SFM). This study confirms that quadrapole sea surface temperature (SST) variability in the extratropical South Pacific triggered by mid-latitude South Pacific atmospheric variability may also have an additional influence on ENSO. The response of the evolution of the ENSO-related zonal wind and SST anomalies in the tropics to the South Pacific extratropical forcing is consistent with the SFM hypothesis. That is, the Pacific–South American (PSA) pattern of the South Pacific extratropical sea level pressure (SLP) anomalies imparts an SST footprint (i.e., a quadrapole SST pattern) onto the ocean during austral summer. This SST footprint subsequently forces the zonal wind anomalies along the equator in the following austral winter that ultimately result in ENSO events during the following austral summer via ocean–atmosphere coupling in the tropics. The present study demonstrates that the influences of extratropical atmospheric variability in the South Pacific and North Pacific on ENSO are different and relatively independent. It is possible that they may, together or separately, influence the occurrence of ENSO events, and the importance of the South Pacific forcing in initiating ENSO events is comparable with that of the North Pacific forcing. An empirical model was established to predict the Nino3.4 index based on the combined South Pacific and North Pacific signals, and results show that it can be used to produce skillful forecasts of the Nino3.4 index with a leading time of up to 1 year.

Cold Tongue and Warm Pool ENSO Events in CMIP5: Mean State and Future Projections

Abstract The representation of the El Niño–Southern Oscillation (ENSO) under historical forcing and future projections is analyzed in 34 models from the Coupled Model Intercomparison Project phase 5 (CMIP5). Most models realistically simulate the observed intensity and location of maximum sea surface temperature (SST) anomalies during ENSO events. However, there exist systematic biases in the westward extent of ENSO-related SST anomalies, driven by unrealistic westward displacement and enhancement of the equatorial wind stress in the western Pacific. Almost all CMIP5 models capture the observed asymmetry in magnitude between the warm and cold events (i.e., El Niños are stronger than La Niñas) and between the two types of El Niños: that is, cold tongue (CT) El Niños are stronger than warm pool (WP) El Niños. However, most models fail to reproduce the asymmetry between the two types of La Niñas, with CT stronger than WP events, which is opposite to observations. Most models capture the observed peak in ENSO amplitude around December; however, the seasonal evolution of ENSO has a large range of behavior across the models. The CMIP5 models generally reproduce the duration of CT El Niños but have biases in the evolution of the other types of events. The evolution of WP El Niños suggests that the decay of this event occurs through heat content discharge in the models rather than the advection of SST via anomalous zonal currents, as seems to occur in observations. No consistent changes are seen across the models in the location and magnitude of maximum SST anomalies, frequency, or temporal evolution of these events in a warmer world.

The seasonal footprinting mechanism in CFSv2: simulation and impact on ENSO prediction

The seasonal footprinting mechanism (SFM) is thought to be a pre-cursor to the El Nino Southern Oscillation (ENSO). Fluctuations in the North Pacific Oscillation (NPO) impact the ocean via surface heat fluxes during winter, leaving a sea-surface temperature (SST) “footprint” in the subtropics. This footprint persists through the spring, impacting the tropical Pacific atmosphere–ocean circulation throughout the following year. The simulation of the SFM in the National Centers for Environmental Prediction (NCEP)/Climate Forecast System, version 2 (CFSv2) is likely to have an impact on operational predictions of ENSO and potentially seasonal predictions in the United States associated with ENSO teleconnection patterns. The ability of the CFSv2 to simulate the SFM and the relationship between the SFM and ENSO prediction skill in the NCEP/CFSv2 are investigated. Results indicate that the CFSv2 is able to simulate the basic characteristics of the SFM and its relationship with ENSO, including extratropical sea level pressure anomalies associated with the NPO in the winter, corresponding wind and SST anomalies that impact the tropics, and the development of ENSO-related SST anomalies the following winter. Although the model is able to predict the correct sign of ENSO associated with the SFM in a composite sense, probabilistic predictions of ENSO following a positive or negative NPO event are generally less reliable than when the NPO is not active.

The effect of ENSO-induced rainfall and circulation changes on the direct and indirect radiative forcing from Indonesian biomass-burning aerosols

Abstract. Emissions of biomass-burning aerosols from the Indonesian region are known to vary in response to rainfall anomalies associated with the El Niño Southern Oscillation (ENSO). For the severe El Niño-related drought in 1997, there have been several attempts to estimate the direct radiative forcing from increased aerosol emissions over Indonesia, as well as the associated feedbacks on climate. However, these estimates have not considered indirect aerosol effects. Another question that has not been addressed is whether the effect of ENSO-related circulation and rainfall anomalies on radiative forcing is significant relative to the effect of changes in emissions. In this study, we analyse the direct and first indirect radiative forcing from El Niño-related increased emissions of Indonesian biomass-burning aerosols, with and without the influence of ENSO-related rainfall and circulation anomalies. We compare two experiments that are performed with the CSIRO-Mk3.6 atmospheric global climate model (GCM). The first experiment (AMIP) consists of a pair of runs that respectively represent El Niño and La Niña conditions. In these runs, the distribution of aerosols is simulated under the influence of realistic Indonesian biomass-burning aerosol emissions and sea surface temperatures (SSTs) for 1997 (El Niño) and 2000 (La Niña). The second experiment (CLIM) is identical to AMIP, but is forced by climatological SSTs, so that in CLIM meteorological differences between 1997 and 2000 are suppressed. The comparison of AMIP and CLIM shows that the aerosol radiative forcing anomalies associated with ENSO (El Niño minus La Niña) are substantially stronger when ENSO-related SST anomalies are taken into account. For the first indirect effect, the influence of SST-induced changes in rainfall and circulation exceeds that of changes in emissions. For the direct aerosol forcing, the influence of changes in SSTs and emissions are of comparable magnitude. Averaged over the Indonesian region (5.6° N–11.2° S, 96.6° E–150.9° E), the first indirect forcing is −0.7 Wm−2 in CLIM and −2.2 Wm−2 in AMIP during the months July to November. The direct aerosol forcing at the top of the atmosphere (surface) is −1.0 (−5.3) Wm−2 in CLIM and −1.8 (−9.1) Wm−2 in AMIP during the same period. Our results suggest that (a) the indirect aerosol effect from biomass-burning aerosols is strong enough to play an important role for impact assessments, and (b) that impacts of biomass-burning aerosols would be considerably underestimated if feedbacks of ENSO-related SST variations on radiative forcing are not taken into account.

ENSO’s Impact on the Gap Wind Regions of the Eastern Tropical Pacific Ocean*

Abstract The recently released NCEP Climate Forecast System Reanalysis (CFSR) is used to examine the response to ENSO in the northeast tropical Pacific Ocean (NETP) during 1979–2009. The normally cool Pacific sea surface temperatures (SSTs) associated with wind jets through the gaps in the Central American mountains at Tehuantepec, Papagayo, and Panama are substantially warmer (colder) than the surrounding ocean during El Niño (La Niña) events. Ocean dynamics generate the ENSO-related SST anomalies in the gap wind regions as the surface fluxes damp the SSTs anomalies, while the Ekman heat transport is generally in quadrature with the anomalies. The ENSO-driven warming is associated with large-scale deepening of the thermocline; with the cold thermocline water at greater depths during El Niño in the NETP, it is less likely to be vertically mixed to the surface, particularly in the gap wind regions where the thermocline is normally very close to the surface. The thermocline deepening is enhanced to the south of the Costa Rica Dome in the Papagayo region, which contributes to the local ENSO-driven SST anomalies. The NETP thermocline changes are due to coastal Kelvin waves that initiate westward-propagating Rossby waves, and possibly ocean eddies, rather than by local Ekman pumping. These findings were confirmed with regional ocean model experiments: only integrations that included interannually varying ocean boundary conditions were able to simulate the thermocline deepening and localized warming in the NETP during El Niño events; the simulation with variable surface fluxes, but boundary conditions that repeated the seasonal cycle, did not.

El-Nino Southern Oscillation simulated and predicted in SNU coupled GCMs

The characteristics of the El-Nino Southern Oscillation (ENSO) simulated in free integrations using two versions of the Seoul National University (SNU) ocean–atmosphere coupled global climate model (CGCM) are examined. A revised version of the SNU CGCM is developed by incorporating a reduced air–sea coupling interval (from 1 day to 2 h), a parameterization for cumulus momentum transport, a minimum entrainment rate threshold for convective plumes, and a shortened auto-conversion time scale of cloud water to raindrops. With the revised physical processes, lower tropospheric zonal wind anomalies associated with the ENSO-related sea surface temperature anomalies (SSTA) are represented with more realism than those in the original version. From too weak, the standard deviation of SST over the eastern Pacific becomes too strong in the revised version due to the enhanced air–sea coupling strength and intraseasonal variability associated with ENSO. From the oceanic side, the stronger stratification and the shallower-than-observed thermocline over the eastern Pacific also contribute to the excessive ENSO. The impacts of the revised physical processes on the seasonal predictability are investigated in two sets of the hindcast experiment performed using the two versions of CGCMs. The prediction skill measured by anomaly correlation coefficients of monthly-mean SSTA shows that the new version has a higher skill over the tropical Pacific regions compared to the old version. The better atmospheric responses to the ENSO-related SSTA in the revised version lead to the basin-wide SSTA maintained and developed in a manner that is closer to observations. The symptom of an excessively strong ENSO of the new version in the free integration is not prominent in the hindcast experiment because the thermocline depth over the eastern Pacific is maintained as initialized over the arc of time of the hindcast (7 months).

Delayed ENSO impact on spring precipitation over North/Atlantic European region

The delayed impact of winter sea-surface temperature (SST) anomalies in tropical Pacific on spring precipitation over the North Atlantic/European (NAE) region is examined using both measured and modeled data for the period 1901–2002. In an AMIP-type Atmospheric General Circulation Model (AGCM) ensemble, the observed delayed spring precipitation response in Europe to winter ENSO-related SST anomalies is well reproduced. A series of targeted AGCM/coupled GCM experiments are performed to further investigate the mechanisms for this delayed influence. It is found that late winter ENSO SST anomalies lead to the well-documented Rossby wave train arching from the Pacific into the Atlantic region. A positive (negative) ENSO event leads to a quasi-barotropic trough (ridge) in the North Atlantic region. The resulting wind and cloud changes cause anomalies in the surface heat fluxes that result in negative (positive) SST anomalies in the central North Atlantic and anomalies of the opposite sign further to the south. The SST anomalies persist into spring and the atmospheric response to these anomalies is an extension of the ENSO-induced trough (ridge) into the European region, leading to enhanced (reduced) moisture flux and low-level convergence (divergence) and thus positive (negative) precipitation anomalies. Although the signal is overall relatively weak (correlation coefficients of European spring rainfall with winter ENSO SSTs of about 0.3), a proper representation of the outlined mechanism in seasonal forecasting systems may lead to improved seasonal predictions.

An Analysis of ENSO Prediction Skill in the CFS Retrospective Forecasts

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.

Two-way coupling of an ENSO model to the global climate model CLIMBER-3 [formula omitted

We present a model study that investigates to what extent it is possible to introduce ENSO variability to an Earth system Model of Intermediate Complexity (EMIC). The Zebiak–Cane ENSO model is dynamically coupled to the EMIC CLIMBER-3 α , which by itself exhibits no interannual or multidecadal variability. ENSO variability is introduced to CLIMBER-3 α by adding ENSO-related sea surface temperature anomalies to the upper layers of the model ocean. For the other coupling direction, changes in the mean CLIMBER-3 α climate on decadal time scales are used to change the background state of the ENSO model, achieving a two-way coupling. We compare typical ENSO-related patterns of a fully coupled pre-industrial model run to reanalysis data and point out the possibilities and limitations of this model configuration. Although introduced ENSO-related SST anomalies and other related variables like the Southern Oscillation Index are well reproduced by the EMIC in the forcing domain, teleconnections to other regions are damped, especially in meridional direction. The reason for this limitation is the atmospheric model, which does not sufficiently resolve the necessary transport mechanisms. Despite this limitation the presented coupling method may still be a useful tool in combination with higher resolution atmospheric models as being in development for the successor model CLIMBER-3 and possibly other EMICs.

Observations of Large-Scale Ocean–Atmosphere Interaction in the Southern Hemisphere

Abstract The authors provide a detailed examination of observed ocean–atmosphere interaction in the Southern Hemisphere (SH). Focus is placed on the observed relationships between variability in SH extratropical sea surface temperature (SST) anomalies, the Southern Annular Mode (SAM), and the El Niño–Southern Oscillation (ENSO). Results are examined separately for the warm (November–April) and cold (May–October) seasons and for monthly and weekly time scales. It is shown that the signatures of the SAM and ENSO in the SH SST field vary as a function of season, both in terms of their amplitudes and structures. The role of surface turbulent and Ekman heat fluxes in driving seasonal variations in the SAM- and ENSO-related SST anomalies is investigated. Analyses of weekly data reveal that variability in the SAM tends to precede anomalies in the SST field by ∼1 week, and that the e-folding time scale of the SAM-related SST field anomalies is at least 4 months. The persistence of the SAM-related SST anomalies is consistent with the passive thermal response of the Southern Ocean to variations in the SAM, and seasonal variations in the persistence of the SAM-related SST anomalies are consistent with the seasonal cycle in the depth of the ocean mixed layer.

The Atmospheric Bridge: The Influence of ENSO Teleconnections on Air–Sea Interaction over the Global Oceans

During El Nino-Southern Oscillation (ENSO) events, the atmospheric response to sea surface temperature (SST) anomalies in the equatorial Pacific influences ocean conditions over the remainder of the globe. This connection between ocean basins via the ''atmospheric bridge'' is reviewed through an examination of previous work augmented by analyses of 50 years of data from the National Centers for Environmental Prediction- National Center for Atmospheric Research (NCEP-NCAR) reanalysis project and coupled atmospheric general circulation (AGCM)-mixed layer ocean model experiments. Observational and modeling studies have now established a clear link between SST anomalies in the equatorial Pacific with those in the North Pacific, north tropical Atlantic, and Indian Oceans in boreal winter and spring. ENSO-related SST anomalies also appear to be robust in the western North Pacific during summer and in the Indian Ocean during fall. While surface heat fluxes are the key component of the atmospheric bridge driving SST anomalies, Ekman transport also creates SST anomalies in the central North Pacific although the full extent of its impact requires further study. The atmospheric bridge not only influences SSTs on interannual timescales but also affects mixed layer depth (MLD), salinity, the seasonal evolution of upper-ocean temperatures, and North Pacific SST variability at lower fre- quencies. The model results indicate that a significant fraction of the dominant pattern of low-frequency (.10 yr) SST variability in the North Pacific is associated with tropical forcing. AGCM experiments suggest that the oceanic feedback on the extratropical response to ENSO is complex, but of modest amplitude. Atmosphere- ocean coupling outside of the tropical Pacific slightly modifies the atmospheric circulation anomalies in the Pacific-North America (PNA) region but these modifications appear to depend on the seasonal cycle and air- sea interactions both within and beyond the North Pacific Ocean.