The influence of tropical Pacific forcing on the Arctic Oscillation

The potential role of tropical Pacific forcing in driving the seasonal variability of the Arctic Oscillation (AO) is explored using both observational data and a simple general circulation model (SGCM). A lead–lag regression technique is first applied to the monthly averaged sea surface temperature (SST) and the AO index. The AO maximum is found to be related to a negative SST anomaly over the tropical Pacific three months earlier. A singular value decomposition (SVD) analysis is then performed on the tropical Pacific SST and the sea level pressure (SLP) over the Northern Hemisphere. An AO-like atmospheric pattern and its associated SST appear as the second pair of SVD modes. Ensemble integrations are carried out with the SGCM to test the atmospheric response to different tropical Pacific forcings. The atmospheric response to the linear fit of the model’s empirical forcing associated with the SST variability in the second SVD modes strongly projects onto the AO. Idealized thermal forcings are then designed based on the regression of the seasonally averaged tropical Pacific precipitation against the AO index. Results indicate that forcing anomalies over the western tropical Pacific are more effective in generating an AO-like response while those over the eastern tropical Pacific tend to produce a Pacific-North American (PNA)-like response. The physical mechanisms responsible for the energy transport from the tropical Pacific to the extratropical North Atlantic are investigated using wave activity flux and vorticity forcing formalisms. The energy from the western tropical Pacific forcing tends to propagate zonally to the North Atlantic because of the jet stream waveguide effect while the transport of the energy from the eastern tropical Pacific forcing mostly concentrates over the PNA area. The linearized SGCM results show that nonlinear processes are involved in the generation of the forced AO-like pattern.     

Northern poleward edge of regional Hadley cell over western Pacific during boreal winter: year-to-year variability,influence factors and associated winter climate anomalies

Compared to the zonal-mean Hadley cell (HC), our knowledge of the characteristics, influence factors and associated climate anomalies of the regional HC remains quite limited. Here, we examine interannual variability of the northern poleward HC edge over western Pacific (WPHCE) during boreal winter. Results suggest that interannual variability of the WPHCE is impacted by the El Niño-Southern Oscillation (ENSO) Modoki, North Pacific Oscillation (NPO) and North Atlantic Oscillation (NAO). The WPHCE tends to shift poleward during negative phase of the ENSO Modoki, and positive phases of the NPO and NAO, which highlights not merely the tropical forcing but also the extratropical signals that modulate the WPHCE. ENSO modoki, NPO and NAO modulate the WPHCE via inducing atmospheric anomalies over the western North Pacific. We further investigate the climatic impacts of the WPHCE on East Asia. The poleward shift of the northern descending branch of the WPHC results in anomalous upward (downward) motions and upper-level divergence (convergence) anomalies over south-central China (northern East-Asia), leading to increased (decreased) rainfall there. Moreover, pronounced cold surface air temperature anomalies appear over south-central China when the sinking branch of the WPHC moves poleward. Based on the temperature diagnostic analysis, negative surface temperature tendency anomalies over central China are mostly attributable to the cold zonal temperature advection and ascent-induced adiabatic cooling, while the negative anomalies over South China are largely due to the cold meridional temperature advection. These findings could improve our knowledge of the WPHCE variability and enrich the knowledge of forcing factors for East Asian winter climate.

     

The response of the North Pacific Decadal Variability to strong tropical volcanic eruptions

In this study, the effects of volcanic forcing on North Pacific climate variability, on interannual to decadal time scales, are examined using climate model simulations covering the last 600?years. The model used is the Bergen Climate Model, a fully coupled atmosphere–ocean general circulation model. It is found that natural external forcings, such as tropical strong volcanic eruptions (SVEs) and variations in total solar irradiance, play an important role in regulating North Pacific Decadal Variability (NPDV). In response to tropical SVEs the lower stratospheric pole–to–equator temperature gradient is enhanced. The North polar vortex is strengthened, which forces a significant positive Arctic Oscillation. At the same time, dipole zonal wind anomalies associated with strong polar vortex propagate downward from the lower stratosphere. Through positive feedbacks in the troposphere, the surface westerly winds across the central North Pacific are significantly weakened, and positive sea level pressure anomalies are formed in the North Pacific. This anomalous surface circulation results in changes in the net heat fluxes and the oceanic advection across the North Pacific. As a result of this, warm water converges in the subtropical western North Pacific, where the surface waters in addition are heated by significantly reduced latent and sensible heat fluxes from the ocean. In the eastern and high–latitude North Pacific the ocean loses more heat, and large–scale decreases in sea surface temperatures are found. The overall response of this chain of events is that the North Pacific enters a negative phase of the Pacific decadal oscillation (PDO), and this negative phase of the PDO is maintained for several years. It is thus concluded that the volcanic forcing plays a key role in the phasing of the PDO. The model results furthermore highlight the important role of troposphere–stratosphere coupling, tropical–extratropical teleconnections and extratropical ocean–atmosphere interactions for describing NPDV.     

Interannual variations and future change of wintertime extratropical cyclone activity over North America in CCSM3

Climatology and interannual variations of wintertime extratropical cyclone frequency in CCSM3 twentieth century simulation are compared with the NCEP/NCAR reanalysis during 1950–1999. CCSM3 can simulate the storm tracks reasonably well, although the model produces slightly less cyclones at the beginning of the Pacific and Atlantic storm tracks and weaker poleward deflection over the Pacific. As in the reanalysis, frequency of cyclones stronger than 980 hPa shows significant correlation with the Pacific/North America (PNA) teleconnection pattern over the Pacific region and with the North Atlantic Oscillation (NAO) in the Atlantic sector. Composite maps are constructed for opposite phases of El Nino-Southern Oscillation (ENSO) and the NAO and all anomalous patterns coincide with observed. One CCSM3 twenty-first century A1B scenario realization indicates there is significant increase in the extratropical cyclone frequency on the US west coast and decrease in Alaska. Meanwhile, cyclone frequency increases from the Great Lakes region to Quebec and decreases over the US east coast, suggesting a possible northward shift of the Atlantic storm tracks under the warmer climate. The cyclone frequency anomalies are closely linked to changes in seasonal mean states of the upper-troposphere zonal wind and baroclinicity in the lower troposphere. Due to lack of 6-hourly outputs, we cannot apply the cyclone-tracking algorithm to the other eight CCSM3 realizations. Based on the linkage between the mean state change and the cyclone frequency anomalies, it is likely a common feature among the other ensemble members that cyclone activity is reduced on the East Coast and in Alaska as a result of global warming.     

北大西洋年际变率的海气耦合模式模拟II：热带太平洋强迫

利用一个全球海气耦合模式(BCM),结合观测资料,讨论了热带太平洋强迫对北大西洋年际气候变率的影响。研究表明,BCM能够相对合理地模拟赤道太平洋的年际变率模态及相应的海温距平型和大气遥相关型,尽管其准3年的振荡周期过于规则。来自数值模式和观测上的证据都表明,北大西洋冬季海温的主导性变率模态,即自北而南出现的“- -”的海温距平型,受到来自热带太平洋强迫的显著影响,其正位相与赤道中东太平洋冷事件相对应。换言之,赤道太平洋暖事件的发生,在太平洋-北美沿岸激发出PNA遥相关型,进而通过在北大西洋产生类似NAO负位相的气压距平型,削弱本来与NAO正位相直接联系的三核型海温距平。北大西洋三核型海温距平对热带太平洋强迫的响应,要滞后2—3个月的时间。     

A summer teleconnection pattern over the extratropical Northern Hemisphere and associated mechanisms

Using monthly data from the European Center for Medium-Range Weather Forecast 40-year reanalysis (ERA-40), we have revealed a teleconnection pattern over the extratropical Northern Hemisphere through the empirical orthogonal function analysis of summer upper-tropospheric eddy temperature. When temperature is higher (lower) over the Eastern Hemisphere (EH), it is lower (higher) over the Western Hemisphere (WH). The teleconnection manifested by this out-of-phase relationship is referred to as the Asian–Pacific oscillation (APO). The values of an index measuring the teleconnection are high before 1976 and low afterwards, showing a downward trend of the stationary wave at a rate of 4% per year during 1958–2001. The index also exhibits apparent interannual variations. When the APO index is high, anomalous upper-tropospheric highs (lows) appear over EH (WH). The formation of APO is likely associated with a zonal vertical circulation in the troposphere. Unforced control runs of both the NCAR Community Atmospheric Model version 3 and the Community Climate System Model version 3 capture the major characteristics of the teleconnection pattern and its associated vertical structure. The APO variability is closely associated with sea surface temperature (SST) in the Pacific, with a significantly positive correlation between APO and SST in the extratropical North Pacific and a significantly negative correlation in the tropical eastern Pacific. Sensitivity experiments show that the anomalies of SST over these two regions influence the APO intensity, but their effects are opposite to each other. Compared to the observation, the positive and negative anomalous centers of the extratropical tropospheric temperature triggered by the SST anomalies have a smaller spatial scale.     

Atmospheric bridge on orbital time scales

Large-scale atmospheric patterns are examined on orbital timescales using a climate model which explicitly resolves the atmosphere–ocean–sea ice dynamics. It is shown that, in contrast to boreal summer where the climate mainly follows the local radiative forcing, the boreal winter climate is strongly determined by modulation of circulation modes linked to the Arctic Oscillation/North Atlantic Oscillation (AO/NAO) and the Northern/Southern Annular Modes. We find that during a positive phase of the AO/NAO the convection in the tropical Pacific is below normal. The related atmospheric circulation provides an atmospheric bridge for the precessional forcing inducing a non-uniform temperature anomalies with large amplitudes over the continents. We argue that this is important for mechanisms responsible for multi-millennial climate variability and glacial inception.     

The impact of combined ENSO and PDO on the PNA climate: a 1,000-year climate modeling study

This study analyzes the atmospheric response to the combined Pacific interannual ENSO and decadal–interdecadal PDO variability, with a focus on the Pacific-North American (PNA) sector, using a 1,000-year long integration of the Canadian Center for Climate Modelling and Analysis (CCCma) coupled climate model. Both the tropospheric circulation and the North American temperature suggest an enhanced PNA-like climate response and impacts on North America when ENSO and PDO variability are in phase. The anomalies of the centers of action for the PNA-like pattern are significantly different from zero and the anomaly pattern is field significant. In association with the stationary wave anomalies, large stationary wave activity fluxes appear in the mid-high latitudes originating from the North Pacific and flowing downstream toward North America. There are significant Rossby wave source anomalies in the extratropical North Pacific and in the subtropical North Pacific. In addition, the axis of the Pacific storm track shifts southward with the positive PNA. Atmospheric heating anomalies associated with ENSO variability are confined primarily to the tropics. There is an anomalous heating center over the northeast Pacific, together with anomalies with the same polarity in the tropical Pacific, for the PDO variability. The in-phase combination of ENSO and PDO would in turn provide anomalous atmospheric energy transports towards North America from both the Tropical Pacific and the North Pacific, which tends to favor the occurrence of stationary wave anomalies and would lead to a PNA-like wave anomaly structure. The modeling results also confirm our analysis based on the observational record in the twentieth century.     

Variability of seasonal-mean fields arising from intraseasonal variability. Part 3: Application to SH winter and summer circulations

A study has been made, using the National Centers for Environmental Prediction and National Center for Atmospheric Research re-analysis 500 hPa geopotential height data, to determine how intraseasonal variability influences, or can generate, coherent patterns of interannual variability in the extratropical summer and winter Southern Hemisphere atmospheric circulation. In addition, by separating this intraseasonal component of interannual variability, we also consider how slowly varying external forcings and slowly varying (interannual and longer) internal dynamics might influence the interannual variability of the Southern Hemisphere circulation. This slow component of interannual variation is more likely to be potentially predictable. How sea surface temperatures are related to the slow components is also considered. The four dominant intraseasonal modes of interannual variability have horizontal structures similar to those seen in both well-known intraseasonal dynamical modes and statistical modes of intraseasonal variability. In particular, they reflect intraseasonal variability in the high latitudes associated with the Southern Annular Mode, and wavenumber 4 (summer) and wavenumber 3 (winter) patterns associated with south Pacific regions of persistent anomalies and blocking, and possibly variability related to the Madden-Julian Oscillation (MJO). The four dominant slow components of interannual variability, in both seasons, are related to high latitude variability associated with the Southern Annular Mode, El Nino Southern Oscillation (ENSO) variability, and South Pacific Wave variability associated with Indian Ocean SSTs. In both seasons, there are strong linear trends in the first slow mode of high latitude variability and these are shown to be related to similar trends in the Indian Ocean. Once these are taken into account there is no significant sea surface temperature forcing of these high latitude modes. The second and third ENSO related slow modes, in each season, have high correlations with tropical sea surface temperature variability in the Pacific and Indian Oceans, both contemporaneously and at one season lag. The fourth slow mode has a characteristic South Pacific wave structure of either a wavenumber 4 (summer) or wavenumber 3 (winter) pattern, with strongest loadings in the South Pacific sector, and an association simultaneously with a dipole SST temperature gradient in the subtropical Indian Ocean.     

Impact of a projected future Arctic Sea Ice reduction on extratropical storminess and the NAO

The impact of a reduced Arctic sea ice cover on wintertime extratropical storminess is investigated by conducting atmospheric general circulation model (AGCM) experiments. The AGCM ECHAM5 is forced by the present and a projected future seasonal cycle of Arctic sea ice. In the experiment with projected sea-ice concentrations significant reductions in storminess were found during December and January in both midlatitudes and towards the Arctic. However, a substantially larger reduction in extratropical storminess was found in March, despite a smaller change in surface energy fluxes in March than in the other winter months. The projected decrease in storminess is also related to the negative phase of the North Atlantic Oscillation (NAO). The March response is consistent with a forcing from transient and quasi-stationary eddies associated with negative NAO events. The greater sensitivity to sea-ice anomalies in late winter sets this study apart from earlier ones.     

Interdecadal change in the relationship of southern China summer rainfall with tropical Indo-Pacific SST

The present study investigates the interdecadal change in the relationship between southern China (SC) summer rainfall and tropical Indo-Pacific sea surface temperature (SST). It is found that the pattern of tropical Indo-Pacific SST anomalies associated with SC summer rainfall variability tends to be opposite between the 1950–1960s and the 1980-1990s. Above-normal SC rainfall corresponds to warmer SST in the tropical southeastern Indian Ocean (SEIO) and cooler SST in the equatorial central Pacific (ECP) during the 1950–1960s but opposite SST anomalies in these regions during the 1980–1990s. A pronounced difference is also found in anomalous atmospheric circulation linking SEIO SST and SC rainfall between the two periods. In the 1950–1960s, two anomalous vertical circulations are present between ascent over SEIO and ascent over SC, with a common branch of descent over the South China Sea that is accompanied by an anomalous low-level anticyclone. In the 1980–1990s, however, a single anomalous vertical circulation directly connects ascent over SC to descent over SEIO. The change in the rainfall–SST relationship is likely related to a change in the magnitude of SEIO SST forcing and a change in the atmospheric response to the SST forcing due to different mean states. A larger SEIO SST forcing coupled with a stronger and more extensive western North Pacific subtropical high in recent decades induce circulation anomalies reaching higher latitudes, influencing SC directly. Present analysis shows that the SEIO and ECP SST anomalies can contribute to SC summer rainfall variability both independently and in concert. In comparison, there are more cases of concerted contributions due to the co-variability between the Indian and Pacific Ocean SSTs.     

The North Atlantic Oscillation—What Role for the Ocean?

The extent to which the North Atlantic Oscillation (NAO) is influenced by changes in the ocean state is an issue that has attracted much recent attention. Although there have been counter claims, the weight of evidence clearly suggests that forcing by the ocean of year-to-year changes in the NAO is a weak influence by comparison with atmospheric internal variability. The NAO is thus very different in character to the Southern Oscillation (SO), and its predictability—at least on seasonal-to-interannual timescales—is almost certainly much lower.Although weak, the influence of the ocean on the NAO is not negligible. In a previous study we found that wintertime North Atlantic climate, including the NAO, was significantly influenced by a tripole pattern of North Atlantic SST anomalies. Here we report the results of experiments to further elucidate the nature of this influence. We show that the tripole pattern induces a significant response both in the tropical Atlantic and at mid-to-high latitudes. The low latitude response is forced by the low latitude SST anomalies, but the high latitude response is influenced by the extratropical SST anomalies as well as those in the tropics. Furthermore, we find evidence of nonlinear interaction between the influence of the tropical and extratropical SST anomalies. Lastly, we investigate the feedback from the atmosphere onto the SST tripole. We find that the expected negative feedback is significantly modified at low latitudes by the dynamical response of the atmosphere.     

Association between Tropical Convection and Boreal Wintertime Extratropical Circulation in 1982／83 and 1988／89

Boreal wintertime extratropical circulation is studied in relation to the tropical convection during the 1982/83 El Nino and 1988/89 La Nina. The anomaly structure of 1982/83 and 1988/89 over the extratropics reveals remarkably different features as the longitudinal tropical forcing region changes. The Rossby wave source (Positive) shows the largest maximum over East Asia in both years due to the persistent heating from the western Pacific warm pool area. However, the sink term shows contrasting features over the subtropics and extratropics between the two years. In the El Nino year, enhanced tropical convection over the eastern Pacific produces the Rossby wave sink at 10?N and shifted eastward over the North Pacific, while in the La Nina year, the sink area is shifted westward over the North Pacific. The contrasting features between the two events in mean-eddy interaction appears especially over the downstream area of the East Asian Jet. The extension (retraction) of the meanflow eastward (westward) to     

Characteristics of the Asian–Pacific Oscillation in Boreal Summer Simulated by BCC_CSM with Different Horizontal Resolutions

The summer Asian–Pacific Oscillation(APO) is a major teleconnection pattern that reflects the zonal thermal contrast between East Asia and the North Pacific in the upper troposphere. The performance of Beijing Climate Center Climate System Models(BCC CSMs) with different horizontal resolutions, i.e., BCC CSM1.1 and BCC CSM1.1(m), in reproducing APO interannual variability, APO-related precipitation anomalies, and associated atmospheric circulation anomalies, is evaluated.The results show that BCC CSM1.1(m) can successfully capture the interannual variability of the summer APO index. It is also more capable in reproducing the APO's spatial pattern, compared to BCC CSM1.1, due to its higher horizontal resolution. Associated with a positive APO index, the northward-shifted and intensified South Asian high, strengthened extratropical westerly jet, and tropical easterly jet in the upper troposphere, as well as the southwesterly monsoonal flow over North Africa and the Indian Ocean in the lower troposphere, are realistically represented by BCC CSM1.1(m), leading to an improvement in reproducing the increased precipitation over tropical North Africa, South Asia, and East Asia, as well as the decreased precipitation over subtropical North Africa, Japan, and North America. In contrast, these features are less consistent with observations when simulated by BCC CSM1.1. Regression analysis further indicates that surface temperature anomalies over the North Pacific and the southern and western flanks of the Tibetan Plateau are reasonably reproduced by BCC CSM1.1(m), which contributes to the substantial improvement in the simulation of the characteristics of summer APO compared to that of BCC CSM1.1.     

Tropical–extratropical climate interaction as revealed in idealized coupled climate model experiments

Tropical–extratropical climate interactions are studied by idealized experiments with a prescribed 2°C SST anomaly at different latitude bands in a coupled climate model. Instead of focusing on intrinsic climate variability, this work investigates the mean climate adjustment to remote external forcing. The extratropical impact on tropical climate can be as strong as the tropical impact on extratropical climate, with the remote sea surface temperature (SST) response being about half the magnitude of the imposed SST change in the forcing region. The equatorward impact of extratropical climate is accomplished by both the atmospheric bridge and the oceanic tunnel. About two-thirds of the tropical SST change comes from the atmospheric bridge, while the remaining one-third comes from the oceanic tunnel. The equatorial SST increase is first driven by the reduced latent heat flux and the weakened poleward surface Ekman transport, and then enhanced by the decrease in subtropical cells’ strength and the equatorward subduction of warm anomalies. In contrast, the poleward impact of tropical climate is accomplished mainly by the atmospheric bridge, which is responsible for extratropical temperature changes in both the surface and subsurface. Sensitivity experiments also show the dominant role of the Southern Hemisphere oceans in the tropical climate change. CCR contribution number 829; DAS-PKU contribution number 002.     

ENSO forcing of the Northern Hemisphere climate in a large ensemble of model simulations based on a very long SST record

The January–March (JFM) climate response of the Northern Hemisphere atmosphere to observed sea surface temperature (SST) anomalies for the period 1855–2002 is analysed from a 35-member ensemble made with SPEEDY, an atmospheric general circulation model (AGCM) of intermediate complexity. The model was run at the T30-L8 resolution, and initial conditions and the early stage of model runs differ among ensemble members in the definition of tropical diabatic heating. SST anomalies in the Niño3.4 region were categorised into five classes extending from strong cold to strong warm. Composites based on such a categorisation enabled an analysis of the influence of the tropical Pacific SST on the Northern Hemisphere atmospheric circulation with an emphasis on the Pacific-North America (PNA) and the North Atlantic-Europe (NAE) regions. As expected, the strongest signal was detected over the PNA region. An “asymmetry” in the model response was found for the opposite polarity of the Niño3.4 index; however, this asymmetry stems mainly from the difference in the amplitude of model response rather than from the phase shift between responses to warm and cold El Niño-Southern Oscillation (ENSO) events. The extratropical signal associated with warm ENSO events was found to be stronger than that related to cold events. The results also reveal that, for the PNA region, the amplitude of the response is positively correlated with the strength of ENSO, irrespective of the sign of ENSO. With almost no phase shift between model responses to El Niño and La Niña, the linear component of the response is much stronger than the non-linear component. Although the model climate response over the NAE region is much weaker than that over the PNA region, some striking similarities with the PNA are found. Both sea level pressure and precipitation responses are positively correlated with the strength of ENSO. This is not true for the 200-hPa geopotential heights, and no plausible explanation for such a result could be offered. An appreciable linear component in model response over the NAE was also found. The model results over the NAE region agree reasonably well with observational studies. An additional analysis of the remote atmospheric response to very weak ENSO forcing (defined from the interval between 0.5σ and 1.0σ of the interannual variance) was also carried out. A discernible model response in the Northern Hemisphere to such a weak SST forcing was found.     

The Combined Effects of the Tropical and Extratropical Quasi-biweekly Oscillations on the Record-setting Mei-yu Rainfall in the Summer of 2020

During June–July 2020, the strongest recorded mei-yu rainfall occurred in the middle and lower reaches of the Yangtze River. The rainfall processes exhibited an obvious quasi-biweekly(biweekly in brief) variability, and there are altogether five cycles. It is found that the biweekly rainfall cycle mainly arises from the collaborative effects of biweekly variabilities from both the tropics and extratropics. As for the tropics, the biweekly meridional march and retreat of the western Pacific subtr...     

Simulating the winter North Atlantic Oscillation: the roles of internal variability and greenhouse gas forcing

Analysis of simulations with seven coupled climate models demonstrates that the observed variations in the winter North Atlantic Oscillation (NAO), particularly the increase from the 1960s to the 1990s, are not compatible with either the internally generated variability nor the response to increasing greenhouse gas forcing simulated by these models. The observed NAO record can be explained by a combination of internal variability and greenhouse gas forcing, though only by the models that simulate the strongest variability and the strongest response. These models simulate inter-annual variability of the NAO index that is significantly greater than that observed, and can no longer explain the observed record if the simulated NAO indices are scaled so that they have the same high-frequency variance as that observed. It is likely, therefore, that other external forcings also contributed to the observed NAO index increase, unless the climate models are deficient in their simulation of inter-decadal NAO variability or their simulation of the response to greenhouse gas forcing. These conclusions are based on a comprehensive analysis of the control runs and transient greenhouse-gas-forced simulations of the seven climate models. The simulations of mean winter circulation and its pattern of inter-annual variability are very similar to the observations in the Atlantic half of the Northern Hemisphere. The winter atmospheric circulation response to increasing greenhouse gas forcing shows little inter-model similarity at the regional scale, and the NAO response is model-dependent and sensitive to the index used to measure it. At the largest scales, however, sea level pressure decreases over the Arctic Ocean in all models and increases over the Mediterranean Sea in six of the seven models, so that there is an increase of the NAO in all models when measured using a pattern-based index.     

The interannual leading modes of the extratropical variability in the Southern Hemisphere simulated by the ECHAM-4 atmospheric model

 An ensemble of twenty-three 14-year experiments conducted with the ECHAM-4 GCM has been examined to test the model's capability to simulate the principal modes of interannual variability. The integrations were performed under specified monthly SST between 1979–1993. The analysis was focused on the Southern Hemisphere (SH) extratropics. Empirical orthogonal functions analysis (EOF) using seasonal anomaly fields has been performed to isolate the principal modes that dominate the southern extratropical variability at the interannual time scale. Leading patterns of 500 hPa geopotential height (z500) have been compared with those estimated from the ECMWF re-analysis dataset. The model is able to adequately reproduce the spatial pattern of the annular mode, but it represents the temporal variations of the oscillation less satisfactorily. The model simulation of the Pacific South American (PSA) pattern is better, both in the shape of the pattern and in the temporal evolution. To verify if the capability of the model to adequately simulate the temporal oscillation of the propagating patterns is related to the increased influence of the tropical external forcing, covarying SST-atmospheric modes have been identified by singular value decomposition (SVD). In winter (July-August-September, JAS) the tropical SST variability is highly correlated with the ENSO mode. In summer (January-February-March, JFM) the strength of the teleconnections is related to strong westerly anomalies, disrupted by a meridional out of phase relation near to South America. The large size of the ensemble was exploited by comparing the time-varying model spread and degrees of freedom of the simulated extratropical circulation. Results show that when the extratropical circulation has a few degrees of freedom, the reproducibility is relatively low and the ensemble is governed by a fairly robust zonally symmetric structure of dispersion. Received: 9 May 2000 / Accepted: 30 January 2001     

QBO influence on extratropical predictive skill

The quasi-biennial oscillation (QBO) in the zonal wind in the tropical stratosphere is one of the most predictable aspects of the circulation anywhere in the atmosphere and can be accurately forecast for many months in advance. If the stratospheric QBO systematically (and significantly) affects the tropospheric circulation, it potentially provides a predictable signal useful for seasonal forecasting. The stratospheric QBO itself is generally not well represented in current numerical models, however, including those used for seasonal prediction and this potential may not be exploited by current numerical-model based forecast systems. The purpose of the present study is to ascertain if a knowledge of the state of the QBO can contribute to extratropical boreal winter seasonal forecast skill and, if so, to motivate further research in this area. The investigation is in the context of the second Historical Forecasting Project (HFP2), a state-of-the-art multimodel two-tier ensemble seasonal forecasting system. The first tier, consisting of a prediction of sea surface temperature anomalies (SSTAs), is followed by the second tier which is a prediction of the state of the atmosphere and surface using an AGCM initialized from atmospheric analyses and using the predicted SSTs as boundary conditions. The HFP2 forecasts are successful in capturing the extratropical effects of sea surface temperature anomalies in the equatorial Pacific to the extent that a linear statistical correction based on the NINO3.4 index does not provide additional extratropical skill. By contrast, knowledge of the state of the stratospheric QBO can be used statistically to add extratropical skill centred in the region of the North Atlantic Oscillation. Although the additional skill is modest, the result supports the contention that taking account of the QBO could improve extratropical seasonal forecasting skill. This might be done statistically after the fact, by forcing the QBO state into the forecast model as it runs or, preferably, by using models which correctly represent the physical processes and behaviour of the QBO.