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Good intentions by the middle class are not always well guided and do not always lead to measurable or significant results. For example, efforts to limit greenhouse gas emissions may hold broad appeal but can still have negligible impact. Therefore, it is suggested to embark on “Apollo projects”, which bundle the potential and willingness of the middle class. These projects should focus on the development of specific technologies, with economic advantages to support their spread throughout the world. Doing so will harness the middle class in support of greenhouse gas emission reductions in the gigaton-range. Such pan-national projects, for example, could address emission-free ship- or air-propulsion, the electrification of heating or of processes in the chemical industry.

We have designed a method for testing the quality of multidecadal analyses of sea surface temperature (SST) in regional seas by using a set of high-quality local SST observations. In recognizing that local data may reflect local effects, we focus on the dominant empirical orthogonal functions (EOFs) of the local data and of the localized data of the gridded SST analyses. We examine the patterns, variability, and trends of the principal components. This method is applied to examine three different SST analyses, i.e., HadISST1, ERSST, and COBE SST. They have been assessed using a newly constructed high-quality dataset of SST at 26 coastal stations along the Chinese coast in 1960-2015, which underwent careful examination with respect to quality and a number of corrections for inhomogeneities. The three gridded analyses perform generally well from 1960 to 2015, in particular since 1980. However, for the pre-satellite period prior to the 1980s, the analyses differ among each other and show some inconsistencies with the local data, such as artificial break points, periods of bias, and differences in trends. We conclude that gridded SST analyses need improvement in the pre-satellite period (prior to the 1980s) by reexamining in detail archives of local quality-controlled SST data in many data-sparse regions of the world.. © Author(s) 2019.

In view of the need to curb greenhouse gases, the question arises as to the functions of liability in providing effective incentives for emitters in order to change their behavior. Liability for emitting greenhouse gases exists (or can exist) in the area of public law and private law and can be subdivided into international, administrative, and criminal liability (public law liabilities) and tort law liability (private law liability). Actions for holding individual and legal persons (such as states, authorities, and companies) liable can, depending on the specific jurisdiction, be triggered by citizens but also by legal persons, such as authorities, companies, and non-governmental organizations (NGOs), particularly environmental NGOs. The central question in this article is how climate liability is arranged under public law and whether there would be any role for climate liability to play under private law, thereby applying a legal and economic methodology. That so-called law and economics doctrine is a useful approach as it has given a lot of attention, for example, to the different functions of specific legal instruments (more particularly regulation, including taxation and emissions trading and tort law liability) for mitigating greenhouse gases. Meanwhile, in practice, various examples can be identified whereby tort law liability is used as a complement to greenhouse gas regulation. This specific use of tort liability is analyzed in the light of the law and economics literature, thereby pointing at prospects but also at remaining core questions. The success of tort law actions will most likely greatly depend on the (lack of) ambition vested into the emissions regulations at international and national levels. One of the exciting questions for the near future is to what extent judges feel able to step into the regulation of the climate change problem, in an ex ante way. The most difficult cases are obviously those where a regulatory system concerning greenhouse gas mitigation has been put in place and where the court system is strong, but where particular groups consider the regulations to be insufficient.

This paper demonstrates two important aspects of regional dynamical downscaling of multidecadal atmospheric reanalysis. First, that in this way skillful regional descriptions of multidecadal climate variability may be constructed in regions with little or no local data. Second, that the concept of large‐scale constraining allows global downscaling, so that global reanalyses may be completed by additions of consistent detail in all regions of the world. Global reanalyses suffer from inhomogeneities. However, their large‐scale componenst are mostly homogeneous; Therefore, the concept of downscaling may be applied to homogeneously complement the large‐scale state of the reanalyses with regional detail—wherever the condition of homogeneity of the description of large scales is fulfilled. Technically, this can be done by dynamical downscaling using a regional or global climate model, which\'s large scales are constrained by spectral nudging. This approach has been developed and tested for the region of Europe, and a skillful representation of regional weather risks—in particular marine risks—was identified. We have run this system in regions with reduced or absent local data coverage, such as Central Siberia, the Bohai and Yellow Sea, Southwestern Africa, and the South Atlantic. Also, a global simulation was computed, which adds regional features to prescribed global dynamics. Our cases demonstrate that spatially detailed reconstructions of the climate state and its change in the recent three to six decades add useful supplementary information to existing observational data for midlatitude and subtropical regions of the world.

Hans von Storch beschäftigt sich als naturwissenschaftlicher Klimaforscher seit vielen Jahren mit dem Verhältnis von Wissenschaft und Gesellschaft. In zahlreichen Publikationen stellt er sich einem kritischen Dialog mit den Sozialwissenschaften, um folgende Fragen zu klären: Was wissen wir über Eigenlogiken im Wissenschaftssystem und in der Politik, und was sagt das über die Möglichkeiten von wissenschaftlicher Politikberatung aus? Welche Rolle spielen die Massenmedien für die gesellschaftliche Wahrnehmung der Klimaforschung? Und wie können wir die Klimaforschung selbst als einen sozialen Prozess verstehen? Seine wesentlichen Thesen des Vortrages: a) Naturwissenschaftliche Klimaforschung ohne sozialwissenschaftliche Kontextualisierung macht wenig Sinn, sofern es um die Erforschung des menschgemachten Klimawandels und des der damit verbundenen Klimawirkung geht – und darum geht es fast immer. Die Klimaforschung ist fast ausschließlich auf dieses Thema ausgerichtet. Sie wird auch in diese Richtung gepusht, sowohl was Mittel als auch öffentliche Anerkennung angeht. b) Die Naturwissenschaftliche Klimaforschung hat in den letzten 30 Jahren wesentliche Ergebnisse erarbeitet und abgesichert: Das Klima verändert sich stärker als aufgrund natürlicher Vorgänge zu erwarten ist. Eine Erklärung für die Änderungen gelingt nur, wenn die Wirkung der stark erhöhten Konzentrationen atmosphärischen Treibhausgase als dominant verstanden wird. Da der Zuwachs dieser Konzentration durch menschlichen Emissionen zustande kommt, sind die Änderungen grundsätzlich steuerbar durch entsprechende gesellschaftliche Eingriffe c) Naturwissenschaftliche Klimaforschung spricht oft direkt zu Gesellschaft. Sie befindet sich in einer postnormalen Situation, wo bisweilen die politische Nützlichkeit vor der methodischen Qualität steht. d) Klimaforschung, und öfter noch Klimaforscher sind also soziale Akteure. Bisweilen findet man diese Akteure stark involviert in die politische Willensbildung, wobei politische Präferenzen als wissenschaftliche Notwendigkeiten präsentiert werden.

We investigate whether the recently observed temperature and precipitation trends over the Baltic Sea Basin are consistent with state-of-the-art regional climate model projections. To address this question we use several data sources: (1) multidecadal trends derived from various observational data sets, (2) estimates of natural variability provided by a 2000 year paleoclimatic model simulation, and (3) response to greenhouse gas forcing derived from regional climate simulations driven by the A1B and RCP4.5 scenarios (from ENSEMBLES and CORDEX projects). Results indicate that, over the past decades, the climate in the Baltic Sea Basin has undergone a change that is beyond the estimated range of natural variability. We test the hypothesis that this change may be understood as a manifestation of global warming due to increasing concentrations of greenhouse gases (GHGs). We find that changes in near-surface temperature support our hypothesis that the effect of GHG is needed to simulate the observed changes. The pattern correlation and regression results clearly illustrate the concerted emergence of an anthropogenic signal consistent with the GHG signal in summer and autumn in the 21st century. However, none of the 19 regional climate simulations used in this study reproduce the observed warming. The observed trends in precipitation and surface solar radiation are also partially inconsistent with the expected changes due to GHG forcing. We conclude that, besides the regional response to GHG forcing, other human-made drivers have had an imprint. Regional emission of industrial aerosols has been strongly reduced in this region, and we suggest that this reduction may be the missing driver.

This book is an update of the first BACC assessment, published in 2008. It offers new and updated scientific findings in regional climate research for the Baltic Sea basin. These include climate changes since the last glaciation (approx. 12,000 years ago), changes in the recent past (the last 200 years), climate projections up until 2100 using state-of-the-art regional climate models and an assessment of climate-change impacts on terrestrial, freshwater and marine ecosystems. There are dedicated new chapters on sea-level rise, coastal erosion and impacts on urban areas. A new set of chapters deals with possible causes of regional climate change along with the global effects of increased greenhouse gas concentrations, namely atmospheric aerosols and land-cover change. The evidence collected and presented in this book shows that the regional climate has already started to change and this is expected to continue. Projections of potential future climates show that the region will probably become considerably warmer and wetter in some parts, but dryer in others. Terrestrial and aquatic ecosystems have already shown adjustments to increased temperatures and are expected to undergo further changes in the near future. The BACC II Author Team consists of 141 scientists from 12 countries, covering various disciplines related to climate research and related impacts. BACC II is a project of the Baltic Earth research network and contributes to the World Climate Research Programme.

We investigated whether anthropogenic forcing is a plausible explanation for the observed warming in the Baltic Sea catchment area. Therefore, we compared the most recent trends in the surface temperature over land with anthropogenic climate change projections from regional climate model simulations. We analyzed patterns of change with different spatio-temporal resolutions. The observed annual area-mean change in the daily-mean temperature was consistent with the anthropogenic climate change signal. This finding was robust to the removal of the signal of the North Atlantic Oscillation. In contrast to the annual area-mean change, we found little consistency in both annual cycle and spatial variability of the observed and projected changes.

This report presents the findings of two surveys of climate scientists' perceptions of the globalwarming issue. The first survey was conducted in 1996 and the second survey in 2003. Abrief text section demonstrates some of the significant findings. The surveys investigate the meansby which scientific conclusions are reached and the climate scientists interpretations of whatthese conclusions might mean. The complete responses to the surveys are presented in Appendix A:Tables and Appendix B: Figures. Each table and figure is presented in a manner to indicatestatistically significant change in scientists perspectives over the period of the two surveys.

A heat-conduction forward model driven by ground surface temperature from three 1000-year climate simulations with the state-of-the-art ECHO-g model has been used to simulate underground temperature perturbation profiles. An inversion approach has been applied to reconstruct ground surface temperature histories from the simulated profiles and to compare them with the climate model temperatures. Results support the skill of borehole inversion methods to retrieve long-term temperature trends, and the robustness of using the present-day borehole network for reconstructing SAT variations.

The ECHAM-1 T21/LSG coupled ocean-atmosphere general circulation model (GCM) is used to simulate climatic conditions at the last interglacial maximum (Eemian. 125 kyr BP). The results reflect thc expected surface temperature changes (with respect to the control run) due to the amplification (reduction) of the seasonal cycle of insolation in the Northern (Southern) Hemisphere. A number of simulated features agree with previous results from atmospheric GCM simulations e.g. intensified summer southwest monsoons) except in the Northern Hemisphere poleward of 30 degrees N. where dynamical feedback, in the North Atlantic and North Pacific increase zonal temperatures about 1 degrees C above what would be predicted from simple energy balance considerations. As this is the same area where most of the terrestrial geological data originate, this result suggests that previous estimates of Eemian global average temperature might have been biased by sample distribution. This conclusion is supported by the fact that the estimated global temperature increase of only 0.3 degrees C greater than the control run ha, been previously shown to be consistent a with CLIMAP sea surface temperature estimates. Although the Northern Hemisphere summer monsoon is intensified. globally averaged precipitation over land is within about 1% of the present, contravening some geological inferences bur not the deep-sea delta(13)C estimates of terrestrial carbon storage changes. Winter circulation changes in the northern Arabian Sea. driven by strong cooling on land, are as large as summer circulation changes that are the usual focus of interest, suggesting that interpreting variations in the Arabian Sea. sedimentary record solely in terms of the summer monsoon response could sometimes lead to errors. A small monsoonal response over northern South America suggests that interglacial paleotrends in this region were not just due to El Nino variations.

A structurally highly simplified, globally integrated coupled climate-economic costs model SIAM (Structural Integrated Assessment Model) is used to compute optimal paths of global CO2 emissions that minimize the net sum of climate damage and mitigation costs. The model is used to study the sensitivity of the computed optimal emission paths with respect to various critical input assumptions. The climate module is represented by a linearized impulse-response model calibrated against a coupled ocean-atmosphere general circulation climate model and a three-dimensional global carbon-cycle model. The cost terms are represented by strongly simplified expressions designed for maximal transparency with respect to sensitive input assumptions. These include the discount rates for mitigation and damage costs, the inertia of the socio-economic system, and the dependence of climate damages on the change in temperature and the rate of change of temperature. Different assumptions regarding these parameters are believed to be the cause of the marked divergences of existing cost-benefit analyses based on more sophisticated economic models. The long memory of the climate system implies that very long time horizons of several hundred years need to be considered to optimize CO2 emissions on time scales relevant for a policy of sustainable development. Cost-benefit analyses over shorter time scales of a century or two can lead to dangerous underestimates of the long term climatic impact of increasing greenhouse-gas emissions. To avert a major long term global warming, CO2 emissions need to be reduced ultimately to very low levels. However, the draw-down can be realized as a gradual transition process over many decades and even centuries. This should nevertheless not be interpreted as providing a time cushion for inaction: the transition becomes more costly the longer the necessary mitigation policies are delayed. However, the long time horizon provides adequate flexibility for later adjustments. Short term energy conservation alone is insufficient and can be viewed only as a useful measure in support of the necessary long term transition to carbon-free energy technologies. For standard climate damage cost expressions, optimal emission paths limiting long term global warming to acceptable sustainable development levels are recovered only if climate damage costs are not significantly discounted. Discounting of climate damages at normal economic rates yields emission paths that are only weakly reduced relative to business as usual scenarios, resulting in high global warming levels that are incompatible with the generally accepted requirements of sustainable development. The solutions are nevertheless logically consistent with the underlying discounting assumption, namely that the occurrence of global warming damages in the distant future as a result of present human activities is of negligible concern today. It follows that a commitment to long term sustainable development, if it in fact exists, should be expressed by an intertemporal relation for the value of the earth's future climate which does not degrade significantly over the time horizon relevant for climate change. Since the future climate is a common assett whose value cannot be determined on the market, the appropriate discount rate for future climate damages should be determined by an assessment of the public willingness to pay, today for the mitigation of futura climate change. To translate our general conclusions into quantitative cost estimates required by decision makers, the present exploratory study needs to be extended using more detailed disaggregated climate damage and mitigation cost estimates and more realistic socio-economic models, including multi-actor interactions, inherent variability, the role of uncertainty and adaptive control strategies.

In this paper we present a preliminary and exploratory analysis of a series of in- depth interviews conducted with twenty eight climate scientists. After justifying the need for such research we present some preliminary results. These are in the form of: (1) the perceptions of risk and hazard as they relate to global warming; (2) the typification of issues drawn from the triadic relationship of science-politics-public and; (3) the typification of ‘scientific personalities’. The perceptions of risk and hazard and typifications of issues are presented as brief edited excerpts from a broader range of the interviews. The typification of ‘scientific personalities’ are presented as three lengthy edited excerpts from interviews with three scientists. Finally, based on the qualitative data in the interviews, we draw a brief conclusion regarding the nature of the research of global climate change. While the data demonstrates that, for the most part, the risk of global climate change is a consensual product of scientific practice, the hazards associated with the event are determined to have a much closer aflinity with the scientist’s personal belief system. It is often these beliefs that come to play a role in the application of science to the public and political spheres.

A fingerprint method for detecting anthropogenic climate change is applied to new simulations with a coupled ocean-atmosphere general circulation model (CGCM) forced by increasing concentrations of greenhouse gases and aerosols covering the years 1880 to 2050. in addition to the anthropogenic climate change signal, the space-time structure of the natural climate variability for near-surface temperatures is estimated from instrumental data over the last 134 years and two 1000 year simulations with CGCMs. The estimates are compared with paleoclimate data over 570 years. The space—time information on both the signal and the noise is used to maximize the signal—to—noise ratio of a detection variable obtained by applying an optimal filter (fingerprint) to the observed data. The inclusion of aerosols slows the predicted future warming. The probability that the observed increase in nearusurface temperatures in recent decades is of natural origin is estimated to be less than 5 %. However, this number is dependent on the estimated natural variability level, which is still subject to some uncertainty.

The concepts of climate variability, due to natural processes, and of cli— mate change, related to man’s activitites, are illustrated. Different types of internal mechanisms, such as multiple equilibria and the integration of noise, are discussed. A systematic approach to discriminate between natu- ral variability and anthropogenic change indicates that the latest warming trends in the observational record can no longer be understood as “vari- ability” but is likely related to external processes, and in particular to the enhanced greenhouse effect. Finally we discuss briefly major challenges climate research will face in the coming decades.

Toward the end of the nineteenth and at the beginning of twentieth century significant discussions occurred among geographers, meteorologists and "climatologists" concerned with the notion of climate variability (Klimaschwankungen) and anthropogenic climate change (Klimawandcllimaéinderungen), for instance, due to deforestation and reforestation. We identify two protagonists of this debate, Eduard Brückner and Julius Hann, who both accept the notion of climate variability on the decadal scale, but respond in very different ways to the discovery of climate change. Bruckner assessed the impact of climate variability on society (e.g., on health, the balance of trade, emigration to the USA), arid. tried to bring these to the attention of the public, whereas Harm limited himself to the immediate natural scientific problem of monitoring and documenting climate variability. We suggest that these discussions and the formation of national governmental and parliamentary committees almost 100 hundred years ago, are not merely of historical interest. In View of presents discussion of climate variability and anthropogenic climate change, and the need for of adequate socio-economic response strategies, past and now neglected arguments may prove important for methodological and theoretical as well as for practical reasons. The past discussions represent a significant social and intellectual analogy for the present situation.

The effect of cold conditions in the central and eastern Equatorial Pacific during Northern Summer is examined in a series of numerical experiments with the low resolution (T21) atmospheric general circulation model ECHAM2. Anomalous sea surface temperatures (SST) as observed in June 1988 were prescribed and the effect on the global circulation is examined. In the model atmosphere, the anomalous cold water in the Equatorial Pacific excites a strong and stable response over the tropical Central and East Pacific. From here stationary Rossby waves radiate into both hemispheres. The Northern Hemisphere wave train is weak and affects only the Northeast Pacific area; the Southern Hemisphere wave train arches from the Central Pacific over the southern tip of South America to the South Atlantic. This response is not only present in the basic anomaly experiment with the T21 GCM but also in experiments with SST anomalies confined to the tropics and with an envelope- forrnulation of the SST anomalies, in experiments with a linear model, and in high resolution (T42) model experiments. The model output is also compared to the actually observed atmospheric state in June 1988. The model simulations do not reproduce the global circulation anomalies which were observed in June 1988. The model experiments are inconclusive with respect to the question of whether the North American drought observed in summer 1988 was related to the anomalous SST conditions in the Subtropical and Tropical Pacific. An explanatory analysis with a linear model reacting to prescribed heating anomalies as well as with the high-resolution GCM indicates that the model overreacted to the equatorial SST anomalies but almost ignored the contemporaneous subtropical SST anomalies on the Southern Hemisphere.

The principal oscillation pattern (POP) analysis is a technique to empirically identify time-dependent spatial patterns in a multivariate time series of geophysical or other data. In order to investigate medium-scale and synoptic waves in the atmosphere it has been applied to tropospheric geopotential height fields of ECMWF analyses from 1984 to 1987. The data have been subjected to zonal Fourier decomposition and to time filtering so that variations with periods between 3 and 25 days were retained. Analyses have been performed separately for each zonal wavenumber 5-9 on the Northern Hemisphere in winter and on the Southern Hemisphere in summer (DJF). POPs can be seen as normal modes of a linear approximation to a more complex dynamical system. The system matrix is estimated from observations of nature. This concept is compared with conventional stability analysis where the system matrix of the linear system is derived from theoretical, in this case quasigeostrophic, reasoning. Only the mean basic flow depends on time- and space-averaged fields of observed wind and temperature from the ECMWF data. It turns out that the most significant POPs are very similar in time and spatial structure to the most unstable waves in the stability analysis. They describe the linear growth phase of baroclinic, unstable waves that propagate eastward with periods of 3-7 days. Since the POPs are purely derived from observations, the results indicate the appropriateness of the assumptions usually made in linear stability analysis of zonally symmetric flows to explain high-frequency atmospheric fluctuations. Moreover, the POP analysis reveals patterns that are not found in the linear stability analysis. These can possibly be attributed to the nonlinear decay phase of baroclinic waves. Eliassen-Palm cross sections help clarify the interpretation of the POPs in terms of the life cycle of nonlinear baroclinic waves.

Problems in the present discussion about increasing storminess in the North At- lantic area are discussed. Observational data so far available do not indicate a change in the storm statistics. Output from climate models points to an intensified storm track in the North Atlantic, but because of the limited skill of present—day climate models in simulating high-frequency variability and regional details any such “fore— cast” has to be considered with caution. A downscaling procedure which relates large—scale time-mean aspects of the state of the atmosphere and ocean to the local statistics of storms is proposed to reconstruct past variations of high-frequency variability in the atmosphere (storminess) and in the sea state (wave statistics). First results are presented.

The ocean surface-atmosphere relationships in the North Atlantic area in northern winter are empirically examined by canonical correlation analysis (CCA). This analysis is performed from two different points of view. First, the connection between atmospheric circulation anomalies, in terms of monthly mean sea level pressure (SLP) and monthly standard deviation of SLP (sigma(SLP)), and sea surface temperature (SST) anomalies of the Atlantic Ocean are directly examined. Second, the air-sea relationships are indirectly studied through their influence upon precipitation in an area likely to be influenced by the North Atlantic, the Iberian Peninsula. The canonical correlation analysis yields two pairs of patterns that describe the coherent variations of the combined SST-SLP fields; one pair of patterns for the SST-sigma(SLP) fields and one pair of patterns for the SLP-sigma(SLP) fields. All patterns are dominant in describing variance. A lag cross-correlation analysis of the time coefficients indicates that monthly mean SLP varies simultaneously with sigma(SLP) but is leading monthly mean SST slightly. These results are consistent with the hypothesis that anomalies of the atmospheric circulation are mainly responsible for the appearance of anomalous wintertime SST and with the notion that the intramonthly variability of the atmosphere (sigma(SLP)) is coupled to the mean flow (SLP). With respect to Iberian precipitation, one well-defined CCA pair of patterns of regional rainfall and, respectively, SLP and SST is found. Above-normal Iberian precipitation is connected with a ''high-index'' North Atlantic SLP distribution and below-normal SST in most of the Atlantic north of 20-degrees-N. The dominant process responsible for the variability of rainfall appears to be the intensity of the westerly wind and the frequency of storms imbedded in it, not the presence of regional or remote SST anomalies. It is concluded that a large-scale SLP pattern in the North Atlantic, similar to the first EOF of the SLP field, is instrumental in generating both the Iberian precipitation and Atlantic SST variability on the seasonal time scale.

The relationship between the genesis of tropical cyclones in the Western Paci- fic and the phase of the tropical 30- to 60 day oscillation (MJO) is analysed using 5 years of data. The phase of the MJO is described by the POP index of the MJO. A statistically significant signal is identified for both the North and South Pacific: More (less) cyclones than on average occur when the MJO is causing negative (positive) regional outgoing long wave radiation (OLR) anomalies. The signal is considerably better defined in the Southern Hemisphere than in the Northern Hemisphere. In the Southern (Northern) hemisphere the expected number of cyclones formed in the convectively active MJO—phase is 4 (2.25) times the expected number in the inactive phase.

In Part I (Storch and Xu, 1990) the Principal Oscillation Pattern (POP) analysis of 200 mb equatorial velocity potential leads to the definition of a bivariate (POP—) index of the tropical 30- to 60—day oscillation. Using the POP prediction scheme this index is predictable for a few days in advance. In this Part II, the prediction of the equatorial velocity potential field, made by the POP method and made by two GCMs, is investigated. The POP index forecast can incorporate skillful forecasts of the equatorial velocity potential (x) field. The 0.50 level of the ensemble correlation skill score is passed after 7 days. If there is a strong signal in the initial state, useful forecasts of more than 20 days are sometimes possible. General— ly, the forecasts are best in northern winter (DJF) and when the initial signal is strong. Apart from day 0 and day 1, the POP forecast scores higher skills than persistence. Only if the 30— to 60—day oscillation is weak in the initial state does the POP forecast fail. Parallel to the POP forecast scheme, two GCMs are considered in terms of their ability to predict the equatorial velocity potential. The NCAR T31 GCM uses regular NMC analyses as initial conditions, and the CNRM T42 uses the initialized analyses of ECMWF. In a series of 15 experimental wintertime forecasts the NCAR T31 GCM appears to be quite skillful in predicting the equatorial x—field, and in particular the 30— to 60—day oscillation. Its skill, however, is less than that of the POP scheme. The CNRM T42 GCM, which was tested with 9 wintertime forecasts, seems not to be able to predict the regular development associated with the tropical 30— to 60—day oscillation. The power of the POP index in explaining the equatorial x—field is a measure of the strength and dominance of the 30— to 60—day oscillation. In the case of the NCAR T31 GCM, this measure at day 0 is an a-priori indicator of the GCM‘s skill in predicting the equatorial velocity potential field.

The merits of the analysis techniques "POP" (Principal Oscillation Pattern) and "PIP" (Principal Interaction Pattern) are demonstrated by applying it to a combined data set of surface wind and sea surface temperature along the equator. The POP analysis detects a dominant low—frequency oscillatory mode with a time scale of about 2 to 3 years which can be identified as the Southern Oscillation. With respect to SST, the mode behaves approximately as a standing patttern with maximum extremes in the Central and Eastern Pacific. In the zonal surface wind, however, a eastward propagating feature can be detected. The meridional wind component is less organized, except for the occurrence of southerly winds in the Eastern Pacific during the "peak phase" of E1 Nifio. The result of the PIP analysis is essentially the same as that of the POP analysis. except for a seasonal dependence of the system’s stability: disturbances tend to be amplified during April to July and damped throughout the rest of the year.

The "Principal Oscillation Pattern" technique is used to derive an index of the tropical 30— to 60—day oscillation. In the 200 mb equatorial velocity potential field one dominant pair of POPs is found. Its properties compare very Well with the properties of the oscillation identified in previous studies. In particular, a good correlation between the time evolution of the POP coefficients and area averaged OLR is found. The POPS are derived from a two year subinterval of the whole five year data set. This leaves independent data for subsequent verification. The patterns and their characteristic numbers are almost unchanged if the whole data set is analysed. Also, the analysis is insensitive to changes of the analysis area: If the analysis is limited to 900 longitude equatorial sectors the signal is identified also and its patterns are consistent with the patterns derived from the full data set. Interestingly, the signal is best defined in the eastern hemisphere. The POPs may be used to derive "associated correlation patterns" of other quantities in winter and summer separately. The path of the oscillation has a marked annual cycle: In northern winter it migrates from the Indian Ocean across Northern Australia into the region of the SPCZ and in northern summer it moves from the Indian Ocean across South Asia along the ITCZ to South America. The POP coefficient may be seen as a bivariate index of the state (phase and strength) of the 30— to 60—day oscillation. Since the POP technique incorporates a prediction equation for the phase of the POP coefficients our POP model allows for the prediction of the complex amplitude of the oscillation. In a sequence of forecast experiments, of which about two thirds used independent data, the POP forecasts were found to be useful in about half of all cases for lead times of several days. The correlation and RMS skills were calculated for the POP forecast and for persistence. The POP forecast appears to be considerably better with respect to both measures. The correlation skill scores 60% after 7 days. The POP forecast is most skillful in northern winter and if strong signals are present with minima of velocity potential in the eastern hemisphere.