Twentieth century Walker Circulation change: data analysis and model experiments
Recent studies indicate a weakening of the Walker Circulation during the twentieth century. Here, we present evidence from an atmospheric general circulation model (AGCM) forced by the history of observed sea surface temperature (SST) that the Walker Circulation may have intensified rather than weakened. Observed Equatorial Indo-Pacific Sector SST since 1870 exhibited a zonally asymmetric evolution: While the eastern part of the Equatorial Pacific showed only a weak warming, or even cooling in one SST dataset, the western part and the Equatorial Indian Ocean exhibited a rather strong warming. This has resulted in an increase of the SST gradient between the Maritime Continent and the eastern part of the Equatorial Pacific, one driving force of the Walker Circulation. The ensemble experiments with the AGCM, with and without time-varying external forcing, suggest that the enhancement of the SST gradient drove an anomalous atmospheric circulation, with an enhancement of both Walker and Hadley Circulation. Anomalously strong precipitation is simulated over the Indian Ocean and anomalously weak precipitation over the western Pacific, with corresponding changes in the surface wind pattern. Some sensitivity to the forcing SST, however, is noticed. The analysis of twentieth century integrations with global climate models driven with observed radiative forcing obtained from the Coupled Model Intercomparison Project (CMIP) database support the link between the SST gradient and Walker Circulation strength. Furthermore, control integrations with the CMIP models indicate the existence of strong internal variability on centennial timescales. The results suggest that a radiatively forced signal in the Walker Circulation during the twentieth century may have been too weak to be detectable.
KeywordsTropical atmospheric circulationTwentieth century Walker Circulation
Global Warming is well underway and at least half of the surface warming observed during the twentieth century can be attributed to anthropogenic forcing (IPCC 2007). The global-scale surface air temperature (SAT) exhibits a strong land-sea contrast, an inter-hemispheric asymmetry with stronger warming of the Northern Hemisphere, and a poleward amplification of the warming in the latter, features global climate models successfully simulate. On regional scales, however, climate models do not agree with regard to the SAT response (Meehl et al. 2008). In particular, Latif and Keenlyside (2009) by reviewing the refereed literature describe a large uncertainty in the Tropical Pacific response to Global Warming. They investigated not only the changes in the interannual variability by studying the El Niño/Southern Oscillation (ENSO), but also the changes in the mean state. Concerning the latter, climate models simulate both an El Niño- and a La Niña-like response, and there seems not to be a clear preference for either of them in the ensemble of models. Such behaviour is expected given the large spread of representing mean state in the models, since there is a subtle balance of the processes governing the sea surface temperature (SST) along the Equatorial Pacific. Both the shallow mixed layer depth in the east and the temperature dependence of the nonlinear cloud-albedo feedback, for instance, support an eastward amplification of the Global Warming response. On the other hand, the negative feedback by the mean equatorial upwelling is strongest in the east, which may favour a westward amplification. One can expect the nature of the SST response to have a strong impact on the thermodynamically driven Walker Circulation, one of the most important circulation systems in the Tropics and the focus of this study.
Observational studies also differ strongly among each other even concerning the sign of the change in the SST gradient along the Pacific Equator. Meehl and Washington (1996), for instance, find evidence for an El Niño-like, while Cane et al. (1997) and very recently Karnauskas et al. (2009) for a La Niña-like SST trend pattern during the twentieth century. Furthermore, Xue et al. (2003) report strong interdecadal SST variability in the tropical Indian and Pacific Oceans during the last 150 years and differences between datasets, which hampers the estimation of the twentieth century inter-basin SST gradient trend. Finally, Deser et al. (2010) report a large uncertainty in the SST trends of the equatorial East Pacific during the twentieth century. As different signs in the change of the SST gradient along the Equator may drive completely different atmospheric responses in the Walker and Hadley Circulation, it is of primary importance to obtain further insight into the observed changes.
A number of studies investigated decadal and longer trends in the tropical circulation (e.g., Clarke and Lebedev 1996; Zhang and Song 2006; Power and Smith 2007; Bunge and Clarke 2009; Deser et al. 2010) and most of those have argued that the Walker Circulation has weakened over the twentieth century. In particular, Vecchi et al. (2006) investigated sea level pressure (SLP) observations and results from one climate model driven with observed twentieth century external forcing. They inferred a weakening of the Walker Circulation from both observations and their simulations. This is consistent with the study of Held and Soden (2006) who investigated the effect of radiative forcing on the hydrological cycle from a number of climate change experiments generated for the Fourth Assessment of the Intergovernmental Panel on Climate Change (IPCC). They conclude that by the fundamental measure provided by the average vertical exchange of mass between the boundary layer and the free troposphere, the atmospheric circulation must slow down. On the contrary, Sohn and Park (2010) analyzing water vapour transports described strengthened tropical circulations in the past three decades. Likewise the trend pattern of altimetry sea level during the two recent decades featuring a rather strong increase in the western Equatorial Pacific and even a slight drop in the east implies a strengthened Walker Circulation, as the sea level gradient across the Equator is strongly controlled by the zonal surface winds. Individual tide gauge stations support the steepening of the Equatorial Pacific sea level gradient with high statistical significance from the 1940s onward (http://www.tidesandcurrents.noaa.gov/sltrends/sltrends.shtml). Many processes, however, may they be internal or external, atmospheric or oceanic, control the strength of the Walker Circulation, including changes in the tropical hydrological cycle in response to radiative forcing (see the discussion by DiNezio et al. 2010 and references therein), which partly explains the competing results.
Here we re-investigate the behaviour of the Walker Circulation during the twentieth century using different observational datasets and ensemble integrations with an atmospheric general circulation model (AGCM) forced by the observed history of SST. We conducted also a number of sensitivity experiments in order to test the experimental setup and to investigate the sensitivity to the choice of the SST dataset used to drive the AGCM and to radiative forcing. Finally, we investigate pre-industrial control and twentieth century runs with global climate models in order to obtain further insight into the dynamics of the tropical atmospheric circulation, specifically the link between the Indo-Pacific SST gradient and Walker Circulation strength. Our main focus is the determination of the spatial structure of the long-term trend in tropical SST and the atmospheric response to it during the twentieth century.
In Sect. 2, we describe the data, the statistical methods used in this study, and the AGCM. The results of the statistical analyses of the observed data are presented in Sect. 3. The atmosphere model results are shown in Sect. 4. The outcome of the different sensitivity experiments with the AGCM is described in Sect. 5. The results obtained concerning the link between the SST and SLP gradient in the Indo-Pacific Region in the different AGCM experiments and in the twentieth century integrations with the Coupled Model Intercomparison Project (CMIP) models are summarized in Sect. 6. A comparison of the SST-forced AGCM results with observations is given in Sect. 7. The main conclusions are drawn in Sect. 8.
2 Data, atmosphere model, and statistical method
We used two observational SST datasets in this study. The first is the ERSST.v3b dataset (Smith et al. 2008; http://www.ncdc.noaa.gov/oa/climate/research/sst/ersstv3.php, in the following ERSST), which was available for the period 1854–2007, providing monthly data on a 2° × 2° grid. These data were used in the statistical analyses described below. The second SST dataset used in this study is HadISST (Rayner et al. 2006; http://www.hadobs.metoffice.com/hadisst/data/download.html) providing monthly data on 1° × 1° grid.
List of atmospheric observations used in this study
Space and time intervals
Near-surface zonal wind velocity
2° × 2°
0.5° × 0.5°
2.5° × 2.5°
T62 Gaussian grid
Model reanalysis output
Sea level pressure (SLP)
2° × 2°
2° × 2°
Reduced-space optimal interpolation
2.5° × 2.5°
Model reanalysis output
2.2 Atmosphere model experiments
The atmospheric data, however, are rather sparse both in time and space. In order to derive a consistent atmospheric response to the observed SST variability, we analyzed the output of an AGCM forced by the history of the observed global monthly (HadISST) SSTs and sea ice distributions for the period 1870–2007. Time varying external forcing is not considered in this particular ensemble experiment. Greenhouse gas concentrations are fixed at “present-day” values, with a carbon dioxide concentration of 348 ppm. As described below, several additional integrations were performed in order to test the experimental strategy, the sensitivity to the choice of the SST dataset and to radiative forcing.
The model, ECHAM5 (Roeckner et al. 2003), is a state-of-the-art AGCM and was used with a horizontal resolution of T106, corresponding to a 1.125° × 1.125° grid resolution with 31 vertical levels. Five integrations were performed with different initial, but identical boundary conditions. This allows to distinguish the boundary (SST) forced signal, estimated by the ensemble mean, from the internal (chaotic) variability of the atmosphere. The same model runs were used by Latif et al. (2007) to study the competing effects of warming in the different tropical oceans on Atlantic hurricane activity. In particular, the model successfully simulated the vertical wind shear over the tropical North Atlantic, an important region for tropical storm development. The AGCM also successfully reproduces the decadal fluctuations in Sahel rainfall during the twentieth century (not shown), which provides some additional confidence in the ability of the model to realistically simulate the main tropical circulation systems and their variability.
A relatively large number of sensitivity integrations were performed. Most of them were conducted with a coarse-resolution version of the AGCM (ECHAM5-T31) that employs a horizontal resolution of T31 (3.75° × 3.75°) with 19 vertical levels. First, in order to test the applicability of the uncoupled experimental setup three positive and three negative centennial SST trends were computed from a pre-industrial control simulation with the Max-Planck-Institut für Meteorologie (MPI) climate model obtained from the Coupled Model Intercomparison Project (CMIP3) database, and ECHAM5-T31 was driven with these. Second, in order to investigate the sensitivity of the results to the choice of the SST dataset the long-term SST trend patterns for each calendar month were derived from the two SST datasets (ERSST, HadISST) and added to the SST climatology to drive the coarse-resolution model in equilibrium-response simulations. Third, in order to investigate the influence of the SST trends in the Indian Ocean on the model results, these two runs were repeated with (1) SST trends specified only in the Pacific and Atlantic, and (2) with trends prescribed only in the Indian Ocean. All equilibrium response integrations are each 25 years long, and the last 20 years were used in the subsequent analyses. Furthermore, we repeated the ensemble of transient ECHAM-T106 integrations (with HadISST) using ERSST. Finally, fourth, in order to investigate the role of time-varying external forcing the ECHAM5-T106 ensemble experiments (with HadISST) were repeated with ECHAM5-T31 with and without time-varying external forcing. The latter includes variable greenhouse gas, aerosol, and ozone forcing. The two ensembles consist of seven realizations each.
2.3 Statistical method
One advantage of the POP method relevant here is that no priori information has to be provided about the space–time structure of the long-term SST trend. Prior to the POP analysis, the monthly values were averaged to annual means, and the long-term mean computed from the complete record was removed. No other time filtering was applied to the SST observations. As will be shown below and as expected, the long-term trend is represented by a real POP mode. We note that we applied POP analysis to both SST datasets (ERSST and HadISST) to investigate the robustness of the results. The findings, however, were rather similar and we concentrate here on the results from the POP analysis of HadISST. The time evolutions of the “trend-POPs” obtained from the two different SST datasets, for instance, are rather similar, as shown below.
3 Results of the statistical analyses
3.1 Trend analysis
A cooling trend in the eastern Equatorial Pacific has been previously described by Cane et al. (1997) and Karnauskas et al. (2009) who used different SST datasets. Cane et al. (1997) originally proposed such behaviour on the basis of the so called dynamical thermostat (Clement et al. 1996), in which the mean equatorial upwelling acts as a negative feedback on the SST and thus on Global Warming. We find a similar cooling trend only in HadISST (see Fig. 1a). This indicates a strong sensitivity of the equatorial SST trend pattern during the twentieth century to the choice of the dataset (Vecchi et al. 2008). The existence of an enhanced zonal SST gradient across the Equatorial Pacific, however, appears to be a robust result, and this is important to the following discussion.
The trend pattern in land precipitation (Fig. 1c) is rather patchy. We would like to note the reduction in rainfall over western North Africa, the reduction over western South America and a consistent increase around the Indian Ocean. The SLP trend (Fig. 1d) can be estimated only at rather few stations. Most obvious is the reduction in SLP over India and eastern Asia. The near-surface zonal wind trend pattern (Fig. 1e) is also rather noisy and shows easterly anomalies over most of the Equatorial Pacific and westerly anomalies over the Equatorial Indian Ocean. We note, however, that this trend is computed over a much shorter time period. As will be shown below, the change in SST gradient evolved rather gradually during the twentieth century, which partly justifies this. The Equatorial Pacific SST trend pattern derived from the different SST products suggests that the Walker Circulation may have strengthened, since such a change in the SST gradient would drive a stronger Walker Circulation considering the type of large-scale ocean–atmosphere interactions acting on interannual timescales. On the other hand, as has been shown by Held and Soden (2006), the radiative forcing during the twentieth century may have a competing effect. The trend analyses of the land precipitation, station SLP, and ship-based near-surface zonal wind, however, are inconclusive, but not entirely inconsistent with the picture of a strengthened Walker Circulation.
3.2 POP analysis of observed SST
The question arises whether the trend structure obtained by the POP method is more reliable than that derived using other statistical methods when applied to the same SST dataset. Standard Empirical Orthogonal Function (EOF) analysis yields quite different results and compounds the interannual variability and the long-term trend. The leading EOF (48%) of HadISST is mostly associated with ENSO (not shown), with strong loadings in the Equatorial East Pacific. However, the corresponding principal component displays a long-term trend, on which strong interannual variability is superimposed. The second EOF mode of HadISST accounts for 22% of the variance. It is also ENSO-like in the Pacific. In fact, in the Pacific, the first two EOF modes are mirror images, with the same pattern but with opposite sign. Its pattern (Fig. 3b) shares some similarities to that of the second POP mode (Fig. 3a). However, the meridional extent of the cooling in the Pacific is much larger. Furthermore, the warming in the Indian Ocean is considerably weaker than indicated by the linear SST trend pattern (Fig. 1a). The principal component of the second EOF mode (Fig. 3d) depicts (like that of the leading EOF mode) a long-term trend, but it also displays considerable interannual variability. In contrast, the interannual variability in the timeseries of the second POP mode is much suppressed. All this indicates that the POP method appears to be superior to the EOF method in distinguishing between the interannual (ENSO) variability and the long-term trend in the observed SST. The results of the POP analysis are, however, largely consistent with those of Guan and Nigam (2008) who investigated SST from HadISST by means of rotated extended EOF analysis. This method follows a similar philosophy as POPs, because it also considers the full space–time structure of the variability.
4 Atmosphere model simulations with observed SSTs
We now turn to the changes in atmospheric circulation which go along with the long-term changes in SST, as obtained from the AGCM forced by the history of the observed SSTs. We have chosen HadISST to drive the model, as it exhibits the stronger change in the Equatorial Pacific SST gradient. The sensitivity of the model results to the choice of the SST dataset is discussed further below.
The uncoupled approach assumes that the ocean and atmosphere are strongly coupled in the Equatorial Region, and that the model and driving SST are realistic. It is well accepted that the ocean and atmosphere are tightly coupled in the Equatorial Pacific and Atlantic, at least at interannual timescales. In the case of the Equatorial Indian Ocean, this has been questioned at decadal timescales by Copsey et al. (2006) reporting a clear discrepancy between the observed and AGCM-simulated trends in SLP in the Indian Ocean Region during the second half of the twentieth century. The results presented next, however, indicate that even on centennial scales the tight relationship between the SST and SLP is supported by a rather large number of global climate models obtained from the CMIP database.
The transient ECHAM5-T106 ensemble integrations forced by HadISST using constant radiative forcing yield a detailed picture of the atmospheric anomalies for the period 1870–2007, provided the made assumptions hold. As time-varying radiative forcing is not considered in this particular ensemble experiment, we can obtain only the SST-forced component of the complete response during the twentieth century. The time evolution of the second most energetic POP mode of HadISST is almost linear during the twentieth century (Fig. 3d), which justifies the calculation of linear trends (1900–2007) to derive the centennial-scale changes in atmospheric circulation. As a consistency check, the map of linear trend coefficients (Fig. 1a) for the SST itself which drives the model can be compared with the pattern of the second most energetic POP mode (Fig. 3c), and they are indeed very similar. The explained variances amount to up to 60% in the western Equatorial Indian Ocean and the Tropical Southeast Atlantic (not shown). Not much variance is explained by the SST trend in the Equatorial East Pacific, which is not surprising given the strong interannual (ENSO) variability in this region, which considerably reduces the signal-to-noise ratio.
5 Sensitivity experiments
5.1 Sensitivity to experimental setup
The uncoupled results are entirely consistent with those from the coupled model. This can be inferred from the symbols forming pairs (uncoupled/coupled) being reasonably close to each other in the scatter diagram (Fig. 6), which provides some confidence in our uncoupled experimental setup. Figure 6a provide all cases that are used in the present study, and Fig. 6c–d for individual cases that will be discussed later. The SLP and near-surface wind velocity responses indicate clear changes in the Walker Circulation (Fig. 7b), similar to those in the original coupled model simulation (Fig. 7c). We would like to point out, however, two caveats. First, there are noticeable differences between the patterns simulated in forced mode and in the coupled model. As suggested by Fig. 6a, however, the large-scale structures are rather similar. Also, the SST patterns exhibiting a positive change in the SST gradient are not that similar to the observed twentieth century trend pattern, with relatively strong changes in the model found in the Equatorial Pacific (Fig. 7a), where we know ocean and atmosphere are tightly coupled. Thus the wind and SLP patterns are quite different to those simulated using observed SSTs (Fig. 4), but the results basically agree in terms of the two gradient indices (Fig. 6a). Second, the mechanism in the MPI coupled model that produces the SST anomalies in the Indian Ocean could be different to that in reality, which would enable a reproduction of the coupled model results in uncoupled mode with prescribed SST. Further investigation of this issue will be the topic of future research and is in our view beyond the scope of this paper.
5.2 Sensitivity to forcing SST
When the SST forcing is restricted only to the Indian Ocean (Fig. 10), the model simulates a strengthened Walker Circulation in both experiments (black and light blue crosses in Fig. 6b). We find a strong sensitivity of the SLP over the Indian Ocean: While the model simulates a high pressure anomaly in the case with no Indian Ocean SST forcing (Fig. 9c, d), it simulates a low pressure anomaly when the forcing is restricted only to the Indian Ocean (Fig. 10c, d). These results show that the Indian Ocean SST basically determines the sign of the Walker Circulation response in our model.
5.3 Sensitivity to radiative forcing
6 Relationship between SST and SLP changes
We additionally investigate now the behaviour of a number of atmosphere–ocean coupled general circulation models, to assess the performance of AGCM ECHAM5 in forced mode and to study the link between the gradients in SST and SLP in the Equatorial Pacific Sector. We computed 100-year trends of SST differences and corresponding SLP differences [using the definition of Vecchi et al. (2006) from various twentieth century integrations (20C3M) with global climate models forced by all known external forcing and from pre-industrial control runs (PICNTRL)]. The data were obtained from the CMIP3 database. We computed trends for the entire twentieth century from the 20C3M-ensemble, and from the pre-industrial control runs we chose the strongest positive and negative 100-year SST trend in each simulation and the corresponding SLP trends.
All analyzed climate model simulations are shown together with all forced AGCM experiments in a scatter diagram (Fig. 6a) with SST difference and (inverted) SLP difference as axes. The trend line in this summarizing scatter plot is adopted from Fig. 6d which shows the results from the unforced (pre-industrial) control runs with the coupled models. The forced AGCM integrations alone are presented in Fig. 6b. All twentieth century integrations with the coupled models are shown separately in Fig. 6c and all pre-industrial control runs with the coupled models separately in Fig. 6d. The SLP gradient change from the transient HadISST-forced ECHAM-T106 experiment amounts to about 0.8 hPa/century (Fig. 6a, b). This constitutes the strongest change in terms of the SLP gradient, which is not surprising given the rather strong SST gradient change. The major result, however, is the existence of a consistent linear relationship: An anomalously strong SST gradient goes along with an anomalously strong (inverted) SLP gradient, and this is independent of the type of model experiment that is considered. This finding indicates that the SST gradient is a good measure of the strength of the Walker Circulation during the twentieth century in virtually all climate models, independent of whether or not time-varying external forcing is included. Even the model (gfdl cm 2.1) that could explain the observed twentieth century SLP gradient change (Vecchi et al. 2006) supports the SST/SLP relationship. The SST gradient change in this model is inconsistent with observations (blue square in Fig. 6c), suggesting a stronger role of SST and that external forcing may have been less important than stated by Vecchi et al. (2006). In fact only one (hadgem1, light blue diamond in Fig. 6a) of the 24 analyzed twentieth century integrations is somewhat consistent with the changes inferred from the gridded datasets shown as black and red hexagons in Fig. 6a. This indicates that there is no externally driven signal in the Walker Circulation during the twentieth century which can be robustly simulated with global climate models when prescribing observed radiative forcing.
We note that the SST gradient change obtained from HadISST is considerably larger than that from ERSST, and also exceeds those simulated in the 20C3M integrations with the CMIP3 models. Yet the magnitude of the change can be simulated internally, as shown in Fig. 6d displaying the results from the pre-industrial control runs. We further note a sensitivity of the SST trends to the horizontal model resolution. This is due to the box averages used in the calculation of the SST gradients that are not exactly the same at the two used horizontal resolutions.
The trends derived from the gridded SLP datasets (HadSLP2 and ICOADS) are inconsistent with those simulated by the vast majority of the global climate models in twentieth century simulations (Fig. 6c), as noted above. On the other hand, the NCEP estimate (blue hexagon in Fig. 6a) is consistent with the models, which may have been expected as NCEP is also a model-based estimate. Moreover, the results from the pre-industrial control integrations (Fig. 6d) display a relatively large internal variability on centennial timescales in the two indices. All this may suggest that that a radiatively forced signal in the Walker Circulation strength remains too weak to be detected. The trend line in Fig. 6c, however, is shifted downward relative to that obtained from the (unforced) control runs (Fig. 6d), which suggests some influence of the radiative forcing along the lines suggested by Held and Soden (2006).
7 Comparison with data
We have shown that the evolution of SST in the Equatorial Region during the twentieth century is zonally asymmetric with a relatively steady increase of the zonal SST contrast between the Maritme Continent and the eastern and central Equatorial Pacific. Although there are large uncertainties in the observational datasets concerning SST trends during the twentieth century, the strengthening of the large-scale SST gradient seems to be robust. The enhanced SST gradient may have strengthened the Walker Circulation during the twentieth century, which was demonstrated by SST-forced experiments with the ECHAM5 AGCM.
We note a sensitivity of the AGCM to the forcing SST dataset: The HadISST forcing is characterized by a stronger SST gradient change and yields a stronger intensification of the Walker Circulation than the ERSST forcing. The changes in the SST gradient force near-surface wind, precipitation and upper air circulation anomalies over the Equatorial Pacific and the Equatorial Indian Ocean in both experiments that are entirely consistent with the picture of a strengthened Walker Circulation during the twentieth century. Furthermore, the model integrations also suggest an intensification of the Hadley Circulation.
We find that the model results are strongly influenced by the warming of the Indian Ocean. Atmosphere models may suffer from serious problems in simulating the response to SST variations in warm pool regions, and we cannot rule out the possibility that our model results are flawed. The uncoupled experimental setup, however, was carefully tested and proven reliable within a coupled model framework. Moreover, the forced AGCM used in this study exhibits similar response characteristics to those simulated by a number of global climate models in twentieth century simulations with observed radiative forcing or in pre-industrial control integrations. In particular, the role of time-varying external forcing in driving changes of the Walker Circulation is found to be weak in both our uncoupled AGCM simulations and twentieth century integrations with the coupled models. Aside from that the internal variability simulated by the coupled models is relatively large, even on multidecadal and longer timescales. This makes detection of an externally driven signal in the Walker Circulation a challenge.
With the caveat in mind that climate models may suffer from large biases and given the extremely large uncertainties in the various observational datasets, any conclusion about the behaviour of the Walker Circulation during the twentieth century is subject to large uncertainties and therefore more or less hypothetical. Many twentieth century integrations cluster around zero change and internal variability in control integrations is rather large. Only one of the 24 analyzed twentieth century integrations is consistent with the changes inferred from the gridded datasets, although its sea level pressure response is weaker and the magnitude of the change could be internally simulated.
This work was supported by the DFG-supported project SFB 754 (www.sfb754.de), the TROIA Project of the Deutsche Forschungsgemeinschaft, and by the European Project ENSEMBLES. Qingjia Meng was supported by the China Scholarship Council. The model runs were performed at the North-German Supercomputing Alliance (HLRN). This is a contribution to the Excellence Cluster “The Future Ocean” (www.ozean-der-zukunft.de).