Abstract

Anomalous rainfall in the tropical Pacific driven by El Niño–Southern Oscillation (ENSO) is a crucial pathway of ENSO’s global impacts. The changes in ENSO rainfall under global warming vary among the models, even though previous studies have shown that many models project that ENSO rainfall will likely intensify and shift eastward in response to global warming. The present study evaluates the robustness of the changes in ENSO rainfall in 32 CMIP5 models forced under the representative concentration pathway 8.5 (RCP8.5) scenario. The robust increase in mean-state moisture dominates the robust intensification of ENSO rainfall. The uncertain amplitude changes in ENSO-related SST variability are the largest source of the uncertainty in ENSO rainfall changes through influencing the amplitude changes in ENSO-driven circulation variability, whereas the structural changes in ENSO SST and ENSO circulation enhancement in the central Pacific are more robust than the amplitude changes. The spatial pattern of the mean-state SST changes—the departure of local SST changes from the tropical mean—with an El Niño–like pattern is a relatively robust factor, although it also contains pronounced intermodel differences. The intermodel spread of historical ENSO circulation is another noteworthy source of the uncertainty in ENSO rainfall changes. The intermodel standard deviation of ENSO rainfall changes increases along with the increase in global-mean surface temperature. However, the robustness of enhanced ENSO rainfall changes in the central-eastern Pacific is almost unchanged, whereas the eastward shift of ENSO rainfall is increasingly robust along with the increase in global-mean surface temperature.

Abstract

El Niño–Southern Oscillation (ENSO) is one of the most important drivers of climatic variability on the global scale. Much of this variability arises in response to ENSO-driven changes in tropical Pacific rainfall. Previous research has shown that the ENSO-driven tropical Pacific rainfall variability can shift east and intensify in response to global warming, even if ENSO-related SST variability remains unchanged. Here, the twenty-first century changes in ENSO-driven tropical Pacific rainfall variability in 32 CMIP5 models forced under the representative concentration pathway 8.5 (RCP8.5) scenario are examined, revealing that the pattern of changes in ENSO-driven rainfall is not only gradually enhanced but also shifts steadily eastward along with the global-mean temperature increase. Using a recently developed moisture budget decomposition method, it is shown that the projected changes in ENSO-driven rainfall variability in the tropical Pacific can be primarily attributed to a projected increase in both mean-state surface moisture and spatially relative changes in mean-state SST, defined as the departure of local SST changes from the tropical mean. The enhanced moisture increase enlarges the thermodynamic component of ENSO rainfall changes. The enhanced El Niño–like changes in mean-state SST steadily move the dynamic component of changes in ENSO-driven rainfall variability to the central-eastern Pacific, along with increasing global-mean temperature.

Abstract

The seasonal changes in tropical SST under global warming are investigated based on the representative concentration pathway 8.5 (RCP8.5) and historical runs in 31 models from phase 5 of CMIP (CMIP5). The tropical SST changes show three pronounced seasonal patterns: the peak locking to the equator throughout the year and the weaker equatorial changes and stronger hemispheric asymmetric changes (HACs) in boreal autumn. The magnitude of the seasonal patterns is comparable to the tropical-mean warming and the annual-mean patterns, implying great impacts on global climate changes. The peak locking to the equator is a result of the equatorial locking of the minimum damping of climatological latent heat flux and the ocean heat transport changes. Excluding the role of ocean heat transport suggested in previous studies, the weaker equatorial warming in boreal autumn is contributed by stronger evaporation damping as a result of stronger climatological evaporation and increased surface wind speed. The seasonal variations of the HAC are driven by the variations of the damping effect of climatological evaporation. In boreal summer, the damping effect of climatological evaporation, which is greater in the Southern Hemisphere, promotes the development of the HAC. Consequently, the HAC peaks in boreal autumn when the damping effect of climatological evaporation transforms to a reverse meridional pattern, which is greater in the Northern Hemisphere. The wind–evaporation–SST feedback, as the key process of the annual-mean HAC, amplifies the seasonal variations of the HAC in tropical SST.

Abstract

Climatology and interannual variability of convectively coupled equatorial wave (CCEW) activity, including the mixed Rossby–gravity (MRG), tropical-depression-type (TD-type), equatorial Rossby (ER), and Kelvin waves, are investigated using the satellite-observed brightness temperature data from the Cloud Archive User Service. The monthly activity of CCEWs is represented by the root mean square of the daily filtered convections in each month based on the Wheeler–Kiladis filtering method. More precise seasonal cycles of CCEW activity are obtained from the meridional and zonal mean climatology.

Interannual variance of CCEW activity is further investigated. Kelvin wave activity has maximum interannual variance over the eastern Pacific, while the other three waves are most variable in the intertropical convergence zone. The four active CCEWs all have significant correlation with the background convection and local sea surface temperature (SST) over the central and eastern Pacific, but they are not significantly correlated over other regions. The El Niño events may induce more trapped and active CCEWs over the central and eastern Pacific but weaker MRG and TD-type waves over the warm pool. In contrast, the El Niño Modoki has much weaker correlation with CCEW activity. CCEW activity over the southeastern Indian Ocean is negatively correlated with the Indian Ocean dipole, while that over the western and northern Indian Ocean may be determined by atmospheric internal disturbances. The tropical southern Atlantic mode is the strongest Atlantic SST anomaly mode correlated with the Atlantic CCEW activity.

Abstract

The scale-dependent characteristics of the optimal perturbations in a zonally asymmetric barotropic model are examined. The dependence of the optimal energy growth on the initial scale is investigated through the calculations of spectrally constrained optimal perturbations. Considering an optimization time of τ = 3 days, and a basic state containing an idealized Asian jet, the optimal amplification factor generally increases with the decrease of the imposed initial scale. In the absence of diffusion, the most amplifying scale becomes the smallest scale in the model. An energetics analysis shows that the energy conversion in the optimal excitation process is dominated by the shear straining term, with a sharp increase in the scale of the perturbation accompanying the explosive energy growth. These results show the similarity between the optimally growing process in the zonally asymmetric system and the shear straining process in a parallel shear flow. Except when a small τ is considered or a sufficiently strong diffusion is used in the system, the optimal energy growth for small-scale disturbances sensitively depends on the zonally varying feature of the basic state. With τ = 3 days, the optimal amplification factors for small-scale disturbances are reduced significantly when the idealized Asian jet is shortened by only one-fifth. At the same time, those for medium- and large-scale disturbances are almost unaffected by the change of the basic state. The reasons for this contrast of the sensitivity property between the small and large scales are discussed.

Abstract

The evolution of El Niño–Southern Oscillation (ENSO) is an essential predictor for global climate anomalies, but ENSO’s effect is regulated by the considerable diversity of post-ENSO evolution. This study identified that there coexist a decaying mode and a developing mode of ENSO in the ENSO decaying periods (from September to August) through empirical orthogonal function (EOF) and extended EOF analyses. The diversity of ENSO evolution during the decaying periods is actually determined by the ENSO developing mode, which is unsteady and susceptible to other signals, whereas the ENSO decaying mode is quite steady. Using multiple datasets, segmented datasets, and sea surface temperature anomalies (SSTAs)-residual EOF, the key signals related to the diversity of ENSO evolution are identified and verified in the South Atlantic and equatorial South Pacific. Based on the decaying and developing modes, a prediction model was constructed to predict the boreal summer Niño-3 index. The ENSO developing mode, explaining very small variance, plays a decisive role in determining the prediction of the summer SSTAs in the Pacific. The forecasting skill of summer ENSO can also be greatly improved when continuous SSTAs and interbasin interactions in the whole tropics are considered. This prediction result indicates that identified precursory disturbance signals and interbasin interactions for ENSO development are crucial for boreal summer ENSO prediction.

Abstract

El Niño–Southern Oscillation (ENSO) is one of the most important sources of climate interannual variability. A prominent characteristic of ENSO is the asymmetric, or so-called nonlinear, local rainfall response to El Niño (EN) and La Niña (LN), in which the maximum rainfall anomalies during EN are located farther east than those during LN. In this study, the changes in rainfall anomalies during EN and LN are examined based on the multimodel ensemble mean results of 32 CMIP5 models under the representative concentration pathway 8.5 (RCP8.5) scenario. It is found that robust EN–LN asymmetric changes in rainfall anomalies exist. The rainfall anomalies during EN and LN both shift eastward and intensify under global warming, but the eastward shift during EN is farther east than that during LN. A simplified moisture budget decomposition method is applied to study the mechanism of the asymmetric response. The results show that the robust increase in mean-state moisture can enlarge the EN–LN asymmetry of the rainfall anomalies, and the spatial relative changes in mean-state SST with an El Niño–like pattern can shift the rainfall anomalies farther east during EN than during LN, enlarging the difference in the zonal locations of the rainfall response to EN and LN. The role of the relative changes in mean-state SST can also be interpreted as follows: the decreased zonal gradient of mean-state SST due to El Niño–like warming leads to a larger EN–LN asymmetry of rainfall anomalies under a future warming climate.

Abstract

This study investigates how intermodel differences in large-scale ocean dynamics affect the tropical Pacific sea surface temperature (SST) warming (TPSW) pattern under global warming, as projected by 32 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). The largest cause of intermodel TPSW differences is related to the cloud–radiation feedback. After removing the effect of cloud–radiation feedback, the authors find that differences in ocean advection play the next largest role, explaining around 14% of the total intermodel variance in TPSW. Of particular importance are differences in climatological zonal overturning circulation among the models. With the robust enhancement of ocean stratification across models, models with relatively strong climatological upwelling tend to have relatively weak SST warming in the eastern Pacific. Meanwhile, the pronounced intermodel differences in ocean overturning changes contribute little to uncertainty in the TPSW pattern. The intermodel differences in climatological zonal overturning are found to be associated with the intermodel spread in climatological SST. In most CMIP5 models, there is a common cold tongue associated with an overly strong overturning in the climatology simulation, implying a La Niña–like bias in the TPSW pattern projected by the MME of the CMIP5 models. This provides further evidence for the projection that the TPSW pattern should be closer to an El Niño–like pattern than the MME projection.

Abstract

This study develops a new observational constraint method, called multimodel ensemble pattern regression (EPR), to correct the projections of regional climate change by the conventional unweighted multimodel mean (MMM). The EPR method first extracts leading modes of historical bias using intermodel EOF analysis, then builds up the linear correlated modes between historical bias and change bias using multivariant linear regression, and finally estimates the common change bias induced by common historical bias. Along with correcting common change bias, the EPR method implicitly removes the intermodel uncertainty in the change projection deriving from the intermodel diversity in background simulation.

The EPR method is applied to correct the patterns of tropical Pacific SST changes using the historical and representative concentration pathway 8.5 (RCP8.5) runs in 30 models from phase 5 of CMIP (CMIP5) and observed SSTs. The common bias patterns of the tropical Pacific SSTs in historical runs, including the excessive cold tongue, the southeastern warm bias, and the narrower warm pool, are estimated to induce La Niña–like change biases. After the estimated common change biases are removed, the corrected SST changes display a pronounced El Niño–like pattern and have much greater zonal gradients. The bias correction decreases by around half of the intermodel uncertainties in the MMM SST projections. The patterns of corrected tropical precipitation and circulation change are dominated by the enhanced SST change patterns, displaying a pronounced warmer-get-wetter pattern and a decreased Walker circulation with decreased uncertainties.

Abstract

The role of the intermodel spread of cloud–radiation feedback in the uncertainty in the tropical Pacific SST warming (TPSW) pattern under global warming is investigated based on the historical and RCP8.5 runs from 32 models participating in CMIP5. The large intermodel discrepancies in cloud–radiation feedback contribute 24% of the intermodel uncertainty in the TPSW pattern over the central Pacific. The mechanism by which the cloud–radiation feedback influences the TPSW pattern is revealed based on an analysis of the surface heat budget. A relatively weak negative cloud–radiation feedback over the central Pacific cannot suppress the surface warming as greatly as in the multimodel ensemble and thus induces a warm SST deviation over the central Pacific, producing a low-level convergence that suppresses (enhances) the evaporative cooling and zonal cold advection in the western (eastern) Pacific. With these processes, the original positive SST deviation over the central Pacific will move westward to the western and central Pacific, with a negative SST deviation in the eastern Pacific. Compared with the observed cloud–radiation feedback from six sets of reanalysis and satellite-observed data, the negative cloud–radiation feedback in the models is underestimated in general. It implies that the TPSW pattern should be closer to an El Niño–like pattern based on the concept of observational constraint. However, the observed cloud–radiation feedback from the various datasets also demonstrates large discrepancies in magnitude. Therefore, the authors suggest that more effort should be made to improve the precision of shortwave radiation observations and the description of cloud–radiation feedback in models for a more reliable projection of the TPSW pattern in future.