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El Niño–Southern Oscillation (ENSO) is the leading mode of interannual ocean–atmosphere coupling in the tropical Pacific, greatly influencing the global climate system. Seasonal phase locking, which means that ENSO events usually peak in boreal winter, is a distinctive feature of ENSO. In model future projections, the ENSO sea surface temperature (SST) amplitude in winter shows no significant change with a large intermodel spread. However, whether and how ENSO phase locking will respond to global warming are not fully understood. In this study, using Community Earth System Model Large Ensemble (CESM-LE) projections, we found that the seasonality of ENSO events, especially its peak phase, has advanced under global warming. This phenomenon corresponds to the seasonal difference in the changes in the ENSO SST amplitude with an enhanced (weakened) amplitude from boreal summer to autumn (winter). Mixed layer ocean heat budget analysis revealed that the advanced ENSO seasonality is due to intensified positive meridional advective and thermocline feedback during the ENSO developing phase and intensified negative thermal damping during the ENSO peak phase. Furthermore, the seasonal variation in the mean El Niño–like SST warming in the tropical Pacific favors a weakened zonal advective feedback in boreal autumn–winter and earlier decay of ENSO. The advance of the ENSO peak phase is also found in most CMIP5/6 models that simulate the seasonal phase locking of ENSO well in the present climate. Thus, even though the amplitude response in the winter shows no model consensus, ENSO also significantly changes during different stages under global warming. [ABSTRACT FROM AUTHOR]

A currently popular idea is that El Niño–Southern Oscillation (ENSO) can be viewed as a linear deterministic system forced by noise representing processes with periods shorter than ENSO. Also, there is observational evidence to suggest that the Madden–Julian oscillation (MJO) acts to trigger and/or amplify the warm phase of ENSO in this way. The feedback of the slower process, ENSO, to higher-frequency atmospheric phenomena, of which a large part of the variability in the intraseasonal band is due to the MJO, has received little attention. This paper considers the hypothesis that the probability of an El Niño event is modified by high MJO activity and that, in turn, the MJO is regulated by ENSO activity. If this is indeed the case, then viewing ENSO as a low-frequency oscillation forced by additive stochastic noise would not present a complete picture. This paper tests the above hypothesis using a stochastically forced intermediate coupled model by allowing ENSO to directly influence the stochastic forcing. The model response to a variety of stochastic forcing types is found to be sensitive to the type of forcing applied. When the model is operated beyond its intrinsic Hopf bifurcation, its probability distribution function (PDF) is fundamentally altered when the stochastic forcing is changed from additive to multiplicative. The model integration period also influences the shape of the PDF, which is also compared to the PDF derived from observations. It is found that multiplicative stochastic forcing reproduces some measures of the observations better than the additive stochastic forcing. [ABSTRACT FROM AUTHOR]

Constraining the low-frequency (LF) behavior of general circulation models (GCMs) requires reliable observational estimates of LF variability. This two-part paper presents multiproxy reconstructions of Niño-3.4 sea surface temperature over the last millennium, applying two techniques [composite plus scale (CPS) and hybrid regularized expectation maximization (RegEM) truncated total least squares (TTLS)] to a network of tropical, high-resolution proxy records. This first part presents the data and methodology before evaluating their predictive skill using frozen network analysis (FNA) and pseudoproxy experiments. The FNA results suggest that about half of the Niño-3.4 variance can be reconstructed back to A.D. 1000, but they show little LF skill during certain intervals. More variance can be reconstructed in the interannual band where climate signals are strongest, but this band is affected by dating uncertainties (which are not formally addressed here). The CPS reliably estimates interannual variability, while LF fluctuations are more faithfully reconstructed with RegEM, albeit with inevitable variance loss. The RegEM approach is also tested on representative pseudoproxy networks derived from two millennium-long integrations of a coupled GCM. The pseudoproxy study confirms that reconstruction skill is significant in both the interannual and LF bands, provided that sufficient variance is exhibited in the target Niño-3.4 index. It also suggests that FNA severely underestimates LF skill, even when LF variability is strong, resulting in overly pessimistic performance assessments. The centennial-scale variance of the historical Niño-3.4 index falls somewhere between the two model simulations, suggesting that the network and methodology presented here would be able to capture the leading LF variations in Niño-3.4 for much of the past millennium, with the caveats noted above. [ABSTRACT FROM AUTHOR]

In this paper the extensive integrations produced for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) are used to examine the relationship between ENSO and monsoons at interannual and decadal time scales. The study begins with an analysis of the monsoon simulation in the twentieth-century integrations. Six of the 18 models were found to have a reasonably realistic representation of monsoon precipitation climatology. For each of these six models SST and anomalous precipitation evolution along the equatorial Pacific during El Niño events display considerable differences when compared to observations. Out of these six models only four [Geophysical Fluid Dynamics Laboratory Climate Model versions 2.0 and 2.1 (GFDL_CM_2.0 and GFDL_CM_2.1), Meteorological Research Institute (MRI) model, and Max Planck Institute ECHAM5 (MPI_ECHAM5)] exhibit a robust ENSO–monsoon contemporaneous teleconnection, including the known inverse relationship between ENSO and rainfall variations over India. Lagged correlations between the all-India rainfall (AIR) index and Niño-3.4 SST reveal that three models represent the timing of the teleconnection, including the spring predictability barrier, which is manifested as the transition from positive to negative correlations prior to the monsoon onset. Furthermore, only one of these three models (GFDL_CM_2.1) captures the observed phase lag with the strongest anticorrelation of SST peaking 2–3 months after the summer monsoon, which is partially attributable to the intensity of the simulated El Niño itself. The authors find that the models that best capture the ENSO–monsoon teleconnection are those that correctly simulate the timing and location of SST and diabatic heating anomalies in the equatorial Pacific and the associated changes to the equatorial Walker circulation during El Niño events. The strength of the AIR-Niño-3.4 SST correlation in the model runs waxes and wanes to some degree on decadal time scales. The overall magnitude and time scale for this decadal modulation in most of the models is similar to that seen in observations. However, there is little consistency in the phase among the realizations, suggesting a lack of predictability of the decadal modulation of the monsoon–ENSO relationship. The analysis was repeated for each of the four models using results from integrations in which the atmospheric CO2 concentration was raised to twice preindustrial values. From these “best” models in the double CO2 simulations there are increases in both the mean monsoon rainfall over the Indian subcontinent (by 5%–25%) and in its interannual variability (5%–10%). For each model the ENSO–monsoon correlation in the global warming runs is very similar to that in the twentieth-century runs, suggesting that the ENSO–monsoon connection will not weaken as global climate warms. This result, though plausible, needs to be taken with some caution because of the diversity in the simulation of ENSO variability in the coupled models that have been analyzed. Implications of the present results for monsoon prediction are discussed. [ABSTRACT FROM AUTHOR]

This study investigates the formation mechanism of the ocean surface warming pattern in response to a doubling CO2 with a focus on the role of ocean heat uptake (or ocean surface heat flux change, ΔQnet). We demonstrate that the transient patterns of surface warming and rainfall change simulated by the dynamic ocean–atmosphere coupled model (DOM) can be reproduced by the equilibrium solutions of the slab ocean–atmosphere coupled model (SOM) simulations when forced with the DOM ΔQnet distribution. The SOM is then used as a diagnostic inverse modeling tool to decompose the CO2-induced thermodynamic warming effect and the ΔQnet (ocean heat uptake)–induced cooling effect. As ΔQnet is largely positive (i.e., downward into the ocean) in the subpolar oceans and weakly negative at the equator, its cooling effect is strongly polar amplified and opposes the CO2 warming, reducing the net warming response especially over Antarctica. For the same reason, the ΔQnet-induced cooling effect contributes significantly to the equatorially enhanced warming in all three ocean basins, while the CO2 warming effect plays a role in the equatorial warming of the eastern Pacific. The spatially varying component of ΔQnet, although globally averaged to zero, can effectively rectify and lead to decreased global mean surface temperature of a comparable magnitude as the global mean ΔQnet effect under transient climate change. Our study highlights the importance of air–sea interaction in the surface warming pattern formation and the key role of ocean heat uptake pattern. [ABSTRACT FROM AUTHOR]

El Niño induces an anomalous easterly wind along the equator and a pair of anomalous anticyclones straddling the equator over the tropical Indian Ocean (TIO) during the autumn of its developing phase. Based on 30 coupled models participating in CMIP5, these atmospheric circulation anomalies over TIO are substantially weakened by about 12%–13% K−1 under global warming scenarios, associated with a weakened zonal gradient of the sea surface temperature (SST) anomaly. The mechanism for the response is investigated based on a hierarchy of model experiments. Based on stand-alone atmospheric model experiments under uniform and patterned mean-state SST warming, the atmospheric circulation anomaly over TIO during the autumn of the developing El Niño is also substantially weakened by about 8% K−1 even if the interannual variability of SST remains exactly unchanged, suggesting that the primary cause resides in the atmosphere rather than the SST anomaly. The tropospheric static stability is robustly enhanced under global warming, and experiments performed by a linear baroclinic model show that a much weaker atmospheric circulation anomaly over TIO is stimulated by an unchanged diabatic heating anomaly under a more stable atmosphere. The weakened atmospheric circulation anomaly due to enhanced static stability weakens the zonal gradient of the SST anomaly within TIO through local air–sea interaction, and it acts to further weaken the atmospheric circulation anomaly. The enhanced static stability of the troposphere is probably the primary cause and the air–sea interaction within TIO is a secondary cause for the weakened impact of the developing El Niño on atmospheric circulation variability over TIO. [ABSTRACT FROM AUTHOR]

By decomposing the variations of the Hadley circulation (HC) and tropical zonal-mean sea surface temperature (SST) into the equatorially asymmetric (HEA for HC, SEA for SST) and symmetric (HES for HC, SES for SST) components, the varying response of the HC to different SST meridional structures under warm and cold conditions of the Indo-Pacific warm pool (IPWP) is investigated over the period 1979–2016. The response of the HC to SST evidences an asymmetric variation between warm and cold IPWP conditions; that is, the response ratio of HEA to SEA relative to that of HES to SES is ~5 under warm conditions and ~2 under cold conditions. This asymmetry is primarily due to a decrease in the HEA-to-SEA ratio under cold IPWP conditions, and is driven by changes in the meridional distribution of SST anomalies. Equatorial asymmetric (symmetric) SST anomalies are dominated by warm (cold) IPWP conditions. Thus, variations of SEA are suppressed under cold IPWP conditions, contributing to the observed weakening of the HEA-to-SEA ratio. The results presented here indicate that the HC is more sensitive to the underlying SST when the IPWP is warmer, during which the variation of SEA is enhanced, suggesting a recent strengthening of the response of the HC to SST, as the IPWP has warmed over the past several decades, and highlighting the importance of the IPWP meridional structures rather than the overall warming of the HC. [ABSTRACT FROM AUTHOR]

Horn of Africa rainfall varies on multiple time scales, but the underlying climate system controls on this variability have not been examined comprehensively. This study therefore investigates the linkages between June–September Horn of Africa (especially Ethiopian) rainfall and regional atmospheric circulation and global sea surface temperature (SST) variations on several key time scales. Wavelet analysis of 5-day average or monthly total rainfall for 1970–99 identifies the dominant coherent modes of rainfall variability. Several regional atmospheric variables and global SST are then identically wavelet filtered, based on the rainfall frequency bands. Regression, correlation, and composite analyses are subsequently used to identify the most important rainfall–climate system time-scale relationships. The results show that Ethiopian monsoon rainfall variation is largely linked with annual time-scale atmospheric circulation patterns involving variability in the major components of the monsoon system. Although variability on the seasonal (75–210 days), quasi-biennial (QB; 1.42–3.04 yr), and El Niño–Southern Oscillation (ENSO; 3.04–4.60 yr) time scales accounts for much less variance than the annual mode (210 days–1.42 yr), they significantly affect Ethiopian rainfall by preferentially modulating the major regional monsoon components and remote teleconnection linkages. The seasonal time scale largely acts in phase with the annual mode, by enhancing or reducing the lower-tropospheric southwesterlies from the equatorial Atlantic during wet or dry periods. The wet QB phase strengthens the Azores and Saharan high and the tropical easterly jet (TEJ) over the Arabian Sea, while the wet ENSO phase enhances the Mascarene high, the TEJ, and the monsoon trough more locally. The effects of tropical SST on Ethiopian rainfall also are prominent on the QB and ENSO time scales. While rainfall–SST correlations for both the QB and ENSO modes are strongly positive (negative) over the equatorial western (eastern) Pacific, only ENSO exhibits widespread strong negative correlations over the Indian Ocean. Opposite QB and ENSO associations tend to characterize dry Ethiopian conditions. The relationships identified on individual time scales now are being used to develop and validate statistical prediction models for Ethiopia. [ABSTRACT FROM AUTHOR]

Surface temperatures have been observed in East Africa for more than 100 yr, but heretofore have not been subject to a rigorous climate analysis. To pursue this goal monthly averages of maximum (TMax), minimum (TMin), and mean (TMean) temperatures were obtained for Kenya and Tanzania from several sources. After the data were organized into time series for specific sites (60 in Kenya and 58 in Tanzania), the series were adjusted for break points and merged into individual gridcell squares of 1.25°, 2.5°, and 5.0°. Results for the most data-rich 5° cell, which includes Nairobi, Mount Kilimanjaro, and Mount Kenya, indicate that since 1905, and even recently, the trend of TMax is not significantly different from zero. However, TMin results suggest an accelerating temperature rise. Uncertainty estimates indicate that the trend of the difference time series (TMax - TMin) is significantly less than zero for 1946–2004, the period with the highest density of observations. This trend difference continues in the most recent period (1979–2004), in contrast with findings in recent periods for global datasets, which generally have sparse coverage of East Africa. The differences between TMax and TMin trends, especially recently, may reflect a response to complex changes in the boundary layer dynamics; TMax represents the significantly greater daytime vertical connection to the deep atmosphere, whereas TMin often represents only a shallow layer whose temperature is more dependent on the turbulent state than on the temperature aloft. Because the turbulent state in the stable boundary layer is highly dependent on local land use and perhaps locally produced aerosols, the significant human development of the surface may be responsible for the rising TMin while having little impact on TMax in East Africa. This indicates that time series of TMax and TMin should become separate variables in the study of long-term changes. [ABSTRACT FROM AUTHOR]

Climate change over the last several decades is suggested to cause a decrease in the magnitude of the uptake of CO2 by the Southern Ocean (Le Quere et al.). In this study, the atmospheric fields from NCEP R1 for the years 1948–2003 are used to drive an ocean biogeochemical model to probe how changes in the heat and freshwater fluxes and in the winds affect the Southern Ocean’s uptake of carbon. Over this period, the model simulations herein show that the increases in heat and freshwater fluxes drive a net increase in Southern Ocean uptake (south of 40°S) while the increases in wind stresses drive a net decrease in uptake. The total Southern Ocean response is nearly identical with the simulation without climate change because the heat and freshwater flux response is approximately both equal and opposite to the wind stress response. It is also shown that any change in the Southern Ocean anthropogenic carbon uptake is always opposed by a much larger change in the natural carbon air–sea exchange. For the 1948–2003 period, the changes in the natural carbon cycle dominate the Southern Ocean carbon uptake response to climate change. However, it is shown with a simple box model that when atmospheric CO2 levels exceed the partial pressure of carbon dioxide (pCO2) of the upwelled Circumpolar Deep Water (≈450 μatm) the Southern Ocean uptake response will be dominated by the changes in anthropogenic carbon uptake. Therefore, the suggestion that the Southern Ocean carbon uptake is a positive feedback to global warming is only a transient response that will change to a negative feedback in the near future if the present climate trend continues. Associated with the increased outgassing of carbon from the natural carbon cycle was a reduction in the aragonite saturation state of the high-latitude Southern Ocean (south of 60°S). In the simulation with just wind stress changes, the reduction in the high-latitude Southern Ocean aragonite saturation state (≈0.2) was comparable to the magnitude of the decline in the aragonite saturation state over the last 4 decades because of rising atmospheric CO2 levels (≈0.2). The simulation showed that climate change could significantly impact aragonite saturation state in the Southern Ocean. [ABSTRACT FROM AUTHOR]

Climate feedback analysis constitutes a useful framework for comparing the global mean surface temperature responses to an external forcing predicted by general circulation models (GCMs). Nevertheless, the contributions of the different radiative feedbacks to global warming (in equilibrium or transient conditions) and their comparison with the contribution of other processes (e.g., the ocean heat uptake) have not been quantified explicitly. Here these contributions from the classical feedback analysis framework are defined and quantified for an ensemble of 12 third phase of the Coupled Model Intercomparison Project (CMIP3)/Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) coupled atmosphere–ocean GCMs. In transient simulations, the multimodel mean contributions to global warming associated with the combined water vapor–lapse-rate feedback, cloud feedback, and ocean heat uptake are comparable. However, intermodel differences in cloud feedbacks constitute by far the most primary source of spread of both equilibrium and transient climate responses simulated by GCMs. The spread associated with intermodel differences in cloud feedbacks appears to be roughly 3 times larger than that associated either with the combined water vapor–lapse-rate feedback, the ocean heat uptake, or the radiative forcing. [ABSTRACT FROM AUTHOR]

The decadal-scale variability in the tropical Pacific has been analyzed herein by means of observations and numerical model simulations. The two leading modes of the sea surface temperature (SST) variability in the central western Pacific are a decadal mode with a period of about 10 yr and the ENSO mode with a dominant period of about 4 yr. The SST anomaly pattern of the decadal mode is ENSO like. The decadal mode, however, explains most variance in the western equatorial Pacific and off the equator. A simulation with an ocean general circulation model (OGCM) forced by reanalysis data is used to explore the origin of the decadal mode. It is found that the variability of the shallow subtropical–tropical overturning cells is an important factor in driving the decadal mode. This is supported by results from a multicentury integration with a coupled ocean–atmosphere general circulation model (CGCM) that realistically simulates tropical Pacific decadal variability. Finally, the sensitivity of the shallow subtropical–tropical overturning cells to greenhouse warming is discussed by analyzing the results of a scenario integration with the same CGCM. [ABSTRACT FROM AUTHOR]

In a simulation of enhanced tropical cyclones in a warmer world, Knutson and Tuleya make several assumptions that are not borne out in the real world. They include an unrealistically large carbon dioxide growth rate, an overly strong relationship between sea surface temperature and hurricane intensity, and the use of a mesoscale model that has shown little to no useful skill in predicting current-day hurricane intensity. After accounting for these inaccuracies, a detectable increase in Atlantic hurricane intensity in response to growing atmospheric greenhouse gas levels during this century becomes unlikely. [ABSTRACT FROM AUTHOR]