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The zonal sea surface temperature (SST) gradient across the tropical Pacific is a pacemaker of the variable rates of global warming. In both historical simulations and future projections, the current state-of-the-art climate models show evident spread in the changes of zonal SST gradient, but the reasons remain unknown. Here, we quantify the contributions of internal variability and model spread to the uncertainty of zonal SST gradient changes by analyzing 342 realizations from CMIP5 and CMIP6 models and several sets of large-ensemble simulations. We found that the internal variability dominates the total uncertainty at multidecadal time scales (∼31-yr trends). Although the ratio of internal uncertainty to the total uncertainty declines along with higher emission of greenhouse gases under global warming, it is still over 80% at the multidecadal time scales in the future. The Pacific decadal oscillation is identified as the key internal mode responsible for the multidecadal uncertainty. For the future projections at centurial time scales, the uncertainty of zonal SST gradient changes is mainly from the intermodel spread in response to external forcing, accounting for about 70% of the uncertainty based on the difference between 2070–99 and 1970–99. The model spread in the cloud–shortwave radiation–SST feedback over the tropical Pacific is important in the uncertainty of zonal SST gradient changes. In particular, the intensity of negative convective cloud feedback in the western Pacific dominates the spread in CMIP5 models, while the intensity of stratocumulus cloud feedback over the southeastern Pacific is the primary process influencing the uncertainty in CMIP6 models. [ABSTRACT FROM AUTHOR]

Rainfall projections from the Coupled Model Intercomparison Project (CMIP) models are strongly tied to projected sea surface temperature (SST) spatial patterns through the "warmer-gets-wetter" mechanism. While these models consistently project an enhanced equatorial warming, they, however, indicate much more uncertain changes in zonal SST gradients. That translates into large uncertainties on rainfall projections. Here, we force an atmospheric model with synthetic SSTs whose zonal SST gradient changes span the range of CMIP5 uncertainties in the presence and in the absence of the robust equatorially enhanced warming. Our results confirm that projected rainfall changes are dominated by the effect of circulation changes, which are tied to SST through the "warmer-gets-wetter" mechanism. We show that SPCZ rainfall changes are entirely driven by the uncertain zonal SST gradient changes. The western equatorial Pacific rainfall increase is largely controlled by the robust enhanced equatorial warming for modest zonal SST gradient changes. However, for larger values, the effect of the zonal SST gradient change on rainfall projections becomes dominant due to nonlinear interactions with the enhanced equatorial warming. Overall, our study demonstrates that uncertainties in the zonal SST gradient changes strongly contribute to uncertainties in rainfall projections over both the South Pacific convergence zone and western equatorial Pacific. It is thus critical to reduce these uncertainties to produce more robust precipitation estimates. [ABSTRACT FROM AUTHOR]

Previous work identified an anthropogenic fingerprint pattern in TAC(x, t), the amplitude of the seasonal cycle of mid- to upper-tropospheric temperature (TMT), but did not explicitly consider whether fingerprint identification in satellite TAC(x, t) data could have been influenced by real-world multidecadal internal variability (MIV). We address this question here using large ensembles (LEs) performed with five climate models. LEs provide many different sequences of internal variability noise superimposed on an underlying forced signal. Despite differences in historical external forcings, climate sensitivity, and MIV properties of the five models, their TAC(x, t) fingerprints are similar and statistically identifiable in 239 of the 240 LE realizations of historical climate change. Comparing simulated and observed variability spectra reveals that consistent fingerprint identification is unlikely to be biased by model underestimates of observed MIV. Even in the presence of large (factor of 3–4) intermodel and inter-realization differences in the amplitude of MIV, the anthropogenic fingerprints of seasonal cycle changes are robustly identifiable in models and satellite data. This is primarily due to the fact that the distinctive, global-scale fingerprint patterns are spatially dissimilar to the smaller-scale patterns of internal TAC(x, t) variability associated with the Atlantic multidecadal oscillation and El Niño–Southern Oscillation. The robustness of the seasonal cycle detection and attribution results shown here, taken together with the evidence from idealized aquaplanet simulations, suggest that basic physical processes are dictating a common pattern of forced TAC(x, t) changes in observations and in the five LEs. The key processes involved include GHG-induced expansion of the tropics, lapse-rate changes, land surface drying, and sea ice decrease. [ABSTRACT FROM AUTHOR]

During El Niño events, a strong tropics-wide warming of the free troposphere is observed (of order 1 K at 300 hPa). This warming plays an important role for the teleconnection processes associated with El Niño but it remains unclear what initiates this warming. Since convective quasi-equilibrium only holds in regions of deep convection, the strong free-tropospheric warming implies that the warmest surface waters (where atmospheric deep convection occurs) must warm during El Niño. We analyze the evolution of the oceanic mixed layer heat budget over El Niño events as function of sea surface temperature (SST). Data from the ERA5 and an unforced simulation of a coupled climate model both confirm that SSTs during an El Niño event increase at the high end of the SST distribution. The data show that this is due to an anomalous heat flux from the atmosphere into the ocean caused by a decrease in evaporation due anomalously weak lowlevel winds (i.e., relative to the wind speed observed in the domain of deep convection in the climatological base state). It is hypothesized that the more zonally symmetric circulation during El Niño is responsible for the weakening of low-level winds. The result of a substantial heat flux into the ocean in the domain of atmospheric deep convection (the opposite of the canonical heat flux out of the ocean into the atmosphere observed in the cold eastern Pacific) caused by a decrease in low-level wind speed implies that the prominent tropospheric warming results from mechanical forcing. [ABSTRACT FROM AUTHOR]

The intraseasonal oscillations (ISOs) over the tropical western North Pacific (WNP) modulate atmospheric convection and heating and affect weather and climate in remote regions through atmospheric teleconnection. The present study unravels the fact that the ISO intensity increase over the tropical WNP in El Niño developing summers is larger, with the center located eastward, compared with the decrease in La Niña developing summers. The asymmetric ISO intensity changes are attributed to the eastward shift of regions of anomalous low-level westerly winds, ascent, easterly shear of zonal winds, and large moisture in El Niño developing summers and westward shift of regions of opposite anomalies in La Niña developing summers, respectively. The asymmetric atmospheric mean anomalies, in turn, are due to the westward shift of anomalous cooling in La Niña developing summers compared to anomalous warming in El Niño developing summers. The 10–20- and 30–60-day ISOs show different patterns of intensity variations due to their different source regions and propagation paths. Atmospheric model simulations confirm the asymmetric response of boreal summer ISO intensity over the tropical WNP to El Niño and La Niña events and the role of asymmetric atmospheric background field changes. Sensitivity experiments illustrate that asymmetric changes in the ISO intensity and atmospheric background fields over the tropical WNP are due to their asymmetric response to opposite tropical central-eastern Pacific SST anomalies. The asymmetry in tropical central-eastern Pacific SST anomalies in El Niño and La Niña events has a small impact on asymmetric ISO intensity changes over the tropical WNP. [ABSTRACT FROM AUTHOR]

The impacts of freshwater flux (FWF) forcing on interannual variability in the tropical Pacific climate system are investigated using a hybrid coupled model (HCM), constructed from an oceanic general circulation model (OGCM) and a simplified atmospheric model, whose forcing fields to the ocean consist of three components. Interannual anomalies of wind stress and precipitation minus evaporation, (P – E), are calculated respectively by their statistical feedback models that are constructed from a singular value decomposition (SVD) analysis of their historical data. Heat flux is calculated using an advective atmospheric mixed layer (AML) model. The constructed HCM can well reproduce interannual variability associated with ENSO in the tropical Pacific. HCM experiments are performed with varying strengths of anomalous FWF forcing. It is demonstrated that FWF can have a significant modulating impact on interannual variability. The buoyancy flux (QB) field, an important parameter determining the mixing and entrainment in the equatorial Pacific, is analyzed to illustrate the compensating role played by its two contributing parts: one is related to heat flux (QT) and the other to freshwater flux (QS). A positive feedback is identified between FWF and SST as follows: SST anomalies, generated by El Niño, nonlocally induce large anomalous FWF variability over the western and central regions, which directly influences sea surface salinity (SSS) and QB, leading to changes in the mixed layer depth (MLD), the upper-ocean stability, and the mixing and the entrainment of subsurface waters. These oceanic processes act to enhance the SST anomalies, which in turn feedback to the atmosphere in a coupled ocean–atmosphere system. As a result, taking into account anomalous FWF forcing in the HCM leads to an enhanced interannual variability and ENSO cycles. It is further shown that FWF forcing is playing a different role from heat flux forcing, with the former acting to drive a change in SST while the latter represents a passive response to the SST change. This HCM-based modeling study presents clear evidence for the role of FWF forcing in modulating interannual variability in the tropical Pacific. The significance and implications of these results are further discussed for physical understanding and model improvements of interannual variability in the tropical Pacific ocean–atmosphere system. [ABSTRACT FROM AUTHOR]

North Pacific decadal oceanic and atmospheric variability is examined from a 650-yr control integration of the Community Climate System Model version 2. The dominant pattern of winter sea surface temperature (SST) variability is similar to the observed “Pacific decadal oscillation,” with maximum amplitude along the Kuroshio Extension. SST anomalies in this region exhibit significant spectral peaks at approximately 16 and 40 yr. Lateral geostrophic heat flux divergence, caused by a meridional shift of the Kuroshio Extension forced by basin-scale wind stress curl anomalies 3–5 yr earlier, is responsible for the decadal SST variability; local surface heat flux and Ekman heat flux divergence act as a damping and positive feedback, respectively. A simple linear Rossby wave model is invoked to explicitly demonstrate the link between the wind stress curl forcing and decadal variability in the Kuroshio Extension. The Rossby wave model not only successfully reproduces the two decadal spectral peaks, but also illustrates that only the low-frequency (>10-yr period) portion of the approximately white noise wind stress curl forcing is relevant. This model also demonstrates that the weak and insignificant decadal spectral peaks in the wind stress curl forcing are necessary for producing the corresponding strong and significant oceanic peaks in the Kuroshio Extension. The wind stress curl response to decadal SST anomalies in the Kuroshio Extension is similar in structure but opposite in sign and somewhat weaker than the wind stress curl forcing pattern. These results suggest that the simulated North Pacific decadal variability owes its existence to two-way ocean–atmosphere coupling. [ABSTRACT FROM AUTHOR]

To elucidate the unique seasonality in the coupled system of the subtropical Mascarene high and low-level clouds, the present study compares wintertime cloud radiative impacts on the high with their summertime counterpart through coupled and atmospheric general circulation model simulations. A comparison of a fully coupled control simulation with another simulation in which the radiative effects of low-level clouds are artificially switched off demonstrates that the low-cloud effect on the formation of the Mascarene high is much weaker in winter. Background climatology plays an important role in this seasonality of the Mascarene high reinforcement. Relative to summer, the suppression of deep convection due to low-level clouds that acts to reinforce the high is much weaker in winter. This arises from 1) seasonally lower sea surface temperature in concert with the smaller sea surface temperature reduction due to the deeper ocean mixed layer and the weaker cloud radiative effect under weaker insolation and 2) seasonally stronger subtropical subsidence associated with the Hadley circulation in winter. As verified through atmospheric dynamical model experiments, enhanced cloud-top radiative cooling by low-level clouds acts to reinforce the wintertime Mascarene high in comparable magnitude as in summer. The present study reveals that the self-sustaining feedback with low-level clouds alone is insufficient for replenishing the full strength of the wintertime Mascarene high. This implies that another internal feedback pathway and/or external driver must be operative in maintaining the wintertime high. [ABSTRACT FROM AUTHOR]

The subtropical oceans between 35° and 20°S in the Southern Hemisphere (SH) have exhibited prevailingly rapid sea level rise (SLR) rates since the mid-twentieth century, amplifying damages of coastal hazards and exerting increasing threats to South America, Africa, and Australia. Yet, mechanisms of the observed SLR have not been firmly established, and its representation in climate models has not been examined. By analyzing observational sea level estimates, ocean reanalysis products, and ocean model hindcasts, we show that the steric SLR of the SH subtropical oceans between 35° and 20°S is faster than the global mean rate by 18.2% ± 9.9% during 1958–2014. However, present climate models—the fundamental bases for future climate projections—generally fail to reproduce this feature. Further analysis suggests that the rapid SLR in the SH subtropical oceans is primarily attributable to the persistent upward trend of the southern annular mode (SAM). Physically, this trend in SAM leads to the strengthening of the SH subtropical highs, with the strongest signatures observed in the southern Indian Ocean. These changes in atmospheric circulation promote regional SLR in the SH subtropics by driving upper-ocean convergence. Climate models show systematic biases in the simulated structure and trend magnitude of SAM and significantly underestimate the enhancement of subtropical highs. These biases lead to the inability of models to correctly simulate the observed subtropical SLR. This work highlights the paramount necessity of reducing model biases to provide reliable regional sea level projections. [ABSTRACT FROM AUTHOR]

The effect of Eurasian spring snowmelt on surface air temperature (SAT) in late spring (April-May) and early summer (June-July) and the relevant physical mechanisms during 1981-2016 are investigated. Results show that the first mode of the interannual Eurasian spring snowmelt represents an east-west dipole anomaly pattern, with an intense center over Siberia and another moderate center around eastern Europe. The European spring snowmelt shows an insignificant relation to the local SAT, whereas the Siberian spring snowmelt has a significant impact on the SAT in late spring and early summer. More Siberian spring snowmelt contributes to higher SAT in late spring and lower SAT in early summer via different mechanisms. In late spring, increased Siberian spring snowmelt corresponds to less local surface albedo and cloud cover, leading to the surface absorbing more shortwave radiation and thereby higher SAT. The subsurface and deep soil moisture anomalies generated from Siberian spring snowmelt can persist into early summer. Excessive Siberian spring snowmelt corresponds to positive soil moisture anomalies, contributing to decreased sensible heat and increased cloud cover in early summer. Increased cloud cover leads to the surface receiving less shortwave radiation. Thus, lower SAT appears over Siberia in early summer due to reduced sensible heat and shortwave radiation. However, the simulation of Eurasian spring snowmelt variability and its influences on SAT via the snow hydrological effect is still a challenge for the climate models that participated in phase 6 of the Coupled Model Intercomparison Project. [ABSTRACT FROM AUTHOR]

This work quantitatively evaluates the fidelity with which the northern annular mode (NAM), southern annular mode (SAM), Pacific-North American pattern (PNA), El Niño-Southern Oscillation (ENSO), Pacific decadal oscillation (PDO), Atlantic multidecadal oscillation (AMO), and the first-order mode interactions are represented in Earth system model (ESM) output from the CMIP6 archive. Several skill metrics are used as part of a differential credibility assessment (DCA) of both spatial and temporal characteristics of the modes across ESMs, ESM families, and specific ESM realizations relative to ERA5. The spatial patterns and probability distributions are generally well represented but skill scores that measure the degree to which the frequencies of maximum variance are captured are consistently lower for most ESMs and climatemodes. Substantial variability in skill scoresmanifests across realizations fromindividual ESMs for the PNA and oceanic modes. Further, the ESMs consistently overestimate the strength of the NAM-PNA first-order interaction and underestimate the NAM-AMO connection. These results suggest that the choice of ESMand ESMrealizations will continue to play a critical role in determining climate projections at the global and regional scale at least in the near term. SIGNIFICANCE STATEMENT: Internal climate variability occurs over multiple spatial and temporal scales and is encapsulated in a series of internal climate modes. The representation of such modes in climate models is a critically important aspect of model fidelity. Analyses presented herein uses several skill scores to evaluate both the spatial and temporal manifestations of these climate modes in the CMIP6 generation of Earth system models (ESMs). There is marked variability in model fidelity for these modes and this variability in credibility within the current climate has important implications for the choice of specific ESMs and ESM realizations in making climate projections. [ABSTRACT FROM AUTHOR]

An objective front detection method is applied to ERA5, CMIP5 historical, and RCP8.5 simulations to evaluate climate model performance in simulating front frequency and to understand future projections of seasonal front activities. The study area is East Asia for two natural seasons, defined as winter (2 December–14 February) and spring (15 February–15 May), in accordance with regional circulation and precipitation patterns. Seasonal means of atmospheric circulation and thermal structures are analyzed to understand possible factors responsible for future front changes. The front location and frequency in CMIP5 historical simulations are captured reasonably. Frontal precipitation accounts for more than 30% of total precipitation over subtropical regions. Projections suggest that winter fronts will decrease over East Asia, especially over southern China. Frontal precipitation is projected to decrease for 10%–30%. Front frequency increases in the South China Sea and tropical western Pacific because of more tropical moisture supply, which enhances local moisture contrasts. During spring, southern China and Taiwan will experience fewer fronts and less frontal precipitation while central China, the Korean Peninsula, and Japan may experience more fronts and more frontal precipitation due to moisture flux from the south that enhances wet-bulb potential temperature θw gradients. Consensus among CMIP5 models in front frequency tendency is evaluated. The models exhibit relatively high consensus in the decreasing trend over polar and subtropical frontal zone in winter and over southern China and Taiwan in spring that may prolong the dry season. Spring front activities are crucial for water resource and risk management in the southern China and Taiwan. [ABSTRACT FROM AUTHOR]

Previous studies have shown the existence of internal multidecadal variability in the Southern Ocean using multiple climate models. This variability, associated with deep ocean convection, can have significant climate impacts. In this work, we use sensitivity studies based on Geophysical Fluid Dynamics Laboratory (GFDL) models to investigate the linkage of this internal variability with the background ocean mean state. We find that mean ocean stratification in the subpolar region that is dominated by mean salinity influences whether this variability occurs, as well as its time scale. The weakening of background stratification favors the occurrence of deep convection. For background stratification states in which the low-frequency variability occurs, weaker ocean stratification corresponds to shorter periods of variability and vice versa. The amplitude of convection variability is largely determined by the amount of heat that can accumulate in the subsurface ocean during periods of the oscillation without deep convection. A larger accumulation of heat in the subsurface reservoir corresponds to a larger amplitude of variability. The subsurface heat buildup is a balance between advection that supplies heat to the reservoir and vertical mixing/convection that depletes it. Subsurface heat accumulation can be intensified both by an enhanced horizontal temperature advection by the Weddell Gyre and by an enhanced ocean stratification leading to reduced vertical mixing and surface heat loss. The paleoclimate records over Antarctica indicate that this multidecadal variability has very likely happened in past climates and that the period of this variability may shift with different climate background mean state. [ABSTRACT FROM AUTHOR]

Observations and climate models are combined to identify an anthropogenic warming signature in the upper ocean heat content (OHC) changes since 1971. We apply a new detection and attribution analysis developed by Ribes et al. that uses a symmetric treatment of the magnitude and the pattern of the climate response to each radiative forcing. A first estimate of the OHC response to natural, anthropogenic, greenhouse gas, and other forcings is derived from a large ensemble of CMIP5 simulations. Observational datasets from historical reconstructions are then used to constrain this estimate. A spatiotemporal observational mask is applied to compare simulations with actual observations and to overcome reconstruction biases. Results on the 0–700-m layer from 1971 to 2005 show that the global OHC would have increased since 1971 by 2.12 ± 0.21 × 107 J m−2 yr−1 in response to GHG emissions alone. But this has been compensated for by other anthropogenic influences (mainly aerosol), which induced an OHC decrease of 0.84 ± 0.18 × 107 J m−2 yr−1. The natural forcing has induced a slight global OHC decrease since 1971 of 0.13 ± 0.09 × 107 J m−2 yr−1. Compared to previous studies we have separated the effect of the GHG forcing from the effect of the other anthropogenic forcing on OHC changes. This has been possible by using a new detection and attribution (D&A) method and by analyzing simultaneously the global OHC trends over 1957–80 and over 1971–2005. This bivariate method takes advantage of the different time variation of the GHG forcing and the aerosol forcing since 1957 to separate both effects and reduce the uncertainty in their estimates. [ABSTRACT FROM AUTHOR]

The wintertime ENSO teleconnection over the North Pacific region consists of an intensified (weakened) low pressure center during El Niño (La Niña) events both in observations and in climate models. Here, it is demonstrated that this teleconnection persists too strongly into late winter and spring in the Community Earth System Model (CESM). This discrepancy arises in both fully coupled and atmosphere-only configurations, when observed SSTs are specified, and is shown to be robust when accounting for the sampling uncertainty due to internal variability. Furthermore, a similar problem is found in many other models from piControl simulations of the Coupled Model Intercomparison Project (23 out of 43 in phase 5 and 11 out of 20 in phase 6). The implications of this bias for the simulation of surface climate anomalies over North America are assessed. The overall effect on the ENSO composite field (El Niño minus La Niña) resembles an overly prolonged influence of ENSO into the spring with anomalously high temperatures over Alaska and western Canada, and wet (dry) biases over California (southwest Canada). Further studies are still needed to disentangle the relative roles played by diabatic heating, background flow, and other possible contributions in determining the overly strong springtime ENSO teleconnection intensity over the North Pacific. [ABSTRACT FROM AUTHOR]

Studies seeking to identify a human-caused global warming signal generally rely on climate model estimates of the "noise" of intrinsic natural variability. Assessing the reliability of these noise estimates is of critical importance. We evaluate here the statistical significance of differences between climate model and observational natural variability spectra for global-mean mid- to upper-tropospheric temperature (TMT). We use TMT information from satellites and large multimodel ensembles of forced and unforced simulations. Our main goal is to explore the sensitivity of model-versus-data spectral comparisons to a wide range of subjective decisions. These include the choice of satellite and climate model TMT datasets, the method for separating signal and noise, the frequency range considered, and the statistical model used to represent observed natural variability. Of particular interest is the amplitude of the interdecadal noise against which an anthropogenic tropospheric warming signal must be detected. We find that on time scales of 5-20 years, observed TMT variability is (on average) overestimated by the last two generations of climate models participating in the Coupled Model Intercomparison Project. This result is relatively insensitive to different plausible analyst choices, enhancing confidence in previous claims of detectable anthropogenic warming of the troposphere and indicating that these claims may be conservative. A further key finding is that two commonly used statistical models of short-term and long-term memory have deficiencies in their ability to capture the complex shape of observed TMT spectra. [ABSTRACT FROM AUTHOR]

Temperature trends across Antarctica over the last few decades reveal strong and statistically significant warming in West Antarctica and the Antarctic Peninsula (AP) contrasting with no significant change overall in East Antarctica. However, recent studies have documented cooling in the AP since the late 1990s. This study aims to place temperature changes in the AP and West Antarctica into a larger spatial and temporal perspective by analyzing monthly station-based surface temperature observations since 1957 across the extratropical Southern Hemisphere, along with sea surface temperature (SST) data and mean sea level pressure reanalysis data. The results confirm statistically significant cooling in station observations and SST trends throughout the AP region since 1999. However, the full 60-yr period shows statistically significant, widespread warming across most of the Southern Hemisphere middle and high latitudes. Positive SST trends broadly reflect these warming trends, especially in the midlatitudes. After confirming the importance of the southern annular mode (SAM) on southern high-latitude climate variability, the influence is removed from the station temperature records, revealing statistically significant background warming across all of the extratropical Southern Hemisphere. Antarctic temperature trends in a suite of climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are then investigated. Consistent with previous work the CMIP5 models warm Antarctica at the background temperature rate that is 2 times faster than that observed. However, removing the SAM influence from both CMIP5 and observed temperatures results in Antarctic trends that differ only modestly, perhaps due to natural multidecadal variability remaining in the observations. [ABSTRACT FROM AUTHOR]

This study investigates the mechanism of the large intermodel uncertainty in the change of ENSO's amplitude under global warming based on 31 CMIP5 models. We find that the uncertainty in ENSO's amplitude is significantly correlated to that of the change in the response of atmospheric circulation to SST anomalies (SSTAs) in the eastern equatorial Pacific Niño-3 region. This effect of the atmospheric response to SSTAs mainly influences the uncertainty in ENSO's amplitude during El Niño (EN) phases, but not during La Niña (LN) phases, showing pronounced nonlinearity. The effect of the relative SST warming and the present-day response of atmospheric circulation to SSTAs are the two major contributors to the intermodel spread of the change in the atmospheric response to SSTAs, of which the latter is more important. On the one hand, models with a stronger (weaker) mean-state SST warming in the eastern equatorial Pacific, relative to the tropical-mean warming, favor a larger (smaller) increase in the change in the response of atmospheric circulation to SSTAs in the eastern equatorial Pacific during EN. On the other hand, models with a weaker (stronger) present-day response of atmospheric circulation to SSTAs during EN tend to exhibit a larger (smaller) increase in the change under global warming. The result implies that an improved simulation of the present-day response of atmospheric circulation to SSTAs could be effective in lowering the uncertainty in ENSO's amplitude change under global warming. [ABSTRACT FROM AUTHOR]

Recent research suggests cloud–radiation interaction as key for intermodel differences in tropical precipitation change with warming. This motivates the hypothesis that intermodel differences in the climatology of precipitation, and in its response to warming, should reduce in the absence of cloud–radiation interaction. The hypothesis is explored with the aquaplanet simulations by the Clouds On-Off Klimate Intercomparison Experiment performed by seven general circulation models, wherein atmospheric cloud radiative effects (ACREs) are active (ACRE-on) and inactive (ACRE-off). Contrary to expectation, models' climatology of tropical precipitation are more diverse in the ACRE-off experiments, as measured by the position of the intertropical convergence zone (ITCZ), the subtropical precipitation minima, and the associated organization of the tropical circulation. Also the direction of the latitudinal shift of the ITCZ differs more in simulations with inactive cloud radiative effects. Nevertheless, both in ACRE-on and ACRE-off, the same relationship between tropical precipitation and the mean vertical velocity (zonally, temporally, and vertically averaged) emerges in all models. An analysis framework based on the moist static energy budget and used in the moisture space is then developed to understand what controls the distribution of the mean vertical velocity. The results suggest that intermodel differences in tropical circulation and zonal-mean precipitation patterns are most strongly associated with intermodel differences in the representation of shallow circulations that connect dry and moist regions. [ABSTRACT FROM AUTHOR]

Drought has severe consequences for humans and their environment, yet we have a limited understanding of the drivers of drought across the full range of time scales on which it occurs. Here, the atmosphere and ocean conditions that drive this continuum of drought variability in southwestern North America (SWNA) are studied using the latest observationally based products, paleoclimate reconstructions, and state-of-the-art Earth system model simulations of the last millennium. A novel application of the self-organizing maps (SOM) methodology allows for a visualization of the continuum of climate states coinciding with thousands of droughts of varying lengths in last millennium simulations from the Community Earth System Model (CESM), the Goddard Institute for Space Studies Model E2-R (GISS E2-R), and eight other members from phase 5 of the Coupled Model Intercomparison Project (CMIP5). It is found that most droughts are associated with a cool Pacific decadal oscillation (PDO) pattern, but persistent droughts can coincide with a variety of ocean–atmosphere states, including time periods showing a warm PDO or weak ocean–atmosphere anomalies. Many CMIP5 models simulate similar SWNA teleconnection patterns, but the SOM analysis demonstrates that models simulate different continuums of ocean–atmosphere states coinciding with droughts of different lengths, suggesting fundamental differences in their drought dynamics. These findings have important implications for our understanding and simulation of the drivers of persistent drought, and for their potential predictability. [ABSTRACT FROM AUTHOR]

The contribution of buoyancy (thermal + freshwater fluxes) versus momentum (wind driven) coupling to SST variance in climate models is a longstanding question. Addressing this question has proven difficult because a gap in the model hierarchy exists between the fully coupled (momentum + buoyancy + ocean dynamics) and slab–mixed layer ocean coupled (thermal with no ocean dynamics) versions. The missing piece is a thermally coupled configuration that permits anomalous ocean heat transport convergence decoupled from the anomalous wind stress. A mechanically decoupled model configuration is provided to fill this gap and diagnose the impact of momentum coupling on SST variance in NCAR CESM. A major finding is that subtropical SST variance increases when momentum coupling is disengaged. An ‘‘opposing flux hypothesis’’ may explain why the subtropics (midlatitudes) experience increased (reduced) variance without momentum coupling. In a subtropical easterly wind regime, Ekman fluxes (Q'ek) oppose thermal fluxes (Q'th), such that when the air and sea are mechanically decoupled (Q'ek = 0), Q'ek + Q'th variance increases. As a result, SST variance increases. In a midlatitude westerly regime where Q'ek and Q'th typically reinforce each other, SST variance is reduced. Changes in mean surface winds with climate change could impact the Q'ek and Q'th covariance relationships. A by-product of mechanically decoupling the model is the absence of ENSO variability. The Pacific decadal oscillation operates without momentum coupling or tropical forcing, although the pattern is modified with enhanced (reduced) variability in the subtropics (midlatitudes). Results show that Ekman fluxes are an important component to tropical, subtropical, and midlatitude SST variance. [ABSTRACT FROM AUTHOR]

To understand sources of variability in the eastern equatorial Pacific, a region integral to modulating global temperatures, the waters upwelling from the Equatorial Undercurrent (EUC) are characterized. Past work is updated using temperature and salinity measurements from the Argo array and current measurements from Tropical Atmosphere Ocean (TAO) buoys. A larger hemispheric asymmetry is found in the water mass contribution through the ventilated thermocline to the EUC than previously reported, with 80%-90% of waters in the western Pacific originating from the Southern Hemisphere. South Pacific subtropical waters are the dominant source feeding the EUC, although in the central equatorial Pacific upper layers of the EUC experience freshening due to the addition of North Pacific waters. Anomalous volume transport, advection of anomalous waters, and shifts in hemispheric contributions contribute to variability in the EUC. These results suggest that variability in the EUC caused by anomalies in the South Pacific ventilated thermocline can explain variability in the eastern equatorial Pacific. [ABSTRACT FROM AUTHOR]

A striking trend of the Indian Ocean interhemispheric gradient in sea surface temperatures (SSTs) developed during the recent global warming hiatus. The contributions of external forcing and internal variability to this trend are examined in forced climate model experiments. Results indicate that the observed negative trend was strong by historical standards and most likely due to internal variability rather than to external forcing. Anthropogenic aerosol forcing favors negative gradient trends, but its effects are countered by greenhouse gas forcing, and both are weak relative to internal variability. The observed interhemispheric gradient trend occurred in parallel with a negative phase of the interdecadal Pacific oscillation (IPO), a linkage that is also found in climate models. However, the physical mechanisms responsible for these gradient trends in models differ from those in ocean reanalysis products. In particular, oceanic processes via an increased Indonesian Throughflow (ITF) transport into the Indian Ocean forced by stronger Pacific trade winds are the principal cause of the observed negative SST gradient trend during 2000-13. In contrast, atmospheric processes via changing surface wind stress over the southern Indian Ocean remotely forced by the IPO appear to play a dominant role in changing the interhemispheric SST gradients in climate models. The models underestimate the magnitude of the IPO and produce changes in the ITF that are too weak owing to their coarse spatial resolution. These model deficiencies may account for the differences between the simulations and observations. [ABSTRACT FROM AUTHOR]

Canonical correlation analysis (CCA)-based statistical corrections are applied to seasonal mean precipitation and temperature hindcasts of the individual models from the North American Multimodel Ensemble project to correct biases in the positions and amplitudes of the predicted large-scale anomaly patterns. Corrections are applied in 15 individual regions and then merged into globally corrected forecasts. The CCA correction dramatically improves the RMS error skill score, demonstrating that model predictions contain correctable systematic biases in mean and amplitude. However, the corrections do not materially improve the anomaly correlation skills of the individual models for most regions, seasons, and lead times, with the exception of October-December precipitation in Indonesia and eastern Africa. Models with lower uncorrected correlation skill tend to benefit more from the correction, suggesting that their lower skills may be due to correctable systematic errors. Unexpectedly, corrections for the globe as a single region tend to improve the anomaly correlation at least as much as the merged corrections to the individual regions for temperature, and more so for precipitation, perhaps due to better noise filtering. The lack of overall improvement in correlation may imply relatively mild errors in large-scale anomaly patterns. Alternatively, there may be such errors, but the period of record is too short to identify them effectively but long enough to find local biases in mean and amplitude. Therefore, statistical correction methods treating individual locations (e.g., multiple regression or principal component regression) may be recommended for today's coupled climate model forecasts. The findings highlight that the performance of statistical postprocessing can be grossly overestimated without thorough cross validation or evaluation on independent data. [ABSTRACT FROM AUTHOR]

The Earth's ice-ocean-atmosphere system exhibits nonlinear responses, such as the difference in the magnitude of the atmospheric response to positive or negative ocean or sea ice anomalies. Two classes of methods that have previously been used to investigate the nonlinear dependence between climate fields are kernel methods and neural network methods. In this paper, a third methodology is introduced: gradient-based kernel dimension reduction. Gradient-based kernel methods are an extension of conventional kernel methods, but gradient-based methods focus on the directional derivatives of the regression surface between two fields. Specifically, a new gradient-based method is developed here: gradient kernel canonical correlation analysis (gKCCA). In gKCCA, the canonical directions maximize the directional derivatives between the predictor field and the response field, while the canonical components of the response field maximize the correlation with a nonlinear augmentation of the predictor canonical components. Gradient-based kernel methods have several advantages: their components can be directly related to the original fields (unlike in conventional kernel methods), and the projection vectors are determined by analytical solution (unlike in neural networks). Here gKCCA is applied to the question of nonlinear coupling between high-frequency (2-3 years) and low-frequency (4-6 years) modes in the Pacific Ocean. The leading gKCCA subspace shows a significant nonlinear coupling between the low-pass and high-pass fields. The paper also shows that the results of gKCCA are robust to different levels of noise and different kernel functions. [ABSTRACT FROM AUTHOR]

Using long-term observational data and numerical model experiments, this study found that the Atlantic multidecadal oscillation (AMO) affects the influence of ENSO-like sea surface temperature anomalies (SSTAs, which contain the variability of both El Niño–Southern Oscillation and Pacific decadal oscillation) on the interannual change in the East Asian winter monsoon (EAWM). In the observations, the out-of-phase relationship between the variations in ENSO and the EAWM was significantly intensified when the AMO and ENSO-like SSTAs were in phase. Warmer-than-normal winters occurred across East Asia when the ENSO-like SSTAs and AMO were positively in phase, with a significantly weakened Siberian high and anomalous anticyclones over the western North Pacific. The opposite patterns occurred under negative in-phase conditions. In contrast, when the ENSO-like and AMO SSTAs were out of phase, the anomalies related to the EAWM tended to exhibit relatively weaker features. Numerical model experiments confirmed these observational results. When the models were perturbed with warm ENSO-like SSTAs and warm AMO SSTAs, the atmosphere showed a weakened Siberian high, strong anticyclonic anomalies over the Philippine Sea, a weakened East Asian trough, and dominant positive temperature anomalies over East Asia, implying a weaker EAWM. Reverse responses to negative in-phase temperature anomalies were observed. However, the atmospheric signals that responded to the out-of-phase conditions were less robust. This phenomenon may be attributed to the superposition of the interannual variability of the EAWM caused by ENSO-like SSTAs upon the influence of AMO on background Eurasian climate and the Walker circulation response to the heating source provided by the AMO, which induced changes in ENSO-like variability through the surface wind anomalies and modulated the anomalous anticyclone/cyclone over the Philippine Sea in warm–cold ENSO-like events. [ABSTRACT FROM AUTHOR]

By performing two sets of high-resolution atmospheric general circulation model (AGCM) experiments, the authors find that the atmospheric response to a sea surface temperature (SST) anomaly in the extratropical North Pacific is sensitive to decadal variations of the background SST on which the SST anomaly is superimposed. The response in the first set of experiments, in which the SST anomaly is superimposed on the observed daily SST of 1981-90, strongly differs from the response in the second experiment, in which the same SST anomaly is superimposed on the observed daily SST of 1991-2000. The atmospheric response over the North Pacific during 1981-90 is eddy mediated, equivalent barotropic, and concentrated in the east. In contrast, the atmospheric response during 1991-2000 is weaker and strongest in the west. The results are discussed in terms of Rossby wave dynamics, with the proposed primary wave source switching from baroclinic eddy vorticity forcing over the eastern North Pacific in 1981-90 to mean-flow divergence over the western North Pacific in 1991-2000. The wave source changes are linked to the decadal reduction of daily SST variability over the eastern North Pacific and strengthening of the Oyashio Extension front over the western North Pacific. Thus, both daily and frontal aspects of the background SST variability in determining the atmospheric response to extratropical North Pacific SST anomalies are emphasized by these AGCM experiments. [ABSTRACT FROM AUTHOR]

Updated and improved satellite retrievals of the temperature of the mid-to-upper troposphere (TMT) are used to address key questions about the size and significance of TMT trends, agreement with model-derived TMT values and whether models and satellite data show similar vertical profiles of warming. A recent study claimed that TMT trends over 1979 and 2015 are 3 limes larger in climate models than in satellite data but did not correct for the contribution TMT trends receive from stratospheric cooling. Here, it is shown that the average ratio of modeled and observed TMT trends is sensitive to both satellite data uncertainties and model-data differences in stratospheric cooling. When the impact of lower-stratospheric cooling on TMT is accounted for, and when the most recent versions of satellite datasets are used, the previously claimed ratio of three between simulated and observed near-global TMT trends is reduced to approximately 1.7. Next, the validity of the statement that satellite data show no significant tropospheric warming over the last 18 years is assessed. This claim is not supported by the current analysis: in five out of six corrected satellite TMT records, significant global-scale tropospheric warming has occurred within the last 18 years. Finally long-standing concerns are examined regarding discrepancies in modeled and observed vertical profiles of warming in the tropical atmosphere. It is shown that amplification of tropical warming between the lower and mid-to-upper tropo-sphere is now in close agreement in the average of 37 climate models and in one updated satellite record. [ABSTRACT FROM AUTHOR]

Projections of twenty-first-century Northern Hemisphere (NH) spring snow cover extent (SCE) from two climate model ensembles are analyzed to characterize their uncertainty. Phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel ensemble exhibits variability resulting from both model differences and internal climate variability, whereas spread generated from a Canadian Earth System Model-Large Ensemble (CanESM-LE) experiment is solely a result of internal variability. The analysis shows that simulated 1981-2010 spring SCE trends are slightly weaker than observed (using an ensemble of snow products). Spring SCE is projected to decrease by −3.7% ± 1.1% decade−1 within the CMIP5 ensemble over the twenty-first century. SCE loss is projected to accelerate for all spring months over the twenty-first century, with the exception of June (because most snow in this month has melted by the latter half of the twenty-first century). For 30-yr spring SCE trends over the twenty-first century, internal variability estimated from CanESM-LE is substantial, but smaller than intermodel spread from CMIP5. Additionally, internal variability in NH extratropical land warming trends can affect SCE trends in the near future ( R2 = 0.45), while variability in winter precipitation can also have a significant (but lesser) impact on SCE trends. On the other hand, a majority of the intermodel spread is driven by differences in simulated warming (dominant in March-May) and snow cover available for melt (dominant in June). The strong temperature-SCE linkage suggests that model uncertainty in projections of SCE could be potentially reduced through improved simulation of spring season warming over land. [ABSTRACT FROM AUTHOR]

Tropical cyclones (TCs), particularly those that are intense and/or slow moving, induce sea surface temperature (SST) reductions along their tracks (commonly referred to as cold wakes) that provide a negative feedback on storm energetics by weakening surface enthalpy fluxes. While computing gains have allowed for simulated TC intensity to increase in global climate models as a result of increased horizontal resolution, many configurations utilize prescribed, noninteractive SSTs as a surface boundary condition to minimize computational cost and produce more accurate TC climatologies. Here, an idealized slab ocean is coupled to a 0.25° variable-resolution version of the Community Atmosphere Model (CAM) to improve closure of the surface energy balance and reproduce observed Northern Hemisphere cold wakes. This technique produces cold wakes that are realistic in structure and evolution and with magnitudes similar to published observations, without impacting large-scale SST climatology. Multimember ensembles show that the overall number of TCs generated by the model is reduced by 5%-9% when allowing for two-way air-sea interactions. TC intensity is greatly impacted; the strongest 1% of all TCs are 20-30 hPa (4-8 m s−1) weaker, and the number of simulated Saffir-Simpson category 4 and 5 TCs is reduced by 65% in slab ocean configurations. Reductions in intensity are in line with published thermodynamic theory. Additional offline experiments and sensitivity simulations demonstrate this response is both significant and robust. These results imply caution should be exercised when assessing high-resolution prescribed SST climate simulations capable of resolving intense TCs, particularly if discrete analysis of extreme events is desired. [ABSTRACT FROM AUTHOR]

Climate models robustly imply that some significant change in precipitation patterns will occur. Models consistently project that the intensity of individual precipitation events increases by approximately 6%-7% K−1, following the increase in atmospheric water content, but that total precipitation increases by a lesser amount (1%-2% K−1 in the global average in transient runs). Some other aspect of precipitation events must then change to compensate for this difference. The authors develop a new methodology for identifying individual rainstorms and studying their physical characteristics-including starting location, intensity, spatial extent, duration, and trajectory-that allows identifying that compensating mechanism. This technique is applied to precipitation over the contiguous United States from both radar-based data products and high-resolution model runs simulating 80 years of business-as-usual warming. In the model study the dominant compensating mechanism is a reduction of storm size. In summer, rainstorms become more intense but smaller; in winter, rainstorm shrinkage still dominates, but storms also become less numerous and shorter duration. These results imply that flood impacts from climate change will be less severe than would be expected from changes in precipitation intensity alone. However, these projected changes are smaller than model-observation biases, implying that the best means of incorporating them into impact assessments is via 'data-driven simulations' that apply model-projected changes to observational data. The authors therefore develop a simulation algorithm that statistically describes model changes in precipitation characteristics and adjusts data accordingly, and they show that, especially for summertime precipitation, it outperforms simulation approaches that do not include spatial information. [ABSTRACT FROM AUTHOR]

The causes of decadal time-scale variations in global mean temperature are currently under debate. Proposed mechanisms include both processes internal to the climate system as well as external forcing. Here, the robustness of spatial and time scale characteristics of unforced (internal) decadal variability among phase 5 of the Coupled Model Intercomparison Project (CMIP5) preindustrial control runs is examined. It is found that almost all CMIP5 models produce an interdecadal Pacific oscillation-like pattern associated with decadal variability, but the frequency of decadal-scale change is model dependent. To assess the roles of atmosphere and ocean dynamics in producing decadal variability, two preindustrial control Community Climate System model (version 4) configurations are compared: one with an atmosphere coupled to a slab ocean and the other fully coupled to a dynamical ocean. Interactive ocean dynamics are not necessary to produce an IPO-like pattern but affect the magnitude and frequency of the decadal changes primarily by impacting the strength of El Niño-Southern Oscillation. However, low-frequency El Niño-Southern Oscillation variability and skewness explains up to only 54% of the spread in frequency of decadal swings in global mean temperature among CMIP5 models; there may be other internal mechanisms that can produce such diversity. The spatial pattern of decadal changes in surface temperature are robust and can be explained by atmospheric processes interacting with the upper ocean, while the frequency of these changes is not well constrained by models. [ABSTRACT FROM AUTHOR]

The impact of climate change on the Pacific decadal oscillation (PDO) is studied using a fully coupled climate model. The model results show that the PDO has a similar spatial pattern in altered climates, but its amplitude and time scale of variability change in response to global warming or cooling. In response to global warming the PDO amplitude is significantly reduced, with a maximum decrease over the Kuroshio-Oyashio Extension (KOE) region. This reduction appears to be associated with a weakened meridional temperature gradient in the KOE region. In addition, reduced variability of North Pacific wind stress, partially due to reduced air-sea feedback, also helps to weaken the PDO amplitude by reducing the meridional displacements of the subtropical and subpolar gyre boundaries. In contrast, the PDO amplitude increases in response to global cooling. In the control simulations the model PDO has an approximately bidecadal peak. In a warmer climate the PDO time scale becomes shorter, changing from ~20 to ~12 yr. In a colder climate the time scale of the PDO increases to ~34 yr. Physically, global warming (cooling) enhances (weakens) ocean stratification. The increased (decreased) ocean stratification acts to increase (reduce) the phase speed of internal Rossby waves, thereby altering the time scale of the simulated PDO. [ABSTRACT FROM AUTHOR]

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 FROM AUTHOR]

This paper investigates the predictive relation between the Atlantic multidecadal oscillation (AMO) and Atlantic meridional overturning circulation across different climate models. Three overturning patterns that are significantly coupled to the AMO on interannual time scales across all climate models are identified using a statistical optimization technique. Including these structures in an autoregressive model extends AMO predictability by 2-9 years, relative to an autoregressive model without these structures. [ABSTRACT FROM AUTHOR]