The effect of stratospheric ozone depletion is simulated in GFDL AM4 model with three ozone schemes: Prescribing monthly zonal mean ozone concentration, full interactive stratospheric chemistry, and a simplified linear ozone chemistry scheme but with full dynamical interactions. While similar amounts of ozone loss are simulated by the three schemes, the two interactive ozone schemes produce significantly stronger stratospheric cooling than the prescribed one. We find that this temperature difference is driven by the dynamical responses to ozone depletion. In particular, the existence of ozone hole leads to strong ozone eddies that are in‐phase with the temperature eddies. The coherence between ozone and temperature anomalies leads to a weaker radiative damping as ozone absorbs shortwave radiation that compensates for the longwave cooling. As a result, less wave dissipates at the lower stratosphere, leading to a weaker descending and dynamical heating over the polar lower stratosphere, and hence a stronger net cooling there. The covariance between ozone and temperature is largely suppressed when ozone is prescribed as monthly zonal mean time series, as is the reduction in the radiative damping following ozone depletion. With much lower computational cost, the simplified ozone scheme is capable of producing similar magnitude of ozone loss and the consequent dynamical responses to those simulated by the full chemistry.
The Coronavirus Disease 2019 (COVID‐19) pandemic led to a widespread reduction in aerosol emissions. Using satellite observations and climate model simulations, we study the underlying mechanisms of the large decreases in solar clear‐sky reflection (3.8 W m−2 or 7%) and aerosol optical depth (0.16 W m−2 or 32%) observed over the East Asian Marginal Seas in March 2020. By separating the impacts from meteorology and emissions in the model simulations, we find that about one‐third of the clear‐sky anomalies can be attributed to pandemic‐related emission reductions, and the rest to weather variability and long‐term emission trends. The model is skillful at reproducing the observed interannual variations in solar all‐sky reflection, but no COVID‐19 signal is discerned. The current observational and modeling capabilities will be critical for monitoring, understanding, and predicting the radiative forcing and climate impacts of the ongoing crisis.
Ayarzagüena, Blanca, Andrew J Charlton-Perez, Amy Butler, Peter Hitchcock, Isla Simpson, Lorenzo M Polvani, Neal Butchart, Edwin P Gerber, Lesley Gray, Birgit Hassler, and Pu Lin, et al., March 2020: Uncertainty in the Response of Sudden Stratospheric Warmings and Stratosphere‐Troposphere Coupling to Quadrupled CO2 Concentrations in CMIP6 Models. JGR Atmospheres, 125(6), DOI:10.1029/2019JD032345. Abstract
Major sudden stratospheric warmings (SSWs), vortex formation, and final breakdown dates are key highlight points of the stratospheric polar vortex. These phenomena are relevant for stratosphere‐troposphere coupling, which explains the interest in understanding their future changes. However, up to now, there is not a clear consensus on which projected changes to the polar vortex are robust, particularly in the Northern Hemisphere, possibly due to short data record or relatively moderate CO2 forcing. The new simulations performed under the Coupled Model Intercomparison Project, Phase 6, together with the long daily data requirements of the DynVarMIP project in preindustrial and quadrupled CO2 (4xCO2) forcing simulations provide a new opportunity to revisit this topic by overcoming the limitations mentioned above. In this study, we analyze this new model output to document the change, if any, in the frequency of SSWs under 4xCO2 forcing. Our analysis reveals a large disagreement across the models as to the sign of this change, even though most models show a statistically significant change. As for the near‐surface response to SSWs, the models, however, are in good agreement as to this signal over the North Atlantic: There is no indication of a change under 4xCO2 forcing. Over the Pacific, however, the change is more uncertain, with some indication that there will be a larger mean response. Finally, the models show robust changes to the seasonal cycle in the stratosphere. Specifically, we find a longer duration of the stratospheric polar vortex and thus a longer season of stratosphere‐troposphere coupling.
Fu, Qiang, R H White, Mingcheng Wang, B Alexander, S Solomon, Andrew Gettelman, D S Battisti, and Pu Lin, March 2020: The Brewer‐Dobson Circulation during the Last Glacial Maximum. Geophysical Research Letters, 47(5), DOI:10.1029/2019GL086271. Abstract
The Brewer‐Dobson circulation (BDC) during the last glacial maximum (LGM) is investigated in simulations using the Whole Atmosphere Community Climate Model version 6 (WACCM6). We examine vertical mass fluxes, age of stratospheric air, and the transformed Eulerian mean stream function and find that the modeled annual‐mean BDC during the LGM is almost everywhere slower than that in the modern climate (with or without anthropogenic ozone depleting substances). Compared to the modern climate, the annual‐mean tropical upwelling in the LGM is 11.3‐16.9%, 11.2‐15.8%, and 4.4‐10.2% weaker, respectively, at 100, 70, and 30 hPa. Simulated decreases in annual‐mean mass fluxes at 70 and 100 hPa are caused by a weaker parameterized orographic gravity wave drag and resolved wave drag, respectively.
We describe the baseline model configuration and simulation characteristics of the Geophysical Fluid Dynamics Laboratory (GFDL)'s Atmosphere Model version 4.1 (AM4.1), which builds on developments at GFDL over 2013–2018 for coupled carbon‐chemistry‐climate simulation as part of the sixth phase of the Coupled Model Intercomparison Project. In contrast with GFDL's AM4.0 development effort, which focused on physical and aerosol interactions and which is used as the atmospheric component of CM4.0, AM4.1 focuses on comprehensiveness of Earth system interactions. Key features of this model include doubled horizontal resolution of the atmosphere (~200 to ~100 km) with revised dynamics and physics from GFDL's previous‐generation AM3 atmospheric chemistry‐climate model. AM4.1 features improved representation of atmospheric chemical composition, including aerosol and aerosol precursor emissions, key land‐atmosphere interactions, comprehensive land‐atmosphere‐ocean cycling of dust and iron, and interactive ocean‐atmosphere cycling of reactive nitrogen. AM4.1 provides vast improvements in fidelity over AM3, captures most of AM4.0's baseline simulations characteristics, and notably improves on AM4.0 in the representation of aerosols over the Southern Ocean, India, and China—even with its interactive chemistry representation—and in its manifestation of sudden stratospheric warmings in the coldest months. Distributions of reactive nitrogen and sulfur species, carbon monoxide, and ozone are all substantially improved over AM3. Fidelity concerns include degradation of upper atmosphere equatorial winds and of aerosols in some regions.
Fu, Qiang, S Solomon, H A Pahlavan, and Pu Lin, November 2019: Observed changes in Brewer–Dobson circulation for 1980–2018. Environmental Research Letters, 14(11), DOI:10.1088/1748-9326/ab4de7. Abstract
Previous work has examined the Brewer–Dobson circulation (BDC) changes for 1980–2009 based on satellite Microwave Sounding Unit (MSU/AMSU) lower-stratospheric temperature (T LS ) observations and ERA-Interim reanalysis data. Here we examine the BDC changes for the longer period now available (1980–2018), which also allows analysis of both the ozone depletion (1980–1999) and ozone healing (2000–2018) periods. We provide observational evidence that the annual mean BDC accelerated for 1980–1999 but decelerated for 2000–2018, with the changes largely driven by the Southern Hemisphere (SH), which might be partly contributed by the effects of ozone depletion and healing. We also show that the annual mean BDC has accelerated in the last 40 years (at the 90% confidence level) with a relative strengthening of ~1.7% per decade. This overall acceleration was driven by both Northern Hemisphere (40%) and SH (60%) cells. Significant SH radiative warming is also identified in September for 2000–2018 after excluding the year 2002 when a very rare SH stratospheric sudden warming occurred, supporting the view that healing of the Antarctic ozone layer has now begun to occur during the month of September.
We describe GFDL's CM4.0 physical climate model, with emphasis on those aspects that may be of particular importance to users of this model and its simulations. The model is built with the AM4.0/LM4.0 atmosphere/land model and OM4.0 ocean model. Topics include the rationale for key choices made in the model formulation, the stability as well as drift of the pre‐industrial control simulation, and comparison of key aspects of the historical simulations with observations from recent decades. Notable achievements include the relatively small biases in seasonal spatial patterns of top‐of‐atmosphere fluxes, surface temperature, and precipitation; reduced double Intertropical Convergence Zone bias; dramatically improved representation of ocean boundary currents; a high quality simulation of climatological Arctic sea ice extent and its recent decline; and excellent simulation of the El Niño‐Southern Oscillation spectrum and structure. Areas of concern include inadequate deep convection in the Nordic Seas; an inaccurate Antarctic sea ice simulation; precipitation and wind composites still affected by the equatorial cold tongue bias; muted variability in the Atlantic Meridional Overturning Circulation; strong 100 year quasi‐periodicity in Southern Ocean ventilation; and a lack of historical warming before 1990 and too rapid warming thereafter due to high climate sensitivity and strong aerosol forcing, in contrast to the observational record. Overall, CM4.0 scores very well in its fidelity against observations compared to the Coupled Model Intercomparison Project Phase 5 generation in terms of both mean state and modes of variability and should prove a valuable new addition for analysis across a broad array of applications.
An unprecedented disruption of the Quasi-Biennial Oscillation (QBO) started to develop from late 2015. The early development of this event is analyzed using the space-time spectra of eddies from reanalysis data. While the extratropical waves propagating horizontally into the tropics were assumed to be the main driver for the disruption, it was not clear why these waves dissipated near the jet core instead of jet edge as linear theory predicts. This study shows that the drastic deceleration of the equatorial jet was largely brought about by a single strong wave packet, and the local winds experienced by the wave packet served as a better indicator of the wave breaking latitude than the zonal mean winds.
Surprisingly, tropical mixed Rossby gravity waves also made an appreciable contribution to the deceleration of the equatorial westerly jet by the horizontal eddy momentum fluxes, especially before January 2016. The horizontal eddy momentum fluxes associated with the tropical waves arise from the deformation of the wave structure when background westerlies increase with height. These horizontal eddy momentum anomalies from the tropical waves are commonly observed in the reanalysis data, but are typically much weaker than those in the 2015/2016 winter. The possibility exists that exceptionally strong equatorially trapped waves precondition the flow to disruption by an extratropical disturbance.
In this two-part paper, a description is provided of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). This version, with roughly 100km horizontal resolution and 33 levels in the vertical, contains an aerosol model that generates aerosol fields from emissions and a “light” chemistry mechanism designed to support the aerosol model but with prescribed ozone. In Part I, the quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode – with prescribed sea surface temperatures (SSTs) and sea ice distribution – is described and compared with previous GFDL models and with the CMIP5 archive of AMIP simulations. The model's Cess sensitivity (response in the top-of-atmosphere radiative flux to uniform warming of SSTs) and effective radiative forcing are also presented. In Part II, the model formulation is described more fully and key sensitivities to aspects of the model formulation are discussed, along with the approach to model tuning.
In Part II of this two-part paper, documentation is provided of key aspects of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode has been provided in Part I. Part II provides documentation of key components and some sensitivities to choices of model formulation and values of parameters, highlighting the convection parameterization and orographic gravity wave drag. The approach taken to tune the model's clouds to observations is a particular focal point. Care is taken to describe the extent to which aerosol effective forcing and Cess sensitivity have been tuned through the model development process, both of which are relevant to the ability of the model to simulate the evolution of temperatures over the last century when coupled to an ocean model.
Geng, L, Lee T Murray, L J Mickley, and Pu Lin, et al., June 2017: Isotopic evidence of multiple controls on atmospheric oxidants over climate transitions. Nature, 546(7656), DOI:10.1038/nature22340. Abstract
The abundance of tropospheric oxidants, such as ozone (O3) and hydroxyl (OH) and peroxy radicals (HO2 + RO2), determines the lifetimes of reduced trace gases such as methane and the production of particulate matter important for climate and human health. The response of tropospheric oxidants to climate change is poorly constrained owing to large uncertainties in the degree to which processes that influence oxidants may change with climate1 and owing to a lack of palaeo-records with which to constrain levels of atmospheric oxidants during past climate transitions2. At present, it is thought that temperature-dependent emissions of tropospheric O3 precursors and water vapour abundance determine the climate response of oxidants, resulting in lower tropospheric O3 in cold climates while HOx (= OH + HO2 + RO2) remains relatively buffered3. Here we report observations of oxygen-17 excess of nitrate (a proxy for the relative abundance of atmospheric O3 and HOx) from a Greenland ice core over the most recent glacial–interglacial cycle and for two Dansgaard–Oeschger events. We find that tropospheric oxidants are sensitive to climate change with an increase in the O3/HOx ratio in cold climates, the opposite of current expectations. We hypothesize that the observed increase in O3/HOx in cold climates is driven by enhanced stratosphere-to-troposphere transport of O3, and that reactive halogen chemistry is also enhanced in cold climates. Reactive halogens influence the oxidative capacity of the troposphere directly as oxidants themselves and indirectly4 via their influence on O3 and HOx. The strength of stratosphere-to-troposphere transport is largely controlled by the Brewer–Dobson circulation5, which may be enhanced in colder climates owing to a stronger meridional gradient of sea surface temperatures6, with implications for the response of tropospheric oxidants7 and stratospheric thermal and mass balance8. These two processes may represent important, yet relatively unexplored, climate feedback mechanisms during major climate transitions.
Hardiman, S C., and Pu Lin, et al., March 2017: The influence of dynamical variability on the observed Brewer-Dobson circulation trend. Geophysical Research Letters, 44(6), DOI:10.1002/2017GL072706. Abstract
The strength of the Brewer-Dobson circulation (BDC) is predicted to increase due to climate change. However, this increase has yet to be robustly detected in observational analyses. In this study a long control simulation is used to calculate the Time of Emergence of the BDC trend and how much of that trend may be masked by dynamical variability in current observations. A Time of Emergence of around 30 years is found (assuming a 2%/decade trend in the BDC), similar to the length of current reanalysis data sets. However, the discrepancies in vertical velocities between different reanalysis products remain far larger than any predicted trend. Furthermore, dynamical variability can completely mask the BDC trend on time scales shorter than around 12 years. Thus, more robust observational analyses of vertical velocity are likely to be needed for at least the next decade before detection of a statistically significant trend can be expected.
This study explores the role of the stratosphere as a source of seasonal predictability of surface climate over Northern Hemisphere extra-tropics both in the observations and climate model predictions. A suite of numerical experiments, including climate simulations and retrospective forecasts, are set up to isolate the role of the stratosphere in seasonal predictive skill of extra-tropical near surface land temperature. We show that most of the lead-0 month spring predictive skill of land temperature over extra-tropics, particularly over northern Eurasia, stems from stratospheric initialization. We further reveal that this predictive skill of extra-tropical land temperature arises from skillful prediction of the Arctic Oscillation (AO). The dynamical connection between the stratosphere and troposphere is also demonstrated by the significant correlation between the stratospheric polar vortex and sea level pressure anomalies, as well as the migration of the stratospheric zonal wind anomalies to the lower troposphere.
This paper investigates changes in the tropical tropopause layer (TTL) in response to carbon dioxide increase and surface warming separately in an atmospheric general circulation model, finding that both effects lead to a warmer tropical tropopause. Surface warming also results in an upward shift of the tropopause. A detailed heat budget analysis is performed to quantify the contributions from different radiative and dynamic processes to changes in the TTL temperature. When carbon dioxide increases with fixed surface temperature, a warmer TTL mainly results from the direct radiative effect of carbon dioxide increase. With surface warming, the largest contribution to the TTL warming comes from the radiative effect of the warmer troposphere, which is partly canceled by the radiative effect of the moistening at the TTL. Strengthening of the stratospheric circulation following surface warming cools the lower stratosphere dynamically and radiatively via changes in ozone. These two effects are of comparable magnitudes. This circulation change is the main cause of temperature changes near 63 hPa but is weak near 100 hPa. Contributions from changes in convection and clouds are also quantified. These results illustrate the heat budget analysis as a useful tool to disentangle the radiative–dynamical–chemical–convective coupling at the TTL and to facilitate an understanding of intermodel difference.
We contrast the responses to ozone depletion in two climate models: CAM3 and GFDL AM3. Although both models are forced with identical ozone concentration changes, the stratospheric cooling simulated in CAM3 is 30% stronger than in AM3 in annual mean, and twice as strong in December. We find that this difference originates from the dynamical response to ozone depletion, and its strength can be linked to the timing of the climatological springtime polar vortex breakdown. This mechanism is further supported by a variant of the AM3 simulation in which the Southern stratospheric zonal wind climatology is nudged to be CAM3-like. Given that the delayed breakdown of the Southern polar vortex is a common bias among many climate models, previous model-based assessments of the forced responses to ozone depletion may have been somewhat overestimated.
Pan, Fang, X Huang, S S Leroy, Pu Lin, L Larrabee Strow, Yi Ming, and V Ramaswamy, August 2017: The stratospheric changes inferred from 10 years of AIRS and AMSU-A radiances. Journal of Climate, 30(15), DOI:10.1175/JCLI-D-17-0037.1. Abstract
We analyze global-mean radiances observed by AIRS (Atmospheric Infrared Sounder) and AMSU-A (Advanced Microwave Sounding Unit) from 2003 to 2012. We focus on channels sensitive to emission and absorption in the stratosphere. Optimal fingerprinting is used to obtain estimates of changes of stratospheric temperature in five vertical layers due to external forcing in the presence of natural variability. Natural variability is estimated using synthetic radiances based on the 500-year GFDL CM3 and 240-year HADGEM2-CC control runs. The results show a cooling rate of 0.65±0.11(2σ) K decade-1 in the upper stratosphere above 6hPa, ~0.46±0.24 K decade-1 in two middle stratospheric layers between 6hPa and 30hPa, and 0.39±0.32 K decade-1 in the lower stratosphere (30-60hPa). The cooling rate in the lowest part of the stratosphere (60-100hPa) is -0.014±0.22 K decade-1, which is smallest among all five layers and statistically insignificant. The synergistic use of well-calibrated passive infrared and microwave radiances permits disambiguation of trends of carbon dioxide and stratospheric temperature, increases vertical resolution of detected stratospheric temperature trends, and effectively reduces uncertainties of estimated temperature trends.
Solomon, S, D Ivy, M Gupta, J Bandoro, B D Santer, Qiang Fu, and Pu Lin, et al., August 2017: Mirrored Changes in Antarctic Ozone and Stratospheric Temperature in the late 20th versus early 21st Centuries. Journal of Geophysical Research: Atmospheres, 122(16), DOI:10.1002/2017JD026719. Abstract
Observed and modeled patterns of lower stratospheric seasonal trends in Antarctic ozone and temperature in the late 20th (1979-2000) and the early 21st (2000-2014) centuries are compared. Patterns of pre-2000 observed Antarctic ozone decreases and stratospheric cooling as a function of month and pressure are followed by opposite-signed (i.e., “mirrored”) patterns of ozone increases and warming post-2000. An interactive chemistry-climate model forced by changes in anthropogenic ozone depleting substances produces broadly similar mirrored features. Statistical analysis of unforced model simulations (from long term model control simulations of a few centuries up to 1000 years) suggests that internal and solar natural variability alone is unable to account for the pattern of observed ozone trend mirroring, implying that forcing is the dominant driver of this behavior. Radiative calculations indicate that ozone increases have contributed to Antarctic warming of the lower stratosphere over 2000-2014, but dynamical changes that are likely due to internal variability over this relatively short period also appear to be important. Overall, the results support the recent finding that the healing of the Antarctic ozone hole is underway, and that coupling between dynamics, chemistry, and radiation is important for a full understanding of the causes of observed stratospheric temperature and ozone changes.
Guan, Xiaodan, Jianping Huang, Ruixia Guo, and Pu Lin, July 2015: The role of dynamically induced variability in the recent warming trend slowdown over the Northern Hemisphere. Scientific Reports, 5, 12669, DOI:10.1038/srep12669. Abstract
Since the slowing of the trend of increasing surface air temperature (SAT) in the late 1990 s, intense interest and debate have arisen concerning the contribution of human activities to the warming observed in previous decades. Although several explanations have been proposed for the warming-trend slowdown (WTS), none has been generally accepted. We investigate the WTS using a recently developed methodology that can successfully identify and separate the dynamically induced and radiatively forced SAT changes from raw SAT data. The dynamically induced SAT changes exhibited an obvious cooling effect relative to the warming effect of the adjusted SAT in the hiatus process. A correlation analysis suggests that the changes are dominated primarily by the North Atlantic Oscillation (NAO), Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO). Our results confirm that dynamically induced variability caused the WTS. The radiatively forced SAT changes are determined mainly by anthropogenic forcing, indicating the warming influence of greenhouse gases (GHGs), which reached levels of 400 ppm during the hiatus period. Therefore, the global SAT will not remain permanently neutral. The increased radiatively forced SAT will be amplified by increased dynamically induced SAT when the natural mode returns to a warming phase in the next period.
Guan, Xiaodan, Jianping Huang, Ruixia Guo, Haipeng Yu, Pu Lin, and Yanting Zhang, December 2015: Role of radiatively forced temperature changes in enhanced semi-arid warming in the cold season over east Asia. Atmospheric Chemistry and Physics, 15, DOI:10.5194/acp-15-13777-201513777-13786. Abstract
As climate change has occurred over east Asia since the 1950s, intense interest and debate have arisen concerning the contribution of human activities to the observed warming in past decades. In this study, we investigate regional surface temperature change during the boreal cold season using a recently developed methodology that can successfully identify and separate the dynamically induced temperature (DIT) and radiatively forced temperature (RFT) changes in raw surface air temperature (SAT) data. For regional averages, DIT and RFT contribute 44 and 56 % to the SAT over east Asia, respectively. The DIT changes dominate the SAT decadal variability and are mainly determined by internal climate variability, represented by the North Atlantic Oscillation (NAO), Pacific Decadal Oscillation (PDO), and Atlantic Multi-decadal Oscillation (AMO). Radiatively forced SAT changes have made a major contribution to the global-scale warming trend and the regional-scale enhanced semi-arid warming (ESAW). Such enhanced warming is also found in radiatively forced daily maximum and minimum SAT. The long-term global-mean SAT warming trend is mainly related to radiative forcing produced by global well-mixed greenhouse gases. The regional anthropogenic radiative forcing, however, caused the enhanced warming in the semi-arid region, which may be closely associated with local human activities. Finally, the relationship between the so-called "global warming hiatus" and regional enhanced warming is discussed.
The behavior of the Brewer-Dobson circulation is investigated using a suite of global climate model simulations with different forcing agents, in conjunction with observation-based analysis. We find that the variations in the Brewer-Dobson circulationare strongly correlated with those in the tropical-mean surface temperature through changes in the upper tropospheric temperature and zonal winds. This correlation is seen on both interannual and multi-decadal timescales, and holds for natural and forced variations alike. The circulation change is relatively insensitive to the spatial pattern of the forcings. Consistent changes in the Brewer-Dobson circulation with respect to those in the tropical-mean surface temperature prevail across timescales and forcings, and constitute an important attribution element of the atmospheric adjustment to global climate change.
Fueglistaler, Stephan, M Abalos, Thomas J Flannaghan, Pu Lin, and W J Randel, December 2014: Variability and trends in dynamical forcing of tropical lower stratospheric temperatures. Atmospheric Chemistry and Physics, 14(24), DOI:10.5194/acp-14-13439-2014. Abstract
We analyse the relation between tropical lower stratospheric temperatures and dynamical forcing over the period 1980–2011 using NCEP, MERRA and ERA-Interim reanalyses. The tropical mean thermodynamic energy equation with Newtonian cooling for radiation is forced with two dynamical predictors: (i) the average eddy heat flux of both hemispheres; and (ii) tropical upwelling estimated from momentum balance following Randel et al. (2002). The correlation (1995–2011) for deseasonalised tropical average temperatures at 70 hPa with the eddy heat flux based predictor is 0.84 for ERA-Interim (0.77 for the momentum balance calculation), and 0.87 for MERRA. The eddy heat flux based predictor indicates a dynamically forced cooling of the tropics of ∼−0.1 K decade−1 (∼−0.2 K decade−1 excluding volcanic periods) for the period 1980–2011 in MERRA and ERA-Interim. ERA-Interim eddy heat fluxes drift slightly relative to MERRA in the 2000's, possibly due to onset of GPS temperature data assimilation. While NCEP gives a small warming trend, all 3 reanalyses show a similar seasonality, with strongest cooling in January/February (∼−0.4 K decade−1, from northern hemispheric forcing) and October (∼−0.3 K decade−1, from southern hemispheric forcing). Months preceding and following the peaks in cooling trends show pronounced smaller, or even warming, trends. Consequently, the seasonality in the trends arises in part due to a temporal shift in eddy activity. Over all months, the Southern Hemisphere contributes more to the tropical cooling in both MERRA and ERA-Interim. The residual time series (observed minus estimate of dynamically forced temperature) are well correlated between ERA-Interim and MERRA, with differences largely due to temperature differences. The residual time series is dominated by the modification of the radiative balance by volcanic aerosol following the eruption of El Chichon (maximum warming of ∼3 K at 70 hPa) and Pinatubo (maximum warming of ∼4 K at 70 hPa), with a strong dynamical response during Pinatubo partially masking the aerosol heating.
