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Archive for February, 2017

Papers on the warming hole of the United States

Posted by Ari Jokimäki on February 8, 2017

This is a list of papers on the warming hole of the United States. The list is not complete, and will most likely be updated in future in order to make it more thorough and more representative.

The United States “warming hole”: quantifying the forced aerosol response given large internal variability (Banerjee et al. 2017)
Abstract: “Twenty-five years of large summer cooling over the southeastern United States ending in the mid-1970s coincided with rapidly increasing anthropogenic aerosol emissions. Here, we assess the claim that the cooling in that period was predominantly due to such aerosols. We utilize two 50-member sets of coupled climate model simulations, one with only anthropogenic aerosol forcings and another with all known natural and anthropogenic forcings, together with a long control integration. We show that, in the absence of aerosol forcing, none of the model simulations capture the observed surface cooling rate (∼0.56∘C decade−1), whereas with increasing aerosol emissions two (of fifty) of the simulations do. More importantly, however, we find that the cooling from aerosols (0.20° C decade−1) is insufficient to explain the observation. Our results therefore suggest that, while aerosols may have played a role, the observed cooling was a rare event that contained a large contribution from unforced internal variability.”
Citation: Banerjee A., L.M Polvani, and J.C. Fyfe (2017), The United States “warming hole’: quantifying the forced aerosol response given large internal variability, Geophys. Res. Lett., 44, doi:10.1002/2016GL071567.

Tracking regional temperature projections from the early 1990s in light of variations in regional warming, including ‘warming holes’ (Grose et al. 2017)
Abstract: “The perception of the accuracy of regional climate projections made in the early 1990s about climate change by 2030 may be influenced by how the temperature trend has changed in the 25 years since their publication. However, temperature trends over this period were influenced not only by external forcings such as greenhouse gases but also natural variations. The temperature of Southern Australia, the Sahel, South Asia and Southern Europe are currently within the warming estimates from statements in the early 1990s from the IPCC and CSIRO, assuming a linear trend between 1990 and 2030. However, northern Australia and central North America are currently at the lower limit or below these projections, having featured areas of multi-year regional cooling during global warming, sometimes called ‘warming holes’. Recent climate model simulations suggest that cooling can be expected in the recent past and near future in some regions, including in Australia and the US, and that cooling is less likely over 1990–2030 than in 1990–2015, bringing observations closer to the IPCC and CSIRO warming estimates by 2030. Cooling at the 25-year scale in some regions can be associated with cyclic variability such as the Inter-decadal Pacific Oscillation, or low trend such as in the Southern Ocean. Explicitly communicating the variability in regional warming rates in climate projections, including the possibility of regional warming ‘holes’ (or the opposite of ‘surges’ or ‘peaks’) would help to set more reliable expectations by users of those projections.”
Citation: Grose, M.R., Risbey, J.S. & Whetton, P.H. Climatic Change (2017) 140: 307. doi:10.1007/s10584-016-1840-9.

North Pacific SST Forcing on the Central United States “Warming Hole” as Simulated in CMIP5 Coupled Historical and Uncoupled AMIP Experiments (Pan et al. 2017)
Abstract: “The central United States experienced a cooling trend during the twentieth century, called the “warming hole,” most notably in the last quarter of the century when global warming accelerated. The coupled simulations of the models that participated in the Coupled Model Intercomparison Project, Phases 3 and 5 (CMIP3/5), have been unable to reproduce this abnormal cooling phenomenon satisfactorily. An unrealistic representation of the observed phasing of the Pacific Decadal Oscillation (PDO)—one of the proposed forcing mechanisms for the warming hole—in the models is considered to be one of the main causes of this effect. The CMIP5’s uncoupled Atmospheric Model Intercomparison Project (AMIP) experiment, whose duration approximately coincides with the peak warming hole cooling period, provides an opportunity, when compared with the coupled historical experiment, to examine the role of the variation in Pacific Ocean sea surface temperature (SST) in the warming hole’s formation and also to assess the skill of the models in simulating the teleconnection between Pacific SST and the continental climate in North America. Accordingly, this study compared AMIP and historical runs in the CMIP5 suite thereby isolating the role of SST forcing in the formation of the warming hole and its maintenance mechanisms. It was found that, even when SST forcing in the AMIP run was “perfectly” prescribed in the models, the skill of the models in simulating the warming hole cooling in the central United States showed little improvement over the historical run, in which SST is calculated interactively, even though the AMIP run overestimated the anti-correlation between temperature in the central United States and the PDO index. The fact that better simulation of the PDO phasing in the AMIP run did not translate into an improved summer cooling trend in the central United States suggests that the inability of the coupled CMIP5 models to reproduce the warming hole under the historical run is not mainly a result of the mismatch between simulated and observed PDO phasing, as believed.”
Citation: Zaitao Pan, Chunhua Shi, Sanjiv Kumar, and Zhiqiu Gao (2017) Atmosphere-Ocean, doi: 10.1080/07055900.2016.1261690.

Disappearance of the southeast U.S. “warming hole” with the late 1990s transition of the Interdecadal Pacific Oscillation (Meehl et al. 2015) [FULL TEXT]
Abstract: “Observed surface air temperatures over the contiguous U.S. for the second half of the twentieth century showed a slight cooling over the southeastern part of the country, the so-called “warming hole,” while temperatures over the rest of the country warmed. This pattern reversed after 2000. Climate model simulations show that the disappearance of the warming hole in the early 2000s is likely associated with the transition of the Interdecadal Pacific Oscillation (IPO) phase from positive to negative in the tropical Pacific in the late 1990s, coincident with the early 2000s slowdown of the warming trend in globally averaged surface air temperature. Analysis of a specified convective heating anomaly sensitivity experiment in an atmosphere-only model traces the disappearance of the warming hole to negative sea surface temperature anomalies and consequent negative precipitation and convective heating anomalies in the central equatorial Pacific Ocean associated with the negative phase of the IPO after 2000.”
Citation: Meehl, G. A., J. M. Arblaster, and C. T. Y. Chung (2015), Disappearance of the southeast U.S. “warming hole” with the late 1990s transition of the Interdecadal Pacific Oscillation, Geophys. Res. Lett., 42, 5564–5570, doi:10.1002/2015GL064586.

Sedimentary proxy evidence of a mid-Holocene hypsithermal event in the location of a current warming hole, North Carolina, USA (Tanner et al. 2015) [FULL TEXT]
Abstract: “A wetland deposit from the southern Appalachian mountains of North Carolina, USA, has been radiocarbon dated and shows continuous deposition from the early Holocene to the present. Non-coastal records of Holocene paleoenvironments are rare from the southeastern USA. Increased stable carbon isotope ratios (δ13C) of sedimentary organic matter and pollen percentages indicate warm, dry early- to mid-Holocene conditions. This interpretation is also supported by n-alkane biomarker data and bulk sedimentary C/N ratios. These warm, dry conditions coincide with a mid-Holocene hypsithermal, or altithermal, documented elsewhere in North America. Our data indicate that the southeastern USA warmed concurrently with much of the rest of the continent during the mid-Holocene. If the current “warming hole” in the southeastern USA persists, during a time of greenhouse gas-induced warming elsewhere, it will be anomalous both in space and time.”
Citation: Benjamin R. Tanner, Chad S. Lane, Elizabeth M. Martin, Robert Young, Beverly Collins (2015) Quaternary Research, Volume 83, Issue 2, March 2015, Pages 315–323, doi: 10.1016/j.yqres.2014.11.004.

Attribution of the United States “warming hole”: Aerosol indirect effect and precipitable water vapor (Yu et al. 2014) [FULL TEXT]
Abstract: “Aerosols can influence the climate indirectly by acting as cloud condensation nuclei and/or ice nuclei, thereby modifying cloud optical properties. In contrast to the widespread global warming, the central and south central United States display a noteworthy overall cooling trend during the 20th century, with an especially striking cooling trend in summertime daily maximum temperature (Tmax) (termed the U.S. “warming hole”). Here we used observations of temperature, shortwave cloud forcing (SWCF), longwave cloud forcing (LWCF), aerosol optical depth and precipitable water vapor as well as global coupled climate models to explore the attribution of the “warming hole”. We find that the observed cooling trend in summer Tmax can be attributed mainly to SWCF due to aerosols with offset from the greenhouse effect of precipitable water vapor. A global coupled climate model reveals that the observed “warming hole” can be produced only when the aerosol fields are simulated with a reasonable degree of accuracy as this is necessary for accurate simulation of SWCF over the region. These results provide compelling evidence of the role of the aerosol indirect effect in cooling regional climate on the Earth. Our results reaffirm that LWCF can warm both winter Tmax and Tmin.”
Citation: Shaocai Yu, Kiran Alapaty, Rohit Mathur, Jonathan Pleim, Yuanhang Zhang, Chris Nolte, Brian Eder, Kristen Foley & Tatsuya Nagashima (2014), Scientific Reports, 4, doi:10.1038/srep06929.

Multidecadal Climate Variability and the “Warming Hole” in North America: Results from CMIP5 Twentieth- and Twenty-First-Century Climate Simulations (Kumar et al. 2013) [FULL TEXT]
Abstract: “The ability of phase 5 of the Coupled Model Intercomparison Project (CMIP5) climate models to simulate the twentieth-century “warming hole” over North America is explored, along with the warming hole’s relationship with natural climate variability. Twenty-first-century warming hole projections are also examined for two future emission scenarios, the 8.5 and 4.5 W m−2 representative concentration pathways (RCP8.5 and RCP4.5). Simulations from 22 CMIP5 climate models were analyzed, including all their ensemble members, for a total of 192 climate realizations. A nonparametric trend detection method was employed, and an alternative perspective emphasizing trend variability. Observations show multidecadal variability in the sign and magnitude of the trend, where the twentieth-century temperature trend over the eastern United States appears to be associated with low-frequency (multidecadal) variability in the North Atlantic temperatures. Most CMIP5 climate models simulate significantly lower “relative power” in the North Atlantic multidecadal oscillations than observed. Models that have relatively higher skill in simulating the North Atlantic multidecadal oscillation also are more likely to reproduce the warming hole. It was also found that the trend variability envelope simulated by multiple CMIP5 climate models brackets the observed warming hole. Based on the multimodel analysis, it is found that in the twenty-first-century climate simulations the presence or absence of the warming hole depends on future emission scenarios; the RCP8.5 scenario indicates a disappearance of the warming hole, whereas the RCP4.5 scenario shows some chance (10%–20%) of the warming hole’s reappearance in the latter half of the twenty-first century, consistent with CO2 stabilization.”
Citation: Sanjiv Kumar, James Kinter, Paul A. Dirmeyer, Zaitao Pan, Jennifer Adams (2013), Journal of Climate, 26, 11, 3511-3527, doi: 10.1175/JCLI-D-12-00535.1.

Intermodel Variability and Mechanism Attribution of Central and Southeastern U.S. Anomalous Cooling in the Twentieth Century as Simulated by CMIP5 Models (Pan et al. 2013) [FULL TEXT]
Abstract: “Some parts of the United States, especially the southeastern and central portion, cooled by up to 2°C during the twentieth century, while the global mean temperature rose by 0.6°C (0.76°C from 1901 to 2006). Studies have suggested that the Pacific decadal oscillation (PDO) and the Atlantic multidecadal oscillation (AMO) may be responsible for this cooling, termed the “warming hole” (WH), while other works reported that regional-scale processes such as the low-level jet and evapotranspiration contribute to the abnormity. In phase 3 of the Coupled Model Intercomparison Project (CMIP3), only a few of the 53 simulations could reproduce the cooling. This study analyzes newly available simulations in experiments from phase 5 of the Coupled Model Intercomparison Project (CMIP5) from 28 models, totaling 175 ensemble members. It was found that 1) only 19 out of 100 all-forcing historical ensemble members simulated negative temperature trend (cooling) over the southeast United States, with 99 members underpredicting the cooling rate in the region; 2) the missing of cooling in the models is likely due to the poor performance in simulating the spatial pattern of the cooling rather than the temporal variation, as indicated by a larger temporal correlation coefficient than spatial one between the observation and simulations; 3) the simulations with greenhouse gas (GHG) forcing only produced strong warming in the central United States that may have compensated the cooling; and 4) the all-forcing historical experiment compared with the natural-forcing-only experiment showed a well-defined WH in the central United States, suggesting that land surface processes, among others, could have contributed to the cooling in the twentieth century.”
Citation: Zaitao Pan, Xiaodong Liu, Sanjiv Kumar, Zhiqiu Gao, James Kinter (2013) Journal of Climate, 26, 17, 6215-6237, doi: 10.1175/JCLI-D-12-00559.1.

Mechanisms Contributing to the Warming Hole and the Consequent U.S. East–West Differential of Heat Extremes (Meehl et al. 2012) [FULL TEXT]
Abstract: “A linear trend calculated for observed annual mean surface air temperatures over the United States for the second-half of the twentieth century shows a slight cooling over the southeastern part of the country, the so-called warming hole, while temperatures over the rest of the country rose significantly. This east–west gradient of average temperature change has contributed to the observed pattern of changes of record temperatures as given by the ratio of daily record high temperatures to record low temperatures with a comparable east–west gradient. Ensemble averages of twentieth-century climate simulations in the Community Climate System Model, version 3 (CCSM3), show a slight west–east warming gradient but no warming hole. A warming hole appears in only several ensemble members in the Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel dataset and in one ensemble member of simulated twentieth-century climate in CCSM3. In this model the warming hole is produced mostly from internal decadal time-scale variability originating mainly from the equatorial central Pacific associated with the Interdecadal Pacific Oscillation (IPO). Analyses of a long control run of the coupled model, and specified convective heating anomaly experiments in the atmosphere-only version of the model, trace the forcing of the warming hole to positive convective heating anomalies in the central equatorial Pacific Ocean near the date line. Cold-air advection into the southeastern United States in winter, and low-level moisture convergence in that region in summer, contribute most to the warming hole in those seasons. Projections show a disappearance of the warming hole, but ongoing greater surface temperature increases in the western United States compared to the eastern United States.”
Citation: Gerald A. Meehl, Julie M. Arblaster, Grant Branstator (2012) Journal of Climate, 25, 18, 6394-6408, doi: 10.1175/JCLI-D-11-00655.1.

Can CGCMs Simulate the Twentieth-Century “Warming Hole” in the Central United States? (Kunkel et al. 2006) [FULL TEXT]
Abstract: “The observed lack of twentieth-century warming in the central United States (CUS), denoted here as the “warming hole,” was examined in 55 simulations driven by external historical forcings and in 19 preindustrial control (unforced) simulations from 18 coupled general circulation models (CGCMs). Twentieth-century CUS trends were positive for the great majority of simulations, but were negative, as observed, for seven simulations. Only a few simulations exhibited the observed rapid rate of warming (cooling) during 1901–40 (1940–79). Those models with multiple runs (identical forcing but different initial conditions) showed considerable intramodel variability with trends varying by up to 1.8°C century−1, suggesting that internal dynamic variability played a major role at the regional scale. The wide range of trend outcomes, particularly for those models with multiple runs, and the small number of simulations similar to observations in both the forced and unforced experiments suggest that the warming hole is not a robust response of contemporary CGCMs to the estimated external forcings. A more likely explanation based on these models is that the observed warming hole involves external forcings combined with internal dynamic variability that is much larger than typically simulated. The observed CUS temperature variations are positively correlated with North Atlantic (NA) sea surface temperatures (SSTs), and both NA SSTs and CUS temperature are negatively correlated with central equatorial Pacific (CEP) SSTs. Most models simulate rather well the connection between CUS temperature and NA SSTs. However, the teleconnections between NA and CEP SSTS and between CEP SSTs and CUS temperature are poorly simulated and the models produce substantially less NA SST variability than observed, perhaps hampering their ability to reproduce the warming hole.”
Citation: Kenneth E. Kunkel, Xin-Zhong Liang, Jinhong Zhu, and Yiruo Lin (2006) Journal of Climate, 19, 17, 4137-4153, doi: 10.1175/JCLI3848.1.

Altered hydrologic feedback in a warming climate introduces a “warming hole” (Pan et al. 2004) [FULL TEXT]
Abstract: “In the last 25 years of the 20th century most major land regions experienced a summer warming trend, but the central U.S. cooled by 0.2–0.8 K. In contrast most climate projections using GCMs show warming for all continental interiors including North America. We examined this discrepancy by using a regional climate model and found a circulation-precipitation coupling under enhanced greenhouse gas concentrations that occurs on scales too small for current GCMs to resolve well. Results show a local minimum of warming in the central U.S. (a “warming hole”) associated with changes in low-level circulations that lead to replenishment of seasonally depleted soil moisture, thereby increasing late-summer evapotranspiration and suppressing daytime maximum temperatures. These regional-scale feedback processes may partly explain the observed late 20th century temperature trend in the central U.S. and potentially could reduce the magnitude of future greenhouse warming in the region.”
Citation: Pan, Z., R. W. Arritt, E. S. Takle, W. J. Gutowski Jr., C. J. Anderson, and M. Segal (2004), Altered hydrologic feedback in a warming climate introduces a “warming hole”, Geophys. Res. Lett., 31, L17109, doi:10.1029/2004GL020528.

General circulation model simulations of recent cooling in the east-central United States (Robinson et al. 2002) [FULL TEXT]
Abstract: “In ensembles of retrospective general circulation model (GCM) simulations, surface temperatures in the east-central United States cool between 1951 and 1997. This cooling, which is broadly consistent with observed surface temperatures, is present in GCM experiments driven by observed time varying sea-surface temperatures (SSTs) in the tropical Pacific, whether or not increasing greenhouse gases and other time varying climate forcings are included. Here we focus on ensembles with fixed radiative forcing and with observed varying SST in different regions. In these experiments the trend and variability in east-central U.S. surface temperatures are tied to tropical Pacific SSTs. Warm tropical Pacific SSTs cool U.S. temperatures by diminishing solar heating through an increase in cloud cover. These associations are embedded within a year-round response to warm tropical Pacific SST that features tropospheric warming throughout the tropics and regions of tropospheric cooling in midlatitudes. Precipitable water vapor over the Gulf of Mexico and the Caribbean and the tropospheric thermal gradient across the Gulf Coast of the United States increase when the tropical Pacific is warm. In observations, recent warming in the tropical Pacific is also associated with increased precipitable water over the southeast United States. The observed cooling in the east-central United States, relative to the rest of the globe, is accompanied by increased cloud cover, though year-to-year variations in cloud cover, U.S. surface temperatures, and tropical Pacific SST are less tightly coupled in observations than in the GCM.”
Citation: Robinson, W. A., R. Reudy, and J. E. Hansen, General circulation model simulations of recent cooling in the east-central United States, J. Geophys. Res., 107(D24), 4748, doi:10.1029/2001JD001577, 2002.

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