Amplification Of Warming Research Articles

Waterside convection and stratification control methane spreading in supersaturated Arctic fjords (Spitsbergen)

Seasonally ice covered in the past, the fjords in West Spitsbergen turn into being perennially ice free in the present. This feedback to Arctic amplification of global warming changes gas fluxes at the atmosphere-ocean interface. Furthermore, in this Polar region, coupled feedbacks likely enhance Arctic amplification of global warming as numerous gas seepages provide evidence for active gas emissions at the sediment-water interface. We present a time series (2015–2017) of dissolved methane concentrations combined with hydrographic data in Adventfjorden and Tempelfjorden, two sub-fjords of Isfjorden located at the west coast of Spitsbergen. While both sub-fjords remained permanently supersaturated, we detected pronounced temporal and spatial variations in the methane excess level. Our study revealed that seasonal water transformations were key to seasonally changing methane pathways including potential sea-air flux (efflux). We suggest that a cascade of feedback processes, seasonally triggered by waterside convection and stratification, adjusts the amount of methane released and transported within fjord water. When sea ice was missing, strong winter cooling affected the methane supersaturation in contrary directions: first a drop and then a strong increase. In early winter, convective mixing favoured efflux, which reduced the supersaturation. Later in winter, the thermal convection resulted in a continuous overturning of the water column. When the thermal convection reached the bottom, sediment resuspension by turbulence increased, which in turn encouraged enhanced methane release. Subsequently transported along vertical isopycnals, methane from the bottom water reached the water-atmosphere interface. These coupled events created a steady state, simultaneously maintaining supersaturation and efflux. During the warm season, the fjord water became stratified and methane transport occurred mainly laterally in the bottom water. The seasonally changing hydrographic conditions strongly triggered the methane spreading in both sub-fjords and point to a switch between the atmosphere and ocean as main sinks in winter and summer, respectively. Upcoming variations in seasonality, i.e. warmer/cooler summer compared to colder/warmer winter will influence these pathways and the final fate of methane discharged into Arctic fjords.

Changes in elevation and mass of Arctic glaciers and ice caps, 2010–2017

Arctic glaciers and ice caps (GIC) are losing mass rapidly, and this process is expected to continue during the 21st century owing to polar amplification of climate warming. Here, we use seven years of CryoSat-2 swath interferometric altimetry to track changes in the volume of Arctic GIC. From these data, we produce a pan-Arctic assessment of GIC mass imbalance, and we partition their losses into signals associated with atmospheric processes and glacier dynamics. Between 2010 and 2017, Arctic GIC lost 609 ± 7 Gt of ice, contributing 0.240 ± 0.007 mm per year to global sea level rise. While surface ablation is responsible for 87% of losses across the Arctic, dynamic imbalance is increasing in the Barents and Kara Sea region where it now accounts for 43% of total ice loss. Arctic GIC's dynamic imbalance is associated with a northward shift of Atlantic climate, and this effect should be considered in global sea level projections.

Atmospheric Circumglobal Teleconnection Triggered by Spring Land Thermal Anomalies Over West Asia and Its Possible Impacts on Early Summer Climate Over Northern China

AbstractEurasian continent experienced significant warming during the past decades. West Asia locates in arid/semi-arid zone and its warming amplification has drawn lots of attention. However, the climatic effect of such a warming is not clear yet. In this study, we explored the possible impacts of recent land surface warming over West Asia on the atmospheric general circulation and climate. Results show that abnormal spring land surface warming over West Asia tends to increase precipitation over North China but decrease (increase) precipitation (air temperature) over Northeast China in early summer (June). It is noted that the precipitation anomalies are much stronger over the eastern region of North/Northeast China. Further analysis suggests abnormal spring land surface warming can trigger eastward-propagating disturbance via diabatic heating, which benefits intensified the atmospheric circumglobal teleconnection (CGT) pattern, causing anomalous circulation and climate in early summer over northern China. Sensitivity experiments demonstrate that abnormal spring land surface warming can increase the atmospheric baroclinic instability and trigger Rossby waves that propagate along the westerly jet stream (WJS), resulting in the formation of CGT. Due to persistent land surface thermal forcing and the interaction between the basic flow (especially WJS) and CGT, the CGT tends to be intensified. The anomalous wave center over East Asia in early summer is responsible for the precipitation increases (decreases) over North (Northeast) China and the evident warming in Northeast China. Our results suggest that the spring land surface thermal anomalies over West Asia can be a potential signal for short-term prediction of early summer climate over northern China.

Changes in polar amplification in response to increasing warming in CMIP6

The climate in polar regions has experienced an obvious warming amplification due to global warming. In this study, the changes in polar amplification are analyzed in response to feedback mechanisms (including Planck, lapse rate, cloud, water vapor, albedo feedback, CO2 radiative forcing, ocean heat uptake, and atmospheric heat transport) under three warming scenarios in CMIP6—namely, SSP1-2.6, SSP2-4.5, and SSP5-8.5. The results show that, by quantifying the warming contribution of different feedback mechanisms to surface air temperature with the “radiative kernel” method, Arctic amplification (AA) is stronger than Antarctic amplification (ANA), mostly resulting from the lapse rate feedback, followed by the albedo and Planck feedbacks. Furthermore, ocean heat uptake causes stronger polar warming in winter than in summer. During winter, the lapse rate feedback causes a larger AA than ANA. The intermodel spread for both AA and ANA decrease with increasing strength of global warming from SSP1-2.6 to SSP5-8.5, and the dominant mechanisms are the Planck, lapse rate, albedo, and ocean heat uptake feedbacks. These findings help to enhance our understanding of polar regions’ responses to different strengths of global warming.摘要由于全球变暖, 极地地区的气候经历了明显的变暖放大.在本项研究中, 我们根据CMIP6模式的三种变暖情景 (SSP1-2.6,SSP2-4.5和SSP5-8.5) 下, 极地放大变化对各个反馈机制 (包括普朗克,温度递减率,云,水蒸气,反照率反馈,CO2强迫,海洋热吸收和大气热传输) 的响应进行了分析.结果表明, 通过用"辐射核"方法量化不同反馈机制对地表温度的增温贡献, 北极放大 (AA) 强于南极放大 (ANA) , 由温度递减率反馈主导, 其次是反照率和普朗克反馈.此外, 海洋的热吸收导致冬季比夏季有更强的极地变暖.在冬季, 温度递减率反馈主导了AA大于ANA.AA和ANA的模式间差异随着全球变暖的增强而减小.

Quantifying the Tropical Upper Tropospheric Warming Amplification Using Radio Occultation Measurements

AbstractIn the upper troposphere (UT), Global Navigation Satellite System (GNSS) Radio Occultations (ROs) provide accurate air temperatures (<0.5 K) every ∼200 m vertically. We use RO observations from 01/2002 until 12/2018 at 300, 250, and 200 hPa to quantify the tropical upper tropospheric amplification (defined as the ratio of temperature trends in the UT relative to the surface). We compare the RO‐derived results against Atmospheric Infrared Sounder (AIRS), Coupled Model Intercomparison Project Phase 6 (CMIP6) Atmospheric Model Intercomparison Project (AMIP) models, Modern‐Era Retrospective Analysis for Research and Applications version 2 (MERRA‐2), and European Center for Medium‐range Weather Forecasts Re‐Analysis Interim (ERA‐Interim) data. We find that CMIP6 AMIP models show excellent agreement with independent AIRS and RO observations on the magnitude of the UT warming and show warming that is significantly faster than both reanalyses above 250 hPa. Additionally, AIRS and CMIP6 AMIP present excellent agreement with the RO‐measured tropical tropospheric amplification.

Emerging Pacific Quasi‐Decadal Oscillation Over the Past 70 Years

AbstractQuasi‐decadal variation in various Pacific regions has been recognized for decades but its spatial‐temporal characteristics and generation mechanisms remain poorly understood. Here we show that the sea surface temperature (SST) in the equatorial central Pacific (ECP) exhibits a prominent 11‐year periodicity since 1951 but not before. An episodic‐like quasi‐decadal warm event is initiated in a northeast‐southwest tilted belt from the United States' west coast to the ECP. The initial development involves positive feedback between atmospheric heating‐induced Rossby waves and underlying SST in the tropical North Pacific. The amplification of ECP warming is primarily due to equatorward heat advection and deepening thermocline, while the zonal advective feedback mainly controls its decay. The quasi‐decadal oscillation (QDO) tends to follow the 11‐years solar cycle with a 1‐to‐2‐year phase delay, meanwhile, it possibly links to the nonlinear behavior of ENSO. The cause of intensification of the QDO over the past 70 years, however, remains elusive.

Large‐scale connection to deadly Indian heatwaves

AbstractIn the present study, we analyse large‐scale connections to deadly heatwaves in India in spring. Deadly Indian heatwaves occur in the spring months of April and May, and are increasing in recent years. These events are associated with the amplification of planetary Rossby waves. This amplification is called quasi‐resonant amplification (QRA). An increase of amplitude or resonance occurs when stationary and moving waves coincide in their size. Previous studies stressed QRA's role in summer because persistent extreme weather events such as the 2003 European heatwave, the 2010 Pakistan flood, Russian heatwave, the 2011 Texas and Oklahoma heat waves, and drought occurred in summer. QRA seems to be related to the amplification of Arctic warming in winter and summer, but observations show that Arctic warming occurs in spring also. The novel aspect of our study is that we found strong evidence for QRA in the boreal spring, the season of Indian heatwaves. Amplified Arctic warming or Arctic amplification thought to be responsible for QRA occurs in spring also. Our results confirm that maximum surface temperatures in spring over India show predictability with high statistical significance, about 4 days ahead. Maximum surface temperatures can be used to predict heatwaves, as shown earlier by some authors.

Wind amplifies the polar sea ice retreat

The rapid polar sea ice retreat and its drivers are challenging and still unresolved questions in climate change research. In particular, the relationship between near-surface wind speed and sea ice extent remains unclear for two main reasons: (1) observed wind speeds over Polar Regions are very sparse, and (2) simulated winds by climate models are dependent on subjective parameterizations of boundary layer stratification, ultimately leading to large uncertainty. Here, we use observation-based data (passive microwave sea ice concentration and six different reanalysis datasets) together with output from 26 climate models (from the CMIP5 archive) to quantify the relationships between near-surface wind speed and sea ice concentration over the past 40 years. We find strong inverse relationships between near-surface wind speed and sea ice concentration that are consistent among the six reanalysis datasets. The poleward wind component is particularly increasing in years of reduced sea ice concentration, which contributes to the enhancement of the atmospheric (surface oceanic) poleward heat flux by up to 24 ± 1% (29 ± 2%) in the Arctic and 37 ± 3% (51 ± 3%) in the Antarctic seas, therefore boosting the impact of polar sea ice loss and contributing to polar amplification of climate warming. In addition, our results show a marginal contribution of the dynamical (pushing/opening/compacting) effects of wind on sea ice compared to the thermodynamic effects which in turn play a lower role than the associated change in local surface Autumn–Winter turbulent and Spring–Summer radiative fluxes. Climate models generally produce similar results but with lower magnitude, and one model even simulates the opposite relationship wind/sea-ice. Given the rapid changes in polar climate and the potential impacts on the mid-latitudes, it is urgent that model developments make use of evidence from satellite observations and reanalysis datasets to reduce uncertainties in the representation of relationships between polar winds and sea ice.

Evaluation of Arctic warming in mid-Pliocene climate simulations

Abstract. Palaeoclimate simulations improve our understanding of the climate, inform us about the performance of climate models in a different climate scenario, and help to identify robust features of the climate system. Here, we analyse Arctic warming in an ensemble of 16 simulations of the mid-Pliocene Warm Period (mPWP), derived from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2). The PlioMIP2 ensemble simulates Arctic (60–90∘ N) annual mean surface air temperature (SAT) increases of 3.7 to 11.6 ∘C compared to the pre-industrial period, with a multi-model mean (MMM) increase of 7.2 ∘C. The Arctic warming amplification ratio relative to global SAT anomalies in the ensemble ranges from 1.8 to 3.1 (MMM is 2.3). Sea ice extent anomalies range from −3.0 to -10.4×106 km2, with a MMM anomaly of -5.6×106 km2, which constitutes a decrease of 53 % compared to the pre-industrial period. The majority (11 out of 16) of models simulate summer sea-ice-free conditions (≤1×106 km2) in their mPWP simulation. The ensemble tends to underestimate SAT in the Arctic when compared to available reconstructions, although the degree of underestimation varies strongly between the simulations. The simulations with the highest Arctic SAT anomalies tend to match the proxy dataset in its current form better. The ensemble shows some agreement with reconstructions of sea ice, particularly with regard to seasonal sea ice. Large uncertainties limit the confidence that can be placed in the findings and the compatibility of the different proxy datasets. We show that while reducing uncertainties in the reconstructions could decrease the SAT data–model discord substantially, further improvements are likely to be found in enhanced boundary conditions or model physics. Lastly, we compare the Arctic warming in the mPWP to projections of future Arctic warming and find that the PlioMIP2 ensemble simulates greater Arctic amplification than CMIP5 future climate simulations and an increase instead of a decrease in Atlantic Meridional Overturning Circulation (AMOC) strength compared to pre-industrial period. The results highlight the importance of slow feedbacks in equilibrium climate simulations, and that caution must be taken when using simulations of the mPWP as an analogue for future climate change.

Possible Influence of Tropical Indian Ocean Sea Surface Temperature on the Proportion of Rapidly Intensifying Western North Pacific Tropical Cyclones during the Extended Boreal Summer

AbstractThis study examines the possible impact of tropical Indian Ocean (TIO) sea surface temperature anomalies (SSTAs) on the proportion of rapidly intensifying tropical cyclones (PRITC) over the western North Pacific (WNP) during the extended boreal summer (July–November). There is a robust interannual association (r= 0.46) between TIO SSTAs and WNP PRITC during 1979–2018. Composite analyses between years with warm and cold TIO SSTAs confirm a significant impact of TIO SSTA on WNP PRITC, with PRITC over the WNP basin being 50% during years with warm TIO SSTAs and 37% during years with cold TIO SSTAs. Tropical cyclone heat potential appears to be one of the most important factors in modulating the interannual change of PRITC over the WNP with a secondary role from midlevel moisture changes. Interannual changes in these large-scale factors respond to SSTA differences characterized by a tropics-wide warming, implying a possible global warming amplification on WNP PRITC. The possible footprint of global warming amplification of the TIO is deduced from 1) a significant correlation between TIO SSTAs and global mean SST (GMSST) and a significant linear increasing trend of GMSST and TIO SSTAs, and 2) an accompanying small difference of PRITC (~8%) between years with detrended warm and cold TIO SSTAs compared to the difference of PRITC (~13%) between years with nondetrended warm and cold TIO SSTAs. Global warming may contribute to increased TCHP, which is favorable for rapid intensification, but increased vertical wind shear is unfavorable for TC genesis, thus amplifying WNP PRITC.

Decadal variability and recent summer warming amplification of the sea surface temperature in the Red Sea.

Under climate change, regional Sea Surface Temperature (SST) changes are a crucial factor affecting marine ecosystems, which thrive only within a certain thermal limit. Thirty-seven years of monthly gridded Optimum Interpolation SST data from 1982 to 2017 were used to investigate the decadal variability of this parameter in the Red Sea during the summer season, in relation to large-scale climate variability. We identified a non-uniform warming trend beginning around the mid-1990s over the whole basin, with a predominant amplified warming over the northern half (0.04°C year-1), which is approximately four times higher than the global warming trend, but much weaker warming over southern end (0.01°C year-1). It was found that the Atlantic Multi-Decadal Oscillation (AMO) and the Silk Road Pattern (SRP) are shaping the RS SST, since their phase shift concurs with the timing of the significant non-uniform warming over the basin. The AMO triggers the SRP-related vertical and horizontal temperature advection that leads to opposite changes in the SST. We suggest that warming is amplified over the basin due to an overlap with global warming signals. Our results have important implications for interannual and decadal SST predictions based on the predictability of AMO and SRP patterns.

Arctic Amplification: A Rapid Response to Radiative Forcing

AbstractArctic amplification (AA) of surface warming is a prominent feature of anthropogenic climate change with important implications for human and natural systems. Despite its importance, the underlying causes of AA are not fully understood. Here, analyzing coupled climate model simulations, we show that AA develops rapidly (within the first few months) following an instantaneous quadrupling of atmospheric CO2. This rapid AA response—which occurs before any significant loss of Arctic sea ice—is produced by a positive lapse rate feedback over the Arctic. Sea ice loss is therefore not needed to produce polar‐amplified warming, although it contributes significantly to this warming after the first few months. Our results provide new and compelling evidence that AA owes its existence, fundamentally, to fast atmospheric processes.

Unique biodiversity in Arctic marine forests is shaped by diverse recolonization pathways and far northern glacial refugia

The Arctic is experiencing a rapid shift toward warmer regimes, calling for a need to understand levels of biodiversity and ecosystem responses to climate cycles. This study presents genetic data for 109 Arctic marine forest species (seaweeds), which revealed contiguous populations extending from the Bering Sea to the northwest Atlantic, with high levels of genetic diversity in the east Canadian Arctic. One-fifth of the species sampled appeared restricted to Arctic waters. Further supported by hindcasted species distributions during the Last Glacial Maximum, we hypothesize that Arctic coastal systems were recolonized from many geographically disparate refugia leading to enriched diversity levels in the east Canadian Arctic, with important contributions stemming from northerly refugia likely centered along southern Greenland. Our results suggest Arctic marine biomes persisted through cycles of glaciation, leading to unique assemblages in polar waters, rather than being entirely derived from southerly (temperate) areas following glaciation. As such, Arctic marine species are potentially born from selective pressures during Cenozoic global cooling and eventual ice conditions beginning in the Pleistocene. Arctic endemic diversity was likely additionally driven by repeated isolations into globally disparate refugia during glaciation. This study highlights the need to take stock of unique Arctic marine biodiversity. Amplification of warming and loss of perennial ice cover are set to dramatically alter available Arctic coastal habitat, with the potential loss of diversity and decline in ecosystem resilience.

Arctic Amplification of Global Warming Strengthened by Sunlight Oxidation of Permafrost Carbon to CO2

AbstractOnce thawed, up to 15% of the ∼1,000 Pg of organic carbon (C) in arctic permafrost soils may be oxidized to carbon dioxide (CO2) by 2,100, amplifying climate change. However, predictions of this amplification strength ignore the oxidation of permafrost C to CO2 in surface waters (photomineralization). We characterized the wavelength dependence of permafrost dissolved organic carbon (DOC) photomineralization and demonstrate that iron catalyzes photomineralization of old DOC (4,000–6,300 a BP) derived from soil lignin and tannin. Rates of CO2 production from photomineralization of permafrost DOC are twofold higher than for modern DOC. Given that model predictions of future net loss of ecosystem C from thawing permafrost do not include the loss of CO2 to the atmosphere from DOC photomineralization, current predictions of an average of 208 Pg C loss by 2,299 may be too low by ~14%.

Time‐dependent warming amplification over the Tibetan Plateau during the past few decades

AbstractIt is reported that surface warming over the Tibetan Plateau (TP) has been faster than the global average, but exactly how much faster remains controversial. This study investigates the time dependency of warming amplification over the TP using CRU_TS4.01 grid dataset during 1961–2016 with a consideration of its consistency with the global average. We find that the magnitude of warming on the TP and its consistency with the global average have been variable. Compared to the global average, the TP warming during 1983–2016 is faster than in the period of 1961–1983, and has a higher consistency with the global average in the period of 1983–2016. The TP warming has a seasonal amplification of 1.1–1.4 times than the global average during 1983–2016, while warming amplification during 1961–1983 is relatively less evident. Generally, the magnitude of warming on the TP is smaller than in the northern high‐latitudes, but larger than in the Southern Hemisphere and the Tropics. Based on current scientific understanding, the possible contribution of snow/ice‐albedo feedback may have played an important role in warming amplification on the TP since the 1980s.

Contrasting Community Composition of Active Microbial Eukaryotes in Melt Ponds and Sea Water of the Arctic Ocean Revealed by High Throughput Sequencing.

Melt ponds (MPs), form as the result of thawing of snow and sea ice in the summer, have lower albedo than the sea ice and are thus partly responsible for the polar amplification of global warming. Knowing the community composition of MP organisms is key to understanding their roles in the biogeochemical cycles of nutrients and elements. However, the community composition of MP microbial eukaryotes has rarely been studied. In the present study, we assessed the microbial eukaryote biodiversity, community composition, and assembly processes in MPs and surface sea water (SW) using high throughput sequencing of 18S rRNA of size-fractionated samples. Alpha diversity estimates were lower in the MPs than SW across all size fractions. The community composition of MPs was significantly different from that of SW. The MP communities were dominated by members from Chrysophyceae, the ciliate classes Litostomatea and Spirotrichea, and the cercozoan groups Filosa-Thecofilosea. One open MP community was similar to SW communities, which was probably due to the advanced stage of development of the MP enabling the exchange of species between it and adjacent SW. High portions of shared species between MPs and SW may indicate the vigorous exchange of species between these two major types of environments in the Arctic Ocean. SW microbial eukaryote communities are mainly controlled by dispersal limitation whereas those of MP are mainly controlled by ecological drift.

Spatial heterogeneity of climate variation and vegetation response for Arctic and high-elevation regions from 2001–2018

In recent decades, an amplification of warming in Arctic and high-elevation regions has been widely observed, along with a general enhancement of vegetation growth. However, driven by variability in controlling factors and complex mechanisms, climate and vegetation changes can be highly heterogeneous in space and time. In this study, an analysis is performed separating Arctic and Tibetan Plateau (TP) vegetated areas into various units according to a map of terrestrial ecoregions. The most recent variations of heat, moisture, and vegetation growth (MODIS Normalized Difference Vegetation Index) are evaluated over 2001–2018. Relationships among the climate and vegetation variables are assessed. Six distinct change patterns are identified: (1) synchronized increase of day and night temperature and precipitation during April to October coinciding with strong vegetation greening, (2) profound warming with no change in precipitation and vegetation, (3) an increase of summer temperature and vegetation with a negative latitudinal gradient, (4) greening under increased precipitation without warming, (5) browning not likely being driven by climate, (6) warming only during the nighttime and moderately enhanced vegetation growth. It is demonstrated that vegetation growth in the Arctic and TP is largely controlled by nighttime temperature and precipitation, as opposed to daytime temperature. The exception is the Canadian Arctic, where greening is directly related to summer daytime warming, and a contrasting relationship is observed on the TP. The underlying causes of these patterns are discussed, relating them to multiple mechanisms reported in the literature. These findings may help to further understand the changing Arctic and high-elevation climate and its effects on vegetation growth.

Modelling the mid-Pliocene warm period using HadGEM2

Abstract Here, for the first time, we present simulations of mid-Pliocene climate using a UK IPCC AR5-class model (HadGEM2). The global annual mean surface air temperature increases by 3.4∘C compared to the preindustrial control, with warming amplified towards the poles. The overall sensitivity of surface air temperature and polar amplification in response to the specification of the Pliocene boundary conditions is greater in HadGEM2 than in a previously utilised UK model (HadCM3). The simulated temperature anomaly is also at the upper range of that produced by the first phase of the Pliocene Model Intercomparison Project ensemble. Energy balance analysis indicates that the polar amplification of the mid-Pliocene warming in HadGEM2 is due to greenhouse gas emissivity changes and surface albedo changes. Approximately 5 × 106km2 of Arctic sea-ice is lost in the HadGEM2 Pliocene simulation and the global precipitation increases by 0.18 mm/day, these anomalies are approximately twice as large as seen in HadCM3. HadGEM2 can retain a much larger amount of soil moisture than HadCM3, such that the amount of evaporation (and precipitation) over the land surface in the mid-Pliocene simulation is not as strongly constrained by water availability. These results highlight the importance of using more recently developed climate and Earth System Models to simulate the past. They further underline that our appreciation of Pliocene climate is model dependant and ultimately limited by our physical understanding of the climate and the way this is represented in models.

Seasonal Changes in the North Atlantic Cold Anomaly: The Influence of Cold Surface Waters From Coastal Greenland and Warming Trends Associated With Variations in Subarctic Sea Ice Cover

AbstractWorldwide sea surface temperatures (SST) have increased on average by about 1 °C since 1900 with the exception of a region of the North Atlantic subpolar gyre near 50°N which has cooled by up to 0.9 °C over the same period, generating the negative feature on temperature anomaly maps which has been colloquially described by Rahmstorf et al. (2015, https://doi.org/10.1038/nclimate2554) as the “cold blob” (abbreviated here CB). This unique long‐term surface cooling trend is most evident in February, but in August net warming is observed even at CB epicenter and the CB itself is reduced to a mere “warming hole.” These seasonal changes in the intensity of the CB are the product of two separate factors: (1) a long‐term winter cooling specific for the CB region which appears to be associated with cooling of Greenland coastal waters in autumn, plausibly linked to summer meltwater from icebergs and sea ice and (2) summer warming effects which derive from (a) dramatic reduction in summer sea ice cover in the sub‐Arctic over the last 30 years that allows enhanced absorption of sunlight by the new open water in summer and (b) an unusual period of increased summer sub‐Arctic ice cover in the early twentieth century, which lowers the SST baseline measured from 1900, thus increasing the calculated linear rate of change of SST with time. Both of these effects could contribute to the observed Arctic amplification of warming.

Projected Changes in Daily Variability and Seasonal Cycle of Near-Surface Air Temperature over the Globe during the Twenty-First Century

AbstractIncreases in atmospheric greenhouse gases will not only raise Earth’s temperature but may also change its variability and seasonal cycle. Here CMIP5 model data are analyzed to quantify these changes in surface air temperature (Tas) and investigate the underlying processes. The models capture well the mean Tas seasonal cycle and variability and their changes in reanalysis, which shows decreasing Tas seasonal amplitudes and variability over the Arctic and Southern Ocean from 1979 to 2017. Daily Tas variability and seasonal amplitude are projected to decrease in the twenty-first century at high latitudes (except for boreal summer when Tas variability increases) but increase at low latitudes. The day of the maximum or minimum Tas shows large delays over high-latitude oceans, while it changes little at low latitudes. These Tas changes at high latitudes are linked to the polar amplification of warming and sea ice loss, which cause larger warming in winter than summer due to extra heating from the ocean during the cold season. Reduced sea ice cover also decreases its ability to cause Tas variations, contributing to the decreased Tas variability at high latitudes. Over low–midlatitude oceans, larger increases in surface evaporation in winter than summer (due to strong winter winds, strengthened winter winds in the Southern Hemisphere, and increased winter surface humidity gradients over the Northern Hemisphere low latitudes), coupled with strong ocean mixing in winter, lead to smaller surface warming in winter than summer and thus increased seasonal amplitudes there. These changes result in narrower (wider) Tas distributions over the high (low) latitudes, which may have important implications for other related fields.