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Abstract
El Niño–Southern Oscillation (ENSO) is the strongest interannual signature in the tropical air–sea system and can affect global atmospheric variables, but ENSO’s climatic impact on land surface variables such as skin temperature, snow cover, and soil moisture that can serve as seasonal climate predictors remains relatively little known. Here we examine ENSO’s impact on the three land surface variables in Northern Hemisphere extratropics by regression analysis, using a combination of ERA5 reanalysis, GLDAS, satellite-based observations, and CESM2 simulations. Our results show that during El Niño winters anomalous land surface warming occurs in Northern Hemisphere midlatitudes, accompanied by snow cover reduction, especially in northern North America and eastern Europe, while anomalous cooling occurs in southern North America and the Tibetan Plateau, accompanied by increased snow cover. Meanwhile, increased soil moisture is observed in southern North America, central Asia, and southeast China. Further analysis indicates that atmospheric moisture processes dominate the formation of the land surface anomalies, for which the changes in water vapor and precipitation induced by the ENSO-related large-scale atmospheric teleconnection are critical. The anomalous land surface warming in midlatitudes mainly results from the increased downward longwave radiation due to the increase of water vapor, whereas the physical pathway causing water vapor anomalies is different over North America and Eurasia. Controlled by ENSO-induced changes in atmospheric circulation and moisture transport, anomalous precipitation explains most of the snow cover and soil moisture anomalies, which play a certain role in shaping the skin temperature anomalies in some regions through snow–albedo feedback and evaporation, respectively.
Abstract
El Niño–Southern Oscillation (ENSO) is the strongest interannual signature in the tropical air–sea system and can affect global atmospheric variables, but ENSO’s climatic impact on land surface variables such as skin temperature, snow cover, and soil moisture that can serve as seasonal climate predictors remains relatively little known. Here we examine ENSO’s impact on the three land surface variables in Northern Hemisphere extratropics by regression analysis, using a combination of ERA5 reanalysis, GLDAS, satellite-based observations, and CESM2 simulations. Our results show that during El Niño winters anomalous land surface warming occurs in Northern Hemisphere midlatitudes, accompanied by snow cover reduction, especially in northern North America and eastern Europe, while anomalous cooling occurs in southern North America and the Tibetan Plateau, accompanied by increased snow cover. Meanwhile, increased soil moisture is observed in southern North America, central Asia, and southeast China. Further analysis indicates that atmospheric moisture processes dominate the formation of the land surface anomalies, for which the changes in water vapor and precipitation induced by the ENSO-related large-scale atmospheric teleconnection are critical. The anomalous land surface warming in midlatitudes mainly results from the increased downward longwave radiation due to the increase of water vapor, whereas the physical pathway causing water vapor anomalies is different over North America and Eurasia. Controlled by ENSO-induced changes in atmospheric circulation and moisture transport, anomalous precipitation explains most of the snow cover and soil moisture anomalies, which play a certain role in shaping the skin temperature anomalies in some regions through snow–albedo feedback and evaporation, respectively.
Abstract
The response of tropical cyclone (TC) frequency to sea surface warming is uncertain in climate models. We hypothesize that one source of uncertainty is the anomalies of large-scale atmospheric radiation in response to climate change, and whose influence on TC frequency is investigated. Given two atmospheric models with opposite TC frequency responses to uniform sea surface warming, we interchange their atmospheric radiation anomalies in experiments with prescribed radiative heating rates. The largest model discrepancy occurs in the western North Pacific, where the TC frequency tends to increase with anomalous large-scale ascent caused by prescribed positive radiation anomalies, while the TC frequency tends to decrease with anomalous large-scale descent caused by prescribed negative radiation anomalies. The model spread in TC frequency response is approximated by the model spread in the frequency response of pre-TC vortices (seeds), which is explained by changes in the large-scale circulation using a downscaling formula known as the seed propensity index. We further generalize the index to predict the influence of large-scale radiation anomalies on TC seed frequency. The results show that model spread in TC and seed frequency response can be reduced when constraining the large-scale radiation anomalies.
Significance Statement
It is difficult to predict whether tropical cyclones will occur more or less frequently in the future and by how much. We show that tropical cyclone frequency is strongly influenced by the global pattern of heating and cooling due to radiation, a process that has been neglected in existing theories. Our theory improves understanding of how tropical cyclones respond to climate change, explaining why one model may predict a frequency increase while a different but equally realistic model may predict a frequency decrease. One reason for the difficulty in predicting tropical cyclone frequency is found to be the difficulty in predicting how global cloud distribution will change in the future.
Abstract
The response of tropical cyclone (TC) frequency to sea surface warming is uncertain in climate models. We hypothesize that one source of uncertainty is the anomalies of large-scale atmospheric radiation in response to climate change, and whose influence on TC frequency is investigated. Given two atmospheric models with opposite TC frequency responses to uniform sea surface warming, we interchange their atmospheric radiation anomalies in experiments with prescribed radiative heating rates. The largest model discrepancy occurs in the western North Pacific, where the TC frequency tends to increase with anomalous large-scale ascent caused by prescribed positive radiation anomalies, while the TC frequency tends to decrease with anomalous large-scale descent caused by prescribed negative radiation anomalies. The model spread in TC frequency response is approximated by the model spread in the frequency response of pre-TC vortices (seeds), which is explained by changes in the large-scale circulation using a downscaling formula known as the seed propensity index. We further generalize the index to predict the influence of large-scale radiation anomalies on TC seed frequency. The results show that model spread in TC and seed frequency response can be reduced when constraining the large-scale radiation anomalies.
Significance Statement
It is difficult to predict whether tropical cyclones will occur more or less frequently in the future and by how much. We show that tropical cyclone frequency is strongly influenced by the global pattern of heating and cooling due to radiation, a process that has been neglected in existing theories. Our theory improves understanding of how tropical cyclones respond to climate change, explaining why one model may predict a frequency increase while a different but equally realistic model may predict a frequency decrease. One reason for the difficulty in predicting tropical cyclone frequency is found to be the difficulty in predicting how global cloud distribution will change in the future.
Abstract
The West Antarctic Ice Sheet (WAIS) may have collapsed during the last interglacial period, between 132 000 and 116 000 years ago. The changes in topography resulting from WAIS collapse would be accompanied by significant changes in Antarctic surface climate, atmospheric circulation, and ocean conditions. Evidence of these changes may be recorded in water-isotope ratios in precipitation archived in the ice. We conduct high-resolution simulations with an isotope-enabled version of the Weather Research and Forecasting Model over Antarctica, with boundary conditions provided by climate model simulations with both present-day and lowered WAIS topography. The results show that while there is significant spatial variability, WAIS collapse would cause detectable isotopic changes at several locations where ice-core records have been obtained or could be obtained in the future. The most robust signals include elevated δ 18O at SkyTrain Ice Rise in West Antarctica and elevated deuterium excess and δ 18O at Hercules Dome in East Antarctica. A combination of records from multiple sites would provide constraints on the timing, rate, and magnitude of past WAIS collapse.
Abstract
The West Antarctic Ice Sheet (WAIS) may have collapsed during the last interglacial period, between 132 000 and 116 000 years ago. The changes in topography resulting from WAIS collapse would be accompanied by significant changes in Antarctic surface climate, atmospheric circulation, and ocean conditions. Evidence of these changes may be recorded in water-isotope ratios in precipitation archived in the ice. We conduct high-resolution simulations with an isotope-enabled version of the Weather Research and Forecasting Model over Antarctica, with boundary conditions provided by climate model simulations with both present-day and lowered WAIS topography. The results show that while there is significant spatial variability, WAIS collapse would cause detectable isotopic changes at several locations where ice-core records have been obtained or could be obtained in the future. The most robust signals include elevated δ 18O at SkyTrain Ice Rise in West Antarctica and elevated deuterium excess and δ 18O at Hercules Dome in East Antarctica. A combination of records from multiple sites would provide constraints on the timing, rate, and magnitude of past WAIS collapse.
Abstract
This study investigates the transient evolution of tropical Pacific sea surface temperature (SST) responses to a constant northern high-latitude solar heating in fully coupled CESM 1.2. The study identifies two stages through multiple ensemble runs. 1) In the first 3 years, a hemispherically asymmetric pattern emerges, caused by air–sea interactions associated with the anomalous cross-equatorial Hadley cell. The northern tropics experience warming that is blocked north of the equator by the intertropical convergence zone. The southeast Pacific cooling reaches the equatorial region and is amplified by the equatorial Ekman divergence. 2) Within a decade, the equatorial cooling is replaced by warming in the eastern equatorial basin. The anomalous warming that appears faster than the time scales of the oceanic ventilation is attributed to anomalous meridional heat convergence and weakening of the northern subtropical cell. Our findings highlight the influence of ocean dynamics on the temporal and spatial evolution of tropical SST response to hemispherically asymmetric heating. The initial cooling caused by Ekman divergence delays the arrival of slow warming, while initial wind and temperature anomalies set the stage for the weakening of the subtropical cell. The results have important implications for understanding the evolution of tropical SST patterns in observational records and future climate change simulations, as they show strong interhemispheric temperature asymmetry in the extratropics.
Abstract
This study investigates the transient evolution of tropical Pacific sea surface temperature (SST) responses to a constant northern high-latitude solar heating in fully coupled CESM 1.2. The study identifies two stages through multiple ensemble runs. 1) In the first 3 years, a hemispherically asymmetric pattern emerges, caused by air–sea interactions associated with the anomalous cross-equatorial Hadley cell. The northern tropics experience warming that is blocked north of the equator by the intertropical convergence zone. The southeast Pacific cooling reaches the equatorial region and is amplified by the equatorial Ekman divergence. 2) Within a decade, the equatorial cooling is replaced by warming in the eastern equatorial basin. The anomalous warming that appears faster than the time scales of the oceanic ventilation is attributed to anomalous meridional heat convergence and weakening of the northern subtropical cell. Our findings highlight the influence of ocean dynamics on the temporal and spatial evolution of tropical SST response to hemispherically asymmetric heating. The initial cooling caused by Ekman divergence delays the arrival of slow warming, while initial wind and temperature anomalies set the stage for the weakening of the subtropical cell. The results have important implications for understanding the evolution of tropical SST patterns in observational records and future climate change simulations, as they show strong interhemispheric temperature asymmetry in the extratropics.
Abstract
In early 2018, due in part to a severe and extended meteorological drought, Cape Town was at risk of being one of the first major metropolitan areas in the world to run out of water. The magnitude of the crisis was exacerbated by the fact that such a prolonged and severe drought was both unanticipated and unpredicted. In this work, we analyze data from both observations and seasonal forecasts made as part of the North American Multimodel Ensemble (NMME) to better understand the predictability of rainfall in the Cape Town (CT) region. We find that there are statistically significant correlations between observed CT rainfall and sea surface temperatures in the tropical Atlantic (∼0.45) as well as a pattern of 200-mb geopotential height (z200) anomalies resembling the Southern Annular Mode (SAM; ∼0.4). Examination of hindcasts from the NMME demonstrates that the models accurately reproduce the observed correlation between CT rainfall and z200 anomalies. However, they fail to reproduce correlations between CT rainfall and the tropical South Atlantic. Decomposition of the correlations into contributions from predictable and unpredictable components indicates that CT rainfall in the models is dominated by unpredicted atmospheric variability (correlation ∼ 0.84) relative to predicted (correlation ∼ 0.14), which may be related to the failure to simulate the connection with the tropical Atlantic.
Significance Statement
Water crises are occurring with increasing severity and frequency around the globe. The ability to accurately forecast wet season rainfall would be invaluable to water managers and other decision-makers. Here, we explore the reasons behind the failure of a suite of operational seasonal forecast models to accurately predict rainfall in the Cape Town region of South Africa.
Abstract
In early 2018, due in part to a severe and extended meteorological drought, Cape Town was at risk of being one of the first major metropolitan areas in the world to run out of water. The magnitude of the crisis was exacerbated by the fact that such a prolonged and severe drought was both unanticipated and unpredicted. In this work, we analyze data from both observations and seasonal forecasts made as part of the North American Multimodel Ensemble (NMME) to better understand the predictability of rainfall in the Cape Town (CT) region. We find that there are statistically significant correlations between observed CT rainfall and sea surface temperatures in the tropical Atlantic (∼0.45) as well as a pattern of 200-mb geopotential height (z200) anomalies resembling the Southern Annular Mode (SAM; ∼0.4). Examination of hindcasts from the NMME demonstrates that the models accurately reproduce the observed correlation between CT rainfall and z200 anomalies. However, they fail to reproduce correlations between CT rainfall and the tropical South Atlantic. Decomposition of the correlations into contributions from predictable and unpredictable components indicates that CT rainfall in the models is dominated by unpredicted atmospheric variability (correlation ∼ 0.84) relative to predicted (correlation ∼ 0.14), which may be related to the failure to simulate the connection with the tropical Atlantic.
Significance Statement
Water crises are occurring with increasing severity and frequency around the globe. The ability to accurately forecast wet season rainfall would be invaluable to water managers and other decision-makers. Here, we explore the reasons behind the failure of a suite of operational seasonal forecast models to accurately predict rainfall in the Cape Town region of South Africa.
Abstract
The Northern Hemisphere snow-cover (NHSC) interannual variability can serve as a crucial source of climate predictability. However, the spatiotemporal characteristics of the NHSC interannual variability and associated formation mechanism lack a thorough investigation using the latest reliable datasets. In this observationally based study, we identify the leading mode of the wintertime NHSC interannual variability and reveal the major determinants and mechanisms for forming the mode. Results show that the leading mode exhibits a hemispheric dipole pattern characterized by snow-cover anomalies in the central-eastern United States and Europe, contrary to those in the Tibetan Plateau (TP). Importantly, the leading mode of TP-only snow-cover variability is essentially a regional manifestation of the dipole NHSC anomaly pattern. The wintertime hemispheric atmospheric circulation anomaly mode, that is, the Arctic Oscillation (AO), primarily determines the dipole NHSC anomaly pattern via altering snowfall and snowmelt. During a positive phase of AO, an anomalous atmospheric anticyclone over the midlatitude North Atlantic favors increased warm-air advection over the central-eastern United States and Europe, reducing the snowfall and the snow cover there. Meanwhile, the anomalously warm atmospheric thermal condition in Europe accompanied by enhanced westerly moisture flux convergence tends to heat the underlying land surface via reducing upward sensible heat flux and increasing downward longwave radiation, thereby causing increased snowmelt and decreased snow cover in northeastern Europe. Associated with the aforementioned atmospheric circulation anomalies, an anomalous atmospheric cyclone prevailing over the TP tends to cause increased southeasterly moisture flux convergence, favoring snowfall and snow increase in the TP.
Abstract
The Northern Hemisphere snow-cover (NHSC) interannual variability can serve as a crucial source of climate predictability. However, the spatiotemporal characteristics of the NHSC interannual variability and associated formation mechanism lack a thorough investigation using the latest reliable datasets. In this observationally based study, we identify the leading mode of the wintertime NHSC interannual variability and reveal the major determinants and mechanisms for forming the mode. Results show that the leading mode exhibits a hemispheric dipole pattern characterized by snow-cover anomalies in the central-eastern United States and Europe, contrary to those in the Tibetan Plateau (TP). Importantly, the leading mode of TP-only snow-cover variability is essentially a regional manifestation of the dipole NHSC anomaly pattern. The wintertime hemispheric atmospheric circulation anomaly mode, that is, the Arctic Oscillation (AO), primarily determines the dipole NHSC anomaly pattern via altering snowfall and snowmelt. During a positive phase of AO, an anomalous atmospheric anticyclone over the midlatitude North Atlantic favors increased warm-air advection over the central-eastern United States and Europe, reducing the snowfall and the snow cover there. Meanwhile, the anomalously warm atmospheric thermal condition in Europe accompanied by enhanced westerly moisture flux convergence tends to heat the underlying land surface via reducing upward sensible heat flux and increasing downward longwave radiation, thereby causing increased snowmelt and decreased snow cover in northeastern Europe. Associated with the aforementioned atmospheric circulation anomalies, an anomalous atmospheric cyclone prevailing over the TP tends to cause increased southeasterly moisture flux convergence, favoring snowfall and snow increase in the TP.
Abstract
El Niño–Southern Oscillation (ENSO) is the most influential interannual climate variability on Earth. The tendency of the mature phase of ENSO, characterized by the strongest sea surface temperature (SST) anomalies, to appear during the boreal winter is known as seasonal phase locking. Climate models are challenged by biases in simulating ENSO seasonal phase locking. Here, we evaluated the ENSO phase-locking simulation performance in 50 models of phase 6 of the Coupled Model Intercomparison Project (CMIP6) and found that the models with the intertropical convergence zone (ITCZ) poleward bias tended to simulate more ENSO events that peaked out of the boreal winter season. The contributions of the ITCZ poleward bias to the ENSO phase-locking bias were also evaluated, yielding a correlation coefficient of 0.55, which can explain approximately 30% of the ENSO seasonal phase-locking bias. The mechanism that influences the simulation of ENSO seasonal phase locking was also assessed. The ITCZ poleward bias induces a dry bias over the equatorial Pacific, especially during the boreal summer. During ENSO events, the meridional movement of the ITCZ is prevented, and the equatorial precipitation and convection anomalies that respond to ENSO events are also restrained. The restrained convection anomaly weakens the ENSO-related zonal wind anomaly, triggering a weaker eastern tropical Pacific thermocline anomaly during the following autumn. The weakened thermocline anomaly cannot sustain further development of ENSO-related SST anomalies. Therefore, ENSO events in models containing the ITCZ poleward bias are restrained during the boreal summer and autumn and, thus, tend to peak out of the winter season.
Significance Statement
We aimed to better understand the mechanism that induces bias when simulating ENSO seasonal phase locking, that is, what disturbs the simulated ENSO events peaking during the boreal winter. As previous studies have primarily focused on the South Pacific convergence zone (SPCZ) bias and other biases, this study is the first to propose the effects of the poleward ITCZ latitude bias and clarify the corresponding mechanism. We show that latitudinal bias can explain approximately 30% of the ENSO seasonal phase-locking bias. This is important because the biases in simulating ENSO seasonal phase locking have long hampered the prediction of ENSO. Our study highlights the importance of the latitude of the ITCZ and provides a basis for the future development of climate models.
Abstract
El Niño–Southern Oscillation (ENSO) is the most influential interannual climate variability on Earth. The tendency of the mature phase of ENSO, characterized by the strongest sea surface temperature (SST) anomalies, to appear during the boreal winter is known as seasonal phase locking. Climate models are challenged by biases in simulating ENSO seasonal phase locking. Here, we evaluated the ENSO phase-locking simulation performance in 50 models of phase 6 of the Coupled Model Intercomparison Project (CMIP6) and found that the models with the intertropical convergence zone (ITCZ) poleward bias tended to simulate more ENSO events that peaked out of the boreal winter season. The contributions of the ITCZ poleward bias to the ENSO phase-locking bias were also evaluated, yielding a correlation coefficient of 0.55, which can explain approximately 30% of the ENSO seasonal phase-locking bias. The mechanism that influences the simulation of ENSO seasonal phase locking was also assessed. The ITCZ poleward bias induces a dry bias over the equatorial Pacific, especially during the boreal summer. During ENSO events, the meridional movement of the ITCZ is prevented, and the equatorial precipitation and convection anomalies that respond to ENSO events are also restrained. The restrained convection anomaly weakens the ENSO-related zonal wind anomaly, triggering a weaker eastern tropical Pacific thermocline anomaly during the following autumn. The weakened thermocline anomaly cannot sustain further development of ENSO-related SST anomalies. Therefore, ENSO events in models containing the ITCZ poleward bias are restrained during the boreal summer and autumn and, thus, tend to peak out of the winter season.
Significance Statement
We aimed to better understand the mechanism that induces bias when simulating ENSO seasonal phase locking, that is, what disturbs the simulated ENSO events peaking during the boreal winter. As previous studies have primarily focused on the South Pacific convergence zone (SPCZ) bias and other biases, this study is the first to propose the effects of the poleward ITCZ latitude bias and clarify the corresponding mechanism. We show that latitudinal bias can explain approximately 30% of the ENSO seasonal phase-locking bias. This is important because the biases in simulating ENSO seasonal phase locking have long hampered the prediction of ENSO. Our study highlights the importance of the latitude of the ITCZ and provides a basis for the future development of climate models.
Abstract
Asian large-scale orography profoundly influences circulation in the North Hemisphere. Considerable spring top-of-the-atmosphere (TOA) radiative cooling over Southeast China (SEC) is very likely related to upstream orography forcing. Here we investigate the mechanical and thermal forcings of Asian large-scale orography, particularly the Tibetan Plateau (TP), on downstream East Asian cloud amount and atmospheric radiation budget during March–April using the Global Monsoons Model Intercomparison Project simulations. The thermal forcing drives significant surface heating and a low-level cyclone over the TP, pumping low-level air to the middle troposphere. Ascent and water vapor convergence triggered by the thermal forcing favor air condensation, low–middle clouds, and resultant strong spring cloud radiative cooling over SEC. Moreover, the thermal forcing moves the position of cloud radiative cooling westward toward the TP. The TP’s blocking role weakens low-level westerlies over SEC, but its deflecting role increases downstream high-level westerlies, dynamically favoring cloud formation over SEC and the eastward ocean. In addition, the TP can force ascent and increase cloud amounts over the western and central TP. The thermal forcing contributes to 57.1% of total cloud amount and 47.6% of TOA cloud radiative cooling induced by the combined orography forcing over SEC while the mechanical one accounts for 79.4% and 95.8% of the counterparts over the ocean to the east of SEC. Our results indicate that Asian large-scale orography shapes the contemporary geographical distribution of spring East Asian cloud amount and atmospheric radiation budget to a large extent.
Significance Statement
Clouds tied to large-scale topography and circulation exhibit some remarkable geographical distributions. The global strongest cloud radiative cooling, with an intensity of up to −90 W m−2, occurs over Southeast China (SEC) during March–April. The primary purpose of this study is to understand the influences of Asian large-scale orography, particularly the Tibetan Plateau (TP), on this unique climatic phenomenon using the latest climate model simulations. Our results show that Asian large-scale orography forcing significantly increases ascent, low–middle cloud formation, and resultant strong spring cloud radiative cooling over SEC and downstream ocean. The sensible-heat-driven air pump induced by the TP’s thermal forcing maintains strong cloud radiative cooling over SEC. This study provides valuable insights that link Asian large-scale orography forcing to downstream cloud–radiation characteristics.
Abstract
Asian large-scale orography profoundly influences circulation in the North Hemisphere. Considerable spring top-of-the-atmosphere (TOA) radiative cooling over Southeast China (SEC) is very likely related to upstream orography forcing. Here we investigate the mechanical and thermal forcings of Asian large-scale orography, particularly the Tibetan Plateau (TP), on downstream East Asian cloud amount and atmospheric radiation budget during March–April using the Global Monsoons Model Intercomparison Project simulations. The thermal forcing drives significant surface heating and a low-level cyclone over the TP, pumping low-level air to the middle troposphere. Ascent and water vapor convergence triggered by the thermal forcing favor air condensation, low–middle clouds, and resultant strong spring cloud radiative cooling over SEC. Moreover, the thermal forcing moves the position of cloud radiative cooling westward toward the TP. The TP’s blocking role weakens low-level westerlies over SEC, but its deflecting role increases downstream high-level westerlies, dynamically favoring cloud formation over SEC and the eastward ocean. In addition, the TP can force ascent and increase cloud amounts over the western and central TP. The thermal forcing contributes to 57.1% of total cloud amount and 47.6% of TOA cloud radiative cooling induced by the combined orography forcing over SEC while the mechanical one accounts for 79.4% and 95.8% of the counterparts over the ocean to the east of SEC. Our results indicate that Asian large-scale orography shapes the contemporary geographical distribution of spring East Asian cloud amount and atmospheric radiation budget to a large extent.
Significance Statement
Clouds tied to large-scale topography and circulation exhibit some remarkable geographical distributions. The global strongest cloud radiative cooling, with an intensity of up to −90 W m−2, occurs over Southeast China (SEC) during March–April. The primary purpose of this study is to understand the influences of Asian large-scale orography, particularly the Tibetan Plateau (TP), on this unique climatic phenomenon using the latest climate model simulations. Our results show that Asian large-scale orography forcing significantly increases ascent, low–middle cloud formation, and resultant strong spring cloud radiative cooling over SEC and downstream ocean. The sensible-heat-driven air pump induced by the TP’s thermal forcing maintains strong cloud radiative cooling over SEC. This study provides valuable insights that link Asian large-scale orography forcing to downstream cloud–radiation characteristics.
Abstract
Studying temperature probability distributions and the physical processes that shape them is important for understanding extreme temperature events. Previous work has used a conditional mean temperature framework to reveal whether horizontal temperature advection drives temperature to extreme or median values at a specific location as a method to dynamically interpret temperature probability distributions. In this paper, we generalize this method to study how other processes shape temperature probability distributions and explore the diverse effects of horizontal temperature advection on temperature probability distributions at different locations and different temperature percentiles. We apply this generalized method to several representative regions to demonstrate its use. We find that temperature advection drives temperatures toward more extreme values over most land in the midlatitudes (i.e., cold air advection occurs during cold anomalies and warm air advection occurs during warm anomalies). In contrast, we find that horizontal temperature advection dampens temperature anomalies in some coastal summer monsoon regions, where extreme temperatures result from other processes, such as horizontal humidity advection and vertical temperature advection. By calculating the mean of processes conditioned on the temperature percentile, this method enables composite analysis of processes that contribute to events for all percentiles and a range of processes. We show examples of composites at different percentiles for certain processes and regions to illustrate the conditional mean analysis. This general approach may benefit future studies related to temperature probability distributions and extreme events.
Abstract
Studying temperature probability distributions and the physical processes that shape them is important for understanding extreme temperature events. Previous work has used a conditional mean temperature framework to reveal whether horizontal temperature advection drives temperature to extreme or median values at a specific location as a method to dynamically interpret temperature probability distributions. In this paper, we generalize this method to study how other processes shape temperature probability distributions and explore the diverse effects of horizontal temperature advection on temperature probability distributions at different locations and different temperature percentiles. We apply this generalized method to several representative regions to demonstrate its use. We find that temperature advection drives temperatures toward more extreme values over most land in the midlatitudes (i.e., cold air advection occurs during cold anomalies and warm air advection occurs during warm anomalies). In contrast, we find that horizontal temperature advection dampens temperature anomalies in some coastal summer monsoon regions, where extreme temperatures result from other processes, such as horizontal humidity advection and vertical temperature advection. By calculating the mean of processes conditioned on the temperature percentile, this method enables composite analysis of processes that contribute to events for all percentiles and a range of processes. We show examples of composites at different percentiles for certain processes and regions to illustrate the conditional mean analysis. This general approach may benefit future studies related to temperature probability distributions and extreme events.
Abstract
We investigated the interannual variations in the annual mean and seasonal cycle of upward shortwave radiation at the top of the atmosphere (TOA SW↑) over the Arctic using the Clouds and the Earth’s Radiant Energy System (CERES) observation data during 2001–20. The annual mean TOA SW↑ over the Arctic showed a decreasing trend from 2001 to 2012 (−2.5 W m−2 decade−1) and had a large interannual variability after 2012. The standard deviation of detrended TOA SW↑ increased from 0.4 W m−2 in 2001–12 to 1.1 W m−2 in 2012–20. Over land, TOA SW↑ variation was related to snow cover in May; snow cover, cloud fraction, and cloud optical depth (COD) in June; and cloud fraction and COD in July. Over ocean, TOA SW↑ variation in June and July was linked to sea ice cover. TOA SW↑ variation over ocean in June and July after 2012 was highly related to the North Atlantic Oscillation (NAO). This study suggests that changes in the large annual mean TOA SW↑ variability after 2012 are explained by the timing of land snow and sea ice melt in spring and summer and cloud variability over land in summer.
Abstract
We investigated the interannual variations in the annual mean and seasonal cycle of upward shortwave radiation at the top of the atmosphere (TOA SW↑) over the Arctic using the Clouds and the Earth’s Radiant Energy System (CERES) observation data during 2001–20. The annual mean TOA SW↑ over the Arctic showed a decreasing trend from 2001 to 2012 (−2.5 W m−2 decade−1) and had a large interannual variability after 2012. The standard deviation of detrended TOA SW↑ increased from 0.4 W m−2 in 2001–12 to 1.1 W m−2 in 2012–20. Over land, TOA SW↑ variation was related to snow cover in May; snow cover, cloud fraction, and cloud optical depth (COD) in June; and cloud fraction and COD in July. Over ocean, TOA SW↑ variation in June and July was linked to sea ice cover. TOA SW↑ variation over ocean in June and July after 2012 was highly related to the North Atlantic Oscillation (NAO). This study suggests that changes in the large annual mean TOA SW↑ variability after 2012 are explained by the timing of land snow and sea ice melt in spring and summer and cloud variability over land in summer.
