Search Results
Abstract
The causes of the high pressure ridge at the North American west coast during winter 2013/14, the driest winter of the recent California drought, are examined. The ridge was part of an atmosphere–ocean state that included anomalies, defined relative to a 1979–2014 mean, of circulation across the Northern Hemisphere, warm sea surface temperatures (SSTs) in the tropical western and northeastern Pacific and the south Indian Ocean, and cool SSTs in the central tropical Pacific. The SST anomalies differ sufficiently between datasets that, when used to force atmosphere models, the simulated circulation anomalies vary notably in realism. Recognizing uncertainty in the SST field, the authors use idealized tropical SST anomaly experiments to identify an optimal combination of SST anomalies that forces a circulation response that best matches observations. The optimal SST pattern resembles that observed but the associated circulation pattern is much weaker than observed, suggesting an important but limited role for ocean forcing. Analysis of the equilibrium and transient upper-troposphere vorticity balance indicates that the SST-forced component of the ridge arose as a summed effect of Rossby waves forced by SST anomalies across the tropical Indo-Pacific oceans and drives upper-troposphere convergence and subsidence at the west coast. The ridge, in observations and model, is associated with northward and southward diversion of storms. The results suggest that tropical Indo-Pacific ocean SSTs helped force the west coast ridge and drought of winter 2013/14.
Abstract
The causes of the high pressure ridge at the North American west coast during winter 2013/14, the driest winter of the recent California drought, are examined. The ridge was part of an atmosphere–ocean state that included anomalies, defined relative to a 1979–2014 mean, of circulation across the Northern Hemisphere, warm sea surface temperatures (SSTs) in the tropical western and northeastern Pacific and the south Indian Ocean, and cool SSTs in the central tropical Pacific. The SST anomalies differ sufficiently between datasets that, when used to force atmosphere models, the simulated circulation anomalies vary notably in realism. Recognizing uncertainty in the SST field, the authors use idealized tropical SST anomaly experiments to identify an optimal combination of SST anomalies that forces a circulation response that best matches observations. The optimal SST pattern resembles that observed but the associated circulation pattern is much weaker than observed, suggesting an important but limited role for ocean forcing. Analysis of the equilibrium and transient upper-troposphere vorticity balance indicates that the SST-forced component of the ridge arose as a summed effect of Rossby waves forced by SST anomalies across the tropical Indo-Pacific oceans and drives upper-troposphere convergence and subsidence at the west coast. The ridge, in observations and model, is associated with northward and southward diversion of storms. The results suggest that tropical Indo-Pacific ocean SSTs helped force the west coast ridge and drought of winter 2013/14.
Abstract
The diagnostic evaluation of moisture budgets in archived atmosphere model data is examined. Sources of error in diagnostic computation can arise from the use of numerical methods different from those used in the atmosphere model, the time and vertical resolution of the archived data, and data availability. These sources of error are assessed using the climatological moisture balance in the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-Interim) that archives vertically integrated moisture fluxes and convergence. The largest single source of error arises from the diagnostic evaluation of divergence. The chosen second-order accurate centered finite difference scheme applied to the actual vertically integrated moisture fluxes leads to significant differences from the ERA-Interim reported moisture convergence. Using daily data, instead of 6-hourly data, leads to an underestimation of the patterns of moisture divergence and convergence by midlatitude transient eddies. A larger and more widespread error occurs when the vertical resolution of the model data is reduced to the 8 levels that is quite common for daily data archived for the Coupled Model Intercomparison Project (CMIP). Dividing moisture divergence into components due to the divergent flow and advection requires bringing the divergence operator inside the vertical integral, which introduces a surface term for which a means of accurate evaluation is developed. The analysis of errors is extended to the case of the spring 1993 Mississippi valley floods, the causes of which are discussed. For future archiving of data (e.g., by CMIP), it is recommended that monthly means of time-step-resolution flow–humidity covariances be archived at high vertical resolution.
Abstract
The diagnostic evaluation of moisture budgets in archived atmosphere model data is examined. Sources of error in diagnostic computation can arise from the use of numerical methods different from those used in the atmosphere model, the time and vertical resolution of the archived data, and data availability. These sources of error are assessed using the climatological moisture balance in the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-Interim) that archives vertically integrated moisture fluxes and convergence. The largest single source of error arises from the diagnostic evaluation of divergence. The chosen second-order accurate centered finite difference scheme applied to the actual vertically integrated moisture fluxes leads to significant differences from the ERA-Interim reported moisture convergence. Using daily data, instead of 6-hourly data, leads to an underestimation of the patterns of moisture divergence and convergence by midlatitude transient eddies. A larger and more widespread error occurs when the vertical resolution of the model data is reduced to the 8 levels that is quite common for daily data archived for the Coupled Model Intercomparison Project (CMIP). Dividing moisture divergence into components due to the divergent flow and advection requires bringing the divergence operator inside the vertical integral, which introduces a surface term for which a means of accurate evaluation is developed. The analysis of errors is extended to the case of the spring 1993 Mississippi valley floods, the causes of which are discussed. For future archiving of data (e.g., by CMIP), it is recommended that monthly means of time-step-resolution flow–humidity covariances be archived at high vertical resolution.
Abstract
The trends over recent decades in tropical Pacific sea surface and upper ocean temperature are examined in observations-based products, an ocean reanalysis and the latest models from the Coupled Model Intercomparison Project phase six and the Multimodel Large Ensembles Archive. Comparison is made using three metrics of sea surface temperature (SST) trend—the east–west and north–south SST gradients and a pattern correlation for the equatorial region—as well as change in thermocline depth. It is shown that the latest generation of models persist in not reproducing the observations-based SST trends as a response to radiative forcing and that the latter are at the far edge or beyond the range of modeled internal variability. The observed combination of thermocline shoaling and lack of warming in the equatorial cold tongue upwelling region is similarly at the extreme limit of modeled behavior. The persistence over the last century and a half of the observed trend toward an enhanced east–west SST gradient and, in four of five observed gridded datasets, to an enhanced equatorial north–south SST gradient, is also at the limit of model behavior. It is concluded that it is extremely unlikely that the observed trends are consistent with modeled internal variability. Instead, the results support the argument that the observed trends are a response to radiative forcing in which an enhanced east–west SST gradient and thermocline shoaling are key and that the latest generation of climate models continue to be unable to simulate this aspect of climate change.
Abstract
The trends over recent decades in tropical Pacific sea surface and upper ocean temperature are examined in observations-based products, an ocean reanalysis and the latest models from the Coupled Model Intercomparison Project phase six and the Multimodel Large Ensembles Archive. Comparison is made using three metrics of sea surface temperature (SST) trend—the east–west and north–south SST gradients and a pattern correlation for the equatorial region—as well as change in thermocline depth. It is shown that the latest generation of models persist in not reproducing the observations-based SST trends as a response to radiative forcing and that the latter are at the far edge or beyond the range of modeled internal variability. The observed combination of thermocline shoaling and lack of warming in the equatorial cold tongue upwelling region is similarly at the extreme limit of modeled behavior. The persistence over the last century and a half of the observed trend toward an enhanced east–west SST gradient and, in four of five observed gridded datasets, to an enhanced equatorial north–south SST gradient, is also at the limit of model behavior. It is concluded that it is extremely unlikely that the observed trends are consistent with modeled internal variability. Instead, the results support the argument that the observed trends are a response to radiative forcing in which an enhanced east–west SST gradient and thermocline shoaling are key and that the latest generation of climate models continue to be unable to simulate this aspect of climate change.
Abstract
Changes of the Asian summer monsoon in response to anthropogenic forcing are examined using observations and phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel, multirealization ensemble. In the twentieth century, CMIP5 models indicate a predominantly drying Asian monsoon, while in the twenty-first century under the representative concentration pathway 8.5 (RCP8.5) scenario, monsoon rainfall enhances across the entire Asian domain. The thermodynamic and dynamic mechanisms causing the changes are evaluated using specific humidity and winds, as well as the moisture budget. The drying trend in the CMIP5 historical simulations and the wetting trend in the RCP8.5 projections can be explained by the relative importance of dynamic and thermodynamic contributions to the total mean moisture convergence. While the thermodynamic mechanism dominates in the future, the historical rainfall changes are dominated by the changes in circulation. The relative contributions of aerosols and greenhouse gases (GHGs) on the historical monsoon change are further examined using CMIP5 single-forcing simulations. Rainfall reduces under aerosol forcing and increases under GHG forcing. Aerosol forcing dominates over the greenhouse effect during the historical period, leading to the general drying trend in the all-forcing simulations. While the thermodynamic change of mean moisture convergence in the all-forcing case is dominated by the GHG forcing, the dynamic change of mean moisture convergence in the all-forcing case is dominated by the aerosol forcing.
Abstract
Changes of the Asian summer monsoon in response to anthropogenic forcing are examined using observations and phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel, multirealization ensemble. In the twentieth century, CMIP5 models indicate a predominantly drying Asian monsoon, while in the twenty-first century under the representative concentration pathway 8.5 (RCP8.5) scenario, monsoon rainfall enhances across the entire Asian domain. The thermodynamic and dynamic mechanisms causing the changes are evaluated using specific humidity and winds, as well as the moisture budget. The drying trend in the CMIP5 historical simulations and the wetting trend in the RCP8.5 projections can be explained by the relative importance of dynamic and thermodynamic contributions to the total mean moisture convergence. While the thermodynamic mechanism dominates in the future, the historical rainfall changes are dominated by the changes in circulation. The relative contributions of aerosols and greenhouse gases (GHGs) on the historical monsoon change are further examined using CMIP5 single-forcing simulations. Rainfall reduces under aerosol forcing and increases under GHG forcing. Aerosol forcing dominates over the greenhouse effect during the historical period, leading to the general drying trend in the all-forcing simulations. While the thermodynamic change of mean moisture convergence in the all-forcing case is dominated by the GHG forcing, the dynamic change of mean moisture convergence in the all-forcing case is dominated by the aerosol forcing.
Abstract
During the summer, the midwestern United States, which covers the main U.S. corn belt, has a net loss of surface water as evapotranspiration exceeds precipitation. The net moisture gain into the atmosphere is transported out of the region to the northern high latitudes through transient eddy moisture fluxes. How this process may change in the future is not entirely clear despite the fact that the corn-belt region is responsible for a large portion of the global supply of corn and soybeans. We find that increased CO2 and the associated warming increase evapotranspiration while precipitation reduces in the region, leading to further reduction in precipitation minus evaporation in the future. At the same time, the poleward transient moisture flux increases, leading to enhanced atmospheric moisture export from the corn-belt region. However, storm-track intensity is generally weakened in the summer because of a reduced north–south temperature gradient associated with amplified warming in the midlatitudes. The intensified transient eddy moisture transport as the storm track weakens can be reconciled by the stronger mean moisture gradient in the future. This is found to be caused by the climatological low-level jet transporting more moisture into the Great Plains region as a result of the thermodynamic mechanism under warmer conditions. Our results, for the first time, show that in the future the U.S. Midwest corn belt will experience more hydrological stress due to intensified transient eddy moisture export, leading to drier soils in the region.
Abstract
During the summer, the midwestern United States, which covers the main U.S. corn belt, has a net loss of surface water as evapotranspiration exceeds precipitation. The net moisture gain into the atmosphere is transported out of the region to the northern high latitudes through transient eddy moisture fluxes. How this process may change in the future is not entirely clear despite the fact that the corn-belt region is responsible for a large portion of the global supply of corn and soybeans. We find that increased CO2 and the associated warming increase evapotranspiration while precipitation reduces in the region, leading to further reduction in precipitation minus evaporation in the future. At the same time, the poleward transient moisture flux increases, leading to enhanced atmospheric moisture export from the corn-belt region. However, storm-track intensity is generally weakened in the summer because of a reduced north–south temperature gradient associated with amplified warming in the midlatitudes. The intensified transient eddy moisture transport as the storm track weakens can be reconciled by the stronger mean moisture gradient in the future. This is found to be caused by the climatological low-level jet transporting more moisture into the Great Plains region as a result of the thermodynamic mechanism under warmer conditions. Our results, for the first time, show that in the future the U.S. Midwest corn belt will experience more hydrological stress due to intensified transient eddy moisture export, leading to drier soils in the region.
Abstract
The net surface water budget, precipitation minus evaporation (P − E), shows a clear seasonal cycle in the U.S. Southwest with a net gain of surface water (positive P − E) in the cold half of the year (October–March) and a net loss of water (negative P − E) in the warm half (April–September), with June and July being the driest months of the year. There is a significant shift of the summer drying toward earlier in the year under a CO2 warming scenario, resulting in substantial spring drying (March–May) of the U.S. Southwest from the near-term future to the end of the current century, with gradually increasing magnitude. While the spring drying has been identified in previous studies, its mechanism has not been fully addressed. Using moisture budget analysis, it was found that the drying is mainly due to decreased mean moisture convergence, partially compensated by the increase in transient eddy moisture flux convergence. The decreased mean moisture convergence is further separated into components as a result of changes in circulation (dynamic changes) and changes in atmospheric moisture content (thermodynamic changes). The drying is found to be dominated by the thermodynamic-driven changes in column-averaged moisture convergence, mainly due to increased dry zonal advection caused by the climatological land–ocean thermal contrast, rather than by the well-known “dry get drier” mechanism. Furthermore, the enhanced dry advection in the warming climate is dominated by the robust zonal mean atmospheric warming, leading to equally robust spring drying in the southwestern United States.
Abstract
The net surface water budget, precipitation minus evaporation (P − E), shows a clear seasonal cycle in the U.S. Southwest with a net gain of surface water (positive P − E) in the cold half of the year (October–March) and a net loss of water (negative P − E) in the warm half (April–September), with June and July being the driest months of the year. There is a significant shift of the summer drying toward earlier in the year under a CO2 warming scenario, resulting in substantial spring drying (March–May) of the U.S. Southwest from the near-term future to the end of the current century, with gradually increasing magnitude. While the spring drying has been identified in previous studies, its mechanism has not been fully addressed. Using moisture budget analysis, it was found that the drying is mainly due to decreased mean moisture convergence, partially compensated by the increase in transient eddy moisture flux convergence. The decreased mean moisture convergence is further separated into components as a result of changes in circulation (dynamic changes) and changes in atmospheric moisture content (thermodynamic changes). The drying is found to be dominated by the thermodynamic-driven changes in column-averaged moisture convergence, mainly due to increased dry zonal advection caused by the climatological land–ocean thermal contrast, rather than by the well-known “dry get drier” mechanism. Furthermore, the enhanced dry advection in the warming climate is dominated by the robust zonal mean atmospheric warming, leading to equally robust spring drying in the southwestern United States.
Abstract
Persistent multiyear cold states of the tropical Pacific Ocean drive hydroclimate anomalies worldwide, including persistent droughts in the extratropical Americas. Here, the atmosphere and ocean dynamics and thermodynamics of multiyear cold states of the tropical Pacific Ocean are investigated using European Centre for Medium-Range Weather Forecasts reanalyses and simplified models of the ocean and atmosphere. The cold states are maintained by anomalous ocean heat flux divergence and damped by increased surface heat flux from the atmosphere to ocean. The anomalous ocean heat flux divergence is contributed to by both changes in the ocean circulation and thermal structure. The keys are an anomalously shallow thermocline that enhances cooling by upwelling and anomalous westward equatorial currents that enhance cold advection. The thermocline depth anomalies are shown to be a response to equatorial wind stress anomalies. The wind stress anomalies are shown to be a simple dynamical response to equatorial SST anomalies as mediated by precipitation anomalies. The cold states are fundamentally maintained by atmosphere–ocean coupling in the equatorial Pacific. The physical processes that maintain the cold states are well approximated by linear dynamics for ocean and atmosphere and simple thermodynamics.
Abstract
Persistent multiyear cold states of the tropical Pacific Ocean drive hydroclimate anomalies worldwide, including persistent droughts in the extratropical Americas. Here, the atmosphere and ocean dynamics and thermodynamics of multiyear cold states of the tropical Pacific Ocean are investigated using European Centre for Medium-Range Weather Forecasts reanalyses and simplified models of the ocean and atmosphere. The cold states are maintained by anomalous ocean heat flux divergence and damped by increased surface heat flux from the atmosphere to ocean. The anomalous ocean heat flux divergence is contributed to by both changes in the ocean circulation and thermal structure. The keys are an anomalously shallow thermocline that enhances cooling by upwelling and anomalous westward equatorial currents that enhance cold advection. The thermocline depth anomalies are shown to be a response to equatorial wind stress anomalies. The wind stress anomalies are shown to be a simple dynamical response to equatorial SST anomalies as mediated by precipitation anomalies. The cold states are fundamentally maintained by atmosphere–ocean coupling in the equatorial Pacific. The physical processes that maintain the cold states are well approximated by linear dynamics for ocean and atmosphere and simple thermodynamics.
Abstract
The recent California drought was associated with a persistent ridge at the west coast of North America that has been associated with, in part, forcing from warm SST anomalies in the tropical west Pacific. Here it is considered whether there is a role for human-induced climate change in favoring such a west coast ridge. The models from phase 5 of the Coupled Model Intercomparison Project do not support such a case either in terms of a shift in the mean circulation or in variance that would favor increased intensity or frequency of ridges. The models also do not support shifts toward a drier mean climate or more frequent or intense dry winters or to tropical SST states that would favor west coast ridges. However, reanalyses do show that over the last century there has been a trend toward circulation anomalies over the Pacific–North American domain akin to those during the height of the California drought. The trend has been associated with a trend toward preferential warming of the Indo–west Pacific, an arrangement of tropical oceans and Pacific–North American circulation similar to that during winter 2013/14, the driest winter of the California drought. These height trends, however, are not reproduced in SST-forced atmosphere model ensembles. In contrast, idealized atmosphere modeling suggests that increased tropical Indo-Pacific zonal SST gradients are optimal for forcing height trends that favor a west coast ridge. These results allow a tenuous case for human-driven climate change driving increased gradients and favoring the west coast ridge, but observational data are not sufficiently accurate to confirm or reject this case.
Abstract
The recent California drought was associated with a persistent ridge at the west coast of North America that has been associated with, in part, forcing from warm SST anomalies in the tropical west Pacific. Here it is considered whether there is a role for human-induced climate change in favoring such a west coast ridge. The models from phase 5 of the Coupled Model Intercomparison Project do not support such a case either in terms of a shift in the mean circulation or in variance that would favor increased intensity or frequency of ridges. The models also do not support shifts toward a drier mean climate or more frequent or intense dry winters or to tropical SST states that would favor west coast ridges. However, reanalyses do show that over the last century there has been a trend toward circulation anomalies over the Pacific–North American domain akin to those during the height of the California drought. The trend has been associated with a trend toward preferential warming of the Indo–west Pacific, an arrangement of tropical oceans and Pacific–North American circulation similar to that during winter 2013/14, the driest winter of the California drought. These height trends, however, are not reproduced in SST-forced atmosphere model ensembles. In contrast, idealized atmosphere modeling suggests that increased tropical Indo-Pacific zonal SST gradients are optimal for forcing height trends that favor a west coast ridge. These results allow a tenuous case for human-driven climate change driving increased gradients and favoring the west coast ridge, but observational data are not sufficiently accurate to confirm or reject this case.
Abstract
During the strong 2015/16 El Niño, only normal to below-average precipitation fell across California in the late winter. This disagrees with both predictions by the ensemble mean of forecast models and expectations for strong El Niños. The authors examine one of the possible reasons why this event did not bring expected precipitation to California in the late winter. The maximum equatorial Pacific sea surface temperature anomalies (SSTAs) were located, compared to the 1982/83 and 1997/98 strong El Niños, farther to the west in the 2015/16 winter, which possibly caused less convection in the eastern tropical Pacific and shifted the teleconnection patterns westward in the North Pacific, thus weakening the influences on California. The SSTA and precipitation forecast for February–April 2016, based on the North American Multimodel Ensemble, showed large discrepancies from observations, with the ensemble mean of most of the models overestimating SSTAs in the eastern tropical Pacific and California precipitation. Atmospheric general circulation model (AGCM) experiments were conducted to test the hypothesis that the warmer eastern tropical Pacific SSTA forecast may have caused the wetter forecast in California in 2015/16 compared to observations. The AGCM experiments suggest it is difficult to assert that the eastern tropical Pacific SSTAs caused the too-wet California precipitation forecast, especially in Southern California, given that the models disagree. Results indicate forecast error can be influenced by atmosphere-model sensitivity to forecast SSTs, but they also indicate atmospheric internal variability may have been responsible for the combination of a strong El Niño and near-normal California precipitation.
Abstract
During the strong 2015/16 El Niño, only normal to below-average precipitation fell across California in the late winter. This disagrees with both predictions by the ensemble mean of forecast models and expectations for strong El Niños. The authors examine one of the possible reasons why this event did not bring expected precipitation to California in the late winter. The maximum equatorial Pacific sea surface temperature anomalies (SSTAs) were located, compared to the 1982/83 and 1997/98 strong El Niños, farther to the west in the 2015/16 winter, which possibly caused less convection in the eastern tropical Pacific and shifted the teleconnection patterns westward in the North Pacific, thus weakening the influences on California. The SSTA and precipitation forecast for February–April 2016, based on the North American Multimodel Ensemble, showed large discrepancies from observations, with the ensemble mean of most of the models overestimating SSTAs in the eastern tropical Pacific and California precipitation. Atmospheric general circulation model (AGCM) experiments were conducted to test the hypothesis that the warmer eastern tropical Pacific SSTA forecast may have caused the wetter forecast in California in 2015/16 compared to observations. The AGCM experiments suggest it is difficult to assert that the eastern tropical Pacific SSTAs caused the too-wet California precipitation forecast, especially in Southern California, given that the models disagree. Results indicate forecast error can be influenced by atmosphere-model sensitivity to forecast SSTs, but they also indicate atmospheric internal variability may have been responsible for the combination of a strong El Niño and near-normal California precipitation.
Abstract
The hydrological cycle in the Mediterranean region, as well as its change over the coming decades, is investigated using the Interim European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-Interim) and phase 5 of the Coupled Model Intercomparison Project (CMIP5) historical simulations and projections of the coming decades. The Mediterranean land regions have positive precipitation minus evaporation, P − E, in winter and negative P − E in summer. According to ERA-Interim, positive P − E over land in winter is sustained by transient eddy moisture convergence and opposed by mean flow moisture divergence. Dry mean flow advection is important for opposing the transient eddy moisture flux convergence in the winter half year and the mass divergent mean flow is a prime cause of negative P − E in the summer half year. These features are well reproduced in the CMIP5 ensemble. The models predict reduced P − E over the Mediterranean region in the future year-round. For both land and sea, a common cause of drying is increased mean flow moisture divergence. Changes in transient eddy moisture fluxes largely act diffusively and cause drying over the sea and moistening over many land areas to the north in winter and drying over western land areas and moistening over the eastern sea in summer. Increased mean flow moisture divergence is caused by both the increase in atmospheric humidity in a region of mean flow divergence and strengthening of the mass divergence. Increased mass divergence is related to increased high pressure over the central Mediterranean in winter and over the Atlantic and northern Europe in summer, which favors subsidence and low-level divergence over the Mediterranean region.
Abstract
The hydrological cycle in the Mediterranean region, as well as its change over the coming decades, is investigated using the Interim European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-Interim) and phase 5 of the Coupled Model Intercomparison Project (CMIP5) historical simulations and projections of the coming decades. The Mediterranean land regions have positive precipitation minus evaporation, P − E, in winter and negative P − E in summer. According to ERA-Interim, positive P − E over land in winter is sustained by transient eddy moisture convergence and opposed by mean flow moisture divergence. Dry mean flow advection is important for opposing the transient eddy moisture flux convergence in the winter half year and the mass divergent mean flow is a prime cause of negative P − E in the summer half year. These features are well reproduced in the CMIP5 ensemble. The models predict reduced P − E over the Mediterranean region in the future year-round. For both land and sea, a common cause of drying is increased mean flow moisture divergence. Changes in transient eddy moisture fluxes largely act diffusively and cause drying over the sea and moistening over many land areas to the north in winter and drying over western land areas and moistening over the eastern sea in summer. Increased mean flow moisture divergence is caused by both the increase in atmospheric humidity in a region of mean flow divergence and strengthening of the mass divergence. Increased mass divergence is related to increased high pressure over the central Mediterranean in winter and over the Atlantic and northern Europe in summer, which favors subsidence and low-level divergence over the Mediterranean region.
