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- Author or Editor: Fengfei Song x
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Abstract
This study investigates the role of internal variability in modulating the East Asian summer monsoon (EASM)–ENSO relationship using Twentieth-Century Reanalysis (20CR) data and simulations from phase 5 of CMIP (CMIP5). Analysis of 20CR data reveals an unstable EASM–ENSO relationship during the twentieth century. During the high-correlation periods of 1892–1912 and 1979–99, an evident western Pacific anticyclone (WPAC) and dipole sea level pressure (SLP) pattern are present in the decaying El Niño summer, accompanied by Indian Ocean warming and a tropospheric temperature Matsuno–Gill pattern. However, these are weaker or absent during low-correlation periods (1914–34 and 1958–78). After removing the external forcings based on historical simulations from 15 CMIP5 models, all the above features remain almost unchanged, suggesting the crucial role of internal variability. In a 501-yr preindustrial control (piControl) simulation without external forcing variation from CCSM4, the EASM–ENSO relationship also shows significant decadal variation, with a magnitude comparable to the 20CR data. The analysis demonstrates that the EASM–ENSO relationship’s variation is modulated by the interdecadal Pacific oscillation (IPO). Compared to negative IPO phases, the warmer East China Sea in positive IPO phases weakens the western North Pacific subtropical high (WNPSH), inducing more precipitation. Thus, the Kelvin wave–induced interannual divergence suppresses more mean-state precipitation and leads to a stronger WPAC. Hence, the IPO modulates the EASM–ENSO relationship through the WNPSH, which is evident in both 20CR and the piControl simulation.
Abstract
This study investigates the role of internal variability in modulating the East Asian summer monsoon (EASM)–ENSO relationship using Twentieth-Century Reanalysis (20CR) data and simulations from phase 5 of CMIP (CMIP5). Analysis of 20CR data reveals an unstable EASM–ENSO relationship during the twentieth century. During the high-correlation periods of 1892–1912 and 1979–99, an evident western Pacific anticyclone (WPAC) and dipole sea level pressure (SLP) pattern are present in the decaying El Niño summer, accompanied by Indian Ocean warming and a tropospheric temperature Matsuno–Gill pattern. However, these are weaker or absent during low-correlation periods (1914–34 and 1958–78). After removing the external forcings based on historical simulations from 15 CMIP5 models, all the above features remain almost unchanged, suggesting the crucial role of internal variability. In a 501-yr preindustrial control (piControl) simulation without external forcing variation from CCSM4, the EASM–ENSO relationship also shows significant decadal variation, with a magnitude comparable to the 20CR data. The analysis demonstrates that the EASM–ENSO relationship’s variation is modulated by the interdecadal Pacific oscillation (IPO). Compared to negative IPO phases, the warmer East China Sea in positive IPO phases weakens the western North Pacific subtropical high (WNPSH), inducing more precipitation. Thus, the Kelvin wave–induced interannual divergence suppresses more mean-state precipitation and leads to a stronger WPAC. Hence, the IPO modulates the EASM–ENSO relationship through the WNPSH, which is evident in both 20CR and the piControl simulation.
Abstract
The climatology and interannual variability of East Asian summer monsoon (EASM) are investigated by using 13 atmospheric general circulation models (AGCMs) from phase 3 of the Coupled Model Intercomparison Project (CMIP3) and 19 AGCMs from CMIP5. The mean low-level monsoon circulation is reasonably reproduced in the multimodel ensemble mean (MME) of CMIP3 and CMIP5 AGCMs, except for a northward shift of the western Pacific subtropical high. However, the monsoon rainband known as mei-yu/baiu/changma (28°–38°N, 105°–150°E) is poorly simulated, although a significant improvement is seen from CMIP3 to CMIP5. The interannual EASM pattern is obtained by regressing the precipitation and 850-hPa wind on the observed EASM index. The observed dipole rainfall pattern is partly reproduced in CMIP3 and CMIP5 MME but with two deficiencies: weaker magnitude and southward shift of the dipole rainfall pattern. These deficiencies are closely related to the weaker and southward shift of the western Pacific anticyclone (WPAC). The simulation skill of the interannual EASM pattern has been significantly improved from CMIP3 to CMIP5 MME accompanied by the enhanced dipole rainfall pattern and WPAC. Analyses demonstrate that the tropical eastern Indian Ocean (IO) rainfall response to local warm SST anomalies and the associated Kelvin wave response over the Indo–western Pacific region are important to maintain the WPAC. A successful reproduction of interannual EASM pattern depends highly on the IO–WPAC teleconnection. The significant improvement in the interannual EASM pattern from CMIP3 to CMIP5 MME is also due to a better reproduction of this teleconnection in CMIP5 models.
Abstract
The climatology and interannual variability of East Asian summer monsoon (EASM) are investigated by using 13 atmospheric general circulation models (AGCMs) from phase 3 of the Coupled Model Intercomparison Project (CMIP3) and 19 AGCMs from CMIP5. The mean low-level monsoon circulation is reasonably reproduced in the multimodel ensemble mean (MME) of CMIP3 and CMIP5 AGCMs, except for a northward shift of the western Pacific subtropical high. However, the monsoon rainband known as mei-yu/baiu/changma (28°–38°N, 105°–150°E) is poorly simulated, although a significant improvement is seen from CMIP3 to CMIP5. The interannual EASM pattern is obtained by regressing the precipitation and 850-hPa wind on the observed EASM index. The observed dipole rainfall pattern is partly reproduced in CMIP3 and CMIP5 MME but with two deficiencies: weaker magnitude and southward shift of the dipole rainfall pattern. These deficiencies are closely related to the weaker and southward shift of the western Pacific anticyclone (WPAC). The simulation skill of the interannual EASM pattern has been significantly improved from CMIP3 to CMIP5 MME accompanied by the enhanced dipole rainfall pattern and WPAC. Analyses demonstrate that the tropical eastern Indian Ocean (IO) rainfall response to local warm SST anomalies and the associated Kelvin wave response over the Indo–western Pacific region are important to maintain the WPAC. A successful reproduction of interannual EASM pattern depends highly on the IO–WPAC teleconnection. The significant improvement in the interannual EASM pattern from CMIP3 to CMIP5 MME is also due to a better reproduction of this teleconnection in CMIP5 models.
Abstract
The climatology and interannual variability of the East Asian summer monsoon (EASM) simulated by 34 coupled general circulation models (CGCMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are evaluated. To estimate the role of air–sea coupling, 17 CGCMs are compared to their corresponding atmospheric general circulation models (AGCMs). The climatological low-level monsoon circulation and mei-yu/changma/baiu rainfall band are improved in CGCMs from AGCMs. The improvement is at the cost of the local cold sea surface temperature (SST) biases in CGCMs, since they decrease the surface evaporation and enhance the circulation. The interannual EASM pattern is evaluated by a skill formula and the highest/lowest eight models are selected to investigate the skill origins. The observed Indian Ocean (IO) warming, tropical eastern Indian Ocean (TEIO) rainfall anomalies, and Kelvin wave response are captured well in high-skill models, while these features are not present in low-skill models. Further, the differences in the IO warming between high-skill and low-skill models are rooted in the preceding ENSO simulation. Hence, the IO–western Pacific anticyclone (WPAC) teleconnection is important for CGCMs, similar to AGCMs. However, compared to AGCMs, the TEIO SST anomaly is warmer in CGCMs, since the easterly wind anomalies in the southern flank of the WPAC reduce the climatological monsoon westerlies and decrease the surface evaporation. The warmer TEIO induces the stronger precipitation anomaly and intensifies the teleconnection. Hence, the interannual EASM pattern is better simulated in CGCMs than that in AGCMs.
Abstract
The climatology and interannual variability of the East Asian summer monsoon (EASM) simulated by 34 coupled general circulation models (CGCMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are evaluated. To estimate the role of air–sea coupling, 17 CGCMs are compared to their corresponding atmospheric general circulation models (AGCMs). The climatological low-level monsoon circulation and mei-yu/changma/baiu rainfall band are improved in CGCMs from AGCMs. The improvement is at the cost of the local cold sea surface temperature (SST) biases in CGCMs, since they decrease the surface evaporation and enhance the circulation. The interannual EASM pattern is evaluated by a skill formula and the highest/lowest eight models are selected to investigate the skill origins. The observed Indian Ocean (IO) warming, tropical eastern Indian Ocean (TEIO) rainfall anomalies, and Kelvin wave response are captured well in high-skill models, while these features are not present in low-skill models. Further, the differences in the IO warming between high-skill and low-skill models are rooted in the preceding ENSO simulation. Hence, the IO–western Pacific anticyclone (WPAC) teleconnection is important for CGCMs, similar to AGCMs. However, compared to AGCMs, the TEIO SST anomaly is warmer in CGCMs, since the easterly wind anomalies in the southern flank of the WPAC reduce the climatological monsoon westerlies and decrease the surface evaporation. The warmer TEIO induces the stronger precipitation anomaly and intensifies the teleconnection. Hence, the interannual EASM pattern is better simulated in CGCMs than that in AGCMs.
Abstract
During boreal spring, observations show a double ITCZ over the eastern Pacific, with the northern ITCZ stronger than the southern ITCZ. However, it is opposite in most climate models. It is also evident that there exists a cold bias in tropical North Atlantic (TNA) sea surface temperature (SST) and a warm bias in southeastern Pacific (SEP) SST. In this study, the influences of TNA and SEP SSTs on the double-ITCZ bias are investigated by prescribing the observed SST in these regions in the NCAR CESM1. Results show that when TNA SST is prescribed, the northern ITCZ is substantially enhanced and the southern ITCZ is moderately reduced, although the SST response in these regions is small. When the SEP SST is prescribed, the southern ITCZ is reduced considerably. When both TNA and SEP SSTs are prescribed, the double-ITCZ bias is reduced by ~68%. Moisture budget analysis suggests that dynamics, mainly the low-level convergence change, determines the above precipitation changes. Based on a mixed layer model, changes in low-level convergence are shown to be determined by surface pressure P s changes. With prescribed TNA/SEP SSTs, SST gradients change the P s in the region directly via the Lindzen–Nigam mechanism. The corresponding low-level circulation changes affect the 850-hPa thermodynamic state in a wider region, which in turn not only strengthens the SST-induced P s change locally but also leads to P s changes remotely, including the northern ITCZ region. Furthermore, the low-level convergence changes the vertical structure of moist static energy, altering the atmospheric stability and modulating precipitation distribution.
Abstract
During boreal spring, observations show a double ITCZ over the eastern Pacific, with the northern ITCZ stronger than the southern ITCZ. However, it is opposite in most climate models. It is also evident that there exists a cold bias in tropical North Atlantic (TNA) sea surface temperature (SST) and a warm bias in southeastern Pacific (SEP) SST. In this study, the influences of TNA and SEP SSTs on the double-ITCZ bias are investigated by prescribing the observed SST in these regions in the NCAR CESM1. Results show that when TNA SST is prescribed, the northern ITCZ is substantially enhanced and the southern ITCZ is moderately reduced, although the SST response in these regions is small. When the SEP SST is prescribed, the southern ITCZ is reduced considerably. When both TNA and SEP SSTs are prescribed, the double-ITCZ bias is reduced by ~68%. Moisture budget analysis suggests that dynamics, mainly the low-level convergence change, determines the above precipitation changes. Based on a mixed layer model, changes in low-level convergence are shown to be determined by surface pressure P s changes. With prescribed TNA/SEP SSTs, SST gradients change the P s in the region directly via the Lindzen–Nigam mechanism. The corresponding low-level circulation changes affect the 850-hPa thermodynamic state in a wider region, which in turn not only strengthens the SST-induced P s change locally but also leads to P s changes remotely, including the northern ITCZ region. Furthermore, the low-level convergence changes the vertical structure of moist static energy, altering the atmospheric stability and modulating precipitation distribution.
ABSTRACT
The double-ITCZ bias has puzzled the climate modeling community for more than two decades. Here we show that, over the northeastern Pacific Ocean, precipitation and sea surface temperature (SST) biases are seasonally dependent in the NCAR CESM1 and 37 CMIP5 models, with positive biases during boreal summer–autumn and negative biases during boreal winter–spring, although the easterly wind bias persists year round. This seasonally dependent bias is found to be caused by the model’s failure to reproduce the climatological seasonal wind reversal of the North American monsoon. During winter–spring, the observed easterly wind dominates, so the simulated stronger wind speed enhances surface evaporation and lowers SST. It is opposite when the observed wind turns to westerly during summer–autumn. An easterly wind bias, mainly evident in the lower troposphere, also occurs in the atmospheric model when the observed SST is prescribed, suggesting that it is of atmospheric origin. When the atmospheric model resolution is doubled in the CESM1, both SST and precipitation are improved in association with the reduced easterly wind bias. During boreal spring, when the double-ITCZ bias is most significant, the northern and southern ITCZ can be improved by 29.0% and 18.8%, respectively, by increasing the horizontal resolution in the CESM1. When dividing the 37 CMIP5 models into two groups on the basis of their horizontal resolutions, it is found that both the seasonally dependent biases over the northeastern Pacific and year-round biases over the southeastern Pacific are reduced substantially in the higher-resolution models, with improvement of ~30% in both regions during boreal spring. Close relationships between wind and precipitation biases over the northeastern and southeastern Pacific are also found among CMIP5 models.
ABSTRACT
The double-ITCZ bias has puzzled the climate modeling community for more than two decades. Here we show that, over the northeastern Pacific Ocean, precipitation and sea surface temperature (SST) biases are seasonally dependent in the NCAR CESM1 and 37 CMIP5 models, with positive biases during boreal summer–autumn and negative biases during boreal winter–spring, although the easterly wind bias persists year round. This seasonally dependent bias is found to be caused by the model’s failure to reproduce the climatological seasonal wind reversal of the North American monsoon. During winter–spring, the observed easterly wind dominates, so the simulated stronger wind speed enhances surface evaporation and lowers SST. It is opposite when the observed wind turns to westerly during summer–autumn. An easterly wind bias, mainly evident in the lower troposphere, also occurs in the atmospheric model when the observed SST is prescribed, suggesting that it is of atmospheric origin. When the atmospheric model resolution is doubled in the CESM1, both SST and precipitation are improved in association with the reduced easterly wind bias. During boreal spring, when the double-ITCZ bias is most significant, the northern and southern ITCZ can be improved by 29.0% and 18.8%, respectively, by increasing the horizontal resolution in the CESM1. When dividing the 37 CMIP5 models into two groups on the basis of their horizontal resolutions, it is found that both the seasonally dependent biases over the northeastern Pacific and year-round biases over the southeastern Pacific are reduced substantially in the higher-resolution models, with improvement of ~30% in both regions during boreal spring. Close relationships between wind and precipitation biases over the northeastern and southeastern Pacific are also found among CMIP5 models.
Abstract
The double intertropical convergence zone (ITCZ) is a long-standing bias in the climatology of coupled general circulation models (CGCMs). The warm biases in southeastern Pacific (SEP) sea surface temperature (SST) are also evident in many CGCMs. In this study, the role of SEP SST in the double ITCZ is investigated by prescribing the observed SEP SST in the Community Earth System Model, version 1 (CESM1). Both the double ITCZ and dry equator problems are significantly improved with SEP SST prescribed. Both atmospheric and oceanic processes are involved in the improvements. The colder SST over the SEP decreases the precipitation, which enhances the southeasterly winds outside the prescribed SST region, cooling the ocean via increased evaporation. The enhanced descending motion over the SEP strengthens the Walker circulation. The easterly winds over the equatorial Pacific enhance upwelling and shoal the thermocline over the eastern Pacific. The changes of surface wind and wind curl lead to a weaker South Equatorial Countercurrent and stronger South Equatorial Current, preventing the warm water from expanding eastward, thereby improving both the double ITCZ and dry equator. The enhanced Walker circulation also increases the low-level wind convergence and reduces the wind speed in the tropical western Pacific, leading to warmer SST and stronger convection there. The stronger convection in turn leads to more cloud and reduces the incoming solar radiation, cooling the SST. These competing effects between radiative heat flux and latent heat flux make the atmospheric heat flux secondary to the ocean dynamics in the western Pacific warming.
Abstract
The double intertropical convergence zone (ITCZ) is a long-standing bias in the climatology of coupled general circulation models (CGCMs). The warm biases in southeastern Pacific (SEP) sea surface temperature (SST) are also evident in many CGCMs. In this study, the role of SEP SST in the double ITCZ is investigated by prescribing the observed SEP SST in the Community Earth System Model, version 1 (CESM1). Both the double ITCZ and dry equator problems are significantly improved with SEP SST prescribed. Both atmospheric and oceanic processes are involved in the improvements. The colder SST over the SEP decreases the precipitation, which enhances the southeasterly winds outside the prescribed SST region, cooling the ocean via increased evaporation. The enhanced descending motion over the SEP strengthens the Walker circulation. The easterly winds over the equatorial Pacific enhance upwelling and shoal the thermocline over the eastern Pacific. The changes of surface wind and wind curl lead to a weaker South Equatorial Countercurrent and stronger South Equatorial Current, preventing the warm water from expanding eastward, thereby improving both the double ITCZ and dry equator. The enhanced Walker circulation also increases the low-level wind convergence and reduces the wind speed in the tropical western Pacific, leading to warmer SST and stronger convection there. The stronger convection in turn leads to more cloud and reduces the incoming solar radiation, cooling the SST. These competing effects between radiative heat flux and latent heat flux make the atmospheric heat flux secondary to the ocean dynamics in the western Pacific warming.
Abstract
Using observations from the Green Ocean Amazon (GOAmazon) field campaign, this study aims to improve trigger functions of convection schemes. Results show that the CAPE generation rate (dCAPE)-type triggers are the first tier and that the Bechtold and heated condensation framework (HCF) triggers are a distant second tier. The composite analysis reveals that the undilute dCAPE trigger underpredicts convection when there is bottom-heavy upward motion but overpredicts convection with low-level downward and upper-level upward motions. The empirical orthogonal function (EOF) analysis on vertical velocity shows that EOF1 (62.65%) exhibits upward motion throughout the troposphere and that EOF2 (28.05%) has lower-level upward motion and upper-level downward motion. Both of them have close relationships with precipitation, indicating the role of vertical velocity in triggering convection. The skill sensitivity analysis shows that the inclusion of 700-hPa upward motion significantly enhances the undilute dCAPE trigger. For the dilute dCAPE trigger, entrainment rate and dCAPE threshold are optimized to improve it. Opposite to dCAPE-type triggers, the Bechtold trigger overemphasizes the low-level vertical velocity and underpredicts the mature and decaying phases of long-lasting convection events. The HCF trigger overemphasizes the near-surface moist static energy and overlooks the vertical velocity. The performance of dCAPE-type triggers on various convective systems over the Amazon region is examined. The eastward-propagating systems are best represented, with only a few underpredictions in their decaying stages. The weak locally occurring systems and marginal phases of westward-propagating systems are easy to underpredict. The revised dCAPE-type triggers perform better on different convection systems and the diurnal cycle of convection.
Abstract
Using observations from the Green Ocean Amazon (GOAmazon) field campaign, this study aims to improve trigger functions of convection schemes. Results show that the CAPE generation rate (dCAPE)-type triggers are the first tier and that the Bechtold and heated condensation framework (HCF) triggers are a distant second tier. The composite analysis reveals that the undilute dCAPE trigger underpredicts convection when there is bottom-heavy upward motion but overpredicts convection with low-level downward and upper-level upward motions. The empirical orthogonal function (EOF) analysis on vertical velocity shows that EOF1 (62.65%) exhibits upward motion throughout the troposphere and that EOF2 (28.05%) has lower-level upward motion and upper-level downward motion. Both of them have close relationships with precipitation, indicating the role of vertical velocity in triggering convection. The skill sensitivity analysis shows that the inclusion of 700-hPa upward motion significantly enhances the undilute dCAPE trigger. For the dilute dCAPE trigger, entrainment rate and dCAPE threshold are optimized to improve it. Opposite to dCAPE-type triggers, the Bechtold trigger overemphasizes the low-level vertical velocity and underpredicts the mature and decaying phases of long-lasting convection events. The HCF trigger overemphasizes the near-surface moist static energy and overlooks the vertical velocity. The performance of dCAPE-type triggers on various convective systems over the Amazon region is examined. The eastward-propagating systems are best represented, with only a few underpredictions in their decaying stages. The weak locally occurring systems and marginal phases of westward-propagating systems are easy to underpredict. The revised dCAPE-type triggers perform better on different convection systems and the diurnal cycle of convection.
Abstract
As the resolution of global climate model increases, whether trigger functions in current convective parameterization schemes still work remains unknown. In this study, the scale dependence of undilute and dilute dCAPE, Bechtold, and heated condensation framework (HCF) triggers is evaluated using the cloud-resolving model (CRM) data. It is found that all these trigger functions are scale dependent, especially for dCAPE-type triggers, with skill scores dropping from ~0.6 at the lower resolutions (128, 64, and 32 km) to only ~0.1 at 4 km. The average convection frequency decreases from 14.1% at 128 km to 2.3% at 4 km in the CRM data, but it increases rapidly in the dCAPE-type triggers and is almost unchanged in the Bechtold and HCF triggers across resolutions, all leading to large overpredictions at higher resolutions. In the dCAPE-type triggers, the increased frequency is due to the increased rate of dCAPE greater than the threshold (65 J kg−1 h−1) at higher resolutions. The box-and-whisker plots show that the main body of dCAPE in the correct prediction and overprediction can be separated from each other in most resolutions. Moreover, the underprediction is found to be corresponding to the decaying phase of convection. Hence, two modifications are proposed to improve the scale dependence of the undilute dCAPE trigger: 1) increasing the dCAPE threshold and 2) considering convection history, which checks whether there is convection prior to the current time. With these modifications, the skill at 16 km, 8 km, and 4 km can be increased from 0.50, 0.27, and 0.15 to 0.70, 0.61, and 0.53, respectively.
Abstract
As the resolution of global climate model increases, whether trigger functions in current convective parameterization schemes still work remains unknown. In this study, the scale dependence of undilute and dilute dCAPE, Bechtold, and heated condensation framework (HCF) triggers is evaluated using the cloud-resolving model (CRM) data. It is found that all these trigger functions are scale dependent, especially for dCAPE-type triggers, with skill scores dropping from ~0.6 at the lower resolutions (128, 64, and 32 km) to only ~0.1 at 4 km. The average convection frequency decreases from 14.1% at 128 km to 2.3% at 4 km in the CRM data, but it increases rapidly in the dCAPE-type triggers and is almost unchanged in the Bechtold and HCF triggers across resolutions, all leading to large overpredictions at higher resolutions. In the dCAPE-type triggers, the increased frequency is due to the increased rate of dCAPE greater than the threshold (65 J kg−1 h−1) at higher resolutions. The box-and-whisker plots show that the main body of dCAPE in the correct prediction and overprediction can be separated from each other in most resolutions. Moreover, the underprediction is found to be corresponding to the decaying phase of convection. Hence, two modifications are proposed to improve the scale dependence of the undilute dCAPE trigger: 1) increasing the dCAPE threshold and 2) considering convection history, which checks whether there is convection prior to the current time. With these modifications, the skill at 16 km, 8 km, and 4 km can be increased from 0.50, 0.27, and 0.15 to 0.70, 0.61, and 0.53, respectively.
Abstract
The mean precipitation along the U.S. West Coast exhibits a pronounced seasonality change under warming. Here we explore the characteristics of the seasonality change and investigate the underlying mechanisms, with a focus on quantifying the roles of moisture (thermodynamic) versus circulation (dynamic). The multimodel simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5) show a simple “wet-get-wetter” response over Washington and Oregon but a sharpened seasonal cycle marked by a stronger and narrower wet season over California. Moisture budget analysis shows that changes in both regions are predominantly caused by changes in the mean moisture convergence. The thermodynamic effect due to the mass convergence of increased moisture dominates the wet-get-wetter response over Washington and Oregon. In contrast, mean zonal moisture advection due to seasonally dependent changes in land–sea moisture contrast originating from the nonlinear Clausius–Clapeyron relation dominates the sharpened wet season over California. More specifically, the stronger climatological land–sea thermal contrast in winter with warmer ocean than land results in more moisture increase over ocean than land under warming and hence wet advection to California. However, in fall and spring, the future change of land–sea thermal contrast with larger warming over land than ocean induces an opposite moisture gradient and hence dry advection to California. These results have important implications for projecting changes in the hydrological cycle of the U.S. West Coast.
Abstract
The mean precipitation along the U.S. West Coast exhibits a pronounced seasonality change under warming. Here we explore the characteristics of the seasonality change and investigate the underlying mechanisms, with a focus on quantifying the roles of moisture (thermodynamic) versus circulation (dynamic). The multimodel simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5) show a simple “wet-get-wetter” response over Washington and Oregon but a sharpened seasonal cycle marked by a stronger and narrower wet season over California. Moisture budget analysis shows that changes in both regions are predominantly caused by changes in the mean moisture convergence. The thermodynamic effect due to the mass convergence of increased moisture dominates the wet-get-wetter response over Washington and Oregon. In contrast, mean zonal moisture advection due to seasonally dependent changes in land–sea moisture contrast originating from the nonlinear Clausius–Clapeyron relation dominates the sharpened wet season over California. More specifically, the stronger climatological land–sea thermal contrast in winter with warmer ocean than land results in more moisture increase over ocean than land under warming and hence wet advection to California. However, in fall and spring, the future change of land–sea thermal contrast with larger warming over land than ocean induces an opposite moisture gradient and hence dry advection to California. These results have important implications for projecting changes in the hydrological cycle of the U.S. West Coast.
Abstract
Climate models project an enhancement in SST seasonal cycle over the midlatitude oceans under global warming. The underlying mechanisms are investigated using a set of partially coupled experiments, in which the contribution from direct CO2 effects (i.e., the response in the absence of wind change) and wind feedbacks can be isolated from each other. Results indicate that both the direct CO2 and wind effects contribute to the enhancement in the SST seasonal cycle, with the former (latter) being more important in the Northern Hemisphere (Southern Hemisphere). Further decomposition of the wind effect into the wind stress feedback and wind speed feedback reveals the importance of the wind stress–driven ocean response in the change of SST seasonal cycle, a result in contrast to a previous study that ascribed the midlatitude SST seasonal cycle change to the thermodynamic wind speed feedback. The direct CO2 effect regulates the SST seasonal cycle through the warming-induced shoaling in the annual mean mixed layer depth (MLD) as well as the MLD difference between winter and summer. Moreover, the surface wind seasonal cycle changes due solely to the direct CO2 effect are found to bear a great resemblance to the full wind response, suggesting that the root cause for the enhancement of the midlatitude SST seasonal cycle resides in the direct CO2 effect. This notion is further supported by an ocean-alone experiment that reproduces the SST seasonal cycle enhancement under a spatially and temporally homogeneous surface thermal forcing.
Abstract
Climate models project an enhancement in SST seasonal cycle over the midlatitude oceans under global warming. The underlying mechanisms are investigated using a set of partially coupled experiments, in which the contribution from direct CO2 effects (i.e., the response in the absence of wind change) and wind feedbacks can be isolated from each other. Results indicate that both the direct CO2 and wind effects contribute to the enhancement in the SST seasonal cycle, with the former (latter) being more important in the Northern Hemisphere (Southern Hemisphere). Further decomposition of the wind effect into the wind stress feedback and wind speed feedback reveals the importance of the wind stress–driven ocean response in the change of SST seasonal cycle, a result in contrast to a previous study that ascribed the midlatitude SST seasonal cycle change to the thermodynamic wind speed feedback. The direct CO2 effect regulates the SST seasonal cycle through the warming-induced shoaling in the annual mean mixed layer depth (MLD) as well as the MLD difference between winter and summer. Moreover, the surface wind seasonal cycle changes due solely to the direct CO2 effect are found to bear a great resemblance to the full wind response, suggesting that the root cause for the enhancement of the midlatitude SST seasonal cycle resides in the direct CO2 effect. This notion is further supported by an ocean-alone experiment that reproduces the SST seasonal cycle enhancement under a spatially and temporally homogeneous surface thermal forcing.
