Abstract
Fire emissions from the Maritime Continent (MC) over the western tropical Pacific are strongly influenced by El Niño–Southern Oscillation (ENSO), posing various climate effect to the Earth system. In this study, we show that the historical biomass burning emissions of black carbon (BCbb) aerosol in the dry season from the MC are strengthened in El Niño years due to the dry conditions. The Eastern-Pacific type of El Niño exerts a stronger modulation in BCbb emissions over the MC region than the Central-Pacific type of El Niño. Based on simulations using the fully coupled Community Earth System Model (CESM), the impacts of increased BCbb emissions on ENSO variability and frequency are also investigated in this study. With BCbb emissions from the MC scaled up by a factor of 10, which enables the identification of climate response from the internal variability, the increased BCbb heats the local atmosphere and changes land-sea thermal contrast, which suppresses the westward transport of the eastern Pacific surface water. It leads to an increase of sea surface temperature in the eastern tropical Pacific, which further enhances ENSO variability and increases the frequency of extreme El Niño and La Niña events. This study highlights the potential role of BCbb emissions on extreme ENSO frequency and this role may be increasingly important in the warming future with higher wildfire risks.
Abstract
Fire emissions from the Maritime Continent (MC) over the western tropical Pacific are strongly influenced by El Niño–Southern Oscillation (ENSO), posing various climate effect to the Earth system. In this study, we show that the historical biomass burning emissions of black carbon (BCbb) aerosol in the dry season from the MC are strengthened in El Niño years due to the dry conditions. The Eastern-Pacific type of El Niño exerts a stronger modulation in BCbb emissions over the MC region than the Central-Pacific type of El Niño. Based on simulations using the fully coupled Community Earth System Model (CESM), the impacts of increased BCbb emissions on ENSO variability and frequency are also investigated in this study. With BCbb emissions from the MC scaled up by a factor of 10, which enables the identification of climate response from the internal variability, the increased BCbb heats the local atmosphere and changes land-sea thermal contrast, which suppresses the westward transport of the eastern Pacific surface water. It leads to an increase of sea surface temperature in the eastern tropical Pacific, which further enhances ENSO variability and increases the frequency of extreme El Niño and La Niña events. This study highlights the potential role of BCbb emissions on extreme ENSO frequency and this role may be increasingly important in the warming future with higher wildfire risks.
Abstract
This study identifies that cold surges over the South China Sea (SCS) have experienced a significant change on decadal time scales. The results indicate that cold surges occur more frequently after the early 2000s than before and are at least partially explained by changes in circulation patterns. Both the negative phase of the Scandinavian (SCA) pattern and the cold phase of Interdecadal Pacific Oscillation (IPO) can induce increased cold surges and the IPO effect dominates in recent decades. When the IPO shifts to its cold phase, low-level cyclones are induced over the western North Pacific through a Gill response. The northeasterlies along the northwest flank of the cyclones further lead to intensified cold surges over the SCS. The above processes can be reproduced in coupled models, suggesting a robust connection between IPO and cold surges. The present findings highlight the role of tropical forcing and bring an insight into understanding of the future climate variability and change over East Asia during boreal winter.
Abstract
This study identifies that cold surges over the South China Sea (SCS) have experienced a significant change on decadal time scales. The results indicate that cold surges occur more frequently after the early 2000s than before and are at least partially explained by changes in circulation patterns. Both the negative phase of the Scandinavian (SCA) pattern and the cold phase of Interdecadal Pacific Oscillation (IPO) can induce increased cold surges and the IPO effect dominates in recent decades. When the IPO shifts to its cold phase, low-level cyclones are induced over the western North Pacific through a Gill response. The northeasterlies along the northwest flank of the cyclones further lead to intensified cold surges over the SCS. The above processes can be reproduced in coupled models, suggesting a robust connection between IPO and cold surges. The present findings highlight the role of tropical forcing and bring an insight into understanding of the future climate variability and change over East Asia during boreal winter.
Abstract
In order to understand the intensification process of tropical cyclones (TCs), we analyzed the relationship between TC intensification rate (I) and environmental variables along TC tracks during the time from TC genesis (tG ) to maximum TC strength (tX ), hereinafter τGX ≡ tX − tG , using a state-of-the-art general circulation model (GCM), observed TC tracks, and ERA5 reanalysis data. During τGX , strong TCs with high I (sTCs) consume more convective available potential energy (CAPE) than weak TCs with low I (wTCs), and bring more CAPE from the equator to sustain sTCs. Compared to wTCs, sTCs prefer an unstable atmosphere with higher sea surface temperature (SST), stronger grid-mean upward flow at 500 hPa (ω 500), more moisture convergence (MC), and weaker wind shear (Vs ). Our GCM simulation shows that MC and CAPE have a single regression slope with I applicable both within and across climate regimes. Using machine learning, we found that the best combination of environmental variables (V6) for predicting I consists of ω 500, MC, SST, mid-tropospheric stability (MTS), Vs , and latitude (|ƒ|). Machine learning with V6 reproduces well the spatial distribution and inter-climate changes of I: TCs are intensified in regions of stronger upward ω 500, more MC, warmer SST, weaker MTS, smaller Vs , and larger |ƒ|; TCs in a warmer climate have higher I than TCs in a colder climate due to more MC, warmer SST, but stronger MTS. These results are consistent with the conceptual understanding that TCs are intensified by the release of latent heat.
Abstract
In order to understand the intensification process of tropical cyclones (TCs), we analyzed the relationship between TC intensification rate (I) and environmental variables along TC tracks during the time from TC genesis (tG ) to maximum TC strength (tX ), hereinafter τGX ≡ tX − tG , using a state-of-the-art general circulation model (GCM), observed TC tracks, and ERA5 reanalysis data. During τGX , strong TCs with high I (sTCs) consume more convective available potential energy (CAPE) than weak TCs with low I (wTCs), and bring more CAPE from the equator to sustain sTCs. Compared to wTCs, sTCs prefer an unstable atmosphere with higher sea surface temperature (SST), stronger grid-mean upward flow at 500 hPa (ω 500), more moisture convergence (MC), and weaker wind shear (Vs ). Our GCM simulation shows that MC and CAPE have a single regression slope with I applicable both within and across climate regimes. Using machine learning, we found that the best combination of environmental variables (V6) for predicting I consists of ω 500, MC, SST, mid-tropospheric stability (MTS), Vs , and latitude (|ƒ|). Machine learning with V6 reproduces well the spatial distribution and inter-climate changes of I: TCs are intensified in regions of stronger upward ω 500, more MC, warmer SST, weaker MTS, smaller Vs , and larger |ƒ|; TCs in a warmer climate have higher I than TCs in a colder climate due to more MC, warmer SST, but stronger MTS. These results are consistent with the conceptual understanding that TCs are intensified by the release of latent heat.
Abstract
Genesis potential index (GPI) has been used widely to estimate the influence of large-scale conditions on tropical cyclone (TC) genesis. Here we find that two GPIs, the Emanuel-Nolan GPI (ENGPI) and dynamic GPI (DGPI), show opposite skills in quantifying decadal variability of TC genesis in the western North Pacific (WNP). During 1979-2020, ENGPI shows a reverse decadal variation to the WNP TC genesis with a significant negative correlation of -0.61, while DGPI can reasonably reproduce the decadal variation of the WNP TC genesis with a significant correlation of 0.66. The opposite skills of the two indices arise from the opposed effects of dynamical and thermodynamical parameters on TC genesis induced by a WNP anomalous cyclonic circulation that controls the decadal variation of TC genesis. On the one hand, the cyclonic circulation leads to favorable dynamical conditions including ascending motion, cyclonic vorticity, and weakened vertical shear, and thus tends to increase the DGPI, On the other hand, the cyclonic circulation leads to unfavorable thermodynamical conditions (decreased maximum potential intensity and mid-level humidity) that tends to decrease the ENGPI. As a result, the DGPI and ENGPI are reversely evolved and eventually lead to their opposite correlation between TC genesis. The significant positive correlation between DGPI and TC genesis suggests a critical role in the large-scale dynamical control of the decadal variability of the WNP TC genesis.
Abstract
Genesis potential index (GPI) has been used widely to estimate the influence of large-scale conditions on tropical cyclone (TC) genesis. Here we find that two GPIs, the Emanuel-Nolan GPI (ENGPI) and dynamic GPI (DGPI), show opposite skills in quantifying decadal variability of TC genesis in the western North Pacific (WNP). During 1979-2020, ENGPI shows a reverse decadal variation to the WNP TC genesis with a significant negative correlation of -0.61, while DGPI can reasonably reproduce the decadal variation of the WNP TC genesis with a significant correlation of 0.66. The opposite skills of the two indices arise from the opposed effects of dynamical and thermodynamical parameters on TC genesis induced by a WNP anomalous cyclonic circulation that controls the decadal variation of TC genesis. On the one hand, the cyclonic circulation leads to favorable dynamical conditions including ascending motion, cyclonic vorticity, and weakened vertical shear, and thus tends to increase the DGPI, On the other hand, the cyclonic circulation leads to unfavorable thermodynamical conditions (decreased maximum potential intensity and mid-level humidity) that tends to decrease the ENGPI. As a result, the DGPI and ENGPI are reversely evolved and eventually lead to their opposite correlation between TC genesis. The significant positive correlation between DGPI and TC genesis suggests a critical role in the large-scale dynamical control of the decadal variability of the WNP TC genesis.
Abstract
Biomass burning aerosol (BBA) emissions in the Coupled Model Intercomparison Project Phase 6 (CMIP6) historical forcing fields have enhanced temporal variability during the years 1997–2014 compared to earlier periods. Recent studies document that the corresponding inhomogeneous shortwave forcing over this period can cause changes in clouds, permafrost, and soil moisture, which contribute to a net terrestrial Northern Hemisphere warming relative to earlier periods. Here, we investigate the ocean response to the hemispherically asymmetric warming, using a 100-member ensemble of the Community Earth System Model version 2 Large Ensemble forced by two different BBA emissions (CMIP6 default and temporally smoothed over 1990–2020). Differences between the two subensemble means show that ocean temperature anomalies occur during periods of high BBA variability and subsequently persist over multiple decades. In the North Atlantic, surface warming is efficiently compensated for by decreased northward oceanic heat transport due to a slowdown of the Atlantic Meridional Overturning Circulation. In the North Pacific, surface warming is compensated for by an anomalous cross-equatorial cell (CEC) that reduces northward oceanic heat transport. The heat that converges in the South Pacific through the anomalous CEC is shunted into the subsurface and contributes to formation of long-lasting ocean temperature anomalies. The anomalous CEC is maintained through latitude-dependent contributions from narrow western boundary currents and basin-wide near-surface Ekman transport. These results indicate that interannual variability in forcing fields may significantly change the background climate state over long timescales, presenting a potential uncertainty in CMIP6-class climate projections forced without interannual variability.
Abstract
Biomass burning aerosol (BBA) emissions in the Coupled Model Intercomparison Project Phase 6 (CMIP6) historical forcing fields have enhanced temporal variability during the years 1997–2014 compared to earlier periods. Recent studies document that the corresponding inhomogeneous shortwave forcing over this period can cause changes in clouds, permafrost, and soil moisture, which contribute to a net terrestrial Northern Hemisphere warming relative to earlier periods. Here, we investigate the ocean response to the hemispherically asymmetric warming, using a 100-member ensemble of the Community Earth System Model version 2 Large Ensemble forced by two different BBA emissions (CMIP6 default and temporally smoothed over 1990–2020). Differences between the two subensemble means show that ocean temperature anomalies occur during periods of high BBA variability and subsequently persist over multiple decades. In the North Atlantic, surface warming is efficiently compensated for by decreased northward oceanic heat transport due to a slowdown of the Atlantic Meridional Overturning Circulation. In the North Pacific, surface warming is compensated for by an anomalous cross-equatorial cell (CEC) that reduces northward oceanic heat transport. The heat that converges in the South Pacific through the anomalous CEC is shunted into the subsurface and contributes to formation of long-lasting ocean temperature anomalies. The anomalous CEC is maintained through latitude-dependent contributions from narrow western boundary currents and basin-wide near-surface Ekman transport. These results indicate that interannual variability in forcing fields may significantly change the background climate state over long timescales, presenting a potential uncertainty in CMIP6-class climate projections forced without interannual variability.
Abstract
During 2013-16 and 2018-22, marine heat waves (MHWs) occurred in the North Pacific, exhibiting similar extensive coverage, lengthy duration, and significant intensity but with different warming centers. The warming center of the 2013-16 event was in the Gulf of Alaska (GOA), while the 2018-22 event had warming centers in both the GOA and the Coast of Japan (COJ). Our observational analysis indicates that these two events can be considered as two MHW variants induced by a basin-wide MHW conditioning mode in the North Pacific. Both variants were driven thermodynamically by atmospheric wavetrains propagating from the tropical Pacific to the North Pacific, within the conditioning mode. The origin and propagating path of these wavetrains play a crucial role in determining the specific type of MHW variant. When a stronger wavetrain originates from the tropical central (western) Pacific, it leads to the GOA (COJ) variant. The cross-basin nature of the wavetrains enables the two MHW variants to be accompanied by a tri-polar pattern of sea surface temperature anomalies in the North Atlantic but with opposite phases. The association of these two MHW variants with the Atlantic Ocean also manifests in the decadal variations of their occurrence. Both variants tend to occur more frequently during the positive phase of the Atlantic Multidecadal Oscillation but less so during the negative phase. This study underscores the importance of cross-basin associations between the North Pacific and North Atlantic in shaping the dynamics of North Pacific MHWs.
Abstract
During 2013-16 and 2018-22, marine heat waves (MHWs) occurred in the North Pacific, exhibiting similar extensive coverage, lengthy duration, and significant intensity but with different warming centers. The warming center of the 2013-16 event was in the Gulf of Alaska (GOA), while the 2018-22 event had warming centers in both the GOA and the Coast of Japan (COJ). Our observational analysis indicates that these two events can be considered as two MHW variants induced by a basin-wide MHW conditioning mode in the North Pacific. Both variants were driven thermodynamically by atmospheric wavetrains propagating from the tropical Pacific to the North Pacific, within the conditioning mode. The origin and propagating path of these wavetrains play a crucial role in determining the specific type of MHW variant. When a stronger wavetrain originates from the tropical central (western) Pacific, it leads to the GOA (COJ) variant. The cross-basin nature of the wavetrains enables the two MHW variants to be accompanied by a tri-polar pattern of sea surface temperature anomalies in the North Atlantic but with opposite phases. The association of these two MHW variants with the Atlantic Ocean also manifests in the decadal variations of their occurrence. Both variants tend to occur more frequently during the positive phase of the Atlantic Multidecadal Oscillation but less so during the negative phase. This study underscores the importance of cross-basin associations between the North Pacific and North Atlantic in shaping the dynamics of North Pacific MHWs.
Diagnosing Seasonal Forecast Skill of the Indian Ocean Dipole Mode Using Model AnalogsAbstract
A retrospective tropical Indian Ocean dipole mode (IOD) hindcast for 1958–2014 was conducted using 20 models from the sixth phase of the Coupled Model Intercomparison Project (CMIP6), with a model-based analog forecast (MAF) method. In the MAF approach, forecast ensembles are extracted from preexisting model simulations by finding the states that initially best match an observed anomaly and tracking their subsequent evolution, with no additional model integrations. By optimizing the key factors in the MAF method, we suggest that the optimal domain for the analog criteria should be concentrated in the tropical Indian Ocean region for IOD predictions. Including external forcing trends improves the skills of the east and west poles of the IOD, but not the IOD prediction itself. The MAF IOD prediction showed comparable skills to the assimilation-initialized hindcast, with skillful predictions corresponding to a 4- and 3-month lead, respectively. The IOD forecast skill had significant decadal variations during the 55-yr period, with low skill after the early 2000s and before 1985 and high skill during 1985–2000. This work offers a computational efficient and practical approach for seasonal prediction of the tropical Indian Ocean sea surface temperature.
Abstract
A retrospective tropical Indian Ocean dipole mode (IOD) hindcast for 1958–2014 was conducted using 20 models from the sixth phase of the Coupled Model Intercomparison Project (CMIP6), with a model-based analog forecast (MAF) method. In the MAF approach, forecast ensembles are extracted from preexisting model simulations by finding the states that initially best match an observed anomaly and tracking their subsequent evolution, with no additional model integrations. By optimizing the key factors in the MAF method, we suggest that the optimal domain for the analog criteria should be concentrated in the tropical Indian Ocean region for IOD predictions. Including external forcing trends improves the skills of the east and west poles of the IOD, but not the IOD prediction itself. The MAF IOD prediction showed comparable skills to the assimilation-initialized hindcast, with skillful predictions corresponding to a 4- and 3-month lead, respectively. The IOD forecast skill had significant decadal variations during the 55-yr period, with low skill after the early 2000s and before 1985 and high skill during 1985–2000. This work offers a computational efficient and practical approach for seasonal prediction of the tropical Indian Ocean sea surface temperature.
Abstract
The effect of tropical cyclone (TC) size on TC-induced sea surface temperature (SST) cooling and subsequent TC intensification is an intriguing issue without much exploration. Via compositing satellite-observed SST over the western North Pacific during 2004–19, this study systematically examined the effect of storm size on the magnitude, spatial extension, and temporal evolution of TC-induced SST anomalies (SSTA). Consequential influence on TC intensification is also explored. Among the various TC wind radii, SSTA are found to be most sensitive to the 34-kt wind radius (R34) (1 kt ≈ 0.51 m s−1). Generally, large TCs generate stronger and more widespread SSTA than small TCs (for category 1–2 TCs, R34: ∼270 vs 160 km; SSTA: −1.7° vs −0.9°C). Despite the same effect on prolonging residence time of TC winds, the effect of doubling R34 on SSTA is more profound than halving translation speed, due to more wind energy input into the upper ocean. Also differing from translation speed, storm size has a rather modest effect on the rightward shift and timing of maximum cooling. This study further demonstrates that storm size regulates TC intensification through an oceanic pathway: large TCs tend to induce stronger SST cooling and are exposed to the cooling for a longer time, both of which reduce the ocean’s enthalpy supply and thereby diminish TC intensification. For larger TCs experiencing stronger SST cooling, the probability of rapid intensification is half of smaller TCs. The presented results suggest that accurately specifying storm size should lead to improved cooling effect estimation and TC intensity prediction.
Significance Statement
Storm size has long been speculated to play a crucial role in modulating the TC self-induced sea surface temperature (SST) cooling and thus potentially influence TC intensification through ocean negative feedback. Nevertheless, systematic analysis is lacking. Here we show that larger TCs tend to generate stronger SST cooling and have longer exposure to the cooling effect, both of which enhance the strength of the negative feedback. Consequently, larger TCs undergo weaker intensification and are less likely to experience rapid intensification than smaller TCs. These results demonstrate that storm size can influence TC intensification not only from the atmospheric pathway, but also via the oceanic pathway. Accurate characterization of this oceanic pathway in coupled models is important to accurately forecast TC intensity.
Abstract
The effect of tropical cyclone (TC) size on TC-induced sea surface temperature (SST) cooling and subsequent TC intensification is an intriguing issue without much exploration. Via compositing satellite-observed SST over the western North Pacific during 2004–19, this study systematically examined the effect of storm size on the magnitude, spatial extension, and temporal evolution of TC-induced SST anomalies (SSTA). Consequential influence on TC intensification is also explored. Among the various TC wind radii, SSTA are found to be most sensitive to the 34-kt wind radius (R34) (1 kt ≈ 0.51 m s−1). Generally, large TCs generate stronger and more widespread SSTA than small TCs (for category 1–2 TCs, R34: ∼270 vs 160 km; SSTA: −1.7° vs −0.9°C). Despite the same effect on prolonging residence time of TC winds, the effect of doubling R34 on SSTA is more profound than halving translation speed, due to more wind energy input into the upper ocean. Also differing from translation speed, storm size has a rather modest effect on the rightward shift and timing of maximum cooling. This study further demonstrates that storm size regulates TC intensification through an oceanic pathway: large TCs tend to induce stronger SST cooling and are exposed to the cooling for a longer time, both of which reduce the ocean’s enthalpy supply and thereby diminish TC intensification. For larger TCs experiencing stronger SST cooling, the probability of rapid intensification is half of smaller TCs. The presented results suggest that accurately specifying storm size should lead to improved cooling effect estimation and TC intensity prediction.
Significance Statement
Storm size has long been speculated to play a crucial role in modulating the TC self-induced sea surface temperature (SST) cooling and thus potentially influence TC intensification through ocean negative feedback. Nevertheless, systematic analysis is lacking. Here we show that larger TCs tend to generate stronger SST cooling and have longer exposure to the cooling effect, both of which enhance the strength of the negative feedback. Consequently, larger TCs undergo weaker intensification and are less likely to experience rapid intensification than smaller TCs. These results demonstrate that storm size can influence TC intensification not only from the atmospheric pathway, but also via the oceanic pathway. Accurate characterization of this oceanic pathway in coupled models is important to accurately forecast TC intensity.
Abstract
The generation of multiple wave couplets with deep tropospheric downdrafts/updrafts by convection is explored through idealized 2D moist numerical simulations as well as dry experiments with prescribed artificial latent heating. These wave couplets are capable of horizontally propagating over a long distance at a fast speed with vertical motions spanning the entire troposphere. The timing of wave generation is determined by the variation in the local heating rate, which arose from the imbalances among latent heating, nonlinear advection, and adiabatic heating/cooling. The amplitudes of wave couplets also correspond well with the strength of the local heating rate. The heat budget analysis highlights the crucial roles of both latent heating and nonlinear advection in the generation of the tropospheric wave couplets. Strong latent heating induces the thermodynamic imbalance and thus triggers waves. Meanwhile, latent heating also increases vertical motion in the source region and thus enhances nonlinear advection through transferring heat upward. Nonlinear advection, which has a comparable magnitude to latent heating in the upper troposphere, partially offsets the balancing effect of adiabatic heating/cooling, and results in a more persistent imbalance at high levels, allowing for the emission of consecutive waves even when latent heating becomes weak. In the simulation with weak nonlinear advection, fewer wave couplets are found, as the effect of latent heating is more easily offset by adiabatic cooling before it weakens.
Significance Statement
The generation of gravity waves in the troposphere by convection is of significant importance in the fields of atmospheric science and meteorology. The waves play a crucial role in the initiation and organization of convection, and the parameterization of wave momentum flux in global numerical models. This study aimed to investigate the generation of wave couplets in the troposphere through idealized numerical simulations with varying prescribed latent heating. The results showed that gravity wave couplets were generated in succession as a result of the imbalances among latent heating, nonlinear advection, and adiabatic heating/cooling. This study highlighted an important but yet complex issue of gravity waves being generated within convection by nonlinear sources other than latent heating, which had been neglected in many recent studies on the topic. These findings deepened our understanding of convectively generated gravity waves and paved the way for coupled wave–convection relationship studies.
Abstract
The generation of multiple wave couplets with deep tropospheric downdrafts/updrafts by convection is explored through idealized 2D moist numerical simulations as well as dry experiments with prescribed artificial latent heating. These wave couplets are capable of horizontally propagating over a long distance at a fast speed with vertical motions spanning the entire troposphere. The timing of wave generation is determined by the variation in the local heating rate, which arose from the imbalances among latent heating, nonlinear advection, and adiabatic heating/cooling. The amplitudes of wave couplets also correspond well with the strength of the local heating rate. The heat budget analysis highlights the crucial roles of both latent heating and nonlinear advection in the generation of the tropospheric wave couplets. Strong latent heating induces the thermodynamic imbalance and thus triggers waves. Meanwhile, latent heating also increases vertical motion in the source region and thus enhances nonlinear advection through transferring heat upward. Nonlinear advection, which has a comparable magnitude to latent heating in the upper troposphere, partially offsets the balancing effect of adiabatic heating/cooling, and results in a more persistent imbalance at high levels, allowing for the emission of consecutive waves even when latent heating becomes weak. In the simulation with weak nonlinear advection, fewer wave couplets are found, as the effect of latent heating is more easily offset by adiabatic cooling before it weakens.
Significance Statement
The generation of gravity waves in the troposphere by convection is of significant importance in the fields of atmospheric science and meteorology. The waves play a crucial role in the initiation and organization of convection, and the parameterization of wave momentum flux in global numerical models. This study aimed to investigate the generation of wave couplets in the troposphere through idealized numerical simulations with varying prescribed latent heating. The results showed that gravity wave couplets were generated in succession as a result of the imbalances among latent heating, nonlinear advection, and adiabatic heating/cooling. This study highlighted an important but yet complex issue of gravity waves being generated within convection by nonlinear sources other than latent heating, which had been neglected in many recent studies on the topic. These findings deepened our understanding of convectively generated gravity waves and paved the way for coupled wave–convection relationship studies.
