Abstract

A novel method was developed to define the regional Hadley circulation (HC) in terms of the meridional streamfunction. The interannual variability of the Atlantic HC in boreal summer was examined using EOF analysis. The leading mode (M1), explaining more than 45% of the variances, is associated with the intensity change of the ITCZ. M1 is significantly correlated to multiple climate factors and has strong impacts on Atlantic tropical cyclone (TC) activity. In the positive (negative) phase of M1, the ITCZ is stronger (weaker) than normal, and more (fewer) TCs form over the main development region (MDR) with a larger (smaller) fraction of storms intensifying into major hurricanes. Analyses showed that the large-scale dynamic and thermodynamic conditions associated with a stronger ITCZ are more favorable for TC activity.

The roles of tropical easterly waves in modulating the Atlantic TC activity are highlighted. In the positive phase of M1, the wave activity is significantly enhanced over the MDR and the Caribbean Sea, which can be attributed to stronger coastal convection and mean flow structure changes. In the context of the recently proposed marsupial paradigm, the frequency and structure of wave pouches were examined. In the positive phase of M1, the pouch frequency increases, and the number of pouches with a vertically coherent structure also rises significantly. A deep and vertically aligned wave pouch has been shown to be highly favorable for TC formation.

The HC perspective thus unifies both dynamic and thermodynamic conditions impacting Atlantic TC activity and helps explain the statistical linkages between Atlantic TC activity and different climate factors.

Abstract

Many coupled climate models suffer from a late retreat bias in North American monsoon (NAM) simulations, which is manifested by overestimated precipitation in October. The overestimated precipitation has long been attributed to the negative sea surface temperature (SST) biases in the tropical Atlantic and insufficient model resolution to resolve mesoscale features. However, we found little correlation between CMIP6 model resolutions and the simulated NAM retreat-season precipitation in October. Instead, we showed that tropical eastern North Pacific SST biases and the associated large-scale circulation biases play a dominant role in inducing the retreat-season biases, with SST biases in other ocean basins playing a secondary role. As revealed by simulations using a hierarchy of models, the positive SST biases in the tropical eastern North Pacific enhance local convection and lead to positive diabatic heating biases throughout the troposphere; the diabatic heating biases generate a Matsuno–Gill type of response that strengthens the subtropical high over the North Atlantic and weakens the subtropical high over the North Pacific, enhancing the low-level northward moisture transport from the tropics to the NAM region. The conclusion is robust across phase 6 of CMIP (CMIP6) models. The precipitation seasonality in the NAM region is used to constrain future projection. The “good” CMIP6 models project that the timing of the NAM peak season remains the same, but the peak-season precipitation is reduced and monsoon retreat is delayed, while the “poor” CMIP6 models project a delayed monsoon peak season with slightly enhanced peak-season precipitation. Both model groups project a drier dry season in the NAM region.

Abstract

Atmospheric general circulation model (AGCM) simulations are carried out to test a hypothesis (Chang et al.) for the asymmetric monsoon transition in which the maximum convection marches gradually from the Asian summer monsoon to the Asian winter monsoon during boreal fall but experiences a sudden transition in the reverse during boreal spring. In the control run, the AGCM is driven by the climatological mean sea surface temperature (SST) with a realistic annual cycle, and it reproduces the observed asymmetric monsoon transition. In the sensitivity test, the model is driven by a similarly realistic SST but whose annual cycle is symmetric. The northwestward march of the maximum convection in boreal spring becomes more gradual, resulting in an overall near-symmetric pattern for the monsoon seasonal transition. The AGCM simulations confirm the hypothesis that the atmospheric mass redistribution due to the different land–ocean thermal memories leads to a seasonally different horizontal convergence field and it facilitates the southeastward monsoon march in boreal fall, while it hinders the northwestward monsoon march in boreal spring, contributing to the asymmetric monsoon transition.

Abstract

A regional climate model is used to simulate the summer monsoon onset in South and Southeast Asia during the year 2000 to explore the interaction between orographic precipitation and the large-scale monsoon circulation. In the control run, the model uses the U. S. Geological Survey topography data and simulates the observed monsoon onset reasonably well. In the sensitivity tests, mountains are removed within different regions south of the Tibetan Plateau. It is found that the Indochina Peninsula monsoon onset is closely related to the local wind–terrain–precipitation interaction, while the Indian monsoon onset is more controlled by the large-scale land–sea thermal contrast.

The sensitivity tests suggest two opposite effects of high terrain on the monsoon circulation and precipitation. When the terrain height is below the lifted condensation level (LCL), the low-level westerlies and the orographic precipitation weaken with increasing terrain height due to the surface drag effect. When the terrain height is above the LCL, the positive feedback associated with the diabatic forcing of orographic precipitation is dominant, and a large mountain height leads to heavier orographic precipitation and stronger low-level westerlies. The sensitivity tests also show that the impact of orographic precipitation in the Indochina Peninsula extends up to 30° longitude upstream and affects monsoon precipitation along the western coast of India.

Abstract

A hybrid statistical–dynamical model is developed to predict multiyear variability of Atlantic tropical cyclone (TC) activity. A Poisson model takes sea surface temperature (SST) averaged over the Atlantic main development region (MDR) and the Atlantic subpolar gyre region (SPG) from the initialized CESM prediction as predictors, and skillfully predicts the basinwide TC frequency, accumulated cyclone energy (ACE), landfalling TC frequency, and hurricane and major hurricane days. Further analysis shows that the SPG SST is a more important source of predictability than the MDR SST for multiyear Atlantic TC activity. The comparison between the uninitialized and initialized CESM predictions suggests that the SPG SST is better predicted by the initialized CESM owing to the better prediction of Atlantic meridional overturning circulation, which contributes to the overall more skillful TC predictions. On the other hand, the skillful prediction of the basinwide TC frequency by the uninitialized CESM suggests the role of external forcing in the variability of Atlantic TC activity. The dependence of the hybrid prediction skills on the dynamic model ensemble size is also explored, and an ensemble size of ~20 is suggested as optimal. Further analysis shows that the SPG SST is associated with the variability of vertical wind shear and precipitable water over the tropical Atlantic even when the influence of the MDR SST is controlled. The spatial patterns of vertical wind shear and precipitable water suggest a strong modulation of ACE and hurricane frequency but a relatively weak influence on the basinwide TC frequency. The physical mechanisms between the SPG SST and Atlantic TC activity are discussed.

Abstract

This study explores the connection of Rossby wave breaking (RWB) with tropical and extratropical variability during the Atlantic hurricane season. The exploration emphasizes subtropical anticyclonic RWB events over the western North Atlantic, which strongly affect tropical cyclone (TC) activity. The first part of the study investigates the link between RWB and tropical sea surface temperature (SST) variability. Tropical SST variability affects tropical precipitation and modulates the large-scale atmospheric circulation over the subtropical Atlantic, which influences the behaviors of Rossby waves and the frequency of RWB occurrence. Meanwhile, RWB regulates surface heat fluxes and helps to sustain SST anomalies in the western North Atlantic. The second part of the study explores the connections between RWB and extratropical atmosphere variability by leveraging weather regime analysis. The weather regimes over the North Atlantic are closely associated with RWB over the eastern North Atlantic and western Europe, but show weak associations with RWB over the western North Atlantic. Instead, RWB over the western basin is closely related to the weather regimes in the North Pacific–North America sector. The finding helps clarify why the correlation between the Atlantic TC activity and the summertime North Atlantic Oscillation is tenuous. The relations between the extratropical weather regimes and tropical climate modes are also discussed. The findings suggest that both tropical and extratropical variability are important for understanding variations of RWB events and their impacts on Atlantic TC activity.

Abstract

The impacts of El Niño and La Niña on the U.S. climate during northern summer are analyzed separately. Composite analyses reveal that a continental-scale anomalous high dominates over most of North America during La Niña events and leads to hot and dry summers over the central United States. However, the impacts of El Niño over North America are weaker and more variable.

A linear barotropic model is used to explore the maintenance of the anomalous patterns. Various forcing terms derived from observations via a single-level vorticity budget analysis are used to drive the model. When the barotropic model is driven by the total forcing (Rossby wave source plus transient eddy forcing plus nonlinear interactions), the model simulations resemble the observed patterns, and a strong and extensive anticyclone is reproduced in the La Niña simulation. The model responses to the individual forcing terms suggested that the vorticity stretching term ( fD) and the transient eddy forcing contribute most to the responses over North America. The stretching term ( fD) excites a low in the El Niño simulation and a high in the La Niña simulation over North America. However, the transient eddy forcing favors an anomalous high over North America in both El Niño and La Niña simulations, such that it weakens the El Niño pattern and strengthens the La Niña pattern.

Abstract

The representation of ENSO and NAO are examined in the Climate Forecast System, version 2 (CFSv2), reforecasts with a focus on the physical processes related to teleconnections and predictability. CFSv2 predicts ENSO well, but an eastward shift of the tropical Pacific sea surface temperature (SST) anomalies is evident. Although it appears minor on the global scale, the shift in convection and the large-scale wave train affects the model prediction of regional climate. In contrast, NAO is predicted poorly. The anomaly correlation coefficient (ACC) between the model ensemble mean and the observation is 0.27 during 1982–2010, and the ensemble spread is large. The representation of three sources of NAO predictability—SST, the stratospheric polar vortex, and the Arctic sea ice concentration—is investigated. It is found that the link between tropical Pacific SST and NAO is not well represented in CFSv2, and that the tropospheric–stratospheric interactions are too weak, both contributing to the poor prediction of NAO. Additionally, the impact of ENSO and NAO on prediction skill of CFSv2 in boreal winter is analyzed in terms of the spatial ACC of geopotential height. Active ENSO events exhibit larger prediction skill than neutral years, especially during the ENSO+/NAO− and ENSO−/NAO+ winters. Spatial patterns of prediction skill are also examined, and larger skill of geopotential height and 2-m air temperature is found outlined by the nodes of the PNA pattern, consistent with the large signal-to-noise ratios associated with the ENSO teleconnection.

Abstract

Large sea ice loss on the synoptic time scale is examined in various subregions in the Arctic as well as at the pan-Arctic scale. It is found that the frequency of large daily sea ice loss (LDSIL) days is significantly correlated with the September sea ice extent over the Beaufort–Chukchi–Siberian Seas, the Laptev–Kara Seas, the central Arctic, and the all-Arctic regions, indicating a link between the synoptic sea ice variability and the interannual variability of the annual minimum sea ice extent. A composite analysis reveals dipoles of anomalous cyclones and anticyclones associated with LDSIL days. Different from the well-known Arctic dipole pattern, the east–west dipoles are found over the corresponding regions of LDSIL in the Arctic marginal seas and are associated with the increasing occurrence of Rossby wave breaking and atmospheric rivers. The anticyclones of the dipoles are persistent and quasi-stationary, reminiscent of blocking. The anomalous poleward flow between the cyclone and the anticyclone enhances the poleward transport of heat and water vapor in the lower troposphere. Although enhanced downward shortwave radiation, associated with reduced cloud fraction, is found in some regions, it is not collocated with the regions of LDSIL. In contrast, enhanced downward longwave radiation owing to increasing column water vapor shows good spatial correspondence with LDSIL, indicating the importance of atmospheric rivers in LDSIL events. Lead/lag composites with respect to the onset of LDSIL episodes reveal precursor wave trains spanning the midlatitudes. The wave trains have predominantly zonal energy propagation in the midlatitudes and do not show a clear link to tropical or subtropical forcing.

Abstract

Polar lows (PLs) are intense mesoscale cyclones over high-latitude oceans. The PL motion and track characteristics over the North Atlantic in the cold season are examined based on ∼1700 PLs during 1979–2016. Our analysis shows that PL motion is mainly controlled by the environmental flow in the lower troposphere. In particular, the steering flow defined over 950–550 hPa within a 450-km radius best matches the PL motion. Meanwhile, 700 hPa is an optimal single level to assess the steering flow. Cluster analysis based on a linear regression mixture model is utilized to describe the PL tracks. Four distinct clusters are identified, and they are characterized by northeastward motion (NE), eastward motion (E), southward motion (S), and slow motion without a dominating direction (SM), respectively. Although PLs in the four clusters have similar lifespans, SM-type PLs have much shorter tracks than the other clusters, due to their slow translation speeds. Further analysis shows that there are no distinctive geographic differences in genesis locations for the four clusters. The track differences can be largely explained by the associated synoptic-scale environmental circulations. Additionally, the S-type PLs tend to develop in a reverse shear environment, the NE-type and E-type PLs are associated with forward shear and left shear environments, respectively, while the SM-type PLs do not show a preference in the environmental shear. The link between the PL tracks and the North Atlantic weather regimes is also investigated. The NAO+ regime is associated with the most frequent PL occurrence over the North Atlantic, while the Scandinavian blocking regime is associated with lowest PL frequency.