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Satoru Okajima, Hisashi Nakamura, and Yohai Kaspi

Abstract

Storm-track activity over the North Pacific (NP) climatologically exhibits a clear minimum in midwinter, when the westerly jet speed sharply maximizes. This counterintuitive phenomenon, referred to as the “midwinter minimum (MWM),” has been investigated from various perspectives, but the mechanisms are still to be unrevealed. Toward better understanding of this phenomenon, the present study delineates the detailed seasonal evolution of climatological-mean Eulerian statistics and energetics of migratory eddies along the NP storm track over 60 years. As a comprehensive investigation of the mechanisms for the MWM, this study has revealed that the net eddy conversion/generation rate normalized by the eddy total energy, which is independent of eddy amplitude, is indeed reduced in midwinter. The reduction from early winter occurs mainly due to the decreased effectiveness of the baroclinic energy conversion through seasonally weakened temperature fluctuations and the resultant poleward eddy heat flux. The reduced net normalized conversion/generation rate in midwinter is also found to arise in part from the seasonally enhanced kinetic energy conversion from eddies into the strongly diffluent Pacific jet around its exit. The seasonality of the net energy influx also contributes especially to the spring recovery of the net normalized conversion/generation rate. The midwinter reduction in the normalized rates of both the net energy conversion/generation and baroclinic energy conversion was more pronounced in the period before the late 1980s, during which the MWM of the storm-track activity was climatologically more prominent.

Open access
Tianbao Zhao and Aiguo Dai

Abstract

Drought is projected to become more severe and widespread as global warming continues in the twenty-first century, but hydroclimatic changes and their drivers are not well examined in the latest projections from phase 6 of the Coupled Model Intercomparison Project (CMIP6). Here, precipitation (P), evapotranspiration (E), soil moisture (SM), and runoff (R) from 25 CMIP6 models, together with self-calibrated Palmer drought severity index with Penman–Monteith potential evapotranspiration (scPDSIpm), are analyzed to quantify hydroclimatic and drought changes in the twenty-first century and the underlying causes. Results confirm consistent drying in these hydroclimatic metrics across most of the Americas (including the Amazon), Europe and the Mediterranean region, southern Africa, and Australia, although the drying magnitude differs, with the drying being more severe and widespread in surface SM than in total SM. Global drought frequency based on surface SM and scPDSIpm increases by ∼25%–100% (50%–200%) under the SSP2-4.5 (SSP5-8.5) scenario in the twenty-first century together with large increases in drought duration and areas, which result from a decrease in the mean and flattening of the probability distribution functions of SM and scPDSIpm, while the R-based drought changes are relatively small. Changes in both P and E contribute to the SM change, whereas scPDSIpm decreases result from ubiquitous PET increases and P decreases over subtropical areas. The R changes are determined primarily by P changes, while the PET change explains most of the E increase. Intermodel spreads in surface SM and R changes are large, leading to large uncertainties in the drought projections.

SIGNIFICANCE STATEMENT

Drought may become more severe and widespread under greenhouse gas (GHG)-induced global warming in the twenty-first century based on model projections. However, there are still large uncertainties in projected future drought changes, especially regarding the extent to which drought changes depend on drought indices and the future emissions scenarios analyzed. The latest projections from CMIP6 models reaffirm the widespread drying and increases in agricultural drought by up to 200% over most of the Americas (including the Amazon), Europe and the Mediterranean region, southern Africa, Southeast Asia, and Australia under moderate-to-high emissions scenarios in the twenty-first century, despite large uncertainties in individual projections partly due to internal variability. Ubiquitous increases in atmospheric demand for moisture under rising temperatures and precipitation decreases over many subtropical regions are the main driver of the projected drying and drought increases.

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Raphaël Rousseau-Rizzi, Timothy M. Merlis, and Nadir Jeevanjee

Abstract

Tropical cyclone (TC) potential intensity (PI) theory has a well-known form, consistent with a Carnot cycle interpretation of TC energetics, which relates PI to mean environmental conditions: the difference between surface and TC outflow temperatures and the air–sea enthalpy disequilibrium. PI has also been defined as a difference in convective available potential energy (CAPE) between two parcels, and quantitative assessments of future changes make use of a numerical algorithm based on this definition. Here, an analysis shows the conditions under which these Carnot and CAPE-based PI definitions are equivalent. There are multiple conditions, not previously enumerated, which in particular reveal a role for irreversible entropy production from surface evaporation. This mathematical analysis is verified by numerical calculations of PI’s sensitivity to large changes in surface-air relative humidity. To gain physical insight into the connection between the CAPE and Carnot formulations of PI, we use a recently developed analytic theory for CAPE to derive, starting from the CAPE-based definition, a new approximate formula for PI that nearly recovers the previous Carnot PI formula. The derivation shows that the difference in undilute buoyancies of saturated and environmental parcels that determines CAPE PI can in fact be expressed as a difference in the parcels’ surface moist static energy, providing a physical link between the Carnot and CAPE formulations of PI. This combination of analysis and physical interpretation builds confidence in previous numerical CAPE-based PI calculations that use climate model projections of the future tropical environment.

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Hiroaki Ueda, Masaya Kuramochi, Koutarou Takaya, Yuhei Takaya, Saki Asano, and Shuhei Maeda

Abstract

Upper-tropospheric anticyclones (UTACs) emerge throughout the seasons with changing location and intensity. Here, the formation mechanisms of these UTACs, especially in the Asian–Australian–western Pacific sector, were investigated based on the diagnosis of the vorticity equation as well as the contribution of the planetary waves. During June–July–August (JJA), a vigorous UTAC corresponding to the South Asian high (SAH) forms over South Asia, to the south of the Tibetan Plateau, where intense heating associated with the Asian summer monsoon rainfall and the resultant baroclinic Rossby response are the important physical processes. Meanwhile, the produced anticyclonic vorticity is farther transported by the interhemispheric divergent wind toward the Southern Hemisphere (SH), creating the SH UTAC centered over the Maritime Continent. During December–January–February (DJF), two zonally elongated UTACs reside on each side of the equator (∼10° poleward), mainly over the Maritime Continent–western Pacific sector. Upon a closer look at the NH winter, we observed that the northern parts of UTAC cannot be explained by this vorticity balance alone. Diagnosis of the wave activity flux indicated that planetary waves emanating from the cold Eurasian continent converges around the northern parts of the UTAC with its peak in the NH winter, which weakens the subtropical jet, thus generating UTAC. Configuration of the SH summer (DJF) UTAC bears resemblance with that of SAH. These results suggest that the creation of anticyclonic vorticity and its interhemispheric transportation as well as the propagation of planetary wave are the selectively important agents for the genesis of seasonally varying UTACs.

Significance Statement

Recent studies have provided evidence that the South Asian high (formerly Tibetan high) is not a purely thermally driven system only maintained over the elevated Tibetan Plateau. This study aims to understand the physical processes responsible for the genesis of the upper-tropospheric anticyclone, especially in the Asian–Australian–western Pacific sector, throughout the season. During summer in the Northern Hemisphere, deep heating caused by South Asian monsoon rainfall plays a crucial role in the genesis of the South Asian high. The wintertime anticyclone emerging over the subtropical western North Pacific is caused via remote influences anchored with tropical convection and the cold Eurasian continent in which atmospheric teleconnections are important. These findings provide new avenues for research on tropical–extratropical interactions with respect to the formation and variability of important climate phenomena.

Open access
Hugh Morrison, John M. Peters, Kamal Kant Chandrakar, and Steven C. Sherwood

Abstract

This study examines two factors impacting initiation of moist deep convection: free-tropospheric environmental relative humidity (ϕE) and horizontal scale of subcloud ascent (R sub), the latter exerting a dominant control on cumulus cloud width. A simple theoretical model is used to formulate a “scale selection” hypothesis: that a minimum R sub is required for moist convection to go deep, and that this minimum scale decreases with increasing ϕE. Specifically, the ratio of Rsub2 to saturation deficit (1 − ϕE) must exceed a certain threshold value that depends on cloud-layer environmental lapse rate. Idealized, large-eddy simulations of moist convection forced by horizontally varying surface fluxes show strong sensitivity of maximum cumulus height to both ϕE and R sub consistent with the hypothesis. Increasing R sub by only 300–400 m can lead to a large increase (>5 km) in cloud height. A passive tracer analysis shows that the bulk fractional entrainment rate decreases rapidly with R sub but depends little on ϕE. However, buoyancy dilution increases as either R sub or ϕE decreases; buoyancy above the level of free convection is rapidly depleted in dry environments when R sub is small. While deep convective initiation occurs with an increase in relative humidity of the near environment from moistening by earlier convection, the importance of this moisture preconditioning is inconclusive as it is accompanied by an increase in R sub. Overall, it is concluded that small changes to R sub driven by external forcing or by convection itself could be a dominant regulator of deep convective initiation.

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Juncong Li, Zhiping Wen, Xiuzhen Li, and Yuanyuan Guo

Abstract

Interdecadal variations of the relationship between El Niño–Southern Oscillation (ENSO) and the Indo-China Peninsula (ICP) surface air temperature (SAT) in winter are investigated in the study. Generally, there exists a positive correlation between them during 1958–2015 because the ENSO-induced anomalous western North Pacific anticyclone (WNPAC) is conducive to pronounced temperature advection anomalies over the ICP. However, such correlation is unstable in time, having experienced a high-to-low transition around the mid-1970s and a recovery since the early 1990s. This oscillating relationship is owing to the anomalous WNPAC intensity in different decades. During the epoch of high correlation, the anomalous WNPAC and associated southwesterly winds over the ICP are stronger, which brings amounts of warm temperature advection and markedly heats the ICP. In contrast, a weaker WNPAC anomaly and insignificant ICP SAT anomalies are the circumstances for the epoch of low correlation. It is also found that substantial southwesterly wind anomalies over the ICP related to the anomalous WNPAC occur only when large sea surface temperature (SST) anomalies over the northwest Indian Ocean (NWIO) coincide with ENSO (viz., when the ENSO–NWIO SST connection is strong). The NWIO SST anomalies are capable of driving favorable atmospheric circulation that effectively alters ICP SAT and efficiently modulates the ENSO–ICP SAT correlation, which is further supported by numerical simulations utilizing the Community Atmospheric Model, version 4 (CAM4). This paper emphasizes the non-stationarity of the ENSO–ICP SAT relationship and also uncovers the underlying modulation factors, which has important implications for the seasonal prediction of the ICP temperature.

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Zixiang Yan, Bo Wu, Tim Li, and Guirong Tan

Abstract

The longitudinal location of precipitation anomalies over the equatorial Pacific shows a distinctive feature with the westernmost location for La Niña, the easternmost location for eastern Pacific (EP) El Niño, and somewhere between for central Pacific (CP) El Niño, even though the center of the sea surface temperature anomaly (SSTA) for La Niña is located slightly east of that of CP El Niño. The mechanisms for such a precipitation diversity were investigated through idealized model simulations and moisture and moist static energy budget analyses. It is revealed that the boundary layer convergence anomalies associated with the precipitation diversity are mainly induced by underlying SSTA through the Lindzen–Nigam mechanism, that is, their longitudinal locations are mainly controlled by the meridional and zonal distributions of the ENSO SSTA. The westward shift of the precipitation anomaly center during La Niña relative to that during CP El Niño is primarily caused by the combined effects of nonlinear zonal moist enthalpy advection anomalies and the Lindzen–Nigam mechanism mentioned above. Such a zonal diversity is further enhanced by the “convection–cloud–longwave radiation” feedback, the SST-induced latent heat flux anomalies, and the advection of mean moist enthalpy by anomalous winds. This diversity in the longitudinal location of precipitation anomalies has contributions to the diversities in the longitudinal locations of anomalous Walker circulation and western North Pacific anomalous anticyclone/cyclone among the three types of ENSO.

Open access
Guiwan Chen, Jian Ling, Yuanwen Zhang, Xin Wang, and Chongyin Li

Abstract

This study explores the impacts of background states on the propagation of the Madden–Julian oscillation (MJO) in 24 CMIP5 models using a precipitation-based MJO tracking method. The ability of the model to reproduce the MJO propagation is reflected in the occurrence frequency of individual MJO events. Moisture budget analysis suggests that the occurrence frequencies of MJO events that propagate across the Indian Ocean (IO-MJO) and western Pacific (WP-MJO) in the models are mainly related to the low-level meridional moisture advection ahead of the MJO convection center. This advection is tightly associated with the background distribution of low-level moisture. Drier biases in background low-level moisture over the entire tropical regions account for underestimated MJO occurrence frequency in the bottom-tier simulations. This study highlights the importance of reproducing the year-to-year background states for the simulations of MJO propagation in the models by further decomposing the background states into the climatology and anomaly components. The background meridional moisture gradient accounting for the IO-MJO occurrence frequency is closely related to its climatology component; however, the anomaly component regulated by El Niño–Southern Oscillation (ENSO) is also important for the WP-MJO occurrence frequency. The year-to-year variations of background zonal and meridional gradients associated with ENSO account for the IO-MJO occurrence frequency tend to be offset from each other. As a result, ENSO shows no significant impact on the IO-MJO occurrence frequency. However, the MJO events are more likely to propagate across the western Pacific during El Niño years.

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Jingfang Fan, Jun Meng, Josef Ludescher, Zhaoyuan Li, Elena Surovyatkina, Xiaosong Chen, Jürgen Kurths, and Hans Joachim Schellnhuber

Abstract

Despite the development of sophisticated statistical and dynamical climate models, a relative long-term and reliable prediction of the Indian summer monsoon rainfall (ISMR) has remained a challenging problem. Toward achieving this goal, here we construct a series of dynamical and physical climate networks based on the global near-surface air temperature field. We show that some characteristics of the directed and weighted climate networks can serve as efficient long-term predictors for ISMR forecasting. The developed prediction method produces a forecasting skill of 0.54 (Pearson correlation) with a 5-month lead time by using the previous calendar year’s data. The skill of our ISMR forecast is better than that of operational forecasts models, which have, however, quite a short lead time. We discuss the underlying mechanism of our predictor and associate it with network–ENSO and ENSO–monsoon connections. Moreover, our approach allows predicting the all-India rainfall, as well as the rainfall different homogeneous Indian regions, which is crucial for agriculture in India. We reveal that global warming affects the climate network by enhancing cross-equatorial teleconnections between the southwest Atlantic, the western part of the Indian Ocean, and the North Asia–Pacific region, with significant impacts on the precipitation in India. A stronger connection through the chain of the main atmospheric circulations patterns benefits the prediction of the amount of rainfall. We uncover a hotspot area in the midlatitude South Atlantic, which is the basis for our predictor, the southwest Atlantic subtropical index (SWAS index). Remarkably, the significant warming trend in this area yields an improvement of the prediction skill.

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Oscar Chimborazo, Justin R. Minder, and Mathias Vuille

Abstract

Many mountain regions around the world are exposed to enhanced warming when compared to their surroundings, threatening key environmental services provided by mountains. Here we investigate this effect, known as elevation-dependent warming (EDW), in the Andes of Ecuador, using observations and simulations with the Weather Research and Forecasting (WRF) Model. EDW is discernible in observations of mean and maximum temperature in the Andes of Ecuador, but large uncertainties remain due to considerable data gaps in both space and time. WRF simulations of present-day (1986–2005) and future climate (RCP4.5 and RCP8.5 for 2041–60) reveal a very distinct EDW signal, with different rates of warming on the eastern and western slopes. This EDW effect is the combined result of multiple feedback mechanisms that operate on different spatial scales. Enhanced upper-tropospheric warming projects onto surface temperature on both sides of the Andes. In addition, changes in the zonal mean midtropospheric circulation lead to enhanced subsidence and warming over the western slopes at high elevation. The increased subsidence also induces drying, reduces cloudiness, and results in enhanced net surface radiation receipts, further contributing to stronger warming. Finally, the highest elevations are also affected by the snow-albedo feedback, due to significant reductions in snow cover by the middle of the twenty-first century. While these feedbacks are more pronounced in the high-emission scenario RCP8.5, our results indicate that high elevations in Ecuador will continue to warm at enhanced rates in the twenty-first century, regardless of emission scenario.

Significance Statement

Mountains are often projected to experience stronger warming than their surrounding lowlands going forward, a phenomenon known as elevation-dependent warming (EDW), which can threaten high-altitude ecosystems and lead to accelerated glacier retreat. We investigate the mechanisms associated with EDW in the Andes of Ecuador using both observations and model simulations for the present and the future. A combination of factors amplify warming at mountain tops, including a stronger warming high in the atmosphere, reduced cloudiness, and a reduction of snow and ice at high elevations. The latter two factors also favor enhanced absorption of sunlight, which promotes warming. The degree to which this warming is enhanced at high elevations in the future depends on the greenhouse gas emission pathway.

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