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Pei-Ning Feng, Hai Lin, Jacques Derome, and Timothy M. Merlis

Abstract

The prediction skill of the North Atlantic Oscillation (NAO) in boreal winter is assessed in the operational models of the WCRP/WWRP Subseasonal-to-Seasonal (S2S) prediction project. Model performance in representing the contribution of different processes to the NAO forecast skill is evaluated. The S2S models with relatively higher stratospheric vertical resolutions (high-top models) are in general more skillful in predicting the NAO than those models with relatively lower stratospheric resolutions (low-top models). Comparison of skill is made between different groups of forecasts based on initial condition characteristics: phase and amplitude of the NAO, easterly and westerly phases of the quasi-biennial oscillation (QBO), warm and cold phases of ENSO, and phase and amplitude of the Madden–Julian oscillation (MJO). The forecasts with a strong NAO in the initial condition are more skillful than with a weak NAO. Those with negative NAO tend to have more skillful predictions than positive NAO. Comparisons of NAO skill between forecasts during easterly and westerly QBO and between warm and cold ENSO show no consistent difference for the S2S models. Forecasts with strong initial MJO tend to be more skillful in the NAO prediction than weak MJO. Among the eight phases of MJO in the initial condition, phases 3–4 and phase 7 have better NAO forecast skills compared with the other phases. The results of this study have implications for improving our understanding of sources of predictability of the NAO. The situation dependence of the NAO prediction skill is likely useful in identifying “windows of opportunity” for subseasonal to seasonal predictions.

Open access
Erin B. Munsell, Scott A. Braun, and Fuqing Zhang

Abstract

This study utilizes brightness temperatures (T bs) observed by the infrared longwave window band (channel 14; 11.2 μm) from the Geostationary Operational Environmental Satellite-16 (GOES-16) to examine the structure of Hurricanes Harvey, Maria, and Michael throughout their lifetimes. During the times leading up to their rapid intensifications (RI), two-dimensional inner-core structures are examined to analyze the strength and location of the developing convection. Moderate vertical wind shear in the environments of Harvey and Michael induced a pronounced convective asymmetry prior to RI, followed by a rapid axisymmetrization that occurred essentially in conjunction with RI. The evolutions of the tropical cyclones’ (TCs’) coldest T bs indicate that the inner-core convective activity began to increase in the 12 h prior to RI onset, primarily in 2–4-h substantial “bursts,” while substantial convection dominated essentially the entirety of the region within 100 km of the surface center within 12 h of the onset of intensification. Azimuthally averaged T b evolutions illustrate the development of each TC’s eye and eyewall, the variability of the radial extent of the central dense overcast associated with the diurnal cycle, as well as details of the evolving convective structures throughout intensification. Hovmöller diagrams of data at constant radii reveal areas of cold T bs propagating around the TCs on time scales of 2–3 h. The examination of these features in a deep-layer shear-relative sense reveals that they initiate primarily downshear of the TCs’ surface centers. As RI is reached, these areas of convection are able to propagate into the upshear quadrants, which helps facilitate the onset of more substantial intensification.

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Yujia You, Mingfang Ting, and Suzana J. Camargo

Abstract

The synoptic low pressure systems (LPSs) formed over the downwind side of the Tibetan Plateau explain a substantial portion of summer rainfall extremes along their paths. Recent studies have found that the total extreme rainfall trend over the East Asian landmass, which features the “south flood–north drought” pattern, can be understood to a great extent by the changes in terrestrial LPSs. Yet, the energy sources fueling these storms and the environmental drivers of their long-term trends remain unclear. Utilizing a probabilistic clustering method, three clusters of terrestrial LPS tracks for the period 1979–2018 are identified. Besides the differences in trajectories that distinguish the clusters into northeastward-migrating and quasi-stationary types, prominent intercluster differences are found in the LPS evolution, energetics, and trends. The Lorenz energetics suggest that while condensational heating is indispensable for all three clusters, the migratory type, which has greater intensity and faster development, is more closely tied to baroclinicity. Nonetheless, the summer baroclinicity alone is not enough to sustain these LPSs as these storms dissipate quickly after propagating out of the humid monsoon region and into the drier extratropics. Over time, occurrences of migratory LPSs decrease, and those of quasi-stationary LPSs increase. Using a Poisson model that links the LPS genesis to local environmental conditions, the decreasing occurrence of migratory LPSs is shown to result from the weakened baroclinicity, whereas the increasing occurrence of quasi-stationary LPSs is primarily driven by enhanced relative humidity and reduced steering flow in the mid-to-lower troposphere over East Asia.

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Samuel Smith, Paul W. Staten, and Jian Lu

Abstract

Models disagree on how much the hydrologic cycle could intensify under climate change. These changes are expected to scale with the Clausius–Clapeyron relation but may locally diverge due in part to the uncertain response of the general circulation, causing the hydrologic cycle to inherit this uncertainty. To identify how the circulation contributes, we link circulation changes to changes in the higher moments of the hydrologic cycle using the novel dynamical framework of the local hydrologic cycle, the portion of the hydrologic cycle driven by moist or dry intrusions. We expand this dynamical framework, developing a closed budget that diagnoses thermodynamic, advective, and overturning contributions to future hydrologic cycle changes. In analyzing these changes for the Community Earth System Model Large Ensemble, we show that overturning is the main dynamic contributor to the tropical and subtropical annual response, consistent with a weakening of this circulation. In the extratropics, we show that advective contributions, likely from storm track changes, dominate the response. We achieve a cleaner separation between dynamic and thermodynamic contributions through a semiempirical scaling, which reveals the robustness of the Clausius–Clapeyron scaling for the local hydrologic cycle. This scaling also demonstrates the slowing of the local hydrologic cycle and how changing subtropical dynamics asymmetrically impact wave breaking and suppress meridional moisture transport. We conclude that dynamic changes in the subtropics are predominantly responsible for the annual, dynamic response in the extratropics and thus a significant contributor to uncertainty in future projections.

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Pier Luigi Vidale, Kevin Hodges, Benoit Vannière, Paolo Davini, Malcolm J. Roberts, Kristian Strommen, Antje Weisheimer, Elina Plesca, and Susanna Corti

Abstract

The role of model resolution in simulating geophysical vortices with the characteristics of realistic tropical cyclones (TCs) is well established. The push for increasing resolution continues, with general circulation models (GCMs) starting to use sub-10-km grid spacing. In the same context it has been suggested that the use of stochastic physics (SP) may act as a surrogate for high resolution, providing some of the benefits at a fraction of the cost. Either technique can reduce model uncertainty, and enhance reliability, by providing a more dynamic environment for initial synoptic disturbances to be spawned and to grow into TCs. We present results from a systematic comparison of the role of model resolution and SP in the simulation of TCs, using EC-Earth simulations from project Climate-SPHINX, in large ensemble mode, spanning five different resolutions. All tropical cyclonic systems, including TCs, were tracked explicitly. As in previous studies, the number of simulated TCs increases with the use of higher resolution, but SP further enhances TC frequencies by ~30%, in a strikingly similar way. The use of SP is beneficial for removing systematic climate biases, albeit not consistently so for interannual variability; conversely, the use of SP improves the simulation of the seasonal cycle of TC frequency. An investigation of the mechanisms behind this response indicates that SP generates both higher TC (and TC seed) genesis rates, and more suitable environmental conditions, enabling a more efficient transition of TC seeds into TCs. These results were confirmed by the use of equivalent simulations with the HadGEM3-GC31 GCM.

Open access
Song Yang, Vincent Lao, Richard Bankert, Timothy R. Whitcomb, and Joshua Cossuth

Abstract

An accurate precipitation climatology is presented for tropical depression (TD), tropical storm (TS), and tropical cyclone (TC) occurrences over oceans using recently released, consistent, and high-quality precipitation datasets from all passive microwave sensors covering 1998–2012 along with the Automated Rotational Center Hurricane Eye Retrieval (ARCHER)-based TC center positions. Impacts with respect to the direction of both TC movement and the 200–850-hPa wind shear on the spatial distributions of TC precipitation are analyzed. The TC eyewall contraction process during its intensification is noted by a decrease in the radius of maximum rain rate with an increase in TC intensity. For global TCs, the maximum rain rate with respect to the direction of TC movement is located in the down-motion quadrants for TD, TS, and category-1–3 TCs, and in a concentric pattern for category-4/5 TCs. A consistent maximum TC precipitation with respect to the direction of the 200–850-hPa wind shear is shown in the downshear left quadrant (DSLQ). With respect to direction of TC movement, spatial patterns of TC precipitation vary with basins and show different features for weak and strong storms. The maximum rain rate is always located in DSLQ for all TC categories and basins, except the Southern Hemisphere basin where it is in the downshear right quadrant. This study not only confirms previously published results on TC precipitation distributions relative to vertical wind shear direction, but also provides a detailed distribution for each TC category and TS, while TD storms display an enhanced rainfall rate ahead of the downshear quadrants.

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John C. King, John Turner, Steve Colwell, Hua Lu, Andrew Orr, Tony Phillips, J. Scott Hosking, and Gareth J. Marshall

Abstract

Commencing in 1956, observations made at Halley Research Station in Antarctica provide one of the longest continuous series of near-surface temperature observations from the Antarctic continent. Since few other records of comparable length are available, the Halley record has been used extensively in studies of long-term Antarctic climate variability and change. The record does not, however, come from a single location but is a composite of observations from a sequence of seven stations, all situated on the Brunt Ice Shelf, that range from around 10 to 50 km in distance from the coast. Until now, it has generally been assumed that temperature data from all of these stations could be combined into a single composite record with no adjustment. Here, we examine this assumption of homogeneity. Application of a statistical changepoint algorithm to the composite record detects a sudden cooling associated with the move from Halley IV to Halley V station in 1992. We show that this temperature step is consistent with local temperature gradients measured by a network of automatic weather stations and with those simulated by a high-resolution atmospheric model. These temperature gradients are strongest in the coastal region and result from the onshore advection of maritime air. The detected inhomogeneity could account for the weak cooling trend seen in the uncorrected composite record. In future, studies that make use of the Halley record will need to account for its inhomogeneity.

Open access
Jingzhuo Wang, Jing Chen, Hanbin Zhang, Hua Tian, and Yining Shi

Abstract

Ensemble forecasting is a method to faithfully describe initial and model uncertainties in a weather forecasting system. Initial uncertainties are much more important than model uncertainties in the short-range numerical prediction. Currently, initial uncertainties are described by the ensemble transform Kalman filter (ETKF) initial perturbation method in Global and Regional Assimilation and Prediction Enhanced System–Regional Ensemble Prediction System (GRAPES-REPS). However, an initial perturbation distribution similar to the analysis error cannot be yielded in the ETKF method of the GRAPES-REPS. To improve the method, we introduce a regional rescaling factor into the ETKF method (we call it ETKF_R). We also compare the results between the ETKF and ETKF_R methods and further demonstrate how rescaling can affect the initial perturbation characteristics as well as the ensemble forecast skills. The characteristics of the initial ensemble perturbation improve after applying the ETKF_R method. For example, the initial perturbation structures become more reasonable, the perturbations are better able to explain the forecast errors at short lead times, and the lower kinetic energy spectrum as well as perturbation energy at the initial forecast times can lead to a higher growth rate of themselves. Additionally, the ensemble forecast verification results suggest that the ETKF_R method has a better spread–skill relationship, a faster ensemble spread growth rate, and a more reasonable rank histogram distribution than ETKF. Furthermore, the rescaling has only a minor impact on the assessment of the sharpness of probabilistic forecasts. The above results all suggest that ETKF_R can be effectively applied to the operational GRAPES-REPS.

Open access
Jun-Chao Yang, Yu Zhang, Ingo Richter, and Xiaopei Lin

Abstract

Moisture transport from the Atlantic to Pacific is important for the basin-scale freshwater budget and the formation of meridional ocean circulation. Although the climatological tropical Atlantic-to-Pacific moisture transport (TAPMORT) has been well investigated, few studies have focused on its variability. Here we investigate the interannual variability of TAPMORT based on the atmospheric reanalysis datasets. The TAPMORT interannual variability is dominated by the variations of transbasin winds across Central America, and peaks in late boreal summer and late boreal winter. 1) In late summer, a developing El Niño and a mature Atlantic Niña set up an interbasin sea surface temperature (SST) gradient that strengthens the low-level jet across Central America and therefore TAPMORT (with weakened TAPMORT for opposite signed events). This process typically occurs from July to September, with a peak in August. 2) In late winter, the strengthened southern North American center of the Pacific–North American (PNA)-like pattern intensifies the TAPMORT variations. Although atmospheric interannual variability dominates these variations, extreme El Niño events are also important for the teleconnections. This process shows a single peak in February, in contrast to the persistent peak in late summer. We further demonstrate that the persistent TAPMORT variability in late summer dominates the moisture divergence over the northwestern tropical Atlantic and modulates freshwater flux there. Thus, our study improves the understanding of how TAPMORT interannual variability and the related interbasin SST gradient regulate the northwestern tropical Atlantic freshwater budget and the related salinity variability.

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Lei Zhang, Weiqing Han, and Zeng-Zhen Hu

Abstract

An unprecedented extreme positive Indian Ocean dipole event (pIOD) occurred in 2019, which has caused widespread disastrous impacts on countries bordering the Indian Ocean, including the East African floods and vast bushfires in Australia. Here we investigate the causes for the 2019 pIOD by analyzing multiple observational datasets and performing numerical model experiments. We find that the 2019 pIOD was triggered in May by easterly wind bursts over the tropical Indian Ocean associated with the dry phase of the boreal summer intraseasonal oscillation, and it was sustained by the local atmosphere–ocean interaction thereafter. During September–November, warm sea surface temperature anomalies (SSTA) in the central-western tropical Pacific Ocean further enhanced the Indian Ocean’s easterly winds, bringing the pIOD to an extreme magnitude. The central-western tropical Pacific warm SSTA was strengthened by two consecutive Madden–Julian oscillation (MJO) events that originated from the tropical Indian Ocean. Our results highlight the important roles of cross-basin and cross-time-scale interactions in generating extreme IOD events. The lack of accurate representation of these interactions may be the root for a short lead time in predicting this extreme pIOD with a state-of-the-art climate forecast model.

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