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Shaohua Chen, Haikun Zhao, Philp J. Klotzbach, Graciela B. Raga, Jian Cao, and Chao Wang

Abstract

This study analyzes decadal modulation of trans-basin variability (TBV) on extended boreal summer (May-October) tropical cyclone frequency (TCF) over the western North Pacific (WNP), central-eastern North Pacific (CENP) and North Atlantic (NATL) basins. There are distinct decadal regimes (P1:1979-1997, P2:1998-2008, and P3:2009-2019) with changes in the interannual relationship between TBV and TCF over these three basins. During P1 and P3, there is a significant inter-annual TBV-TCF relationship over the CENP and NATL, but these relationships become insignificant during P2. Changes in the interannual TBV-TCF relationship over the WNP are opposite to those over the CENP and NATL basins, with significant relationship during P2 but insignificant relationship during P1 and P3. Changes in all three basins coincide with decadal changes in large-scale parameters associated with TBV. Consistent basin-wide changes in lower-tropospheric vorticity (vertical wind shear) associated with TBV appear to be largely responsible for changes in total TCF over the NATL (CENP) during P1 and P3. In contrast, a dipole pattern in lower-tropospheric vorticity and vertical wind shear anomalies associated with TBV over the NATL and CENP basins occurs during P2, leading to an insignificant interannual TBV-TCF relationship over the NATL and CENP basins. Over the WNP, a basin-wide consistent distribution of lower-tropospheric vorticity associated with TBV is consistent with changes in total TCF during P2, while a dipole correlation pattern between TBV-associated factors and TCF during P1 and P3 leads to a weak correlation between TBV and WNP TCF. These three distinct observed decadal regimes may be associated with interactions between ENSO and the Pacific Decadal Oscillation on decadal timescales.

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Mustafa Hakkı Aydoğdu, Mehmet Reşit Sevinç, and Mehmet Cançelik

Abstract

In Şanlıurfa, Turkey, agriculture is the most important source of income. This study aimed to determine Şanlıurfa farmers’ willingness to pay for drought adaptation policies and the factors affecting their willingness. The data were obtained from face-to-face surveys with farmers, selected using a simple random sampling method. According to the results, 50.26% perceive a risk of drought, and 35.86% are willing to pay for adaptation policies. Among those willing to pay, the average amount was 22.63 $/ha (1$=5.676 Turkish Lira), while the average for all participants was 13.55 $/ha. This adds up to a total of 14.363 million $/year for Şanlıurfa. This amount is 1.47% of the annual average income of the participants and is thus within their ability to pay. Age, amount of land farmed, education level, experience, and income were factors affecting willingness to pay. Many respondents, however, were unaware of drought adaptation policies. Because there is concern that drought risk is increasing, awareness needs to be increased, for example through extension services. To the best of our knowledge, this is the first study of its kind, and the results may be useful for creating and applying drought adaptation policies in both Turkey and other regions with similar socio-economic characteristics.

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Eric Bembenek, Timothy M. Merlis, and David N. Straub

Abstract

A large fraction of tropical cyclones (TCs) are generated near the intertropical convergence zone (ITCZ), and barotropic instability of the related wind shear has been shown to be an important generation mechanism. The latitudinal position of the ITCZ shifts seasonally and may shift poleward in response to global warming. Aquaplanet GCM simulations have shown TC-generation frequency to vary with position of the ITCZ. These results, and that moisture plays an essential role in the dynamics, motivate the present study on the growth rates of barotropic instability in ITCZ-like zonal wind profiles. Base-state zonal wind profiles are generated by applying a prescribed forcing (representing zonally-averaged latent heat release in the ITCZ) to a shallow-water model. Shifting the latitudinal position of the forcing alters these profiles, with a poleward shift leading to enhanced barotropic instability. Next, an examination of how latent release impacts the barotropic breakdown of these profiles is considered. To do this, moisture is explicitly represented using a tracer variable. Upon supersaturation, precipitation occurs and the related latent heat release is parameterized as a mass transfer out of the dynamically active layer. Whether moisture serves to enhance or reduce barotropic growth rates is found to depend on how saturation humidity is represented. In particular, taking it to be constant or a function of the layer thickness (related to temperature) leads to a reduction, whereas taking it to be a specified function of latitude leads to an enhancement. Simple arguments are given to support the idea that moisture effects should lead to a reduction in the moist shallow water model and that a poleward shift of the ITCZ should lead to an enhancement of barotropic instability.

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Emilien Jolly, Fabio D’Andrea, Gwendal Rivière, and Sebastien Fromang

Abstract

The changes of midlatitude Rossby waves and cold extreme temperature events (cold spells) during warm Arctic winters are analysed using a dry three-level quasi-geostrophic model on the sphere. Two long-term simulations are compared: the first run has the observed wintertime climatology, while the second run includes the composite of the global anomalies associated with the six hottest Arctic winters. A spectral analysis shows a large increase in wave amplitude for near-zero and westward phase speeds and a more moderate decrease for high eastward phase speeds. The increase in low-frequency variability (periods greater than a week) associated with the power shift to slower waves is largely responsible for an increase in mid-latitude long-lasting cold spells. In mid-latitude regions, in presence of a mean warming, that increase in low-frequency variance compensates the increase of the mean temperature, resulting at places in a frequency of cold spells that remains by and large unaltered. In presence of mean cooling, both the increase in variance and the decrease in the mean temperature participate in an increased frequency of cold spells. Sensitivity experiments show that the power shift to slower waves is mainly due to the tropical anomalies that developed during those particular winters and less importantly to changes in the background flow at higher latitudes associated with the Arctic Amplification pattern.

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Ming-Dah Chou, Kyu-Tae Lee, Il-Sung Zo, Wei-Liang Lee, Chein-Jung Shiu, and Joon-Bum Jee

Abstract

A new k-distribution scheme without the assumption of the correlation between the absorption coefficients at different pressures is developed for solar heating due to water vapor and CO2. Grouping of spectral points is based on the observation that radiation at spectral points with a large absorption coefficient is quickly absorbed to heat the stratosphere, and the heating below is attributable to the absorption of the solar radiation at the remaining spectral points. By grouping the spectral points with a large absorption coefficient at low pressures, the range of the absorption coefficient of the remaining spectral points is narrowed, and the k-distribution approximation can be accurately applied to compute solar heating in both the stratosphere and troposphere. Grouping of the spectral points is based on the absorption coefficient at a couple of reference pressures where heating is significant. With a total number of 52 spectral groups in the water vapor and CO2 bands, fluxes and heating rates were calculated for various solar zenith angles in some typical and sampled atmospheres in diverse climatic regimes and seasons. The maximum heating rate difference between the k-distribution and line-by-line calculations is < 0.09 K day-1 for water vapor, and < 0.2 K day-1 for CO2. The difference in the surface radiation is ~ 1.4 W m-2 for water vapor and 0.6 W m-2 for CO2, while it could increase to 2.6 W m-2 due to overlapping absorption. These results can be improved by increasing the number of spectral groups at the expense of computational economy.

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Yi Jin, Xuebin Zhang, John A. Church, and Xianwen Bao

Abstract

Projections of future sea-level changes are usually based on global climate models (GCMs). However, the changes in shallow coastal regions, like the marginal seas near China, cannot be fully resolved in GCMs. To improve regional sea-level simulations, a high-resolution (~8 km) regional ocean model is set up for the marginal seas near China for both the historical (1994-2015) and future (2079-2100) periods under representative concentration pathways (RCPs) 4.5 and 8.5. The historical ocean simulations are evaluated at different spatiotemporal scales, and the model is then integrated for the future period, driven by projected monthly climatological climate change signals from 8 GCMs individually via both surface and open boundary conditions. The downscaled ocean changes derived by comparing historical and future experiments reveal greater spatial details than those from GCMs, e.g., a low dynamic sea level (DSL) centre of -0.15 m in the middle of the South China Sea (SCS). As a novel test, the downscaled results driven by the ensemble mean forcings are almost identical with the ensemble average results from individually downscaled cases. Forcing of the DSL change and increased cyclonic circulation in the SCS are dominated by the climate change signals from the Pacific, while the DSL change in the East China marginal seas is caused by both local atmosphere forcing and signals from the Pacific. The method of downscaling developed in this study is a useful modelling protocol for adaptation and mitigation planning for future oceanic climate changes.

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Fengmin Wu, Wenkai Li, Peng Zhang, and Wei Li

Abstract

Superimposed on a warming trend, Arctic winter surface air temperature (SAT) exhibits substantial interannual variability, whose underlying mechanisms are unclear, especially regarding the role of sea-ice variations and atmospheric processes. Here, atmospheric reanalysis data and idealized atmospheric model simulations are used to reveal the mechanisms by which sea-ice variations and atmospheric anomalous conditions affect interannual variations in wintertime Arctic SAT. Results show that near-surface interannual warming in the Arctic is accompanied by comparable warming throughout large parts of the Arctic troposphere and large-scale anomalous atmospheric circulation patterns. Within the Arctic, changes in large-scale atmospheric circulations due to internal atmospheric variability explain a substantial fraction of interannual variation in SAT and tropospheric temperatures, which lead to an increase in moisture and downward longwave radiation, with the rest likely coming from sea ice-related and other surface processes. Arctic winter sea-ice loss allows the ocean to release more heat and moisture, which enhances Arctic warming; however, this effect on SAT is confined to the ice-retreat area and has a limited influence on large-scale atmospheric circulations.

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Kamal Tewari, Saroj K. Mishra, Anupam Dewan, Abhishek Anand, and In-Sik Kang

Abstract

Earth’s orography profoundly influences its climate and is a major reason behind the zonally asymmetric features observed in the atmospheric circulation. The response of the atmosphere to orographic forcing, when idealized aqua mountains are placed individually and in pairs (180° apart) at different latitudes, is investigated in the present study using a simplified general circulation model. The investigation reveals that the atmospheric response to orography is dependent on its latitudinal position: orographically triggered stationary waves in the mid-latitudes are most energetic compared to the waves generated due to anomalous divergence in the tropics. The impact on precipitation is confined to the latitude of the orography when it is placed near the tropics, but when it is situated at higher latitudes, it also has a significant remote impact on the tropics. In general, the tropical mountains block the easterly flow, resulting in a weakening of the Hadley cells and a local reduction in the total poleward flux transport by the stationary eddies. On the other hand, the mid-latitudinal orography triggers planetary-scale Rossby waves and enhances the poleward flux transport by stationary eddies. The twin mountains experiments, which are performed by placing orography in pairs at different latitudes, show that the energy fluxes, stationary wave, and precipitation pattern are not merely the linear additive sum of individual orographic responses at these latitudes. The non-linearity in a diagnostic sense is a product interaction of flow between the two mountains, which depends on the background flow, the separation distance between mountains, and wind shear worldwide.

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Helene Asbjørnsen, Helen L. Johnson, and Marius Årthun

Abstract

The inflow across the Iceland-Scotland Ridge determines the amount of heat supplied to the Nordic Seas from the subpolar North Atlantic (SPNA). Consequently, variable inflow properties and volume transport at the ridge influence marine ecosystems and sea ice extent further north. Here, we identify the upstream pathways of the Nordic Seas inflow, and assess the mechanisms responsible for interannual inflow variability. Using an eddy-permitting ocean model hindcast and a Lagrangian analysis tool, numerical particles are released at the ridge during 1986-2015 and tracked backward in time. We find an inflow that is well-mixed in terms of its properties, where 64% comes from the subtropics and 26% has a subpolar or Arctic origin. The local instantaneous response to the NAO is important for the overall transport of both subtropical and Arctic-origin waters at the ridge. In the years before reaching the ridge, the subtropical particles are influenced by atmospheric circulation anomalies in the gyre boundary region and over the SPNA, forcing shifts in the North Atlantic Current (NAC) and the subpolar front. An equatorward shifted NAC and westward shifted subpolar front correspond to a warmer, more saline inflow. Atmospheric circulation anomalies over the SPNA also affect the amount of Arctic-origin water re-routed from the Labrador Current toward the Nordic Seas. A high transport of Arctic-origin water is associated with a colder, fresher inflow across the Iceland-Scotland Ridge. The results thus demonstrate the importance of gyre dynamics and wind forcing in affecting the Nordic Seas inflow properties and volume transport.

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Sonia Lasher-Trapp, Enoch Jo, Luke R. Allen, Bryan N. Engelsen, and Robert J. Trapp

Abstract

The current study identifies and quantifies various mechanisms of entrainment, and their diluting effects, in the developing and mature stages of a simulated supercell thunderstorm. The two stages, differentiated by the lack or presence of a rotating updraft, are shown to entrain air by different, but related mechanisms that result from the strong vertical wind shear of the environment. The greatest entrainment rates in the developing stage result from the asymmetric overturning of large eddies near cloud top on the down-shear side. These rates are greater than those published in the literature for cumuli developing in environments lacking strong shear. Although the entrainment rate increases exponentially in time throughout the developing stage, successive cloud turrets help to replenish some of the lost buoyancy and condensate, allowing the nascent storm to develop further. During the mature stage, the greatest entrainment rates occur via “ribbons” of horizontal vorticity wrapping around the rotating updraft that ascend in time. The smaller width of the ribbons in comparison to the wider storm core limits their dilutive effects. Passive tracers placed in the low-level air ingested by the mature storm indicate that on average 20% of the core contains some undiluted air ingested from below the storm base, unaffected by any entrainment mechanism.

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