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Ryan J. Longman, Oliver Elison Timm, Thomas W. Giambelluca, and Lauren Kaiser

Abstract

Undisturbed trade-wind conditions compose the most prevalent synoptic weather pattern in Hawai‘i and produce a distinct pattern of orographic rainfall. Significant total rainfall contributions and extreme events are linked to four types of atmospheric disturbances: cold fronts, kona lows, upper-tropospheric disturbances, and tropical cyclones. In this study, a 20-yr (1990–2010) categorical disturbance time series is compiled and analyzed in relation to daily rainfall over the same period. The primary objective of this research is to determine how disturbances contribute to total wet-season rainfall on the Island of O‘ahu, Hawai‘i. On average, 41% of wet-seasonal rainfall occurs on disturbance days. A total of 17% of seasonal rainfall can be directly attributed to disturbances (after a background signal is removed) and as much as 48% in a single season. The intensity of disturbance rainfall (mm day−1) is a stronger predictor (r 2 = 0.49; p < 0.001) of the total seasonal rainfall than the frequency of occurrence (r 2 = 0.11; p = 0.153). Cold fronts are the most common disturbance type; however, the rainfall associated with fronts that cross the island is significantly higher than rainfall produced from noncrossing fronts. In fact, noncrossing fronts produce significantly less rainfall than under mean nondisturbance conditions 76% of the time. While the combined influence of atmospheric disturbances can account for almost one-half of the rainfall received during the wet season, the primary factor in determining a relatively wet or dry season/year on Oʻahu is the frequency and rainfall intensity of kona low events.

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Tuomas Naakka, Tiina Nygård, and Timo Vihma

Abstract

Atmospheric moisture is a key component in the water cycle and radiative transfer. In this study, a comprehensive picture of air moisture climatology and related physical processes is presented for the first time for the circumpolar area south of 50°S. The results are based on the most modern global reanalysis, ERA5, which manages reasonably well to close the Antarctic water budget. We show that over the ocean transient cyclones have the dominant role in determining moisture conditions, whereas over the continent the moisture conditions are largely affected by the mean circulation. Over the open sea, moisture transport from lower latitudes is an equally important source of moisture compared to the local evaporation, but practically all precipitating moisture over the plateau is provided by the horizontal transport. Over the ocean and continental slopes, southward moisture transport brings warm and moist air masses from lower latitudes, notably increasing atmospheric water vapor and cloud water, and simultaneously decreasing local evaporation over the open sea. On the Antarctic plateau, radiative cooling leads to high relative humidity and causes condensation of moisture especially near the surface, causing a nearly permanent specific humidity inversion layer. As a consequence, dry air masses with extremely low specific humidity are formed. These dry air masses are transported downward from the plateau by katabatic winds, experiencing adiabatic warming. This leads to a decrease in relative humidity and to a downward-directed sensible heat flux, which enable efficient surface evaporation on the coastal slopes and farther over coastal polynyas and leads.

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Ming Zhang, Yonggang Liu, Jian Zhang, and Qin Wen

Abstract

North Africa was green during the mid-Holocene [about 6000 years ago (6 ka)] and emitted much less dust to the atmosphere than in the present day. Here we use a fully coupled atmosphere–ocean general circulation model, CESM1.2.2, to test the impact of dust reduction and greening of the Sahara on the Atlantic meridional overturning circulation (AMOC) during this period. Results show that dust removal leads to a decrease of AMOC by 6.2% while greening of the Sahara with 100% shrub (100% grass) cover causes an enhancement of the AMOC by 6.1% (4.8%). The AMOC is increased by 5.3% (2.3%) when both the dust reduction and green Sahara with 100% shrub (100% grass) are considered. The AMOC changes are primarily due to the precipitation change over the west subtropical North Atlantic, from where the salinity anomaly is advected to the deep-water formation region. Global-mean surface temperature increases by 0.09° and 0.40°C (0.25°C) when global dust is removed and when North Africa and the Arabian region are covered by shrub (grass), respectively, showing a dominating effect of vegetation over dust. The comparison between modeled and reconstructed sea surface temperature is improved when the effect of vegetation is considered. The results may have implications for climate impact of future wetting over North Africa, either through global warming or through building of solar farms and wind farms.

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Samar Minallah and Allison L. Steiner

Abstract

This study evaluates the historical climatology and future changes of the atmospheric water cycle for the Laurentian Great Lakes region using 15 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6). While the models have unique seasonal characteristics in the historical (1981–2010) simulations, common patterns emerge in the midcentury SSP2–4.5 scenario (2041–70), including a prevalent shift in the precipitation seasonal cycle with summer drying and wetter winter and spring months, and a ubiquitous increase in the magnitudes of convective precipitation, evapotranspiration, and moisture inflow into the region. The seasonal cycle of moisture flux convergence is amplified (i.e., the magnitude of winter convergence and summer divergence increases), which is the primary driver of future total precipitation changes. The precipitation recycling ratio is also projected to decline in summer and increase in winter by midcentury, signifying a larger contribution of the regional moisture (via evapotranspiration) to total precipitation in the colder months. Most models (10/15) either do not represent the Great Lakes or have major inconsistencies in how the lakes are simulated both in terms of spatial representation and treatment of lake processes. In models with some lake presence, the contribution of lake grid cells to the regional evapotranspiration magnitude can be more than 50% in winter. In the future, winter months have a larger increase in evaporation over water surfaces than the surrounding land, which corroborates past findings of sensitivity of deep lakes to climate warming and highlights the importance of lake representation in these models for reliable regional hydroclimatic assessments.

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Ziyi Cai, Qinglong You, Fangying Wu, Hans W. Chen, Deliang Chen, and Judah Cohen

Abstract

The Arctic has experienced a warming rate higher than the global mean in the past decades, but previous studies show that there are large uncertainties associated with future Arctic temperature projections. In this study, near-surface mean temperatures in the Arctic are analyzed from 22 models participating in phase 6 of the Coupled Model Intercomparison Project (CMIP6). Compared with the ERA5 reanalysis, most CMIP6 models underestimate the observed mean temperature in the Arctic during 1979–2014. The largest cold biases are found over the Greenland Sea the Barents Sea, and the Kara Sea. Under the SSP1-2.6, SSP2-4.5, and SSP5-8.5 scenarios, the multimodel ensemble mean of 22 CMIP6 models exhibits significant Arctic warming in the future and the warming rate is more than twice that of the global/Northern Hemisphere mean. Model spread is the largest contributor to the overall uncertainty in projections, which accounts for 55.4% of the total uncertainty at the start of projections in 2015 and remains at 32.9% at the end of projections in 2095. Internal variability uncertainty accounts for 39.3% of the total uncertainty at the start of projections but decreases to 6.5% at the end of the twenty-first century, while scenario uncertainty rapidly increases from 5.3% to 60.7% over the period from 2015 to 2095. It is found that the largest model uncertainties are consistent cold bias in the oceanic regions in the models, which is connected with excessive sea ice area caused by the weak Atlantic poleward heat transport. These results suggest that large intermodel spread and uncertainties exist in the CMIP6 models’ simulation and projection of the Arctic near-surface temperature and that there are different responses over the ocean and land in the Arctic to greenhouse gas forcing. Future research needs to pay more attention to the different characteristics and mechanisms of Arctic Ocean and land warming to reduce the spread.

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Jingyi Li, Fei Li, Shengping He, Huijun Wang, and Yvan J Orsolini

Abstract

The Tibetan Plateau (TP), referred to as the “Asian water tower,” contains one of the largest land ice masses on Earth. The local glacier shrinkage and frozen-water storage are strongly affected by variations in surface air temperature over the TP (TPSAT), especially in springtime. This study reveals that the relationship between the February North Atlantic Oscillation (NAO) and March TPSAT is unstable with time and regulated by the phase of the Atlantic multidecadal variability (AMV). The significant out-of-phase connection occurs only during the warm phase of AMV (AMV+). The results show that during the AMV+, the negative phase of the NAO persists from February to March, and is accompanied by a quasi-stationary Rossby wave train trapped along a northward-shifted subtropical westerly jet stream across Eurasia, inducing an anomalous adiabatic descent that warms the TP. However, during the cold phase of the AMV, the negative NAO cannot persist into March. The Rossby wave train propagates along the well-separated polar and subtropical westerly jets, and the NAO–TPSAT connection is broken. Further investigation suggests that the enhanced synoptic eddy and low-frequency flow (SELF) interaction over the North Atlantic in February and March during the AMV+, caused by the southward-shifted storm track, helps maintain the NAO pattern via positive eddy feedback. This study provides a new detailed perspective on the decadal variability of the North Atlantic–TP connection in late winter to early spring.

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Ge Song, Bohua Huang, Rongcai Ren, and Zeng-Zhen Hu

Abstract

In this article, the interannual variability of upper-ocean temperature in the equatorial Indian Ocean (IO) and its basinwide connections are investigated using 58-yr (1958–2015) comprehensive monthly mean ocean reanalysis data. Three leading modes of an empirical orthogonal function (EOF) analysis dominate the variability of upper-ocean temperature in the equatorial IO over a wide range of time scales. A coherent interannual band within the first two EOF modes identifies an oscillation between the zonally tilting thermocline across the equatorial IO in its peak phases and basinwide displacement of the equatorial thermocline in its transitional phases. Consistent with the recharge oscillation paradigm, this oscillation is inherent in the equatorial IO with a quasi-periodicity around 15 months, in which the wind-induced off-equatorial Rossby waves near 5°–10°S provide the phase-transition mechanism. This intrinsic IO oscillation provides the biennial component in the observed IOD variations. The third leading mode shows a nonlinear long-term trend of the upper-ocean temperature, including the near-surface warming along the equatorial Indian Ocean, accompanied by cooling trend in the lower thermocline originating farther south. Such vertical contrary trends may lead to an enhanced stratification in the equatorial IO.

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Arjun Jagannathan, Kaushik Srinivasan, James C. McWilliams, M. Jeroen Molemaker, and Andrew L. Stewart

Abstract

Current–topography interactions in the ocean give rise to eddies spanning a wide range of spatial and temporal scales. The latest modeling efforts indicate that coastal and underwater topography are important generation sites for submesoscale coherent vortices (SCVs), characterized by horizontal scales of O(0.110)km. Using idealized, submesoscale and bottom boundary layer (BBL)-resolving simulations and adopting an integrated vorticity balance formulation, we quantify precisely the role of BBLs in the vorticity generation process. In particular, we show that vorticity generation on topographic slopes is attributable primarily to the torque exerted by the vertical divergence of stress at the bottom. We refer to this as the bottom stress divergence torque (BSDT). BSDT is a fundamentally nonconservative torque that appears as a source term in the integrated vorticity budget and is to be distinguished from the more familiar bottom stress curl (BSC). It is closely connected to the bottom pressure torque (BPT) via the horizontal momentum balance at the bottom and is in fact shown to be the dominant component of BPT in solutions with a well-resolved BBL. This suggests an interpretation of BPT as the sum of a viscous, vorticity-generating component (BSDT) and an inviscid, “flow-turning” component. Companion simulations without bottom drag illustrate that although vorticity generation can still occur through the inviscid mechanisms of vortex stretching and tilting, the wake eddies tend to have weaker circulation, be substantially less energetic, and have smaller spatial scales.

Open access
Annick Terpstra, Ian A. Renfrew, and Denis E. Sergeev

Abstract

Equatorward excursions of cold polar air masses into ice-free regions, so-called cold-air outbreak (CAO) events, are frequently accompanied by the development of severe mesoscale weather features. Focusing on two key regions, the Labrador Sea and the Greenland–Norwegian Seas, we apply objective detection for both CAO events and polar mesoscale cyclones to outline the temporal evolution of CAO events and quantify associated mesoscale cyclogenesis. We introduce a novel metric, the CAO depth, which incorporates both the static stability and the temperature of the air mass. The large-scale atmospheric conditions during the onset of CAO events comprise a very cold upper-level trough over the CAO region and a surface cyclone downstream. As the CAO matures, the cold air mass extends southeastward, accompanied by lower static stability and enhanced surface fluxes. Despite the nearly 20° difference in latitude, CAO events over both regions exhibit similar evolution and characteristics including surface fluxes and thermodynamic structure. About two-thirds of the identified CAO events are accompanied by polar mesoscale cyclogenesis, with the majority of mesoscale cyclones originating inside the cold air masses. Neither the duration nor the maturity of the CAO event seems relevant for mesoscale cyclogenesis. Mesoscale cyclogenesis conditions during CAO events over the Labrador Sea are warmer, moister and exhibit stronger surface latent heat fluxes than their Norwegian Sea counterparts.

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Hidetaka Hirata

Abstract

We examined the climatological features of strong winds associated with extratropical cyclones around Japan during 40 seasons between November and April from 1979/80 to 2018/19 using reanalysis data. Our assessments revealed that the extratropical cyclones caused most of the strong winds around Japan (80%–90%). Notably, the contribution of explosively developing extratropical cyclones is larger (70%–80%). The strong winds are mainly related to the warm conveyor belt (WCB) and cold conveyor belt (CCB) inside the explosive cyclones. Moreover, the strong winds tend to be distributed widely over the southwestern quadrant of the cyclones. This is due to the intensification of the horizontal pressure gradient between the mature cyclones and the Siberian high extending from the Eurasian continent to Japan. We investigated the regionality of strong winds by highlighting the three areas with high frequencies of strong winds: the area around Hokkaido (i.e., the northernmost island of Japan; area A), and the areas around the Japan Sea side (area B) and the Pacific Ocean side (area C) of the main island of Japan. The features of the seasonal change in the frequency of the strong winds differ in each area, which reflects the seasonal change in the activities of the explosive cyclones. Moreover, the CCB, the head of the CCB and WCB, and the CCB and WCB bring the strong winds to areas A, B, and C, respectively. The timing of the appearance of these windstorms during the life cycles of typical cases highlighted in this study is consistent with that observed in Europe.

Open access