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Kevin Boyd
,
Zhuo Wang
, and
John E. Walsh

Abstract

Polar lows (PLs) are intense maritime mesocyclones that typically develop during marine cold-air outbreak events over the high latitudes. The impacts posed by these systems to humans and the broader environment demand a robust understanding of the environmental factors that promote PL formation and, in turn, skillful prediction of PL activity. We hypothesize that the variability of PL activity is associated with some key large-scale climate variables skewed toward “extreme” values, which can provide predictable information on PL activity beyond the synoptic time scale. A PL genesis potential index (PGI) is developed that relates the climatological spatial distribution of PL genesis frequency and key climate variables in a Poisson regression framework. The optimal set of predictors consists of a static stability parameter and an environmental baroclinicity parameter. The optimal predictor categories are shown to be robust across different reanalyses and PL track datasets. The observed spatial distribution and seasonal cycle of PL genesis frequency are represented well by the PGI, and the interannual variability of PL activity is captured skillfully. The effects of the Arctic Oscillation (AO), El Niño–Southern Oscillation (ENSO), and a few other climate modes on the interannual variability of PL activity are explored. Overall, our results suggest that the PGI may be used to inform skillful subseasonal to seasonal prediction of PL activity.

Significance Statement

Polar lows are intense mesocyclones over high-latitude oceans, and they have destructive impacts on coastal and island communities, and maritime and air operations. However, skillful prediction of polar lows on the subseasonal and longer time scales remains challenging. This study links polar low activity to large-scale environmental conditions in the Arctic through a statistical modeling approach. This work is based on the hypothesis that a shared statistical relationship exists between the large-scale climate variables and polar low activity across the Arctic, which enables a geographical unification of the controlling factors on polar low activity. Our results reveal two dominant factors, one related to the lower-tropospheric stratification and the other to the hydrodynamic instability of the lower-tropospheric flow. This statistical framework has potential applications to climate prediction and projection of polar low activity.

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Neelesh Rampal
,
Andrew Lorrey
, and
Nicolas Fauchereau

Abstract

Weather regimes (WRs), also known as synoptic types, are defined as recurrent patterns that have been used to categorize variability in atmospheric circulation. However, defining the optimal number of patterns can often be arbitrary, and there are common shortcomings when oversimplifying a wide range of synoptic conditions and weather outcomes. We build on previous work that has defined regional WRs and objectively ascribe an optimal number of once-daily weather patterns for Aotearoa New Zealand (ANZ) using affinity propagation combined with K-means clustering. Nine primary WRs for ANZ were classified based on once-daily geopotential height spatial patterns, but these patterns still retained a wide degree of spatial variability. Subsidiary clusters were subsequently defined within each primary WR by applying affinity propagation and K-means clustering to reveal the largest within-cluster differences based on joint daily temperature and precipitation anomalies. Up to three subsidiary patterns in each of the primary regimes were revealed, with a total of 21 unique daily patterns emerging from the two-tier classification. Subsidiary WRs reveal subtle differences in the location and intensity of regional-scale pressure anomalies, pressure gradients, and wind flow over both main islands that lead to large differences in surface weather anomalies. Impacts of atmospheric variability related to each subsidiary WR are exemplified by different spatial outcomes for rainfall and temperature (including intensity of anomalies) at regional and subregional levels. The approach presented in this study has utility for enhancing prediction of weather outcomes, including extreme weather, and can also be applied more widely over a range of time scales to improve understanding of weather and climate linkages.

Open access
Lauren Vorhees
,
Jane Harrison
,
Michael O’Driscoll
,
Charles Humphrey Jr.
, and
Jared Bowden

Abstract

Nearly one-half of the residents of North and South Carolina use decentralized or onsite wastewater treatment systems (OWTS). As the climate changes, coastal communities relying on OWTS are particularly vulnerable, as soil-based wastewater treatment may be reduced by water inundation from storm surge, sea level rise and associated groundwater rise, and heavy rainfall. Despite the vulnerabilities of OWTS to increased precipitation and sea level rise, there is little known about how onsite wastewater managers are responding to current and future climate risks. We conducted interviews with wastewater operators and installers and health regulators to understand the functioning, management, and regulation of OWTS in the current climate, challenges with rising sea levels and increases in extreme weather events, and what adaptation strategies could be implemented to mitigate negative impacts. Our results indicate that heavy precipitation and storm surges cause malfunctions for conventional septic systems where traditional site variables (e.g., soil type or groundwater level) are undesirable. Weather and climate are not required regulatory factors to consider in system selection and site approval, but many OWTS managers are aware of their impacts on the functioning of systems, and some are preemptively taking action to mitigate those impacts. Our findings suggest that filling gaps in the current communication structure between regulators and homeowners relying on OWTS is critical for coastal communities in the Carolinas to build climate resilience into decentralized wastewater infrastructure.

Significance Statement

This research aims to understand the functioning, management, and regulation of onsite wastewater treatment systems in the current climate, the challenges to these systems caused by rising sea levels and increases in extreme weather events, and the adaptation strategies that can be implemented to mitigate negative climate impacts. These results can be used by state government agencies, municipalities, and private sector wastewater managers to improve the resiliency of onsite wastewater treatment systems.

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Xiaojun Chu
and
Jing Xu

Abstract

Climate change increases the probability and intensity of disaster and brings adverse impacts on social and economic activities. This paper presents the impact of climate risk on the cost of equity capital (COE) and sheds light on the influence mechanisms and moderating factors between climate disaster shocks and the COE in a developing country. We first explain how climate risk represented by drought impacts the COE theoretically. Using the sample data listed in A-share market from 2004 to 2019, we find that drought leads to the rise of the COE due to the deterioration of information environment and the rise of business risk. Specifically, the influence mechanism is tested, and the results show that 1) drought increases firms’ real earnings management 2) and drought has a negative impact on the firms’ return on asset (ROA). Namely, the influence mechanism of drought on the COE is that drought changes the firms’ information environment and business activities. Further analysis shows that the impact of drought on the COE is different in a heterogeneous firm. The drought has a significant impact on the COE in firms with low-ability managers, state-owned enterprises, and politically connected firms, but the impact is not significant in firms with high-ability managers, non-state-owned enterprises, and nonpolitically connected firms. Our research helps people to understand the consequences of climate change from the microeconomic-level firm’s perspective.

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Stuart Jenkins
,
Adam Povey
,
Andrew Gettelman
,
Roy Grainger
,
Philip Stier
, and
Myles Allen

Abstract

Estimates of the anthropogenic effective radiative forcing (ERF) trend have increased by 50% since 2000 (from +0.4 W m−2 decade−1 in 2000–09 to +0.6 W m−2 decade−1 in 2010–19), the majority of which is driven by changes in the aerosol ERF trend, as a result of aerosol emissions reductions. Here we study the extent to which observations of the climate system agree with these ERF assumptions. We use a large ERF ensemble from the IPCC’s Sixth Assessment Report (AR6) to attribute the anthropogenic contributions to global mean surface temperature (GMST), top-of-atmosphere radiative flux, and we use aerosol optical depth observations. The GMST trend has increased from +0.18°C decade−1 in 2000–09 to +0.35°C decade−1 in 2010–19, coinciding with the anthropogenic warming trend rising from +0.19°C decade−1 in 2000–09 to +0.24°C decade−1 in 2010–19. This, as well as observed trends in top-of-atmosphere radiative fluxes and aerosol optical depths, supports the claim of an aerosol-induced temporary acceleration in the rate of warming. However, all three observation datasets additionally suggest that smaller aerosol ERF trend changes are compatible with observations since 2000, since radiative flux and GMST trends are significantly influenced by internal variability over this period. A zero-trend-change aerosol ERF scenario results in a much smaller anthropogenic warming acceleration since 2000 but is poorly represented in AR6’s ERF ensemble. Short-term ERF trends are difficult to verify using observations, so caution is required in predictions or policy judgments that depend on them, such as estimates of current anthropogenic warming trend, and the time remaining to, or the outstanding carbon budget consistent with, 1.5°C warming. Further systematic research focused on quantifying trends and early identification of acceleration or deceleration is required.

Open access
Yueyang Lu
,
Igor Kamenkovich
, and
Pavel Berloff

Abstract

Lateral mesoscale eddy-induced tracer transport is traditionally represented in coarse-resolution models by the flux–gradient relation. In its most complete form, the relation assumes the eddy tracer flux as a product of the large-scale tracer concentration gradient and an eddy transport coefficient tensor. However, several recent studies reported that the tensor has significant spatiotemporal complexity and is not uniquely defined, that is, it is sensitive to the tracer distributions and to the presence of nondivergent (“rotational”) components of the eddy flux. These issues could lead to significant biases in the representation of the eddy-induced transport. Using a high-resolution tracer model of the Gulf Stream region, we examine the diffusive and advective properties of lateral eddy-induced transport of dynamically passive tracers, reevaluate the utility of the flux–gradient relation, and propose an alternative approach based on modeling the local eddy forcing by a combination of diffusion and generalized eddy-induced advection. Mesoscale eddies are defined by a scale-based spatial filtering, which leads to the importance of new eddy-induced terms, including eddy-mean covariances in the eddy fluxes. The results show that the biases in representing these terms are noticeably reduced by the new approach. A series of targeted simulations in the high-resolution model further demonstrates that the approach outperforms the flux–gradient model in reproducing the stirring and dispersing effect of eddies. Our study indicates potential to upgrade the traditional flux–gradient relation for representing the eddy-induced tracer transport.

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Chen Zhao
,
Tim Li
, and
Mingyu Bi

Abstract

The Advanced version of the Weather Research and Forecasting (WRF-ARW) Model is used to investigate the influence of an easterly wave (EW) on the genesis of Typhoon Hagupit (2008) in the western North Pacific. Observational analysis indicates that the precursor disturbance of Typhoon Hagupit (2008) is an easterly wave (EW) in the western North Pacific, which can be detected at least 7 days prior to the typhoon genesis. In the control experiment, the genesis of the typhoon is well captured. A sensitivity experiment is conducted by filtering out the synoptic-scale (3–8-day) signals associated with the EW. The absence of the EW eliminates the typhoon genesis. Two mechanisms are proposed regarding the effect of the EW on the genesis of Hagupit. First, the background cyclonic vorticity of the EW could induce the small-scale cyclonic vorticities to merge and develop into a system-scale vortex. Second, the EW provides a favorable environment in situ for the rapid development of the typhoon disturbance through a positive moisture–convection feedback.

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L. Mahrt
,
Erik Nilsson
, and
Anna Rutgersson

Abstract

We analyze approximately four years of heat-flux measurements at two levels, profiles of air temperature, and multiple measurements of the water temperature collected at a coastal zone site. Our analysis considers underestimation of the sea surface flux resulting from vertical divergence of the heat flux between the surface and the lowest flux level. We examine simple relationships of the heat flux to the wind speed and stratification and the potential influence of fetch and temperature advection. The fetch ranges from about 4 to near 400 km. For a given wind-direction sector, the transfer coefficient varies only slowly with increasing instability but decreases significantly with increasing stability. The intention here is not to recommend a new parameterization but rather to establish relationships that underlie the bulk formula that could lead to assessments of uncertainty and improvement of the bulk formula.

Significance Statement

The behavior of surface heat fluxes in the coastal zone is normally more complex than over the open ocean but has a large impact on human activity. Our study examines extensive flux measurements on a tower in the Baltic Sea that allows partitioning of the fluxes according to wind direction without seriously depleting the data for a given wind-direction sector. Because some of the normal assumptions for the usual parameterization are not met, our study examines relationships behind the parameterization of the surface fluxes.

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Amy F. Waterhouse
,
Tyler Hennon
,
Eric Kunze
,
Jennifer A. MacKinnon
,
Matthew H. Alford
,
Robert Pinkel
,
Harper Simmons
,
Caitlin B. Whalen
,
Elizabeth C. Fine
,
Jody Klymak
, and
Julia M. Hummon

Abstract

Internal waves are predominantly generated by winds, tide–topography interactions, and balanced flow–topography interactions. Observations of vertical shear of horizontal velocity (uz , υz ) from lowered acoustic Doppler current profilers (LADCP) profiles conducted during GO-SHIP hydrographic surveys, as well as vessel-mounted sonars, are used to interpret these signals. Vertical directionality of intermediate-wavenumber [ λ z O ( 100 )  m ] internal waves is inferred in this study from rotary-with-depth shears. Total shear variance and vertical asymmetry ratio (Ω), i.e., the normalized difference between downward- and upward-propagating intermediate wavenumber shear variance, where Ω > 0 (<0) indicates excess downgoing (upgoing) shear variance, are calculated for three depth ranges: 200–600 m, 600 m–1000 mab (meters above bottom), and below 1000 mab. Globally, downgoing (clockwise-with-depth in the Northern Hemisphere) exceeds upgoing (counterclockwise-with-depth in the Northern Hemisphere) shear variance by 30% in the upper 600 m of the water column (corresponding to the globally averaged asymmetry ratio of Ω ¯ = 0.13 ), with a near-equal distribution below 600-m depth ( Ω ¯ 0 ). Downgoing shear variance in the upper water column dominates at all latitudes. There is no statistically significant correlation between the global distribution of Ω and internal wave generation, pointing to an important role for processes that redistribute energy within the internal wave continuum on wavelengths of O ( 100 )  m .

Open access
Zhiwu Chen
,
Gengbin Liu
,
Zhiyu Liu
,
Shaomin Chen
,
Huaihao Lu
,
Jiexin Xu
,
Yankun Gong
,
Jieshuo Xie
,
Yinghui He
,
Ju Chen
,
Yunkai He
, and
Shuqun Cai

Abstract

Tide-induced near-inertial internal waves (NIWs) are generated by tide–topography interaction and are energized by internal tides through triadic resonant interaction of internal waves. They are located above topography and could be in close contact with wind-induced NIWs when the topography is a tall ridge, like in the Luzon Strait of the northern South China Sea (SCS). A natural question arises as to whether there is significant interaction between wind- and tide-induced NIWs. By using moored velocity observations, a satellite-tracked surface drifter dataset, and idealized numerical simulations, we find that in the presence of tide-induced NIWs, the wind can inject slightly more near-inertial energy (NIE), while in the presence of wind-induced NIWs, significantly more tidal energy is transferred to NIWs. Thus, wind- and tide-induced NIWs can mutually enhance each other, producing more NIE than a linear superposition of that generated by wind and tide forcing alone. Increasing wind intensity and tidal excursion lead to saturation of NIE enhancement, while a taller ridge leads to stronger enhancement. The high mixed layer NIE near Luzon Strait is mostly generated by the wind, while the mutual enhancement between wind- and tide-induced NIWs can further enhance this pattern. The interaction between wind- and tide-induced NIWs leads to an enhancement of 25% more NIE. If tide-induced NIWs are neglected, as is usually the case in the estimation of NIE, the total NIE will be underestimated by almost 50%. This might imply that tide-induced NIWs are important for the energetics of NIWs in Luzon Strait.

Significance Statement

Near-inertial internal waves (NIWs) usually occupy the most kinetic energy of internal waves and contribute significantly to ocean mixing. Near the surface they are usually generated by wind forcing, but near the bottom they can be generated by geostrophic or tidal flow interacting with topography. Above the tall ridge in Luzon Strait, wind- and tide-induced NIWs are in close contact, leading to potential interactions. It is found that these NIWs can mutually enhance each other, with most of the additional near-inertial energy (NIE) coming from the tides. If tide-induced NIWs are neglected, the total NIE will be underestimated by almost 50%. This suggests that tide-induced NIWs are important for the energetics of NIWs in Luzon Strait.

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