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Peng Wang
,
James C. McWilliams
,
Dongxiao Wang
, and
Daling Li Yi

Abstract

Upwelling brings deep, cold, and nutrient-rich water to the euphotic zone, enhancing biological primary productivity. Coastal upwelling is affected by various factors, such as winds, topography, and tides. However, it remains unclear how the upwelling is affected by surface waves, particularly the Stokes drift and its related forces, that is, conservative wave effects. Here using a coupled wave–circulation model, we examined how conservative wave effects impact the wind-driven coastal upwelling system over an idealized continental shelf. We showed that conservative wave effects reduce upwelling but enhance downwelling; consequently, the amount of deep cold water brought up to the surface by upwelling is reduced with waves, leading to a weaker upwelling front than that without waves. Conservative wave effects also change the potential vorticity (PV) fluxes across the sea surface/bottom and alter the thickness of surface/bottom negative-PV layers. In addition, conservative wave effects modify the turbulent thermal wind (TTW) associated with the upwelling front, forming a Stokes–TTW balance. Further, we studied sensitivities of the upwelling and downwelling magnitudes to four parameters: wave height, wind stress, shelf slope, and wave incident angle. We combined these parameters into a single nondimensional number that can indicate when conservative wave effects need to be included in the upwelling and downwelling.

Significance Statement

Upwelling is important to the marine ecosystem because it enhances biological primary productivity by bringing nutrient-rich water to the euphotic zone from depths. However, it remains unclear how the upwelling is affected by ubiquitous surface waves. Here using numerical simulations, we showed that Stokes drift and its related forces due to surface waves reduce upwelling but enhance downwelling. It implies that there could be a substantial bias in the estimation of upwelling and downwelling if surface waves are not considered. Further, we proposed a nondimensional number to indicate when surface waves need to be considered in the upwelling and downwelling.

Open access
Hans Burchard
,
Karsten Bolding
,
Xaver Lange
, and
Alexander Osadchiev

Abstract

For Arctic estuaries that are characterized by landfast sea ice cover during the winter season, processes generating estuarine circulation and residual stratification have not yet been investigated, although some of the largest estuaries in the world belong to this class. Landfast sea ice provides a no-slip surface boundary condition in addition to the bottom boundary, such that frictional effects are expected to be increased. For this study of estuarine circulation and residual stratification under landfast sea ice, first, a simple linear analytical model is used. To include tidally varying scenarios, a water-column model is applied with a second-moment turbulence closure to juxtapose free-surface and ice-covered estuaries. Well-mixed and strongly stratified tidally periodic scenarios are analyzed by means of a decomposition of estuarine circulation into contributions from gravitational circulation, eddy viscosity–shear covariance (ESCO), surface stress, and river runoff. A new method is developed to also decompose tidal residual salinity anomaly profiles. Estuarine circulation intensity and tidally residual potential energy anomaly are studied for a parameter space spanned by the Simpson number and the unsteadiness number. These are the major results of this study that will support future scenario studies in Arctic estuaries under conditions of accelerated warming: (i) residual surface drag under ice opposes estuarine circulation; (ii) residual differential advection under ice destabilizes the near-surface flow; (iii) reversal of ESCO during strong stratification does not occur under landfast sea ice; (iv) tidal pumping (s-ESCO) contributes dominantly to residual stratification also with sea ice cover.

Significance Statement

Our work gives a first qualitative and quantitative understanding of how landfast sea ice cover on tidal estuaries impacts on the generation of estuarine circulation and residual stratification. Along the Arctic coasts, where some of the world’s largest estuaries are located, these processes play a significant role for the economy and ecology by means of transports of sediments, nutrients and pollutants. Due to Arctic amplification, the conditions for ice-covered estuaries are strongly changing in a way that the ice-covered periods may be shorter in the future. Our results intend to motivate field observations and realistic model studies to allow for better predicting the consequences of these changes.

Open access
David J. Wiersema
,
Katherine A. Lundquist
,
Jeffrey D. Mirocha
, and
Fotini Katopodes Chow

Abstract

This paper evaluates the representation of turbulence and its effect on transport and dispersion within multiscale and microscale-only simulations in an urban environment. These simulations, run using the Weather Research and Forecasting Model with the addition of an immersed boundary method, predict transport and mixing during a controlled tracer release from the Joint Urban 2003 field campaign in Oklahoma City, Oklahoma. This work extends the results of a recent study through analysis of turbulence kinetic energy and turbulence spectra and their role in accurately simulating wind speed, direction, and tracer concentration. The significance and role of surface heat fluxes and use of the cell perturbation method in the numerical simulation setup are also examined. Our previous study detailed the model development necessary for our multiscale simulations, examined model skill at predicting wind speeds and tracer concentrations, and demonstrated that dynamic downscaling from mesoscale to microscale through a sequence of nested simulations can improve predictions of transport and dispersion relative to a microscale-only simulation forced by idealized meteorology. Here, predictions are compared with observations to assess qualitative agreement and statistical model skill at predicting wind speed, wind direction, tracer concentration, and turbulent kinetic energy at locations throughout the city. We also investigate the scale distribution of turbulence and the associated impact on model skill, particularly for predictions of transport and dispersion. Our results show that downscaled large-scale turbulence, which is unique to the multiscale simulations, significantly improves predictions of tracer concentrations in this complex urban environment.

Significance Statement

Simulations of atmospheric transport and mixing in urban environments have many applications, including pollution modeling for urban planning or informing emergency response following a hazardous release. These applications include phenomena with spatial scales spanning from millimeters to kilometers. Most simulations resolve flow only within the urban area of interest, omitting larger scales of turbulence and regional influences. This study examines a method that resolves both the small and large-scale flow features. We evaluate simulation accuracy by comparing predictions with observations from an experiment involving the release of a tracer gas in Oklahoma City, Oklahoma, with emphasis on correctly modeling turbulent fluctuations. Our results demonstrate the importance of resolving large-scale flow features when predicting transport and dispersion in urban environments.

Open access
Zhenyu You
and
Yi Deng

Abstract

Mesoscale convective systems (MCSs) play a key role in regulating variability in the U.S. water and energy cycle. Here a hierarchical dissection of the multiscale forcing of springtime MCSs is carried out through a two-step classification process. Hierarchical clustering is first applied to spatiotemporally evolving upper-tropospheric height fields to reveal large-scale forcing patterns of MCSs. Five distinct forcing patterns (clusters) are identified with three being “remotely forced” and two associated with “local growth.” The upper-level troughs associated with these forcing patterns create broad envelopes downstream within which large-scale ascent and MCS genesis tend to occur. Further classification of MCSs based on MCS track locations reveals that local dynamic and thermodynamic forcing determines the precise locations of MCS genesis in the envelope created by large-scale forcing. Specifically, MCSs often occur near surface fronts in warm sectors of surface low pressure systems and are accompanied by low-level kinematic and moisture convergence driven by low-level jets (LLJs). Nearly 50% of spring MCSs are associated with potential instability realized through frontal lifting, and the highest probability of MCS genesis is seen with an environmental CAPE of ∼1400 J kg−1 and CIN of ∼150 J kg−1. The positive trend of the U.S. MCS genesis frequency observed in recent decades is found to be driven by the cluster of MCSs forced at large scale by the Pacific storm track. Regression analysis further suggests that the growing phase of the Pacific decadal oscillation (PDO) modulates the associated MCS large-scale forcing and is ultimately responsible for the positive MCS trend.

Significance Statement

The purpose of this study is to provide a systematic classification of multiscale forcing factors triggering mesoscale convective system development over the United States. These storms are very active in spring and often lead to intense rainfall and other weather hazards such as lightning, hail, and tornadoes. They play a key role in the U.S. hydrological cycle and have been occurring more frequently over the past several decades. Our study reveals the detailed characteristics of atmospheric forcing leading to these storms. Such information lays theoretical grounds for designing prediction schemes of warm season severe weather and provides guidance for model development to improve climate models’ simulation and long-term projection of these storms.

Restricted access
Youtong Zheng
and
Yi Ming

Abstract

Interpreting behaviors of low-level clouds (LLCs) in a climate model is often not straightforward. This is particularly so over polar oceans where frozen and unfrozen surfaces coexist, with horizontal winds streaming across them, shaping LLCs. To add clarity to this interpretation issue, we conduct budget analyses of LLCs using a global atmosphere model with a fully prognostic cloud scheme. After substantiating the model’s skill in reproducing observed LLCs, we use the modeled budgets of cloud fraction and water content to elucidate physics governing changes of LLCs across sea ice edges. Contrasting LLC regimes between open water and sea ice are found. LLCs over sea ice are primarily maintained by large-scale condensation: intermittent intrusions of maritime humid air and surface radiative cooling jointly sustain high relative humidity near the surface, forming extensive but tenuous stratus. This contrasts with the LLCs over open water where the convection and boundary layer condensation sustain the LLCs on top of deeper boundary layers. Such contrasting LLC regimes are influenced by the direction of horizontal advection. During on-ice flow, large-scale condensation dominates the regions, both open water and sea ice regions, forming clouds throughout the lowest several kilometers of the troposphere. During off-ice flow, as cold air masses travel over the open water, the cloud layer lifts and becomes denser, driven by increased surface fluxes that generate LLCs through boundary layer condensation and convective detrainment. These results hold in all seasons except summer when the atmosphere–surface decoupling substantially reduces the footprints of surface type changes.

Open access
Xueli Yin
,
Dongliang Yuan
,
Xiang Li
,
Zheng Wang
,
Yao Li
,
Corry Corvianawatie
,
Adhitya Kusuma Wardana
,
Dewi Surinati
,
Adi Purwandana
,
Mochamad Furqon Azis Ismail
,
Asep Sandra Budiman
,
Ahmad Bayhaqi
,
Praditya Avianto
,
Edi Kusmanto
,
Priyadi Dwi Santoso
,
Dirhamsyah
, and
Zainal Arifin

Abstract

The mean circulation and volume budgets in the upper 1200 m of the Maluku Sea are studied using multiyear current meter measurements of four moorings in the Maluku Channel and of one synchronous mooring in the Lifamatola Passage. The measurements show that the mean current in the depth range of 60–450 m is northward toward the Pacific Ocean with a mean transport of 2.07–2.60 Sv (1 Sv ≡ 106 m3 s−1). In the depth range of 450–1200 m, a mean western boundary current (WBC) flows southward through the western Maluku Sea and connects with the southward flow in the Lifamatola Passage. The mean currents in the central-eastern Maluku Channel are found to flow northward at this depth range, suggesting an anticlockwise western intensified gyre circulation in the middle layer of the Maluku Sea. Budget analyses suggest that the mean transport of the intermediate WBC is 1.83–2.25 Sv, which is balanced by three transports: 1) 0.62–0.93 Sv southward transport into the Seram–Banda Seas through the Lifamatola Passage, 2) 0.97–1.01 Sv returning to the western Pacific Ocean through the central-eastern Maluku Channel, and 3) a residual transport surplus, suggested to upwell to the upper layer joining the northward transport into the Pacific Ocean. The dynamics of the intermediate gyre circulation are explained by the potential vorticity (PV) integral constraint of a semienclosed basin.

Significance Statement

The Indonesian Throughflow plays an important role in the global ocean circulation and climate variations. Existing studies of the Indonesian Throughflow have focused on the upper thermocline currents. Here we identify, using mooring observations, an intermediate western boundary current with the core at 800–1000-m depth in the Maluku Sea, transporting intermediate waters from the Pacific into the Seram–Banda Seas through the Lifamatola Passage. Potential vorticity balance suggests an anticlockwise gyre circulation in the intermediate Maluku Sea, which is evidenced by the mooring and model data. Transport estimates suggest northward countercurrent in the upper Maluku Sea toward the Pacific, supplied by the Lifamatola Passage transport and upwelling from the intermediate layer in the Maluku Sea. Our results suggest the importance of the intermediate Indonesian Throughflow in global ocean circulation and overturn. More extensive investigations of the Indo-Pacific intermediate ocean circulation should be conducted to improve our understanding of global ocean overturn and heat and CO2 storages.

Restricted access
Lisa Bengtsson
,
Luc Gerard
,
Jongil Han
,
Maria Gehne
,
Wei Li
, and
Juliana Dias

Abstract

A prognostic closure is introduced to, and evaluated in, NOAA’s Unified Forecast System. The closure addresses aspects that are not commonly represented in traditional cumulus convection parameterizations, and it departs from the previous assumptions of a negligible subgrid area coverage and statistical quasi-equilibrium at steady state, the latter of which becomes invalid at higher resolution. The new parameterization introduces a prognostic evolution of the convective updraft area fraction based on a moisture budget, and, together with the buoyancy-driven updraft vertical velocity, it completes the cloud-base mass flux. In addition, the new closure addresses stochasticity and includes a representation of subgrid convective organization using cellular automata as well as scale-adaptive considerations. The new cumulus convection closure shows potential for improved Madden–Julian oscillation (MJO) prediction. In our simulations we observe better propagation, amplitude, and phase of the MJO in a case study relative to the control simulation. This improvement can be partly attributed to a closer coupling between low-level moisture flux convergence and precipitation as revealed by a space–time coherence spectrum. In addition, we find that enhanced organization feedback representation and stochastic effects, represented using cellular automata, further enhance the amplitude and propagation of the MJO, and they provide realistic uncertainty estimates of convectively coupled equatorial waves at seasonal time scales. The scale-adaptive behavior of the scheme is also studied by running the global model with 25-, 13-, 9-, and 3-km grid spacing. It is found that the convective area fraction and the convective updraft velocity are both scale adaptive, leading to a reduction of subgrid convective precipitation in the higher-resolution simulations.

Restricted access
John P. O’Brien
and
Clara Deser

Abstract

While much attention has been given to understanding how anthropogenic radiative forcing influences the mean state of the climate system, far less scrutiny has been paid to how it may modulate naturally occurring modes of variability. This study investigates forced changes to unforced modes of wintertime atmospheric circulation variability and associated impacts on precipitation over the North Pacific and adjacent regions based on the 40-member Community Earth System Model version 1 Large Ensemble across the 1920–2100 period. Each simulation is subject to the same radiative forcing protocol but starts from a slightly different initial condition, leading to different sequences of internal variability. Evolving forced changes in the amplitude and spatial character of the leading internal modes of 500-hPa geopotential height variability are determined by applying empirical orthogonal function analysis across the ensemble dimension at each time step. The results show that the leading modes of internal variability intensify and expand their region of influence in response to anthropogenic forcing, with concomitant impacts on precipitation. Linkages between the Pacific and Atlantic sectors, and between the tropics and extratropics, are also enhanced in the future. These projected changes are driven partly by teleconnections from amplified ENSO activity and partly by dynamical processes intrinsic to the extratropical atmosphere. The marked influence of anthropogenic forcing on the characteristics of internal extratropical atmospheric circulation variability presents fundamental societal challenges to future water resource planning, flood control, and drought mitigation.

Open access
Kewei Lyu
,
Xuebin Zhang
,
John A. Church
,
Quran Wu
,
Russell Fiedler
, and
Fabio Boeira Dias

Abstract

A rapid warming and freshening of the Southern Ocean have been observed over the past several decades and are attributed to anthropogenic climate change. In this study, ocean model perturbation experiments are conducted to separate roles of individual surface forcing in the Southern Ocean temperature and salinity changes. Model-based findings are compared with results from a theoretical framework including three idealized processes defined on the θS diagram. Under the future scenario of CO2 doubling, the heat flux forcing dominates the large-scale warming, deepening of isopycnals, and spiciness changes along isopycnals, which can be captured by an idealized pure warming process to represent the subduction of surface heat uptake. The poleward-intensifying westerly winds account for 24% of the enhanced warming between 35° and 50°S and would have comparable contribution as the heat flux forcing after removing the global ocean warming effect. In contrast, the widespread freshening in the Southern Ocean driven by increased surface freshwater input is largely compensated by the wind-driven saltening. The response to freshwater forcing could not be approximated as a similar pure freshening process as the induced cooling and freshening have comparable effects on density. The wind-driven changes are primarily through the local heave of isopycnals, thus resembling an idealized pure heave process, but contain considerable spiciness signals especially in the midlatitude Southern Ocean, resulting from anomalous northward transport and subduction of heat and salt that are largely density-compensating. These distinct signatures of individual surface forcing help us to better understand observed and projected changes in the Southern Ocean.

Significance Statement

Considerable changes including a rapid warming and freshening have been observed in the Southern Ocean as it absorbs most of the extra heat from the anthropogenic climate change, receives increased surface freshwater input, and experiences a poleward shift and intensification of the westerly winds. The purpose of this study is to distinguish different contributions from surface heat flux, freshwater flux, and wind forcing to the Southern Ocean temperature and salinity changes, based on ocean model experiments and three idealized processes from a theoretical framework. Our study reveals distinct signatures of individual surface forcing that help us to understand linkages between changes seen at the surface and in the interior Southern Ocean.

Restricted access
Artur Gevorgyan
,
Luis Ackermann
,
Yi Huang
,
Steven Siems
, and
Michael Manton

Abstract

The case study of a heavy precipitation event associated with the passage of cold front over the Australian Snowy Mountains (ASM) on 3 August 2018 has been examined using the observational data from an intensive field campaign and high-resolution (1 km) Weather Research and Forecasting (WRF) simulation. We divided this event into prefrontal, cold front, and postfrontal periods. The cold front and postfrontal periods were characterized by higher production of graupel, while relatively low graupel was produced in the prefrontal period. Overall, aggregation along with deposition are likely the main growth mechanisms of snow in the prefrontal clouds, while heavy rain was produced below the melting level over windward slopes of the ASM. The simulated melting level is lower compared to the observations, which is consistent with model cold bias. Stronger orographic uplift and frontal forcing were mainly responsible for the enhanced supercooled liquid water (SLW) production over the ASM in the cold front period. A drop in elevation of the freezing level and increase in low-level relative humidity further enhanced the SLW production. The production of graupel through riming processes was highly efficient in the cold front period given the high concentration of ice-phase hydrometeors in the frontal clouds and the development of clouds comprising supercooled liquid water. The orographic updrafts and embedded convection were the main dynamical processes generating postfrontal SLW clouds and graupel. Ice initiation processes were activated once SLW cloud tops reached −15°C level followed by graupel production through riming processes.

Restricted access