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Jun Ying, Tao Lian, Ping Huang, Gang Huang, Dake Chen, and Shangfeng Chen

Abstract

The surface heat flux anomalies during El Niño events have always been treated as an atmospheric response to sea surface temperature anomalies (SSTAs). However, whether they play roles in the formation of SSTAs remains unclear. In this study, we find that the surface net heat flux anomalies in different El Niño types have different effects on the development of the spatial pattern of SSTAs. By applying the fuzzy clustering method, El Niño events during 1982–2018 are classified into two types: 1) extreme El Niños with strong positive SSTAs, with the largest SSTAs in the eastern equatorial Pacific, and 2) moderate El Niños with moderate positive SSTAs, with the largest SSTAs in the central equatorial Pacific. The surface net heat flux anomalies in extreme El Niños generally display a “larger warming gets more damping” zonal paradigm, and essentially do not impact the formation of the spatial pattern of SSTAs. Those in moderate El Niños, however, can impact the formation of the spatial pattern of SSTAs by producing more damping effects in the eastern than in the central equatorial Pacific, thus favoring the largest SSTAs being confined to the central equatorial Pacific. More damping effects of net heat flux anomalies in the eastern equatorial Pacific in moderate El Niños are contributed by the surface latent heat flux anomalies, which are mainly regulated by the negative relative humidity–SST feedback and the positive wind–evaporation–SST feedback. Therefore, we highlight that these two atmospheric adjustments should be considered during the development of moderate El Niños in order to obtain a comprehensive understanding of the formation of El Niño diversity.

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Chiung-Yin Chang and Malte F. Jansen

Abstract

Although the reconfiguration of the abyssal overturning circulation has been argued to be a salient feature of Earth’s past climate changes, our understanding of the physical mechanisms controlling its strength remains limited. In particular, existing scaling theories disagree on the relative importance of the dynamics in the Southern Ocean versus the dynamics in the basins to the north. In this study, we systematically investigate these theories and compare them with a set of numerical simulations generated from an ocean general circulation model with idealized geometry, designed to capture only the basic ingredients considered by the theories. It is shown that the disagreement between existing theories can be partially explained by the fact that the overturning strengths measured in the channel and in the basin scale distinctly with the external parameters, including surface buoyancy loss, diapycnal diffusivity, wind stress, and eddy diffusivity. The overturning in the reentrant channel, which represents the Southern Ocean, is found to be sensitive to all these parameters, in addition to a strong dependence on bottom topography. By contrast, the basin overturning varies with the integrated surface buoyancy loss rate and diapycnal diffusivity but is mostly unaffected by winds and channel topography. The simulated parameter dependence of the basin overturning can be described by a scaling theory that is based only on basin dynamics.

Open access
Xinyan Zhang and Weixin Xu

Abstract

This study investigates diurnal variations of tropical cyclone precipitation in the northwest Pacific (NWP) region, including the South China Sea (SCS) and adjacent landmasses. Diurnal cycles of TC rainfall show significant land–sea contrasts. The primary peak of areal mean TC rain rate occurs in the early morning [0600 local time (LT)] and the afternoon (1500 LT) over the ocean and land, respectively. Both the total and heavy TC precipitation extend farther inland in the afternoon, while nocturnal heavy TC rain is more confined to the coast. A significant semidiurnal cycle of TC precipitation is observed over the ocean (i.e., a secondary peak near 1800 LT). The diurnal cycle of TC rainfall also depends on precipitation frequency, intensity, and radial distance from the TC center. Over the ocean, although TC precipitation intensity shows a pronounced diurnal cycle, its precipitation frequency exhibits virtually no diurnal variation. Over land, TC precipitation frequency markedly peaks in the afternoon (1500 LT), whereas its precipitation intensity interestingly maximizes in the early morning (0300–0600 LT). Diurnal variations of TC asymmetric rainfall structure are consistent with diurnal changes of vertical wind shear. Over the SCS, maximum precipitation located in the downshear-left quadrant and is the most extensive in the morning. However, this heavy rain area shrinks and shifts downshear-ward in the afternoon, consistent with changes of the magnitude (reduced) and direction (clockwise) of the shear. In contrast, TCs over the open ocean of the NWP have little diurnal variability of precipitation asymmetry, due mainly to a diurnally invariant shear environment.

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Charles W. McMahon, Joseph J. Kuehl, and Vitalii A. Sheremet

Abstract

The dynamics of gap-leaping western boundary currents (e.g., the Kuroshio intrusion, the Loop Current) are explored through rotating table experiments and a numerical model designed to replicate the experimental apparatus. Simplified experimental and numerical models of gap-leaping systems are known to exhibit two dominant states (leaping or penetrating into the gap) as the inertia of the current competes with vorticity constraints (in this case the β effect). These systems are also known to admit multiple states with hysteresis. To advance toward more realistic oceanographic scenarios, recent studies have explored the effects of islands, mesoscale eddies, and variable baroclinic deformation radii on the dynamical system. Here, the effect of throughflow forcing is considered, with particle tracking velocimetry (PTV) used in the laboratory experiments. Mean transport in or out of the gap is found to significantly shift the hysteresis range as well as change its width. Because of these transformations, changes in throughflow can induce transitions in the gap-leaping system when near a critical state (leaping-to-penetrating/penetrating-to-leaping). Results from the study are interpreted within a nonlinear dynamical framework and various properties of the system are explored.

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Michela Biasutti, Rick D. Russotto, Aiko Voigt, and Charles C. Blackmon-Luca

Abstract

The TRACMIP (Tropical Rain Belts with an Annual Cycle and Continent Model Intercomparison Project) ensemble includes slab-ocean aquaplanet controls and experiments with a highly idealized tropical continent, characterized by modified aquaplanet grid cells with increased evaporative resistance, increased albedo, reduced heat capacity, and no ocean heat transport (zero Q-flux). In the annual mean, an equatorial cold tongue develops west of the continent and induces dry anomalies and a split in the oceanic intertropical convergence zone (ITCZ). Ocean cooling is initiated by advection of cold, dry air from the winter portion of the continent; warm, humid anomalies in the summer portion are restricted to the continent by anomalous surface convergence. The surface energy budget suggests that ocean cooling persists and intensifies because of a positive feedback between a colder surface, drier and colder air, reduced downwelling longwave (LW) flux, and enhanced net surface LW cooling (LW feedback). A feedback between wind, evaporation, and SST (so-called WES feedback) also contributes to the establishment and maintenance of the cold tongue. Simulations with a gray-radiation model and simulations that diverge from protocol (with negligible winter cooling) confirm the importance of moist-radiative feedbacks and of rectification effects on the seasonal cycle. This mechanism coupling the continental and oceanic climate might be relevant to the double ITCZ bias. The key role of the LW feedback suggests that the study of interactions between monsoons and oceanic ITCZs requires full-physics models and a hierarchy of land models that considers evaporative processes alongside heat capacity as a defining characteristic of land.

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Preston Spicer, Kelly L. Cole, Kimberly Huguenard, Daniel G. MacDonald, and Michael M. Whitney

Abstract

The mixing of river plumes into the coastal ocean influences the fate of riverborne tracers over the inner shelf, though the relative importance of mixing mechanisms under different environmental conditions is not fully understood. In particular, the contribution to plume mixing from bottom-generated shear stresses, referred to as tidal mixing, is rarely considered important relative to frontal and stratified shear (interfacial) mixing in surface advected plumes. The effect of different mixing mechanisms is investigated numerically on an idealized, tidally pulsed river plume with varying river discharge and tidal amplitudes. Frontal, interfacial, and tidal mixing are quantified via a mixing energy budget to compare the relative importance of each to the overall buoyancy flux over one tide. Results indicate that tidal mixing can dominate the energy budget when the tidal mixing power exceeds that of the input buoyancy flux. This occurs when the nondimensional number, RiER01 (the estuarine Richardson number divided by the mouth Rossby number), is generally less than 1. Tidal mixing accounts for between 60% and 90% of the net mixing when RiER01<1, with the largest contributions during large tides and low discharge. Interfacial mixing varies from 10% to 90% of total mixing and dominates the budget for high discharge events with relatively weaker tides (RiER01>1). Frontal mixing is always less than 10% of total mixing and never dominates the budget. This work is the first to show tidal mixing as an important mixing mechanism in surface advected river plumes.

Open access
Xiaomin Chen, Ming Xue, Bowen Zhou, Juan Fang, Jun A. Zhang, and Frank D. Marks

Abstract

Horizontal grid spacings of numerical weather prediction models are rapidly approaching O(1) km and have become comparable with the dominant length scales of flows in the boundary layer; within such “gray-zones,” conventional planetary boundary layer (PBL) parameterization schemes start to violate basic design assumptions. Scale-aware PBL schemes have been developed recently to address the gray-zone issue. By performing WRF simulations of Hurricane Earl (2010) at subkilometer grid spacings, this study investigates the effect of the scale-aware Shin–Hong (SH) scheme on the tropical cyclone (TC) intensification and structural changes in comparison to the non-scale-aware YSU scheme it is built upon. Results indicate that SH tends to produce a stronger TC with a more compact inner core than YSU. At early stages, scale-aware coefficients in SH gradually decrease as the diagnosed boundary layer height exceeds the horizontal grid spacing. This scale-aware effect is most prominent for nonlocal subgrid-scale vertical turbulent fluxes, in the nonprecipitation regions radially outside of a vortex-tilt-related convective rainband, and from the early stage through the middle of the rapid intensification (RI) phase. Both the scale awareness and different parameterization of the nonlocal turbulent heat flux in SH reduce the parameterized vertical turbulent mixing, which further induces stronger radial inflows and helps retain more water vapor in the boundary layer. The resulting stronger moisture convergence and diabatic heating near the TC center account for a faster inner-core contraction before RI onset and higher intensification rates during the RI period. Potential issues of applying these two PBL schemes in TC simulations and suggestions for improvements are discussed.

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Varvara E. Zemskova, Brian L. White, and Alberto Scotti

Abstract

We present numerical results for an idealized rotating, buoyancy- and wind-forced channel as a simple model for the Southern Ocean branch of the meridional overturning circulation (MOC). Differential buoyancy forcing is applied along the top horizontal surface, with surface cooling at one end (to represent the pole) and surface warming at the other (to represent the equatorial region) and a zonally re-entrant channel to represent the Antarctic Circumpolar Current (ACC). Zonally uniform surface wind forcing is applied with a similar pattern to the westerlies and easterlies with varying magnitude relative to the buoyancy forcing. The problem is solved numerically using a 3D direct numerical simulations (DNS) model based on a finite-volume solver for the Boussinesq Navier–Stokes equations with rotation. The overall dynamics, including large-scale overturning, baroclinic eddying, turbulent mixing, and resulting energy cascades, are studied by calculating terms in the energy budget using the local available potential energy framework. The basic physics of the overturning in the Southern Ocean are investigated at multiple scales and the output from the fully resolved DNS simulations is compared with the results from previous studies of the global (ECCO2) and Southern Ocean eddy-permitting state estimates. We find that both the magnitude and shape of the zonal wind stress profile are important to the spatial pattern of the overturning circulation. However, the available potential energy budget and the diapycnal mixing are not significantly affected by the surface wind stress and are primarily set by the buoyancy forcing at the surface.

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Jing Ming and Jianqi Sun

Abstract

This study investigates the relationship between the central tropical Pacific (CTP) sea surface temperature (SST) and the surface air temperature (SAT) variability unrelated to canonical El Niño–Southern Oscillation (ENSO) over mid–high-latitude Eurasia during boreal summers over the past half-century. The results show that their relationship experienced a decadal shift around the early 1980s. Before the early 1980s, the Eurasian SAT–CTP SST connection was weak; after that time, the relationship became stronger, and the SAT anomalies exhibited a significant wavelike pattern over Eurasia. Such a decadal change in the Eurasian SAT–CTP SST relationship could be attributed to decadal changes in the mean state and variability of CTP SST. The warmer mean state and enhanced SST variability after the early 1980s reinforced the convective activities over the tropical Pacific, leading to significantly anomalous divergence/convergence and Rossby wave sources over the North Pacific. This outcome further excited the wave train propagating along the Northern Hemisphere zonal jet stream to northern Eurasia and then affected the surface heat fluxes and atmospheric circulations over the region, resulting in wavelike SATs over Eurasia. However, during the period before the early 1980s, the CTP SST had a weak impact on the North Pacific atmospheric circulation and was consequently not able to excite the wave train pattern to impact the Eurasian atmospheric circulation and SATs. The physical processes linking the CTP SST and Eurasian SAT are further confirmed by numerical simulations. The results of this study are valuable to understanding the variability of summer Eurasian SATs.

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Will Boyles and Matthias Katzfuss

Abstract

The ensemble Kalman filter (EnKF) is a popular technique for data assimilation in high-dimensional nonlinear state-space models. The EnKF represents distributions of interest by an ensemble, which is a form of dimension reduction that enables straightforward forecasting even for complicated and expensive evolution operators. However, the EnKF update step involves estimation of the forecast covariance matrix based on the (often small) ensemble, which requires regularization. Many existing regularization techniques rely on spatial localization, which may ignore long-range dependence. Instead, our proposed approach assumes a sparse Cholesky factor of the inverse covariance matrix, and the nonzero Cholesky entries are further regularized. The resulting method is highly flexible and computationally scalable. In our numerical experiments, our approach was more accurate and less sensitive to misspecification of tuning parameters than tapering-based localization.

Open access