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Kai-Chih Tseng, Nathaniel C. Johnson, Eric D. Maloney, Elizabeth A. Barnes, and Sarah B. Kapnick

Abstract

The excitation of the Pacific–North American (PNA) teleconnection pattern by the Madden–Julian oscillation (MJO) has been considered one of the most important predictability sources on subseasonal time scales over the extratropical Pacific and North America. However, until recently, the interactions between tropical heating and other extratropical modes and their relationships to subseasonal prediction have received comparatively little attention. In this study, a linear inverse model (LIM) is applied to examine the tropical–extratropical interactions. The LIM provides a means of calculating the response of a dynamical system to a small forcing by constructing a linear operator from the observed covariability statistics of the system. Given the linear assumptions, it is shown that the PNA is one of a few leading modes over the extratropical Pacific that can be strongly driven by tropical convection while other extratropical modes present at most a weak interaction with tropical convection. In the second part of this study, a two-step linear regression is introduced that leverages a LIM and large-scale climate variability to the prediction of hydrological extremes (e.g., atmospheric rivers) on subseasonal time scales. Consistent with the findings of the first part, most of the predictable signals on subseasonal time scales are determined by the dynamics of the MJO–PNA teleconnection while other extratropical modes are important only at the shortest forecast leads.

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Jiayu Zhang, Ping Huang, Fei Liu, and Shijie Zhou

Abstract

This study investigates what forms the spatial pattern of the amplitude changes in tropical intraseasonal and interannual variability—represented by the two most important variables, precipitation (ΔP′) and circulation (Δω′)—under global warming, based on 24 models from the phase 5 of the Coupled Model Intercomparison Project (CMIP5). Diagnostic analyses reveal that the moisture budget and thermodynamic energy equations related to the ΔP′ and Δω′ proposed separately in previous studies are simultaneously tenable. As a result, we investigate the mechanism for the spatial pattern of Δω′ from the perspective of the moist static energy (MSE) balance mainly considering the positive contribution from vertical advection. Therefore, based on the simplified MSE balance, the spatial pattern of Δω′ can be approximately projected based on three factors: background circulation variability ω′, the vertical gradient of mean-state MSE M¯, and its future change ΔM¯. Under global warming, the middle-level vertical gradient of MSE increases slightly over the Indian Ocean and the Maritime Continent and decreases over the equatorial Pacific where the increase in sea surface temperature (SST) exceeds the tropical mean. The vertical gradient of mean-state MSE is modified by the increase in vertical gradients of moisture and dry static energy (DSE) simultaneously. In short, the change in the vertical gradient of mean-state MSE under global warming can influence the moisture budget and thermodynamic energy balances, resulting in the spatial pattern of ΔP′ and Δω′ at intraseasonal and interannual time scales consequently, mainly determined by the lower boundary moisture condition in the response of the SST change pattern.

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Dylan Oldenburg, Robert C. J. Wills, Kyle C. Armour, LuAnne Thompson, and Laura C. Jackson

Abstract

Ocean heat transport (OHT) plays a key role in climate and its variability. Here, we identify modes of low-frequency North Atlantic OHT variability by applying a low-frequency component analysis (LFCA) to output from three global climate models. The first low-frequency component (LFC), computed using this method, is an index of OHT variability that maximizes the ratio of low-frequency variance (occurring at decadal and longer time scales) to total variance. Lead–lag regressions of atmospheric and ocean variables onto the LFC time series illuminate the dominant mechanisms controlling low-frequency OHT variability. Anomalous northwesterly winds from eastern North America over the North Atlantic act to increase upper ocean density in the Labrador Sea region, enhancing deep convection, which later increases OHT via changes in the strength of the Atlantic meridional overturning circulation (AMOC). The strengthened AMOC carries warm, salty water into the subpolar gyre, reducing deep convection and weakening AMOC and OHT. This mechanism, where changes in AMOC and OHT are driven primarily by changes in Labrador Sea deep convection, holds not only in models where the climatological (i.e., time-mean) deep convection is concentrated in the Labrador Sea, but also in models where the climatological deep convection is concentrated in the Greenland–Iceland–Norwegian (GIN) Seas or the Irminger and Iceland Basins. These results suggest that despite recent observational evidence suggesting that the Labrador Sea plays a minor role in driving the climatological AMOC, the Labrador Sea may still play an important role in driving low-frequency variability in AMOC and OHT.

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Jonathan A. Baker, Andrew J. Watson, and Geoffrey K. Vallis

Abstract

The response of the meridional overturning circulation (MOC) to changes in Southern Ocean (SO) zonal wind forcing and Pacific Ocean basin vertical diffusivity is investigated under varying buoyancy forcings, corresponding to “warm,” “present day,” and “cold” states, in a two-basin general circulation model connected by a southern circumpolar channel. We find that the Atlantic MOC (AMOC) strengthens with increased SO wind stress or diffusivity in the model Pacific, under all buoyancy forcings. The sensitivity of the AMOC to wind stress increases as the buoyancy forcing is varied from a warm to a present-day or cold state, whereas it is most sensitive to the Pacific diffusivity in a present-day or warm state. Similarly, the AMOC is more sensitive to buoyancy forcing over the Southern Ocean under reduced wind stress or enhanced Pacific diffusivity. These results arise because of the increased importance of the Pacific pathway in the warmer climates, giving an increased linkage between the basins and so the opportunity for the diffusivity in the Pacific to affect the overturning in the Atlantic. In cooler states, such as in glacial climates, the two basins are largely decoupled and the wind strength over the SO is the primary determinant of the AMOC strength. Both wind- and diffusively driven upwelling sustain the AMOC in the warmer (present day) state. Changes in SO wind stress alone do not shoal the AMOC to resemble that observed at the last glacial maximum; changes in the buoyancy forcing are also needed to decouple the two basins.

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Zane Martin, Clara Orbe, Shuguang Wang, and Adam Sobel

Abstract

Observational studies show a strong connection between the intraseasonal Madden–Julian oscillation (MJO) and the stratospheric quasi-biennial oscillation (QBO): the boreal winter MJO is stronger, more predictable, and has different teleconnections when the QBO in the lower stratosphere is easterly versus westerly. Despite the strength of the observed connection, global climate models do not produce an MJO–QBO link. Here the authors use a current-generation ocean–atmosphere coupled NASA Goddard Institute for Space Studies global climate model (Model E2.1) to examine the MJO–QBO link. To represent the QBO with minimal bias, the model zonal-mean stratospheric zonal and meridional winds are relaxed to reanalysis fields from 1980 to 2017. The model troposphere, including the MJO, is allowed to freely evolve. The model with stratospheric nudging captures QBO signals well, including QBO temperature anomalies. However, an ensemble of nudged simulations still lacks an MJO–QBO connection.

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Dingwen Zeng and Xing Yuan

Abstract

Persistent drought events that cause serious damage to the economy and environment are usually intensified by the feedback between the land surface and atmosphere. Therefore, reasonably modeling land–atmosphere coupling is critical for skillful prediction of persistent droughts. However, most high-resolution regional climate modeling has focused on the amplification effect of land–atmosphere coupling on local anticyclonic circulation anomalies, while less attention has been paid to the nonlocal influence through altering large-scale atmospheric circulation. Here we investigate how the antecedent land–atmosphere coupling over the area south of Lake Baikal (ASLB) influences the drought events occurring over its downstream region [i.e., Northeast China (NEC)] by using the Weather Research and Forecasting (WRF) Model and a linear baroclinic model (LBM). When the ASLB region is artificially forced to be wet in the WRF simulations during March–May, the surface sensible heating is weakened and results in a cooling anomaly in low level atmosphere during May–July. Consequently, the anticyclonic circulation anomalies over ASLB and NEC are weakened, and the severity of NEC drought during May–July cannot be captured due to the upstream wetting in March–May. In the LBM experiments, idealized atmospheric heating anomaly that mimics the diabatic heating associated with surface wetness is imposed over ASLB, and the quasi-steady response pattern of 500-hPa geopotential height to the upstream wetting is highly consistent with that in the WRF simulation. In addition, the lower-level heating instead of the upper-level cooling makes a major contribution to the high pressure anomaly over NEC. This study implies the critical role of modeling upstream land–atmosphere coupling in capturing downstream persistent droughts.

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Brett Chrisler and Justin P. Stachnik

Abstract

Recent studies have examined moist entropy (ME) as a proxy for moist static energy (MSE) and the relative role of the underlying processes responsible for changes in ME that potentially affect MJO propagation. This study presents an analysis of the intraseasonally varying (ISV) ME anomalies throughout the lifetime of observed MJO events. A climatology of continuing and terminating MJO events is created from an event identification algorithm using common tracking indices including the OLR-based MJO index (OMI), filtered OMI (FMO), real-time multivariate MJO (RMM), and velocity potential MJO (VPM) index. ME composites for all indices show a statistically significant break in the wavenumber-1 oscillation at day 0 for terminating events in nearly all domains except RMM phase 6 and phase 7. The ME tendency is decomposed into horizontal and vertical advection, sensible and latent heat fluxes, and shortwave and longwave radiative fluxes using ERA-Interim data. The relative role of each processes toward the eastward propagation is discussed as well as their effects on MJO stabilization. Statistically significant differences occur for all terms by day −10. A domain sensitivity test is performed where eastward propagation is favored for vertical advection given a larger, asymmetric domain for continuing events. A reduced eastward propagation from vertical advection is evident 2–3 days before similar differences in horizontal advection for terminating events. The importance of horizontal advection for the eastward propagation of the MJO is discussed in addition to the relative destabilization from vertical advection in the convectively suppressed region downstream of future terminating MJOs.

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Lixin Qu, Leif N. Thomas, and Robert D. Hetland

Abstract

This study describes a specific type of critical layer for near-inertial waves (NIWs) that forms when isopycnals run parallel to sloping bathymetry. Upon entering this slantwise critical layer, the group velocity of the waves decreases to zero and the NIWs become trapped and amplified, which can enhance mixing. A realistic simulation of anticyclonic eddies on the Texas–Louisiana shelf reveals that such critical layers can form where the eddies impinge onto the sloping bottom. Velocity shear bands in the simulation indicate that wind-forced NIWs are radiated downward from the surface in the eddies, bend upward near the bottom, and enter critical layers over the continental shelf, resulting in inertially modulated enhanced mixing. Idealized simulations designed to capture this flow reproduce the wave propagation and enhanced mixing. The link between the enhanced mixing and wave trapping in the slantwise critical layer is made using ray tracing and an analysis of the waves’ energetics in the idealized simulations. An ensemble of simulations is performed spanning the relevant parameter space that demonstrates that the strength of the mixing is correlated with the degree to which NIWs are trapped in the critical layers. While the application here is for a shallow coastal setting, the mechanisms could be active in the open ocean as well where isopycnals align with bathymetry.

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Xingchi Wang and Tobias Kukulka

Abstract

Turbulence driven by wind and waves controls the transport of heat, momentum, and matter in the ocean surface boundary layer (OSBL). For realistic ocean conditions, winds and waves are often neither aligned nor constant, for example, when winds turn rapidly. Using a large-eddy simulation (LES) method, which captures shear-driven turbulence (ST) and Langmuir turbulence (LT) driven by the Craik–Leibovich vortex force, we investigate the OSBL response to abruptly turning winds. We design idealized LES experiments in which winds are initially constant to equilibrate OSBL turbulence before abruptly turning 90° either cyclonically or anticyclonically. The transient Stokes drift for LT is estimated from a spectral wave model. The OSBL response includes three successive stages that follow the change in direction. During stage 1, turbulent kinetic energy (TKE) decreases as a result of reduced TKE production. Stage 2 is characterized by TKE increasing, with TKE shear production recovering and exceeding TKE dissipation. Transient TKE levels may exceed their stationary values because of inertial resonance and nonequilibrium turbulence. Turbulence relaxes to its equilibrium state at stage 3, but LT still adjusts as a result of slowly developing waves. During stages 1 and 2, greatly misaligned wind and waves lead to Eulerian shear TKE production exceeding Stokes drift shear TKE production. A Reynolds stress budget analysis and Reynolds-averaged Navier–Stokes equation models indicate that Stokes drift shear production furthermore drives the OSBL response. The Coriolis effects result in asymmetrical OSBL responses to wind turning directions. Our results suggest that transient wind conditions play a key role in understanding realistic OSBL dynamics.

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Xu Zhang, Yuhua Yang, Baode Chen, and Wei Huang

Abstract

The quantitative precipitation forecast in the 9-km operational modeling system (without the use of a convection parameterization scheme) at the Shanghai Meteorological Service (SMS) usually suffers from excessive precipitation at the grid scale and less-structured precipitation patterns. Two scale-aware convection parameterizations were tested in the operational system to mitigate these deficiencies. Their impacts on the warm-season precipitation forecast over China were analyzed in case studies and two-month retrospective forecasts. The results from case studies show that the importance of convection parameterization depends on geographical regions and weather regimes. Considering a proper magnitude of parameterized convection can produce more realistic precipitation distribution and reduce excessive gridscale precipitation in southern China. In northeast and southwest China, however, the convection parameterization plays an insignificant role in precipitation forecast because of strong synoptic-scale forcing. A statistical evaluation of the two-month retrospective forecasts indicates that the forecast skill for precipitation in the 9-km operational system is improved by choosing proper convection parameterization. This study suggests that improvement in contemporary convection parameterizations is needed for their usage for various meteorological conditions and reasonable partitioning between parameterized and resolved convection.

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