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Tsz-Kin Lai, Eric A. Hendricks, M. K. Yau, and Konstantinos Menelaou

Abstract

Intense tropical cyclones (TCs) often experience secondary eyewall formations and the ensuing eyewall replacement cycles. Better understanding of the underlying dynamics is crucial to make improvements to the TC intensity and structure forecasting. Radar imagery of some double-eyewall TCs and a real-case simulation study indicated that the barotropic instability (BI) across the moat (aka type-2 BI) may play a role in inner eyewall decay. A three-dimensional numerical study accompanying this paper pointed out that type-2 BI is able to withdraw the inner eyewall absolute angular momentum (AAM) and increase the outer eyewall AAM through the eddy radial transport of eddy AAM. This paper explores the reason why the eddy radial transport of eddy AAM is intrinsically nonzero. Linear and nonlinear shallow water experiments are performed and they produce expected evolutions under type-2 BI. It will be shown that only nonlinear experiments have changes in AAM over the inner and outer eyewalls, and the changes solely originate from the eddy radial transport of eddy AAM. This result highlights the importance of nonlinearity of type-2 BI. Based on the distribution of vorticity perturbations and the balanced-waves arguments, it will be demonstrated that the nonzero eddy radial transport of eddy AAM is an essential outcome from the intrinsic interaction between the mutually growing vortex Rossby waves across the moat under type-2 BI. The analyses of the most unstable mode support the findings and will further attribute the inner eyewall decay and outer eyewall intensification to the divergence and convergence of the eddy angular momentum flux, respectively.

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Yen-Ting Hwang and Po-Chun Chung

Abstract

This study explores the seasonal sensitivity of tropical circulation responses to an idealized extratropical thermal forcing using the Community Atmosphere Model version 5 coupled to a slab ocean. The thermal heating over the Southern Ocean is held constant, and the tropical responses in each month of the year are investigated. An anomalous cross-equatorial cell and a southward tropical rain belt shift occur every month. The anomalous cross-equatorial cell has a strong influence on the strengths of the Hadley cell and the subtropical jet in the winter hemisphere; in contrast, it has nearly no impact on the Hadley cell and the subtropical jet strengths in the summer hemisphere. The seasonal variation of the anomalous cross-equatorial cell is small (30% of the annual mean change), and could be understood via the energetic and the sea surface temperature gradient perspectives. Both perspectives point to the seasonality of the anomalous ocean heat uptake within the deep tropics as the key factor explaining the weak seasonality of the anomalous cross-equatorial cell. We propose a hypothesis explaining about 75% of this seasonal variation via the climatological position of the ITCZ relative to the anomalous cross-equatorial cell. The results suggest a modest seasonality in tropical precipitation and circulation responses to extratropical forcing. Also, such seasonality may be partly predicted by the climatological seasonal cycle of the tropical circulations.

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V. Krishnamurthy, Jessica Meixner, Lydia Stefanova, Jiande Wang, Denise Worthen, Shrinivas Moorthi, Bin Li, Travis Sluka, and Cristiana Stan

Abstract

The predictability of the Unified Forecast System (UFS) Coupled Model Prototype 2 developed by the National Centers for Environmental Prediction is assessed for the boreal summer over the continental United States (CONUS). The retrospective forecasts of low-level horizontal wind, precipitation and 2-m temperature for 2011–17 are examined to determine the predictability at subseasonal time scale. Using a data-adaptive method, the leading modes of variability are obtained and identified to be related to El Niño–Southern Oscillation (ENSO), intraseasonal oscillation (ISO), and warming trend. In a new approach, the sources of enhanced predictability are identified by examining the forecast errors and correlations in the weekly averages of the leading modes of variability. During the boreal summer, the ISO followed by the trend in UFS are found to provide better predictability in weeks 1–4 compared to the ENSO mode and the total anomaly. The western CONUS seems to have better predictability on weekly time scale in all three modes.

Open access
Motoki Nagura

Abstract

This study investigates spreading and generation of spiciness anomalies of the Subantarctic Mode Water (SAMW) located on 26.6 to 26.8 σ θ in the south Indian Ocean, using in situ hydrographic observations, satellite measurements, reanalysis datasets, and numerical model output. The amplitude of spiciness anomalies is about 0.03 psu or 0.13°C and tends to be large along the streamline of the subtropical gyre, whose upstream end is the outcrop region south of Australia. The speed of spreading is comparable to that of the mean current, and it takes about a decade for a spiciness anomaly in the outcrop region to spread into the interior up to Madagascar. In the outcrop region, interannual variability in mixed layer temperature and salinity tends to be density compensating, which indicates that Eulerian temperature or salinity changes account for the generation of isopycnal spiciness anomalies. It is known that wintertime temperature and salinity in the surface mixed layer determine the temperature and salinity relationship of a subducted water mass. Considering this, the mixed layer heat budget in the outcrop region is estimated based on the concept of effective mixed layer depth, the result of which shows the primary contribution from horizontal advection. The contributions from Ekman and geostrophic currents are comparable. Ekman flow advection is caused by zonal wind stress anomalies and the resulting meridional Ekman current anomalies, as is pointed out by a previous study. Geostrophic velocity is decomposed into large-scale and mesoscale variability, both of which significantly contribute to horizontal advection.

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Yuhui Li, Yun Qiu, Jianyu Hu, Cherry Aung, Xinyu Lin, Chunsheng Jing, and Junpeng Zhang

ABSTRACT

Multisource satellite remote sensing data have been used to analyze the strong upwelling event off the southern coast of Sri Lanka in 2013 and its relationship with Indian Ocean dipole (IOD) events. The upwelling area in 2013 is 5.7 times larger than that in a normal year and lasts from June to August, with the peaks of the cooling anomaly reaching −1.5°C and the positive chlorophyll a concentration anomaly exceeding 3.1 mg m−3. In 2013, the negative unseasonable IOD (IODJJA) event enhances the southwest monsoon, while the blocking of the monsoon wind by the island results in a stronger westerly/northwesterly wind stress off the southern coast of Sri Lanka and a weaker westerly/northwesterly wind stress over the eastern Sri Lanka waters. This causes stronger offshore transport and positive Ekman pumping off the southern coast, forming a strong upwelling event there. Further analysis indicates that the interannual variability of the upwelling, as represented by a newly constructed index based on satellite observations, is primarily caused by the variations of local wind associated with the IOD. The upwelling off the southern coast of Sri Lanka weakens (strengthens) in the positive (negative) IOD years. However, an analysis based on 21 IOD events during 1982–2019 demonstrates that the effects of the three types of IOD events, including IODJJA, prolonged IOD (IODLONG), and normal IOD (IODSON), on the upwelling are different. Compared to the IODSON events, the IODJJA and IODLONG events tend to have stronger influences due to their earlier developing phases.

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Luke Phillipson, Yi Li, and Ralf Toumi

Abstract

The forecast of tropical cyclone (TC) intensity is a significant challenge. In this study, we showcase the impact of strongly coupled data assimilation with hypothetical ocean currents on analyses and forecasts of Typhoon Hato (2017). Several observation simulation system experiments (OSSE) were undertaken with a regional coupled ocean–atmosphere model. We assimilated combinations of (or individually) a hypothetical coastal current HF radar network, a dense array of drifter floats, and minimum sea level pressure. During the assimilation, instant updates of many important atmospheric variables (winds and pressure) are achieved from the assimilation of ocean current observations using the cross-domain error covariance, significantly improving the track and intensity analysis of Typhoon Hato. Relative to a control experiment (with no assimilation), the error of minimum pressure decreased by up to 13 hPa (4 hPa/57% on average). The maximum wind speed error decreased by up to 18 kt (5 kt/41% on average) (1 kt ≈ 0.5 m s−1). By contrast, weakly coupled implementations cannot match these reductions (10% on average). Although traditional atmospheric observations were not assimilated, such improvements indicate that there is considerable potential in assimilating ocean currents from coastal HF radar and surface drifters within a strongly coupled framework for intense landfalling TCs.

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Christopher D. McCray, John R. Gyakum, and Eyad H. Atallah

Abstract

Though prolonged freezing rain events are rare, they can result in substantial damage when they occur. While freezing rain occurs less frequently in the south-central United States than in some regions of North America, a large number of extremely long-duration events lasting at least 18 h have been observed there. We explore the key synoptic–dynamic conditions that lead to these extreme events through a comparison with less severe short-duration events. We produce synoptic–dynamic composites and 7-day backward trajectories for parcels ending in the warm and cold layers for each event category. The extremely long-duration events are preferentially associated with a deeper and more stationary 500-hPa longwave trough centered over the southwestern United States at event onset. This trough supports sustained flow of warm, moist air from within the planetary boundary layer over the Gulf of Mexico northward into the warm layer. The short-duration cases are instead characterized by a more transient upper-level trough axis centered over the south-central U.S. region at onset. Following event onset, rapid passage of the trough leads to quasigeostrophic forcing for descent and the advection of cold, dry air that erodes the warm layer and ends precipitation. While trajectories ending in the cold layer are very similar between the two categories, those ending in the warm layer have a longer history over the Gulf of Mexico in the extreme cases compared with the short-duration ones, resulting in warmer and moister onset warm layers.

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Amélie Simon, Guillaume Gastineau, Claude Frankignoul, Clément Rousset, and Francis Codron

Abstract

The impact of Arctic sea ice loss on the ocean and atmosphere is investigated focusing on a gradual reduction of Arctic sea ice by 20% of the annual mean, occurring within 30 years, starting from present-day conditions. Two ice-constraining methods are explored to melt Arctic sea ice in a coupled climate model, while keeping present-day conditions for external forcing. The first method uses a reduction of sea ice albedo, which modifies the incoming surface shortwave radiation. The second method uses a reduction of thermal conductivity, which changes the heat conduction flux inside ice. Reduced thermal conductivity inhibits oceanic cooling in winter and sea ice basal growth, reducing the seasonality of sea ice thickness. For similar Arctic sea ice area loss, decreasing the albedo induces larger Arctic warming than reducing the conductivity, especially in spring. Both ice-constraining methods produce similar climate impacts, but with smaller anomalies when reducing the conductivity. In the Arctic, the sea ice loss leads to an increase of the North Atlantic water inflow in the Barents Sea and eastern Arctic, while the salinity decreases and the gyre intensifies in the Beaufort Sea. In the North Atlantic, the subtropical gyre shifts southward and the Atlantic meridional overturning circulation weakens. A dipole of sea level pressure anomalies sets up in winter over northern Siberia and the North Atlantic, which resembles the negative phase of the North Atlantic Oscillation. In the tropics, the Atlantic intertropical convergence zone shifts southward as the South Atlantic Ocean warms. In addition, Walker circulation reorganizes and the southeastern Pacific Ocean cools.

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Yonatan Givon, Chaim I. Garfinkel, and Ian White

Abstract

An intermediate complexity general circulation model is used to investigate the transient response of the NH winter stratosphere to modulated ultraviolet (UV) radiation by imposing a stepwise, deliberately exaggerated UV perturbation and analyzing the lagged response. Enhanced UV radiation is accompanied by an immediate warming of the tropical upper stratosphere. The warming then spreads into the winter subtropics due to an accelerated Brewer–Dobson circulation in the tropical upper stratosphere. The poleward meridional velocity in the subtropics leads to an increase in zonal wind in midlatitudes between 20° and 50°N due to Coriolis torque. The increase in midlatitude zonal wind is accompanied by a dipole in Eliassen–Palm flux convergence, with decreased convergence near the winter pole and increased convergence in midlatitudes (where winds are strengthening due to the Coriolis torque); this dipole subsequently extends the anomalous westerlies to subpolar latitudes within the first 10 days. The initial radiatively driven acceleration of the Brewer–Dobson circulation due to enhanced shortwave absorption is replaced in the subpolar winter stratosphere by a wave-driven deceleration of the Brewer–Dobson circulation, and after a month the wave-driven deceleration of the Brewer–Dobson circulation encompasses most of the winter stratosphere. Approximately a month after UV is first modified, a significant poleward jet shift is evident in the troposphere. The results of this study may have implications for the observed stratospheric and tropospheric responses to solar variability associated with the 27-day solar rotation period, and also to solar variability on longer time scales.

Open access
Z. E. Gillett, H. H. Hendon, J. M. Arblaster, and E.-P. Lim

Abstract

Interannual variability of the Southern Hemisphere subtropical jet (STJ) is assessed using atmospheric reanalyses during 1979–2018. The focus is on the austral winter season when the STJ is strongest and most distinct from the midlatitude eddy-driven jet (EDJ). Variations in the intensity and latitudinal position of the STJ are diagnosed using an index developed to discriminate between variations associated with the EDJ. STJ intensity and position variations are found to be tied to different mechanisms. An intensification of the STJ is associated with enhanced divergent outflow from diabatic heating over the equatorial Pacific Ocean, primarily resulting from eastern Pacific or canonical El Niño. This intensification is associated with a narrowing of the STJ and an in-place weakening of the EDJ. An equatorward-shifted STJ, however, appears to be eddy driven and is associated with an acceleration and poleward displacement of the EDJ, which projects onto the positive polarity of the southern annular mode. As has previously been reported, El Niño Modoki (or central Pacific El Niño) can act to shift the EDJ poleward during austral winter; thus, a possible pathway for changes in the position of the STJ is via tropically forced changes in the position of the EDJ. In contrast to previous studies, we also highlight a weakening and poleward shift of the STJ in association with an expansion of the Hadley circulation.

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