Baroclinic Growth Rates Research Articles

A global climatology of polar lows investigated for local differences and wind-shear environments

Abstract. Polar lows are intense mesoscale cyclones developing in marine polar air masses. This study presents a new global climatology of polar lows based on the ERA5 reanalysis for the years 1979–2020. Criteria for the detection of polar lows are derived based on a comparison of five polar-low archives with cyclones derived by a mesoscale tracking algorithm. The characteristics associated with polar lows are considered by the following criteria: (i) intense cyclone (large relative vorticity), (ii) mesoscale (small vortex diameter), and (iii) development in the marine polar air masses (a combination of low potential static stability and low potential temperature at the tropopause). Polar lows develop in all marine areas adjacent to sea ice or cold landmasses, mainly in the winter half year. The length and intensity of the season are regionally dependent. The highest density appears in the Nordic Seas. For all ocean sub-basins, forward-shear polar lows are the most common, whereas weak-shear polar lows and those propagating towards warmer environments are second and third most frequent, depending on the area. Reverse-shear polar lows and those propagating towards colder environments are rather seldom, especially in the Southern Ocean. Generally, polar lows share many characteristics across ocean basins and wind-shear categories. The most remarkable difference is that forward-shear polar lows often occur in a stronger vertical wind shear, whereas reverse-shear polar lows feature lower static stability. Hence, the contribution to a fast baroclinic growth rate is slightly different for the shear categories.

How the Speed and Latitude of the Jet Stream Affect the Downstream Response to Recurving Tropical Cyclones

Abstract Recurving tropical cyclones (TCs) that interact with the jet stream can trigger Rossby wave packets that amplify the flow far downstream, but the extent to which the jet stream modulates TC–jet interactions and the development of the downstream response remains unclear. This study uses 25 idealized simulations from the COAMPS-TC model to examine how the latitude and maximum wind speed of an initially zonal jet stream affect the downstream response to recurving TCs. During the first 5 days of the simulations, the formation of a jet streak and a ridge immediately downstream of the TC occurs earlier on low-latitude jets than on high-latitude jets. This is due to weaker TC inertial stability at low latitudes, which promotes negative potential vorticity advection by the irrotational outflow along the jet. Increasing the speed of the jet locally reduces inertial stability poleward of the TC, but does not profoundly affect the ability of the TC to perturb the jet. Beyond 5 days, the highest-latitude and fastest jets, which have the largest baroclinic growth rates, exhibit the highest-amplitude Rossby waves and the most rapidly intensifying surface cyclones farther downstream of the TCs. Both measures of the downstream response are more sensitive to changing the speed than the latitude of the jet. Deactivating condensational heating, shortly after TCs trigger a Rossby wave packet, decreases the amplitude and variability of the downstream flow by up to 3 times relative to the fully moist simulations. This result emphasizes the importance of moist diabatic processes for generating an amplified downstream response to recurving TCs within 7–10 days.

A New Metric to Diagnose Precipitation Distribution in Transitioning Tropical Cyclones

A new coupled dynamic and thermodynamic metric is developed based on the Eady Moist Baroclinic Growth Rate (EMBGR), to discriminate between left-of-track (LOT) and right-of-track (ROT) precipitation distributions in transitioning tropical cyclones (TCs). LOT events pose a major flood risk even when a TC tracks along a coastline or just offshore, as flash flooding can occur hundreds of kilometers inland from the cyclone center. The EMBGR can improve human-produced quantitative precipitation forecasts (QPF) because it is dependent on relatively well-forecast large-scale mass fields. The ability of the EMBGR to identify precipitation distribution is first explored in a case study of TC Matthew (2016), using reanalysis and numerical model forecasts. Subsequently, a composite analysis of 36 years (1979–2014) of United States landfalling TCs using reanalysis data shows that the EMBGR is an effective discriminator between LOT and ROT distributions. The utility of the EMBGR is quantified using a pattern correlation analysis for both TC Matthew and the composites. Finally, a conceptual schematic is developed for LOT cases so that forecasters can most effectively utilize the EMBGR to improve human QPF skill during transitioning TCs.

Antarctic Thermocline Dynamics along a Narrow Shelf with Easterly Winds

AbstractDetermining the role of Southern Ocean warm intermediate water for driving melting of the Antarctic ice sheet is a major challenge in assessing future sea level rise. Analysis of 2859 CTD profiles obtained between 1977 and 2016 by ships and instrumented seals at the Weddell Sea continental slope reveals a seasonal rise of the Antarctic Slope Front thermocline by more than 100 m during the summer. The signal at Kapp Norvegia (17°W) corresponds with a seasonal warming downstream at the Filchner Trough (40°W), indicating that a coherent evolution of the slope front along the shelf break regulates the onshore flow of warm deep water. Climatological cross sections of the slope front hydrography show that downwelling of Antarctic Surface Water forms a secondary front above the warm deep water interface during summer. Enhanced baroclinic growth rates at this front suggest that the wind-driven suppression of the thermocline is partially compensated by a shallower eddy overturning cell when surface water is present. A simple model of the Weddell Gyre boundary current reveals that wintertime densification of surface waters is crucial for maintaining the deep thermocline along the eastern Weddell Sea coast. The sensitivity of the warm inflow to the cross-frontal density gradient implies a positive feedback with ice shelf melting that may lead to an abrupt transition into a high melting state once warm water rises over the shelf break depth. Despite its regional focus, this study highlights the role of upper ocean buoyancy fluxes for controlling the thermocline depth along seasonally ice-covered narrow shelf regions with cyclonic along-slope winds.

Decadal Variability in the South Pacific Subtropical Countercurrent and Regional Mesoscale Eddy Activity

AbstractDecadal variability of eddy activity in the western, subtropical South Pacific is examined using the past two decades of satellite altimetry data. Between 21° and 29°S, there is a band of heightened eddy activity. In this region, the eastward South Pacific Subtropical Countercurrent (STCC) overlays the westward South Equatorial Current (SEC). This vertically sheared STCC–SEC system is subject to baroclinic instabilities. By using the European Centre for Medium-Range Weather Forecasts (ECMWF) Ocean Reanalysis System, version 4 (ORAS4), data and verifying with the gridded Argo float data, low-frequency variations in the state of the ocean in this region are investigated. It is found that the low-frequency changes in the shearing and stratification of the STCC–SEC region simultaneously work to modulate the strength of baroclinic instabilities, as measured through the baroclinic growth rate. These changes in the strength of the instabilities consequently affect the observed eddy activity. Using a linearization of the baroclinic growth rate, the contribution to the variability from the changes in shearing is found to be roughly twice as large as those from changes in stratification. Additionally, changes in the temperature and salinity fields are both found to have significant impacts on the low-frequency variability of shearing and stratification, for which salinity changes are responsible for 50%–75% of the variability as caused by temperature changes. However, the changes in all these parameters do not occur concurrently and can alternately work to negate or augment each other.

The solsticial pause on Mars: 2 modelling and investigation of causes

The martian solsticial pause, presented in a companion paper (Lewis et al., 2016), was investigated further through a series of model runs using the UK version of the LMD/UK Mars Global Climate Model. It was found that the pause could not be adequately reproduced if radiatively active water ice clouds were omitted from the model. When clouds were used, along with a realistic time-dependent dust opacity distribution, a substantial minimum in near-surface transient eddy activity formed around solstice in both hemispheres. The net effect of the clouds in the model is, by altering the thermal structure of the atmosphere, to decrease the vertical shear of the westerly jet near the surface around solstice, and thus reduce baroclinic growth rates. A similar effect was seen under conditions of large dust loading, implying that northern midlatitude eddy activity will tend to become suppressed after a period of intense flushing storm formation around the northern cap edge. Suppression of baroclinic eddy generation by the barotropic component of the flow and via diabatic eddy dissipation were also investigated as possible mechanisms leading to the formation of the solsticial pause but were found not to make major contributions. Zonal variations in topography were found to be important, as their presence results in weakened transient eddies around winter solstice in both hemispheres, through modification of the near-surface flow. The zonal topographic asymmetry appears to be the primary reason for the weakness of eddy activity in the southern hemisphere relative to the northern hemisphere, and the ultimate cause of the solsticial pause in both hemispheres. The meridional topographic gradient was found to exert a much weaker influence on near-surface transient eddies.

Exploring the isopycnal mixing and helium–heat paradoxes in a suite of Earth system models

Abstract. This paper uses a suite of Earth system models which simulate the distribution of He isotopes and radiocarbon to examine two paradoxes in Earth science, each of which results from an inconsistency between theoretically motivated global energy balances and direct observations. The helium–heat paradox refers to the fact that helium emissions to the deep ocean are far lower than would be expected given the rate of geothermal heating, since both are thought to be the result of radioactive decay in Earth's interior. The isopycnal mixing paradox comes from the fact that many theoretical parameterizations of the isopycnal mixing coefficient ARedi that link it to baroclinic instability project it to be small (of order a few hundred m2 s−1) in the ocean interior away from boundary currents. However, direct observations using tracers and floats (largely in the upper ocean) suggest that values of this coefficient are an order of magnitude higher. Helium isotopes equilibrate rapidly with the atmosphere and thus exhibit large gradients along isopycnals while radiocarbon equilibrates slowly and thus exhibits smaller gradients along isopycnals. Thus it might be thought that resolving the isopycnal mixing paradox in favor of the higher observational estimates of ARedi might also solve the helium paradox, by increasing the transport of mantle helium to the surface more than it would radiocarbon. In this paper we show that this is not the case. In a suite of models with different spatially constant and spatially varying values of ARedi the distribution of radiocarbon and helium isotopes is sensitive to the value of ARedi. However, away from strong helium sources in the southeastern Pacific, the relationship between the two is not sensitive, indicating that large-scale advection is the limiting process for removing helium and radiocarbon from the deep ocean. The helium isotopes, in turn, suggest a higher value of ARedi below the thermocline than is seen in theoretical parameterizations based on baroclinic growth rates. We argue that a key part of resolving the isopycnal mixing paradox is to abandon the idea that ARedi has a direct relationship to local baroclinic instability and to the so-called "thickness" mixing coefficient AGM.

What Is the Source of the Stratospheric Gravity Wave Belt in Austral Winter?

Abstract During austral winter, and away from orographic maxima or “hot spots,” stratospheric gravity waves in both satellite observations and Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data reveal enhanced amplitudes in a broad midlatitude belt extending across the Southern Ocean from east of the Andes to south of New Zealand. The peak latitude of this feature slowly migrates poleward from 50° to 60°S. Wave amplitudes are much weaker across the midlatitude Pacific Ocean. These features of the wave field are in striking agreement with diagnostics of baroclinic growth rates in the troposphere associated with midlatitude winter storm tracks and the climatology of the midlatitude jet. This correlation suggests that these features of the stratospheric gravity wave field are controlled by geographical variations of tropospheric nonorographic gravity wave sources in winter storm tracks: spontaneous adjustment emission from the midlatitude winter jet, frontogenesis, and convection.

Responses of the leading mode of coldwave intensity in China to a warming climate

Regional extreme cold events have changed notably with recent global warming. Understanding how these cold extremes change in China is an urgent issue. This study examines the responses of the dominant mode of China coldwave intensity (CWI) to global warming by comparing observations with simulations from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The leading modes of the CWI derived from empirical orthogonal function (EOF) analysis have different features in different epochs. During the cold period (1957–1979), the leading mode is characterized by centers of extreme values of CWI in northern China; while during the warm period (1980–2009), the leading mode features two maximum loading centers over northern and southern China. The southward extension of the extreme value center is associated with an increase in the intensity of coldwave variations in southern China relative to previous decades. A multi-model ensemble of seven state-of-the-art climate models shows an extension of the maximum loading of the CWI leading mode into southern China by the end of the 21st century (2080–2099) under the A1B global warming scenario (atmospheric CO2 concentration of 720 ppm). These results indicate that the primary response of the leading mode of CWI to global warming might be a southward extension of the extreme value center. This response may be associated with the southward shift of the storm track observed during recent decades. A significant change in the baroclinic growth rates around 40°N is accompanied by a consistent change in synoptic eddies in the troposphere, which may indicate a shift in the preferred latitude for the growth of eddies. As a result, the storm track tends to move southward, suggesting that southern China may experience increased storminess due to increased baroclinic instability in the troposphere.

The Effect of Compressibility on Barotropic and Baroclinic Instability

Abstract The effect of compressibility on two-dimensional barotropic and baroclinic growth rates is examined by means of a linearized nonhydrostatic compressible model. It is shown that the growth rates are diminished when compressibility is included because perturbation internal energy resents a sink of basic-state kinetic energy when work is done to compress the medium. Nonlinear simulations provided by compressible and incompressible versions of the ZETA model show that the solutions are nearly identical, but the compressible solution develops more slowly than the incompressible one, consistent with the linear analysis.

Moist Stability of a Baroclinic Zonal Flow with Conditionally Unstable Stratification

Abstract The moist stability of a midlatitude zonal flow with a conditionally unstable layer in the presence of an Ekman layer is investigated. The vertical velocity employed in a simplified Kuo's parameterization is sustained by baroclinic wave forcing, diabatic heating and Ekman pumping. A general dispersion relation and eigenfunction are derived analytically for a class of flows with various vertical heating profiles. The moist unstable mode may be regarded as a baroclinic wave modified by the bulk effect of the convective heating, for which the fundamental dependences of the baroclinic growth rate on the Burger number and vertical shear remain qualitatively valid. Waves longer than the Rossby radius of deformation are not appreciably affected, while the shorter waves are significantly destablized by the convective heating. The growth rates and wavelengths of the most unstable modes are nonlinear functions of the averaged specific humidity of the moist layer, and there is an optimum specific humidity t...