Strong Meridional Circulation Research Articles

Influence of the Antarctic oscillation on summer precipitation over East Asia

As the strongest atmospheric mode in the extratropical Southern Hemisphere, the Antarctic oscillation (AAO) can affect regional and even global climate through zonal asymmetric and symmetric dynamic mechanisms. Observational analysis indicates that AAO can affect the zonal-mean vertical velocity of the Northern Hemisphere through eddy-induced meridional circulation in boreal summer, corresponding to the out-of-phase zonal precipitation anomaly at approximately 20°N and the in-phase zonal precipitation anomalies at approximately 30°N and 50°N. In particular, the precipitation anomalies related to AAO in East Asia are much stronger than those in the other regions at the same latitude, and the AAO-related precipitation anomaly agrees well with the precipitation leading mode over East Asia. The strong meridional circulation anomaly in the East Asia sector associated with the asymmetry of AAO results in strong subsidence over the Philippine Sea, and the tropical cooling triggers a Pacific-Japan-like (PJ-like) pattern anomaly, which is conducive to moisture transport over East Asia. In addition, the transient eddy forcing related to AAO causes abnormal circulation over East Asia, which further strengthens the PJ-like pattern. The zonal precipitation anomalies in the Northern Hemisphere are superimposed on the moisture transport and precipitation anomalies related to the PJ-like pattern, hence promoting positive precipitation anomalies in the Yangtze River valley, northeastern China, South Korea and southern Japan. In contrast, negative precipitation anomalies occur in southern China and North China. Our results indicate that AAO is a potential indicator for the East Asian summer precipitation anomaly.

A baseline evaluation of oceanographic and sea ice conditions in the Hudson Bay Complex during 2016–2018

In this paper, we examine sea surface temperatures (SSTs) and sea ice conditions in the Hudson Bay Complex as a baseline evaluation for the BaySys 2016–2018 field program time frame. Investigated in particular are spatiotemporal patterns in SST and sea ice state and dynamics, with rankings of the latter to highlight extreme conditions relative to the examined 1981–2010 climatology. Results from this study show that SSTs in northwestern Hudson Bay from May to July, 2016–2018, are high relative to the climatology for SST (1982–2010). SSTs are also warmer in 2016 and 2017 than in 2018 relative to their climatology. Similarly, unusually low sea ice cover existed from August to December of 2016 and July to September of 2017, while unusually high sea ice cover existed in January, February, and October of 2018. The ice-free season was approximately 20 days longer in 2016 than in 2018. Unusually high ice-drift speeds occurred in April of 2016 and 2017 and in May of 2018, coinciding with strong winds in 2016 and 2018 and following strong winds in March 2017. Strong meridional circulation was observed in spring of 2016 and winter of 2017, while weak meridional circulation existed in 2018. In a case study of an extreme event, a blizzard from 7 to 9 March 2017, evaluated using Lagrangian dispersion statistics, is shown to have suppressed sea ice deformation off the coast of Churchill. These results are relevant to describing and planning for possible future pathways and scenarios under continued climate change and river regulation.

Rossby Number Dependence of Venus/Titan‐Type Superrotation and Its Related Intermittency

AbstractVenus/Titan‐type superrotation driven by stratospheric heating and intermittency seen in the superrotation dynamics are investigated using an idealized general circulation model in a high Rossby number regime (Ro = 7.5 to 23 for the strongest zonal jet) where the superrotation is formed by the meridional circulation and equatorward eddy momentum flux. When the jet core has a Rossby number of ∼23 on a slowly rotating or small planet, fast planetary‐scale Rossby waves are transiently amplified by both barotropic and baroclinic energy conversion and intermittently produce equatorward eddy momentum fluxes. At high latitudes, poleward eddy momentum transport also occurs when the poleward heat flux of baroclinic eddies is strong. On such a slowly rotating or small‐sized planet, equatorial superrotation is developed efficiently by weak, intermittent equatorward momentum flux in the presence of polar indirect circulation and the speed of the zonal flow is roughly constant over the low‐ and mid‐latitudes. In contrast, on a relatively fast rotating or large‐sized planet when the Rossby number is ∼7.5 for the jet core, although the zonal jets and equatorward eddy momentum fluxes intensify, equatorial superrotation is not developed efficiently (i.e., the superrotation intensity and its equatorial efficiency are small). Strong equatorward eddy momentum fluxes are produced continuously by slow barotropic Rossby waves on the equatorward flanks of the jets developed by the strong meridional circulation. The poleward heat fluxes of baroclinic waves are negligible because it is much smaller than the heat flux of the zonal‐mean meridional circulation.

Simulating Jupiter’s weather layer. Part II: Passive ammonia and water cycles

We examine the ammonia and water cycles in Jupiter’s upper troposphere and lower stratosphere during spin-up of a multiple zonal jet global circulation using Oxford’s Jupiter General Circulation Model. Jupiter’s atmosphere is simulated at 512 × 256 horizontal resolution with 33 vertical levels between 0.01 and 18 bar, putting the lowest level well below the expected water cloud base. Simulations with and without a 5.7 Wm−2 interior heat source were run for 130000–150000 d to allow the deep atmosphere to come into radiative-convective-dynamical equilibrium, with variants on the interior heating case including varying the initial tracer distribution, particle condensate diameter, and cloud process timescales. The cloud scheme includes simple representations of the ammonia and water cycles. Ammonia vapour changes phase to ice, and reacts with hydrogen sulphide to produce ammonium hydrosulphide. Water changes phases between vapour, liquid, and ice depending on local environmental conditions, and all condensates sediment at their respective Stokes velocities. With interior heating, clouds of ammonia ice, ammonium hydrosulphide ice, and water ice form with cloud bases around 0.4 bar, 1.5 bar, and 3 bar, respectively. Without interior heating the ammonia cloud base forms in the same way, but the ammonium hydrosulphide and water clouds sediment to the bottom of the domain. The liquid water cloud is either absent or extremely sparse. Zonal structures form that correlate regions of strong latitudinal shear with regions of constant condensate concentration, implying that jets act as barriers to the mixing. Regions with locally high and low cloud concentrations also correlated with regions of upwelling and downwelling, respectively. Shortly after initialisation, the ammonia vapour distribution up to the cloud base resembles the enhanced concentration seen in Juno observations, due to strong meridional mean circulation at the equator. The resemblance decays rapidly over time, but suggests that at least some of the relevant physics is captured by the model. The comparison should improve with additional microphysics and better representation of the deep ammonia reservoir.

Evolution of the Ofanto River delta from the ‘Little Ice Age’ to modern times: Implications of large-scale synoptic patterns

We reconstruct the evolution of the Ofanto River delta from the 17th century to the present using historical maps (1600–1850), official IGM topographic maps (1850–1980) and recent aerial photographs (2015), and we compare long-term morphological changes with the evolution of the delta of the Volturno River during the same time period. The aim of this study is to define the role of climatic (flood frequency, synoptic pressure patterns) and anthropogenic factors (deforestation, anthropogenic sediment subtraction of river sediment) in the evolution of the Ofanto delta. We analysed the importance of each factor on the evolution of the delta and compared them with the simultaneous behaviour of the Volturno delta to highlight the role of large-scale synoptic pressure patterns. We found that the main driver of different delta evolution phases is weather-climatic condition, while anthropogenic factors interacted with the delta evolution in different ways but did not control the first-order evolution. In particular, analysing the data on recent floods, we found that the most favourable situations for both rivers are omega-blocking, deep low-pressure trough and strong meridional circulation (mode Ω) which create Mediterranean low-pressure systems. Instead, a zonal circulation (mode W) can only cause floods on Volturno. Because the evolution of a delta is driven by the frequency of floods, and because we found that the frequency of floods is guided by synoptic patterns, a relationship can be established between delta evolution and synoptic patterns in the past. Consequently, past phases of the contemporary progradation of the Ofanto and Volturno deltas suggest the increasing frequency of mode Ω, while phases of simultaneous progradation of the Volturno delta and stability and/or retreat of the Ofanto delta are indicative of the increasing frequency of mode W. The only exception occurred during the last evolutionary phase (60 years), when anthropogenic sediment subtraction was prevalent.

Paleoenvironments in the Fram Strait during Marine Isotope Stages 2–6 based on planktonic paleobiological and stable-isotope proxies and ice-rafted debris

Two sediment cores from the Knipovich Ridge in the Fram Strait have been studied to examine paleoceanographic and climatic changes in the Polar North Atlantic during the late Middle and Late Pleistocene. As inferred from the data on foraminifers, nannofossils, diatoms, IRD, oxygen isotope values, and AMS 14C ages, the recovered deposits span the interval of MIS 2–6. Several beds marked by a high productivity of microplankton and often by a great IRD content are recorded in MIS 2, MIS 5a, 5c, 5e, and MIS 6 suggesting a repeated occurrence of seasonally ice-free waters and strong meridional circulation in the eastern Fram Strait. The planktonic foraminifers Globigerinoides ruber, Globorotalia scitula, G. crassaformis, and pink-colored Globigerina rubescens, exotic for this latitude, were identified in three points of the section. A hiatus in one of the cores studied was revealed from comparison of IRD variations and the occurrence of Pullenia bulloides.

四川巴中地区引发滑坡地质灾害的强降水类型及典型案例分析

运用巴中地区区域自动站逐日、逐小时降水资料和NCEP 1˚ × 1˚间隔为6 h的再分析资料分析了巴中地区降水气候特征，并具体对比分析巴中地区引发滑坡的三种不同类型强降水特征，得出结论如下：1) 巴中地区年降水量整体表现为“南北多、东西少”的分布特征。强降水事件集中发生在6~9月，可分为持续性强降水、突发性暴雨和连续性暴雨三种类型。2) 窑厂坪的滑坡由持续性强降水引发，特点是降雨持续时间长，小时降水强度变化不大，累积降水量大。黑马山滑坡由突发性暴雨引发，其特点是降水持续时间短，累积降水量大。老河沟后山滑坡由连续性暴雨引发，特点为降水过程中出现多次强降水，强降水间有数小时的时间间歇。3) 三次降水过程前期中高纬经向环流强盛，且均出现高空急流，但其主要影响系统略有不同。4) 三次强降水过程前期在低层均有明显的偏北气流存在，冷空气由北向南侵入，存在较强的对流性不稳定能量，垂直上升运动较强，但上升运动出现的高度存在差异。 By using the weather station precipitation and NCEP 1˚ × 1˚ reanalysis data, this paper analyzed the climate characteristics of precipitation and the characteristics among three different types of heavy rain events which triggered the landslides in Sichuan Bazhong area, and the results show that: 1) The annual precipitation in southern and northern is more than eastern and western of Bazhong area. 2) Yaochangping landslide triggered by persistent heavy rainfall, which characterized by long duration, little change of hourly precipitation and large accumulation precipitation. Heimashan landslide is triggered by sudden rainstorm, which is characterized by short duration and large accumulation precipitation. Laohegou landslide triggered by continuous rainstorm, characterized by two or more times of precipitation events with several hours of intermittent during the entire precipitation process, the landslide occurred in the mid-late period of the precipitation process. 3) Strong meridional circulation occurs early in three rainfall events in high latitudes, but its main impact system is slightly different. 4) There was significantly northern airflow existing in the low level during the prophase of the three heavy precipitations and the cold airflow irrupted from north to south direction, with strong convective instability energy and with strong vertical upward movement. However, the height differences emerged during the upward motion.

Excursions to C<sub>4</sub> vegetation recorded in the Upper Pleistocene loess of Surduk (Northern Serbia): an organic isotope geochemistry study

Abstract. Loess sequences have been intensively studied to characterize past glacial climates of the 40–50° north and south latitude zones. Combining different approaches of sedimentology, magnetism, geochemistry, geochronology and malacology allows the general pattern of the climate and environment of the last interglacial–glacial cycle in Eurasia and America to be characterized. Previous studies performed in Europe have highlighted the predominance (if not the sole occurrence) of C3 vegetation. The presence of C3 plants suggests a regular distribution of precipitation along the year. Therefore, even if the mean annual precipitation remained very low during the most extensive glacial times, free water was available for more than 2 months per year. Contrarily, the δ13C record of Surduk (Serbia) clearly shows the occurrence and dominance of C4 plants during at least 4 episodes of the last glacial times at 28.0–26.0 kyr cal BP, 31.4–30.0 kyr cal BP, 53.4–44.5 kyr cal BP and 86.8–66.1 kyr. The C4 plant development is interpreted as a specific atmospheric circulation pattern that induces short and dry summer conditions. As possible explanation, we propose that during "C4 episodes", the Mediterranean Sea would have been under the combined influence of the following: (i) a strong meridional circulation unfavorable to water evaporation that reduced the Mediterranean precipitation on the Balkans; and (ii) a high positive North Atlantic Western Russian (NA/WR)-like atmospheric pattern that favored northerlies over westerlies and reduced Atlantic precipitation over the Balkans. This configuration would imply very dry summers that did not allow C3 plants to grow, thus supporting C4 development. The intra-"C4 episode" periods would have occurred under less drastic oceanic and atmospheric patterns that made the influence of westerlies on the Balkans possible.

An advective solar-type dynamo without the $\sf \alpha$ effect

<i>Context. <i/> Most solar and stellar dynamo models use the scenario where the magnetic field is generated by the interplay between differential rotation (the <i>Ω<i/> effect) and a mean electromotive force due to helical turbulent convection flows (the <i>α<i/> effect). There are, however, turbulent dynamo mechnisms that may complement the <i>α<i/> effect or may be an alternative to it.<i>Aims. <i/>We investigate models of solar-type dynamos where the <i>α<i/> effect is completely replaced by two other turbulent dynamo mechanisms, namely the effect and the shear-current effect, which both result from an inhomogeneity of the mean magnetic field.<i>Methods. <i/> We studied axisymmetric mean-field dynamo models containing differential rotation, the and shear-current effects, and a meridional circulation. The model calculations were carried out using the rotation profile of the Sun as obtained from helioseismic measurements and radial profiles of other quantities according to a standard model of the solar interior.<i>Results. <i/>Without meridional flow, no satisfactory agreement of the models with the solar observations can be obtained. With a sufficiently strong meridional circulation included, however, the main properties of the large-scale solar magnetic field, namely, its oscillatory behavior, its latitudinal drift towards the equator within each half cycle, and its dipolar parity with respect to the equatorial plane, are correctly reproduced.<i>Conclusions. <i/> We have thereby constructed the first mean-field models of solar-type dynamos that do not use the <i>α<i/> effect.

Multiple Equilibrium States Appearing in a Venus-Like Atmospheric General Circulation Model

We investigated the existence of multiple equilibrium states in the Venusian atmospheric general circulation suggested by Matsuda (1980, 1982) using a Venus-like atmospheric general circulation model. We ran the model from two initial conditions: a state with large zonal wind increasing with height and a motionless state. For the large zonal wind initial state, a strong zonal wind with weak meridional circulation, i.e., super-rotation, appears. However for the motionless initial condition, slow zonal wind with strong meridional circulation relative to the farmer state appears. Each circulation reached a quasi-steady state. These results have the same features suggested by Matsuda. The presence of multiple equilibrium states was sensitive to the horizontal eddy viscosity parameter. For the strong zonal wind state, the acceleration of the zonal mean zonal wind results from the horizontal EP flux divergence from the wavenumber one component on the equator, which is mainly maintained by the Gierasch mechanism (1975). The presence of multiple equilibrium states suggests that an alternative slow zonal wind state could appear in the Venusian atmosphere if an appropriate initial condition or drastic fluctuation is assigned.

Superrotation in a Venus general circulation model

A superrotating atmosphere with equatorial winds of ∼35 m s−1 is simulated using a simplified Venus general circulation model (GCM). The equatorial superrotation in the model atmosphere is maintained by barotropic instabilities in the midlatitude jets which transport angular momentum toward the equator. The midlatitude jets are maintained by the mean meridional circulation, and the momentum transporting waves are qualitatively similar to observed midlatitude waves; an equatorial Kelvin wave is also present in the atmosphere. The GCM is forced by linearized cooling and friction parameterizations, with hyperdiffusion and a polar Fourier filter to maintain numerical stability. Atmospheric superrotation is a robust feature of the model and is spontaneously produced without specific tuning. A strong meridional circulation develops in the form of a single Hadley cell, extending from the equator to the pole in both hemispheres, and from the surface to 50 km altitude. The zonal jets produced by this circulation reach 45 m s−1 at 60 km, with peak winds of 35 m s−1 at the equator. A warm pole and cold collar are also found in the GCM, caused by adiabatic warming in the mean meridional circulation. Wave frequencies and zonal wind speeds are smaller than in observations by cloud tracking but are consistent with a Doppler shifting by wind speeds in the generating region of each wave. Magnitudes of polar temperature anomalies are smaller than the observed features, suggesting dynamical processes alone may not be sufficient to maintain the large observed temperature contrasts at the magnitudes and periods found in this GCM.

Symmetry breaking in free-surface cylinder flows

The flow in a stationary open cylinder driven by the constant rotation of the bottom endwall is unstable to three-dimensional perturbations for sufficiently large rotation rates. The bifurcated state takes the form of a rotating wave. Two distinct physical mechanisms responsible for the symmetry breaking are identified, which depend on whether the fluid depth is sufficiently greater or less than the cylinder radius. For deep systems, the rotating wave results from the instability of the near-wall jet that forms as the boundary layer on the rotating bottom endwall is turned into the interior. In this case the three-dimensional perturbations vanish at the air/water interface. On the other hand, for shallow systems, the fluid at radii less than about half the cylinder radius is in solid-body rotation whereas the fluid at larger radii has a strong meridional circulation. The interface between these two regions of flow is unstable to azimuthal disturbances and the resulting rotating wave persists all the way to the air/water interface. The flow dynamics are explored using three-dimensional Navier–Stokes computations and experimental results obtained via digital particle image velocimetry. The use of a flat stress-free model for the air/water interface reproduces the experimental results in the deep system but fails to capture the primary instability in the shallow system, even though the experimental imperfections, i.e. departures from a perfectly flat and clean air/water interface, are about the same for the deep and the shallow systems. The flat stress-free model boundary conditions impose a parity condition on the numerical solutions, and the consideration of an extended problem which reveals this hidden symmetry provides insight into the symmetry-breaking instabilities.

Mixing Characteristics in a Conical Taylor-Couette Flow System at Low Reynolds Numbers

Mixing behaviors in a conical Taylor–Couette flow system were investigated at low Reynolds numbers using flow-visualization and salt-solution tracer techniques. Owing to sensitivity during the start-up procedure, the acceleration rate of the inner conical cylinder was precisely controlled by computer. Two different flow modes were observed in the laminar flow regime. When the inner conical cylinder was accelerated at a low rate (0.05 rad/s2), the whole fluid column was filled with helical vortices. In the case of a high acceleration rate (0.5 rad/s2), an upward traveling motion of the Taylor vortices was observed. The vortices started to display a global upward motion due to the strong meridional circulation. These two flow modes showed quite different mixing behaviors even at nearly equal Reynolds numbers. In the case of the upward traveling motion, the tracer substance was convected with the traveling vortices in the main flow cross-section and with the meridional flow at the outer-edge cell boundaries. In the helical motion, on the other hand, the tracer was transported by diffusion-like motion along the helical vortex filament and by convection along the secondary flow streamlines at outer-edge cell boundaries.

A large-eddy simulation of turbulent compressible convection: differential rotation in the solar convection zone

We present the results of two simulations of the convection zone, obtained by solving the full hydrodynamic equations in a section of a spherical shell. The first simulation has cylindrical rotation contours (parallel to the rotation axis) and a strong meridional circulation, which traverses the entire depth. The second simulation has isorotation contours about mid-way between cylinders and cones, and a weak meridional circulation, concentrated in the uppermost part of the shell. We show that the solar differential rotation is directly related to a latitudinal entropy gradient, which pervades into the deep layers of the convection zone. We also offer an explanation of the angular velocity shear found at low latitudes near the top. A non-zero correlation between radial and zonal velocity fluctuations produces a significant Reynolds stress in that region. This constitutes a net transport of angular momentum inwards, which causes a slight modification of the overall structure of the differential rotation near the top. In essence, the thermodynamics controls the dynamics through the Taylor–Proudman momentum balance. The Reynolds stresses only become significant in the surface layers, where they generate a weak meridional circulation and an angular velocity ‘bump’.

Simulation of large‐scale dynamics in the mesosphere and lower thermosphere with the Doppler‐spread parameterization of gravity waves: 1. Implementation and zonal mean climatologies

Small‐scale gravity waves (GWs) originating in the lower atmosphere maintain the large‐scale structure and circulation in the mesosphere and lower thermosphere (MLT). At solstice the drag imposed by these waves supports a strong meridional circulation from summer pole to winter pole that acts as a giant “refrigerator” transporting heat from a colder summer mesosphere to a warmer winter mesosphere. The Doppler‐spread parameterization (DSP) of gravity waves developed by C. O. Hines has been implemented into the Spectral Mesosphere/Lower Thermosphere Model. Simulations for solstice and equinox conditions compare favorably with empirical models and observations. A particularly good agreement for both seasons is found with recent in situ observations of polar mesospheric temperatures. The thermal structure at high latitudes, especially at solstice, is almost entirely driven by the GW momentum deposition in the MLT. The DSP appears to provide a very realistic distribution of the GW drag imposed on the large‐scale circulation. A modification is suggested for the calculation of GW energy deposition rate that appears to be more consistent with the physics of Doppler spreading. The simulations exhibit strong seasonal variations in the energy deposition rates with a summer maximum of ∼200 mW kg−1 at middle and high latitudes in the upper mesosphere in very good agreement with recent in situ estimates.

The Posidonia Shale (Lower Toarcian) of SW-Germany: an oxygen-depleted ecosystem controlled by sea level and palaeoclimate

The Lower Toarcian Posidonia Shale is famous for its excellently preserved fossils and its high amount of organic matter (up to 16%). Both quality of preservation and accumulation of organic matter have been explained by permanent anoxic bottom water conditions. High-resolution geochemical, sedimentological and palaeoecological investigations of various sections of the Posidonia Shale in SW-Germany, however, indicate that oxygen availability was variable and ranged from short oxygenated periods to longer-term anoxia. The benthic macrofauna consists of nine fossil communities and was used, in combination with geochemical data, to reconstruct a time-averaged oxygen curve. Anoxic conditions prevailed during the deposition of the Toarcian black shales; they were, however, punctuated by various short periods (weeks to years) with oxygenated bottom water conditions. Sedimentological (e.g. distinctiveness of microlamination, siliciclastic content) and geochemical parameters (e.g. organic matter content, isotopic signatures: δ 18O and δ 13C, molecular redox parameters: pristane/phytane ratio, arylisoprenoids) exhibit a remarkable covariation and seem to be controlled by sea level fluctuations. Maximum oxygen depletion and an extreme negative shift of δ 13C org values (−34‰) occurred during the early falciferum-zone. This is explained by the recycling of 12C-enriched carbon derived from remineralization of organic matter on and within the substrate during low sea level stand and a highly elevated redox boundary including photic zone anoxia. The subsequent transgression permitted enhanced water exchange with the Tethyan Ocean and caused improvement of living conditions at the end of the falciferum-zone. Other important factors controlling the depositional environment are the overall palaeogeographic situation and climate. The early Jurassic is the latest period before break-up of Pangaea and probably was ruled by a strong meridional atmospheric circulation system with pronounced seasonal changes of prevailing trade- and monsoon-wind systems. An estuarine circulation with a positive water balance and surface water with slightly reduced salinity in the summer alternated with an anti-estuarine circulation and a negative water balance in the winter. During the summer months a stratified water column with anoxic conditions below the halocline developed. δ 18O data indicate low salinity in the surface water during the monsoon-influenced summer. High productivity was then located in the photic zone and the corresponding isotopically light δ 18O-signal was fixed in the calcareous nannoplankton. During the winter months a saline circulation system brought oxygen to the benthic environment, favouring temporary benthic colonization, especially during times of relative sea level high stand.

The Influence of Wave– and Zonal Mean–Ozone Feedbacks on the Quasi-biennial Oscillation

The effects of wave and zonal mean ozone heating on the evolution of the quasi-biennial oscillation (QBO) are examined using a two-dimensional mechanistic model of the equatorial stratosphere. The model atmosphere is governed by coupled equations for the zonal mean and (linear) wave fields of ozone, temperature, and wind, and is driven by specifying the amplitudes of a Kelvin wave and a Rossby‐gravity wave at the lower boundary. Wave‐mean flow interactions are accounted for in the model, but not wave‐wave interactions. A reference simulation (RS) of the QBO, in which ozone feedbacks are neglected, is carried out and the results compared with Upper Atmosphere Research Satelliteobservations. The RS is then compared with three model experiments, which examine separately and in combination the effects of wave ozone and zonal mean ozone feedbacks. Wave‐ozone feedbacks alone increase the driving by the Kelvin and Rossby‐gravity waves by up to 10%, producing stronger zonal wind shear zones and a stronger meridional circulation. Zonal mean‐ ozone feedbacks (ozone QBO) alone decrease the magnitude of the temperature QBO by up to 15%, which in turn affects the momentum deposition by the wave fields. Overall, the zonal mean‐ozone feedbacks increase the magnitude of the meridional circulation by up to 30%. The combined effects of wave‐ozone and ozone QBO feedbacks generally produce a larger response then either process alone. Moreover, these combined ozone feedbacks produce a temperature QBO amplitude that is up to 30% larger than simulations without the feedbacks. Correspondingly, significant changes are also observed in the zonal wind and ozone QBOs. When ozone feedbacks are included in the model, the Kelvin and Rossby‐gravity wave amplitudes can be reduced by ;10% and still produce a QBO similar to simulations without ozone.

Three‐dimensional isoneutral potential vorticity structure in the Indian Ocean

The three‐dimensional isoneutral potential vorticity structure of the Indian Ocean is examined using World Ocean Circulation Experiment and National Oceanic and Atmospheric Administration conductivity‐temperature‐depth data and historical bottle data. The distribution of the potential vorticity is set by the Indian Ocean's source waters and their circulation inside the basin. The lower thermocline has a high potential vorticity signal extending westward from northwest of Australia and a low signal from the Subantarctic Mode Water in the south. The Antarctic Intermediate Water inflow creates patches of high potential vorticity at intermediate depths in the southern Indian Ocean, below which the field becomes dominated by planetary vorticity, indicating a weaker meridional circulation and weaker potential vorticity sources. Wind‐driven gyre depths have lower potential vorticity gradients primarily due to same‐source waters. Homogenization and western shadow zones are not observed. The β‐effect dominates the effect of the Somali Current and the Red Sea Water on the potential vorticity distribution. Isopleths tilt strongly away from latitude lines in the deep and abyssal waters as the Circumpolar Deep Water fills the basins in deep western boundary currents, indicating a strong meridional circulation north of the Antarctic Circumpolar Current. The lower‐gradient intermediate layer surrounded vertically by layers with higher meridional potential vorticity gradients in the subtropical Indian Ocean suggests that Rossby waves will travel ∼1.3 times faster than standard theory predicts. To the south, several pools of homogenized potential vorticity appear in the upper 2000 m of the Southern Ocean where gyres previously have been identified. South of Australia the abyssal potential vorticity structure is set by a combination of the Antarctic Circumpolar Current and the bathymetry.

Effects of the seasonal variation on forming the steady state of an atmosphere-ocean coupled system

A three-dimensional, coupled atmosphere-ocean general circulation model is developed and effects of the seasonal variation on forming a steady-state coupled atmosphere-ocean system are studied. Case studies are carried out for a basin of idealized geometry by changing the period of the seasonal variation in the solar forcing. The coupled system shows significant differences depending on the existence of the seasonal variation, where the heat transport by the oceanic circulation plays a central role. For the regular seasonal variation, a stronger oceanic meridional circulation, which accompanies larger poleward heat transport, is realized compared with that for the annual-mean solar forcing. The stronger meridional circulation is associated with a larger wintertime meridional gradient of the sea surface temperature. The meridional gradient of the annual-mean sea surface temperature is smaller for the regular seasonal variation, which results in a smaller atmospheric poleward heat transport. In consequence, the total of the atmospheric and the oceanic poleward heat transport is almost identical for the two cases. Cases with the seasonal variation of the doubled period and with the time-filtered flux exchange between the atmosphere and the ocean are also studied, showing that the system is not sensitive to those factors.

The Response of a Zonally Symmetric Atmosphere to Subtropical Thermal Forcing: Threshold Behavior

We consider the response of a zonally symmetric atmosphere to a thermal forcing that is localized in the subtropics. Specifically, the equilibrium temperature distribution has a local subtropical peak and is flat elsewhere, including at the equator. On the basis of inviscid steady-state theory, it is argued that the response to such forcing is one of two distinct types. Below a threshold forcing, the atmosphere adopts a steady state of thermal equilibrium with no meridional flow. With supercritical forcing, this state breaks down and a strong meridional circulation is predicted. The threshold forcing value is that at which the absolute vorticity of the zonal flow (in gradient balance with the equilibrium temperatures) vanishes at the upper boundary. These inviscid predictions are tested in a zonally symmetric numerical model; while the model viscosity shifts the threshold and otherwise modifies the response, the threshold is clearly evident in the model behavior.