Quasi-equilibrium Tropical Circulation Model Research Articles

El Niño–Southern Oscillation Sea Level Pressure Anomalies in the Western Pacific: Why Are They There?*

Abstract Although sea level pressure (SLP) anomalies in the western Pacific have long been recognized as an integral part of the classic Southern Oscillation pattern associated with El Niño–Southern Oscillation (ENSO), there is an unresolved question regarding the dynamics that maintain these anomalies. Traditional studies of the ENSO response in the tropics assume a single deep baroclinic mode associated with the tropospheric temperature anomalies. However, the SLP anomalies in the western Pacific are spatially separated from the baroclinic signal in the NCEP–NCAR reanalysis, CMIP5 models, and an intermediate complexity model [a quasi-equilibrium tropical circulation model (QTCM)]. Separation of ENSO SLP anomalies in the tropical Pacific into baroclinic and barotropic components indicates that the barotropic component contributes throughout the tropics and constitutes the primary contribution in the western Pacific. To demonstrate the roles of baroclinic and barotropic modes in ENSO teleconnections within the tropics, a series of QTCM experiments is performed, where anomalies in the interactions between baroclinic and barotropic modes are suppressed over increasingly wider latitudinal bands in the tropical Pacific. If this suppression is done in the 15°N–15°S band, the pressure signals in the western Pacific are only partly removed, whereas if it is done in the 30°N–30°S band, the anomalies in the western Pacific are almost entirely removed. This suggests the following pathway: interactions with SST anomalies create the baroclinic response in the central and eastern Pacific, but baroclinic–barotropic interactions, arising substantially in the subtropical Pacific, generate a barotropic response that yields the SLP anomalies in the western Pacific.

Interhemispheric Teleconnections from Tropical Heat Sources in Intermediate and Simple Models

Abstract The mechanisms that control the interhemispheric teleconnections from tropical heat sources are investigated using an intermediate complexity model [a quasi-equilibrium tropical circulation model (QTCM)] and a simple linear two-level model with dry dynamics. Illustrating the interhemispheric teleconnection process with an Atlantic warm pool principal case, the heat source directly excites a baroclinic response that spreads across the equator. Then, three processes involving baroclinic–barotropic interactions—shear advection, surface drag, and vertical advection—force a cross-equatorial barotropic Rossby wave response. An analysis of these processes in QTCM simulations indicates that 1) shear advection has a pattern that roughly coincides with the baroclinic signal in the tropics and subtropics, 2) surface drag has large amplitude and spatial extent and can be very effective in forcing barotropic motions around the globe, and 3) vertical advection has a significant contribution locally and remotely where large vertical motions and vertical shear occur. The simple model is modified to perform experiments in which each of these three mechanisms may be included or omitted. By adding surface drag and vertical advection, and comparing each to shear advection, the effects of the three mechanisms on the generation and propagation of the barotropic Rossby waves are shown to be qualitatively similar to the results in QTCM. It is also found that the moist processes included in the QTCM can feed back on the teleconnection process and alter the teleconnection pattern by enlarging the prescribed tropical heating in both intensity and geographical extent and by inducing remote precipitation anomalies by interaction with the basic state.

Mechanisms of Mid-Holocene Precipitation Change in the South Pacific Convergence Zone

Abstract The variability of the South Pacific convergence zone (SPCZ) during the mid-Holocene is investigated using models archived in the Paleoclimate Modelling Intercomparison Project Phase II (PMIP2) database. Relative to preindustrial conditions, mid-Holocene top-of-atmosphere insolation was relatively lower during austral summer [December–February (DJF)], which is the season when the SPCZ is at its peak intensity. In response to this perturbation, the PMIP2 models simulate a displacement of the SPCZ to the southwest. This SPCZ shift is associated with a sea surface temperature (SST) dipole, with increased rainfall collocated with warm SST anomalies. Decomposing the DJF precipitation changes in terms of a diagnostic moisture budget indicates that the SPCZ shift is balanced to leading order by a change in the mean moisture convergence. Changes to the broad area of upper-level negative zonal stretching deformation, where transient eddies can become trapped and subsequently generate deep convection, support the notion that the SPCZ shift in the subtropics is tied to eddy forcing. Idealized experiments performed with an intermediate-level complexity model, the Quasi-Equilibrium Tropical Circulation Model (QTCM), suggest that the mid-Holocene change in rainfall in the SPCZ region as well as the equatorial Pacific is dominated by a change in the underlying SST. The tropical portion of the SPCZ is further remotely affected by the orbitally induced weakening of the Australian monsoon, even though this effect is weaker compared to the effect from SSTs.

Monsoon intraseasonal oscillation and land–atmosphere interaction in an idealized model

A simple coupled model is used in a zonally-symmetric configuration to investigate the effect of land–atmosphere coupling on the Asian monsoon intraseasonal oscillation. The atmospheric model is a version of the Quasi-equilibrium Tropical Circulation Model with a prognostic atmospheric boundary layer, as well as two free-tropospheric modes in momentum, and one each in moisture and temperature. The land model is the simple one-layer model SLand. The complete nonlinear version and a linear version of the model are used to understand how land–atmosphere interaction influences the northward-propagating intraseasonal oscillation that has been documented in the atmospheric model (Bellon and Sobel in J Geophys Res 113, 2008a, J Atmos Sci 65:470–489, 2008b). Our results show that this interaction damps the intraseasonal variability in most cases. The small heat capacity of land surfaces is the main factor that intervenes directly in the dynamics of the intraseasonal oscillation and explains the damping of intraseasonal variability. But in a few peculiar cases, the small heat capacity of land can also cause a strong interaction between the intraseasonal oscillation and the mean state via the nonlinearity of precipitation, that enhances the monsoon intraseasonal variability. High land albedo indirectly influences the intraseasonal variability by setting the seasonal mean circulation to conditions unfavorable for the monsoon intraseasonal oscillation.

Breakdown and Reformation of the Intertropical Convergence Zone in a Moist Atmosphere

Abstract The effects of moisture on the intertropical convergence zone (ITCZ) over the eastern Pacific on the synoptic time scale are investigated using an intermediate complexity atmospheric circulation model, the quasi-equilibrium tropical circulation model (QTCM1), on an aquaplanet. The dry simulation shows results consistent with those of simple dynamic models, except that a slightly stronger heating rate is needed owing to different model designs. In the moist simulations, the most important result is the formation of a tail southwest of a vortex during and after the ITCZ breakdown. This tail may extend zonally more than 60° longitude and last for more than two weeks in an idealized simulation. In the eastern North Pacific, this phenomenon is often observed in cases that involve easterly waves. In a sense, the formation of the tail suggests a possible mechanism that forms an ITCZ efficiently. This study shows that the surface convergent flow induced by a disturbance initializes a positive wind–evaporation feedback that forms the tail. In the tail, the most important energy source is surface evaporation, and the latent heat is nicely balanced by an adiabatic cooling of the ascending motion. In other words, the energy is redistributed vertically by vertical energy convergence. The lifespan of the tail is controlled by the propagation of tropical waves that modify the surface wind pattern, leading to a decrease in surface wind speed and corresponding surface fluxes. It may explain the absence of the tail in some of the events in the real atmosphere.

Multiple Equilibria of the Hadley Circulation in an Intermediate-Complexity Axisymmetric Model

Abstract A model of intermediate complexity based on quasi-equilibrium theory—a version of the Quasi-Equilibrium Tropical Circulation Model with a prognostic atmospheric boundary layer, as well as two free-tropospheric modes in momentum, and one each in moisture and temperature—is used in a zonally symmetric aquaplanet configuration to study the sensitivity of the Hadley circulation to the sea surface temperature (SST) latitudinal distribution. For equatorially symmetric SST forcing with large SST gradients in the tropics, the model simulates the classical double Hadley cell with one equatorial intertropical convergence zone (ITCZ). For small SST gradients in the tropics, the model exhibits multiple equilibria, with one equatorially symmetric equilibrium and two asymmetric equilibria (mirror images of each other) with an off-equatorial ITCZ. Further investigation of the feedbacks at play in the model shows that the assumed vertical structure of temperature variations is crucial to the existence and stability of the asymmetric equilibria. The free-tropospheric moisture–convection feedback must also be sufficiently strong to sustain asymmetric equilibria. Both results suggest that the specific physics of a given climate model condition determine the existence of multiple equilibria via the resulting sensitivity of the convection to free-tropospheric humidity and the vertical structure of adiabatic heating. The symmetry-breaking mechanism and resulting multiple equilibria have their origin in the local multiple equilibria that can be described by a single-column model using the weak temperature gradient approximation. An additional experiment using an SST latitudinal distribution with a relative minimum at the equator shows that the feedbacks controlling these multiple equilibria might be relevant to the double-ITCZ problem.

The Moisture Mode in the Quasi-Equilibrium Tropical Circulation Model. Part II: Nonlinear Behavior on an Equatorial β Plane

Abstract Numerical calculations of a simplified quasi-equilibrium tropical circulation model (QTCM) on the equatorial β plane have been performed to explore the nonlinear regime of the moisture mode. Sensitivity tests have examined the effects of nonlinear advection and nonlinear wind-induced surface heat exchange (WISHE). Starting from a spatially homogeneous radiative–convective equilibrium with some background gustiness, the model develops quasi-stationary moisture modes, as expected from linear analysis. Upon nonlinear saturation due to thermodynamic limiting processes, a different regime emerges. A classical Gill model augmented with a prognostic humidity variable is found to be able to capture the nonlinear dynamics of the moisture mode; the time evolution solely resides in the humidity variable, and it is possible to understand its dynamics by examining the humidity budget. A scaling analysis shows that the approximation with a Gill model is valid for disturbances whose time scale is much longer than the damping time scale of equatorial waves. When nonlinear WISHE is included, a large-scale disturbance of wavenumber 1 grows and moves westward because evaporation is enhanced to the west of increased convection. Turning on nonlinear advection leads to disturbances of wavenumber 10 that translate eastward via advection of dry air by Rossby gyres. Combining nonlinear WISHE and nonlinear advection leads to gross moist instability and prohibits long-term numerical integration, which suggests that QTCM must be refined to fully describe the nonlinear dynamics of the moisture mode.

The Moisture Mode in the Quasi-Equilibrium Tropical Circulation Model. Part I: Analysis Based on the Weak Temperature Gradient Approximation

Abstract The moisture mode in a simplified version of the quasi-equilibrium tropical circulation model (QTCM) of Neelin and Zeng is analyzed. Perturbation expansion based on the ratio of temperature tendency to adiabatic cooling simplifies the system and dispersion relationship. The weak temperature gradient (WTG) approximation of Sobel, Nilsson, and Polvani naturally emerges as the dynamical balance of the moisture mode. The condition of the expansion can be phrased in terms of the nondimensional wavenumber and is satisfied in the tropics even for the planetary scale. The WTG growth rate equation demonstrates that the moisture mode is unstable when moist static energy sources such as cloud radiative forcing and gustiness-enhanced evaporation exceed its export. Wind-induced surface heat exchange does not affect the growth rate at the leading order, although it propagates the mode eastward in the mean easterly wind. For typical values of parameters, the time scale of moisture mode instability is several days. Nonlinear WTG calculations show that the moisture mode nonlinearly saturates by a thermodynamic limiting process. In the standard parameter regime, a phase diagram reveals two stable fixed points in addition to the unstable solution of radiative–convective equilibrium.

Qtcm 0.1.2: a Python implementation of the Neelin-Zeng Quasi-Equilibrium Tropical Circulation Model

Abstract. Historically, climate models have been developed incrementally and in compiled languages like Fortran. While the use of legacy compiled languages results in fast, time-tested code, the resulting model is limited in its modularity and cannot take advantage of functionality available with modern computer languages. Here we describe an effort at using the open-source, object-oriented language Python to create more flexible climate models: the package qtcm, a Python implementation of the intermediate-level Neelin-Zeng Quasi-Equilibrium Tropical Circulation model (QTCM1) of the atmosphere. The qtcm package retains the core numerics of QTCM1, written in Fortran to optimize model performance, but uses Python structures and utilities to wrap the QTCM1 Fortran routines and manage model execution. The resulting "mixed language" modeling package allows order and choice of subroutine execution to be altered at run time, and model analysis and visualization to be integrated in interactively with model execution at run time. This flexibility facilitates more complex scientific analysis using less complex code than would be possible using traditional languages alone, and provides tools to transform the traditional "formulate hypothesis → write and test code → run model → analyze results" sequence into a feedback loop that can be executed automatically by the computer.

Poleward-Propagating Intraseasonal Monsoon Disturbances in an Intermediate-Complexity Axisymmetric Model

Abstract A model of intermediate complexity based on quasi-equilibrium theory—a version of the quasi-equilibrium tropical circulation model with a prognostic atmospheric boundary layer, as well as two free-tropospheric modes in momentum, and one each in moisture and temperature—is used in a zonally symmetric aquaplanet configuration to simulate aspects of the South Asian monsoon and its variability. Key qualitative features of both the mean state and the 30–60-day mode of the intraseasonal variability are simulated satisfactorily. The model has two limit cycles of similar period and structure that can account for this mode. Both feature northward propagation of the tropical convergence zone from 5°S to 25°N with a period of about 50 days. The dynamics of the oscillations are investigated. The system reaches a Hopf bifurcation when the asymmetry of the sea surface temperature (SST) forcing is increased. Beyond the bifurcation, the mean flow is linearly unstable, and the one linearly unstable mode is similar in structure and period to the nonlinear mode. The wind-induced surface heat fluxes are necessary to obtain the instability of the mean monsoon flow, as are the 2 degrees of freedom in the vertical structure of both humidity and wind.

The boundary layer contribution to intertropical convergence zones in the quasi-equilibrium tropical circulation model framework

Theories for the position and intensity of precipitation over tropical oceans on climate time scales have a perplexing disagreement between those that focus on the momentum budget of the atmospheric boundary layer (ABL) and those that focus on thermodynamic factors. In the case of narrow intertropical convergence zones (ITCZs), there is some evidence for both classes of theories, and there are large open questions on the interpretation of the moist static energy (MSE) and momentum budgets of these regions. We develop a model in which both types of mechanisms can operate and the interaction between them can be analyzed. The model includes a mixed-layer ABL, coupled to a free troposphere whose vertical structure follows the quasi-equilibrium tropical circulation model (QTCM) of Neelin and Zeng. The case analyzed here is axisymmetric, using a fixed sea surface temperature (SST) lower boundary condition with an idealized off-equatorial SST maximum. We examine a regime with small values of the gross moist stability associated with tropospheric motions, which is realistic but poses theoretical challenges. In both rotating (equatorial β-plane) and nonrotating cases, the model ITCZ width and intensity are substantially controlled by the horizontal diffusion of moisture, which is hypothesized to be standing in for nonaxisymmetric transients. The inclusion of the ABL increases the amplitude and sharpness of the ITCZ, contributing to the importance of diffusion. Analytical solutions under simplifying assumptions show that the ABL contribution is not singular in the nondiffusive limit; it just features an ITCZ more intense than observed. A negative gross moist stability contribution associated with the flow component driven by ABL momentum dynamics plays a large role in this. Because of the ABL contribution, the flow imports, rather than exports, MSE in the ITCZ, but we show that this can be understood rather simply. The ABL contribution can be approximately viewed as a forcing to the tropospheric thermodynamics. The ABL forcing term is in addition to thermodynamic forcing by net flux terms in the MSE budget, which otherwise is much as in the standard QTCM. The ABL momentum budget suggests that divergent flow in the ABL is controlled to a significant extent by the pressure gradient imprinted on the ABL by the SST gradient—termed the Lindzen–Nigam contribution—although we also find that the thermodynamics mediating this is nontrivial, especially in the rotating case. Nonetheless, when this component of the pressure gradient is artificially removed, the peak ITCZ precipitation is reduced by a fraction on the order of 15 to 25%, less than might have been expected based on the diagnosis of the ABL momentum budget.

Interpretation of simple and cloud-resolving simulations of moist convection–radiation interaction with a mock-Walker circulation

An idealized two-dimensional mock-Walker circulation in the tropical atmosphere forced by prescribed horizontal gradients in sea-surface temperature (SST) is discussed. This model problem includes feedbacks between cumulus convection and tropical large-scale circulations that have proved challenging for global climate models to predict accurately. Three-dimensional cloud-resolving model (CRM) simulations that explicitly simulate turbulent circulations within individual cloud systems across 4,096 and 1,024 km-wide Walker circulations are compared with a simple theoretical model, the Simplified Quasiequilibrium Tropical Circulation Model (SQTCM). This theoretical model combines the weak-temperature-gradient approximation with a unimodal truncation of tropospheric vertical structure coupled to highly simplified formulations of moist precipitating cumulus convection and its cloud-radiative feedbacks. The rainfall, cloud and humidity distribution, circulation strength, energy fluxes and scaling properties are compared between the models. The CRM-simulated horizontal distribution of rainfall and energy fluxes are adequately predicted by the SQTCM. However, the humidity distribution (drier subsidence regions and high-humidity boundary layers in the CRM), vertical structure and domain-size scaling of the circulation differ significantly between the models. For the SQTCM, the concept of gross moist stability – related to advection of moist static energy (MSE) out of tropospheric columns by the mean divergent circulation – is used to explain the width and intensity of the rainy region. Column MSE budgets averaged across the ascent branch of the simulated Walker circulation provide similar insight into the cloud-resolving simulations after consideration of the more complex horizontal and vertical circulation structure and the role of transient eddies. A nondimensional ascent-region moist stability ratio α, analogous to the SQTCM gross moist stability, is developed. One term of α is related to the vertical profiles of ascent-region mean vertical motion and ascent-region edge MSE; a second term is proportional to eddy export from the ascent region. Smaller α induces a narrower, rainier ascent region. The sensitivity of the SQTCM and CRM to a uniform 2 K increase in SST is compared, and the rainy upward branch of the circulation narrows in both models. MSE budget arguments are used to explain this behavior. In the simple model, the gross moist stability is a decreasing function of tropospheric temperature. Hence gross moist stability reduces and the ascent region narrows as the SST increases. In the CRM, increased atmospheric radiative cooling due to the warmer and moister troposphere destabilizes the MSE profile and decreases α, inducing a narrower ascent region. In the CRM, and to a lesser extent in the SQTCM, intensified shortwave cloud forcing in the warmer climate causes a negative radiative feedback on the SST change.

Asymptotic solutions of the axisymmetric moist Hadley circulation in a model with two vertical modes

A simplified model of the moist axisymmetric Hadley circulation is examined in the asymptotic limit in which surface drag is strong and the meridional wind is weak compared to the zonal wind. Our model consists of the quasi-equilibrium tropical circulation model (QTCM) equations on an axisymmetric aquaplanet equatorial beta-plane. This model includes two vertical momentum modes, one baroclinic and one barotropic. Prior studies use either continuous stratification, or a shallow water system best viewed as representing the upper troposphere. The analysis here focuses on the interaction of the baroclinic and barotropic modes, and the way in which this interaction allows the constraints on the circulation known from the fully stratified case to be satisfied in an approximate way. The dry equations, with temperature forced by Newtonian relaxation towards a prescribed radiative equilibrium, are solved first. To leading order, the resulting circulation has a zonal wind profile corresponding to uniform angular momentum at a level near the tropopause, and zero zonal surface wind, owing to the cancelation of the barotropic and baroclinic modes there. The weak surface winds are calculated from the first-order corrections. The broad features of these solutions are similar to those obtained in previous studies of the dry Hadley circulation. The moist equations are solved next, with a fixed sea surface temperature at the lower boundary and simple parameterizations of surface fluxes, deep convection, and radiative transfer. The solutions yield the structure of the barotropic and baroclinic winds, as well as the temperature and moisture fields. In addition, we derive expressions for the width and strength of the equatorial precipitating region (ITCZ) and the width of the entire Hadley circulation. The ITCZ width is on the order of a few degrees in the absence of any horizontal diffusion and is relatively insensitive to parameter variations.

Remote ENSO forcing versus local air-sea interaction in QTCM: a sensitivity study to intraseasonal variability

Abstract. The skill of a newly designed global atmospheric model of intermediate complexity - QTCM (for quasi-equilibrium tropical circulation model) in simulating the teleconnections is investigated. The role of the ENSO remote forcing over the Pacific surrounding regions is emphasized from sensitivity experiments to critical parameters of the model. The role of the tropical intraseasonal variability (ITV) on the simulated ENSO teleconnection pattern is estimated using the methodology proposed by Lin et al. (2000) allowing to damp the energy of ITV in the model. The reduction of intraseasonal variability allows emphasizing the forced response of the atmosphere and eases the detection of local coupled atmosphere-ocean patterns. It was shown that the simulated ITV has an impact on the ENSO teleconnection pattern both in the mid-latitudes and in regions of ascending and descending branches of Walker circulation cells in the tropics.

Reorganization of Tropical Climate during El Niño: A Weak Temperature Gradient Approach

Abstract The applicability of a weak temperature gradient (WTG) formulation for the reorganization of tropical climate during El Niño–Southern Oscillation (ENSO) events is investigated. This idealized dynamical framework solves for the divergent portion of the tropical circulation by assuming a spatially homogeneous perturbation temperature profile and a mass balance constraint applied over the tropical belt. An intermediate-level complexity model [the Quasi-Equilibrium Tropical Circulation Model (QTCM)] configured with the WTG assumptions is used to simulate El Niño conditions and is found to yield an appropriate level of tropospheric warming, a plausible pattern of precipitation anomalies in the tropical Pacific source region of El Niño, and a gross precipitation deficit over the Tropics outside the Pacific (hereafter the “remote Tropics”). Additional tests of the WTG framework with La Niña forcing conditions and enhanced greenhouse gas concentrations support its applicability. However, the ENSO response under the WTG framework fails in some respects when compared to the standard QTCM: in particular, some regional features of the anomalous precipitation response, especially in the remote Tropics, differ markedly between the two model versions. These discrepancies appear to originate in part from the lack of anomalous tropospheric temperature gradients (and circulations) in the framework presented here. Nevertheless, the WTG approach appears to be a useful lowest-order model for the tropical climate adjustment to ENSO. The WTG framework is also used to argue that El Niño may not represent a good proxy for tropical rainfall changes under greenhouse gas warming scenarios because the large-scale subsidence occurring with the tropospheric warming in the El Niño scenario has an effect on rainfall that is distinct from the effect of increased tropospheric temperatures common to both the greenhouse gas warming and El Niño scenarios.

Local and Remote Impacts of Aerosol Climate Forcing on Tropical Precipitation*

Abstract Mechanisms that determine the direct and indirect effects of aerosols on the tropical climate involve moist dynamical processes and have local and remote impacts on regional tropical precipitation. These mechanisms are examined in a climate model of intermediate complexity [quasi-equilibrium tropical circulation model (QTCM)] forced by prescribed aerosol forcing, which is obtained from a general circulation model (ECHAM4). The aerosol reflection is the dominant aerosol forcing, while the aerosol absorption has complex but much weaker influences on the regional tropical precipitation based on the ECHAM4 aerosol forcing. The local effect associated with aerosols contributes negative precipitation anomalies over convective regions by affecting the net energy flux into the atmospheric column. This net energy flux is controlled by the radiative forcing at the top of the atmosphere on time scales where surface heat flux is near equilibrium, balancing anomalous solar radiation by evaporation, longwave radiation, and sensible heat. Considering the aerosol absorption effect alone, the associated precipitation anomalies are slightly negative but small when surface heat fluxes are near equilibrium. Two effects found in global warming, the upped-ante mechanism and the anomalous gross moist stability mechanism, occur with opposite sign in the aerosol case. Both act as remote effects via the widespread cold tropospheric temperature anomalies induced by the aerosol forcing. In the upped-ante mechanism in global warming, a warm troposphere increases the low-level moisture “ante” required for convection, creating spatially varying moisture anomalies that disfavor precipitation on those margins of convective zones where the mean flow imports air from nonconvective regions. In the aerosol case here, a cool troposphere preferentially decreases moisture in convective regions, creating positive precipitation anomalies at inflow margins. In the anomalous gross moist stability mechanism for the aerosol case, the decrease in moisture in convective regions acts to enhance the gross moist stability, so convection and the associated precipitation are reduced. The partitioning between the aerosol local and remote effects on regional tropical precipitation differs spatially. Over convective regions that have high aerosol concentration, such as the South American region, the aerosol local effect contributes more negative precipitation anomalies than the anomalous gross moist stability mechanism in the QTCM simulations. On the other hand, the remote effect is more important over convective regions with small aerosol concentrations, such as the western Pacific Maritime Continent. Remote effects of midlatitude aerosol forcing have a substantial contribution to tropical anomalies.

Mechanisms of Remote Tropical Surface Warming during El Niño

Abstract The authors demonstrate through atmospheric general circulation model (the Community Climate Model version 3.10) simulations of the 1997/98 El Niño that the observed “remote” (i.e., outside the Pacific) tropical land and ocean surface warming appearing a few months after the peak of the El Niño event is causally linked to the Tropics-wide warming of the troposphere resulting from increased atmospheric heating in the Pacific, with the latter acting as a conduit for the former. Unlike surface temperature, the surface flux behavior in the remote Tropics in response to El Niño is complex, with sizable spatial variation and compensation between individual flux components; this complexity suggests a more fundamental control (i.e., tropospheric temperature) for the remote tropical surface warming. Over the remote oceans, latent heat flux acting through boundary layer humidity variations is the important regulator linking the surface warming in the model simulations to the tropospheric warming over the remote tropical oceans. Idealized 1997/98 El Niño simulations using an intermediate tropical circulation model (the Quasi-Equilibrium Tropical Circulation Model) in which individual surface fluxes are directly manipulated confirms this result. The findings over the remote ocean are consistent with the “tropospheric temperature mechanism” previously proposed for the tropical ENSO teleconnection, with equatorial planetary waves propagating tropospheric temperature anomalies from the eastern Pacific to the remote Tropics and moist convective processes mediating the troposphere-to-remote-surface connection. The latter effectively requires the boundary layer moist static energy to vary in concert with the free tropospheric moist static energy. Over the remote land regions, idealized model simulations suggest that sensible heat flux regulates the warming response to El Niño, though the underlying mechanism has not yet been fully determined.

Mechanisms for Lagged Atmospheric Response to ENSO SST Forcing*

Abstract The mechanism and sensitivity of the lagged response of tropical tropospheric temperature to El Niño–Southern Oscillation (ENSO) SST forcing are examined using the Quasi-Equilibrium Tropical Circulation Model (QTCM) coupled to a slab mixed layer ocean model, along with a simple analytical model. It is found that the lag and amplitude of tropospheric temperature response depend on mixed layer depth (MLD), ENSO SST forcing period, areal fraction of the mixed layer ocean, and the strength of Tropics to midlatitude transports. The phase lag is not a monotonic function of mixed layer depth. It maximizes at moderate MLD and, thus, is not very sensitive to MLD in the realistic range. The phase lag asymptotes to values determined by free-atmospheric time scales, between 1 and 2 months, for small or large values of MLD. The amplitude of the tropospheric temperature response decreases with increasing MLD. The phase lag and amplitude of tropospheric temperature both increase as a specified ENSO SST forcing period increases and they appear to be rather insensitive to the seasonal cycle of SST. On the other hand, the phase lag and amplitude of mixed layer ocean SST change monotonically with MLD and ENSO forcing period, with a deeper mixed layer producing longer lag and smaller amplitude of SST anomalies. Longer ENSO SST forcing periods correspond to longer lag and larger amplitude of mixed layer ocean SST anomalies. While the ENSO region convective heating (precipitation) anomalies are closely tied to SST anomalies, the tropical mean precipitation seems best viewed as a complex by-product of the response rather than as a driver. One useful parameter determining the lag of tropospheric temperature to ENSO SST is the freedecay time scale of the coupled system. This parameter combines the effects of surface flux exchanges, heat loss at the top of the atmosphere and from the Tropics to midlatitudes, and finite ocean heat capacity. It is indicative of the extent to which the lagged response of tropical tropospheric temperature to ENSO SST is a coupled phenomenon. Overall, the contribution of coupling to SST outside the ENSO region substantially increases the amplitude and lag of the tropospheric temperature response to ENSO.

Moist Teleconnection Mechanisms for the Tropical South American and Atlantic Sector*

AbstractTeleconnections have traditionally been studied for the case of dry dynamical response to a given diabatic heat source. Important anomalies often occur within convective zones, for instance, in the observed remote response to El Niño. The reduction of rainfall and teleconnection propagation in deep convective regions poses theoretical challenges because feedbacks involving convective heating and cloud radiative effects come into play. Land surface feedbacks, including variations of land surface temperature, and ocean surface layer temperature response must be taken into account. During El Niño, descent and negative precipitation anomalies often extend across equatorial South America and the Atlantic intertropical convergence zone. Analysis of simulated mechanisms in a case study of the 1997/98 El Niño is used to illustrate the general principals of teleconnections occurring in deep convective zones, contrasting land and ocean regions. Comparison to other simulated events shows similar behavior. Tropospheric temperature and wind anomalies are spread eastward by wave dynamics modified by interaction with the moist convection zones. The traditional picture would have gradual descent balanced by radiative damping, but this scenario misses the most important balances in the moist static energy (MSE) budget. A small “zoo” of mechanisms is active in producing strong regional descent anomalies and associated drought. Factors common to several mechanisms include the role of convective quasi equilibrium (QE) in linking low-level moisture anomalies to free tropospheric temperature anomalies in a two-way interaction referred to as QE mediation. Convective heating feedbacks change the net static stability to a gross moist stability (GMS) M. The large cloud radiative feedback terms may be manipulated to appear as a modified static stability Meff, under approximations that are quantified for the quasi-equilibrium tropical circulation model used here. The relevant measure of Meff differs between land, where surface energy flux balance applies, and short time scales over ocean. For the time scale of an onsetting El Niño, a mixed layer ocean response is similar to a fixed sea surface temperature (SST) case, with surface fluxes lost into the ocean and Meff substantially reduced over ocean-enhancing descent anomalies. Use of Meff aids analysis of terms that act as the initiators of descent anomalies. Apparently modest terms in the MSE budget can be acted on by the GMS multiplier effect, which yields substantial precipitation anomalies due to the large ratio of the moisture convergence to the MSE divergence. Advection terms enter in several mechanisms, with the leading effects here due to advection by mean winds in both MSE and momentum balances. A Kelvinoid solution is presented as a prototype for how easterly flow enhances moist wave decay mechanisms, permitting relatively small damping terms by surface drag and radiative damping to produce the substantial eastward temperature gradients seen in observations and simulations and contributing to precipitation anomalies. The leading mechanism for drought in eastern equatorial South America is the upped-ante mechanism in which QE mediation of teleconnected tropospheric temperature anomalies tends to produce moisture gradients between the convection zone, where low-level moisture increases toward QE, and the neighboring nonconvective region. Over the Atlantic ITCZ, the upped-ante mechanism is a substantial contributor, but on short time scales several mechanisms referred to jointly as troposphere/SST disequilibrium mechanisms are important. While SST is adjusting during passive SST (coupled ocean mixed layer) experiments, or for fixed SST, heat flux to the ocean is lost to the atmosphere, and these mechanisms can induce descent and precipitation anomalies, although they disappear when SST equilibrates. In simulations here, cloud radiative feedbacks, surface heat fluxes induced by teleconnected wind anomalies, and surface fluxes induced by QE-mediated temperature anomalies are significant disequilibrium contributors. At time scales of several months or longer, remaining Atlantic ITCZ rainfall reductions are maintained by the upped-ante mechanism.

Sensitivity of Tropical Tropospheric Temperature to Sea Surface Temperature Forcing*

Abstract During El Niño, there are substantial tropospheric temperature anomalies across the entire tropical belt associated with the warming of sea surface temperatures (SSTs) in the central and eastern Pacific. The quasi-equilibrium tropical circulation model (QTCM) is used to investigate the mechanisms for tropical tropospheric temperature response to SST forcing. In both observations and model simulations, the tropical averaged tropospheric temperature anomaly 〈T̂′〉 is approximately linear with the tropical mean SST anomaly 〈T′s〉 for observed SST forcing. Regional SST anomaly experiments are used to estimate regional sensitivity measures and quantify the degree of nonlinearity. For instance, SST anomalies of 3°C in the central Pacific would give a nonlinear 〈T̂′〉 response about 15% greater than a linear fit to small SST anomaly experiments would predict, but for the maximum observed SST anomaly in this region the response differs by only 5% from linearity. Nonlinearity in 〈T̂′〉 response is modest even when local precipitation response is highly nonlinear. While temperature anomalies have large spatial scales, the main precipitation anomaly tends to be local to the SST anomaly regions. The tropical averaged precipitation anomalies 〈P′〉 do not necessarily have a simple relation to tropical averaged tropospheric temperature anomalies or SST forcing. The approximate linearity of the 〈T̂′〉 response is due to two factors: 1) the strong nonlinearities that occur locally tend to be associated with the transport terms, which become small in the large-area average; and 2) the dependence on temperature of the top-of-atmosphere and surface fluxes has only weak nonlinearity over the range of 〈T̂′〉 variations. Analytical approximations to the QTCM suggest that the direct impact of climatological SST, via flux terms, contributes modestly to regional variations in the sensitivity α of 〈T̂′〉 to 〈T′s〉. Wind speed has a fairly strong effect on α but tends to oppose the direct effect of SST since cold SST regions often have stronger climatological wind, which would yield larger slopes. A substantial contribution to regional variation in α comes from the different reaction of moisture to SST anomalies in precipitating and nonprecipitating regions. Although regions over climatologically warm water have a slightly higher sensitivity, subregions of El Niño SST anomalies even in the colder eastern Pacific contribute substantially to tropospheric temperature anomalies.