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Nicholas J. Lutsko

Abstract

An equatorial heat source mimicking the strong diabatic heating above the west Pacific is added to an idealized, dry general circulation model. For small (<0.5 K day−1) heating rates the responses closely match the expectations from linear Matsuno–Gill theory, though the amplitudes of the responses increase sublinearly. This “linear” regime breaks down for larger heating rates and it is found that this is because the stability of the tropical atmosphere increases. At the same time, the equatorial winds increasingly superrotate. This superrotation is driven by stationary eddy momentum fluxes by the waves excited by the heating and is damped by the vertical advection of low-momentum air by the mean flow and, at large heating rates, by the divergence of momentum by transient eddies.

These dynamics are explored in additional experiments in which the equator-to-pole temperature gradient is varied. Very strong superrotation is produced when a large heating rate is applied to a setup with a relatively weak equator-to-pole temperature gradient, though there is no evidence that this is a case of “runaway” superrotation.

Open access
Nicholas J. Lutsko

Abstract

The nonacceleration theorem states that the torque exerted on the atmosphere by orography is exactly balanced by the convergence of momentum by the stationary waves that the orography excites. This balance is tested in simulations with a stationary wave model and with a dry, idealized general circulation model (GCM), in which large-scale orography is placed at the latitude of maximum surface wind speed. For the smallest mountain considered (maximum height H = 0.5 m), the nonacceleration balance is nearly met, but the damping in the stationary wave model induces an offset between the stationary eddy momentum flux (EMF) convergence and the mountain torque, leading to residual mean flow changes. A stationary nonlinearity appears for larger mountains (H ≥ 10 m), driven by preferential deflection of the flow around the poleward flank of the orography, and causes further breakdown of the nonacceleration balance. The nonlinearity grows as H is increased, and is stronger in the GCM than in the stationary wave model, likely due to interactions with transient eddies. The midlatitude jet shifts poleward for H ≤ 2 km and equatorward for larger mountains, reflecting changes in the transient EMFs, which push the jet poleward for smaller mountains and equatorward for larger mountains. The stationary EMFs consistently force the jet poleward. These results add to our understanding of how orography affects the atmosphere’s momentum budget, providing insight into how the nonacceleration theorem breaks down; the roles of stationary nonlinearities and transients; and how orography affects the strength and latitude of eddy-driven jets.

Free access
Nicholas J. Lutsko

Abstract

Increases in the severity of heat stress extremes are potentially one of the most impactful consequences of climate change, affecting human comfort, productivity, health, and mortality in many places on Earth. Heat stress results from a combination of elevated temperature and humidity, but the relative contributions of each of these to heat stress changes have yet to be quantified. Here, conditions for the baseline specific humidity are derived for when specific humidity or temperature dominates heat stress changes, as measured using the equivalent potential temperature (θ E ). Separate conditions are derived over ocean and over land, in addition to a condition for when relative humidity changes make a larger contribution than the Clausius–Clapeyron response at fixed relative humidity. These conditions are used to interpret the θ E responses in transient warming simulations with an ensemble of models participating in phase 6 of the Climate Model Intercomparison Project. The regional pattern of θ E changes is shown to be largely determined by the pattern of specific humidity changes, with the pattern of temperature changes playing a secondary role. This holds whether considering changes in seasonal-mean θ E or in extreme (98th-percentile) θ E events, and uncertainty in the response of specific humidity to warming is shown to be the leading source of uncertainty in the θ E response at most land locations. Finally, analysis of ERA5 data demonstrates that the pattern of observed θ E changes is also well explained by the pattern of specific humidity changes. These results demonstrate that understanding regional changes in specific humidity is largely sufficient for understanding regional changes in heat stress.

Full access
Pengcheng Zhang
and
Nicholas J. Lutsko

Abstract

Although Earth’s troposphere does not superrotate in the annual mean, for most of the year—from October to May—the winds of the tropical upper troposphere are westerly. We investigate this seasonal superrotation using reanalysis data and a single-layer model for the winds of the tropical upper troposphere. We characterize the temporal and spatial structures of the tropospheric superrotation, and quantify the relationships between the superrotation and the leading modes of tropical interannual variability. We also find that the strength of the superrotation has remained roughly constant over the past few decades, despite the winds of the tropical upper troposphere decelerating (becoming more easterly) in other months. We analyze the monthly zonal-mean zonal momentum budget and use numerical simulations with an axisymmetric, single-layer model of the tropical upper troposphere to study the underlying dynamics of the seasonal superrotation. Momentum flux convergence by stationary eddies accelerates the superrotation, while cross-equatorial easterly momentum transport associated with the Hadley circulation decelerates the superrotation. The seasonal modulations of these two competing factors shape the superrotation. The single-layer model is able to qualitatively reproduce the seasonal progression of the winds in the tropical upper troposphere, and highlights the northward displacement of the intertropical convergence zone in the annual mean as a key factor responsible for the annual cycle of the tropical winds.

Restricted access
Nicholas J. Lutsko
and
Momme C. Hell

Abstract

Annular modes are the leading mode of variability in extratropical atmospheres, and a key source of predictability at midlatitudes. Previous studies of annular modes have primarily used dry atmospheric models, so that moisture’s role in annular mode dynamics is still unclear. In this study, a moist two-layer quasigeostrophic channel model is used to study the effects of moisture on annular mode persistence. Using a channel model allows moisture’s direct effects to be studied, rather than changes in persistence due to geometric effects associated with shifts in jet latitude on the sphere. Simulations are performed in which the strength of latent heat release is varied to investigate how annular mode persistence responds as precipitation becomes a leading term in the thermodynamic budget. At short lags (<20 model days, ≈4 Earth days), moisture increases annular mode persistence, reflecting weaker eddy activity that is less effective at disrupting zonal-mean wind anomalies. Comparisons to dry simulations with weaker mean flows demonstrate that moisture is particularly effective at damping high-frequency eddies, further enhancing short-lag persistence. At long lags (>20 model days), moisture weakly increases persistence, though it decreases the amplitudes of low-frequency annular mode anomalies. In the most realistic simulation, the greater short-lag persistence increases the e-folding time of the zonal index by 21 model days (≈4 Earth days). Moisture also causes a transition to propagating variability, though this does not seem to affect the leading mode’s persistence.

Full access
Nicholas J. Lutsko
and
Isaac M. Held

Abstract

A dry atmospheric general circulation model is forced with large-scale, Gaussian orography in an attempt to isolate a regime in which the model responds linearly to orographic forcing and then to study the departures from linearity as the orography is increased in amplitude. In contrast to previous results, which emphasized the meridional propagation of orographically forced stationary waves, using the standard Held–Suarez (H–S) control climate, it is found that the linear regime is characterized by a meridionally trapped, zonally propagating wave. Meridionally trapped waves of this kind have been seen in other contexts, where they have been termed “circumglobal waves.” As the height of the orography is increased, the circumglobal wave coexists with a meridionally propagating wave and for large-enough heights the meridionally propagating wave dominates the response. A barotropic model on a sphere reproduces this trapped wave in the linear regime and also reproduces the transition to meridional propagation with increasing amplitude. However, mean-flow modification by the stationary waves is very different in the two models, making it difficult to argue that the transitions have the same causes. When adding asymmetry across the equator to the H–S control climate and placing the orography in the cooler hemisphere, it becomes harder to generate trapped waves in the GCM and the trapping becomes sensitive to the shape of the orography. The barotropic model overestimates the trapping in this case. These results suggest that an improved understanding of the role of circumglobal waves will be needed to understand the stationary wave field and its sensitivity to the changes in the zonal-mean climate.

Full access
Nicole K. Neumann
and
Nicholas J. Lutsko

Abstract

The factors controlling the present-day pattern of temperature variance are poorly understood. In particular, it is unclear why the variance of wintertime near-surface temperatures on daily and synoptic time scales is roughly twice as high over North America as over Eurasia. In this study, continental geometry’s role in shaping regional wintertime temperature variance is investigated using idealized climate model simulations run with midlatitude continents of different shapes. An isolated, rectangular midlatitude continent suggests that in the absence of other geographic features, the highest temperature variance will be located in the northwest of the continent, roughly collocated with the region of largest meridional temperature gradients, and just north of the maximum near-surface wind speeds. Simulations with other geometries, mimicking key features of North America and Eurasia, investigate the impacts of continental length and width, sloping coastlines, and inland bodies of water on regional temperature variance. The largest effect comes from tapering the northwest corner of the continent, similar to Eurasia, which substantially reduces the maximum temperature variance. Narrower continents have smaller temperature variance in isolation, implying that the high variances over North America must be due to the nonlocal influence of stationary waves. Support for this hypothesis is provided by simulations with two midlatitude continents, which show how continental geometry and stationary waves can combine to shape regional temperature variance.

Significance Statement

Wintertime temperature variance over North America is roughly twice as high as over Eurasia, but the reasons for this are unknown. Here we use idealized climate model simulations to investigate how continental geometry shapes regional temperature variance. We find that the smaller variance over Eurasia is largely due to the tapering of its northwest coast, which weakens temperature gradients in the continental interior. Our simulations also suggest that in isolation a narrow continent, like North America, should have weak temperature variance, implying that stationary waves are responsible for the high variance over North America. Understanding the controls on regional temperature variance is important for interpreting present-day winter climates and how these will change in the future.

Open access
Nicholas J. Lutsko
and
Max Popp

Abstract

The relative contributions of the meridional gradients in insolation and in longwave optical depth (caused by gradients in water vapor) to the equator-to-pole temperature difference, and to Earth’s climate in general, have not been quantified before. As a first step to understanding these contributions, this study investigates simulations with an idealized general circulation model in which the gradients are eliminated individually or jointly, while keeping the global means fixed. The insolation gradient has a larger influence on the model’s climate than the gradient in optical depth, but both make sizeable contributions and the changes are largest when the gradients are reduced simultaneously. Removing either gradient increases global-mean surface temperature due to an increase in the tropospheric lapse rate, while the meridional surface temperature gradients are reduced. “Global warming” experiments with these configurations suggest similar climate sensitivities; however, the warming patterns and feedbacks are quite different. Changes in the meridional energy fluxes lead to polar amplification of the response in all but the setup in which both gradients are removed. The lapse-rate feedback acts to polar amplify the responses in the Earth-like setup, but is uniformly negative in the other setups. Simple models are used to interpret the results, including a prognostic model that can accurately predict regional surface temperatures, given the meridional distributions of insolation and longwave optical depths.

Open access
Nicholas J. Lutsko
and
Ken Takahashi

Abstract

The relationship between climate models’ internal variability and their response to external forcings is investigated. Frequency-dependent regressions are performed between the outgoing top-of-atmosphere (TOA) energy fluxes and the global-mean surface temperature in the preindustrial control simulations of the CMIP5 archive. Two distinct regimes are found. At subdecadal frequencies the surface temperature and the outgoing shortwave flux are in quadrature, while the outgoing longwave flux is linearly related to temperature and acts as a negative feedback on temperature perturbations. On longer time scales the outgoing shortwave and longwave fluxes are both linearly related to temperature, with the longwave continuing to act as a negative feedback and the shortwave acting as a positive feedback on temperature variability. In addition to the different phase relationships, the two regimes can also be seen in estimates of the coherence and of the frequency-dependent regression coefficients. The frequency-dependent regression coefficients for the total cloudy-sky flux on time scales of 2.5 to 3 years are found to be strongly (r 2 > 0.6) related to the models’ equilibrium climate sensitivities (ECSs), suggesting a potential “emergent constraint” for Earth’s ECS. However, O(100) years of data are required for this relationship to become robust. A simple model for Earth’s surface temperature variability and its relationship to the TOA fluxes is used to provide a physical interpretation of these results.

Open access
Nicholas J. Lutsko
,
John Marshall
, and
Brian Green

Abstract

Motivated by observations of southward ocean heat transport (OHT) in the northern Indian Ocean during summer, the role of the ocean in modulating monsoon circulations is explored by coupling an atmospheric model to a slab ocean with an interactive representation of OHT and an idealized subtropical continent. Southward OHT by the cross-equatorial cells is caused by Ekman flow driven by southwesterly monsoon winds in the summer months, cooling sea surface temperatures (SSTs) south of the continent. This increases the reversed meridional surface gradient of moist static energy, shifting the precipitation maximum over the land and strengthening the monsoonal circulation, in the sense of enhancing the vertical wind shear. However, the atmosphere’s cross-equatorial meridional overturning circulation is also weakened by the presence of southward OHT, as the atmosphere is required to transport less energy across the equator. The sensitivity of these effects to varying the strength of the OHT, fixing the OHT at its annual-mean value, and to removing the land is explored. Comparisons with more realistic models suggest that the idealized model used in this study produces a reasonable representation of the effect of OHT on SSTs equatorward of subtropical continents, and hence can be used to study the role of OHT in shaping monsoon circulations on Earth.

Open access