The summer rainfall climate of East Asia underwent large and abrupt changes during past climates, in response to precessional forcing, glacial–interglacial cycles as well as abrupt changes to the ...North Atlantic during the Last Glacial. However, current interpretations of said changes are typically formulated in terms of modulation of summer monsoon intensity, and do not account for the known complexity in the seasonal evolution of East Asian rainfall, which exhibits sharp transition from the Spring regime to the Meiyu, and then again from the Meiyu to the Summer regime.
We explore the interpretation that East Asian rainfall climate undergoes a modulation of its seasonality during said paleoclimate changes. Following previous suggestions we focus on role of the westerly jet over Asia, namely that its latitude relative to Tibet is critical in determining the stepwise transitions in East Asian rainfall seasons. In support of this linkage, we show from observational data that the interannual co-variation of June (July–August) rainfall and upper tropospheric zonal winds show properties consistent with an altered timing of the transition to the Meiyu (Summer), and with more northward-shifted westerlies for earlier transitions.
We similarly suggest that East Asian paleoclimate changes resulted from an altered timing in the northward evolution of the jet and hence the seasonal transitions, in particular the transition of the jet from south of the Plateau to the north that determines the seasonal transition from Spring rains to the Meiyu. In an extreme scenario – which we speculate the climate system tended towards during stadial (cold) phases of D/O stadials and periods of low Northern Hemisphere summer insolation – the jet does not jump north of the Plateau, essentially keeping East Asia in prolonged Spring conditions.
We argue that this hypothesis provides a viable explanation for a key paleoproxy signature of D/O stadials over East Asia, namely the heavier mean δ18O of precipitation as recorded in speleothem records. The southward jet position prevents the low-level monsoonal flow – which is isotopically light – from penetrating into the interior of East Asia; as such, precipitation there will be heavier, consistent with speleothem records. This hypothesis can also explain other key evidences of East Asian paleoclimate changes, in particular the occurrence of dusty conditions during North Atlantic stadials, and the southward migration of the Holocene optimal rainfall.
•We explore role of seasonal rainfall transitions in East Asian paleoclimate change.•Seasonal regimes determined by meridional position of westerlies relative to Tibet.•East Asian paleoclimate changes reflect systematic meridional shifts to westerlies.•Modern-day analogs and model simulations support this hypothesis.•Hypothesis may partly explain cave records of East Asian paleoclimate.
Abstract
The radiative forcing from carbon dioxide is approximately logarithmic in its concentration, producing about 4 W m
−2
of global-mean forcing for each doubling. Although these are basic facts ...of climate science, competing explanations for them have been given in the literature. Here, the reasons for the logarithmic forcing of carbon dioxide are explored in detail and a simplified model for the forcing is constructed. An essential component is the particular distribution of absorption coefficients within the 15-
μ
m band of carbon dioxide. An alternative explanation, which does not depend on the spectrum of carbon dioxide but instead hinges on the tropospheric lapse rate, is shown to be neither necessary nor sufficient to explain the logarithmic forcing of carbon dioxide and to be generally inapplicable to well-mixed greenhouse gases in Earth’s atmosphere.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A new formulation of the weak pressure gradient approximation (WPG) is introduced for parameterizing large-scale dynamics in limited-domain atmospheric models. This new WPG is developed in the ...context of the one-dimensional, linearized, damped, shallow-water equations and then extended to Boussinesq and compressible fluids. Unlike previous supradomain-scale parameterizations, this formulation of WPG correctly reproduces both steady-state solutions and first baroclinic gravity waves. In so doing, this scheme eliminates the undesirable gravity wave resonance in previous versions of WPG. In addition, this scheme can be extended to accurately model the emission of gravity waves with arbitrary vertical wavenumber.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
The baroclinic-mode decomposition is a fixture of the tropical-dynamics literature because of its simplicity and apparent usefulness in understanding a wide range of atmospheric phenomena. ...However, its derivation relies on the assumption that the tropopause is a rigid lid that artificially restricts the vertical propagation of wave energy. This causes tropospheric buoyancy anomalies of a single vertical mode to remain coherent for all time in the absence of dissipation. Here, the authors derive the Green’s functions for these baroclinic modes in a two-dimensional troposphere (or, equivalently, a three-dimensional troposphere with one translational symmetry) that is overlain by a stratosphere. These Green’s functions quantify the propagation and spreading of gravity waves generated by a horizontally localized heating, and they can be used to reconstruct the evolution of any tropospheric heating. For a first-baroclinic two-dimensional right-moving or left-moving gravity wave with a characteristic width of 100 km, its initial horizontal shape becomes unrecognizable after 4 h, at which point its initial amplitude has also been reduced by a factor of 1/π. After this time, the gravity wave assumes a universal shape that widens linearly in time. For gravity waves on a periodic domain the length of Earth’s circumference, it takes only 10 days for the gravity waves to spread their buoyancy throughout the entire domain.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
The radiative forcing from carbon dioxide is approximately logarithmic in its concentration, producing about 4 W m
−2
of global-mean forcing for each doubling. Although these are basic facts ...of climate science, competing explanations for them have been given in the literature. Here, the reasons for the logarithmic forcing of carbon dioxide are explored in detail and a simplified model for the forcing is constructed. An essential component is the particular distribution of absorption coefficients within the 15-
μ
m band of carbon dioxide. An alternative explanation, which does not depend on the spectrum of carbon dioxide but instead hinges on the tropospheric lapse rate, is shown to be neither necessary nor sufficient to explain the logarithmic forcing of carbon dioxide and to be generally inapplicable to well-mixed greenhouse gases in Earth’s atmosphere.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Previous studies have shown that galaxies with minor companions exhibit an elevated star formation rate. We reverse this inquiry, constructing a volume-limited sample of ∼L
★ (M
r
≤−19.5 + ...5log h) galaxies from the Sloan Digital Sky Survey Data Release 6 that are isolated with respect to other luminous galaxies. Cosmological simulations suggest that 99.8 per cent of these galaxies are alone in their dark matter haloes with respect to other luminous galaxies. We search the area around these galaxies for photometric companions. Matching strongly star forming EW(Hα) ≥ 35 Å and quiescent EW(Hα) < 35 Å samples for stellar mass and redshift using a Monte Carlo resampling technique, we demonstrate that rapidly star forming galaxies are more likely to have photometric companions than other galaxies. The effect is relatively small; about 11 ± 1 per cent of quiescent, isolated galaxies have minor photometric companions at radii ≤60 kpc h
−1 kpc, while about 16 ± 1 per cent of strongly star forming ones do. Though small, the cumulative difference in satellite counts between strongly star forming and quiescent galaxies is highly statistically significant (P
KS= 1.350 × 10−3) out to radii of ∼100 h
−1 kpc. We discuss explanations for this excess, including the possibility that ∼5 per cent of strongly star forming galaxies have star formation which is causally related to the presence of a minor companion.
Large‐scale dynamics parameterizations are tested numerically in cloud‐resolving simulations, including a new version of the weak‐pressure‐gradient approximation (WPG) introduced by Edman and Romps ...(2014), the weak‐temperature‐gradient approximation (WTG), and a prior implementation of WPG. We perform a series of self‐consistency tests with each large‐scale dynamics parameterization, in which we compare the result of a cloud‐resolving simulation coupled to WTG or WPG with an otherwise identical simulation with prescribed large‐scale convergence. In self‐consistency tests based on radiative‐convective equilibrium (RCE; i.e., no large‐scale convergence), we find that simulations either weakly coupled or strongly coupled to either WPG or WTG are self‐consistent, but WPG‐coupled simulations exhibit a nonmonotonic behavior as the strength of the coupling to WPG is varied. We also perform self‐consistency tests based on observed forcings from two observational campaigns: the Tropical Warm Pool International Cloud Experiment (TWP‐ICE) and the ARM Southern Great Plains (SGP) Summer 1995 IOP. In these tests, we show that the new version of WPG improves upon prior versions of WPG by eliminating a potentially troublesome gravity‐wave resonance.
Key Points:
Self‐consistency tests isolate errors due to parameterized large‐scale dynamics
WPG and WTG pass self‐consistency tests based on RCE
A new version of WPG eliminates an undesirable gravity‐wave resonance
Large-scale dynamics parameterizations are tested numerically in cloud-resolving simulations, including a new version of the weak-pressure-gradient approximation (WPG) introduced by Edman and Romps ...(2014), the weak-temperature-gradient approximation (WTG), and a prior implementation of WPG. We perform a series of self-consistency tests with each large-scale dynamics parameterization, in which we compare the result of a cloud-resolving simulation coupled to WTG or WPG with an otherwise identical simulation with prescribed large-scale convergence. In self-consistency tests based on radiative-convective equilibrium (RCE; i.e., no large-scale convergence), we find that simulations either weakly coupled or strongly coupled to either WPG or WTG are self-consistent, but WPG-coupled simulations exhibit a nonmonotonic behavior as the strength of the coupling to WPG is varied. We also perform self-consistency tests based on observed forcings from two observational campaigns: the Tropical Warm Pool International Cloud Experiment (TWP-ICE) and the ARM Southern Great Plains (SGP) Summer 1995 IOP. In these tests, we show that the new version of WPG improves upon prior versions of WPG by eliminating a potentially troublesome gravity-wave resonance.
Moist convective clouds play a key role in the earth's energy and water cycles, but their turbulent and localized nature makes it difficult to study how they interact with the large-scale atmospheric ...circulation. In this dissertation, we study the interaction between convection and the large-scale tropical atmosphere in two ways. First, we develop simple analytical models for how convection interacts with large-scale circulations in the tropical atmosphere. These models are primarily based on a traditional-but-crude approximation of gravity wave dynamics, in which the tropopause is assumed to be a rigid lid. We test these models in cloud-resolving numerical simulations of a small patch of tropical atmosphere and determine that some of these methods pass a simple test of self-consistency, but are far from perfect. Alternately, we take a more sophisticated analytical approach—instead of using the traditional rigid lid model of the tropical atmosphere, we derive novel analytical solutions for pulses of buoyancy (a crude representation of the effects of convective clouds) in an atmosphere with both a troposphere and a stratosphere. This construction allows wave energy to realistically radiate out of the troposphere. We find that allowing wave energy to radiate out of the troposphere causes buoyancy anomalies in the troposphere to decay on timescales of hours to days, in stark contrast to the rigid lid model, which predicts that buoyancy anomalies persist forever in the absence of dissipation.
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