Changes In Meridional Temperature Gradient Research Articles

Warmer SSTs in the Equatorial Eastern Pacific Delay the Seasonal March of the Asian–Pacific Summer Monsoon Onset

Abstract This study investigates the impact of El Niño–Southern Oscillation (ENSO) on the seasonal evolution of the Asian–Pacific summer monsoon. The results show that during the summers when the SSTs in the equatorial eastern Pacific are above normal (i.e., during developing El Niño), the seasonal march of the Asian–Pacific summer monsoon is delayed. Specifically, the stepwise northeastward extension of the western North Pacific monsoon and the northward migration of East Asian and Indian monsoon are all delayed, resulting in rainfall deficiency in the western North Pacific and India, and more rainfall in the Yangtze River valley but less rainfall in North China due to the delayed northward migration and resultant prolonged rainy season in the Yangtze River valley. In addition, the seasonal march of the Asian upper-tropospheric westerly jet, which is mainly characterized by the northward migration during the monsoon development, is also delayed, concurrent with the delayed march of monsoonal rainfall. During La Niña summers, however, the seasonal march of the monsoon and Asian jet does not exhibit clear variations, suggesting asymmetrical impacts between El Niño and La Niña. Furthermore, we explain these variations in the seasonal evolution of rainfall and jet and the asymmetrical impacts between El Niño and La Niña through the variations in tropical tropospheric temperatures and the resultant changes in meridional temperature gradient. Significance Statement The seasonal march of the Asian–Pacific summer monsoon exhibits large spatial and temporal variations, but shows some regular features such as stepwise northward migrations. The impacts of ENSO on the seasonal evolution of the components of Asian–Pacific summer monsoon have been shown in previous studies. However, there have been remarkable uncertainties in the previous results, and more emphasis has been put on the impacts of preceding ENSO. This study starts from El Niño and La Niña summers to reduce uncertainty caused by defining the seasonal march of the monsoon, and includes the variations in circulations that correspond well to monsoonal rainfall, making the conclusions more reliable.

The Role of Temperature Gradients versus Static Stability on the Zonal Wind and Eddy Kinetic Energy Response to Thermal Perturbations

Abstract The relative roles of meridional temperature gradients and static stability with regard to the extratropical response to thermal perturbations are explored in a dry primitive equation (PE) GCM. A quasigeostrophic (QG) model is used to separate the relative roles because the static stability in a QG model is externally prescribed and can therefore be altered independently of the meridional temperature gradient. For most experiments, the changes in meridional temperature gradients make the largest contribution to the zonal-mean zonal wind (U) changes, although the static stability changes are also important. In most cases, the mechanism for the static stability response does not directly involve the eddies. Instead, the advection of the static stability perturbation by the zonal-mean vertical velocity accounts for most of the U response to static stability. For increased static stability, advection by the Ferrel cell increases the meridional temperature gradient, which strengthens the jet via thermal wind. The stronger jet then shifts poleward, consistent with various theories in the literature. For eddy kinetic energy (EKE), the direct effect of static stability makes a nonnegligible contribution in most cases; however, the meridional temperature gradient and advection by Ferrel cell effects together are usually more dominant.

Enhanced precipitation in the Gulf of Mexico during the Eocene–Oligocene transition driven by interhemispherical temperature asymmetry

Abstract Studies reveal that the sea-surface temperature (SST) of the Northern Hemisphere decreased at a smaller amplitude than that of the Southern Hemisphere during the Eocene–Oligocene transition (EOT). This interhemispheric temperature asymmetry has been associated with intensified Atlantic Meridional Overturning Circulation (AMOC) that may have driven enhanced precipitation and weathering in low latitudes and the subsequent drawdown of atmospheric carbon dioxide. However, no quantitative constraints on paleo-precipitation have been reported in low latitudes to characterize the AMOC effect across the EOT. Here, we present the results of high-resolution (ca. 6 k.y. per sample) isotopic and biomarker records from the Gulf of Mexico. Reconstructed precipitation using leaf wax carbon isotopes shows an increase of 44% across the EOT (34.1–33.6 Ma), which is accompanied by a secular increase in SST of ~2 °C during the latest Eocene. We attribute the enhanced precipitation in the Gulf of Mexico to the northward shift of the Intertropical Convergence Zone that was driven by an enlarged polar-tropic temperature gradient in the Southern Hemisphere and an invigorated AMOC. Our findings link changes in meridional temperature gradient and large-scale oceanic circulation to the lowlatitude terrestrial hydroclimate and provide paleohydrological evidence that supports CO2-weathering feedback during the EOT “greenhouse” to “icehouse” transition.

Southern Hemisphere subtropical drying as a transient response to warming

Climate projections1–3 and observations over recent decades4,5 indicate that precipitation in subtropical latitudes declines in response to anthropogenic warming, with significant implications for food production and population sustainability. However, this conclusion is derived from emissions scenarios with rapidly increasing radiative forcing to the year 21001,2, which may represent very different conditions from both past and future ‘equilibrium’ warmer climates. Here, we examine multi-century future climate simulations and show that in the Southern Hemisphere subtropical drying ceases soon after global temperature stabilizes. Our results suggest that twenty-first century Southern Hemisphere subtropical drying is not a feature of warm climates per se, but is primarily a response to rapidly rising forcing and global temperatures, as tropical sea-surface temperatures rise more than southern subtropical sea-surface temperatures under transient warming. Subtropical drying may therefore be a temporary response to rapid warming: as greenhouse gas concentrations and global temperatures stabilize, Southern Hemisphere subtropical regions may experience positive precipitation trends. Warming is altering subtropical precipitation; however, it is not clear whether this will continue in an equilibrium climate. Using projections to 2300, Southern Hemisphere drying is shown to be a transient response to the meridional temperature gradient changes.

Metric-Dependent Tendency of Tropical Belt Width Changes during the Last Glacial Maximum

Motivated by studies of tropical expansion under modern global warming, the behavior of the tropical belt during the Last Glacial Maximum (LGM) relative to the preindustrial period has been investigated in this study, using simulations from phase 3 of the Paleoclimate Modelling Intercomparison Project (PMIP3) under the framework of phase 5 of the Coupled Model Intercomparison Project (CMIP5). The tropical belt width changes determined by multiple metrics present two opposite tendencies. One refers to the poleward migration of the tropical edge as measured by the steep tropopause gradient and the subtropical jet, and the other suggests that the LGM tropics become narrower as measured by the Hadley cell extent, the eddy-driven jet, and the latitude where precipitation minus evaporation equals zero. The magnitude of such changes widely differs across models and metrics. In absolute terms, the multimodel mean total width changes range from 0.6° to 1.7° among metrics, with contributions predominantly from the Northern Hemisphere. Furthermore, the two metrics that indicate tropical widening are located in the upper troposphere. Such widening is closely related to the vertical and meridional temperature gradient changes in the subtropical regions. The other metrics are located in the middle and lower troposphere, and their variations are directly or indirectly related to changes in the low-level baroclinicity. The diverse responses of metrics to the LGM boundary conditions suggest that the tropical belt width changes and their climatic impacts are distinguished by the different measurements. The selection of metrics should correspond to the specific tropical properties of concern.

Sensitivity of idealised baroclinic waves to mean atmospheric temperature and meridional temperature gradient changes

The sensitivity of idealised baroclinic waves to different atmospheric temperature changes is studied. The temperature changes are based on those which are expected to occur in the Northern Hemisphere with climate change: (1) uniform temperature increase, (2) decrease of the lower level meridional temperature gradient, and (3) increase of the upper level temperature gradient. Three sets of experiments are performed, first without atmospheric moisture, thus seeking to identify the underlying adiabatic mechanisms which drive the response of extra-tropical storms to changes in the environmental temperature. Then, similar experiments are performed in a more realistic, moist environment, using fixed initial relative humidity distribution. Warming the atmosphere uniformly tends to decrease the kinetic energy of the cyclone, which is linked both to a weaker capability of the storm to exploit the available potential energy of the zonal mean flow, and less efficient production of eddy kinetic energy in the wave. Unsurprisingly, the decrease of the lower level temperature gradient weakens the resulting cyclone regardless of the presence of moisture. The increase of the temperature gradient in the upper troposphere has a more complicated influence on the storm dynamics: in the dry atmosphere the maximum eddy kinetic energy decreases, whereas in the moist case it increases. Our analysis suggests that the slightly unexpected decrease of eddy kinetic energy in the dry case with an increased upper tropospheric temperature gradient originates from the weakening of the meridional heat flux by the eddy. However, in the more realistic moist case, the diabatic heating enhances the interaction between upper- and low-level potential vorticity anomalies and hence helps the surface cyclone to exploit the increased upper level baroclinicity.

Centennial trends in Northern Hemisphere winter storm tracks over the twentieth century

In this study changes in Northern Hemisphere winter storm tracks during the twentieth century are investigated based on the individual 56 ensemble members of the twentieth century re-analysis dataset. It is found that the twentieth century trends in storm-track activities exhibit large discrepancies between the upper and lower troposphere. In the upper troposphere, a substantial intensification is identified at the poleward and downstream regions of the North Pacific and North Atlantic storm-track activities, indicating a large northeastward expansion of storm tracks in the late twentieth century. However, in the lower troposphere the synoptic eddy activities, especially in terms of the eddy kinetic energy and meridional eddy heat flux, tend to be significantly weakened over the high latitudes of central-western North Pacific and the upstream regions of the North Atlantic storm tracks. Further inspections find that such strengthening (weakening) of storm tracks in the upper (lower) troposphere are attributed mainly to an increase (decrease) of baroclinic instability, which is predominantly determined by the meridional temperature gradient changes. Moreover, from a local energetic perspective, the baroclinic generation and barotropic damping of the synoptic eddies are found to be substantially enhanced at the upstream and downstream regions of the two storm tracks in the upper troposphere, respectively, while in the lower troposphere the baroclinic energy conversion to eddies is generally decreased.

Impact of the Warming Pattern on Global Energetics

Abstract The warming pattern due to higher greenhouse gas concentrations is expected to affect the global atmospheric energetics mainly via changes in the (i) meridional temperature gradient and (ii) mean static stability. Changes in surface meridional temperature gradients have been previously regarded as the determining feature for the energetics response, but recent studies suggest that changes in mean static stability may be more relevant than previously thought. This study aims to determine the relative importance of these two effects by comparing the energetics responses due to different warming patterns using a fully coupled atmosphere–ocean general circulation model. By means of an additional diabatic forcing, experiments with different warming patterns are obtained: one with a 2xCO2-like pattern that validates the method, one with only the tropical upper-tropospheric warming, and one with only the high-latitude surface warming. The study’s findings suggest that the dominant aspect of the warming pattern that alters the global atmospheric energetics is not its associated meridional temperature gradient changes, but the mean static stability changes. The tropical upper warming weakens the energetics by increasing the mean static stability, whereas the surface warming strengthens it by reducing the mean static stability. The combined 2xCO2-like response is dominated by the tropical upper-tropospheric warming effect, hence the weaker energetic activity. Eddy kinetic energy changes consistently, but the two opposite responses nearly cancel each other in the 2xCO2 case. Therefore, estimates of future changes in storminess may be particularly sensitive to the relative magnitude of the main features of the simulated warming pattern.

Dynamic millennial‐scale climate changes in the northwestern Pacific over the past 40,000 years

Ice core records of polar temperatures and greenhouse gases document abrupt millennial‐scale oscillations that suggest the reduction or shutdown of thermohaline Circulation (THC) in the North Atlantic Ocean may induce the abrupt cooling in the northern hemisphere. It remains unknown, however, whether the sea surface temperature (SST) is cooling or warming in the Kuroshio of the Northwestern Pacific during the cooling event. Here we present an AMS 14C‐dated foraminiferal Mg/Ca SST record from the central Okinawa Trough and document that the SST variations exhibit two steps of warming since 21 ka — at 14.7 ka and 12.8 ka, and a cooling (∼1.5°C) during the interval of the Younger Dryas. By contrast, we observed no SST change or oceanic warming (∼1.5–2°C) during the episodes of Northern Hemisphere cooling between ∼21–40 ka. We therefore suggest that the “Antarctic‐like” timing and amplitude of millennial‐scale SST variations in the subtropical Northwestern Pacific between 20–40 ka may have been determined by rapid ocean adjustment processes in response to abrupt wind stress and meridional temperature gradient changes in the North Pacific.

Impacts of tectonic and orbital forcing on East African climate: a comparison based on global climate model simulations

A global atmosphere–ocean model has been forced with topographic and orbital scenarios in order to evaluate the relative role of both factors for the past climate of East Africa. Forcing the model with a significantly reduced topography in Eastern and Southern Africa leads to a distinct increase in moisture transport from the Indian Ocean into the eastern part of the continent and increased precipitation in Eastern Africa. Simulations with step-wise reduced height show that this climate change occurs continuously with the change in topography, i.e., an abrupt change of local climatic features with a critical height is not found. Simulations of the last interglacial (at 125,000 years before present, i.e., the Eemian interglacial) and the last glacial inception (at 115,000 years before present) are used as examples for the role of orbital-induced changes in insolation. Here, changes in meridional temperature gradients lead to modifications in moisture transport of similar order of magnitude, but with different spatial and seasonal structure. For the Eemian interglacial, a distinct increase in summer moisture transport from the Atlantic deep into the continent at around 20°N is simulated.

Pliocene development of the east‐west hydrographic gradient in the equatorial Pacific

Foraminifer count and multispecies isotopic data from Ocean Drilling Program (ODP) Sites 806 (0°N, 165°E) and 847 (0°N, 95°W) are presented for the last 6.4 m.y. Faunal evidence shows a decline of taxa of Miocene origin between 4.5 and 4.0 Ma at both sites. Prior to this decline, Globoturborotalita species and Neogloboquadrina acostaensis dominated Sites 806 and 847 assemblages, respectively. Globoturborotalitids were not replaced by other thermocline dwellers after 4.2 Ma, which suggests a deepening of the thermocline and thickening of the warm pool at Site 806. At Site 847 in the eastern “cold tongue,” N. acostaensis was gradually replaced by N. dutertrei. After 4.2 Ma, there was also a large increase (from 0.7 to 1.3‰) in the difference between the δ18O values of Globigerinoides sacculifer, a surface dweller, and Neogloboquadrina dutertrei, a seasonal thermocline dweller at Site 847; these data, in conjunction with faunal evidence, suggest that the temperature gradient in the upper photic zone increased significantly, largely because of subsurface cooling. The events in planktonic foraminifer evolution and ecology suggest that between 4.5 and 4.0 Ma a significant step was made toward development of the modern east‐west gradient in surface hydrography. The timing of these events suggests that they were related to the closing of the Central American Seaway and subsequent changes in meridional temperature gradients and/or changes in air‐sea interactions that modified tropical winds.

Effects of dynamic heat fluxes on model climate sensitivity: Meridional sensible and latent heat fluxes

We incorporate physically based parameterizations of atmospheric meridional heat fluxes into the climate model of Wang et al. (1984) to study their effects on climate changes caused by increases of CO2 abundance and solar constant variations. The physically based parameterizations calculate both the magnitudes and profiles of the sensible and latent heat fluxes in good agreement with observations. Comparisons of climate sensitivity between the physically based parameterizations and the original model's empirical parameterization for total transport are performed. The results indicate that both parameterizations yield about the same changes in global mean surface temperature and ice line, but the physically based parameterizations produce smaller changes in meridional temperature gradient. Also, the sensible and latent heat fluxes from the physical parameterization produce mutually compensating changes which are consistent with general circulation model results. We also study the separate feedback effects of sensible and latent heat fluxes by allowing only one or the other to change during climate changes. It is found that these variations in the partitioning of changes in the meridional heat transport have relatively small effects on the change of hemispheric mean surface temperature, but they can affect both the sign as well as the magnitude of the change of meridional temperature gradient. The change in total meridional heat transport is also relatively insensitive to the partitioning variations. Further analysis of the model's energy balance indicates that this insensitivity is a consequence of two features of the results: (1) absorption of the latitudinally varying part of the solar heating is affected very little by the partitioning, and (2) thermal structure changes that are affected by the partitioning do not influence greatly the zonal energy balance.

Empirical Relations Between Seasonal Changes in Meridional Temperature Gradients and Meridional Fluxes of Heat

Abstract The correlation of the zonal mean meridional flux of heat in the atmosphere-ocean system and of its various components with the zonal mean 1000 mb meridional temperature gradient are calculated from analyses of the mean seasonal changes in these quantities. The correlations for the total atmospheric flux and its dominant components are typically 90% or more. The correlation of the eddy flux with the temperature gradient is particularly high (typically 97%) but only if the transient and stationary components are added together. There appears to be no correlation between the ocean flux and the temperature gradient in low latitudes. In cases where the correlation is high, the seasonal changes are used to derive empirical relations between the fluxes and the gradient. Seasonal changes in the total flux in the atmosphere-ocean system are twice as strong as would be implied by a linear diffusion law. The empirical relations for the seasonal changes in the eddy flux of sensible heat indicate that it is ...