The dependence of upper ocean gyres on wind and buoyancy forcing

A series of numerical simulations with different forcing conditions are carried out, to investigate the roles played by buoyancy and wind forcing on the upper ocean gyres, and to contrast the laminar and eddying regimes. Model experiments show that the buoyancy-driven eastward geostrophic flow tends to suppress the formation of the wind-driven subpolar gyre, but the northward eddy heat transport can homogenize the subpolar water and reduce the meridional temperature gradient by about two-third, thus counteracting the buoyancy effect and saving the subpolar gyre. For the subtropical gyre, its transport is enhanced by eddy mixing, and the role of buoyancy forcing is very sensitive to the choice of diapycnal diffusivity. Our results suggest that eddy effects must be considered in the dynamics of the subpolar gyre, and vertical diffusivity should be selected carefully in simulating the basin-wide circulations.

Meridional temperature gradient and the midlatitude storm track

<p>The southern hemisphere’s atmospheric circulation experiences several annual and seasonal changes that are well documented and studied. The teleconnections between different variables are verified and used to explain variability in everyday climate and weather. Theories using physics are taught and published in textbooks to help us understand the connectivity and complexity of such a system. One theory is the meridional temperature gradient has a direct impact on the storm track. This thesis investigates that theory using the ERA-Interim dataset. The temperature gradient is a direct result of the temperature field, and depending on the latitudes you decide in which to constrain your gradient, the gradient experiences several changes. In the high latitudes, the southern annual oscillation created a two peaked pattern; the mid-latitudes display the expected seasonal mono peak pattern. The strong correlations seen in the high latitudes means that the gradient is driven by the patterns experienced at higher latitudes. The independence of behaviours displayed by the ocean sectors led to the research investigating the influences, looking at not just the hemisphere, but also each basin separately. The Pacific and Indian Ocean showed in several results to act independently from one another, in temperature gradients, wind field, and storm track position. The strong correlations between the temperature gradient and the wind field, as well as the storm track field show that the two are connected, as the theory suggests. If temperatures rise in the tropics, or decrease in the poles, then the temperature gradient will steepen. The pressure gradient force increases which pushes the thermal wind balance poleward, shifting the position of the westerlies. The area with the largest variation in the wind speed becomes the storm track, which would also shift poleward. Climatic factors such as the southern oscillation index, southern annular mode or Indian Ocean dipole show slight correlations with the temperature field, but have little to no influence on the temperature gradient itself. Precipitation levels in New Zealand are highly variable due to the nature of the countries location and topography. What was found was little connection between the northern part of the country and the storm track. However, closer proximity to the storm track, such as the south of the country, do experience a small amount of variation due to the storm tracks influence.</p>

Meridional temperature gradient and the midlatitude storm track

<p>The southern hemisphere’s atmospheric circulation experiences several annual and seasonal changes that are well documented and studied. The teleconnections between different variables are verified and used to explain variability in everyday climate and weather. Theories using physics are taught and published in textbooks to help us understand the connectivity and complexity of such a system. One theory is the meridional temperature gradient has a direct impact on the storm track. This thesis investigates that theory using the ERA-Interim dataset. The temperature gradient is a direct result of the temperature field, and depending on the latitudes you decide in which to constrain your gradient, the gradient experiences several changes. In the high latitudes, the southern annual oscillation created a two peaked pattern; the mid-latitudes display the expected seasonal mono peak pattern. The strong correlations seen in the high latitudes means that the gradient is driven by the patterns experienced at higher latitudes. The independence of behaviours displayed by the ocean sectors led to the research investigating the influences, looking at not just the hemisphere, but also each basin separately. The Pacific and Indian Ocean showed in several results to act independently from one another, in temperature gradients, wind field, and storm track position. The strong correlations between the temperature gradient and the wind field, as well as the storm track field show that the two are connected, as the theory suggests. If temperatures rise in the tropics, or decrease in the poles, then the temperature gradient will steepen. The pressure gradient force increases which pushes the thermal wind balance poleward, shifting the position of the westerlies. The area with the largest variation in the wind speed becomes the storm track, which would also shift poleward. Climatic factors such as the southern oscillation index, southern annular mode or Indian Ocean dipole show slight correlations with the temperature field, but have little to no influence on the temperature gradient itself. Precipitation levels in New Zealand are highly variable due to the nature of the countries location and topography. What was found was little connection between the northern part of the country and the storm track. However, closer proximity to the storm track, such as the south of the country, do experience a small amount of variation due to the storm tracks influence.</p>

The Regional Hadley Cells Response to the Sea Surface Temperature Distribution Across the Indo-Pacific Ocean

Hadley Cells are thermally driven cell in the tropics. On its occurrence, these cells are strongly influenced by the sea surface temperature (SST) distribution across the tropical ocean or the Pacific Ocean as the investigated location in this study. The SST shifting in the Pacific Ocean is mainly due to the ENSO. An opposite SST polarity between the western and eastern Pacific Ocean are captured during ENSO events. This means that ENSO could trigger an anomalous regional Hadley Cells that behave oppositely between Indonesia or the western Pacific and the eastern Pacific. This study examines the strength of the regional Hadley Cells related to the ENSO event across the Indonesian region and the Pacific Ocean. A significant correlation between the Hadley Cells and ENSO as the tropical climate variability in the Pacific Oceans are found. The strength of the Hadley Cells associated with ENSO event is examined by using the zonally average vertical velocity across the Pacific Ocean. During La Nina, the regional Hadley Cells over Indonesia or the western Pacific strengthened, whereas the regional cells over the eastern Pacific weakened. In contrast, during El Nino where the warm pool shifted to the eastern Pacific, the regional cell in the eastern Pacific strengthened, while the cell over the western Pacific weakened. These anomalous conditions clearly show that the meridional temperature gradient is strongly affecting the regional Hadley Cells strength. The stronger the meridional temperature gradient, the stronger the regional Hadley Cells.

Evaluation of synoptic eddy activities and their feedback onto the midlatitude jet in five atmospheric reanalyses with coarse versus fine model resolutions

Interaction between synoptic eddy and mean flow plays a crucial role in maintaining midlatitude westerly jet. In this study, climatologies of synoptic eddy activities and their feedback onto midlatitude jet for 1980–2016 are evaluated and compared through analyzing daily data from five atmospheric reanalyses with different resolutions including one coarse-resolution reanalysis (NCEP2) and four fine-resolution reanalyses (ERA-Interim, JRA-55, MERRA-2, and CFSR). Horizontal resolutions of the atmospheric models generating those reanalyses are approximately equivalent to 210, 79, 60, 50, and 38 km, respectively. Results show that the eddy activities and their feedback onto the midlatitude jet in those fine-resolution reanalyses are consistently and significantly stronger than those in the coarse-resolution reanalysis (NCEP2). The maximal relative increases that are found to occur primarily in the midlatitudes of the Southern Hemisphere are estimated to be up to 55% for the baroclinicity, 53% for the eddy energetics, 59% for the eddy forcing, and even 85% for the eddy feedback onto the mean flow. Those increases are reasonably conjectured to be related to increased model resolutions, since the synoptic eddy genesis is proportional to the low-level atmospheric meridional temperature gradient which is sensitive to the meridional resolution of atmospheric models. Although the coarse-resolution reanalysis resolves synoptic eddies insufficiently and thus underestimates their feedback onto the mean flow, the magnitudes of eddy-driven jets are almost the same among five reanalyses, implying a mismatch between the eddy feedback and the eddy-driven jet in the coarse-resolution reanalysis. Therefore, the results of this study imply the importance of using fine-resolution reanalyses in accurately understanding the midlatitude synoptic eddy–mean flow interaction.

Evaluation of Synoptic Eddy Activities and their Feedback onto the Midlatitude Jet in Five Atmospheric Reanalyses with Coarse Versus Fine Model Resolutions

Interaction between synoptic eddy and mean flow plays a crucial role in maintaining midlatitude westerly jet. In this study, climatologies of synoptic eddy activities and their feedback onto midlatitude jet for 1980–2016 are evaluated and compared through analyzing daily data from five atmospheric reanalyses with different resolutions including one coarse-resolution reanalysis (NCEP2) and four fine-resolution reanalyses (ERA-Interim, JRA-55, MERRA-2, and CFSR). Horizontal resolutions of the atmospheric models generating those reanalyses are approximately equivalent to 210, 79, 60, 50, and 38 km, respectively. Results show that the eddy activities and their feedback onto mean flow in those fine-resolution reanalyses are consistently and significantly stronger than those in the coarse-resolution reanalysis (NCEP2). The maximal relative increases that are found to occur primarily in the midlatitudes of the Southern Hemisphere are estimated to be up to 55% for the baroclinicity, 53% for the eddy energetics, 59% for the eddy forcing, and even 85% for the eddy feedback onto mean flow. Those increases are reasonably conjectured to be related to increased model resolutions, since the synoptic eddy genesis is proportional to the lower-level atmospheric meridional temperature gradient which is sensitive to the meridional resolution of atmospheric models. Although the coarse-resolution reanalysis resolves synoptic eddies insufficiently and thus underestimates their feedback onto the mean flow, the magnitudes of eddy-driven jets are almost the same among five reanalyses, implying a mismatch between the eddy feedback and the eddy-driven jet in the coarse-resolution reanalysis. Therefore, the results of this study imply the importance of using fine-resolution reanalyses in accurately understanding the midlatitude synoptic eddy-mean flow interaction.

Subseasonal-to-Seasonal Predictability of Onset Dates of South China Sea Summer Monsoon: A Perspective of Meridional Temperature Gradient

The onset of the South China Sea summer monsoon (SCSSM) has traditionally been ascribed to the El Niño–Southern Oscillation (ENSO) on an interannual timescale, but the two do not correspond in some years. The present study applies harmonic analysis on the meridional temperature gradient (MTG) in mid–upper troposphere over South China Sea (SCS) and decomposes the onset process to be a slow-varying seasonal cycle and transient subseasonal component. The ENSO-related air temperature anomaly in the southern SCS provides seasonal predictability of SCSSM onset by a stable and robust relationship between ENSO and MTG seasonal cycle. However, in the northern SCS, the MTG is regulated by an intraseasonal oscillation (ISO) of extratropical air temperature with a significant 10–30-day period. This ISO originates over the western TP, then propagates eastward and gets enhanced by anomalous diabatic heating due to spring rainfall anomaly over South China, as a result of subseasonal thermal forcing of TP. When the ISO arrives to the north of the SCS, it directly changes the tropospheric temperature to modulate the MTG. Meanwhile, the upper-level circulation associated with the ISO alters the meridional potential vorticity advection and pumping effect, followed by the anomalous low-level westerly wind and monsoon convection over the SCS. The SCSSM onset is evidently disrupted from its seasonal cycle when this ISO is more active. Since the independence of its intensity from ENSO, this extratropical ISO over TP and South China provides additional subseasonal predictability of the onset dates of the SCSSM.

Exploring the possible meridional temperature gradient of Early Eocene Climatic Optimum with an energy balance model

&lt;p&gt;Early Eocene Climatic Optimum (EECO, ~53-51 million years) is one of the past warm periods, associated with high CO&lt;sub&gt;2&lt;/sub&gt; concentrations (~900-2500 ppmv), which can serve as an analogue for our possible future, high C0&lt;sub&gt;2 &lt;/sub&gt;climate. One notable feature of this hothouse climate state is the weaker meridional temperature gradient relative to pre-industrial values. This have been confirmed by both proxies and models, but the extent of the temperature gradient still requires more research. Models are challenged to reproduce the stronger than present day polar amplification signal, and it is also shown that high latitude proxy data are often influenced by seasonal bias. Thus, there is an uncertainty regarding both the observed and modelled meridional gradient and the mentioned issues complicate also the comparison between modeled and proxy data.&lt;/p&gt;&lt;p&gt;In our work we aim to investigate the EECO period with a simple energy balance box model and apply the maximum entropy production principle to explore the possible scenarios of meridional temperature gradients. We find that the maximum entropy production principle could be beneficial in the paleoclimate context since it has the utility to give an accurate prediction for non-equilibrium systems with the minimal amount of information. We also assess the heat transport signaled by proxy data and by state-of-the-art model outputs in accordance to our theoretical constrains based on the idealized test case.&lt;/p&gt;

Summer Westerly Jet in Northern Hemisphere during the Mid-Holocene: A Multi-Model Study

The upper-level jet stream, a narrow band of maximum wind speed in the mid-latitude westerlies, exerts a considerable influence on the global climate by modulating the transport and distribution of momentum, heat and moisture. In this study by using four high-resolution models in the Paleoclimate Modelling Intercomparison Project phase 3, the changes of position and intensity of the northern hemisphere westerly jet at 200 hPa in summer during the mid-Holocene (MH), as well as the related mechanisms, are investigated. The four models show similar performance on the westerly jet. At the hemispheric scale, the simulated westerly jet has a poleward shift during the MH compared to the preindustrial period. The warming in arctic and cooling in the tropics during the MH are caused by the orbital changes of the earth and the precipitation changes, and it could lead to the weakened meridional temperature gradient and pressure gradient, which might account for the poleward shift of the westerly jet from the thermodynamic perspective. From the dynamic perspective, two maximum centers of eddy kinetic energy are simulated over the North Pacific and North Atlantic with the north deviation, which could cause the northward movement of the westerly jet. The weakening of the jet stream is associated with the change of the Hadley cell and the meridional temperature gradient. The largest weakening is over the Pacific Ocean where both the dynamic and the thermodynamic processes have weakening effects. The smallest weakening is over the Atlantic Ocean, and it is induced by the offset effects of dynamic processes and thermodynamic processes. The weakening over the Eurasia is mainly caused by the dynamic processes.

Estimating structural and parametric uncertainties in the simulated early Eocene surface warming

&lt;p&gt;Simulating the proxy-derived surface warming and reduced meridional temperature gradient of the early Eocene greenhouse climate still represents a challenge for most atmosphere-ocean general circulation models. A profound understanding of uncertainties associated with the respective model results is thereby essential to reliably identify any similarities or misfits to the proxy record. Besides incomplete knowledge of past greenhouse gas concentrations and other boundary conditions, structural and parametric uncertainties are the main factors that determine our confidence in paleoclimate simulation results.&lt;/p&gt;&lt;p&gt;The recent publication of coordinated model experiments that apply identical paleogeographic boundary conditions for key time periods of the early Eocene (DeepMIP) allows a systematic analysis of inter-model differences and therefore of structural uncertainties in the simulated surface warming. Here we additionally explore the parametric uncertainty of the early Eocene climatic optimum (EECO) surface warming within one DeepMIP model. For this we performed perturbed parameter ensemble (PPE) simulations with HadCM3B at different atmospheric CO&lt;sub&gt;2&lt;/sub&gt; concentrations following the DeepMIP protocol. Twenty-one parameter sets based on changes in six atmospheric parameters, a sea-ice parameter and the ocean background diffusivity were branched off from the respective DeepMIP control simulations and integrated for a further 1500 model years. The selected parameter sets are based on previous results demonstrating their ability to simulate a pre-industrial global-mean surface temperature within &amp;#177;2 &amp;#176;C of the standard configuration.&lt;/p&gt;&lt;p&gt;Preliminary results indicate a large spread of the simulated low-latitude surface warming in the PPE and therefore significant changes of the large-scale meridional temperature gradient for the EECO. Some ensemble members develop numerical instabilities at CO&lt;sub&gt;2&lt;/sub&gt; concentrations of 840 ppmv and above, most likely in consequence of high temperatures in the tropical troposphere. We further compare the magnitude of the parametric uncertainty of the HadCM3B perturbation experiments with the structural differences found in the DeepMIP multi-model ensemble and explore the sensitivity of the results to the strength of the applied greenhouse gas forcing. Model skill of the PPE members is tested against the most recent DeepMIP compilations of marine and terrestrial proxy temperatures.&lt;/p&gt;