Using the coupled climate model CLIMBER-3a, we investigate changes in sea surface elevation due to a weakening of the thermohaline circulation (THC). In addition to a global sea level rise due to a warming of the deep sea, this leads to a regional dynamic sea level change which follows quasi-instantaneously any change in the ocean circulation. We show that the magnitude of this dynamic effect can locally reach up to ~1m, depending on the initial THC strength. In some regions the rate of change can be up to 20-25 mm/yr. The emerging patterns are discussed with respect to the oceanic circulation changes. Most prominent is a south-north gradient reflecting the changes in geostrophic surface currents. Our results suggest that an analysis of observed sea level change patterns could be useful for monitoring the THC strength.
The ocean thermohaline circulation, also called meridional overturning circulation, is caused by water density contrasts. This circulation has large capacity of carrying heat around the globe and it thus affects the energy budget and further affects the climate. We consider a thermohaline circulation model in the meridional plane under external wind forcing. We show that, when there is no wind forcing, the stream function and the density fluctuation (under appropriate metrics) tend to zero exponentially fast as time goes to infinity. With rapidly oscillating wind forcing, we obtain an averagin…
In this work, we study the impact of non-Gaussian alpha-stable Levy motion on transitions between metastable equilibrium states (or attractors) in a stochastic Stommel two-box model for thermohaline circulation (THC). By maximizing the probability density of the solution process associated with a nonlocal Fokker-Planck equation, we compute maximal likely pathways and identify corresponding maximal likely stable equilibrium states. Our numerical results indicate weakened THC may be induced by perturbation with very small noise intensity in a certain range of stability index. Moreover, larger no…
Within a simple model context, the sensitivity and stability of the thermohaline circulation to finite amplitude perturbations is studied. A new approach is used to tackle this nonlinear problem. The method is based on the computation of the so-called Conditional Nonlinear Optimal Perturbation (CNOP) which is a nonlinear generalization of the linear singular vector approach (LSV). It is shown that linearly stable thermohaline circulation states can become nonlinearly unstable and the properties of the perturbations with optimal nonlinear growth are determined. An asymmetric nonlinear response …
The relatively warm climate found in the North- Western Europe is due to the gulf stream that circulates warm saline water from southern latitudes to Europe. In North Atlantic ocean the stream gives out a large amount of heat, cools down and sinks to the bottom to complete the Thermohaline circulation. There is considerable debate on the stability of the stream to inputs of fresh water from the melting ice in Greenland and Arctic. The circulation, being switched off, will have massive impact on the climate of Europe. Intergovernmental panel on climate change (IPCC) has warned of this danger in…
We reconsider the problem of the stability of the thermohaline circulation as described by a two-dimensional Boussinesq model with mixed boundary conditions. We determine how the stability properties of the system depend on the intensity of the hydrological cycle. We define a two-dimensional parameters' space descriptive of the hydrology of the system and determine, by considering suitable quasi-static perturbations, a bounded region where multiple equilibria of the system are realized. We then focus on how the response of the system to finite-amplitude surface freshwater forcings depends on t…
The understanding of the statistical properties and of the dynamics of multistable systems is gaining more and more importance in a vast variety of scientific fields. This is especially relevant for the investigation of the tipping points of complex systems. Sometimes, in order to understand the time series of given observables exhibiting bimodal distributions, simple one-dimensional Langevin models are fitted to reproduce the observed statistical properties, and used to investing-ate the projected dynamics of the observable. This is of great relevance for studying potential catastrophic chang…
A thorough analysis of the stability of the uncoupled Rooth interhemispheric 3-box model of thermohaline circulation (THC) is presented. The model consists of a northern high latitudes box, a tropical box, and a southern high latitudes box, which respectively correspond to the northern, tropical and southern Atlantic ocean. We adopt restoring boundary conditions for the temperature variables and flux boundary conditions for the salinity variables. We study how the strength of THC changes when the system undergoes forcings that are analogous to those of global warming conditions by applying to …
A thorough analysis of the stability of a coupled version of an inter-hemispheric 3-box model of Thermohaline Circulation (THC) is presented. This study follows a similarly structured analysis on an uncoupled version of the same model presented in Part I. We study how the strength of THC changes when the system undergoes forcings representing global warming conditions. Each perturbation to the initial equilibrium is characterized by the total radiative forcing realized, by the rate of increase, and by the North-South asymmetry. The choice of suitably defined metrics allows us to determine the …
The main objective of this and its accompanying articles is to derive a mathematical theory associated with the thermohaline circulations (THC). This article provides a general transition and stability theory for the Boussinesq system, governing the motion and states of the large-scale ocean circulation. First, it is shown that the first transition is either to multiple steady states or to oscillations (periodic solutions), determined by the sign of a nondimensional parameter $K$, depending on the geometry of the physical domain and the thermal and saline Rayleigh numbers. Second, for both the…
A combination of analytical and numerical techniques are used to efficiently determine the qualitative and quantitative behaviour of a one-basin zonally averaged thermohaline circulation ocean model. In contrast to earlier studies which use time stepping to find the steady solutions, the steady state equations are first solved directly to obtain the multiple equilibria under identical mixed boundary conditions. This approach is based on the differentiability of the governing equations and especially the convection scheme. A linear stability analysis is then performed, in which the normal modes…
Predictability of the North Atlantic thermohaline circulation (THC) variability as simulated in the GFDL coupled ocean-atmosphere general circulation model is established for a set of ensemble experiments. The ensembles consist of identical oceanic initial conditions underneath a model atmosphere chosen randomly from the model climatology. This experimental design is based on the separation in time scales present in the model which motivates the assumption that the predictability deduced from these ensembles provides an upper limit to the model's THC predictability. The climatology is taken fr…
Persistent memory and tail-risk amplification in Atlantic Meridional Overturning Circulation variability: a Volterra integral framework calibrated with CMIP6
Faculty of Engineering, Universidad del Desarrollo（发展大学工程学院）
The interdecadal variability of a stochastically forced four-box model of the oceanic meridional thermohaline circulation (THC) is described and compared to the THC variability in the coupled ocean-atmosphere GCM of Delworth, Manabe, and Stouffer (1993). The box model is placed in a linearly stable thermally dominant mean state under mixed boundary conditions. A linear stability analysis of this state reveals one damped oscillatory THC mode in addition to purely damped modes. The variability of the model under a moderate amount of stochastic forcing, meant to emulate the random variability of …
The seasonal and spatial evolution of mixed layers (MLs) in the Red Sea (RS) is analyzed for the 2001-2015 period using the results of a high resolution (~1km horizontal, 50 vertical layers) ocean circulation model forced by a novel regional high resolution (5 km) atmospheric reanalysis dataset. The simulation reproduces the main features of the near-surface stratification, as described by the available observations. The seasonal evolution of the modeled mixed layer depths (MLDs) in the RS is predominantly driven by atmospheric buoyancy forcing, especially its heat flux component. Everywhere i…
Climate and ocean circulation changes toward a modern snowball Earth
Japan Agency for Marine-Earth Science and Technology（日本海洋地球科学技术局）；Earth-Life Science Institute (ELSI), Institute of Future Science, Institute of Science Tokyo（东京科学大学未来科学研究所生命起源研究所）；Atmosphere and Ocean Research Institute, The University of Tokyo（东京大学大气海洋研究所）；University of the Ryukyus（冲绳大学）；Department of Geophysics, Graduate School of Science, Tohoku University（东北大学理学研究科地球物理学科）；National Institute of Polar Research（国立极地研究所）；The Graduate University for Advanced Studies, SOKENDAI（综合研究大学院大学）
The impact of uncertainties in surface layer physics on the atmospheric general circulation is comparatively unexplored. Here the sensitivity of the zonal-mean circulation to reduced air-sea momentum roughness ($Z_{0m}$) at low flow speed is investigated with the Community Atmosphere Model (CAM3). In an aquaplanet framework with prescribed sea surface temperatures, the response to reduced $Z_{0m}$ resembles the La Ni$\tilde{\text{n}}$a minus El Ni$\tilde{\text{n}}$o response to El Ni$\tilde{\text{n}}$o Southern Oscillation variability with: i) a poleward shift of the mid-latitude westerlies ex…
Objectives: This study intends to describe the Andaman Sea circulation and investigate the dominant modes of variability in the basin. Analysis/Observations: The domain experiences stronger South-westerly winds from May to September and relatively weaker North-easterlies from November to February. A strong negative Ekman pumping along the north coast of Indonesia is observed during Summer. The transport of water across the straits of Andaman and Nicobar Islands (ANI) is computed by simple mass balance and is found to be in phase with the monthly averaged Mean Sea Level Anomalies (MSLA) of Anda…
Near the ocean surface, river plumes influence stratification, buoyancy and transport of tracers, nutrients and pollutants. The extent to which river plumes influence the overall circulation, however, is generally poorly constrained. This work focuses on the South China Sea (SCS) and quantifies the dynamical impacts of the Mekong River plume, which is bound to significantly change in strength and seasonality in the next 20 years if the construction of over hundred dams moves ahead as planned. The dynamic impact of the freshwater fluxes on the SCS circulation are quantified by comparing submeso…
Poleward trends in seasonal-mean latitudes of tropical cyclones (TCs) have been identified in direct observations from 1980 to present. Paleoclimate reconstructions also indicate poleward-equatorward migrations over centennial to millennial timescales. Hadley circulation (HC) is often both implicitly and explicitly invoked to provide dynamical linkages to these shifts, although no direct analysis of concurrent changes in the recent period has been presented. Here the observational TC record (1981-2016) and the ERA-Interim, JRA55 and MERRA2 reanalyses are studied to examine potential relationsh…
Titan's abundant lakes and seas exchange methane vapor and energy with the atmosphere via a process generally known as air-sea interaction. This turbulent exchange process is investigated with an atmospheric mesoscale model coupled to a slab model representation of an underlying lake. The impact of lake size, effective lake mixed layer depth, background wind speed, air-lake temperature differential, and atmospheric humidity on air-sea interaction processes is studied through dozens of two-dimensional simulations. The general, quasi-steady solution is a non-linear superposition of a very weak b…
Density and sea level changes weaken the Atlantic Meridional Overturning Circulation through ocean bottom pressure
Rosenstiel School of Marine, Atmospheric, and Earth Science, University of Miami（迈阿密大学罗斯恩施泰尔海洋、大气和地球科学学院）；National Oceanography Centre, Southampton（南安普敦国家海洋中心）
Particularly challenging classes of heterogeneous surfaces are ones where strong secondary circulations are generated, potentially dominating the flow dynamics. In this study, we focus on land-sea breeze circulations (LSBs) resulting from surface thermal contrasts in the presence of increasing synoptic pressure forcing. The relative importance and orientation of the thermal and synoptic forcings are measured through two dimensionless parameters: a bulk Richardson number (Mg is the geostrophic wind magnitude and Wstar a convective buoyant velocity scale), and the angle alpha between the shore a…
Marine Heatwaves in the Arabian Sea: Drivers and Impacts on Atmospheric Circulation and Extreme Precipitation
University of Colorado, Boulder（科罗拉多大学博尔德分校）；Texas A&M University–Corpus Christi（德克萨斯农工大学科珀斯克里斯蒂分校）；Scripps Institution of Oceanography, University of California, San Diego（加州大学圣地亚哥分校斯克里普斯海洋研究所）；Florida State University（佛罗里达州立大学）
Thousands of exoplanets have been detected to date, and with future planned missions this tally will increase. Understanding the climate dependence on the planetary parameters is vital for the study of terrestrial exoplanet habitability. Using an idealized general circulation model with a seasonal cycle, we study the seasonal response of the surface temperature and Hadley circulation to changes in the orbital, rotational and radiative timescales. Analyzing the climate's seasonal response to variations in these timescales, we find a regime transition between planets controlled by the annual mea…
We introduce bilayer networks in this paper to study the coupled air-sea systems. It proved true that the framework of bilayer networks is powerful for studying the statistical topology structure and dynamics between the fields of ocean and atmosphere. Based on bilayer network, we identify the key correlation regions within and between the air-sea systems. A coupled mechanism between Asia monsoon circulation and Walker circulation is proposed to explain the high correlation phenomenon between the regions on air-sea interaction. We also identify the key regions, which influence the air-sea syst…
The sea-ice cover of the Arctic Ocean is an important element of the climate and ocean system in the Northern Hemisphere as it impacts albedo, atmospheric pressure regimes, CO2-exchange at the ocean/atmosphere interface as well as the North Atlantic freshwater budget and thermohaline circulation [1]. Due to global warming, the Arctic sea-ice cover is presently evolving at an unprecedent rate towards full melt during the summer season, driving the so-called "Arctic amplification" [2]. However, the Arctic sea-ice has also experienced large amplitude variations, from seasonal to orbital (Milankov…
The steady and transient response of "dynamically" dry and moist atmospheres to uniform sea-surface temperature (SST) is studied. Specifically, the latent heat (Lv) of water vapor is varied, so that for small Lv, water substance is essentially a passive tracer from a dynamical point of view. Despite the lack of SST gradients, a general circulation with Hadley and Ferrel cells is observed for relatively stronger moist coupling. Organized precipitation patterns via equatorial waves appear to play a significant role in tropical ascent, and along with the equatorial deformation radius, the Hadley …
Several researchers have previously identified a severe climatic upheaval in tropical North Africa that began just over 4000 years ago and lasted for about three centuries. The upheaval is known to have occurred shortly after a volcanic eruption, and companion work proposes that this eruption was colossal. Here, we suggest how the eruption acted as a trigger for the upheaval: by forcing changes in ocean circulation; although the initial (atmospheric) forcing lasted only a few years, the ocean required three centuries to regain equilibrium. The suggested triggering mechanism is supported by pal…
With growing amounts of wind and solar power in the electricity mix of many European countries, understanding and predicting variations of renewable energy generation at multiple timescales is crucial to ensure reliable electricity systems. At seasonal scale, the balance between supply and demand is mostly determined by the large-scale atmospheric circulation, which is uncertain due to climate change and natural variability. Here we employ four teleconnection indices, which represent a linkage between atmospheric conditions at widely separated regions, to describe the large-scale circulation a…
While surface temperature gradients have been highlighted as drivers of low-level atmospheric circulation, the underlying physical mechanisms remain unclear. Lindzen and Nigam (1987) noted that sea level pressure (SLP) gradients are proportional to surface temperature gradients if isobaric height (the height where pressure does not vary in the horizontal plane) is constant; their own model of low-level circulation assumed that isobaric height in the tropics is around 3 km. Recently Bayr and Dommenget (2013) proposed a simple model of temperature-driven air redistribution from which they derive…
Did recent sea surface temperature warming reinforce the extreme East Asian summer monsoon precipitation in 2020?
Ulsan National Institute of Science and Technology（蔚山科学技术院）；Pohang University of Science and Technology（浦项科技大学）；Yonsei University（延世大学）；Seoul National University（首尔大学）
Transition to double-cell mock Walker circulations with surface warming explained by periodic convection
Courant Institute of Mathematical Sciences, New York University（纽约大学柯朗数学科学研究所）；Institute of Earth Sciences, The Hebrew University of Jerusalem（耶路撒冷希伯来大学地球科学学院）；Department of Geosciences, Princeton University（普林斯顿大学地质科学系）；Program in Atmospheric and Oceanic Sciences, Princeton University（普林斯顿大学大气与海洋科学项目）
Direct Radiative Impacts of Stratospheric Aerosols on the Tropical Troposphere: Clouds, Precipitation, and Circulation in Convection-Resolving and Global Simulations
We characterize horizontal mixing and transport structures in the surface circulation of the Mediterranean Sea, as obtained from a primitive equation circulation model. We calculate the Finite Size Lyapunov Exponents (FSLEs) of the velocity data set, which gives a direct measure of the local stirring. By proper election of the FSLE parameters, we focus on the mesoscale structures, locating a number of vortices embedded in an intricate network of high-stretching lines. These lines control transport in the system. At the edge of the vortices, a dense tangle of line intersections appears, identif…
During the last ice age several quasi-periodic abrupt warming events took place. Known as Dansgaard-Oeschger (DO) events their effects were felt globally, although the North Atlantic experienced the largest temperature anomalies. Paleoclimate data shows that the fluctuations often occurred right after massive glacial meltwater releases in the North Atlantic and in bursts of three or four with progressively decreasing strengths. In this study a simple dynamical model of an overturning circulation and sea ice is developed with the goal of understanding the fundamental mechanisms that could have …
The mechanism that causes an interdecadal oscillation in a coarse resolution sector ocean model forced by mixed boundary conditions is studied. The oscillation is characterized by large fluctuations in convective activity and air/sea heat exchange on a decadal time scale. When the convective activity is large, a strengthening of the southeastward surface flow advects more relatively fresh water from the northwestern part of the basin into the convective area, which reduces the convective activity. Similarly, when the convective activity is small, the flow of relatively fresh water is weak, whi…
The correlation between increased Sahel rainfall and reduced Saharan surface pressure is well established in observations and global climate models, and has been used to imply that increased Sahel rainfall is caused by a stronger shallow meridional circulation (SMC) over the Sahara. This study uses two atmospheric reanalyses to examine interannual variability of Sahel rainfall and the Saharan SMC, which consists of northward near-surface flow across the Sahel into the Sahara and southward flow near 700 hPa out of the Sahara. During wet Sahel years, the Saharan SMC shifts poleward, producing a …
The processes and instabilities occurring at the ocean surface in the northern Gulf of Mexico are investigated with a regional model at submesoscale-permitting horizontal grid resolution (i.e., HR with dx = 1.6 km) over a three-year period, from January 2010 to December 2012. A mesoscale-resolving, lower resolution run (LR, with dx = 5 km) is also considered for comparison. The HR run is obtained through two-way nesting within the LR run. In HR quantities such local Rossby number, horizontal divergence, vertical velocity, and strain rate are amplified in winter, when the mixed layer is deepest…
Multi-scale decomposition of sea surface height snapshots using machine learning
Columbia University（哥伦比亚大学）；New York University（纽约大学）；Lamont Doherty Earth Observatory（拉蒙特-多尔蒂地球观测所）；Applied Physics and Applied Mathematics（应用物理与应用数学学院）；Courant Institute（柯朗研究所）
We present a set of idealised numerical experiments of a solstitial aquaplanet ocean and examine the thermodynamic and dynamic implications of surface gravity waves (SGWs) upon its mean state. The aquaplanet's oceanic circulation is dominated by an equatorial zonal jet and four Ekman driven meridional overturning circulation (MOC) cells aligned with the westerly atmospheric jet streams and easterly trade winds in both hemispheres. Including SGW parameterization (representing modulations of air-sea momentum fluxes, Langmuir circulation and Stokes-Coriolis force) increases mixed layer vertical m…
Data-driven Global Ocean Modeling for Seasonal to Decadal Prediction
School of Computer Science, Fudan University（复旦大学计算机科学学院）；Shanghai Artificial Intelligence Laboratory（上海人工智能实验室）；Potsdam Institute for Climate Impact Research（波茨坦气候影响研究所）；Japan Agency for Marine-Earth Science and Technology（日本海洋研究开发机构）；Institut de Recherche pour le Développement (IRD)（法国发展研究院）；French National Research Institute for Sustained Development (IRD)（法国可持续发展研究院）；Cooperative Institute for Research in Environmental Sciences（合作环境科学研究所）；NOAA Physical Sciences Laboratory（美国国家海洋和大气管理局物理科学实验室）；Nanjing University of Information Science and Technology（南京信息工程大学）；Technical University of Munich（慕尼黑工业大学）；University of Colorado Boulder（科罗拉多大学博尔德分校）
When assessing the hazard brought by storm surges on the coast, a frequency analysis is often conducted. An issue of particular concern is the sea level rise (SLR), thought to have an impact on maximum water levels. In this study, three gauge stations with long records dating back to the 19th century and located along the Polish Baltic Sea coast were analyzed. Different theoretical distributions were tested in order to find the best-fitting one. Gumbel distribution was chosen and estimated for 40-year moving periods. The location parameter soared in all tide gauges, but the shape parameter was…
A new method of regime shift detection in the correlation coefficient is proposed. The method is designed to find multiple change-points with unknown locations in time series. It signals a possible regime shift in real time and allows for its monitoring. The method is tested on randomly generated time series with predefined change-points. It is applied to examine structural changes in the Bering Sea climate. A major shift is found in 1967, which coincides with a transition from a zonal type of atmospheric circulation to a meridional one. The roles of the Siberian and Alaskan centers of action …
