Variability Of Circulation Research Articles

Mid-Holocene reinforcement of North Atlantic atmospheric circulation variability from a western Baltic lake sediment record

Abstract. Knowledge about the timing, amplitude and spatial gradients of Holocene environmental variability in the circum-Baltic region is key to understanding its responses to ongoing climate change. Based on a multi-dating and proxy approach, we reconstruct changes in productivity using total organic carbon (TOC) contents in sediments of Lake Kälksjön (KKJ) from west–central Sweden spanning the last 9612 (+255/-114) years. An exception is the period from 1878 CE until today, in which sedimentation was dominated by anthropogenic lake level lowering and land use. In-lake productivity was higher during periods of warmer winters with shortened ice cover and prolonged growing seasons. A multi-millennial increase in productivity throughout the last ∼ 9600 years is associated with progressively warmer winters in northwestern Europe, likely triggered by the coinciding increase in Northern Hemisphere winter insolation. Decadal to centennial periods of higher productivity in KKJ tend to correspond to warmer winters during a more positive North Atlantic Oscillation (NAO) polarity, as reconstructed for the last 8000 years. In consequence, we assume our decadal to centennial productivity record from KKJ sediments for the complete ∼ 9600 years to provide a qualitative record of NAO polarity. A shift towards higher productivity variability at ∼ 5450 cal a BP is hypothesized to reflect a reinforcement of NAO-like atmospheric circulation variability, possibly driven by more vigorous changes in North Atlantic deep-water formation.

On using self‐organizing maps and discretized Sammon maps to study links between atmospheric circulation and weather extremes

AbstractClassifications by self‐organizing maps (SOMs) have been used to study links between extreme weather events and atmospheric circulation. However, SOMs may often provide a too generalized image of circulation, which is too coarse to properly resolve meteorological extremes. Several approaches have been used to circumvent this limitation, including an increase in the number of nodes and/or relaxation of the stress on topological ordering of nodes, through adjusting the neighbourhood radius. Here, we (1) test whether weighting of training fields based on extremity of their circulation could lead to SOMs that would better represent circulation extremes without impairing the topological ordering of nodes, and (2) we investigate whether Sammon maps (SM), which have so far been limited to validation of SOMs in climatology, could provide a viable alternative to SOMs in studying weather extremes. We show that removing a certain percentage of circulation fields closest to the climatological mean has a greater effect on the representation of circulation extremes than more than doubling the SOM array size. Additionally, this step allows for decreasing the neighbourhood radius, the parameter responsible for topological ordering, which would not otherwise be possible without impairing the topology of the map. The SOMs are compared with two newly developed methods that discretize SMs based on arbitrarily chosen and distribution‐based bins. We demonstrate the usefulness of both methods in climate model validation and show that both methods are able to capture circulation variability in its extremes in more detail than the SOMs. The distribution‐based method does not suffer from sample‐size issues and outperforms all other classifications in extracting ERA5 patterns leading to extreme temperature minima over central Europe. Our results suggest that classifications based on SM could be a viable alternative to SOMs when SOMs lack the ability to discriminate circulation outliers leading to temperature extremes.

Laboratory Study of Surfactant Film Compression and Boundary Transformation under the Action of Surface Waves

In this paper we present the results of a laboratory experiment simulating the mechanisms of a surfactant film localization under the action of periodic surface waves. It is shown that the film compressed by a surface wave, can be conditionally divided into three sections — the dense film region as the most distant from the wave source, the area of a diluted film and the region of relatively clean water. The paper is focused on the processes occurring in the diluted film region characterized by the transition from clean water to the dense film, where horizontal circulation currents are registered. The character of currents is varying depending on the wave amplitude so that the divergent circulation cells occur at small and large wave amplitudes, but at medium amplitudes the roller currents forming a convergence zone on the basin axis are observed. The experimentally obtained stationary distribution of surfactant concentration is compared with the previously developed model of film compression due to surface tension induced by waves. The comparison shows good agreement between the theory and the experimental results at small and large wave amplitudes. Their discrepancy at medium wave amplitudes in the diluted film region is presumably related to the presence of convergent cells which are not considered in the model.

The variability of atmospheric circulation over Poland in the years 1950–2021

The variability of atmospheric circulation over Poland in the years 1950–2021

Relationship between cold season air temperature in the Baltic sea region and the Arctic sea ice extent

The variability of the atmospheric circulation described by the AO and NAO indices has a significant impact on the fluctuations of climate parameters in the Baltic Sea region. Many research show, that a decline in Arctic Sea ice extent increases the likelihood of negative phases of the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) in winter, and in the future could lead to more frequent relatively cold weather events during winter months in Baltic Sea region. This research is important for understanding possible teleconnections between Arctic Sea ice and atmospheric circulation (AO and NAO). In addition, this research helps to better understand the dynamics of Arctic Sea ice extent and Baltic Sea region air temperature, as well as AO and NAO circulation impact on the air temperature in the Baltic Sea region. A statistically significant decrease in the Arctic Sea ice extent and a statistically significant increase in the air temperature in the Baltic Sea region have been determined. The obtained results show that the expansion of the Arctic Sea ice in January-February with a delay of one month determines the AO and NAO values (correlation coefficients vary from 0.32 to 0.55). This means that if Arctic Sea ice decreases during winter months, AO and NAO indices are more likely to become negative as well. However, as the obtained correlations are not very strong, the results are insufficient to confirm the hypothesis that the Arctic Sea ice extent impacts negative AO and NAO phases likelihood and requires further research. A strong correlation between the Baltic Sea region air temperature and the values of AO/NAO indices has been established. The obtained results show that the Baltic Sea region air temperature variability in December-March is greatly influenced by the AO and NAO phases. Keywords: teleconnections, Arctic Sea ice, AO, NAO, Baltic Sea region, air temperature.

After Two Decades, Argo at PMEL, Looks to the Future

The NOAA Pacific Environmental Laboratory (PMEL) has contributed to the revolutionary Argo ocean observing system since its inception, developing CTD calibration algorithms and software that have been adopted by the international Argo community. PMEL has provided over 1,440 Argo floats—~13% of the global array—with ~500 currently active. PMEL scientific contributions using Argo data have ranged from regional to global analyses of ocean circulation and water-mass variability, to ocean warming and its contributions to sea level rise and Earth’s energy imbalance, to estimates of global ocean deoxygenation. In recent years, PMEL has initiated both Deep Argo (with a regional pilot array of full-ocean-depth profiling floats in the rapidly changing and dynamic western South Atlantic) and Biogeochemical (BGC) Argo (with a pilot array in the biogeochemically diverse and economically important California Current Large Marine Ecosystem). PMEL is also developing innovative near-global maps of ocean physical and biogeochemical parameters using machine learning algorithms that enable investigations of societally important oceanographic phenomena, and an Adopt-A-Float program. Future challenges include growing the financial, infrastructure, and human resources necessary to take the Deep and BGC Argo missions global and to fulfill the One Argo mission of a global, full-depth, multidisciplinary ocean observing array.

Quantifying and Understanding Forced Changes to Unforced Modes of Atmospheric Circulation Variability over the North Pacific in a Coupled Model Large Ensemble

Abstract While much attention has been given to understanding how anthropogenic radiative forcing influences the mean state of the climate system, far less scrutiny has been paid to how it may modulate naturally occurring modes of variability. This study investigates forced changes to unforced modes of wintertime atmospheric circulation variability and associated impacts on precipitation over the North Pacific and adjacent regions based on the 40-member Community Earth System Model version 1 Large Ensemble across the 1920–2100 period. Each simulation is subject to the same radiative forcing protocol but starts from a slightly different initial condition, leading to different sequences of internal variability. Evolving forced changes in the amplitude and spatial character of the leading internal modes of 500-hPa geopotential height variability are determined by applying empirical orthogonal function analysis across the ensemble dimension at each time step. The results show that the leading modes of internal variability intensify and expand their region of influence in response to anthropogenic forcing, with concomitant impacts on precipitation. Linkages between the Pacific and Atlantic sectors, and between the tropics and extratropics, are also enhanced in the future. These projected changes are driven partly by teleconnections from amplified ENSO activity and partly by dynamical processes intrinsic to the extratropical atmosphere. The marked influence of anthropogenic forcing on the characteristics of internal extratropical atmospheric circulation variability presents fundamental societal challenges to future water resource planning, flood control, and drought mitigation.

Spatial changes in two major modes of atmospheric circulation variability during the 20th century

This study presents the time development of two major modes of atmospheric circulation variability in the Northern Hemisphere, the North Atlantic Oscillation (NAO) and the Pacific/North American Pattern (PNA), during the 20th century. We employ 500 hPa winter monthly means of the long-term reanalysis 20CR, which cover the 1871−2011 period. We use a moving Principal Component Analysis calculated for 40-year periods with a one-year step. The spatial structure of NAO is well developed and stable during the whole 20th century using the ensemble mean. However, substantial changes in PNA pattern occur during the late 19th century. Its centres are weaker and smaller, except the centre over the northern Pacific Ocean, which is stronger and larger in the early period. However, these changes are not visible when using ensemble members. Therefore, we suggest that these shifts do not reflect real changes in the atmosphere but are produced by the reanalysis and statistical method itself.

Impact of Yucatan Channel Subsurface Velocity Observations on the Gulf of Mexico State Estimates

Abstract The Gulf of Mexico (GoM) surface circulation variability is dominated by the Loop Current (LC) and the episodically released anticyclonic Loop Current eddies (LCEs). The Yucatan Current feeds the LC through the Yucatan Channel (YC), and its flow structure at the YC is hypothesized to affect the LC evolution critically. This study examines the impact of assimilating YC subsurface velocity observations from a tall mooring array across the YC on the GoM circulation. State estimates and forecasts of the LC circulation were produced using a regional implementation of the Massachusetts Institute of Technology general circulation model (MITgcm) and its adjoint-based four-dimensional variational (4DVAR) assimilation system. The estimates were constrained by combinations of the YC observations and satellite-derived sea surface height (SSH) and sea surface temperature (SST). The results show that assimilation of both moored and satellite data improves the model hindcasts and forecasts for all LC phases. Additionally, one realization of the state estimate that assimilates only moored data matches the LCE detachment timing with that of AVISO SSH. Observations from the moorings close to the Yucatan Peninsula significantly impact the LCE detachment. A finite-time Lyapunov exponent analysis reveals the differences among the assimilation experiments, such as eddylike structures intruding into the GoM through the YC, and its relation to the typical LC sudden growth. Finally, an adjoint sensitivity analysis is used to verify the dynamic link between the LC extension and the intrusion of eddylike structures into the GoM.

Routine Reversal of the AMOC in an Ocean Model Ensemble

AbstractWe describe a form of Atlantic Meridional Overturning Circulation (AMOC) variability that we believe has not previously appeared in observations or models. It is found in an ensemble of eddy‐resolving North Atlantic simulations that the AMOC frequently reverses in sign at ∼35°N with gyre‐wide anomalies in size and that reach throughout the water column. The duration of each reversal is roughly 1 month. The reversals are part of the annual AMOC cycle occurring in boreal winter, although not all years feature an actual reversal in sign. The occurrence of the reversals appears in our ensemble mean, suggesting it is a forced feature of the circulation. A partial explanation is found in an Ekman response to wind stress anomalies. Model ensemble simulations run with different combinations of climatological and realistic forcings argue that it is the atmospheric forcing specifically that results in the reversals, despite the signals extending into the deep ocean.

Distinct Responses of North Pacific and North Atlantic Summertime Subtropical Anticyclones to Global Warming

Abstract The subtropical North Pacific and North Atlantic are controlled by basin-scale anticyclones in boreal summer. Based on a novel metric regarding the strengths of the rotational and the divergent circulation of anticyclones, we investigated the possible future responses in the intensity of these two subtropical anticyclones to global warming. While the North Atlantic subtropical anticyclone (NASA) is projected to strengthen, the North Pacific subtropical anticyclone (NPSA) is projected to weaken, in terms of both the rotational and the divergent circulation. The distinct responses of the NPSA and NASA are corroborated by the models participating in the fifth and sixth phases of the Coupled Model Intercomparison Project (CMIP), under both intermediate and high emission scenarios. We further investigated the possible mechanism for their distinct responses by decomposing the effect of greenhouse gas forcing into the direct effect of increased CO2 concentration and the indirect effect through sea surface temperature (SST). The intensified NASA results from the CO2 direct forcing while the weakened NPSA is dominated by the SST warming. The CO2 direct forcing enhances the NASA by increasing land–ocean thermal contrast anchored by the largest subtropical continental area, the Eurasian–African continent. Both the uniform SST warming and the change in SST pattern act to weaken the NPSA by increasing the latent heating over the subtropical North Pacific basin, as suggested by atmospheric component model simulations. The distinct responses of the NPSA and the NASA may lead to zonal asymmetry of the subtropical climate change.

Wind influence on residual circulation in Patagonian channels and fjords

Circulation in estuaries is typically considered to be driven by horizontal density gradients, called gravitational circulation, but can be modified by other forcing such as the wind. Studies have shown that wind influence can either augment or counteract density-driven residual flows in estuaries, but the temporal and spatial extent of this influence remains unknown, particularly in deep and highly stratified fjords. This study aims to fill this gap by investigating the seasonal variability in residual circulation, wind forcing, and stratification in an extensive fjord and channel system in Chilean Patagonia. Observations of current velocity and wind are supplemented by atmospheric and hydrodynamic circulation models in the Moraleda Channel, a system that connects the fjord system of Northern Patagonia to the Pacific Ocean. Results show a marked seasonality, given that the surface outflow is reversed by wind forcing during the winter, a season characterized by strong southward (up-channel) winds. This produces a near-surface flow directed up-channel in the upper ∼40 m of the water column, in opposition to the horizontal pressure gradient. Cross-channel transects revealed that the vertical structure of the along-channel residual velocity was three-layer during the winter. The near-surface (<50 m) along-channel current on the eastern side of the channel followed the wind direction (southward). Between 50 and 100 m depth the flow was out-channel and below 100 m depth the flow was again in-channel. On the western side, the along-channel residual flow was two-layer, with near surface flow directed northward (against the wind) with a return flow below (deeper than 50 m). During the spring, northward (out-channel) winds dominated and the vertical structure of the along-channel residual velocity was two-layer, augmenting the horizontal pressure gradient, which stabilized the water column, enhancing stratification, inhibiting mixing and elevated gravitational circulation.

The El Niño Southern Oscillation drives multidirectional inter-reef larval connectivity in the Great Barrier Reef.

The El Niño Southern Oscillation (ENSO) is the strongest source of interannual global climate variability, and extreme ENSO events are projected to increase in frequency under climate change. Interannual variability in the Coral Sea circulation has been associated with ENSO, although uncertainty remains regarding ENSO's influence on hydrodynamics and larval dispersal in the adjacent Great Barrier Reef (GBR). We investigated larval connectivity during ENSO events from 2010 to 2017 throughout the GBR, based on biophysical modelling of a widespread predatory reef fish, Lutjanus carponotatus. Our results indicate a well-connected system over the study period with high interannual variability in inter-reef connectivity associated with ENSO. Larval connectivity patterns were highly correlated to variations in the Southern Oscillation Index (SOI). During El Niño conditions and periods of weak SOI, larval dispersal patterns were predominantly poleward in the central and southern regions, reversing to a predominant equatorward flow during very strong SOI and extreme La Niña conditions. These ENSO-linked connectivity patterns were associated with positive connectivity anomalies among reefs. Our findings identify ENSO as an important source of variation in larval dispersal and connectivity patterns in the GBR, which can influence the stability of population dynamics and patterns of biodiversity in the region.

Temperature Structures Associated with Different Components of the Atmospheric Circulation on Tidally Locked Exoplanets

Observations of time-resolved thermal emission from tidally locked exoplanets can tell us about their atmospheric temperature structure. Telescopes such as JWST and ARIEL will improve the quality and availability of these measurements. This motivates an improved understanding of the processes that determine atmospheric temperature structure, particularly atmospheric circulation. The circulation is important in determining atmospheric temperatures, not only through its ability to transport heat, but also because any circulation pattern needs to be balanced by horizontal pressure contrasts, therefore implying a particular temperature structure. In this work, we show how the global temperature field on a tidally locked planet can be decomposed into contributions that are balanced by different components of the atmospheric circulation. These are the superrotating jet, stationary Rossby waves, and the divergent circulation. To achieve this, we partition the geopotential field into components balanced by the divergent circulation and the rotational circulation, with the latter comprising the jet and Rossby waves. The partitioned geopotential then implies a corresponding partitioning of the temperature via the hydrostatic relation. We apply these diagnostics to idealized general circulation model simulations, to show how the separate rotational and divergent circulations together make up the total three-dimensional atmospheric temperature structure. We also show how each component contributes distinct signatures to the thermal phase curve of a tidally locked planet. We conclude that this decomposition is a physically meaningful separation of the temperature field that explains its global structure, and can be used to fit observations of thermal emission.

Twelve-Year Cycle in the Cloud Top Winds Derived from VMC/Venus Express and UVI/Akatsuki Imaging

We present joint analysis of the UV (365 nm) images captured by the cameras on board ESA’s Venus Express and JAXA’s Akatsuki spacecraft. These observations enabled almost continuous characterization of the cloud top circulation over the longest period of time so far (2006–2021). More than 46,000 wind vectors were derived from tracking the UV cloud features and revealed changes in the atmospheric circulation with the period of 12.5 ± 0.5 years. The zonal wind component is characterized by an annual mean of −98.6 ± 1.3 m/s and an amplitude of 10.0 ± 1.6 m/s. The mean meridional wind velocity is −2.3 ± 0.2 m/s and has an amplitude of 3.4 ± 0.3 m/s. Plausible physical explanations of the periodicity include both internal processes and external forcing. Both missions observed periodical changes in the UV albedo correlated with the circulation variability. This could result in acceleration or deceleration of the winds due to modulation of the deposition of the radiative energy in the clouds. The circulation can be also affected by the solar cycle that has a period of approximately 11 years with a large degree of deviation from the mean. The solar cycle correlated with the wind observations can probably influence both the radiative balance and chemistry of the mesosphere. The discovered periodicity in the cloud top circulation of Venus, and especially its similarity with the solar cycle, is strongly relevant to the study of exoplanets in systems with variable “suns”.

A Eurasian Basin sedimentary record of glacial impact on the central Arctic Ocean during MIS 1–4

Pronounced Quaternary circulation and depositional changes in the Arctic Ocean basins remain poorly understood. We investigate sediment core ARC5-ICE4 (ICE4) in the Eurasian (Amundsen) Basin off the Lomonosov Ridge to reconstruct glacial-interglacial variability in sediment sources and circulation. The ICE4 record features variable lithologies including distinct glacigenic deposits with Ice Rafted Debris (IRD). The sediment age is constrained by a combination of the organic carbon 14C with paleomagnetic data and correlation to earlier developed stratigraphies. The derived sedimentation rates constitute several cm/kyr (6.5 cm/kyr average) consistent with estimates from regional geophysical data. The developed age model covers the time interval from ca. 10 to 75 ka including Late and Middle Weichselian/Wisconsinian glaciations. Respective glacigenic sediments were primarily deposited by pulsed iceberg discharge. The Last Glacial Maximum and deglaciation is expressed in sharp IRD peaks including mineral proxies of the Laurentide Ice Sheet with an apparent millennial-scale variability. The older glacigenic sedimentary unit with abundant IRD of predominantly Siberian provenance was deposited during deglaciation of the Middle Weichselian (MIS 4/3) Eurasian Ice Sheet. These sedimentary differences indicate diverging impacts of the two glaciations on the Arctic Ocean, including ice-sheet sizes/geometries and oceanic circulation.

Variability of Sea Level and Circulation in the North Atlantic Based on Satellite Altimetry Data

Variability of Sea Level and Circulation in the North Atlantic Based on Satellite Altimetry Data

Toward Ocean Hindcasts in Earth System Models: AMOC Variability in a Partially Coupled Model at Eddying Resolution

AbstractWhile forced ocean hindcast simulations are useful for a wide range of applications, a key limitation is their inability to simulate ocean‐atmosphere feedbacks. As a consequence, they need to rely on artificial choices such as sea surface salinity restoring and other corrections affecting the surface freshwater fluxes. Fully coupled models overcome these limitations, but lack the correct timing of variability due to weaker observational constraints. This leads to a mismatch between forced and coupled models on interannual to decadal timescales. A possibility to combine the advantages of both modeling strategies is to apply a partial coupling (PCPL), that is, replacing the surface winds stress in the ocean component by wind stress derived from reanalysis. To identify the capabilities, limitations and possible use cases of partial coupling, we perform a fully coupled, two partially coupled and an ocean‐only experiment using an all‐Atlantic nested ocean configuration at eddying resolution in a global climate model. We show that the correct timing of Atlantic Meridional Overturning Circulation (AMOC) variability in PCPL experiments is robust on timescales below 5 years. Mid‐latitude wind stress curl changes contribute to decadal AMOC variability, but North Atlantic buoyancy fluxes are not significantly altered by incorporating reanalyzed wind stress anomalies, limiting the success of PCPL on this timescale. Long term trends of the AMOC in PCPL mode are consistent with fully coupled model experiments under historic atmospheric boundary conditions, suggesting that a partially coupled model is still able to simulate the important ocean‐atmosphere feedbacks necessary to maintain a stable AMOC.

Meridional response of Western North Pacific paleocyclone activity to tropical atmospheric circulation variability over the past millennium

Sedimentary records of past tropical cyclone (TC) activity indicate large variability and spatial heterogeneity on the interannual-century scales. However, the meridional variation patterns and climatic forcing mechanisms for regional TC activity are not well understood because of the short and incomplete observational record. Here, we present a near-annually resolved record of intense TC activity derived from a back-reef lagoon at Anle Atoll in the southern South China Sea since the Little Ice Age (LIA). Our reconstruction indicates that TC activity was higher during the early LIA than what has been observed throughout the instrumental record. Basin-wide compiled records of the western North Pacific (WNP) cyclogenesis revealed strong evidence of a meridional seesaw pattern for storm activity between relatively low and high latitudes over the past millennium. This pattern suggests a centennial-scale link with Pacific hydroclimate atmospheric circulation shifts. A combination of southward displacement of the Intertropical Convergence Zone and westward migration of the Pacific Walker Circulation, as well as damped El Niño Southern Oscillation variability, likely contributed to enhanced TC activity during the LIA in the low latitudes of the WNP. External forcing of solar irradiance budgets profoundly affects the hydroclimate and TC climatology in the WNP.

Discrepancies between observations and climate models of large-scale wind-driven Greenland melt influence sea-level rise projections

While climate models project that Greenland ice sheet (GrIS) melt will continue to accelerate with climate change, models exhibit limitations in capturing observed connections between GrIS melt and changes in high-latitude atmospheric circulation. Here we impose observed Arctic winds in a fully-coupled climate model with fixed anthropogenic forcing to quantify the influence of the rotational component of large-scale atmospheric circulation variability over the Arctic on the temperature field and the surface mass/energy balances through adiabatic processes. We show that recent changes involving mid-to-upper-tropospheric anticyclonic wind anomalies – linked with tropical forcing – explain half of the observed Greenland surface warming and ice loss acceleration since 1990, suggesting a pathway for large-scale winds to potentially enhance sea-level rise by ~0.2 mm/year per decade. We further reveal fingerprints of this observed teleconnection in paleo-reanalyses spanning the past 400 years, which heightens concern about model limitations to capture wind-driven adiabatic processes associated with GrIS melt.