Flux Anomalies Research Articles

Using Orbiting Carbon Observatory-2 (OCO-2) column CO2 retrievals to rapidly detect and estimate biospheric surface carbon flux anomalies

Abstract. The global carbon cycle is experiencing continued perturbations via increases in atmospheric carbon concentrations, which are partly reduced by terrestrial biosphere and ocean carbon uptake. Greenhouse gas satellites have been shown to be useful in retrieving atmospheric carbon concentrations and observing surface and atmospheric CO2 seasonal-to-interannual variations. However, limited attention has been placed on using satellite column CO2 retrievals to evaluate surface CO2 fluxes from the terrestrial biosphere without advanced inversion models at low latency. Such applications could be useful to monitor, in near real time, biosphere carbon fluxes during climatic anomalies like drought, heatwaves, and floods, before more complex terrestrial biosphere model outputs and/or advanced inversion modelling estimates become available. Here, we explore the ability of Orbiting Carbon Observatory-2 (OCO-2) column-averaged dry air CO2 (XCO2) retrievals to directly detect and estimate terrestrial biosphere CO2 flux anomalies using a simple mass-balance approach. An initial global analysis of surface–atmospheric CO2 coupling and transport conditions reveals that the western US, among a handful of other regions, is a feasible candidate for using XCO2 for detecting terrestrial biosphere CO2 flux anomalies. Using the CarbonTracker model reanalysis as a test bed, we first demonstrate that a well-established mass-balance approach can estimate monthly surface CO2 flux anomalies from XCO2 enhancements in the western United States. The method is optimal when the study domain is spatially extensive enough to account for atmospheric mixing and has favorable advection conditions with contributions primarily from one background region. We find that errors in individual soundings reduce the ability of OCO-2 XCO2 to estimate more frequent, smaller surface CO2 flux anomalies. However, we find that OCO-2 XCO2 can often detect and estimate large surface flux anomalies that leave an imprint on the atmospheric CO2 concentration anomalies beyond the retrieval error/uncertainty associated with the observations. OCO-2 can thus be useful for low-latency monitoring of the monthly timing and magnitude of extreme regional terrestrial biosphere carbon anomalies.

Combined oceanic and atmospheric forcing of the 2013/14 marine heatwave in the northeast Pacific

An unprecedented warm sea surface temperature (SST) anomaly event, namely, the Blob, occurred in the northeast Pacific during the winter (October–January) of 2013/2014, causing substantial economic and ecological impacts. Here, we explore the driving forces of the Blob from both atmospheric and oceanic perspectives and show that the Blob primarily resulted from weak wintertime cooling due to the reduced air-sea heat flux transfer from the ocean to the atmosphere and the reduced horizontal advection of cold water in the upper ocean. Both mechanisms were attributed to an anomalous high-pressure system over the study region. Specifically, the anomalous air-sea heat flux, which was dominated by turbulent heat flux anomalies, was mainly induced by the increased air temperature (i.e., with a contribution of approximately 70%) and the weakened wind speed associated with the high-pressure system. The reduced horizontal heat advection was mainly due to the weakened winds acting on the ocean temperature meridional gradient. Using a regional ocean numerical model with different experimental runs, we evaluated the contributions of air temperature and wind drivers to the Blob at both the surface and subsurface of the ocean. The Blob was absent when the model was forced by climatology-air-temperature. Both the SST and integrated ocean heat content (OHC, 0–150 m) decreased, and the mixed layer depth (MLD) was deeper than that in the control run forced by real atmospheric conditions. In the climatology-winds experiment, obvious warm anomalies still existed, which were similar to but weaker than the control run. The SST (OHC) and MLD values in the climatology-winds run were between those of the climatology-air-temperature run and the control run. Compared to former studies that attribute the formation of the Blob to an anomalous air-sea heat flux and horizontal advection mainly induced by reduced winds, our study demonstrates that anomalous warm air temperatures played a more important role in its formation.

Exceptionally persistent Eurasian cold events and their stratospheric link

Persistent boreal winter cold spells (PCEs) can heavily strain the economy and significantly impact everyday life. While sudden stratospheric warmings are considered a precursor for Eurasian (EUR) cold events, these temperature extremes may occur during the full range of stratospheric variability. We investigate PCEs relative to the prevailing stratospheric polar vortex regime before their onset, with a particular focus on extremely weak (SSW) and strong (SPV) stratospheric winds by performing (lagged) composite analysis based on ERA5 reanalysis. On average, SPV PCEs that are concentrated over central EUR, are colder, shorter and set in more abruptly compared to SSW PCEs. A quasi-stationary, mid-tropospheric anticyclone over the Arctic Ocean that blocks warm air advection toward EUR is connected to the canonical downward progression of the negative North Atlantic Oscillation for SSW PCEs. In contrast, during SPV PCEs, the anticyclone is part of a Rossby wave having an origin co-located with negative wave activity flux anomalies over and being influenced by stratospheric wave reflection toward the North Atlantic. Its slow east-ward propagation is likely related to Arctic surface warming and unusually weak zonal winds over EUR.

Fast and Slow Subpolar Ocean Responses to the North Atlantic Oscillation: Thermal and Dynamical Changes

AbstractClimate model hindcasts are analyzed to reveal the impacts of the North Atlantic Oscillation (NAO) on the North Atlantic subpolar ocean, which exhibits variability on seasonal to decadal timescales. The ocean response to a single winter NAO event is separated into fast and slow responses. The fast response persists over winter–spring seasons, during which wind stress and heat flux anomalies associated with the NAO rapidly modify ocean temperatures via changes in Ekman transport and ocean‐atmosphere heat exchanges. The slow response persists for 3–4 years, during which overturning and gyre circulations redistribute opposing‐signed surface temperature anomalies created by the NAO. This redistribution modifies east‐west temperature contrasts altering the meridional heat transport associated with gyres and changing the strength of the overturning circulation. Hence, the fast and slow responses lead to opposing‐signed subpolar temperature anomalies in time from the competing effects of local forcing and horizontal heat convergence.

Subseasonal predictability of the transition of the stratospheric polar vortex: A case study in winter 1987/88

The stratospheric polar vortex (SPV), which is an important factor in subseasonal-to-seasonal climate variability and climateprediction, exhibited a remarkable transition from weak in early winter to strong in late winter in 1987/88 (most significant on the interannual timescale during 1979–2019). Therefore, in this study, the subseasonal predictability of this transition SPV case in 1987/88 was investigated using the hindcasts from a selected model (that of the Japan Meteorological Agency) in the Subseasonal-to-Seasonal Prediction project database. Results indicated that the predictability of both weak and strong SPV stages in winter 1987/88, especially near their peak dates, exhibited large sensitivity to the initial condition, which derived mainly from the sensitivity in capturing the 100-hPa eddy heat flux anomalies. Meanwhile, the key tropospheric precursory systems with respect to the occurrence and predictability of this transition SPV case were investigated. The Eurasian teleconnection wave trains might have been a key precursor for the weak SPV stage, while significant tropospheric precursors for the strong SPV stage were not found in this study. In addition, positive correlation (r = 0.41) existed between the forecast biases of the SPV and the NAO in winter 1987/88, which indicates that reducing the forecast biases of the SPV might help to improve the forecasting of the NAO and tropospheric weather.摘要平流层极涡作为冬季次季节尺度上一个重要的可预测性来源, 其强度在1987/88年冬季表现为1979–2019年最显著的转折, 即在前 (后) 冬极端偏弱 (强). 因此在本文中选取这一个例研究了该年冬季平流层极涡在次季节尺度上的可预测性. 结果表明弱极涡和强极涡事件的预测与模式能否准确预测上传行星波的强度紧密相关. 同时, 发现前期对流层欧亚遥相关波列可能是弱极涡事件发生的关键预兆信号. 此外, 模式对平流层极涡强度和北大西洋涛动预测误差之间存在显著正相关关系, 表明模式减少平流层极涡的预测误差可能可以提高北大西洋涛动及相关对流层气候预测.

The Role of Tibetan Plateau–Indian Ocean Thermal Contrast in the Significant Increasing Precipitation over the Southern Tibetan Plateau in May after the Mid-1990s

Abstract This study investigates the impact of the interannual variation of Tibetan Plateau (TP)–Indian Ocean thermal contrast on southern TP precipitation in May and its changes in the mid-1990s. It is found that the southern TP precipitation is closely related to the TP–Indian Ocean thermal contrast, which could be represented by a thermal contrast index (TCI). The analysis reveals that the southern TP precipitation in May increases significantly after 1996. Meanwhile, the TCI also enhances notably. It is also found that compared with the period in 1979–96 (P1), the relationship between the southern TP precipitation and the TCI is more pronounced in 1997–2014 (P2). Further analysis shows that the meridional water vapor flux anomaly over the Arabian Sea (ABS_QV) acts as a linkage between the southern TP precipitation and the TCI. During P2, associated with the positive ABS_QV, positive water vapor flux anomalies emerge over the Arabian Sea and divide into two branches to the southwest of TP. One branch transports the moisture to the southern TP directly and forms a stronger southwesterly anomaly at the southwest edge of the TP than P1. The other branch stretches northward and imposes positive diabatic heating (Q1) anomalies over the Ural Mountains region (UMR_Q1). The results obtained by composite analysis and an anomaly atmospheric general circulation model both confirm that the UMR_Q1 anomalies can induce the wave activity propagating from the UMR_Q1 area to the TP and form the northerly anomaly over the TP, contributing to the convergence that favors the southern TP precipitation.

Processes Controlling the Southern Ocean Temperature Change

Abstract Observations show that since the 1950s, the subsurface Southern Ocean has been rapidly warming in the south and cooling in the north. However, what drives these opposing latitudinal patterns of change is not well understood. By analyzing the outputs of atmosphere–ocean general circulation models, we examine and quantify the processes controlling the temperature change in the Southern Ocean. We find that subsurface cooling in the north results from the combined effect of weak warming driven by greenhouse gases and cooling forced by aerosols. Contrary to previous work, we find that the influence of stratospheric ozone forcing is small and unnecessary to explain the observed patterns. Analysis of heat budget diagnostics shows that ocean heat content change between 400 and 1200 m in the Southern Ocean is primarily driven by slight imbalances between the large and opposing changes in residual mean advection and isopycnal diffusion. We show that while surface heat flux anomalies are the main contributor to the warming in the Southern Ocean, changes in wind stress are also required to produce the observed pattern of temperature change. These results highlight the importance of ocean dynamics in subsurface ocean heat content change. The counterbalancing effects of aerosols and greenhouse gases on ocean heat content change in the Southern Ocean will change with aerosols declining in the future, while our results suggest that changes in stratospheric ozone forcing will have only a minor effect on future patterns of Southern Ocean thermohaline structure.

Excitation of the Madden‐Julian Oscillation in Response to Transient Ocean Warming in SPCAM

AbstractRecent studies have suggested that the Madden‐Julian oscillation (MJO) could be generated as part of atmospheric adjustment to localized equatorial heating in models properly resolving moist convection. Here we use the Super‐parameterized Community Atmosphere Model (SPCAM) to study atmospheric response to transient sea surface warming in the equatorial Indian Ocean, which leads to the excitation of an MJO event, robust across different experiments, in addition to the expected equatorial Rossby and Kelvin waves. A moist static energy (MSE) budget analysis suggests that longwave and surface turbulent latent heat flux anomalies are predominantly responsible for the MJO excitation; however, these terms oppose its eastward propagation. In contrast, advection of MSE generally weakens the MJO amplitude, but contributes to its eastward propagation. These results highlight the role of the MJO in the moist atmospheric adjustment to localized tropical heating, with implications for the MJO mechanisms and prediction.

What Mainly Drives the Interannual Climate Variability Over the Barents‐Kara Seas in Boreal Early Autumn?

AbstractYear‐to‐year forecasts of the Arctic climate conditions have drawn much interest in recent decades. This study investigates the primary drivers responsible for interannual variations (without multi‐decadal trends) of air temperature at 2 m (T2m) and sea ice concentration (SIC) over the Barents‐Kara Seas (BKS) during September from 1950 to 2021. The Arctic Dipole (AD), a leading factor in the BKS variability, plays an important role via influencing large‐scale atmospheric circulations and local thermodynamical feedbacks (i.e., vapor and albedo feedbacks). During a negative AD phase, the positive surface air temperature advection anomalies warm up the BKS region and promote sea ice melting, which in turn maintains the below‐normal sea ice and warmer‐than‐normal conditions via positive vapor and albedo feedbacks. Moreover, our results suggest that the sea surface temperature (SST) variability in the midlatitudes of the North Atlantic in the preceding month could also be an important driver of the SIC variability over the BKS area. The anomalous temperature advections from the surface to the upper troposphere are accompanied by a Rossby wave‐type atmospheric response, which intensifies the energy exchanges between the ocean and the atmosphere. Local SST anomalies associated with the surface heat flux anomalies further persist in the following month and influence sea ice variations due to their long‐lasting memory. A better understanding of the mechanisms of interannual climate variabilities over the BKS region in boreal early autumn is helpful to improving the Arctic regional climate predictions.

Interannual variations and their dynamic mechanisms of wintertime temperature inversions in the Gulf of Alaska

Using the Simple Ocean Data Assimilation Version 2.2.4 dataset from 1950 to 2010, we analyzed the interannual variations in wintertime temperature inversions in the Gulf of Alaska (GOA), and the linkages to the Aleutian Low are also examined. The results show that the climatological wintertime temperature inversions is predominantly distributed in the northwestern GOA (NWGOA) and northern GOA (NGOA), in which it is stronger in the NWGOA than in the NGOA. Interannual variations in the temperature difference (∆T) are pronounced and are mainly controlled by the temperature minimum (Tmin) of the inversions layer. The temperature inversions layer became warmer and shallower during 1950–2010, among Tmin and the temperature maximum (Tmax) was notably warming, and the upper edge depth (Dmin) and lower edge depth (Dmax) revealed significant shoaling. ∆T decreased by 0.12 °C from 1950 to 2010 in the NWGOA but had no trend in the NGOA. The interannual variations in wintertime temperature inversions are controlled by the mixed-layer temperature anomalies and are closely correlated with the changes in the Aleutian Low. The weakened (deepened) Aleutian Low during strong (weak) ∆T winters could cause the weakening (enhancement) of the cyclone wind field in the North Pacific subarctic region, which is conducive to slowing down (spinning up) the flow of the Alaskan Gyre and the transportation of warm air into the GOA. Thus, a negative (positive) Tmin anomalies within the mixed layer is larger than Tmax anomalies, resulting in a larger (smaller) ∆T; meanwhile, the strong (weak) cooling effect leads to a deeper (shallower) Dmax. The interannual variations in the wintertime temperature inversions could be influenced by net heat flux and advection anomalies in the NWGOA, but only caused by net heat flux anomalies in the NGOA.

A modeling framework to understand historical and projected ocean climate change in large coupled ensembles

Abstract. The ocean responds to climate change through modifications of heat, freshwater and momentum fluxes at its boundaries. Disentangling the specific role of each of these contributors in shaping the changes of the thermohaline structure of the ocean is central for our process understanding of climate change and requires the design of specific numerical experiments. While it has been partly addressed by modeling studies using idealized CO2 forcings, the time evolution of these individual contributions during historical and projected climate change is however lacking. Here, we propose a novel modeling framework to isolate these contributions in coupled climate models for which large ensembles of historical and scenario simulations are available. The first step consists in reproducing a coupled pre-industrial control simulation with an ocean-only configuration, forced by prescribed fluxes at its interface, diagnosed from the coupled model. In a second step, we extract the external forcing perturbations from the historical+scenario ensemble of coupled simulations, and we add them to the prescribed fluxes of the ocean-only configuration. We then successfully replicate the ocean's response to historical and projected climate change in the coupled model during 1850–2100. In a third step, this full response is decomposed in sensitivity experiments in which the forcing perturbations are applied individually to the heat, freshwater and momentum fluxes. Passive tracers of temperature and salinity are implemented to discriminate the addition of heat and freshwater flux anomalies from the redistribution of pre-industrial heat and salt content in response to ocean circulation changes. Here, we first present this general framework and then apply it to the IPSL-CM6A-LR model and its ocean component NEMO3.6. This framework brings new opportunities to precisely explore the mechanisms driving historical and projected ocean changes within single climate models.

Causes and processes of two opposite climatic years in the tropical Atlantic warm pools

A numerical investigation of the interannual Southwestern and Equatorial Atlantic warm pools (WPs) (SST>28 °C) was conducted for the 2000–2014 period, focusing on two major opposite events over the past 39 years (1982–2020): an intense warming event in 2010 (sizeable warm pool) and a moderate-to-low event in 2012 (small warm pool). Some modes of climate variability (remote and local) were confronted with the WP area. Significant remote connections were found for the North Atlantic Oscillation (NAO) and the Atlantic Meridional Mode (AMM). Whereas, the Tropical Northern Atlantic (TNA) and Tropical Southern Atlantic (TSA) indices were locally related to anomalies over the WP area. On the months that preceded the 2010 WP event, an anomalous atmospheric circulation in the tropical North Atlantic basin occurred (extremely negative NAO and positive AMM), causing a major weakening of winds, shallower mixed layer, and higher SST. Consequently, the WP boundaries, which usually developed southward along the equator in the austral fall, were pushed northward as warm waters spread out into the north tropical basin. Locally, both basins (i.e., tropical north and south) were warmer than climatologically expected (positive TNA and TSA), leading to a significant increase in the WP area recorded in this period. On the other hand, during the months that preceded the 2012 WP, the trade winds were faster (positive NAO and positive AMM), leading to a condition close to the climatologically expected. The approximation of the center of the subtropical high in the southern ocean basin practically extinguished the portion of warm waters that usually spreads southwest along the basin. While the TNA was weakly positive, the TSA presented negative anomalies that ensured a significant reduction in the WP area in the southern basin during this time. Numerical results from the 2000–2014 run were contrasted with PIRATA buoys and satellite data to assess the model's performance. Investigations of the mixed layer temperature terms disclosed the predominance of the atmosphere over the oceanic terms during the warm pool area increase/decrease, showing a significantly different composition within the oceanic terms. The persistence of the same sign anomalies during the three months preceding the WP's seasonal development appeared to be a pivotal factor in understanding how different the events in 2010 and 2012 were. Net heat flux anomalies, mainly controlled by anomalous wind patterns, were primarily responsible for the extreme events observed in 2010 and 2012.

Meridional Propagation of Carbon Dioxide (CO2) Growth Rate and Flux Anomalies From the Tropics Due to ENSO

AbstractEl Niño Southern Oscillation (ENSO) influence on carbon dioxide (CO2) growth rates are not spatially uniform or simultaneous around the globe. Using atmospheric CO2 observations and atmospheric chemistry‐transport model (ACTM), we show that anomalies in CO2 fluxes and growth rate originate in the tropics, as an effect of ENSO. The CO2 anomalies are then propagated from the equator toward the poles with asymmetric delays. A maximum delay of about 8 and 4 months are found at the northern hemisphere (NH) and southern hemisphere (SH) high latitudes, respectively. This asymmetric time delay is because of the CO2 flux anomaly mainly originate in the tropical SH land and transport of SH air into the NH is slower than that for NH air into the SH. The poleward increase of time delay is more homogeneous in the upper troposphere, as per the ACTM simulations, but observational evidence suffers from gaps in long‐term measurements.

Unprecedented Arctic sea ice thickness loss and multiyear-ice volume export through Fram Strait during 2010–2011

The satellite-observed sea ice thickness (SIT) records from 2003 to 2020 identify an extreme SIT loss during 2010–2011. Ice thickness budget analysis demonstrates that the thickness loss was associated with an extraordinarily large multiyear ice (MYI) volume export through the Fram Strait during the season of sea ice advance. High cloudiness led to positive anomalies of net longwave radiation, and positive net surface energy flux anomalies supported enhanced sea ice melt from June to August. Due to the MYI loss, the Arctic sea ice became more sensitive to subsequent atmospheric anomalies. The reduced surface albedo triggering a positive ice-albedo amplifying feedback and contributed to the accelerating loss of ice thickness. These tightly coupled events highlight that the increasingly younger and thinner Arctic sea ice is becoming more vulnerable to external forcing and created the precondition for the rapid reduction in sea ice extent in 2012.

Causes of the drastic change in sea ice on the southern northwind ridge in July 2019 and July 2020: From a perspective from atmospheric forcing

Arctic sea ice is a key factor in high–latitude air–sea–ocean interactions. In recent decades, its extent has been decreasing in all seasons with large interannual variability, especially for the Northwind Ridge. After removing the trend in the changes during July 1979 to 2020, 2019 had an abnormally low value, while the following year, 2020, had an abnormally high value. The underlying processes driving this variability in July near the southern Northwind Ridge, which is one of the areas with the most drastic changes in Arctic, are not well understood. There, we demonstrated that the shortwave radiation anomaly in July is the direct reason for the sea ice anomaly in July 2019 and July 2020. Importantly, the total energy surplus in the spring of 2019 (enough to melt ∼18 cm of sea ice) and 2020 (potentially melting ∼11 cm of sea ice) indirectly influenced the sea ice. The abnormal change in moisture and its convergence mainly caused by atmospheric circulation were the main reasons for the longwave radiation and latent flux anomalies. Cloud water mainly affected shortwave radiation, including the positive net shortwave radiation anomaly in May 2019.

Connection between the North Atlantic sea surface temperature and the late autumn snow cover anomalies over the central Tibetan Plateau

Many studies have investigated the effects of the snow anomalies over the Tibetan Plateau (TP), yet the original causes of these anomalies are not fully understood. In this study, we explore the possible connection between the late autumn (November) central TP snow cover anomalies and the monthly sea surface temperature (SST) over the North Atlantic (NA) in autumn and find significant correlations over the mid-latitude NA during October and November. By analyzing the air-sea interaction over the NA, we show that a well-organized horseshoe-like SST anomaly pattern coupled with a surface anomalous anticyclone centered over the mid-latitude NA in October and its persistence in November may play a key role in the November central TP snow cover anomalies. Such an October surface anomalous anticyclone could effectively result in the surface turbulent heat flux anomalies via the surface wind-induced horizontal temperature advection, which is conducive to the formation of the horseshoe-like SST anomalies over the mid-latitude NA. The resultant anomalous SST pattern could further contribute to a North Atlantic Oscillation (NAO)-like pattern in November and stimulate the wave trains eastward to the TP. The anomalous anticyclone (cyclone) associated with the wave trains to the southwest (north) of the TP suppresses the precipitation over the central TP and thus reduces the local snow cover. The atmospheric circulation responses to the persistent NA-SST anomalies are well validated by the numerical experiments. The above result might help to predict the condition of the central TP snow cover anomalies in November, one month ahead.

Changes in the Relationship between ENSO and the Winter Arctic Stratospheric Polar Vortex in Recent Decades

Abstract The impact of El Niño–Southern Oscillation (ENSO) on the winter stratospheric polar vortex intensity (PVI) is re-examined, using JRA-55 datasets and the CESM2 model. Our results show that the negative correlation between ENSO and PVI has weakened in recent decades and is no longer statistically significant after the mid-1990s. This weakening ENSO–PVI relationship after the mid-1990s is associated with wave-1 fluxes entering the stratosphere; that is, the ENSO-induced increasing wave-1 fluxes are weaker after the mid-1990s than before the mid-1990s. Except for the Pacific–North America (PNA)-like wave train, the stratospheric wave flux changes are related to ENSO-induced geopotential height changes over East Asia. The ENSO-induced tropical zonal circulation anomalies are enhanced from the central Pacific to East Asia, leading to more substantial wave flux anomalies propagating westward from the central Pacific to East Asia and a more significant high center over Northeast Asia after the mid-1990s than before the mid-1990s. This ENSO-induced high center after the mid-1990s is out of phase with the climatological center of wave 1, leading to a relatively weaker increase in wave 1. Model results show that changes in the ENSO–PVI relationship and ENSO–Asia circulation are associated with changes in ENSO-related SST combined with global mean SST. Significance Statement In this study, changes in the ENSO–polar vortex relationship and the relevant mechanisms are analyzed from the perspective of the ENSO-induced East Asia circulation. The results can help us better understand and predict variations in the stratospheric polar vortex.

Contrasting projections of the ENSO-driven CO2flux variability in the equatorial Pacific under high-warming scenario

Abstract. The El Niño–Southern Oscillation (ENSO) widely modulates the global carbon cycle. More specifically, it alters the net uptake of carbon in the tropical ocean. Indeed, over the tropical Pacific less carbon is released by oceans during El Niño, while the opposite is the case for La Niña. Here, the skill of Earth system models (ESMs) from the latest Coupled Model Intercomparison Project (CMIP6) to simulate the observed tropical Pacific CO2 flux variability in response to ENSO is assessed. The temporal amplitude and spatial extent of CO2 flux anomalies vary considerably among models, while the surface temperature signals of El Niño and La Niña phases are generally well represented. Under historical conditions followed by the high-warming Shared Socio-economic Pathway (SSP5-8.5) scenarios, about half the ESMs simulate a reversal in ENSO–CO2 flux relationship. This gradual shift, which occurs as early as the first half of the 21st century, is associated with a high CO2-induced increase in the Revelle factor that leads to stronger sensitivity of partial pressure of CO2 (pCO2) to changes in surface temperature between ENSO phases. At the same time, uptake of anthropogenic CO2 substantially increases upper-ocean dissolved inorganic carbon (DIC) concentrations (reducing its vertical gradient in the thermocline) and weakens the ENSO-modulated surface DIC variability. The response of the ENSO–CO2 flux relationship to future climate change is sensitive to the contemporary mean state of the carbonate ion concentration in the tropics. We present an emergent constraint between the simulated contemporary carbonate concentration with the projected cumulated CO2 fluxes. Models that simulate shifts in the ENSO–CO2 flux relationship simulate positive bias in surface carbonate concentrations.

Interannual variability in the Australian carbon cycle over 2015–2019, based on assimilation of Orbiting Carbon Observatory-2 (OCO-2) satellite data

Abstract. In this study, we employ a regional inverse modelling approach to estimate monthly carbon fluxes over the Australian continent for 2015–2019 using the assimilation of the total column-averaged mole fractions of carbon dioxide from the Orbiting Carbon Observatory-2 (OCO-2, version 9) satellite. Subsequently, we study the carbon cycle variations and relate their fluctuations to anomalies in vegetation productivity and climate drivers. Our 5-year regional carbon flux inversion suggests that Australia was a carbon sink averaging −0.46 ± 0.08 PgC yr−1 (excluding fossil fuel emissions), largely influenced by a strong carbon uptake (−1.04 PgC yr−1) recorded in 2016. Australia's semi-arid ecosystems, such as sparsely vegetated regions (in central Australia) and savanna (in northern Australia), were the main contributors to the carbon uptake in 2016. These regions showed relatively high vegetation productivity, high rainfall, and low temperature in 2016. In contrast to the large carbon sink found in 2016, the large carbon outgassing recorded in 2019 coincides with an unprecedented rainfall deficit and higher-than-average temperatures across Australia. Comparison of the posterior column-averaged CO2 concentration with Total Carbon Column Observing Network (TCCON) stations and in situ measurements offers limited insight into the fluxes assimilated with OCO-2. However, the lack of these monitoring stations across Australia, mainly over ecosystems such as savanna and areas with sparse vegetation, impedes us from providing strong conclusions. To a certain extent, we found that the flux anomalies across Australia are consistent with the ensemble means of the OCO-2 Model Intercomparison Project (OCO-2 MIP) and FLUXCOM (2015–2018), which estimate an anomalous carbon sink for Australia in 2016 of −1.09 and −0.42 PgC yr−1 respectively. More accurate estimates of OCO-2 retrievals, with the addition of ocean glint data into our system, and a better understanding of the error in the atmospheric transport modelling will yield further insights into the difference in the magnitude of our carbon flux estimates.

The observed connection between the Quasi‐Biennial Oscillation and the persistence of the North Atlantic Oscillation in boreal winter

AbstractThis study reveals that the persistence of North Atlantic Oscillation (NAO) during boreal winter is closely linked to the phase of the Quasi‐Biennial Oscillation (QBO) at 30 hPa for the period 1958–2020. Our results show that the early winter (i.e., November–December) NAO signal tends to persist into the subsequent January in the easterly phase of the QBO (EQBO). However, in the westerly phase of the QBO (WQBO) there is hardly connection between the early winter and the subsequent January NAO signals. Further analysis suggests that there is stronger stratosphere–troposphere connection and stronger amplitude of the NAO during the early winters of the EQBO compared to those of the WQBO. A stronger NAO in the EQBO tends to induce enhanced positive feedback of air–sea interaction, which may contribute to a prolonged signal of the NAO. Specifically, significant surface heat flux anomalies over the high‐latitude and subtropical regions of North Atlantic induced by a stronger early winter NAO may trigger a tripolar pattern of anomalous sea surface temperature in the North Atlantic in the subsequent January. And this anomalous SST pattern may have a feedback on the atmosphere to strength the anomalous NAO by changing the atmospheric baroclinicity and synoptic eddy activities. In contrast, the NAO in the early winters of the WQBO tends to have a weaker amplitude and be confined to the troposphere. This weaker NAO is shown to induce a weaker air–sea interaction in the North Atlantic, and tends not to favour a prolonged NAO.