Intraseasonal Variability Research Articles

Tropical Intraseasonal Variability Response to Zonally Asymmetric Forcing in an Idealized Moist GCM

Abstract The tropical intraseasonal variability in an idealized moist general circulation model (GCM) coupled to a slab ocean is investigated. The model has a simple moist convection scheme and realistic radiative transfer, but no parameterization of cloud processes. In a zonally symmetric aquaplanet state, variability is dominated by westward-propagating Rossby waves. Enforcing zonal asymmetry through the application of a prescribed ocean heat flux in the bottom boundary leads to the development of a slow, eastward propagating mode that bears some of the characteristics of the observed Madden–Julian oscillation (MJO). When the ocean heat flux is made stronger, high-frequency Kelvin waves exist alongside the MJO mode. The strength of the disturbances and the spatial distribution of their precipitation anomalies are sensitive to the strength of intraseasonal sea surface temperature (SST) anomalies. The greatest resemblance to the MJO is observed when shallow slab ocean depths (1 m) are used, but the mode still exists at deeper slabs. Sensitivity experiments to the parameters of the convection scheme suggest that the simulated MJO mode couples to convection in a way that is distinct from both Kelvin and Rossby waves generated by the model. Analysis of the column moist static energy (CMSE) budget of the MJO mode suggests that column radiative heating plays only a weak role in destabilizing the mode relative to the stabilizing contribution of vertical advection. The CMSE budget analysis highlights the importance of the life cycle of horizontal advection for the destabilization and propagation of the MJO. Synergies between the generated MJO mode and linear theories of the MJO are discussed as well.

Climatology and variability of air mass transport from the boundary layer to the Asian monsoon anticyclone

Abstract. Air masses within the Asian monsoon anticyclone (AMA) show anomalous signatures in various trace gases. In this study, we investigate how air masses are transported from the planetary boundary layer (PBL) to the AMA based on multiannual trajectory analyses. In particular, we focus on the climatological perspective and on the intraseasonal and interannual variability. Further, we also discuss the relation of the interannual east–west displacements of the AMA with the transport from the PBL to the AMA. To this end we employ backward trajectories, which were computed for 14 northern summer (June–August) seasons using reanalysis data. Further, we backtrack forward trajectories from a free-running chemistry–climate model (CCM) simulation, which includes parametrized Lagrangian convection. The analysis of 30 monsoon seasons of this additional model data set helps us to carve out robust or sensitive features of transport from the PBL to the AMA with respect to the employed model. Results from both the trajectory model and the Lagrangian CCM emphasize the robustness of the three-dimensional transport pathways from the top of the PBL to the AMA. Air masses are transported upwards on the south-eastern side of the AMA and subsequently recirculate within the full AMA domain, where they are lifted upwards on the eastern side and transported downwards on the western side of the AMA. The contributions of different PBL source regions to AMA air are robust across the two models for the Tibetan Plateau (TP; 17 % vs. 15 %) and the West Pacific (around 12 %). However, the contributions from the Indian subcontinent and Southeast Asia are considerably larger in the Lagrangian CCM data, which might indicate an important role of convective transport in PBL-to-AMA transport for these regions. The analysis of both model data sets highlights the interannual and intraseasonal variability of the PBL source regions of the AMA. Although there are differences in the transport pathways, the interannual east–west displacement of the AMA – which we find to be related to the monsoon Hadley index – is not connected to considerable differences in the overall transport characteristics. Our results from the trajectory model data reveal a strong intraseasonal signal in the transport from the PBL over the TP to the AMA: there is a weak contribution of TP air masses in early June (less than 4 % of the AMA air masses), whereas in August the contribution is considerable (roughly 24 %). The evolution of the contribution from the TP is consistent across the two modelling approaches and is related to the northward shift of the subtropical jet and the AMA during this period. This finding may help to reconcile previous results and further highlights the need of taking the subseasonal (and interannual) variability of the AMA and associated transport into account.

Linkages between Madden–Julian oscillation and drought events over Kenya

Increased frequencies and intensities of extreme weather events have negatively impacted climate-sensitive socio-economic sectors in Kenya and larger Equatorial East Africa (EEA). Madden–Julian oscillation (MJO) influence intra-seasonal weather variability over Kenya although less attention has been given to its effect on extreme weather events such as droughts and floods, which have increased in frequency and intensity. Outgoing Longwave Radiation (OLR) was used in this work as proxy data for rainfall to study the geographical distribution and circulation anomalies associated with MJOs and their impacts on extreme weather events. Extreme weather events are identified using the self-calibrating Palmer Drought Severity Index (sc-PDSI), based on sc-PDSI, 2013/2014 and 2017/2018 as the drought and flood years, respectively. The background power spectral analysis reveals that MJOs are more dominant during the March–May (MAM) season than other seasons. The variance analysis depicted that the maximum power of MJO-filtered OLR is cantered within the tropical Indian Ocean, maritime continent and the tropical Pacific Ocean. Upper tropospheric (200 hPa) wind signatures give a clear Matsuno-Gill-type circulation compared to the lower tropospheric wind flows. Thus, the signatures can be used to develop a dynamic MJO index for prediction purposes. There exists a weak direct relationship between MJO and sc-PDSI; however, the influence may result from its modulation of atmospheric circulation as illustrated by the wind and velocity potential patterns before and after the passage of the convective MJO phase.

Intraseasonal variability of the equatorial intermediate current in the eastern Indian Ocean

The intraseasonal variability (ISV; 30–105 days) in the equatorial Indian Ocean is important. However, the knowledge of the ISV of the intermediate current remains limited because of the especially sparse observations. This study focuses on the ISV of the Equatorial Intermediate Current (EIC; 200–1000 m) by taking advantage of three deep-sea moorings at (0°, 80°E), (0°, 85°E) and (0°, 93°E). The characteristics and the generation process of EIC ISV in the eastern Indian Ocean are shown in this study, with aid of reanalysis data and ocean model experiments. At the intermediate depths, the standard deviations (STDs) of the intraseasonal zonal current velocity at mooring sites are approximately 35%, 40%, and 59% of the STDs of the EIC velocity, respectively, suggesting the importance of the intraseasonal components in the EIC. The power spectral of observations shows a prominent peak at 90 days and secondary peaks at 30–60 days. The stronger 90-day variability results mainly from an equatorial basin resonance of the second baroclinic modes, with contributions from low-order modes three and four. Associated with the wave energy propagation ray, the EIC ISVs present obvious spatial distribution characteristics in both horizontal and vertical directions. They tend to intensify toward the west and have maximum amplitudes at depths of 400–600 m between 80 and 85°E. Generation of the EIC ISV is attributable to the combined contribution of the directly forced Kelvin and Rossby waves and the reflected Rossby waves driven by intraseasonal equatorial zonal wind forcing. It takes about 40 days to establish the intraseasonal EIC anomaly. This study provides observed evidence and dynamic analysis for the EIC ISV in the Indian Ocean, contributing to our understanding of the intermediate-deep circulation.

Intra-seasonal variation of the wintertime Polar/Eurasia pattern

Intra-seasonal variation of the wintertime Polar/Eurasia pattern

Distinct MJOs Under the Two Types of La Niña

AbstractEl Niño‐Southern Oscillation (ENSO) strongly influences the interannual variations of Madden‐Julian Oscillation (MJO)—the dominant mode of tropical intraseasonal variability. Previous studies have revealed more about the impacts of the diversified El Niño on MJO, but less about those of the La Niña diversity. Using observations and global climate model experiments, this study demonstrates that MJO undergoes distinct variations and Pacific teleconnections when La Niña varies between eastern‐Pacific (EP) and central‐Pacific (CP) types. Compared to the CP La Niña, MJO under the EP La Niña is more energetic over the southern Maritime Continent–South Pacific Convergence Zone and can propagate faster and further with a broader zonal circulation scale over the Indo‐Pacific warm pool. The stronger and faster MJO under the EP La Niña is associated with more vigorous ascending motions, higher moisture content and stronger moistening in the lower‐level atmosphere ahead of the MJO deep convection. Although the meridional moisture advection exists as the fundamental mechanism of MJO eastward propagation, the distinct MJO under the two types of La Niña primarily results from the different lower‐tropospheric moistening due to the zonal moisture advection. Both the background mean and anomalous zonal winds and moisture gradients are stronger under the EP La Niña, thereby causing a remarkable invigoration effect of zonally advective moistening. Diverse extratropical circulation responses of distinct MJOs are also discussed. These results should improve our understanding of the whole ENSO diversity paradigm that influences MJO and teleconnections.

Foreign yield shocks and domestic price variability: the case of maize in developing countries

International trade helps to smooth food price swings caused from seasonal imbalances between domestic supply and demand. Trade also increases the possibility of importing price volatility from abroad. This concern looms large in the face of increased crop yield variability associated with climate change. We assess the extent to which maize yield shocks in exporting countries exacerbate the intra-seasonal variability of maize prices in a cross section of 75 markets in Africa, Asia, and Latin American countries during 2000/01–2017/18. We find that extreme below-trend reductions in maize yields in exporting countries are associated with increased intra-annual maize price variability in the focus countries. In contrast, above-trend maize yields in exporting countries are associated with reduced variability.

Wind Speed as a Dominant Source of Periodicities in Reported Emission Rates of Volcanic SO2

AbstractVolcanoes have been found to display periodicities or cyclic trends in a wide range of phenomena. These include the eruptive activity itself, but also in the time series of geophysical and geochemical monitoring data such as volcanic degassing. Here, we test the existence of periodicities of volcanic degassing at 32 volcanoes using the time series of sulfur dioxide (SO2) emission rates from data of the Network for Observation of Volcanic and Atmospheric Change (NOVAC). We use the Lomb‐Scargle periodogram to analyze the SO2 data which allows efficient computation of a Fourier‐like power spectrum from unevenly sampled data. We were able to calculate False‐Alarm Probabilities in 28 of the 32 volcanoes, and we identified significant periodicities in the SO2 emission rates in 17 of the 28 volcanoes. However, we find that most of these periodicities are also present in the plume speeds used to determine SO2 emission rates. Periodicities at about 30–70, ∼120, and ∼180 days were identified at volcanoes located between 16°N and 16°S and are related to intraseasonality and interseasonality in global trade winds and not volcanic in origin. Periodicities between 30 and 70 days in both plume speed and SO2 emission rates are associated to the Madden‐Julian Oscillation that is responsible for intraseasonal variability in the tropical atmosphere. Our study highlights the importance of using local wind data for deriving realistic SO2 emissions and the identification of short‐term periodicity in volcanic behavior.

The Influence of the Madden–Julian Oscillation on the Wet Season Rainfall over Saudi Arabia

The influence of Madden–Julian oscillation (MJO) is examined on intraseasonal rainfall variability during the wet season (November–April) by using the real-time multivariate (RMM) MJO index, ERA5 reanalysis, and daily observed rainfall dataset from 26 stations in Saudi Arabia for the period 1985–2021. The MJO 8 phases are categorized into wet (phases 1, 2, 7 and 8) and dry (phases 3, 4, 5, and 6) based on the Saudi Arabian intraseasonal rainfall characteristics associated with MJO phases. It is observed that 41% (46%) of total (extreme) rainfall events occur during the MJO wet phases, while only 23% (18%) of such events occur during MJO dry phases. The intraseasonal variability signals are isolated from daily dataset by applying a 30- to 90-day period bandpass filter. The analyses are validated by constructing composites of daily filtered precipitation anomalies during MJO 8 phases. The physical mechanism indicates that the significant intraseasonal wetter conditions are linked with enhanced easterly and southeasterly moisture convergence over Saudi Arabia from the Arabian Sea. The atmospheric cyclonic circulation anomalies during the wet phases favor more moisture convergence and vertical moisture advection, which may lead to enhanced convection and rainfall. However, during the dry phases, anticyclonic circulation anomalies enhance moisture divergence and reduce vertical moisture advection and consequently suppress the convection and rainfall activity over Saudi Arabia. The analyses show that the intraseasonal rainfall variability over Saudi Arabia is significantly influenced by the MJO during the wet season. These findings have important implications for sub-seasonal rainfall forecasting in Saudi Arabia.

Seasonal and intra-seasonal variations of surface hydrological parameters over North Yellow Sea and its implications for applicability of quasi-synchronous ship-based multi-station hydrological investigation data

The quasi-synchronous ship-based multi-station hydrological investigation is the most basic and classic oceanographic research method. Nevertheless, the temporal variability of regional hydrological parameters raises the uncertainty of interpreting quasi-synchronous multi-station datasets. There has been a lack of in-depth assessment of the accuracy of survey results. In this study, two hydrological investigations during the summer of 2016 and winter of 2017 in the North Yellow Sea (NYS), accompanied by time series buoy observations, are presented to elucidate its influencing factors and applicability evaluation of cruise hydrological investigation data. The results indicate that the solar radiation and tidal condition account for the diurnal and semi-diurnal cycle of sea surface temperature (SST) in the summer and winter seasons, respectively. The sea surface salinity shows no significant variation during the cruise survey period, while the seasonal characteristic is dominated by the regional ocean circulation. The vertical hydrological structures (stratified in summer or well-mixed in winter) limit or enhance the upward diffusion of bottom resuspended sediment in the offshore area, furtherly causing higher surface turbidity in winter. The upwelling phenomenon is underestimated in summer cruises due to the diurnal variation of SST, while the cold-water tongue may be overinterpreted in winter cruise because of the northerly wind event. The applicability of multi-station hydrological investigation data depends on the comparison of the spatio-temporal scale of the scientific question being studied with the time-varying of the data. The coefficient of variation (CV) of sea surface parameters ranges from 0.03 to 0.36 for SST, 0.02 to 0.04 for SSS, 0.01 to 1.37 for chl a, and 0.06 to 0.50 for Rrs_555. The sea surface parameters display higher variabilities during spring and autumn season than summer and winter season over NYS, and the coastal areas show larger temporal variation than central NYS. It is suggested that the multi-station investigation should be conducted at the time when the hydrological structure is relatively stable as far as possible, especially for the nearshore stations. This study provides a meaningful reference for oceanographic research that although the background hydrologic field can be well described through the ship-based investigation data, attention should also be paid to the influence of time-varying meteorological and hydrological conditions to achieve more objective data interpretation.

Spatiotemporal Characteristics of Soil Moisture and Land–Atmosphere Coupling over the Tibetan Plateau Derived from Three Gridded Datasets

Soil moisture is a crucial component of the water cycle and plays an important role in regional weather and climate. However, owing to the lack of In Situ observations, an accurate understanding of the spatiotemporal variations of soil moisture (SM) on the Tibetan Plateau (TP) is still lacking. In this study, we used three gridded SM products to characterize the spatiotemporal features of SM on the TP during the warm season (May to August). We analyzed the fifth-generation European Centre for Medium-Range Weather Forecasts atmospheric reanalysis (ERA5), Global Land Data Assimilation System (GLDAS), and Soil Moisture Active Passive (SMAP) datasets and used station observation data and triple collocation to quantify product accuracy and consistency. Results of the evaluation based on observation data show that both ERA5 and GLDAS overestimate SM, while the accuracy of SMAP is high. In terms of capturing the temporal variations of SM measured at stations, the performance of ERA5 and that of SMAP are superior to that of GLDAS. According to the evaluation based on triple collocation, SMAP exhibits the smallest random error over the TP and the highest temporal correlation with the unknown true SM in eastern TP. For SMAP, SM variability is the largest in the southern TP. For ERA5 and GLDAS, variability in the western TP is substantially larger than that for SMAP. Low-frequency (30–90 days) variations are the largest contributor to TP SM intraseasonal variability. Relative to SMAP, the contribution of high-frequency variations is low in ERA5 and GLDAS. Land-atmosphere coupling is stronger (weaker) in the western (southeastern) TP, which is relatively dry (wet). Our evaluation of SM product performance over the TP may facilitate the use of these products for disaster monitoring and climate and hydrological studies.

Multidecadal Intensification of Atlantic Tropical Instability Waves

AbstractTropical Instability Waves (TIWs) are the dominant source of intraseasonal variability in the central equatorial Atlantic and play an important role in the redistribution of heat in the upper ocean. Here we use multidecadal records of sea surface temperature, sea level anomaly, sea surface salinity, and near‐surface currents constructed from in situ and satellite observations to reveal a long‐term intensification of the intraseasonal variability of these variables due to an increase of TIW activity. Enhanced barotropic energy conversion from increased covariance of horizontal current fluctuations, rather than low‐frequency changes of the mean zonal currents, drives the TIW intensification. As a consequence, boreal summer cooling of tropical North Atlantic surface waters through horizontal eddy temperature advection increased by 0.03°C month−1 decade−1 during 1993–2021, a change of 74% ± 53% relative to the long‐term mean. The presented multidecadal TIW trends are strongly modulated by interannual variations like the 2021 Atlantic Niño.

Observation of Bottom-Trapped Topographic Rossby Waves to the West of the Luzon Strait, South China Sea

Abstract Topographic Rossby waves (TRWs) play an important role in deep-ocean dynamics and abyssal intraseasonal variations. Observational records from 15 current- and pressure-recording inverted echo sounders (CPIESs) and two moorings deployed in the northern Manila Trench (MT), South China Sea (SCS), for over 400 days were utilized to analyze the widely existing near-21-day bottom-trapped TRWs in the trench. The TRWs were generally generated in winter and summer, dominated by perturbations in the upper ocean. Kuroshio intrusion and its related variabilities contributed to the perturbations in winter, whereas the perturbations generated north of Luzon Island dominated in summer. Eddies north of Luzon propagated northwestward in the summer of 2018; however, these eddies caused the Kuroshio meanderings in the Luzon Strait (LS) in the summer of 2019. The variations in the Kuroshio path and the Kuroshio-related eddies induced TRWs in the deep ocean in regions with steep topography. However, the spatiotemporal distributions of TRWs were complex owing to the propagation of the waves. Some cases of TRWs showed no relation to the local upper-layer perturbations but propagated from adjacent regions. Some of these TRWs were induced by perturbations in the upper ocean in adjacent regions, and propagated anticlockwise in the MT with shallow water to their right, while others may be related to the intraseasonal variations in deep-water overflow in the LS and propagated northward. This study suggests that the Kuroshio and Kuroshio-related eddies significantly contribute to the dynamic processes associated with intraseasonal variations in the deep SCS through the generation of TRWs. Significance Statement Topographic Rossby waves (TRWs) are fluctuations generated when water columns travel across sloping topography under potential vorticity conservation. Based on observations of 15 current- and pressure-recording inverted echo sounders (CPIESs) and two moorings in the northern Manila Trench (MT) in the South China Sea (SCS), TRWs with periods of approximately 21 days were observed and analyzed. This study describes the generation, propagation, and spatiotemporal distribution of TRWs west of the LS and confirms that regional Kuroshio meanderings and upper eddies play important roles in the dynamic processes associated with intraseasonal variations in the deep SCS; the study may thus contribute to knowledge on the dynamic response of the abyssal current to mesoscale perturbations in the upper ocean.

Intra-seasonal variability of the abyssal currents in COMRA’s contract area in the Clarion-Clipperton Zone

Intra-seasonal variability of the abyssal currents in COMRA’s contract area in the Clarion-Clipperton Zone

Estimating body mass of sperm whales from aerial photographs

AbstractBody mass is a fundamental feature of animal physiology. Although sperm whales (Physeter macrocephalus) are the largest toothed predators on earth, body mass is seldom included in studies of their ecophysiology and bioenergetics due to the inherent difficulties of obtaining direct measurements. We used UAV‐photogrammetry to estimate the weight of free‐ranging sperm whales. Aerial photographs (23 calves, 11 juveniles, 55 nonmother adults, 13 mothers) were collected in the Eastern Caribbean and Mediterranean Sea during 2017–2020. Body length, widths, and heights (dorso‐ventral distance at 5% increments) were measured from dorsal and lateral photographs, while body volume was calculated using an elliptical model. Volume varied noticeably (12.01 ± 4.79 m3) in larger animals (>8 m), indicating fluctuations in body condition of adults and mothers. Volume was converted to mass, using tissue‐density estimates from catch data, animal‐borne tags, and body‐tissue composition. Average total body density ranged from 834 to 1,003 kg/m3, while the weight predictions matched with existing measurements and weight‐length relationships. Our body‐mass models can be used to study sperm whale bioenergetics, including inter‐ and intraseasonal variations in body condition, somatic growth, metabolic rates, and cost of reproduction.

Boreal Summer Intraseasonal Meridional Oscillations Over East Asia and Their Influences on Precipitation

AbstractThe intraseasonal meridional wave trains are the major features of East Asia summer monsoon (EASM), but the complete physical pictures of dynamic pathways from tropical and mid‐high latitudes and their climate impacts have not been fully illustrated. With NCEP‐DOE Reanalysis‐II daily data set, two boreal summer intraseasonal meridional oscillations (ISMOs) are isolated and identified by extended EOF of zonal wind anomalies along the coast of East Asia on 10‐ to 30‐day timescale, that is, ISMO1 and ISMO2. They both demonstrate obvious quasi‐biweekly oscillations with an equivalent barotropic structure in troposphere, and ISMO1 represents the southward propagating wave train with large variances in upper level, which is initiated from the West Europe and primarily forced by the soil moisture anomalies over mid‐high latitudes of Eurasian continent, whereas ISMO2 propagates northward in mid‐lower troposphere from the tropical western North Pacific to East Asia that is closely related to BSISO2. The two ISMOs are capable of explaining a large fraction of the total intraseasonal variances of EASM, wherein, ISMO1 is responsible for a meridional dipole pattern of precipitation anomalies over eastern China, while ISMO2 has major influence in southern China. Generally, ISMO‐caused changes of extreme precipitation probability are in good accordance with the spatial pattern of its related 10‐ to 30‐day intraseasonal precipitation anomalies. Since ISMO1 and ISMO2 can clearly separate the tropical and extratropical‐origin meridional oscillations and further have robust influences on precipitation over East Asia, they can be used as the new monitoring and predicting factors to the intraseasonal variations of EASM and their associated extreme precipitation.

Intraseasonal Variability of Surface Circulation in the Indo‐Pacific Warm Pool Induced by Summer Monsoon Intraseasonal Oscillations

AbstractThe Indian summer monsoon intraseasonal oscillation (MISO) is the most influential mode of tropical atmospheric intraseasonal variability in boreal summer (May–October). This study investigates how the MISO drives the intraseasonal variability (ISV) of zonal currents (U) in the Indo‐Pacific warm pool (IPWP) region. An analysis of satellite observations confirms a close association between U ISV and the MISO, with MISO‐induced surface U anomalies reaching 16–19 cm s−1 near the equator and 4–5 cm s−1 in the Northern Hemisphere portion of the IPWP. These ISVs account for 15%–35% of the intensity changes in the major zonal currents. The spatial distribution of U ISV shows coherent enhancement and suppression of MISO convections with a lag of approximately 7 days as a delayed response to the tropical wind forcing. ISV anomalies show northward emanation following propagation of the MISO. Ocean model experiments confirm that large‐scale ISV in surface U is primarily caused by the wind forcing of the MISO. Owing to the convergence/divergence of MISO‐induced U ISV, significant warm water volume anomalies occur in the western‐to‐central equatorial Pacific.

Diapycnal mixing variations induced by subthermocline eddies observed in the north Pacific western boundary region

Recent observations prove that subthermocline eddies (SEs) are energetic in the north Pacific western boundary region, where interhemispheric waters meet. Our previous study showed that the SEs play an important role in isopycnal mixing of interhemispheric intermediate waters. Whether the SEs can induce diapycnal mixing in the western boundary region is unknown although it has been found true in the interior region. In this study, based on in-situ observations and fine-scale parameterization method, we show spatial structure and variability of diapycnal mixing induced by the SEs in the north Pacific western boundary region. The SEs are located between 200 and 750 m with a maximum swirl speed reaching 0.5 m s-1 and exhibit significant intra-seasonal variability with the period ranging between 50 and 100 days. Compared with shears induced by tides and near-inertial oscillation, sub-inertial shears induced by the SEs are dominant in the subthermocline layer. Consequently, diapycnal diffusivity is elevated up to O(10-4) m2 s-1 about one order higher than the background value when the SEs were passing by. The integrated diapycnal diffusivity between 200 and 750 m is increased by 210%. Modulated by the SEs, diapycnal mixing in the subthermocline has significant intra-seasonal variations. With more and more SEs being observed around the global oceans, we suggest that SE-induced mixing may not be trivial in closing the global ocean energy budget.

Estimating intra-seasonal photosynthetic discrimination and water use efficiency using δ13C of leaf sucrose in Scots pine.

Sucrose has a unique role in recording environmental and physiological signals during photosynthesis in its carbon isotope composition (δ13C) and transport of the signal to tree rings. Yet, instead of sucrose, total organic matter (TOM) or water-soluble carbohydrates (WSC) are typically analysed in studies that follow δ13C signals within trees. To study how the choice of organic material may bias the interpretation of δ13C records, we used mature field-grown Scots pine (Pinus sylvestris) to compare for the first time δ13C of different leaf carbon pools with δ13C of assimilates estimated by a chamber-Picarro system (δ13CA_Picarro), and a photosynthetic discrimination model (δ13CA_model). Compared with sucrose, the other tested carbon pools, such as TOM and WSC, poorly recorded the seasonal trends or absolute values of δ13CA_Picarro and δ13CA_model. Consequently, in comparison with the other carbon pools, sucrose δ13C was superior for reconstructing changes in intrinsic water use efficiency (iWUE), agreeing in both absolute values and intra-seasonal variations with iWUE estimated from gas exchange. Thus, deriving iWUE and environmental signals from δ13C of bulk organic matter can lead to misinterpretation. Our findings underscore the advantage of using sucrose δ13C to understand plant physiological responses in depth.

Spatial and temporal variability of phytoplankton photophysiology in the Atlantic Southern Ocean

Active chlorophyll-a fluorescence was measured during five summer research cruises (2008 – 2016), spanning the Atlantic sector of the Southern Ocean. This unique data set provides information for assessing zonal, inter-annual and intra-seasonal variability (early versus late summer) of photosynthetic efficiency (Fv/Fm). The zonal variability of Fv/Fm showed a typical latitudinal decline from a maximum in the Polar Frontal Zone (PFZ) (0.24±0.03) to a minimum in the Southern Antarctic Circumpolar Current Zone (SACCZ) (0.18±0.07). The inter-annual variability in Fv/Fm (between each cruise) was the highest in the SACCZ, while the Antarctic Zone (AZ) exhibited low inter-annual variability. Intra-seasonal variability between the zones was limited to a significantly higher mean Fv/Fm in the PFZ and AZ in early summer compared to late summer. Intra-seasonal variability between the cruises was, however, inconsistent as higher mean Fv/Fm in early summer were seen during some years as opposed to others. Ancillary physical and biogeochemical parameters were also assessed to investigate potential direct and indirect drivers of co-variability with Fv/Fm through a series of statistical t-tests, where significant differences in Fv/Fm were used as focus points to interrogate the plausibility of co-variance. Inter-zonal variability of surface seawater temperature (SST) and Silicate:Phosphate (Si:P) ratios were highlighted as co-varying with Fv/Fm in all zones, whilst community structure played an indirect role in some instances. Similarly, inter-annual variability in Fv/Fm co-varied with SST, Nitrate:Phosphate (N:P) and Si:P ratios in the PFZ, AZ and SACCZ, while community structure influenced inter-annual variability in the PFZ and SACCZ. Intra-seasonal variability in Fv/Fm was linked to all the ancillary parameters, except community structure in the AZ, whilst different ancillary parameters dominated differences during each of the cruises. These results were further scrutinized with a Principal Component Analysis for a subset of co-located data points, where N:P and Si:P ratios emerged as the principal indirect drivers of Fv/Fm variability. This study highlights the scope for using Fv/Fm to reflect the net response of phytoplankton photophysiology to environmental adjustments and accentuates the complex interplay of different physical and biogeochemical parameters that act simultaneously and oftentimes antagonistically, influencing inter-zonal, inter-annual and intra-seasonal variability of Fv/Fm.