Tree-ring oxygen isotopes record a decrease in Amazon dry season rainfall over the past 40 years

Extant climate observations suggest the dry season over large parts of the Amazon Basin has become longer and drier over recent decades. However, such possible intensification of the Amazon dry season and its underlying causes are still a matter of debate. Here we used oxygen isotope ratios in tree rings (δ18OTR) from six floodplain trees from the western Amazon to assess changes in past climate. Our analysis shows that δ18OTR of these trees is negatively related to inter-annual variability of precipitation during the dry season over large parts of the Amazon Basin, consistent with a Rayleigh rainout model. Furthermore δ18OTR increases by approximately 2‰ over the last four decades (~ 1970–2014) providing evidence of an Amazon drying trend independent from satellite and in situ rainfall observations. Using a Rayleigh rainout framework, we estimate basin-wide dry season rainfall to have decreased by up to 30%. The δ18OTR record further suggests such drying trend may not be unprecedented over the past 80 years. Analysis of δ18OTR with sea surface temperatures indicates a strong role of a warming Tropical North Atlantic Ocean in driving this long-term increase in δ18OTR and decrease in dry season rainfall.

The Synergistic Effect of The Preceding Winter Northern Hemisphere Annular Mode and Spring Tropical North Atlantic SST On Spring Extreme Cold Events in The Mid-high Latitudes of East Asia

In this paper, the synergistic effect of the preceding winter positive Northern Hemisphere annular mode (pNAM) and spring negative tropical North Atlantic (nTNA) sea surface temperature anomaly (SSTA) on spring extreme cold events in the mid-high latitudes of East Asia (MHEA) is investigated. The results show that the co-occurrence of the two factors is unfavorable for extreme cold events during spring in the MHEA via the snow cover and atmospheric bridges. Over the Atlantic, the spring nTNA SSTA can lead to an atmospheric response that is similar to the North Atlantic Oscillation (NAO), which enhances the persistence of the pNAM and in turn amplifies the negative spring Eurasian snow cover extent (EASCE) anomaly caused by the preceding winter pNAM. Meanwhile, the spring EASCE is closely related to the spring MHEA anomalous anticyclone. In addition to storing its signal in the spring EASCE, the spring nTNA SSTA can also lead to the spring MHEA anomalous anticyclone via the eastward Rossby wave train. The evidence shows that the Rossby wave energy can propagate eastward to the MHEA as a result of the enhanced negative spring EASCE anomaly and Rossby wave induced by the spring nTNA SSTA, and the two factors have an obvious synergistic effect on the spring MHEA anomalous anticyclone. This anomalous MHEA anticyclone becomes a barrier that can hinder the intrusion of cold air from the polar region and can increase the thickness of the atmospheric layer. The anomalous sinking motion of the spring MHEA anomalous anticyclone can also lead to an increase in net radiation received at the surface and increase the air temperature through the vertical motion of air. The southerly wind over the west side of the spring MHEA anomalous anticyclone leads to horizontal warm advection. All of the above processes favor an increase in air temperature and dampen extreme cold events, implying the synergistic effect of the preceding winter pNAM and spring nTNA SSTA on spring extreme cold events in the MHEA.

JOANNE: Joint dropsonde Observations of the Atmosphere in tropical North atlaNtic meso-scale Environments

As part of the EUREC4A field campaign which took place over the tropical North Atlantic during January–February 2020, 1215 dropsondes from the HALO and WP-3D aircraft were deployed through 26 flights to characterize the thermodynamic and dynamic environment of clouds in the trade-wind regions. We present JOANNE (Joint dropsonde Observations of the Atmosphere in tropical North atlaNtic meso-scale Environments), the dataset that contains these dropsonde measurements and the products derived from them. Along with the raw measurement profiles and basic post-processing of pressure, temperature, relative humidity and horizontal winds, the dataset also includes a homogenized and gridded dataset with 10 m vertical spacing. The gridded data are used as a basis for deriving diagnostics of the area-averaged mesoscale circulation properties such as divergence, vorticity, vertical velocity and gradient terms, making use of sondes dropped at regular intervals along a circular flight path. A total of 85 such circles, ∼ 222 km in diameter, were flown during EUREC4A. We describe the sampling strategy for dropsonde measurements during EUREC4A, the quality control for the data, the methods of estimation of additional products from the measurements and the different post-processed levels of the dataset. The dataset is publicly available (https://doi.org/10.25326/246, George et al., 2021b) as is the software used to create it (https://doi.org/10.5281/zenodo.4746312, George, 2021).

Glider Observations of Thermohaline Staircases in the Tropical North Atlantic Using an Automated Classifier

Thermohaline staircases are stepped structures of alternating thick mixed layers and thin high gradient interfaces. These structures can be up to several tens of metres thick and are associated with double-diffusive mixing. Thermohaline staircases occur across broad swathes of the Arctic and tropical/subtropical oceans and can increase rates of diapycnal mixing by up to five times the background rate, driving substantial nutrient fluxes to the upper ocean. In this study, we present an improved classification algorithm to detect thermohaline staircases in ocean glider profiles. We use a dataset of 1162 glider profiles from the tropical North Atlantic collected in early 2020 at the edge of a known thermohaline staircase region. The algorithm identifies thermohaline staircases in 97.7 % of profiles that extend deeper than 300 m. We validate our algorithm against previous results obtained from algorithmic classification of Argo float profiles. Using fine resolution temperature data from a fast-response thermistor on one of the gliders, we explore the effect of varying vertical bin sizes on detected thermohaline staircases. Our algorithm builds on previous work with improved flexibility and the ability to classify staircases from profiles with poor salinity data. Using our results, we propose that the incidence of thermohaline staircases is limited by strong background vertical gradients in conservative temperature and absolute salinity.

Tropical North Atlantic response to ENSO: sensitivity to model spatial resolution

In this study the ENSO teleconnection with the Tropical North Atlantic (TNA) sea surface temperatures (SSTs) in boreal spring is analyzed in ocean-atmosphere coupled global circulation models. To assess the role played by horizontal resolution of models on this teleconnection, we used a multi-model dataset which is the first to combine models with both low and high resolution. The TNA response to ENSO projects onto the most significant SST mode of the tropical Atlantic at interannual timescales, the Atlantic meridional mode (AMM). Its evolution is primary driven by the wind-evaporation-SST (WES) feedback, which in turn is based on the development of an initial SST gradient. This study examines and quantifies the relative contribution of a dynamic-related (upwelling) and a thermodynamic-related (evaporation) process in triggering this gradient in the case of the ENSO-TNA teleconnection. While no major contribution is found with the evaporation, a consistent contribution from the coastal upwelling off north-west Africa is identified. This contribution is enhanced in high resolution models and highlights the close link between the upwelling in winter and the development of the AMM in spring. It is further shown that high resolution models present a thinner and more realistic ocean mixed layer within the upwelling area, which enhances the effect of surface winds on upwelling and SSTs. As a consequence high resolution models are more sensitive than low resolution models to surface wind errors, thereby do not ensuring an improved reliability nor predictability of the TNA SST response to ENSO.

Small pigmented eukaryote assemblages of the western tropical North Atlantic around the Amazon River plume during spring discharge

Small pigmented eukaryotes (⩽ 5 µm) are an important, but overlooked component of global marine phytoplankton. The Amazon River plume delivers nutrients into the oligotrophic western tropical North Atlantic, shades the deeper waters, and drives the structure of microphytoplankton (> 20 µm) communities. For small pigmented eukaryotes, however, diversity and distribution in the region remain unknown, despite their significant contribution to open ocean primary production and other biogeochemical processes. To investigate how habitats created by the Amazon river plume shape small pigmented eukaryote communities, we used high-throughput sequencing of the 18S ribosomal RNA genes from up to five distinct small pigmented eukaryote cell populations, identified and sorted by flow cytometry. Small pigmented eukaryotes dominated small phytoplankton biomass across all habitat types, but the population abundances varied among stations resulting in a random distribution. Small pigmented eukaryote communities were consistently dominated by Chloropicophyceae (0.8–2 µm) and Bacillariophyceae (0.8–3.5 µm), accompanied by MOCH-5 at the surface or by Dinophyceae at the chlorophyll maximum. Taxonomic composition only displayed differences in the old plume core and at one of the plume margin stations. Such results reflect the dynamic interactions of the plume and offshore oceanic waters and suggest that the resident small pigmented eukaryote diversity was not strongly affected by habitat types at this time of the year.