Large hemispheric differences in the Hadley cell strength variability due to ocean coupling

By modulating the distribution of heat, precipitation and moisture, the Hadley cell holds large climate impacts at low and subtropical latitudes. Here we show that the interannual variability of the annual mean Hadley cell strength is ~ 30% less in the Northern Hemisphere than in the Southern Hemisphere. Using a hierarchy of ocean coupling experiments, we find that the smaller variability in the Northern Hemisphere stems from dynamic ocean coupling, which has opposite effects on the variability of the Hadley cell in the Southern and Northern Hemispheres; it acts to increase the variability in the Southern Hemisphere, which is inversely linked to equatorial upwelling, and reduce the variability in the Northern Hemisphere, which shows a direct relation with the subtropical wind-driven overturning circulation. The important role of ocean coupling in modulating the tropical circulation suggests that further investigation should be carried out to better understand the climate impacts of ocean-atmosphere coupling at low latitudes.

Seeking natural analogs to fast-forward the assessment of marine CO2 removal

Mitigating global climate change will require gigaton-scale carbon dioxide removal (CDR) as a supplement to rapid emissions reduction. The oceans cover 71% of the Earth surface and have the potential to provide much of the required CDR. However, none of the proposed marine CDR (mCDR) methods is sufficiently well understood to determine their real-world efficiency and environmental side effects. Here, we argue that using natural mCDR analogs should become the third interconnecting pillar in the mCDR assessment as they bridge the gap between numerical simulations (i.e., large scale/reduced complexity) and experimental studies (i.e., small scale/high complexity). Natural mCDR analogs occur at no cost, can provide a wealth of data to inform mCDR, and do not require legal permission or social license for their study. We propose four simple criteria to identify particularly useful analogs: 1) large scale, 2) abruptness of perturbation, 3) availability of unperturbed control sites, and 4) reoccurrence. Based on these criteria, we highlight four examples: 1) equatorial upwelling as a natural analog for artificial upwelling, 2) downstream of Kerguelen Island for ocean iron fertilization, 3) the Black and Caspian Seas for ocean alkalinity enhancement, and 4) the Great Atlantic Sargassum Belt for ocean afforestation. These natural analogs provide a reality check for experimental assessments and numerical modeling of mCDR. Ultimately, projections of mCDR efficacy and sustainability supported by observations from natural analogs will provide the real-world context for the public debate and will facilitate political decisions on mCDR implementation. We anticipate that a rigorous investigation of natural analogs will fast-forward the urgently needed assessment of mCDR.

Observations on Seychelles-Chagos Thermocline Ridge (SCTR) upwelling during April-May 2019 in the western tropical Indian Ocean

<p>The Seychelles-Chagos Thermocline Ridge (SCTR) in the western tropical Indian Ocean is known as a region of off-equatorial upwelling contrasting to equatorial upwelling in the Pacific and Atlantic where the most wide open-ocean upwelling occurs corresponding to ascending branch of one of the meridional overturning cells in the Indian Ocean, yet detailed stratification, upwelling intensity, and dynamics of SCTR upwelling variability are still poorly understood. Here, we present observational results on the SCTR upwelling based on ship-based data collected during April-May 2019 as a part of the Korea-US inDian Ocean Scientific Research Program (KUDOS). The upwelling structure is confirmed from 20 ℃ and 10 ℃ isotherms (D20 and D10) shoaling up in the center of SCTR, from 200 m to 100 m (D20) and from 600 m to 400 m (D10), respectively. Horizonal divergence at the upper 250 m within an 1° by 1° area in the SCTR center (8 °S, 61 °E) estimated from currents measurements along the boundaries (1.0 x 10<sup>-3</sup> Sv) supports a mean upwelling intensity of 7.0 x 10<sup>-3</sup> m day<sup>-1</sup> (1.0 x 10<sup>-3</sup> Sv divided by the area). The upwelling intensity generally decreases with depth but shows multiple peaks within the upper water column, yielding the maximum peak (5.0 x 10<sup>-2</sup> m day<sup>-1</sup>) at 60 m and the minimum peak (1.4 x 10<sup>-2</sup> m day<sup>-1</sup>) at 230 m, with negative peaks (downwelling) at depths around 100 and 210 m. Our results on the observed structure and intensity of SCTR upwelling are discussed in comparison to time-varying local wind stress curl-driven Ekman pumping, D20-based Seychelles Upwelling Index (SUI), and Indian Dipole Mode Index (DMI). Detailed observations on the structure and intensity of SCTR upwelling presented here have important implications on time-varying SCTR upwelling (e.g., weakened upwelling peaked in fall 2019) and climate via meridional overturning circulation in the upper Indian Ocean.</p>

Reconstruction of the surface marine carbonate system at the Western Tropical Atlantic

<p>The Western Tropical Atlantic is a crucial region when it comes to understanding the CO<sub>2</sub> dynamics in the tropics, as it is subject to large inputs of freshwater from the Amazon River and the ITCZ rainfall, as well as the input of CO<sub>2</sub>-rich waters from upwelling of subsurface water. This study aims to reconstruct the surface marine carbonate system from 1998 to 2018 using sea surface temperature (SST) and sea surface salinity (SSS) data from the PIRATA buoy at 8°N 38°W and describe its variability in time. Two empirical models were used to calculate total alkalinity (TA) and dissolved inorganic carbon (DIC) from SSS. From these two parameters and SST data, it was possible to calculate pH and CO<sub>2</sub> fugacity (<em>f</em>CO<sub>2</sub>) values. Only DIC, pH and <em>f</em>CO<sub>2</sub> showed a statistically significant trend in time, where DIC showed an increase of 0.717 µmol kg<sup>-1</sup> year<sup>-1</sup>, pH decreased 0.001394 pH units year<sup>-1</sup>, and <em>f</em>CO<sub>2</sub> had an increase of 1.539 µatm year<sup>-1</sup>. Two different seasons were observed when data were analyzed: a dry season from January to June, when SSTs were lower (around 27°C) and SSS was stable around 36, matching the period when the ITCZ is over the South American continent, Amazon river plume is restricted to western shelf areas and Equatorial upwelling is more active, and a rainy season from July to December, when SSTs were higher (around 28.5°C) and SSS had higher variability (from 31 to 36), matching the period when the ITCZ is at its northern range, the Amazon plume is spread eastwards through the North Brazil Current’s retroflection and the Equatorial upwelling is less intense. Along with that, TA, DIC and pH varied positively with SSS, with higher values (TA around 2350 µmol kg<sup>-1</sup>, DIC around 2025 µmol kg<sup>-1</sup> and pH around 8.060 pH units) during dry season and lower values (TA around 2300 µmol kg<sup>-1</sup>, DIC around 1990 µmol kg<sup>-1</sup> and pH around 8.050 pH units) during rainy season. On the other hand, <em>f</em>CO<sub>2</sub> varied positively with SST, with lower values (around 385 µatm) during dry, upwelling season and higher values (around 390 µatm) during rainy season, showing that both SSS and SST variability play an important role in the CO<sub>2</sub> solubility in the region.</p>

Dynamical and Thermodynamic Changes in the Historical Response of Atlantic Sector Rainfall to Anthropogenic Emissions in the IPSL-5A Model.

<p>CMIP5 models, including IPSL-5A, developed at the Institut Pierre Simon Laplace, largely reproduce the observed post-World War II decline in Sahel precipitation. We use all- and single-forcing historical simulations performed with IPSL-5A to better understand the impact of emissions of aerosols and greenhouse gases in Sahel drought. Specifically, we analyze the moisture budget to assess the two main processes, namely stabilization and moisture supply, that are hypothesized to shape moisture convergence and precipitation in the Atlantic sector.</p><p>1) The net change has the sign of the expected thermodynamic change: an increase in precipitation in GHG-induced warming, and a decrease in aerosol-induced cooling. Thermodynamic change is opposed by dynamical change.</p><p>The rainfall change in GHG-induced warming, in the Sahel as well as across all other regions of climatological precipitation, including the Atlantic Intertropical Convergence Zone (ITCZ), is positive and largely dominated by the change in the thermodynamic term associated with convergence, meaning that the change is consistent with an increase in moisture that assumes no change in the atmospheric circulation: as the ocean warms, it supplies more moisture to the monsoon.</p><p>This wetting thermodynamic term associated with convergence is opposed by drying associated with the corresponding dynamical term, which is especially strong at the margins, and signifies a weakened mass flux, or slow-down of the overturning circulation. This negative change in mass convergence is symptomatic of stabilization in a warmer world.</p><p>The effect of sulfate aerosol-induced cooling is equal and opposite to that of GHG-induced warming.</p><p>2) The ITCZ response is complicated by the dynamical ocean feedback associated with changes in the meridional gradient in sea surface temperature. GHG-induced warming leads not only to an increase in precipitation, but also to a poleward shift of the ITCZ. This poleward shift is accompanied by (south) westerly wind anomalies, which drive an off-equatorial cooling Ekman flow equatorward of the ITCZ. These same westerly anomalies induce a weakening of equatorial upwelling, and warming of the eastern equatorial Atlantic cold tongue.</p><p>Here, too, the effect of sulfate aerosol-induced cooling is equal and opposite to that of GHG-induced warming. An equatorward shift of the ITCZ is accompanied by (north) easterly wind anomalies, which drive off-equatorial warming, and equatorial cooling.</p>

Decoupled changes in upwelling and acidity in the eastern equatorial Pacific during the Pliocene

<p>The Pliocene epoch (5.3-2.6 million years ago) is the last time Earth experienced atmospheric carbon dioxide levels comparable to present day anthropogenic levels. As such, this time interval is a potential analogue for future, warmer Earth system states. One enigmatic feature of Pliocene climate is a reduced east-west sea surface temperature gradient in the equatorial Pacific (indicative of reduced equatorial upwelling) coinciding with enhanced biological productivity in the eastern equatorial Pacific (indicative of enhanced equatorial upwelling).  Here we use boron isotopes to investigate these dynamics and to reconstruct the zonal surface pH gradient across the Pliocene equatorial Pacific. We find a strengthened pH gradient relative to modern (with more acidic conditions in the east than the west) despite a reduced temperature gradient at this time. These findings are in contrast to modern-day dynamics in which temperature and acidity co-vary, such that the reduction of the zonal temperature gradient during an El Niño event is accompanied by reduced acidity (as well as reduced upwelling and productivity) in the eastern equatorial Pacific. We show that this decoupling between changes in the pH and temperature gradients is consistent with biogeochemically enabled model simulations of Pliocene climate containing an active Pacific meridional overturning circulation and a weakly stratified equatorial thermocline. This reorganization of Pacific circulation and the onset of north Pacific deep water formation allows old, acidic, more nutrient-rich waters to reach the eastern equatorial Pacific despite weak wind-driven upwelling rates, accounting for the low pH values we observe there as well as previous evidence of enhanced productivity.</p>

­­Mean Atlantic Subtropical Cells in the CMIP6 models

<p>The Atlantic Subtropical Cells (STCs) consist primarily of poleward Ekman divergence in the surface layer, subduction in the subtropics, and equatorward convergence in the thermocline that largely compensates the surface Ekman divergence through equatorial upwelling. As a result, the STCs play an important role in connecting the tropical and subtropical Atlantic Ocean, in terms of heat, freshwater, oxygen, and nutrients transports. However, their representation in state-of-the-art coupled models has not been systematically evaluated so far. In this study, we investigate the performance of the Coupled Model Intercomparison Project phase 6 (CMIP6) models in simulating the Atlantic STCs. Comparing model results with observations, we first present the simulated mean state with respect to ensembles of the key components participating in the STC loop, i.e., the meridional Ekman and geostrophic flow at 10°N and 10°S, and the Equatorial Undercurrent (EUC) at 23°W. We then examine the inter-model spread and the relationships between these key components. We find that there is a general weak bias in the Southern Hemispheric ensemble Ekman transports and mixed-layer geostrophic transports in comparison to the observations. The inter-model spread of mean EUC strengths are primarily associated with the intensity of the mean wind stress in the tropical South Atlantic among the models. Since the poleward Ekman transports induced by the trade winds are regarded as the driver of the STC loop, our results point out the necessity to improve skills of coupled models to simulate the Southern Hemisphere atmospheric forcing in driving the Atlantic STCs.</p>

How is the increase in the variability of ENSO events over the last 6000 years linked to the mean state change of the tropical Pacific Ocean ?

<div> <div> <div> <p>El Niño events are the dominant mode of tropical interannual climate variability. This phenomenon, coupled with changes in atmospheric pressure related to the Southern Oscillation, modifies the distribution of surface water temperatures and weather conditions via atmospheric teleconnections. To better understand the linkages between changes in ENSO characteristics and changes in the Pacific ocean mean state, we use two transient simulations of the last 6000 years performed with the IPSL model that differ in resolution and presence (or not) of dynamical vegetation. The objective is to test several hypothesis raised in the literature on the role of the thermocline and the different factors constraining its changes with time.</p> <p>This study will put an emphasis on the role of ocean dynamics. Several modelling studies indicate that an insolation-forced reduced equatorial upwelling feedback during the Mid-Holocene may be responsible for the less frequent ENSO events compared to modern. A few hypotheses have been made to explain this reduction in ENSO variability and equatorial upwelling feedback in the Mid-Holocene compared with today : subduction of warmer-than-normal South Pacific mode waters into the equatorial subsurface and tilt of the thermocline in the Warm Pool. Using specific diagnoses, we discuss the relative strength of different processes and highlight the differences between the processes explaining the long-term trend in variability and those characterising multidecadal to centennial variability.</p> </div> </div> </div>

Quantifying the Role of Ocean Dynamics in Ocean Mixed-Layer Temperature Variability

Understanding the role of the ocean in climate variability requires first understanding the role of ocean dynamics in ocean mixed layer and thus sea surface temperature variability. However, key aspects of the spatially and temporally varying contributions of ocean dynamics to such variability remain unclear. Here, the authors quantify the contributions of ocean-dynamical processes to mixed layer temperature variability on monthly to multiannual timescales across the globe. To do so, they use two complementary but distinct methods: 1) a method in which ocean heat transport is estimated directly from a state-of-the-art ocean state estimate spanning 1992-2015; and 2) a method in which it is estimated indirectly from observations between 1980-2017 and the energy budget of the mixed layer. The results extend previous studies by providing quantitative estimates of the role of ocean dynamics in mixed layer temperature variability throughout the globe, across a range of timescales, in a range of available measurements, and using two different methods. Consistent with previous studies, both methods indicate that the ocean-dynamical contribution to mixed layer temperature variance is largest over western boundary currents, their eastward extensions, and regions of equatorial upwelling. In contrast to previous studies, the results suggest that ocean dynamics reduce the variance of Northern Hemisphere mixed layer temperatures on timescales longer than a few years. Hence, in the global-mean, the fractional contribution of ocean dynamics to mixed layer temperature variability decreases at increasingly low-frequencies. Differences in the magnitude of the ocean-dynamical contribution based on the two methods highlight the critical need for improved and continuous observations of the ocean mixed layer.

Mechanisms of Future Changes in Equatorial Upwelling: CMIP5 Intermodel Analysis

The future change in equatorial upwelling between 1971–2000 and 2071–2100 is investigated using data from 24 coupled climate models. The multimodel ensemble (MME) mean exhibits substantial equatorial upwelling decrease in the eastern Pacific and weaker decrease in the western Atlantic Ocean. The MME mean of upwelling change and intermodel variation of that are decomposed into distinct isopycnal and diapycnal components. In the Pacific, the diapycnal upwelling decreases near the surface, associated with a weakened Ekman pumping. The isopycnal upwelling decreases at depths of 75–200 m around the core of the Equatorial Undercurrent (EUC) due to flattening of the density layer in which it flows. Both the weakened Ekman pumping and the EUC flattening are induced by the locally weakened trade wind over the eastern Pacific basin. In the equatorial Atlantic, both the change in MME mean and the intermodel variation of upwellings are significantly related to the weakened trade wind and enhanced stratification, although these drivers are not independent. The results for the Pacific Ocean imply that future reduction in upwelling may have impacts at different depths by different mechanisms. In particular, the rapid warming of sea surface temperature in the eastern Pacific basin may be mainly caused by the near-surface diapycnal upwelling reduction rather than isopycnal upwelling reduction associated EUC flattening, which is important at deeper levels.