Ocean-atmosphere Interactions Research Articles

Oceanic Rossby waves drive inter-annual predictability of net primary production in the central tropical Pacific

In the Pacific Ocean, off-equatorial Rossby waves (RWs), initiated by atmosphere-ocean interaction, modulate the inter-annual variability of the thermocline. In this study, we explore the resulting potential gain in predictability of central tropical Pacific primary production, which in this region strongly depends on the supply of macronutrients from below the thermocline. We use a decadal prediction system based on the Max Planck Institute Earth system model to demonstrate that for the time period 1998–2014 properly initialized RWs explain an increase in predictability of net primary productivity (NPP) in the off-equatorial central tropical Pacific. We show that, for up to 5 years in advance, predictability of NPP derived from the decadal prediction system is significantly larger than that derived from persistence alone, or an uninitialized historical simulation. The predicted signal can be explained by the following mechanism: off-equatorial RWs are initiated in the eastern Pacific and travel towards the central tropical Pacific on a time scale of 2–6 years. On their arrival the RWs modify the depths of both thermocline and nutricline, which is fundamental to the availability of nutrients in the euphotic layer. Local upwelling transports nutrients from below the nutricline into the euphotic zone, effectively transferring the RW signal to the near-surface ocean. While we show that skillful prediction of central off-equatorial tropical Pacific NPP is possible, we open the door for establishing predictive systems for food web and ecosystem services in that region.

Interactions between a Tropical Cyclone and Upper-Tropospheric Cold-Core Lows Simulated by an Atmosphere-Wave-Ocean Coupled Model: A Case Study of Typhoon Jongdari (2018)

Typhoon Jongdari (2018) took an unusual track along the circumference of an upper-tropospheric cold low (UTCL) before making landfall in Japan on 29 July. To investigate the effects of atmosphere–ocean interactions and interactions between the UTCL and Jongdari on the storm's track, numerical simulations were conducted with a 3-km-mesh nonhydrostatic atmosphere model and an atmosphere–wave–ocean coupled model, using different initial conditions created by adopting different start times of numerical integration. The UTCL was characterized by high potential vorticity (PV), low pressure, and low relative humidity on the 355-K isotherm surface. While the UTCL moved southwestward north of Jongdari from 25 to 27 July, simulation results indicate that Jongdari traveled counterclockwise along the circumference of the UTCL. After Jongdari moved westward, the coupled model clearly simulated sea surface cooling along the track. Jongdari weakened after making landfall while the UTCL also weakened south of Japan. In particular, latent heat flux from the sea and the resulting humidification of the upper troposphere through the convection affected the UTCL. When Jongdari redeveloped over the ocean south of Kyushu, some simulations showed that Jongdari merged with the UTCL there as a result of high PV in Jongdari and relatively low upper-tropospheric PV near the UTCL. Ocean coupling helped sustain the uppertropospheric PV near the UTCL and weakened the column of elevated PV associated with Jongdari, which affected the location of the tropopause folding transformed from the UTCL by lowering the PV column of Jongdari and weakening the upper-tropospheric outflow from the center. Because the steering flow of Jongdari was affected by the geostrophic-balanced cyclonic circulation created by the UTCL, a larger difference of the atmospheric initial conditions between the initial times had a stronger influence on track and intensity simulations of both Jongdari and UTCL than ocean coupling.

The IOD–ENSO Interaction: The Role of the Indian Ocean Current’s System

The Indian Ocean dipole (IOD) is one of the main modes characterizing the interannual variability of the large-scale ocean–atmosphere interaction in the equatorial zone of the World Ocean. A dipole manifests itself as an out-of-phase interannual fluctuation of the ocean–atmosphere characteristics in the western and eastern parts of the equatorial–tropical zone of the Indian Ocean. IOD can be a consequence of the ENSO (El Niño–Southern Oscillation) events in the Pacific Ocean, or it can be independent of them and arise due to the Indian Ocean inherent processes. Earlier, it was suggested that the generation of the long planetary waves in the Indian Ocean by the ENSO events is one of the mechanisms of the ENSO impact on the IOD. However, quite often, such a mechanism is not the case and IOD is generated itself as an independent Indian Ocean mode. We hypothesized that this generation is due to the growing oceanic disturbances, as a result of instability of the system of Indian Ocean zonal currents in the vicinity of the critical layer, in which the phase velocity of Rossby waves is equal to the average velocity of the zonal currents. In the present work, the study of the features of the formation of the critical layer in the equatorial–tropical zone of the Indian Ocean is continued using different oceanic re-analyses and standard theory of the Rossby waves. As a result of comparison of different re-analyses data with the RAMA (The Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction) measurements, the operative re-analysis ORAS5 output of European Centre for Medium-Range Weather Forecasts (ECMWF) on potential temperature, salinity, and the zonal component of the currents’ velocity for the period 1979–2018 was used. Monthly profiles of potential temperature, salinity, and the zonal component of the currents’ velocity were selected from the ORAS5 archive for the sections situated between 7.5–15.5° S and 50–100° E. From these data and for each month, using the standard theory of planetary waves, the phase velocity of the lowest baroclinic mode of the Rossby long waves was calculated and the critical layers were determined. For each critical layer, its length was calculated. The obtained time series of the length of the critical layers were compared to the variability of dipole mode index (DMI). It is shown that the majority of the cases of the IOD generation as inherent (independent on the Pacific processes) mode were accompanied by the critical layer formation in the region of interest. Usually, the critical layers occur in spring, one to two months before the onset of the positive IOD events. This indicates that the presence of instability in the system of the zonal currents can be a reason for the generation of IOD and the asymmetry of the amplitude of the dipole mode index between positive and negative events. During the extremely intense ENSO event of 1997–1998, which was accompanied by the strong IOD event, the critical layer in the equatorial–tropical zone of the Indian Ocean was absent. This ENSO event generated the oceanic planetary waves at the eastern edge of the Indian Ocean. Therefore, it is shown that the above mechanism of the ENSO–IOD interaction is a reality.

Exploring the coupled ocean and atmosphere system with a data science approach applied to observations from the Antarctic Circumnavigation Expedition

Abstract. The Southern Ocean is a critical component of Earth's climate system, but its remoteness makes it challenging to develop a holistic understanding of its processes from the small scale to the large scale. As a result, our knowledge of this vast region remains largely incomplete. The Antarctic Circumnavigation Expedition (ACE, austral summer 2016/2017) surveyed a large number of variables describing the state of the ocean and the atmosphere, the freshwater cycle, atmospheric chemistry, and ocean biogeochemistry and microbiology. This circumpolar cruise included visits to 12 remote islands, the marginal ice zone, and the Antarctic coast. Here, we use 111 of the observed variables to study the latitudinal gradients, seasonality, shorter-term variations, geographic setting of environmental processes, and interactions between them over the duration of 90 d. To reduce the dimensionality and complexity of the dataset and make the relations between variables interpretable we applied an unsupervised machine learning method, the sparse principal component analysis (sPCA), which describes environmental processes through 14 latent variables. To derive a robust statistical perspective on these processes and to estimate the uncertainty in the sPCA decomposition, we have developed a bootstrap approach. Our results provide a proof of concept that sPCA with uncertainty analysis is able to identify temporal patterns from diurnal to seasonal cycles, as well as geographical gradients and “hotspots” of interaction between environmental compartments. While confirming many well known processes, our analysis provides novel insights into the Southern Ocean water cycle (freshwater fluxes), trace gases (interplay between seasonality, sources, and sinks), and microbial communities (nutrient limitation and island mass effects at the largest scale ever reported). More specifically, we identify the important role of the oceanic circulations, frontal zones, and islands in shaping the nutrient availability that controls biological community composition and productivity; the fact that sea ice controls sea water salinity, dampens the wave field, and is associated with increased phytoplankton growth and net community productivity possibly due to iron fertilisation and reduced light limitation; and the clear regional patterns of aerosol characteristics that have emerged, stressing the role of the sea state, atmospheric chemical processing, and source processes near hotspots for the availability of cloud condensation nuclei and hence cloud formation. A set of key variables and their combinations, such as the difference between the air and sea surface temperature, atmospheric pressure, sea surface height, geostrophic currents, upper-ocean layer light intensity, surface wind speed and relative humidity played an important role in our analysis, highlighting the necessity for Earth system models to represent them adequately. In conclusion, our study highlights the use of sPCA to identify key ocean–atmosphere interactions across physical, chemical, and biological processes and their associated spatio-temporal scales. It thereby fills an important gap between simple correlation analyses and complex Earth system models. The sPCA processing code is available as open-access from the following link: https://renkulab.io/gitlab/ACE-ASAID/spca-decomposition (last access: 29 March 2021). As we show here, it can be used for an exploration of environmental data that is less prone to cognitive biases (and confirmation biases in particular) compared to traditional regression analysis that might be affected by the underlying research question.

Correction to: Role and response of ocean–atmosphere interactions during Amphan (2020) super cyclone

Correction to: Role and response of ocean–atmosphere interactions during Amphan (2020) super cyclone

Biogenic aerosol in central East Antarctic Plateau as a proxy for the ocean-atmosphere interaction in the Southern Ocean

Ten years of data of biogenic aerosol (methane sulfonic acid, MSA, and non-sea salt sulfate, nssSO42−) collected at Concordia Station in the East Antarctic plateau (75° 06′ S, 123° 20′ E) are interpreted as a function of the Southern Annular Mode (SAM), Chlorophyll-a concentration (Chl-a; a proxy for phytoplankton biomass), sea ice extent and area. It is possible to draw three different scenarios that link these parameters in early, middle, and late summer. In early summer, the biogenic aerosol is significantly correlated to sea ice retreats through the phytoplankton biomass increases. Chl-a shows a significant correlation with nssSO42− in the finest fraction (< 1 μm). In contrast, only Chl-a in West Pacific and Indian Ocean sectors correlates with MSA in the coarse fraction. The transport routes towards the inner Antarctic plateau and aerosol formation processes could explain the different correlation patterns of the two compounds both resulting from the DMS oxidation. In mid-summer, Chl-a concentrations are at the maximum and are not related to sea ice melting. Due to the complexity of transport processes of air masses towards the Antarctic plateau, the MSA concentrations are low and not related to Chl-a concentration. In late summer, MSA and nssSO42− present the highest concentrations in their submicrometric aerosol fraction, and both are significantly correlated with Chl-a but not with the sea ice. In early and mid-summer, the enhanced efficiency of transport processes from all the surrounding oceanic sectors with air masses traveling at low elevation can explain the highest concentrations of nssSO42− and especially MSA. Finally, considering the entire time series, MSA shows significant year-to-year variability. This variability is significantly correlated with SAM but with a different time lag in early (0-month lag) and late summer (4-months lag). This correlation likely occurs through the effect of the SAM on phytoplankton blooms.

Direct astronomical influence on abrupt climate variability

Changes in the magnitude of millennial-scale climate variability (MCV) during the Late Pleistocene occur as a function of changing background climate state over tens of thousands of years, an indirect consequence of slowly varying incoming solar radiation associated with changes in Earth’s orbit. However, whether astronomical forcing can stimulate MCV directly (without a change in the background state) remains to be determined. Here we use a comprehensive fully coupled climate model to demonstrate that orbitally driven insolation changes alone can give rise to spontaneous millennial-scale climate oscillations under intermediate glacial conditions. Our results demonstrate that an abrupt transition from warm interstadial to cold stadial conditions can be triggered directly by a precession-controlled increase in low-latitude boreal summer insolation and/or an obliquity-controlled decrease in high-latitude mean annual insolation, by modulating North Atlantic low-latitude hydroclimate and/or high-latitude sea ice–ocean–atmosphere interactions, respectively. Furthermore, contrasting insolation effects over the tropical versus subpolar North Atlantic, exerted by obliquity or precession, result in an oscillatory climate regime, even within an otherwise stable climate. With additional sensitivity experiments under different glacial–interglacial climate backgrounds, we synthesize a coherent theoretical framework for climate stability, elaborating the direct and indirect (dual) control by Earth’s orbital cycles on millennial-scale climate variability during the Pleistocene. Millennial-scale climate oscillations can arise from orbital forcing alone during relatively stable glacial climate states, according to an analysis of high- and low-latitude climate proxy records as well as climate modelling.

Artificial Intelligence powered forecast of oceanic mesoscale phenomena: A typhoon cold wake case occurring in Northwest Pacific Ocean

The phenomenon of typhoon cold wake has always been a key research object in the field of ocean–atmosphere interaction. Different from the physical equation-based numerical model which has long been dependent on, inspired by largely available forecasting and reanalyzed data and the advance of artificial intelligence technology, we build RC_ViT deep learning model for forecasting the typhoon cold wake. The total RMSE of grid points at each moment was between 0.5 and 0.9 °C, indicating that the model was accurate to forecast the sea surface temperature after the pass of typhoon. During the testing period of 11 years (2010–2020), using the deep learning method, our artificial intelligence model accurately and efficiently forecasts the sea surface temperature field. The cold wake of typhoon Megi (2010) was simulated by a Regional Ocean Model System (ROMS) and the artificial intelligence model separately. The results show that before the typhoon passing, the SST forecasted by the artificial intelligence model is more accurate, and after the typhoon passes, the ROMS forecast result has a smaller RMSE value. The reason may be that the intensity of typhoon Megi was too strong, and the artificial intelligence model training samples were few, resulting in artificial intelligence model forecast accuracy lower than traditional numerical models after the Megi passing. This study demonstrates the strong potential of the data-driven artificial intelligence model as an alternative to traditional numerical models for forecasting oceanic phenomena.

Indian Ocean Dipole leads to Atlantic Ni\xf1o

Atlantic Niño is the Atlantic equivalent of El Niño-Southern Oscillation (ENSO), and it has prominent impacts on regional and global climate. Existing studies suggest that the Atlantic Niño may arise from local atmosphere-ocean interaction and is sometimes triggered by the Atlantic Meridional Mode (AMM), with overall weak ENSO contribution. By analyzing observational datasets and performing numerical model experiments, here we show that the Atlantic Niño can be induced by the Indian Ocean Dipole (IOD). We find that the enhanced rainfall in the western tropical Indian Ocean during positive IOD weakens the easterly trade winds over the tropical Atlantic, causing warm anomalies in the central and eastern equatorial Atlantic basin and therefore triggering the Atlantic Niño. Our finding suggests that the cross-basin impact from the tropical Indian Ocean plays a more important role in affecting interannual climate variability than previously thought.

Interaction of tropical cyclone-like vortices with sea-surface temperature anomalies and topography in a simple shallow-water atmospheric model

In this paper, we show how the thermal effects affect trajectories, intensity, and formation of secondary structures during the passages of strong tropical cyclone-like vortices over oceanic warm and cold pools as well as over an island-type topography. Our results are obtained using the moist-convective thermal rotating shallow-water atmospheric model recently developed in [A. Kurganov et al., “Moist-convective thermal rotating shallow-water model,” Phys. Fluids 32, 066601 (2020)]. This model introduces thermodynamics of the moist air and moist convection in the standard rotating shallow-water models and allows to include in the latter atmosphere–ocean interactions in an elementary way.

Lagged teleconnections of climate variables identified via complex rotated Maximum Covariance Analysis

AbstractA proper description of ocean-atmosphere interactions is key for a correct understanding of climate evolution. The interplay among the different variables acting over the climate is complex, often leading to correlations across long spatial distances (teleconnections). In some occasions, those teleconnections occur with quite significant temporal shifts that are fundamental for the understanding of the underlying phenomena but which are poorly captured by standard methods. Applying orthogonal decomposition such as Maximum Covariance Analysis (MCA) to geophysical data sets allows to extract common dominant patterns between two different variables, but generally suffers from (i) the non-physical orthogonal constraint as well as (ii) the consideration of simple correlations, whereby temporally offset signals are not detected. Here we propose an extension, complex rotated MCA, to address both limitations. We transform our signals using the Hilbert transform and perform the orthogonal decomposition in complex space, allowing us to correctly correlate out-of-phase signals. Subsequent Varimax rotation removes the orthogonal constraints, leading to more physically meaningful modes of geophysical variability. As an example of application, we have employed this method on sea surface temperature and continental precipitation; our method successfully captures the temporal and spatial interactions between these two variables, namely for (i) the seasonal cycle, (ii) canonical ENSO, (iii) the global warming trend, (iv) the Pacific Decadal Oscillation, (v) ENSO Modoki and finally (vi) the Atlantic Meridional Mode. The complex rotated modes of MCA provide information on the regional amplitude, and under certain conditions, the regional time lag between changes on ocean temperature and land precipitation.

Holocene terrestrialization process on the Sanjiang Plain (China) and its significance to the East Asian summer monsoon circulation

A wide-spread terrestrialization process has been reported occurring across the Sanjiang Plain in the Holocene Epoch, while little is known for its detailed process and links to regional climate variation. Here, we present high-resolution palynological and lithological data of a peatland's sedimentary profile in the central Sanjiang Plain. The study aims to reconstruct the local terrestrialization process and discuss its significance to the Holocene East Asian summer monsoon (EASM) evolution. The results indicate that a paleolake surrounded by broadleaved forests developed during ~7.4-4.5 ka BP, corresponding to a wet and warm stage with strengthened EASM. Thereafter, a wetland dominated by Cyperaceae and Poaceae was initiated with the terrestrialization process. This process lasted until 3.9 ka BP when a lithological mud-to-peat transition occurred indicating a peatland began to develop after the extinction of the paleolake. Considering the prevalent EASM climate on the modern Sanjiang Plain, this paleolake-to-peatland transition during 4.5-3.9 ka BP was attributed to the mid-Holocene EASM retreat in addition to the paleolake's autogenic infilling process. During the peatland developing stage, four dry intervals with weakened EASM occurred in 3.9-3.2 ka BP, 2.0-1.3 ka BP, 1.0-0.5 ka BP, and 0.1-0.0 ka BP. They were alternated with three wet stages of the strengthened EASM during 3.2-2.0 ka BP, 1.3-1.0 ka BP, and 0.5-0.1 ka BP. Our findings are supported by a series of paleoclimatic records across the monsoonal regions of China. We suggest that the EASM evolution associated with the tropical ocean-atmosphere interactions was initially modulated by the solar output variation.

Role and response of ocean–atmosphere interactions during Amphan (2020) super cyclone

Role and response of ocean–atmosphere interactions during Amphan (2020) super cyclone

Impact of Large-Scale Ocean–Atmosphere Interactions on Interannual Water Storage Changes in the Tropics and Subtropics

Satellite observations from the Gravity Recovery and Climate Experiment (GRACE) provide unique measurements of global terrestrial water storage (TWS) changes at different spatial and temporal scales. Large-scale ocean–atmosphere interactions might have significant impacts on the global hydrological cycle, resulting in considerable influences on TWS changes. Quantifying the contributions of large-scale ocean–atmosphere interactions to TWS changes would be beneficial to improving our understanding of water storage responses to climate variability. In the study, we investigate the impact of three major global ocean–atmosphere interactions—El Niño and Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Atlantic Meridional Mode (AMM) on interannual TWS changes in the tropics and subtropics, using GRACE measurements and climate indices. Based on the least square principle, these climate indices, and the corresponding Hilbert transformations along with a linear trend, annual and semi-annual terms are fitted to the TWS time series on global 1° × 1° grids. By the fitted results, we analyze the connections between interannual TWS changes and ENSO, IOD, and AMM indices, and estimate the quantitative contributions of these climate phenomena to TWS changes. The results indicate that interannual TWS changes in the tropics and subtropics are related to ENSO, IOD, and AMM climate phenomena. The contribution of each climate phenomenon to TWS changes might vary in different regions, but in most parts of the tropics and subtropics, the ENSO contribution to TWS changes is found to be more dominant than those from IOD and AMM.

Changes in the ENSO–ISMR relationship in the historical and future projection periods based on coupled models

AbstractWe examine the projected changes in Indian summer monsoon rainfall (ISMR) and El Niño Southern Oscillation (ENSO) along with their changing relationships by the end of 21st century using Coupled Model Inter Comparison Project Phase5 (CMIP5) models. The historical (1951–2005) and future (2050–2099) projections of ISMR were assessed in addition to the changes in the ENSO–ISMR relationship during both the periods. The future period was selected as 2050–2099 in order to understand how the increase in greenhouse gas emissions and global warming by the end of 21st century will affect the ocean atmosphere interactions over the tropical Pacific Ocean and the associated rainfall characteristics over the Indian region. For the future projection, we considered Representative Concentration Pathways (RCP) 4.5 and 8.5 scenarios for the five models (CNRM‐CM5, GISS‐E2‐R, GISS‐E2‐R‐CC, HadGEM2‐CC and IPSL‐CM5A‐MR), which were selected from the 25 CMIP5 models based on their ability to capture the seasonal cycle and variability in the historical period. Although, all the five models simulate an increase in ISMR in both the RCPs, the increase is well simulated by CNRM‐CM5, HadGEM2‐CC and IPSL‐CM5A‐MR models mainly over the northeast, west coast, foothills of Himalayas, central India and the peninsular Indian regions. Substantial differences in simulating the ENSO–ISMR relationship are observed among the models in both historical and future RCP scenarios, and the observed relationship is well represented in GISS‐E2‐R‐CC, HadGEM2‐CC and IPSL‐CM5A‐MR models. The rainfall over most of the Indian regions shows a significant decrease (increase) during El Niño (La Niña) events in RCP 4.5. The changes in sea surface temperature (SST) anomalies and Walker circulations over the Indo‐Pacific region during El Niño and La Niña events are connected with the ISMR characteristics during these events in the RCP 4.5 scenario. On the other hand, the Indo‐Pacific region undergoes total warming (cooling) during El Niño (La Niña) events, and the Indo‐Pacific SST does not exhibit significant variability in the RCP 8.5 scenario. Hence, they are not well related with the observed ISMR patterns in that scenario.

Wood Anatomy of Douglas-Fir in Eastern Arizona and Its Relationship With Pacific Basin Climate.

Dendroclimatic reconstructions, which are a well-known tool for extending records of climatic variability, have recently been expanded by using wood anatomical parameters. However, the relationships between wood cellular structures and large-scale climatic patterns, such as El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), are still not completely understood, hindering the potential for wood anatomy as a paleoclimatic proxy. To better understand the teleconnection between regional and local climate processes in the western United States, our main objective was to assess the value of these emerging tree-ring parameters for reconstructing climate dynamics. Using Confocal Laser Scanning Microscopy, we measured cell lumen diameter and cell wall thickness (CWT) for the period 1966 to 2015 in five Douglas-firs [Pseudotsuga menziesii (Mirb.) Franco] from two sites in eastern Arizona (United States). Dendroclimatic analysis was performed using chronologies developed for 10 equally distributed sectors of the ring and daily climatic records to identify the strongest climatic signal for each sector. We found that lumen diameter in the first ring sector was sensitive to previous fall–winter temperature (September 25th to January 23rd), while a precipitation signal (October 27th to February 13th) persisted for the entire first half of the ring. The lack of synchronous patterns between trees for CWT prevented conducting meaningful climate-response analysis for that anatomical parameter. Time series of lumen diameter showed an anti-phase relationship with the Southern Oscillation Index (a proxy for ENSO) at 10 to 14year periodicity and particularly in 1980–2005, suggesting that chronologies of wood anatomical parameters respond to multidecadal variability of regional climatic modes. Our findings demonstrate the potential of cell structural characteristics of southwestern United States conifers for reconstructing past climatic variability, while also improving our understanding of how large-scale ocean–atmosphere interactions impact local hydroclimatic patterns.

Introducing the New Regional Community Earth System Model, R-CESM

AbstractThe development of high-resolution, fully coupled, regional Earth system model systems is important for improving our understanding of climate variability, future projections, and extreme events at regional scales. Here we introduce and present an overview of the newly developed Regional Community Earth System Model (R-CESM). Different from other existing regional climate models, R-CESM is based on the Community Earth System Model version 2 (CESM2) framework. We have incorporated the Weather Research and Forecasting (WRF) Model and Regional Ocean Modeling System (ROMS) into CESM2 as additional components. As such, R-CESM can be conveniently used as a regional dynamical downscaling tool for the global CESM solutions or/and as a standalone high-resolution regional coupled model. The user interface of R-CESM follows that of CESM, making it readily accessible to the broader community. Among countless potential applications of R-CESM, we showcase here a few preliminary studies that illustrate its novel aspects and value. These include 1) assessing the skill of R-CESM in a multiyear, high-resolution, regional coupled simulation of the Gulf of Mexico; 2) examining the impact of WRF and CESM ocean–atmosphere coupling physics on tropical cyclone simulations; and 3) a convection-permitting simulation of submesoscale ocean–atmosphere interactions. We also discuss capabilities under development such as (i) regional refinement using a high-resolution ROMS nested within global CESM and (ii) “online” coupled data assimilation. Our open-source framework (publicly available at https://github.com/ihesp/rcesm1) can be easily adapted to a broad range of applications that are of interest to the users of CESM, WRF, and ROMS.

Oceanography time series reveals annual asynchrony input between oceanic and estuarine waters in Patagonian fjords

The postglacial Patagonian fjord system along the west coast of southern South America is one of the largest stretches of the southern hemisphere (SH) fjord belt, influenced by the SH westerly wind belt and continental freshwater input. This study reports a 3-year monthly time series (2017–2020) of physical and biogeochemical parameters obtained from the Reloncaví Marine Observatory (OMARE, Spanish acronym) at the northernmost embayment and fjord system of Patagonia. The main objective of this work was to understand the land–atmosphere–ocean interactions and to identify the mechanisms that modulate the density of phytoplankton. A key finding of this study was the seasonally varying asynchronous input of oceanic and estuarine water. Surface lower salinity and warmer estuarine water arrived in late winter to summer, contributing to water column stability, followed by subsurface higher salinity and less warmer oceanic water during fall–winter. In late winter 2019, an interannual change above the picnocline due to the record-high polarity of the Indian Ocean Dipole inhibited water column stability. The biogeochemical parameters (NO3−, NO2−, PO43−, Si(OH)4, pH, and dissolved oxygen) responded to the surface annual salinity variations, and oceanic water mass contributed greatly to the subsurface inorganic nutrient input. The water column N/P ratio indicated that no eutrophication occurred, even under intense aquaculture activity, likely because of the high ventilation dynamics of the Reloncaví Sound. Finally, a shift in phytoplankton composition, characterized by surface chlorophyll-a maxima in late winter and deepening of spring–summer blooms related to the physicochemical conditions of the water column, was observed. Our results support the ecosystem services provided by local oceanography processes in the north Patagonian fjords. Here, the anthropogenic impact caused by economic activities could be, in part, chemically reduced by the annual ventilation cycle mediated by the exchange of oceanic water masses into Patagonian fjords.

The essential role of early-spring westerly wind burst in generating the centennial extreme 1997/98 El Niño

AbstractWhile both intrinsic low-frequency atmosphere–ocean interaction and multiplicative burst-like event affect the development of the El Niño–Southern Oscillation (ENSO), the strong nonlinearity in ENSO dynamics has prevented us from separating their relative contributions. Here we propose an online filtering scheme to estimate the role of the westerly wind bursts (WWBs), a type of aperiodic burst-like atmospheric perturbation over the western-central tropical Pacific, in the genesis of the centennial extreme 1997/98 El Niño using the CESM coupled model. This scheme highlights the deterministic part of ENSO dynamics during model integration, and clearly demonstrates that the strong and long-lasting WWB in March 1997 was essential for generating the 1997/98 El Niño. Without this WWB, the intrinsic low-frequency coupling would have only produced a weak warm event in late 1997 similar to the 2014/15 El Niño.

Zonal mean and shift modes of historical climate response to evolving aerosol distribution

Anthropogenic aerosols are effective radiative forcing agents that perturb the Earth’s climate. Major emission sources shifted from the western to eastern hemisphere around the 1980s. An ensemble of single-forcing simulations with an Earth System Model reveals two stages of aerosol-induced climate change in response to the global aerosol increase for 1940–1980 and the zonal shift of aerosol forcing for 1980–2020, respectively. Here, using idealized experiments with hierarchical models, we show that the aerosol increase and shift modes of aerosol-forced climate change are dynamically distinct, governed by the inter-hemispheric energy transport and basin-wide ocean–atmosphere interactions, respectively. The aerosol increase mode dominates in the motionless slab ocean model but is damped by ocean dynamics. Free of zonal-mean energy perturbation, characterized by an anomalous North Atlantic warming and North Pacific cooling, the zonal shift mode is amplified by interactive ocean dynamics through Bjerknes feedback. Both modes contribute to a La Niña-like pattern over the equatorial Pacific. We suggest that a global perspective that accommodates the evolving geographical distribution of aerosol emissions is vital for understanding the aerosol-forced historical climate change.