Through analysis of remotely sensed sea surface temperature (SST) and sea ice concentration data, we investigate the impact of winds related to the Southern Annular Mode (SAM) on sea ice extent ...around Antarctica. We show that positive SAM anomalies in the austral summer are associated with anomalously cold SSTs that persist and lead to anomalous ice growth in the following autumn, while negative SAM anomalies precede warm SSTs and a reduction in sea ice extent during autumn. The largest effect occurs in April, when a unit change in the detrended summertime SAM is followed by a 1.8±0.6 ×105 km2 change in detrended sea ice extent. We find no evidence that sea ice extent anomalies related to the summertime SAM affect the wintertime sea ice extent maximum. Our analysis shows that the wind anomalies related to the negative SAM during the 2016/2017 austral summer contributed to the record minimum Antarctic sea ice extent observed in March 2017.
Key Points
Summertime Southern Annular Mode anomalies modulate the seasonal cycle of sea ice extent
Positive values of the summertime Southern Annular Mode are followed by increased sea ice extent during autumn
The negative Southern Annular Mode during the 2016/2017 austral summer contributed to the record minimum sea ice extent in March 2017
This study investigates the dynamics that led to the repeated cold surges over midlatitude Eurasia, exceptionally warm conditions and sea ice loss over the Arctic, and the unseasonable weakening of ...the stratospheric polar vortex in autumn and early winter 2016–2017. We use ERA‐Interim reanalysis data and COBE sea ice and sea surface temperature observational data to trace the dynamical pathways that caused these extreme phenomena. Following abnormally low sea ice conditions in early autumn over the Pacific sector of the Arctic basin, blocking anticyclones became dominant over Eurasia throughout autumn. Ural blocking (UB) activity was four times above climatological levels and organized in several successive events. UB episodes played a key role in the unprecedented sea ice loss observed in late autumn 2016 over the Barents‐Kara Seas and the weakening of the stratospheric vortex. Each blocking induced circulation anomalies that resulted in cold air advection to its south and warm advection to its north. The near‐surface warming anomalies over the Arctic and cooling anomalies over midlatitude Eurasia varied in phase with the life cycles of UB episodes. The sea ice cover minimum over the Barents‐Kara Seas in 2016 was not observed in late summer but rather in mid‐November and December shortly after the two strongest UB episodes. Each UB episode drove intense upward flux of wave activity that resulted in unseasonable weakening of the stratospheric vortex in November. The surface impact of this weakening can be linked to the migration of blocking activity and cold spells toward Europe in early winter 2017.
Key Points
Record‐breaking blocking activity caused the cold weather over Eurasia in autumn and early winter 2016–2017
Successive Ural blocking episodes drove the unprecedented sea ice loss over the Barents‐Kara Seas in late autumn 2016
Ural blocking episodes triggered the unseasonable weakening of the stratospheric polar vortex in November 2016
ABSTRACT
A synthetic model is presented to enlarge the evolutionary framework of the General Dynamic Model (GDM) and the Glacial Sensitive Model (GSM) of oceanic island biogeography from the ...terrestrial to the marine realm. The proposed ‘Sea‐Level Sensitive’ dynamic model (SLS) of marine island biogeography integrates historical and ecological biogeography with patterns of glacio‐eustasy, merging concepts from areas as diverse as taxonomy, biogeography, marine biology, volcanology, sedimentology, stratigraphy, palaeontology, geochronology and geomorphology. Fundamental to the SLS model is the dynamic variation of the littoral area of volcanic oceanic islands (defined as the area between the intertidal and the 50‐m isobath) in response to sea‐level oscillations driven by glacial–interglacial cycles. The following questions are considered by means of this revision: (i) what was the impact of (global) glacio‐eustatic sea‐level oscillations, particularly those of the Pleistocene glacial–interglacial episodes, on the littoral marine fauna and flora of volcanic oceanic islands? (ii) What are the main factors that explain the present littoral marine biodiversity on volcanic oceanic islands? (iii) How can differences in historical and ecological biogeography be reconciled, from a marine point of view? These questions are addressed by compiling the bathymetry of 11 Atlantic archipelagos/islands to obtain quantitative data regarding changes in the littoral area based on Pleistocene sea‐level oscillations, from 150 thousand years ago (ka) to the present. Within the framework of a model sensitive to changing sea levels, we discuss the principal factors affecting the geographical range of marine species; the relationships between modes of larval development, dispersal strategies and geographical range; the relationships between times of speciation, modes of larval development, ecological zonation and geographical range; the influence of sea‐surface temperatures and latitude on littoral marine species diversity; the effect of eustatic sea‐level changes and their impact on the littoral marine biota; island marine species–area relationships; and finally, the physical effects of island ontogeny and its associated submarine topography and marine substrate on littoral biota. Based on the SLS dynamic model, we offer a number of predictions for tropical, subtropical and temperate volcanic oceanic islands on how rates of immigration, colonization, in‐situ speciation, local disappearance, and extinction interact and affect the marine biodiversity around islands during glacials and interglacials, thus allowing future testing of the theory.
The impact of uncertainties in air‐sea fluxes and ocean model parameters on the ocean circulation and ocean heat uptake (OHU) is assessed in a novel modeling framework. We use an ocean‐only model ...forced with the simulated sea surface fields of the CMIP5 climate models. The simulations are performed using control and 1% CO2 warming scenarios. The ocean‐only ensemble adequately reproduces the mean Atlantic Meridional Overturning Circulation (AMOC) and the zonally integrated OHU. The ensemble spread in AMOC strength, its weakening, and Atlantic OHU due to different air‐sea fluxes is twice as large as the uncertainty range related to vertical and mesocale eddy diffusivities. The sensitivity of OHU to uncertainties in air‐sea fluxes and model parameters differs vastly across basins, with the Southern Ocean exhibiting strong sensitivity to air‐sea fluxes and model parameters. This study clearly demonstrates that model biases in air‐sea fluxes are one of the key sources of uncertainty in climate simulations.
Key Points
Ocean model is forced with air‐sea fluxes from CMIP5 models to examine the drivers of uncertainty in ocean circulation and heat uptake (OHU)
High‐latitude air‐sea fluxes are the dominant source of uncertainty in the spread of Atlantic MOC and OHU over model structural uncertainty
Subgrid‐scale parameters lead to large uncertainty in the circulation and OHU, especially in the Pacific and Southern Oceans
•We developed a new model (BGP) for teleconnection studies.•The BGP classifies monthly rainfall events.•The BGP can forecast rainfall classes based on global SST variations.•The BGP exhibits ...teleconnection signals with explicit mathematical expressions.
The effectiveness of genetic programming (GP) for solving regression problems in hydrology has been recognized in recent studies. However, its capability to solve classification problems has not been sufficiently explored so far. This study develops and applies a novel classification-forecasting model, namely Binary GP (BGP), for teleconnection studies between sea surface temperature (SST) variations and maximum monthly rainfall (MMR) events. The BGP integrates certain types of data pre-processing and post-processing methods with conventional GP engine to enhance its ability to solve both regression and classification problems simultaneously. The model was trained and tested using SST series of Black Sea, Mediterranean Sea, and Red Sea as potential predictors as well as classified MMR events at two locations in Iran as predictand. Skill of the model was measured in regard to different rainfall thresholds and SST lags and compared to that of the hybrid decision tree-association rule (DTAR) model available in the literature. The results indicated that the proposed model can identify potential teleconnection signals of surrounding seas beneficial to long-term forecasting of the occurrence of the classified MMR events.
The Pacific decadal oscillation (PDO) is the leading mode of sea surface temperature (SST) variability over the North Pacific (north of 20°N). Its South Pacific counterpart (south of 20°S) is the ...South Pacific decadal oscillation (SPDO). The effects of tropical eastern Pacific (TEP) SST forcing and internal atmospheric variability are investigated for both the PDO and SPDO using a 10-member ensemble tropical Pacific pacemaker experiment. Each member is forced by the historical radiative forcing and observed SST anomalies in the TEP region. Outside the TEP region, the ocean and atmosphere are fully coupled and freely evolve. The TEP-forced PDO (54% variance) and SPDO (46% variance) are correlated in time and exhibit a symmetric structure about the equator, driven by the Pacific–North American (PNA) and Pacific–South American teleconnections, respectively. The internal PDO resembles the TEP-forced component but is related to internal Aleutian low (AL) variability associated with the Northern Hemisphere annular mode and PNA pattern. The internal variability is locally enhanced by barotropic energy conversion in the westerly jet exit region around the Aleutians. By contrast, barotropic energy conversion is weak associated with the internal SPDO, resulting in weak geographical preference of sea level pressure variability. Therefore, the internal SPDO differs from the TEP-forced component, featuring SST anomalies along ~60°S in association with the Southern Hemisphere annular mode. The limitations on isolating the internal component from observations are discussed. Specifically, internal PDO variability appears to contribute significantly to the North Pacific regime shift in the 1940s.
We present results from a new, global, high‐resolution (∼3‐km for ocean and ∼6‐km for atmosphere) realistic earth system simulation. This simulation allows us to examine aspects of small‐scale ...air‐sea interaction beyond what previous studies have reported. Our study focuses on recurring intermittent wind events in the Gulf Stream region. These events induce local air‐sea heat fluxes above Sea Surface Temperature (SST) anomalies with horizontal scales smaller than 500‐km. In particular, strong latent heat bursts above warm SST anomalies are observed during these wind events. We show that such wind events are associated with a secondary circulation that acts to fuel the latent heat bursts by transferring dry air and momentum down to the surface. The intensity of this secondary circulation is related to the strength of small‐scale SST fronts that border SST anomalies. The study of such phenomena requires high‐resolution in both the atmospheric and oceanic components of the model.
Plain Language Summary
We explore the atmospheric circulation above Sea Surface Temperature (SST) anomalies of less than 500 km‐scale using a new, global, coupled ocean‐atmosphere simulation performed at high horizontal resolution and integrated for 3 months. Our study focuses on intermittent wind events in the Gulf Stream region and the resulting local air‐sea heat fluxes above warm SST anomalies: A strong latent heat burst above these SST anomalies is observed during the intermittent wind events. Furthermore, during these events, a secondary circulation develops up to altitudes of 2,000 m above warm SST anomalies, which results in sinking of warm and dry air and air momentum from upper levels down to the sea‐surface. Such secondary circulation is triggered by the strong wind stress divergences that develop above small‐scale SST fronts bordering the SST anomalies. The consequence is an increase of latent heat fluxes above SST anomalies.
Key Points
Strong turbulent flux discontinuities observed at ocean fronts suggest the importance of small scales for air‐sea interactions
Intermittent large‐scale winds together with mesoscale Sea Surface Temperature variations trigger secondary circulations in the atmospheric boundary layer
Air‐sea interactions are explored under a wider range of periods and wind speeds than previously examined
Using delay‐Doppler maps of Global Navigation Satellite Systems Reflectometry (GNSS‐R) from the TechDemoSat‐1 satellite and considering sea ice and ocean interaction, an innovative method for ...retrieval of Arctic sea ice concentration (SIC) based on a deep neural network is proposed. This retrieval method shows the potential of future GNSS‐R applications for Arctic missions. Compared with SIC products from Hamburg University, the root mean square errors (RMSE) of retrieved results in March and June 2016 are 0.0284 and 0.0415, respectively. When the retrieved GNSS‐R SIC data are added into the assimilation as supplementary passive microwave remote‐sensing data, it has a positive influence on improving the accuracy of the Arctic SIC forecast. Especially in some edge regions of sea ice, when compared to only assimilating the remote‐sensing data, the regional RMSE of joint assimilation has a maximum decrease of approximately 17% in the 24‐hr forecast time, and over 5% in 72‐hr.
Plain Language Summary
Accurate sea ice forecasting is critical to understanding the risks of Arctic maritime activity and to improving climate forecasting in the mid‐high latitudes of the Northern Hemisphere. Data assimilation of sea ice observations is an effective way to improve the numerical model forecast results, and its effect is related to both the quality and the quantity of observations. As a new remote sensing technology, Global Navigation Satellite Systems Reflectometry (GNSS‐R) shows great potential in sea ice remote sensing. Based on GNSS‐R data from TechDemoSat‐1, we combined Delay‐doppler number eigenvalue method and deep neural network to propose an innovative method for estimating sea ice concentration (SIC) at GNSS‐R subsatellite points. The estimated SIC has reasonable accuracy and provides more information at subsatellite points near the grid points of the passive microwave remote sensing product. When it is joint assimilated with passive microwave remote sensing SIC, it has a positive influence on the forecast effect of SIC in the region with rapid change of sea ice. This is the first time that GNSS‐R data has been applied to the prediction of Arctic SIC, which is of great value in promoting the application and development of GNSS‐R in Arctic sea ice forecast.
Key Points
An innovative method for retrieving sea ice concentration (SIC) at Global Navigation Satellite Systems Reflectometry (GNSS‐R) subsatellite points is presented, considering the marine factors
The joint assimilation of retrieved GNSS‐R SIC and passive microwave remote sensing SIC is a useful means to improve the forecast accuracy
More GNSS‐R SIC data in areas with large SIC gradient will bring more useful information to the sea ice forecast
The spatial distribution of summer precipitation anomalies over eastern China often shows a dipole pattern, with anti-phased precipitation anomalies between southern China and northern China, known ...as the “southern flooding and northern drought” (SF-ND) pattern. In 2015, China experienced heavy rainfall in the south and the worst drought since 1979 in the north, which caused huge social and economic losses. Using reanalysis data, the atmospheric circulation anomalies and possible mechanisms related to the summer precipitation anomalies in 2015 were examined. The results showed that both El Nino and certain atmospheric teleconnections, including the Pacific Japan/East Asia Pacific (PJ/EAP), Eurasia pattern (EU), British-Baikal Corridor pattern (BBC), and Silk Road mode (SR), contributed to the dipole pattern of precipitation anomalies. The combination of these factors caused a southwards shift of the western Pacific subtropical high (WPSH) and a weakening of the East Asian summer monsoon. Consequently, it was difficult for the monsoon front and associated rain band to migrate northwards, which meant that less precipitation occurred in northern China while more precipitation occurred in southern China. This resulted in the SF-ND event. Moreover, further analysis revealed that global sea surface temperature anomalies (SSTAs) or sea-ice anomalies were key to stimulating these atmospheric teleconnections.
An extreme positive Indian Ocean Dipole (IOD) event occurred in 2019 boreal autumn, which has induced severe climate impacts around the Indian Ocean basin. In this study, the cause for 2019 IOD event ...and the related mechanism are explored. We find that the remarkable strengthening of Australian high and weakening of sea level pressure over South China Sea/Philippine Sea have been evidently visible since May 2019. Such a record‐breaking interhemispheric pressure gradient (IHPG) induced northward cross‐equatorial flow over the western Maritime Continent, which triggered strong wind‐evaporation‐SST and thermocline feedbacks in 2019. In addition to 2019 case, historical IOD events are highly correlated with IHPG from boreal late spring to summer. We show that skillful IHPG prediction can be made in March by European Centre for Medium‐Range Weather Forecasts seasonal forecast system, which makes it quite possible for the early warning of extreme IOD events by two to three seasons ahead.
Plain Language Summary
During 2019 fall, an extreme Indian Ocean Dipole (IOD) event occurred. In this paper, the possible cause and mechanism of this event are investigated. We find that Australian high was strengthened, and Philippine Sea Anticyclone was weakened since May 2019. This interhemispheric sea level pressure gradient (IHPG) anomaly generates the northward cross equator over the Maritime Continent, which is crucial for the IOD growth. Not only for 2019 IOD, IHPG is a good indicator for almost all the IOD events in history. Since IHPG is predictable in March, the early warning of IOD events and resultant climate disasters could be possible.
Key Points
An extreme positive Indian Ocean Dipole (IOD) event occurred in 2019
The interhemispheric sea level pressure gradient (IHPG) across the Maritime Continent is suggested to be the cause of this IOD event
The skillful prediction of IHPG could give rise to the successful IOD prediction by two to three seasons ahead
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