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Earth System Dynamics

Light absorption by marine cyanobacteria affects tropical climate mean state and variability

We use an Earth system model to study the effects of light absorption by marine cyanobacteria on climate. We find that cyanobacteria have a considerable cooling effect on tropical SST with implications for ocean and atmosphere circulation patterns as well as for climate variability. The results indicate the importance of considering phytoplankton light absorption in climate models, and specifically highlight the role of cyanobacteria due to their regulative effect on tropical SST and climate.

The Cryosphere

Seasonal to decadal variability in ice discharge from the Greenland Ice Sheet

We derive the first continuous record of total ice discharged from all large Greenland outlet glaciers over the 2000–2016 period, resolving a distinct pattern of seasonal variability. We compare these results to glacier retreat and meltwater runoff and find that while runoff has a limited impact on ice discharge in summer, long-term changes in discharge are highly correlated to retreat. These results help to better understand Greenland outlet glacier sensitivity over a range of timescales.

The Cryosphere

Cold-to-warm flow regime transition in snow avalanches

Snow avalanches show complicated flow behaviour, characterized by several flow regimes which coexist in one avalanche. In this work, we analyse flow regime transitions where a powder snow avalanche transforms into a plug flow avalanche by incorporating warm snow due to entrainment. Prediction of such a transition is very important for hazard mitigation, as the efficiency of protection dams are strongly dependent on the flow regime, and our results should be incorporated into avalanche models.

Annales Geophysicae

Solar wind and kinetic heliophysics

This paper originated from the lecture I gave as the Hannes Alfvén medalist at the EGU General Assembly in Vienna in spring 2018. The paper reviews various aspects of modern solar wind physics and elucidates the role Alfvén waves play in solar wind acceleration and turbulence, which prevail in the low corona and inner heliosphere. Our understanding of the solar wind has recently made considerable progress based on remote sensing, in situ measurements, kinetic simulation and fluid modeling.

Ocean Science

What can seabirds tell us about the tide?

Ocean Science
DOI 10.5194/os-14-1483-2018
29 November 2018

This paper describes a feasibility study carried out to determine if information gathered for one discipline could berepurposedto provide insight in another. Data gathered during a study of bird distribution were used to investigate whether these same data could be used to measure tidal current velocities and direction. The paper concludes that there is potential to use GPS-tagged birds asdrifters of opportunityand that interdisciplinary sharing of data can provide additional insight.

Natural Hazards and Earth System Sciences

Field survey of Typhoon Hato (2017) and a comparison with storm surge modeling in Macau

Natural Hazards and Earth System Sciences
DOI 10.5194/nhess-18-3167-2018
29 November 2018

Typhoon Hato was one of the most damaging natural disaster events in the western Pacific region in 2017. It caused the the worst flooding in Macau since its instrumental records began in 1925. We present a high-resolution survey map recording inundation depths and distances at 278 sites in Macau. We provide a series of inundation maps under different tidal and sea levels. The maps that highlight adaptive strategies are essential in order to keep up with the pace of rising sea level.

Climate of the Past

Carbon burial in deep-sea sediment and implications for oceanic inventories of carbon and alkalinity over the last glacial cycle

A data-based reconstruction of carbon-bearing deep-sea sediment shows significant changes in the global burial rate over the last glacial cycle. We calculate the impact of these deep-sea changes, as well as hypothetical changes in continental shelf burial and volcanic outgassing. Our results imply that these geological fluxes had a significant impact on ocean chemistry and the global carbon isotopic ratio, and that the natural carbon cycle was not in steady state during the Holocene.

Climate of the Past

Connecting the Greenland ice-core and U/Th timescales via cosmogenic radionuclides: testing the synchroneity of Dansgaard–Oeschger events

The last glacial period was characterized by a number of rapid climate changes seen, for example, as abrupt warmings in Greenland and changes in monsoon rainfall intensity. However, due to chronological uncertainties it is challenging to know how tightly coupled these changes were. Here we exploit cosmogenic signals caused by changes in the Sun and Earth magnetic fields to link different climate archives and improve our understanding of the dynamics of abrupt climate change.

Atmospheric Chemistry and Physics

Mesospheric bores at southern midlatitudes observed by ISS-IMAP/VISI: a first report of an undulating wave front

Spatial structures of wave disturbances in the upper atmosphere were investigated with space-borne imaging from the International Space Station. The wave disturbance occurred around an altitude of 100 km, and is called a mesospheric bore. The large-scale structure of mesospheric bores has not been fully captured by previous ground-based imagers, but the space-borne imaging captured a bore with a wide field of view, and showed that bores can have a large undulating wave front as long as 2000 km.

Biogeosciences

Distribution and cycling of terrigenous dissolved organic carbon in peatland-draining rivers and coastal waters of Sarawak, Borneo

The carbon cycle is a key control for the Earth’s climate. Every year rivers deliver a lot of organic carbon to coastal seas, but we do not know what happens to this carbon, particularly in the tropics. We show that rivers in Borneo deliver carbon from peat swamps to the sea with at most minimal biological or chemical alteration in estuaries, but sunlight can rapidly oxidise this carbon to CO₂. This means that south-east Asian seas are likely hotspots of terrestrial carbon decomposition.

Hydrology and Earth System Sciences

Global phosphorus recovery from wastewater for agricultural reuse

Phosphorus (P) is important to global food security. Thus it is concerning that natural P reserves are predicted to deplete within the century. Here we explore the potential of P recovery from wastewater (WW) at global scale. We identify high production and demand sites to determine optimal market prices and trade flows. We show that 20% of the agricultural demand can be met, yet only 4 % can be met economically. Nonetheless, this recovery stimulates circular economic development in WW treatment.

Earth System Dynamics

ESD Ideas: The stochastic climate model shows that underestimated Holocene trends and variability represent two sides of the same coin

Long-term sea surface temperature trends and variability are underestimated in models compared to paleoclimate data. The idea is presented that the trends and variability are related, which is elaborated in a conceptual model framework. The temperature spectrum can be used to estimate the timescale-dependent climate sensitivity.

Climate of the Past

A 900-year New England temperature reconstruction from in situ seasonally produced branched glycerol dialkyl glycerol tetraethers (brGDGTs)

We measured biomarker production over a year in a small inland lake in the northeastern USA. Understanding biomarkers in the modern environment helps us improve reconstructions of past climate from lake sediment records. We use these results to interpret a 900-year decadally resolved temperature record from this lake. Our record highlights multi-decadal oscillations in temperature superimposed on a long-term cooling trend, providing novel insight into climate dynamics of the region.

SOIL

Spatial assessments of soil organic carbon for stakeholder decision-making – a case study from Kenya

SOIL
DOI 10.5194/soil-4-259-2018
7 November 2018

Land degradation impacts the health and livelihoods of about 1.5 billion people worldwide. The state of the environment and food security are strongly interlinked in tropical landscapes. This paper demonstrates the integration of soil organic carbon (SOC) and land health maps with socioeconomic datasets into an online, open-access platform called the Resilience Diagnostic and Decision Support Tool for Turkana County in Kenya.

Earth System Dynamics

Cascading transitions in the climate system

We introduce a framework of cascading tipping, i.e. a sequence of abrupt transitions occurring because a transition in one system affects the background conditions of another system. Using bifurcation theory, various types of these events are considered and early warning indicators are suggested. An illustration of such an event is found in a conceptual model, coupling the North Atlantic Ocean with the equatorial Pacific. This demonstrates the possibility of events such as this in nature.

Biogeosciences

Predominance of methanogens over methanotrophs in rewetted fens characterized by high methane emissions

Rewetting drained peatlands may lead to prolonged emission of the greenhouse gas methane, but the underlying factors are not well described. In this study, we found two rewetted fens with known high methane fluxes had a high ratio of microbial methane producers to methane consumers and a low abundance of methane consumers compared to pristine wetlands. We therefore suggest abundances of methane-cycling microbes as potential indicators for prolonged high methane emissions in rewetted peatlands.

Natural Hazards and Earth System Sciences

Towards risk-based flood management in highly productive paddy rice cultivation – concept development and application to the Mekong Delta

Natural Hazards and Earth System Sciences
DOI 10.5194/nhess-18-2859-2018
5 November 2018

In this study we provide an estimation of flood damages and risks to rice cultivation in the Mekong Delta. The derived modelling concept explicitly takes plant phenomenology and timing of floods in a probabilistic modelling framework into account. This results in spatially explicit flood risk maps to rice cultivation, quantified as expected annual damage. Furthermore, the changes in flood risk of two land-use scenarios were estimated and discussed.

Solid Earth

The influence of upper-plate advance and erosion on overriding plate deformation in orogen syntaxes

Solid Earth
DOI 10.5194/se-9-1207-2018
5 November 2018

In this modeling study, we investigate rock uplift at plate corners (syntaxes). These are characterized by a unique bent geometry at subduction zones and exhibit some of the world’s highest rock uplift rates. We find that the style of deformation changes above the plate’s bent section and that active subduction is necessary to generate an isolated region of rapid uplift. Strong erosion there localizes uplift on even smaller scales, suggesting both tectonic and surface processes are important.

Atmospheric Chemistry and Physics

Additional global climate cooling by clouds due to ice crystal complexity

Using light diffraction it is possible to detect microscopic features within ice particles that have not yet been fully characterized. Here, this technique was applied in airborne measurements, where it was found that majority of atmospheric ice particles have features that significantly change the way ice particles interact with solar light. The microscopic features make ice-containing clouds more reflective than previously thought, which could have consequences for predicting our climate.

Atmospheric Chemistry and Physics

Mesoscale fine structure of a tropopause fold over mountains

GLORIA observations during two crossings of the polar front jet stream resolve the fine mesoscale structure of a tropopause fold in high detail. Tracer–tracer correlations of H₂O and O₃ are presented as a function of potential temperature and reveal an active mixing region. Our study confirms conceptual models of tropopause folds, validates the high quality of ECMWF IFS forecasts, and suggests that mountain waves are capable of modulating exchange processes in the vicinity of tropopause folds.

Solid Earth

Oblique rifting: the rule, not the exception

Solid Earth
DOI 10.5194/se-9-1187-2018
26 October 2018

Fragmentation of continents often involves obliquely rifting segments that feature a complex three-dimensional structural evolution. Here we show that more than ~ 70 % of Earth’s rifted margins exceeded an obliquity of 20° demonstrating that oblique rifting should be considered the rule, not the exception. This highlights the importance of three-dimensional approaches in modelling, surveying, and interpretation of those rift segments where oblique rifting is the dominant mode of deformation.

Hydrology and Earth System Sciences

Modelling the water balance of Lake Victoria (East Africa) – Part 1: Observational analysis

Lake Victoria is the largest lake in Africa and one of the two major sources of the Nile river. The water level of Lake Victoria is determined by its water balance, consisting of lake precipitation and evaporation, inflow from rivers and lake outflow, controlled by two hydropower dams. Here, we present a water balance model for Lake Victoria, which closely represents the observed lake levels. The model results highlight the sensitivity of the lake level to human operations at the dam.

Hydrology and Earth System Sciences

Modelling the water balance of Lake Victoria (East Africa) – Part 2: Future projections

Lake Victoria is the second largest freshwater lake in the world and one of the major sources of the Nile River, which is controlled by two hydropower dams. In this paper we estimate the potential consequences of climate change for future water level fluctuations of Lake Victoria. Our results reveal that the operating strategies at the dam are the main controlling factors of future lake levels and that regional climate simulations used in the projections encompass large uncertainties.

The Cryosphere

The internal structure of the Brunt Ice Shelf from ice-penetrating radar analysis and implications for ice shelf fracture

Ice shelves are thick sheets of ice floating on the ocean off the coasts of Antarctica and Greenland. They help regulate the flow of ice off the continent. Ice shelves undergo a natural cycle of seaward flow, fracture, iceberg production and regrowth. The Brunt Ice Shelf recently developed two large cracks. We used ground-penetrating radar to find out how the internal structure of the ice might influence the present crack development and the future stability of the ice shelf.

Biogeosciences

Contrasting biosphere responses to hydrometeorological extremes: revisiting the 2010 western Russian heatwave

Northern forests enhanced their productivity during and before the 2010 Russian mega heatwave. We scrutinize this issue with a novel type of multivariate extreme event detection approach. Forests compensate for 54 % of the carbon losses in agricultural ecosystems due to vulnerable conditions in spring and better water management in summer. The findings highlight the importance of forests in mitigating climate change, while not alleviating the consequences of extreme events for food security.

Hydrology and Earth System Sciences

The importance of small artificial water bodies as sources of methane emissions in Queensland, Australia

Artificial water bodies are a major source of methane and an important contributor to flooded land greenhouse gas emissions. Past studies focussed on large water supply or hydropower reservoirs with small artificial water bodies (ponds) almost completely ignored. This regional study demonstrated ponds accounted for one-third of flooded land surface area and emitted over 1.6 million t CO₂eq. yr⁻¹(10 % of land use sector emissions). Ponds should be included in regional GHG inventories.

Earth System Dynamics

The climate of a retrograde rotating Earth

Model experiments show that changing the sense of Earth’s rotation has relatively little impact on the globally and zonally averaged energy budgets but leads to large shifts in continental climates and patterns of precipitation. The retrograde world is greener as the desert area shrinks. Deep water formation shifts from the North Atlantic to the North Pacific with subsequent changes in ocean overturning. Over large areas of the Indian Ocean, cyanobacteria dominate over bulk phytoplankton.

Solid Earth

GHOST: Geoscientific Hollow Sphere Tessellation

Solid Earth
DOI 10.5194/se-9-1169-2018
10 October 2018

I present the GHOST (Geoscientific Hollow Sphere Tessellation) software which allows for the fast generation of computational meshes in hollow sphere geometries counting up to a hundred million cells. Each mesh is composed of concentric spherical shells made of quadrilaterals or triangles. I focus here on three commonly used meshes used in the geodynamics/geophysics community and further benchmark the gravity and gravitational potential procedures in the simple case of a constant density.

Geoscience Communication

Building a Raspberry Pi school magnetometer network in the UK

As computing and geophysical sensor components have become increasingly affordable over the past decade, it is now possible to build a cost-effective system for monitoring the Earth’s natural magnetic field variations, in particular for space weather events, e.g. aurorae. Sensors available to the general public are ~ 100 times less sensitive than scientific instruments but only 1/100th of the price. We demonstrate a system that allows schools to contribute to a genuine scientific sensor network.

Atmospheric Measurement Techniques

Mapping carbon monoxide pollution from space down to city scales with daily global coverage

On 13 October 2017, the S5-P satellite was launched with TROPOMI as its only payload. One of the primary products is atmospheric CO observed with daily global coverage and spatial resolution of 7 × 7 km². The new dataset allows the sensing of CO enhancements above cities and industrial areas and can track pollution transport from biomass burning regions. Through validation with ground-based TCCON measurements we show that the CO data product is already well within the mission requirement.

Atmospheric Chemistry and Physics

Ice crystal number concentration estimates from lidar–radar satellite remote sensing – Part 1: Method and evaluation

The number concentration of ice crystals (N_i) is a key cloud property that remains very uncertain due to difficulties in determining it using satellites. This lack of global observational constraints limits our ability to constrain this property in models responsible for predicting future climate. This pair of papers fills this gap by showing and analyzing the first rigorously evaluated global climatology of N_i, leading to new information shedding light on the processes that control high clouds.

Biogeosciences

Reviews and syntheses: Carbon use efficiency from organisms to ecosystems – definitions, theories, and empirical evidence

Carbon fixed by plants and phytoplankton through photosynthesis is ultimately stored in soils and sediments or released to the atmosphere during decomposition of dead biomass. Carbon-use efficiency is a useful metric to quantify the fate of carbon – higher efficiency means higher storage and lower release to the atmosphere. Here we summarize many definitions of carbon-use efficiency and study how this metric changes from organisms to ecosystems and from terrestrial to aquatic environments.

Hydrology and Earth System Sciences

Why has catchment evaporation increased in the past 40 years? A data-based study in Austria

We analyze changes in catchment evaporation estimated from the water balances of 156 catchments in Austria over 1977–2014, as well as the possible causes of these changes. Our results show that catchment evaporation increased on average by 29 ± 14 mm yr⁻¹ decade⁻¹. We attribute this increase to changes in atmospheric demand (based on reference and pan evaporation), changes in vegetation (quantified by a satellite-based vegetation index), and changes in precipitation.

Atmospheric Chemistry and Physics

Trends in China’s anthropogenic emissions since 2010 as the consequence of clean air actions

To tackle the problem of severe air pollution, China has implemented active clean air policies in recent years. We quantified China’s anthropogenic emissions during 2010–2017 and identified the major driving forces of these trends by using a combination of bottom-up emission inventory and index decomposition analysis (IDA) approaches. The major air pollutants have reduced their emissions by 17–62 % during 2010–2017. The IDA results suggest that emission control measures are the main drivers.

Biogeosciences

Potential for phenol biodegradation in cloud waters

Biogeosciences
DOI 10.5194/bg-15-5733-2018
28 September 2018

The main objective of this work was to evaluate the potential degradation of phenol, a highly toxic pollutant, by cloud microorganisms. Phenol concentrations measured on five cloud samples collected at the PUY station in France were from 0.15 to 0.74 µg L⁻¹. Metatranscriptomic analysis suggested that phenol could be biodegraded directly in clouds, likely by Gammaproteobacteria. A large screening showed that 93 % of 145 bacterial strains isolated from clouds were able to degrade phenol.

Atmospheric Chemistry and Physics

How well do stratospheric reanalyses reproduce high-resolution satellite temperature measurements?

Reanalyses (RAs) are models which assimilate observations and are widely used as proxies for the true atmospheric state. Here, we resample six leading RAs using the weighting functions of four high-res satellite instruments, allowing a like-for-like comparison. We find that the RAs generally reproduce the satellite data well, except at high altitudes and in the tropics. However, we also find that the RAs more tightly correlate with each other than with observations, even those they assimilate.

Hydrology and Earth System Sciences

Global 5 km resolution estimates of secondary evaporation including irrigation through satellite data assimilation

Evaporation from wetlands, lakes and irrigation areas needs to be measured to understand water scarcity. So far, this has only been possible for small regions. Here, we develop a solution that can be applied at a very high resolution globally by making use of satellite observations. Our results show that 16% of global water resources evaporate before reaching the ocean, mostly from surface water. Irrigation water use is less than 1% globally but is a very large water user in several dry basins.

Ocean Science

World Ocean Circulation Experiment – Argo Global Hydrographic Climatology

Ocean Science
DOI 10.5194/os-14-1127-2018
27 September 2018

The new gridded WOCE-Argo Global Hydrographic Climatology (WAGHC) is described and compared with the NOAA WOA13 atlas. The monthly fields of temperature and salinity for 65 depth levels have a 1/4° spatial resolution. Two versions of the climatology were produced and differ with respect to the spatial interpolation performed on isobaric or isopycnal surfaces, respectively. The climatology characterizes the thermohaline state of the world ocean for the time period from 2008 to 2012.

Atmospheric Measurement Techniques

Cleaning up our water: reducing interferences from nonhomogeneous freezing of “pure” water in droplet freezing assays of ice-nucleating particles

Ice nucleation commonly studied using droplet freezing measurements suffers from artifacts caused by water impurities or substrate effects. We evaluate a series of substrates and water sources to find methods that reduce the background freezing temperature limit. The best performance was obtained from our new microfluidic device and hydrophobic glass surfaces, using filtered HPLC bottled water. We conclude with recommendations for best practices in droplet freezing experiments and data analysis.

Earth System Dynamics

Diurnal land surface energy balance partitioning estimated from the thermodynamic limit of a cold heat engine

Turbulent fluxes represent an efficient way to transport heat and moisture from the surface into the atmosphere. Due to their inherently highly complex nature, they are commonly described by semiempirical relationships. What we show here is that these fluxes can also be predicted by viewing them as the outcome of a heat engine that operates between the warm surface and the cooler atmosphere and that works at its limit.

The Cryosphere

Stopping the flood: could we use targeted geoengineering to mitigate sea level rise?

The Cryosphere
DOI 10.5194/tc-12-2955-2018
20 September 2018

In this paper, we explore the possibility of using locally targeted geoengineering to slow the rate of an ice sheet collapse. We find that an intervention as big as existing large civil engineering projects could have a 30 % probability of stopping an ice sheet collapse, while larger interventions have better odds of success. With more research to improve upon the simple designs we considered, it may be possible to perfect a design that was both achievable and had good odds of success.

Biogeosciences

Tracing water masses with ¹²⁹I and ²³⁶U in the subpolar North Atlantic along the GEOTRACES GA01 section

Biogeosciences
DOI 10.5194/bg-15-5545-2018
18 September 2018

The investigation of water mass transport pathways and timescales is important to understand the global ocean circulation. Following earlier studies, we use artificial radionuclides introduced to the oceans in the 1950s to investigate the water transport in the subpolar North Atlantic (SPNA). For the first time, we combine measurements of the long-lived iodine-129 and uranium-236 to confirm earlier findings/hypotheses and to better understand shallow and deep ventilation processes in the SPNA.

Geoscientific Model Development

sympl (v. 0.4.0) and climt (v. 0.15.3) – towards a flexible framework for building model hierarchies in Python

In the same way that the fruit fly or the yeast cell serve as model systems in biology, climate scientists use a range of computer models to gain a fundamental understanding of our climate system. These models range from extremely simple models that can run on your phone to those that require supercomputers. Sympl and climt are packages that make it easy for climate scientists to build a hierarchy of such models using Python, which facilitates easy to read and self-documenting models.

Hydrology and Earth System Sciences

Speculations on the application of foliar ¹³C discrimination to reveal groundwater dependency of vegetation and provide estimates of root depth and rates of groundwater use

Groundwater is a significant water resource for humans and for groundwater-dependent vegetation. Several challenges to managing both groundwater resources and dependent vegetation include defining the location of dependent vegetation, the rate of groundwater use, and the depth of roots accessing groundwater. In this study we demonstrate a novel application of measurements of stable isotopes of carbon that can be used to identify the location, the rooting depth, and the rate of groundwater use.

Atmospheric Chemistry and Physics

Inverse modelling of CF₄ and NF₃ emissions in East Asia

Emissions of carbon tetrafluoride CF₄, NF₃ and CHF₃ in east Asia have been calculated using atmospheric measurements and an atmospheric transport model. We calculate emissions of CF₄ to be quite constant between the years 2008 and 2015 for both China and South Korea, with 2015 emissions calculated at 4.33 ± 2.65 Gg yr^-1 and 0.36 ± 0.11 Gg yr^-1, respectively. Emission estimates of NF₃ from South Korea could be made with relatively small uncertainty at 0.6 ± 0.07 Gg yr^-1 in 2015.

Atmospheric Chemistry and Physics

The climate effects of increasing ocean albedo: an idealized representation of solar geoengineering

Marine cloud brightening has been proposed as a means of geoengineering/climate intervention, or deliberately altering the climate system to offset anthropogenic climate change. In idealized simulations that highlight contrasts between land and ocean, we find that the globe warms, including the ocean due to transport of heat from land. This study reinforces that no net energy input into the Earth system does not mean that temperature will necessarily remain unchanged.

Climate of the Past

Wet avalanches: long-term evolution in the Western Alps under climate and human forcing

In the context of a warming climate, mountain environments are especially vulnerable to a change in the risk pattern. Our study focuses on the past evolution of wet avalanches, likely triggered by warmer temperatures destabilizing the snow cover. In the last 3300 years we observed an increase of wet avalanche occurrence related to human activities, intensifying pressure on forest cover, as well as favorable climate conditions such as warmer temperatures coinciding with retreating glacier phases.

The Cryosphere

Mechanisms leading to the 2016 giant twin glacier collapses, Aru Range, Tibet

In Tibet, two glaciers suddenly collapsed in summer 2016 and produced two gigantic ice avalanches, killing nine people. This kind of phenomenon is extremely rare. By combining a detailed modelling study and high-resolution satellite observations, we show that the event was triggered by an increasing meltwater supply in the fine-grained material underneath the two glaciers. Contrary to what is often thought, this event is not linked to a change in the thermal condition at the glacier base.

Biogeosciences

Integrated management of a Swiss cropland is not sufficient to preserve its soil carbon pool in the long term

It is of great interest to know whether croplands act as a net source or sink of atmospheric CO₂and if soil carbon © stocks are preserved over long timescales due to the role of C in soil fertility. For a cropland in Switzerland it was found that managing the field under the Swiss framework of the Proof of Ecological Performance (PEP) resulted in soil C losses of 18.0 %. Additional efforts are needed to bring Swiss management practices closer to the goal of preserving soil C in the long term.

Ocean Science

Technical note: Two types of absolute dynamic ocean topography

Ocean Science
DOI 10.5194/os-14-947-2018
4 September 2018

Two types of marine geoid exist with the first type being the average level of sea surface height if the water is at rest (classical definition), and the second type being satellite-determined with the condition that the water is usually not at rest. The associated absolute dynamic ocean topography (DOT) also has two types. Horizontal gradients of the two DOTs are different with the 1st (2nd) type representing the absolute (relative) surface geostrophic currents.