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Globally, surface air and ocean temperatures have warmed by about 1°C since 1900. More than 90% of the additional heat contained in the climate system (atmosphere, ocean, land) due to global warming is stored in the ocean, so what do these increased ocean temperatures mean for our weather? The ocean stores significantly more heat than the atmosphere: the top few metres of the ocean contain more thermal energy than the entire atmosphere. There is a continuous exchange of heat between the ocean and atmosphere and the weather we experience is intrinsically linked to the heat contained in the ocean. As atmospheric winds flow over the ocean, they typically pick up moisture and either gain or release heat. At mid-latitudes, and depending on the season, maritime air masses are usually either comparatively mild and humid (winter) or cool and humid (summer). Regions such as western Europe or the north-western US and western Canada experience maritime climates. These are characterized by reduced seasonal temperature extremes compared to locations at similar latitudes in the interior and along the east coasts of the continents as the prevalent westerly winds either come from the ocean (west coasts) or the interior of the continents (east coasts).

Observational rain gauge/satellite and reanalysis datasets since the 1950s are evaluated for trends in mean and extreme rainfall in and around Mainland Southeast Asia (MSEA). Rain gauge data indicate strong increases exceeding 50% in both annual mean precipitation and various extreme precipitation indices over Vietnam and the northwestern part of the peninsula since 1979. The remote influence of ENSO may partially explain the recent precipitation trend toward a more intense regional hydrological cycle, in response to predominant La Niña states over recent decades. Increasing precipitation in MSEA is also associated with increased monsoon intensity in southeast Asia and a northward shift of the monsoon activity center toward MSEA over 1979–2018. Warming-driven evaporation increases were obtained over the adjacent seas typically feeding precipitation over MSEA associated with a shift toward predominantly positive phases of the two major natural climate variability modes of the tropical Indian Ocean, namely the Indian Ocean Dipole and the Indian Ocean Basin Mode. A moisture budget analysis using ERA5 re-analysis data showed increasing oceanic moisture transports along the typical winter and summer moisture pathways toward the MSEA. However, results show that during summer the major part of increased moisture from the oceanic moisture sources ends up as precipitation over the oceanic regions adjacent to MSEA with ERA5 not being able to produce the observed positive trends in summer continental precipitation. On the other hand, ERA5 reveals pronounced increases in winter precipitation over the MSEA, in accordance with rain-gauge data, associated with strongly increasing transport of moisture originated from the western tropical Pacific and the South China Sea.

The ocean plays a key role in modulating the climate of the Earth system (ES). At the present time it is also a major sink both for the carbon dioxide (CO2) released by human activities and for the excess heat driven by the resulting atmospheric greenhouse effect. Understanding the ocean's role in these processes is critical for model projections of future change and its potential impacts on human societies. A necessary first step in assessing the credibility of such future projections is an evaluation of their performance against the present state of the ocean. Here we use a range of observational fields to validate the physical and biogeochemical performance of the ocean component of UKESM1, a new Earth system model (ESM) for CMIP6 built upon the HadGEM3-GC3.1 physical climate model. Analysis focuses on the realism of the ocean's physical state and circulation, its key elemental cycles, and its marine productivity. UKESM1 generally performs well across a broad spectrum of properties, but it exhibits a number of notable biases. Physically, these include a global warm bias inherited from model spin-up, excess northern sea ice but insufficient southern sea ice and sluggish interior circulation. Biogeochemical biases found include shallow remineralization of sinking organic matter, excessive iron stress in regions such as the equatorial Pacific, and generally lower surface alkalinity that results in decreased surface and interior dissolved inorganic carbon (DIC) concentrations. The mechanisms driving these biases are explored to identify consequences for the behaviour of UKESM1 under future climate change scenarios and avenues for model improvement. Finally, across key biogeochemical properties, UKESM1 improves in performance relative to its CMIP5 precursor and performs well alongside its fellow members of the CMIP6 ensemble.

Rain-gauge datasets indicate strong increases in both annual mean and extreme precipitation over large parts of the Mainland Southeast Asia (MSEA) including Vietnam and the northwestern part of the peninsula over the last 40 years. Increasing precipitation is associated with increased monsoon intensity in southeast Asia and a northward shift of the monsoon activity centre towards MSEA. Warming-driven evaporation increases over the three main oceanic moisture sources - the Arabian Sea, the Bay of Bengal, and the South China Sea- may partially explain increasing precipitation in large parts of MSEA. Changes in the patterns of the two main modes of natural variability in the tropical Indian Ocean – the Indian Ocean Basin Mode (IOBM) and the Indian Ocean Dipole (IOD) – contribute to surface warming in these oceanic moisture source regions supplying precipitation to MSEA. Climate model projections show robust wide-spread trends in wet season precipitation with increasing frequency and intensity of extreme precipitation events throughout MSEA over the 21st century. Similar to observations, the projected precipitation trends are associated with strong warming-driven increases in evaporation in all major oceanic moisture sources supplying precipitation to MSEA.

Numerical model studies have shown the meridional overturning circulation (MOC) to exhibit variability on near-inertial timescales, and also indicate a region of enhanced variability on the equator. We present an analysis of a set of integrations of a global configuration of a numerical ocean model, which show very large amplitude oscillations in the MOCs in the Atlantic, Indian and Pacific oceans confined to the equatorial region. The amplitude of these oscillations is proportional to the width of the ocean basin, typically about 100 (200) Sv in the Atlantic (Pacific). We show that these oscillations are driven by surface winds within 10°N/S of the equator, and their periods (typically 4-10 days) correspond to a small number of low mode equatorially trapped planetary waves. Furthermore, the oscillations can be well reproduced by idealised wind-driven simulations linearised about a state of rest. Zonally integrated linearised equations of motion are solved using vertical normal modes and equatorial meridional modes representing Yanai and inertia-gravity waves. Idealised simulations capture between 85% and 95% of the variance of matching time-series segments diagnosed from the NEMO integrations. Similar results are obtained for the corresponding modes in the Atlantic and Indian Oceans. Our results raise questions about the roles of inertia-gravity waves near the equator in the vertical transfer of heat and momentum and whether these transfers will be explicitly resolved by ocean models or need to be parametrised.

We investigate tropical cyclone (TC) activity and intensity within a 100km radius of Bermuda between 1955 and 2019. The results show a more easterly genesis over time and significant increasing trends in tropical cyclone intensity (maximum wind speed (Vmax)) with a decadal Vmax median value increase of 30kts from 33 to 63kts (r=0.94, p=0.02), together with significant increasing August, September, October (ASO) sea surface temperature (SST) of 1.1°C (0.17°C per decade) r= 0.4 (p<0.01) and increasing average ocean temperature between 0.5–0.7°C (0.08-0.1°C per decade) r=0.3(p<0.01) in the depth range 0-300m. The strongest correlation is found between TC intensity and ocean temperature averaged through the top 50m ocean layer (T50m ) r=0.37 (p<0.01). We show how tropical cyclone potential intensity estimates are closer to actual intensity by using T50m as opposed to SST using the Bermuda Atlantic Timeseries Hydrostation S dataset. We modify the widely used sea surface temperature potential intensity index by using T50m to provide a closer estimate of the observed minimum sea level pressure (MSLP), and associated Vmax than by using SST, creating a T50m potential intensity (T50m_PI) index. The average MSLP difference is reduced by 12mb and proportional (r=0.74, p<0.01) to the SST/(T50m ) temperature difference. We also suggest the index could be used over a wider area of the subtropical/tropical Atlantic where there is a shallow mixed layer depth.

The Atlantic meridional overturning circulation (AMOC) represents the zonally integrated stream function of meridional volume transport in the Atlantic Basin. The AMOC plays an important role in transporting heat meridionally in the climate system. Observations suggest a heat transport by the AMOC of 1.3 PW at 26 degrees N-a latitude which is close to where the Atlantic northward heat transport is thought to reach its maximum. This shapes the climate of the North Atlantic region as we know it today. In recent years there has been significant progress both in our ability to observe the AMOC in nature and to simulate it in numerical models. Most previous modeling investigations of the AMOC and its impact on climate have relied on models with horizontal resolution that does not resolve ocean mesoscale eddies and the dynamics of the Gulf Stream/North Atlantic Current system. As a result of recent increases in computing power, models are now being run that are able to represent mesoscale ocean dynamics and the circulation features that rely on them. The aim of this review is to describe new insights into the AMOC provided by high-resolution models. Furthermore, we will describe how high-resolution model simulations can help resolve outstanding challenges in our understanding of the AMOC.

High-resolution satellite images of ocean color and sea surface temperature reveal an abundance of ocean fronts, vortices and filaments at scales below 10 km but measurements of ocean surface dynamics at these scales are rare. There is increasing recognition of the role played by small scale ocean processes in ocean-atmosphere coupling, upper-ocean mixing and ocean vertical transports, with advanced numerical models and in situ observations highlighting fundamental changes in dynamics when scales reach 1 km. Numerous scientific publications highlight the global impact of small oceanic scales on marine ecosystems, operational forecasts and long-term climate projections through strong ageostrophic circulations, large vertical ocean velocities and mixed layer re-stratification. Small-scale processes particularly dominate in coastal, shelf and polar seas where they mediate important exchanges between land, ocean, atmosphere and the cryosphere, e.g., freshwater, pollutants. As numerical models continue to evolve toward finer spatial resolution and increasingly complex coupled atmosphere-wave-ice-ocean systems, modern observing capability lags behind, unable to deliver the high-resolution synoptic measurements of total currents, wind vectors and waves needed to advance understanding, develop better parameterizations and improve model validations, forecasts and projections. SEASTAR is a satellite mission concept that proposes to directly address this critical observational gap with synoptic two-dimensional imaging of total ocean surface current vectors and wind vectors at 1 km resolution and coincident directional wave spectra. Based on major recent advances in squinted along-track Synthetic Aperture Radar interferometry, SEASTAR is an innovative, mature concept with unique demonstrated capabilities, seeking to proceed toward spaceborne implementation within Europe and beyond.

High-resolution satellite images of ocean color and sea surface temperature reveal an abundance of ocean fronts, vortices and filaments at scales below 10 km but measurements of ocean surface dynamics at these scales are rare. There is increasing recognition of the role played by small scale ocean processes in ocean-atmosphere coupling, upper-ocean mixing and ocean vertical transports, with advanced numerical models and in situ observations highlighting fundamental changes in dynamics when scales reach 1 km. Numerous scientific publications highlight the global impact of small oceanic scales on marine ecosystems, operational forecasts and long-term climate projections through strong ageostrophic circulations, large vertical ocean velocities and mixed layer re-stratification. Small-scale processes particularly dominate in coastal, shelf and polar seas where they mediate important exchanges between land, ocean, atmosphere and the cryosphere, e.g., freshwater, pollutants. As numerical models continue to evolve toward finer spatial resolution and increasingly complex coupled atmosphere-wave-ice-ocean systems, modern observing capability lags behind, unable to deliver the high-resolution synoptic measurements of total currents, wind vectors and waves needed to advance understanding, develop better parameterizations and improve model validations, forecasts and projections. SEASTAR is a satellite mission concept that proposes to directly address this critical observational gap with synoptic two-dimensional imaging of total ocean surface current vectors and wind vectors at 1 km resolution and coincident directional wave spectra. Based on major recent advances in squinted along-track Synthetic Aperture Radar interferometry, SEASTAR is an innovative, mature concept with unique demonstrated capabilities, seeking to proceed toward spaceborne implementation within Europe and beyond

This work investigates the variability of extreme weather events (drought spells, DS15, and daily heavy rainfall, PR99) over East Asia. It particularly focuses on the large scale atmospheric circulation associated with high levels of the occurrence of these extreme events. Two observational datasets (APHRODITE and PERSIANN) are compared with two high-resolution global climate models (HiRAM and HadGEM3-GC2) and an ensemble of other lower resolution climate models from CMIP5. We first evaluate the performance of the high resolution models. They both exhibit good skill in reproducing extreme events, especially when compared with CMIP5 results. Significant differences exist between the two observational datasets, highlighting the difficulty of having a clear estimate of extreme events. The link between the variability of the extremes and the large scale circulation is investigated, on monthly and interannual timescales, using composite and correlation analyses.Both extreme indices DS15 and PR99 are significantly linked to the low level wind intensity over East Asia, i.e. the monsoon circulation. It is also found that DS15 events are strongly linked to the surface temperature over the Siberian region and to the land-sea pressure contrast, while PR99 events are linked to the sea surface temperature anomalies over the West North Pacific. These results illustrate the importance of the monsoon circulation on extremes over East Asia. The dependencies on of the surface temperature over the continent and the sea surface temperature raise the question as to what extent they could affect the occurrence of extremes over tropical regions in future projections.

Variability in the North Atlantic ocean heat transport at 26.5N on short (5-day) timescales is identified and contrasted with different behaviour at monthly intervals using a combination of RAPID/MOCHA/WBTS measurements and the NEMO-LIM2 1/12 ocean circulation/sea ice model. Wind forcing plays the leading role in establishing the heat transport variability through the Ekman transport response of the ocean and the associated driving atmospheric conditions vary significantly with timescale. We find that at 5-day timescales the largest changes in the heat transport across 26.5N coincide with north-westerly airflows originating over the American land mass that drive strong southward anomalies in the Ekman flow. During these events the northward heat transport reduces by 0.5-0.7 PW (i.e. about 50% of the mean). In contrast, the Ekman transport response at longer monthly timescales is smaller in magnitude (0.2-0.3 PW) and consistent with expected variations in the leading mode of North Atlantic atmospheric variability, the North Atlantic Oscillation. The north-westerly airflow mechanism can have a prolonged influence beyond the central 5-day timescale and on occasion can reduce the accumulated winter ocean heat transport into the north Atlantic by ~40%.

The low-frequency variability of the Atlantic meridional overturning circulation (AMOC) is investigated from 2, ¼°, and ° global ocean–sea ice simulations, with a specific focus on its internally generated (i.e., “intrinsic”) component. A 327-yr climatological ¼° simulation, driven by a repeated seasonal cycle (i.e., a forcing devoid of interannual time scales), is shown to spontaneously generate a significant fraction R of the interannual-to-decadal AMOC variance obtained in a 50-yr “fully forced” hindcast (with reanalyzed atmospheric forcing including interannual time scales). This intrinsic variance fraction R slightly depends on whether AMOCs are computed in geopotential or density coordinates, and on the period considered in the climatological simulation, but the following features are quite robust when mesoscale eddies are simulated (at both ¼° and ° resolutions); R barely exceeds 5%–10% in the subpolar gyre but reaches 30%–50% at 34°S, up to 20%–40% near 25°N, and 40%–60% near the Gulf Stream. About 25% of the meridional heat transport interannual variability is attributed to intrinsic processes at 34°S and near the Gulf Stream. Fourier and wavelet spectra, built from the 327-yr ¼° climatological simulation, further indicate that spectral peaks of intrinsic AMOC variability (i) are found at specific frequencies ranging from interannual to multidecadal, (ii) often extend over the whole meridional scale of gyres, (iii) stochastically change throughout these 327 yr, and (iv) sometimes match the spectral peaks found in the fully forced hindcast in the North Atlantic. Intrinsic AMOC variability is also detected at multidecadal time scales, with a marked meridional coherence between 35°S and 25°N (15–30 yr periods) and throughout the whole basin (50–90-yr periods).