Historical Nansen Cast Temperature Profiles and the Ocean Heat Content 1 Change since 19 th Century 2

Improved Global Ocean Heat Content Estimation by Modeling Vertical Spatio-Temporal Dependence

Estimating ocean heat content (OHC) with reliable uncertainties is critical for understanding and monitoring the evolution of Earth's climate, as the ocean has stored most of the energy accumulated in the climate system due to Earth Energy Imbalance. Here, we use Argo profiling float data from 2004-2022 to map OHC. As fewer Argo observations are available deeper in the water column, previous studies have partitioned the ocean into at least two pressure layers and mapped each separately, which complicates the estimation of uncertainties when the maps are summed to get the total OHC. In this work, we consider the case of two pressure layers and propose an improved mapping and uncertainty quantification method using bivariate locally stationary Gaussian processes and conditional simulations to map the two sections jointly while accounting for the correlation between them. We find that modeling this correlation results in improved OHC anomaly mapping and up to a 15 percent reduction of global OHC anomaly uncertainties in comparison to mapping the two layers separately without accounting for their dependence. These estimated uncertainties are essential to analyze the statistical significance of OHC anomalies on both regional and global scales, which we demonstrate using several climatological case studies.

Observations of seawater temperature and salinity as the main source of information on large-scale ocean currents

Abstract A. S. Sarkisyan developed a diagnostic method in the 1960s, based on the inclusion of annual mean temperature and salinity fields in circulation models, which made it possible to simulate climatic currents in the World Ocean and a number of other marine basins. As observations of seawater temperature and salinity accumulated, their climatic seasonal cycle became available on regular grids for several marine areas. Therefore, A. S. Sarkisyan and his followers extended the diagnostic method by proposing a set of algorithms, including nudging, which allowed the simulation of seasonal circulation for any individual marine basin. Recently, the possibility has emerged of using daily eddy-resolving three-dimensional temperature and salinity fields reconstructed from satellite observations on a regular grid. This makes it possible to apply nudging to the reanalysis of the Black Sea state and to off-line field analysis of the World Ocean. Nudging has shown good correspondence with similar results obtained by more complex assimilation schemes. Based on these results and developing the ideas of A. S. Sarkisyan that the key information about ocean currents is contained in the spatial distribution of the density field, an economical algorithm for four-dimensional variational assimilation of temperature and salinity observations in an ocean circulation model is proposed here.

Marine Heatwaves and NAO-Related Ocean–Atmosphere Variability in the North Atlantic

Increasing greenhouse gas concentrations are placing severe pressure on the Earth system, particularly on the ocean, which plays a vital role in carbon and heat uptake, and overall climate regulation. Consequently, the ocean is experiencing an accelerated warming, leading to an increase in the occurrence of extreme seawater temperature events, called Marine Heatwaves (MHWs). According to the most common definition, an MHW event is identified when local temperatures exceed the 90th percentile threshold of the climatology for at least five consecutive days. In this study, the definition was modified by calculating both the mean and the 90th percentile of SST over the entire available historical period (1982–2022), rather than using a fixed 30-year baseline. While MHWs can develop as a function of multiple drivers (including subsurface heat re-emergence, anomalously warm water masses, ocean heat advection, reduced vertical mixing, and mixed-layer stratification associated with surface heat gain), this study focuses on synoptic-scale atmospheric conditions associated with MHW occurrence and characteristics in the North Atlantic basin, from 1982 to 2022, with the objectives of identifying spatial-temporal trends of MHWs, examining the atmospheric conditions associated with their occurrence and exploring their relationship with prevalent climate variability modes. The results show positive trends in MHW frequency, duration, and intensity, albeit characterised by significant zonal and meridional variability, with noticeable differences between composite patterns of frequency and maximum intensity, according to the prevailing North Atlantic Oscillation (NAO) mode. The annual NAO appears to modulate the spatial distribution of MHWs, with its positive phase favouring MHWs in mid-latitude regions, while the negative phase impacts subpolar and tropical regions. Furthermore, concerning case-specific events, the stationarity of high-pressure systems, with weak pressure gradients, reduced wind speeds and increased solar radiation appears to be associated with the occurrence of the analysed events, while atmospheric instability appears to signal their decline, likely linked to enhanced wind-induced ocean mixing.

The Vertical Structure of the Temperature Field and the Spatial Distribution of the Dispersion of Temperature Fluctuations in the Global Ocean on Synoptic Scales

Orographic Quantitative Precipitation Forecast Sensitivity to the Choice of Microphysics Parameterization during a U.S.West Coast Atmospheric River Event

The Center for Western Weather and Water Extremes provides high-resolution forecasts of Atmospheric Rivers (ARs) along the U.S. West Coast. Using a 34-year reforecast dataset, we assess the sensitivity of orographic quantitative precipitation forecasts (QPF) associated with ARs to bulk microphysics parameterization (BMP). A 14-member, 168-hour reforecast ensemble of an AR event from 7–10 January 2017 is produced by varying only the BMP. Advances in microphysics parameterization have led to improved multi-moment schemes as well as a shift in the treatment of ice-phase hydrometeors. Rather than tracking the evolution of a prescribed ice class, some newer schemes track a single ice class and the evolution of its properties. These newer schemes, such as the Predicted Particle Properties schemes, the Ice-Spheroids Habit Model with Aspect-ratio Evolution scheme, and National Taiwan University parameterization may improve operational forecasts. We compare several BMPs to identify potential forecast improvements during a landfalling AR. Compared to precipitation observations and analyses, the QPF from the ensemble exhibited a general positive bias along the windward slopes of the Sierra Nevada Mountains, with a negative QPF bias in the lee (recognizing observational uncertainties). Several of the newer BMPs reduced this bias couplet near the Sierra Nevada. Our analysis identified a strong correlation between QPF and the drying ratio in the ensemble. Some newer schemes featured up to a 5% lower drying ratio, consistent with bias reduction and more realistic representation of cold-season orographic precipitation in this case. Further case studies are needed to generalize findings.

Regions of extremes of decadal surface temperature trends in 1900-1970

Surface layer temperature changes occur on various time scales: interannual, decadal, secular. Decades and longer periods are the most important time scales for studying gradual changes in mean hydrometeorological values. This paper is dedicated to assessing the spatial distribution of decadal trends in mean annual surface layer temperature (°C/yr) for successive decades from 1901 to 1970. The source of the initial data is the land+sea series from the HadCRUT5 Analysis version 5.0.2.0 archive, characterizing 5° x 5° latitude-longitude boxes. The resulting maps of the spatial distribution of trends revealed warming and cooling areas with linear dimensions greater than 1000 km, for which the absolute rate of change exceeded 0.1°C/yr. This value is three times higher than the hemispheric average estimates. Such values are referred to as extreme in the paper. In many cases, they are confined to the Arctic zone. The presence of such areas from 1901 to 1970 is a consequence of natural variability in the climate system, as anthropogenic impacts on it were still minor during this period.