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We construct an upper ocean (0–1000 m) North Atlantic heat budget (26°–67°N) for the period 1950–2020 using multiple observational datasets and an eddy-permitting global ocean model. On multidecadal time scales, ocean heat transport convergence controls ocean heat content (OHC) tendency in most regions of the North Atlantic with little role for diffusive processes. In the subpolar North Atlantic (45°–67°N), heat transport convergence is explained by geostrophic currents, whereas ageostrophic currents make a significant contribution in the subtropics (26°–45°N). The geostrophic contribution in all regions is dominated by anomalous advection across the time-mean temperature gradient although other processes make a significant contribution, particularly in the subtropics. The time scale and spatial distribution of the anomalous geostrophic currents are consistent with a simple model of basin-scale thermal Rossby waves propagating westward/northwestward in the subpolar gyre, and multidecadal variations in regional OHC are explained by geostrophic currents periodically coming into alignment with the mean temperature gradient as the Rossby wave passes through. The global ocean model simulation shows that multidecadal variations in the Atlantic meridional overturning circulation are synchronized with the ocean heat transport convergence consistent with modulation of the west–east pressure gradient by the propagating Rossby wave. Significance Statement: The purpose of the work is to understand why the North Atlantic Ocean warms up and cools down on time scales of about 40 years. The key finding is that the temperature fluctuations are caused by an oceanic wave pushing the ocean surface up and down and causing ocean currents to change direction, pushing heat into and out of different parts of the ocean, and drawing down or emitting heat to the atmosphere. The findings matter because the warm and cool periods affect the climate of the countries surrounding the North Atlantic. Climate models need to account for this oceanic wave process to correctly predict how it will change in the future and affect the large-scale climate in a warming world. [ABSTRACT FROM AUTHOR]

Abstract: Lack of reliable observational metadata represents a key barrier to understanding sea surface temperature (SST) measurement biases, a large contributor to uncertainty in the global surface record. We present a method to identify SST measurement practice by comparing the observed SST diurnal cycle from individual ships with a reference from drifting buoys under similar conditions of wind and solar radiation. Compared to existing estimates, we found a larger number of engine room‐intake (ERI) reports post–World War II and in the period 1960–1980. Differences in the inferred mixture of observations lead to a systematic warmer shift of the bias adjusted SST anomalies from 1980 compared to previous estimates, while reducing the ensemble spread. Changes in mean field differences between bucket and ERI SST anomalies in the Northern Hemisphere over the period 1955–1995 could be as large as 0.5°C and are not well reproduced by current bias adjustment models. [ABSTRACT FROM AUTHOR]

Uncertainty in the bias adjustments applied to historical sea-surface temperature (SST) measurements made using buckets are thought to make the largest contribution to uncertainty in global surface temperature trends. Measurements of the change in temperature of water samples in wooden and canvas buckets are compared with the predictions of models that have been used to estimate bias adjustments applied in widely used gridded analyses of SST. The results show that the models are broadly able to predict the dependence of the temperature change of the water over time on the thermal forcing and the bucket characteristics: volume and geometry; structure and material. Both the models and the observations indicate that the most important environmental parameter driving temperature biases in historical bucket measurements is the difference between the water and wet-bulb temperatures. However, assumptions inherent in the derivation of the models are likely to affect their applicability. We observed that the water sample needed to be vigorously stirred to agree with results from the model, which assumes well-mixed conditions. There were inconsistencies between the model results and previous measurements made in a wind tunnel in 1951. The model assumes non-turbulent incident flow and consequently predicts an approximately square-root dependence on airflow speed. The wind tunnel measurements, taken over a wide range of airflows, showed a much stronger dependence. In the presence of turbulence the heat transfer will increase with the turbulent intensity; for measurements made on ships the incident airflow is likely to be turbulent and the intensity of the turbulence is always unknown. Taken together, uncertainties due to the effects of turbulence and the assumption of well-mixed water samples are expected to be substantial and may represent the limiting factor for the direct application of these models to adjust historical SST observations. [ABSTRACT FROM AUTHOR]

Drifting buoy sea-surface temperature ( SST) records have been used to characterize the diurnal variability of ocean temperature at a depth of order 20 cm. We use measurements covering the period 1986-2012 from the International Comprehensive Ocean-Atmosphere Data Set ( ICOADS) version 2.5, which is a collection of marine surface observations that includes individual SST records from drifting buoys. Appropriately transformed, this dataset is well suited for estimation of the diurnal cycle, since many drifting buoys have high temporal coverage (many reports per day), and are globally distributed. For each drifter for each day, we compute the local-time daily SST variation relative to the local-time daily mean SST. Climatological estimates of subdaily SST variability are found by averaging across various strata of the data: in 10° latitudinal bands as well as globally; and stratified with respect to season, wind speed and cloud cover. A parameterization of the diurnal variability is fitted as a function of the variables used to stratify the data, and the coefficients for this fit are also provided with the data. Results are consistent with expectations based on the previous work: the diurnal temperature cycle peaks in early afternoon (circa 2 pm local time); there is an increase in amplitude and a decrease in seasonality towards the equator. Generally, the ocean at this depth cools on windy days and warms on calm days, so that a component of subdaily variability is the SST tendency on slower timescales. By not 'closing' the diurnal cycle when stratified by environmental conditions, this dataset differs from previously published diurnal-cycle parameterizations. This thorough characterization of the SST diurnal cycle will assist in interpreting SST observations made at different local times of day for climatological purposes, and in testing and constraining models of the diurnal-cycle and air-sea interaction at high temporal resolution. [ABSTRACT FROM AUTHOR]

Wind velocity and air–sea turbulent flux measurements made from shipborne instruments are biased due to the effect of the ship on the flow of air to the instruments. The presence of the ship causes the airflow to a particular instrument site to be either accelerated or decelerated, displaced vertically, and sometimes deflected slightly in the horizontal. Although recognized for some time, it is only recently that the problem has been addressed using three-dimensional computational fluid dynamics (CFD) models to simulate the flow over particular ships, quantify the effects of flow distortion, and hence correct the ship-based measurements. It has previously been shown that this improves the calculated momentum fluxes by removing disparities between data from different ships, or from instruments in different locations on the same ship. This paper provides validation of the CFD model simulations. Two research ships were instrumented with multiple anemometers located in both well-exposed and badly exposed sites. Data are compared to the results of model simulations of the flow at various relative wind directions and wind speeds. Except when the anemometers are in the wake of an upwind obstruction, the model and the in situ wind speed estimates typically agree to within 2%. Direct validation of the model-derived estimates of the vertical displacement of the flow was not possible due to the extreme difficulty of obtaining such measurements in the field. In this study, simulations of flows at 0° and 90° from the bow of the ship were made and displacements of about 1 and 5 m were found, respectively. These results were used to correct the in situ momentum flux data. In one case, the application of the different bow-on and beam-on corrections for vertical displacement successfully removed the disparity seen in the uncorrected data. In a second case, the beam-on vertical displacement overcorrected the flux results. This overcorrection could be caused... [ABSTRACT FROM AUTHOR]

The article offers information on the global condition of surface humidity in 2015. Topics include the record-high level of surface moisture values in 2015, observation of the El Niño-Southern Oscillation (ENSO) region of the Pacific Ocean, and water vapor amounts and levels of saturation in Africa and Australia.