There is debate about slowing of the Atlantic Meridional Overturning Circulation (AMOC), a key component of the global climate system. Some focus is on the sea surface temperature (SST) slightly cooling in parts of the subpolar North Atlantic despite widespread ocean warming. Atlantic SST is influenced by the AMOC, especially on decadal timescales and beyond. The local cooling could thus reflect AMOC slowing and diminishing heat transport, consistent with climate model responses to rising atmospheric greenhouse gas concentrations. Here we show from Atlantic SST the prevalence of natural AMOC variability since 1900. This is consistent with historical climate model simulations for 1900–2014 predicting on average AMOC slowing of about 1 Sv at 30° N after 1980, which is within the range of internal multidecadal variability derived from the models’ preindustrial control runs. These results highlight the importance of systematic and sustained in-situ monitoring systems that can detect and attribute with high confidence an anthropogenic AMOC signal.

Sea-level rise 1 is one of the most pressing aspects of anthropogenic global warming with far-reaching consequences for coastal societies. However, sea-level rise did 2-7 and will strongly vary from coast to coast 8-10 . Here we investigate the long-term internal variability e ects on centennial projections of dynamic sea level (DSL), the local departure from the globally averaged sea level. A large ensemble of global warming integrations has been conducted with a climate model, where each realization was forced by identical CO 2 increase but started from di erent atmospheric and oceanic initial conditions. In large parts of the mid-and high latitudes, the ensemble spread of the projected centennial DSL trends is of the same order of magnitude as the globally averaged steric sea-level rise, suggesting that internal variability cannot be ignored when assessing twenty-first-century DSL trends. The ensemble spread is considerably reduced in the mid-to high latitudes when only the atmospheric initial conditions di er while keeping the oceanic initial state identical; indicating that centennial DSL projections are strongly dependent on ocean initial conditions.Globally averaged sea level has risen by about 20 cm since 1900 and at a rate of about 3 mm yr −1 during the past two decades, but with strong regional variation 1-7 . For example, the western tropical Pacific featured a much stronger rise than the global average during the recent decades, whereas even falling sea levels were observed in the eastern tropical Pacific and along the west coast of the Americas 7 . Dynamic sea surface topography, the departure from the Earth's geoid, is influenced by ocean currents, local mass balance and density changes of the water column 11-14 . The DSL, which is the focus of this study, has been introduced to describe the collective effect of the local steric (thermosteric and halosteric) and dynamical ocean adjustment contribution 9,11,14 . As the observed sealevel changes include the effects of both external forcing (natural, for example, solar; and anthropogenic, for example, CO 2 ) and internal variability, we need to understand both drivers to assess twentieth and twenty-first-century sea-level changes.Climate modes, patterns with identifiable characteristics and specific regional effects, are prominent examples of internal variability. The El Niño/Southern Oscillation 15 , a quasi-periodic fluctuation of the equatorial Pacific sea surface temperature with a period of about 4 years, is the leading mode of tropical interannual variability. El Niño/Southern Oscillation is associated with zonal redistributions of heat, causing large sea-level anomalies across the equatorial Pacific and along the west coasts of the Americas 15 . The Pacific Decadal Oscillation, a decadal climate mode, also strongly affects sea level in the Pacific 2,16,17 and tropical South Indian Ocean 18 . Other regions of strong internal decadal sea-level variations are the North 19,20 and South Atlantic 19 . Here we address the influence of the longer centen...

While the Earth's surface has considerably warmed over the past two decades, the tropical Pacific has featured a cooling of sea surface temperatures in its eastern and central parts, which went along with an unprecedented strengthening of the equatorial trade winds, the surface component of the Pacific Walker Circulation (PWC). Previous studies show that this decadal trend in the trade winds is generally beyond the range of decadal trends simulated by climate models when forced by historical radiative forcing. There is still a debate on the origin of and the potential role that internal variability may have played in the recent decadal surface wind trend. Using a number of long control (unforced) integrations of global climate models and several observational data sets, we address the question as to whether the recent decadal to multidecadal trends are robustly classified as an unusual event or the persistent response to external forcing. The observed trends in the tropical Pacific surface climate are still within the range of the long‐term internal variability spanned by the models but represent an extreme realization of this variability. Thus, the recent observed decadal trends in the tropical Pacific, though highly unusual, could be of natural origin. We note that the long‐term trends in the selected PWC indices exhibit a large observational uncertainty, even hindering definitive statements about the sign of the trends.

Spatial and temporal variations of nutrient-rich upwelled water across the major eastern boundary upwelling systems are primarily controlled by the surface wind with different, and sometimes contrasting, impacts on coastal upwelling systems driven by alongshore wind and offshore upwelling systems driven by the local wind-stress-curl. Here, concurrently measured wind-fields, satellite-derived Chlorophyll-a concentration along with a state-of-the-art ocean model simulation spanning 2008-2018 are used to investigate the connection between coastal and offshore physical drivers of the Benguela Upwelling System (BUS). Our results indicate that the spatial structure of long-term mean upwelling derived from Ekman theory and the numerical model are fairly consistent across the entire BUS and closely followed by the Chlorophyll-a pattern. The variability of the upwelling from the Ekman theory is proportionally diminished with offshore distance, whereas different and sometimes opposite structures are revealed in the model-derived upwelling. Our result suggests the presence of sub-mesoscale activity (i.e., filaments and eddies) across the entire BUS with a large modulating effect on the wind-stress-curl-driven upwelling off Lüderitz and Walvis Bay. In Kunene and Cape Frio upwelling cells, located in the northern sector of the BUS, the coastal upwelling and open-ocean upwelling frequently alternate each other, whereas they are modulated by the annual cycle and mostly in phase off Walvis Bay. Such a phase relationship appears to be strongly seasonally dependent off Lüderitz and across the southern BUS. Thus, our findings suggest this relationship is far more complex than currently thought and seems to be sensitive to climate changes with short- and far-reaching consequences for this vulnerable marine ecosystem.

Climate models generally simulate a long-term slowdown of the Pacific Walker Circulation in a warming world. However, despite increasing greenhouse forcing, there was an unprecedented intensification of the Pacific Trade Winds during 1992–2011, that co-occurred with a temporary slowdown in global surface warming. Using ensemble simulations from three different climate models starting from different initial conditions, we find a large spread in projected 20-year globally averaged surface air temperature trends that can be linked to differences in Pacific climate variability. This implies diminished predictive skill for global surface air temperature trends over decadal timescales, to a large extent due to intrinsic Pacific Ocean variability. We show, however, that this uncertainty can be considerably reduced when the initial oceanic state is known and well represented in the model. In this case, the spatial patterns of 20-year surface air temperature trends depend largely on the initial state of the Pacific Ocean.

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Abstract. In this study we have analysed wind and wave time series data resulting from hourly measurements on the sea surface in Bushehr, the northern part of the Persian Gulf, from 15 July to 4 August 2000. Wind speed (U 10 ) ranged from 0.34 to 10.38 m/s as alternating sea and land breezes. The lowest wind speed occurs at about midnight and the highest at around noon. The calculated autocorrelation of wind speed data shows that when the sea-land breeze is strong, the land-sea breeze is weak and vice versa. The significant wave height (H s ) varies between 0.10 to 1.02 m. The data of the present study reflects mostly the local waves or the sea waves. The calculated correlation between wind and wave parameters is rather weak, due to the continuous change in the wind direction. Wave height distribution follows the well-known Rayleigh distribution law. The cross correlation analyses between U 10 and H s reveal a time lag of 4 h. Finally, we have shown that the time series of U 10 , H s , and wave period are stationary. We have modeled these parameters by an auto regressive moving average (ARMA) and auto regressive integrated moving average (ARIMA) models.

Long‐term predictability of the North Atlantic sea surface temperature (SST) is commonly attributed to buoyancy‐forced changes of the Atlantic Meridional Overturning Circulation. Here we investigate the role of surface wind stress forcing in decadal hindcasts as another source of extratropical North Atlantic SST predictability. For this purpose, a global climate model is forced by reanalysis (ERA‐interim) wind stress anomalies over the period 1979–2017. The simulated climate states serve as initial conditions for decadal hindcasts. Significant skill in predicting detrended observed annual SST anomalies is observed over the extratropical central North Atlantic with anomaly correlation coefficients exceeding 0.6 at lead times of 4 to 7 yrs. The skill is insensitive to the calendar month of initialization and primarily linked to upper ocean heat content anomalies that lead anomalous SSTs by several years.