Description: Sea-level rise1 is one of the most pressing aspects of anthropogenic global warming with far-reaching consequences for coastal societies. However, sea-level rise did2, 3, 4, 5, 6, 7 and will strongly vary from coast to coast8, 9, 10. Here we investigate the long-term internal variability effects 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 CO2 increase but started from different 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 differ while keeping the oceanic initial state identical; indicating that centennial DSL projections are strongly dependent on ocean initial conditions.

Description: 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.

Description: 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.

Description: We explore the predictability of tropical Atlantic sea surface temperature (SST) and the potential influence of climate model bias on SST predictions over the tropical Atlantic. Two statistical methods are used to examine the skill in forecasting tropical Atlantic SST anomalies (SSTAs): linear inverse modeling (LIM) and analogue forecast (AF). The statistical models are trained either with observations or with data from two control integrations of the Kiel Climate Model (KCM), which only differ with respect to the resolution of its atmospheric component. Observed SSTAs suggest that Tropical Atlantic climatic changes are potentially predictable at lead times of up to 6 months over large parts of the Tropical Atlantic. The SSTAs from the KCM version employing a high-resolution atmosphere model (KCM-HRES) is potentially predictable at a level comparable to that derived from the observations, whereas the SSTAs from the KCM version employing a low-resolution atmosphere model (KCM-LRES) is considerably less potentially predictable. We show that the enhanced potential predictability in the former KCM version can be very likely related to the improved representation of ENSO-like dynamics and its seasonality. We used the statistical models in true forecast mode, i.e. the prediction schemes were trained from data independent of the forecast period. Using observed SSTAs to train the LIM yields significant skill in forecasting observed SSTAs at lead times of up to 4 months across all calendar months, which is mostly restricted to the northern and equatorial western Tropical Atlantic. Similar patterns, but with lower skill, are found when the models’ SSTAs are used, in which LIM trained with the KCM-HRES generally yields higher skills than that from the KCM-LRES. Applying AF yields significant skills in predicting observed SSTAs over the same regions, but the forecast skills are considerably smaller. When the SSTAs together with either sea level pressure (SLP) anomalies or dynamic sea level (DSL) anomalies from the KCM are used to construct the statistical models, the prediction of observed equatorial Atlantic SSTAs can be improved, with significant skill enhancement at lead times of up to 4 months in limited regions. An optimal initial SSTA pattern is found, which results in the largest transient anomaly growth over the entire domain. Independent of external forces, this amplification is developed internally; meaning that the seasonal forecast might be more sensitive to initial conditions than currently thought.

Description: 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.

Description: 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.

Description: Surface wind is taken as the primary driver of upwelling in the eastern boundary upwelling systems. The fluctuation of momentum flux associated with the variation in wind regulates the nutrient supply to the euphotic surface layer via changing the properties of oceanic mixed layer depth, the coastal and offshore upwelling, and horizontal advection. Here, the spatial and temporal variability of the surface wind field over the last seven decades across the Peruvian upwelling system is investigated. Strong fluctuations in seasonal to decadal timescales are found over the entire upwelling system. A semi-periodic wind fluctuation on an interannual timescale is found, which is closely related to the regional sea surface temperature and can be attributed to the El Niño Southern Oscillation (ENSO). However, the wind anomaly patterns during positive and negative phases of ENSO are not opposite, which suggests an asymmetric response of local wind to ENSO cycles. In addition, a semi-regular fluctuation on the decadal timescale is evident in the wind field, which can be attributed to the Interdecadal Pacific Oscillation (IPO). Our results show that the sea surface temperature over the Humboldt Upwelling System is closely connected to local wind stress and the wind stress curl. The SST wind stress co-variability seems more pronounced in the coastal upwelling cells, in which equatorward winds are very likely accompanied by robust cooling over the coastal zones. Over the past seven decades, wind speed underwent a slightly positive trend. However, the spatial pattern of the trend features considerable heterogeneity with larger values near the coastal upwelling cells.

Description: 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.

Description: The southeastern tropical Atlantic hosts a coastal upwelling system characterized by high biological productivity. Three subregions can be distinguished based on differences in the physical climate: the tropical Angolan and the northern and southern Benguela upwelling systems (tAUS, nBUS, sBUS). The tAUS, which is remotely forced via equatorial and coastal trapped waves, can be characterized as a mixing-driven system, where the wind forcing plays only a secondary role. The nBUS and sBUS are both forced by alongshore winds and offshore cyclonic wind stress curl. While the nBUS is a permanent upwelling system, the sBUS is impacted by the seasonal cycle of alongshore winds. Interannual variability in the region is dominated by Benguela Niños and Niñas that are warm and cold events observed every few years in the tAUS and nBUS. Decadal and multidecadal variations are reported for sea surface temperature and salinity, stratification and subsurface oxygen. Future climate warming is likely associated with a southward shift of the South Atlantic wind system. While the mixing-driven tAUS will most likely be affected by warming and increasing stratification, the nBUS and sBUS will be mostly affected by wind changes with increasing winds in the sBUS and weakening winds in the northern nBUS.