Mean Sea Surface Temperature Change Research Articles

Persistence or Transition of the North Atlantic Oscillation Across Boreal Winter: Role of the North Atlantic Air‐Sea Coupling

AbstractClimate anomalies over North America and Europe are closely related to the North Atlantic Oscillation (NAO). The persistence, decay or switch of the NAO phase across the boreal winter determines whether the NAO‐related climate anomalies are sustained, weakened or reversed during the boreal winter. This study compares the spatiotemporal evolution of atmospheric wind and sea surface temperature (SST) anomalies over the North Atlantic corresponding to persistence and switch of the NAO phase across the boreal winter. It is revealed that the persistence of the NAO from early to late winter is associated with a strong coupling between the NAO and the North Atlantic tripole SST anomalies. The formation of the tripole SST anomalies is attributed to the wind‐related surface latent heat flux changes. The tripole SST anomalies feedback on the NAO through eddy‐mean flow interaction. In contrast, during the winters with the switch of the NAO phase, the North Atlantic SST anomalies follow the wind changes, but do not have obvious feedback on the NAO. Our analysis identifies a prominent change of the winter NAO evolution in the early 1990s. The NAO switches its phase from early to late winter during 1970s through the early 1990s, but tends to maintain its phase during the boreal winter after the early 1990s. This interdecadal change appears to be related to the mean SST change. The SST increase in both the tropical and mid‐latitude North Atlantic Ocean in the early 1990s enhances the feedback of the NAO‐induced tripole SST anomalies on the NAO.

Mean sea surface temperature changes influence ENSO-related precipitation changes in the mid-latitudes

El Niño profoundly impacts precipitation in high-population regions. This demands an advanced understanding of the changes in El Niño-induced precipitation under the future global warming scenario. However, thus far, consensus is lacking regarding future changes in mid-latitude precipitation influenced by El Niño. Here, by analyzing the Coupled Model Intercomparison Project simulations, we show that future precipitation changes are tightly linked to the response of each type of El Niño to the tropical Pacific mean sea surface temperature (SST) change. A La Niña-like mean SST change intensifies basin-wide El Niño events causing approximately 20% more precipitation over East Asia and North America via enhancing moisture transport. Meanwhile, an El Niño-like mean SST change generates more frequent eastern Pacific El Niño events, enhancing precipitation in North American. Our findings highlight the importance of the mean SST projection in selectively influencing the types of El Niño and their remote impact on precipitation.

Phytoplankton calcifiers control nitrate cycling and the pace of transition in warming icehouse and cooling greenhouse climates

Abstract. Phytoplankton calcifiers contribute to global carbon cycling through their dual formation of calcium carbonate and particulate organic carbon (POC). The carbonate might provide an efficient export pathway for the associated POC to the deep ocean, reducing the particles' exposure to biological degradation in the upper ocean and increasing the particle settling rate. Previous work has suggested ballasting of POC by carbonate might increase in a warming climate, in spite of increasing carbonate dissolution rates, because calcifiers benefit from the widespread nutrient limitation arising from stratification. We compare the biogeochemical responses of three models containing (1) a single mixed phytoplankton class, (2) additional explicit small phytoplankton and calcifiers, and (3) additional explicit small phytoplankton and calcifiers with a prognostic carbonate ballast model, to two rapid changes in atmospheric CO2. The first CO2 scenario represents a rapid (151-year) transition from a stable icehouse climate (283.9 ppm) into a greenhouse climate (1263 ppm); the second represents a symmetric rapid transition from a stable greenhouse climate into an icehouse climate. We identify a slope change in the global net primary production response with a transition point at about 3.5 ∘C global mean sea surface temperature change in all models, driven by a combination of physical and biological changes. We also find that in both warming and cooling scenarios, the application of a prognostic carbonate ballast model moderates changes in carbon export production, suboxic volume, and nitrate sources and sinks, reducing the long-term model response to about one-third that of the calcifier model without ballast. Explicit small phytoplankton and calcifiers, and carbonate ballasting, increase the physical separation of nitrate sources and sinks through a combination of phytoplankton competition and lengthened remineralization profile, resulting in a significantly higher global nitrate inventory in this model compared to the single phytoplankton type model (15 % and 32 % higher for icehouse and greenhouse climates). Higher nitrate inventory alleviates nitrate limitation, increasing phytoplankton sensitivity to changes in physical limitation factors (light and temperature). This larger sensitivity to physical forcing produces stronger shifts in ocean phosphate storage between icehouse and greenhouse climates. The greenhouse climate is found to hold phosphate and nitrate deeper in the ocean, despite a shorter remineralization length scale than the icehouse climate, because of the longer residence times of the deep water masses. We conclude the global biogeochemical impact of calcifiers extends beyond their role in global carbon cycling, and that the ecological composition of the global ocean can affect how ocean biogeochemistry responds to climate forcing.

Mean warming not variability drives marine heatwave trends

Marine heatwaves have been shown to be increasing in frequency, duration and intensity over the past several decades. Are these changes related to rising mean temperatures, changes to temperature variability, or a combination of the two? Here we investigate this question using satellite observations of sea surface temperature (SST) covering 36 years (1982–2017). A statistical climate model is used to simulate SST time series, including realistic variability based on an autoregressive model fit to observations, with specified trends in mean and variance. These simulated SST time series are then used to test whether observed trends in marine heatwave properties can be explained by changes in either mean or variability in SST, or both. We find changes in mean SST to be the dominant driver of the increasing frequency of marine heatwave days over approximately 2/3 of the ocean; while it is the dominant driver of changes in marine heatwave intensity (temperature anomaly) over approximately 1/3 of the ocean. We also find that changes in mean SST explain changes in both MHW properties over a significantly larger proportion of the world’s ocean than changes in SST variance. The implication is that given the high confidence of continued mean warming throughout the twenty-first century due to anthropogenic climate change we can expect the historical trends in marine heatwave properties to continue over the coming decades.

Feedback of 10–20-day intraseasonal oscillations on seasonal mean SST in the tropical Western North Pacific during boreal spring through fall

The present study documents the factors for year-to-year changes in the intensity of 10–20-day intraseasonal oscillations (ISOs) and investigates the feedback of the 10–20-day ISO intensity on seasonal mean sea surface temperature (SST) change in the tropical western North Pacific during boreal spring through fall. An analysis of local correlation reveals a significant negative correlation of the 10–20-day ISO intensity and the seasonal mean SST tendency in the tropical western North Pacific during spring, summer, and fall, suggesting a plausible feedback of the ISO intensity on seasonal mean SST anomaly. The 10–20-day ISO intensity change over the tropical western North Pacific is influenced by El Nino-Southern Oscillation (ENSO) through modulation of vertical shear of zonal winds, lower-level moisture, and upward motion. Due to the phase dependence of location of these ENSO-induced background field changes, the ISO intensity is subject to ENSO influence in different regions during the three seasons. The feedback of the 10–20-day ISO intensity on local seasonal mean SST change in the tropical western North Pacific is demonstrated by separating latent heat flux anomalies into components on different time scales. The ISO-induced latent heat flux anomalies may accumulate in a season and overcome interannual anomalies due to seasonal mean changes. Thus, the ISO-induced surface heat flux change may play an important role in the seasonal mean SST anomaly in the tropical western North Pacific.

Eddy–Wind Interaction in the California Current System: Dynamics and Impacts

AbstractThe summertime California Current System (CCS) is characterized by energetic mesoscale eddies, whose sea surface temperature (SST) and surface current can significantly modify the wind stress and Ekman pumping. Relative importance of the eddy–wind interactions via SST and surface current in the CCS is examined using a high-resolution (7 km) regional coupled model with a novel coupling approach to isolate the small-scale air–sea coupling by SST and surface current. Results show that when the eddy-induced surface current is allowed to modify the wind stress, the spatially averaged surface eddy kinetic energy (EKE) is reduced by 42%, and this is primarily due to enhanced surface eddy drag and reduced wind energy transfer. In contrast, the eddy-induced SST–wind coupling has no significant impact on the EKE. Furthermore, eddy-induced SST and surface current modify the Ekman pumping via their crosswind SST gradient and surface vorticity gradient, respectively. The resultant magnitudes of the Ekman pumping velocity are comparable, but the implied feedback effects on the eddy statistics are different. The surface current-induced Ekman pumping mainly attenuates the amplitude of cyclonic and anticyclonic eddies, acting to reduce the eddy activity, while the SST-induced Ekman pumping primarily affects the propagation. Time mean–rectified change in SST is determined by the altered offshore temperature advection by the mean and eddy currents, but the magnitude of the mean SST change is greater with the eddy-induced current effect. The demonstrated remarkably strong dynamical response in the CCS system to the eddy-induced current–wind coupling indicates that eddy-induced current should play an important role in the regional coupled ocean–atmosphere system.

Potential increasing dominance of heterotrophy in the global ocean

Autotrophy is largely resource-limited in the modern ocean. Paleo evidence indicates this was not necessarily the case in warmer climates, and modern observations as well as standard metabolic theory suggest continued ocean warming could shift global ecology towards heterotrophy, thereby reducing autotrophic nutrient limitation. Such a shift would entail strong nutrient recycling in the upper ocean and high rates of net primary production (NPP), yet low carbon export to the deep ocean and sediments. We demonstrate transition towards such a state in the early 22nd century as a response to business-as-usual representative concentration pathway forcing (RCP8.5) in an intermediate complexity Earth system model in three configurations; with and without an explicit calcifier phytoplankton class and calcite ballast model. In all models nutrient regeneration in the near-surface becomes an increasingly important driver of primary production. The near-linear relationship between changes in NPP and global sea surface temperature (SST) found over the 21st century becomes exponential above a 2–4 global mean SST change. This transition to a more heterotrophic ocean agrees roughly with metabolic theory.

Changes in the variability of the North Pacific sea surface temperature caused by direct sulfate aerosol forcing in China in a coupled general circulation model

We examine the direct effect of anthropogenic sulfate aerosol from China on the sea surface temperature (SST) variability in the North Pacific, especially focusing on the Pacific Decadal Oscillation (PDO) using a coupled general circulation model (CGCM). We conduct two long‐term simulations, i.e., the Exp_Asia and the Exp_Asia_2x. Anthropogenic sulfate aerosols are prescribed in China only in the Exp_Asia. The conditions for the Exp_Asia_2x simulation are the same as in the Exp_Asia simulation, except the concentration of the anthropogenic sulfate aerosols in China is two times higher than that of the Exp_Asia simulation. It is found that the spatial structure of the PDO changes slightly despite an increase in the anthropogenic sulfate aerosol in China. In contrast, the temporal structure of the PDO is significantly altered between the two simulations. The variance in the PDO index in the boreal winter (December‐January‐February) and spring (March‐April‐May) is enhanced for the Exp_Asia_2x on low‐frequency timescales (>10 years) compared to that of the Exp_Asia simulation. Upon further analysis, the results of our study indicate that the change in the PDO index from the Exp_Asia to the Exp_Asia_2x is associated with the change in the mean SST in the Kuroshio‐Oyashio Extension (KOE) region through the modification of atmospheric circulation. The change in SST in the KOE region is primarily associated with a wave‐like temperature structure forced by the diabatic cooling in the low‐level atmosphere in China through the modification of atmospheric circulation, which is caused by an increase in the anthropogenic sulfate aerosols. Therefore, it is possible for the temporal structure of the North Pacific SST variability (i.e., PDO) to be altered through the mean SST change in the North Pacific Ocean caused by an increase in regional sulfate aerosols in China.

Patterns of change in sea surface temperature in the North Atlantic during the last three decades: beyond mean trends

Sea surface temperature (SST) is an important indicator of changes in the climate system and a key driver of marine ecosystems. Here we studied the strength and spatial patterns of changes in North Atlantic SST during the last three decades (1982–2010). Regional and local patterns of change were studied using data derived from the Advanced Very High Resolution Radiometer (AVHRR) sensors. Apart from changes in mean SST, we studied changes in the seasonal cycle, in the spatial patterning of temperature anomalies and in the location of selected isotherms. We quantified the degree of nonlinearity in mean SST as an indicator of the rate at which SST trends changed during the study period. Changes in the timing and intensity of seasonal extremes were explored, and a heuristic method was used to derive the length of the period of stratification and to estimate its variation. Our results were in general coherent with the main impacts predicted by climate change projections, with greatest changes located at northern latitudes and near land. Marked variation in the spatial patterns was also found for different variables, strengthening the view that physical changes could be promoting the arrangement of novel marine biological communities. The observed changes in ocean SST highlighted the need of a more local and regional focus in future climate change studies.

The effect of skeletal mass accumulation in Porites on coral Sr/Ca and δ18O paleothermometry

Paleotemperature estimates based on coral Sr/Ca have not been widely accepted because the reconstructed glacial‐Holocene shift in tropical sea‐surface temperature (∼4–6°C) is larger than that indicated by foraminiferal Mg/Ca (∼2–4°C). We show that corals over‐estimate changes in sea‐surface temperature (SST) because their records are attenuated during skeletogenesis within the living tissue layer. To quantify this process, we microprofiled skeletal mass accumulation within the tissue layer of Porites from Australasian coral reefs and laboratory culturing experiments. The results show that the sensitivity of the Sr/Ca and δ18O thermometers in Porites will be suppressed, variable, and dependent on the relationship between skeletal growth rate and mass accumulation within the tissue layer. Our findings help explain why δ18O‐SST sensitivities for Porites range from −0.08‰/°C to −0.22‰/°C and are always less than the value of −0.23‰/°C established for biogenic aragonite. Based on this observation, we recalibrated the coral Sr/Ca thermometer to determine a revised sensitivity of −0.084 mmol/mol/°C. After rescaling, most of the published Sr/Ca‐SST estimates for the Indo‐Pacific region for the last ∼14,000 years (−7°C to +2°C relative to modern) fall within the 95% confidence envelope of the foraminiferal Mg/Ca‐SST records. We conclude that two types of calibration scales are required for coral paleothermometry; an attenuated Porites‐specific thermometer sensitivity for studies of seasonal to interannual change in SST and, importantly, the rescaled −0.084 mmol/mol/°C Sr/Ca sensitivity for studies of 20th‐century trends and millennial‐scale changes in mean SST. The calibration‐scaling concept will apply to the development of transfer functions for all geochemical tracers in corals.

Future projections of precipitation characteristics in East Asia simulated by the MRI CGCM2

Projected changes in precipitation characteristics around the mid-21st century and end-of-the-century are analyzed using the daily precipitation output of the 3-member ensemble Meteorological Research Institute global ocean-atmosphere coupled general circulation model (MRI-CGCM2) simulations under the Special Report on Emissions Scenarios (SRES) A2 and B2 scenarios. It is found that both the frequency and intensity increase in about 40% of the globe, while both the frequency and intensity decrease in about 20% of the globe. These numbers differ only a few percent from decade to decade of the 21st century and between the A2 and B2 scenarios. Over the rest of the globe (about one third), the precipitation frequency decreases but its intensity increases, suggesting a shift of precipitation distribution toward more intense events by global warming. South China is such a region where the summertime wet-day frequency decreases but the precipitation intensity increases. This is related to increased atmospheric moisture content due to global warming and an intensified and more westwardly extended North Pacific subtropical anticyclone, which may be related with an El Nino-like mean sea surface temperature change. On the other hand, a decrease in summer precipitation is noted in North China, thus augmenting a south-to-north precipitation contrast more in the future.

Regional ocean climate simulations with a 3D ice-ocean model for the Baltic Sea. Part 1: model experiments and results for temperature and salinity

Sea surface temperatures and salinity profiles in the Baltic Sea have been analyzed under different climate conditions using a 3D coupled ice-ocean model. As a reference, hindcast simulations for the period 1980–93 have been performed using observed three-hourly meteorological forcing fields and observed monthly river runoff. The results are compared with available observations from monitoring stations. The observed Baltic Sea climate is well reproduced by the model. Furthermore, two sets of 9-year time slice experiments have been performed using results from an atmospheric regional climate model as forcing. One of the time slice sets represents pre-industrial greenhouse conditions (control simulation), and the other set represents a global warming condition with a 150% increase in equivalent CO2 concentrations (scenario simulation) with lateral boundary conditions from the global atmosphere-ocean general circulation model, HadCM2. To simulate river runoff, a large-scale hydrological model has been applied. As the time slices are too short to properly spin up initial stratification for future climate, salinity is treated as an uncertainty factor. An extreme condition is obtained by integrating the Baltic Sea model for a period of 100 years while assuming that no salt water inflow will occur in the future. Salinity in the Gotland Basin decreases in the surface layer by about 3 to 4 psu and in the bottom layer by about 6 to 6.5 psu. The final quasi-equilibrium is characterized by salinities of 2.8 psu (minimum at the surface) to 6.5 psu (maximum at the bottom). The area averaged annual mean sea surface temperature change between scenario and control run is about 2.3 °C. The warming in different seasons is almost the same. The computational effective time slice approach in dynamical downscaling experiments is regarded as a feasible technique to regionalize global climate change experiments in the Baltic Sea.

Quaternary surface-water stable isotope signal from calcareous nannofossils at DSDP Site 593, southern Tasman Sea

Oxygen isotope values from calcareous nannofossils in four cores spanning the Quaternary from DSDP Site 593 in Tasman Sea are compared with the δ 18O signal of planktonic and benthic foraminifers from the same samples. The classic mid-late Quaternary isotope stages are exhibited with stage 12 particularly well developed. When δ 18O values of nannofossils are adjusted for coccolithophore vital effects they indicate larger (by 1–6°C) surface to bottom paleotemperature gradients and greater (by 1–3°C) changes in mean sea-surface temperature between full glacial and interglacial conditions than do δ 18O values from planktonic foraminifers. Along with the foraminifers, the nannofossils record a bimodal distribution of δ 18O between the early and mid-late Quaternary, indicating a significant change in global ice budget. The δ 13C of nannofossils also shows a bimodal distribution, but is opposite to that for the foraminifers. Nannofossil δ 18O values record a shift of ∼ −0.8‰ at isotope stage 8 corresponding to a major reduction in abundance of the previously dominant gephyrocapsids. A shift in δ 13C of ∼ −1.5‰ also occurs at stage 8, and a shift in δ 13C of ∼ +1.2‰ at around stage 14. The δ 18O shift in nannofossils is at least a Pacific-wide phenomenon; the δ 13C shifts are possibly global. The δ 13C signal of nannofossils exhibits an antipathetic relationship to that of benthic foraminifers back to isotope stage 18 but no significant correlation beyond this level to the base of the Quaternary. This is interpreted as reflecting local productivity dominating global influences on δ 13C since stage 18 at DSDP Site 593. The difference between nannofossil and benthic foraminifer δ 13C signals (Δ 13C) tends to be maximum during glacial stages and minimum during interglacials throughout the section, showing a strong correlation with the nannofossil δ 18O signal. The increased partitioning of 13C between surface and bottom waters during the glacial periods may indicate heightened productivity in surface waters in the southern Tasman Sea at these times.