North Atlantic Sea Surface Temperature Research Articles

The Influence of Atlantic Variability on Asian Summer Climate Is Sensitive to the Pattern of the Sea Surface Temperature Anomaly

AbstractWe simulate the response of Asian summer climate to Atlantic multidecadal oscillation (AMO)-like sea surface temperature (SST) anomalies using an intermediate-complexity general circulation model (IGCM4). Experiments are performed with seven individual AMO SST anomalies obtained from CMIP5/PMIP3 global climate models as well as their multimodel mean, globally and over the North Atlantic Ocean only, for both the positive and negative phases of the AMO. During the positive (warm) AMO phase, a Rossby wave train propagates eastward, causing a high pressure and warm and dry surface anomalies over eastern China and Japan. During the negative (cool) phase of the AMO, the midlatitude Rossby wave train is less robust, but the model does simulate a warm and dry South Asian monsoon, associated with the movement of the intertropical convergence zone in the tropical Atlantic. The circulation response and associated temperature and precipitation anomalies are sensitive to the choice of AMO SST anomaly pattern. A comparison between global SST and North Atlantic SST perturbation experiments indicates that East Asian climate anomalies are forced from the North Atlantic region, whereas South Asian climate anomalies are more strongly affected by the AMO-related SST anomalies outside the North Atlantic region. Experiments conducted with different amplitudes of negative and positive AMO anomalies show that the temperature response is linear with respect to SST anomaly but the precipitation response is nonlinear.

The North Atlantic as a Driver of Summer Atmospheric Circulation

AbstractSkill in seasonal forecasts in the Northern Hemisphere extratropics is mostly limited to winter. Drivers of summer circulation anomalies over the North Atlantic–European (NAE) sector are poorly understood. Here, we investigate the role of North Atlantic sea surface temperatures (SSTs) in driving summer atmospheric circulation changes. The summer North Atlantic Oscillation (SNAO), the leading mode of observed summer atmospheric circulation variability in the NAE sector, is correlated with a distinct SST tripole pattern in the North Atlantic. An atmospheric general circulation model is used to test whether there are robust atmospheric circulation responses over the NAE sector to concurrent SSTs related to the SNAO. The most robust responses project onto the summer east Atlantic (SEA) pattern, the second dominant mode of observed summer atmospheric circulation variability in the NAE sector, and are most evident at the surface in response to tropical SSTs and at altitude in response to extratropical SSTs. The tropical-to-extratropical teleconnection appears to be due to Rossby wave propagation from SST anomalies, and in turn precipitation anomalies, in the Caribbean region. We identify key biases in the model, which may be responsible for the overly dominant SEA pattern variability, compared to the SNAO, and may also explain why the responses resemble the SEA pattern, rather than the SNAO. Efforts to eradicate these biases, perhaps achieved by higher-resolution simulations or with improved model physics, would allow for an improved understanding of the true response to North Atlantic SST patterns.

Why Does a Colder (Warmer) Winter Tend to Be Followed by a Warmer (Cooler) Summer over Northeast Eurasia?

AbstractThis study reveals a pronounced out-of-phase relationship between surface air temperature (SAT) anomalies over northeast Eurasia in boreal winter and the following summer during 1980–2017. A colder (warmer) winter over northeast Eurasia tends to be followed by a warmer (cooler) summer of next year. The processes for the out-of-phase relation of winter and summer SAT involve the Arctic Oscillation (AO), the air–sea interaction in the North Atlantic Ocean, and a Eurasian anomalous atmospheric circulation pattern induced by the North Atlantic sea surface temperature (SST) anomalies. Winter negative AO/North Atlantic Oscillation (NAO)-like atmospheric circulation anomalies lead to continental cooling over Eurasia via anomalous advection and a tripolar SST anomaly pattern in the North Atlantic. The North Atlantic SST anomaly pattern switches to a dipolar pattern in the following summer via air–sea interaction processes and associated surface heat flux changes. The summer North Atlantic dipolar SST anomaly pattern induces a downstream atmospheric wave train, including large-scale positive geopotential height anomalies over northeast Eurasia, which contributes to positive SAT anomalies there via enhancement of downward surface shortwave radiation and anomalous advection. Barotropic model experiments verify the role of the summer North Atlantic SST anomalies in triggering the atmospheric wave train over Eurasia. Through the above processes, a colder winter is followed by a warmer summer over northeast Eurasia. The above processes apply to the years when warmer winters are followed by cooler summers except for opposite signs of SAT, atmospheric circulation, and SST anomalies.

Wind Stress‐Induced Multiyear Predictability of Annual Extratropical North Atlantic Sea Surface Temperature Anomalies

AbstractLong‐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.

Effects of tropical North Atlantic sea surface temperature on intense tropical cyclones landfalling in China

AbstractThis study identifies a significant positive association between tropical North Atlantic (TNA) sea surface temperature (SST) during preceding spring and the frequency of intense tropical cyclones (TCs) making landfall over mainland China during autumn on interannual time scales. Observational analyses show that a warm SST anomaly in TNA during spring can induce a warm SST anomaly in the western part of the western North Pacific (WNP) in subsequent autumn, through a chain of air‐sea coupled processes such as Rossby‐wave response, wind‐evaporation‐SST feedback, and westward advection of warm surface water. The physical mechanisms are verified using a suite of coupled numerical experiments performed by the state‐of‐the‐art Community Earth System Model. The warm SST anomaly in the western part of WNP is responsible for the genesis and intensification of intense TCs in situ, and the climatologically mean southeasterly steering flow is favourable for the intense TCs to make landfall over mainland China.

Relationship between Rainfall Pattern in North Central and Southern Nigeria and Some Tropical Climate Systems

Monthly rainfall data from meteorological stations in Nigeria are analyzed from 1951 to 1992, to establish their relationship with some Tropical climate systems. The climate systems include Tropical South Atlantic (TSA) sea surface temperature index, North Atlantic Ocean (NAO) atmospheric index, Tropical North Atlantic (TNA) sea surface temperature index, Central India Precipitation (CIP) and and Outgoing Longwave Radiation Anomaly (OLRA)). The rainfall pattern was given in terms of the August break (quantified in terms of the monsoon break intensity (MBI) and annual rainfall anomaly index (RAI). Partial correlation analysis was carried out to determine the effect of the tropical climate systems on the linear association between them and the stations in Nigeria. Our results show that the rainfall anomalies although sometimes intense do not have predictable patterns. The tele connection between CIP and total rainfall in Nigeria suggests that the rainfall patterns in Nigeria is likely to be modulated by the Tropical Easterly Jet (TEJ) connecting rainfall pattern in Central India to that in Nigeria. The August break is observed to be highly variable and does not show a clear discernable pattern of variability. Its variability may be connected with multiple forcings from ocean and mesoscale circulations.

Development, Amplification, and Decay of Atlantic/European Summer Weather Patterns Linked to Spring North Atlantic Sea Surface Temperatures

AbstractA recent study identified a relationship between North Atlantic Ocean sea surface temperature (SST) gradients in spring and a specific pattern of atmospheric circulation in the following summer: the summer east Atlantic (SEA) pattern. It was shown that the SEA pattern is closely associated with meridional shifts in the eddy-driven jet in response to anomalous SST gradients. In this study, the physical mechanisms underlying this relationship are investigated further. It is shown that the predictable SEA pattern anomalies appear in June–July and undergo substantial amplification between July and August before decaying in September. The associated SST anomalies also grow in magnitude and spatial extent from June to August. The question of why the predictable atmospheric anomalies should occur in summer is addressed, and three factors are identified. The first is the climatological position of the storm track, which migrates poleward from spring to summer. The second is that the magnitude of interannual SST variability underlying the storm track peaks in summer, both in absolute terms, and relative to the underlying mean SST gradient. The third factor is the most interesting. We identify a positive coupled ocean–atmosphere feedback, which operates in summer and leads to the amplification of both SST and atmospheric circulation anomalies. The extent to which the identified processes are captured in the HadGEM3-GC2 climate model is also assessed. The model is able to capture the relationship between spring North Atlantic SSTs and subsequent ocean–atmosphere conditions in early summer, but the relationship is too weak. The results suggest that the real world might be more predictable than is inferred from the models.

Ocean and land forcing of the record-breaking Dust Bowl heatwaves across central United States

The severe drought of the 1930s Dust Bowl decade coincided with record-breaking summer heatwaves that contributed to the socio-economic and ecological disaster over North America’s Great Plains. It remains unresolved to what extent these exceptional heatwaves, hotter than in historically forced coupled climate model simulations, were forced by sea surface temperatures (SSTs) and exacerbated through human-induced deterioration of land cover. Here we show, using an atmospheric-only model, that anomalously warm North Atlantic SSTs enhance heatwave activity through an association with drier spring conditions resulting from weaker moisture transport. Model devegetation simulations, that represent the wide-spread exposure of bare soil in the 1930s, suggest human activity fueled stronger and more frequent heatwaves through greater evaporative drying in the warmer months. This study highlights the potential for the amplification of naturally occurring extreme events like droughts by vegetation feedbacks to create more extreme heatwaves in a warmer world.

What Leads to Persisting Surface Air Temperature Anomalies from Winter to Following Spring over Mid- to High-Latitude Eurasia?

AbstractSurface air temperature (SAT) anomalies tend to persist from winter to the following spring over the mid- to high latitudes of Eurasia. The present study compares two distinct cases of Eurasian SAT anomaly evolution and investigates the reasons for the persistence of continental-scale mid- to high-latitude Eurasian SAT anomalies from winter to following spring (termed persistent cases). The persisting SAT anomalies are closely associated with the sustenance of large-scale atmospheric circulation anomaly pattern over the North Atlantic and Eurasia, featuring a combination of the North Atlantic Oscillation/Arctic Oscillation (NAO/AO) and the Scandinavian pattern, from winter to spring. The combined circulation anomalies result in SAT warming over most of mid- to high-latitude Eurasia via anomalous wind-induced temperature advection. The sustenance of atmospheric circulation anomaly pattern is related to the maintenance of the North Atlantic triple sea surface temperature (SST) anomaly pattern due to air–sea interaction processes. The Barents Sea ice anomalies, which form in winter and increase in spring, also partly contribute to the sustenance of atmospheric circulation anomalies via modulating thermal state of the lower troposphere. In the cases that notable SAT warming (cooling) in winter is replaced by pronounced SAT cooling (warming) in the subsequent spring—termed reverse cases—the North Atlantic SST anomalies become small and the Greenland Sea ice change is a response to atmospheric change in spring. Without the support of lower boundary forcing, the atmospheric circulation anomaly pattern experiences a reverse in the spatial distribution from winter to spring likely due to internal atmospheric processes.

Coherent Interannual Variations of Springtime Surface Temperature and Temperature Extremes Between Central‐Northern Europe and Northeast Asia

AbstractThis study reveals a strong coherent interannual variation in spring surface air temperature (SAT) and temperature extremes between central‐northern Europe (45–70°N, 0–30°E) and Northeast Asia (40–70°N, 110–140°E). The regional mean spring SAT over the two regions is highly correlated, with a correlation of 0.6 during 1950–2017. Arctic Oscillation (AO) and an atmospheric teleconnection, termed the North Atlantic‐Eurasian (NAE) pattern, both play important roles in connecting SAT anomalies over the two regions. Anticyclonic anomalies are seen over central‐northern Europe and Northeast Asia during positive phases of the spring AO and NAE teleconnections. The associated southerly and westerly wind anomalies to the northwestern side of the anomalous anticyclones lead to surface warming over the two regions via anomalous wind‐induced heat advection from lower latitudes. In addition, a decrease in cloud cover and an accompanying increase in surface downward shortwave radiation also contribute to the coherent SAT warming over central‐northern Europe and Northeast Asia. We suggest that synoptic‐scale eddies forcings are crucial to the maintenance of the spring NAE teleconnection. Atmospheric model experiments indicate that the North Atlantic sea surface temperature anomalies also partly contribute to the maintenance of the NAE‐related atmospheric anomalies over the North Atlantic and European regions. The observed coherent spring SAT variations over central‐northern Europe and Northeast Asia and their associations with the AO and midlatitude atmospheric teleconnections can be reproduced in the long historical simulation of a coupled model. This study has implication for the understanding of concurrent occurrence of extreme temperature events over different regions of Eurasia.

Historical Simulations With HadGEM3‐GC3.1 for CMIP6

AbstractWe describe and evaluate historical simulations which use the third Hadley Centre Global Environment Model in the Global Coupled configuration 3.1 (HadGEM3‐GC3.1) and which form part of the UK's contribution to the sixth Coupled Model Intercomparison Project, CMIP6. These simulations, run at two resolutions, respond to historically evolving forcings such as greenhouse gases, aerosols, solar irradiance, volcanic aerosols, land use, and ozone concentrations. We assess the response of the simulations to these historical forcings and compare against the observational record. This includes the evolution of global mean surface temperature, ocean heat content, sea ice extent, ice sheet mass balance, permafrost extent, snow cover, North Atlantic sea surface temperature and circulation, and decadal precipitation. We find that the simulated time evolution of global mean surface temperature broadly follows the observed record but with important quantitative differences which we find are most likely attributable to strong effective radiative forcing from anthropogenic aerosols and a weak pattern of sea surface temperature response in the low to middle latitudes to volcanic eruptions. We also find evidence that anthropogenic aerosol forcings play a role in driving the Atlantic Multidecadal Variability and the Atlantic Meridional Overturning Circulation, which are key features of the North Atlantic ocean. Overall, the model historical simulations show many features in common with the observed record over the period 1850–2014 and so provide a basis for future in‐depth study of recent climate change.

Anchoring of atmospheric teleconnection patterns by Arctic Sea ice loss and its link to winter cold anomalies in East Asia

AbstractIn this paper, the physical processes underlying recent winter cold anomalies over East Asia (EA) are examined via statistical analysis. It is found that the EA cold anomaly depends on the warming in the North Atlantic, sea ice loss in the Barents–Kara Sea (BKS), and atmospheric teleconnection patterns. Specifically, the sea ice loss in the BKS can anchor teleconnection patterns originating from different North Atlantic sea surface temperature (SST) patterns. Different patterns of North Atlantic warming can affect the position of the cold anomaly region through altering the atmospheric circulations. In addition, whether the relevant teleconnection pattern leads to enhanced cold anomaly over EA crucially depends on the sea ice loss in the BKS, because it can anchor the blocking anticyclone embedded in the teleconnection pattern over the Ural region and make it more persistent and quasi‐stationary. Furthermore, it is found that the role of SST modes in the EA cold anomaly depends on their time scales. Although the strong basin‐scale warming (north–south SST tripolar mode) in the North Atlantic mid‐ to high‐latitudes plays a major role in decadal (interannual) cold anomaly over EA, it appears that the Atlantic east–west SST dipole structure dominates winter temperature variations over EA in recent decades on both the interannual and decadal time scales.

Miocene Evolution of North Atlantic Sea Surface Temperature

AbstractWe reconstruct sea surface temperatures (SSTs) at Deep Sea Drilling Project Site 608 (42.836°N, 23.087°), north of the Azores Front, and Ocean Drilling Program Site 982 (57.516°N, 15.866°), under the North Atlantic Current, in order to track Miocene (23.1–5.3 Ma) development of North Atlantic surface waters. Mean annual SSTs from TEX86and UK′37proxy estimates at both sites were 10–15 °C higher than modern through the Miocene Climatic Optimum (17–14.5 Ma). During the global cooling of the Middle Miocene Climate Transition (~14.5–12.5 Ma), SSTs at midlatitude Site 608 cooled by ~6 °C, whereas high‐latitude Site 982 cooled by only ~2 °C, resulting in an ~4 Myr collapse of the SST gradient between the two sites. This regional pattern is inconsistent with an increased latitudinal surface temperature gradient, as generally associated with global cooling episodes linked to decreasingpCO2levels. Instead, the pattern is best explained by enhanced ocean heat transport into the high‐latitude North Atlantic superimposed on the global cooling trend, probably due to enhanced Atlantic meridional overturning circulation and/or a stronger North Atlantic Current. During global late Miocene cooling (~8–7 Ma), surface waters cooled by ~6 °C at Site 982 while minimal change occurred at Site 608, reestablishing the North Atlantic SST gradient. The collapse and reemergence of the SST gradient between the middle‐ and high‐latitude North Atlantic suggests that interaction between changes in regional ocean circulation and the global response to changes in greenhouse gas concentration was important in Miocene climate evolution.

Modulation of the Relationship between ENSO and Its Combination Mode by the Atlantic Multidecadal Oscillation

AbstractRecent studies demonstrated the existence of a conspicuous atmospheric combination mode (C-mode) originating from nonlinear interactions between El Niño–Southern Oscillation (ENSO) and the Pacific warm pool annual cycle (AC). Here we find that the C-mode exhibits prominent decadal amplitude variations during the ENSO decaying boreal spring season. It is revealed that the Atlantic multidecadal oscillation (AMO) can largely explain this waxing and waning in amplitude. A robust positive correlation between ENSO and the C-mode is detected during a negative AMO phase but not during a positive phase. Similar results can also be found in the relationship of ENSO with 1) the western North Pacific (WNP) anticyclone and 2) spring precipitation over southern China, both of which are closely associated with the C-mode. We suggest that ENSO property changes due to an AMO modulation play a crucial role in determining these decadal shifts. During a positive AMO phase, ENSO events are distinctly weaker than those in an AMO negative phase. In addition, El Niño events concurrent with a positive AMO phase tend to exhibit a westward-shifted sea surface temperature (SST) anomaly pattern. These SST characteristics during the positive AMO phase are both not conducive to the development of the meridionally asymmetric C-mode atmospheric circulation pattern and thus reduce the ENSO/C-mode correlation on decadal time scales. These observations can be realistically reproduced by a coupled general circulation model (CGCM) experiment in which North Atlantic SSTs are nudged to reproduce a 50-yr sinusoidally varying AMO evolution. Our conclusion carries important implications for understanding seasonally modulated ENSO dynamics and multiscale climate impacts over East Asia.

Long-Lead Predictions of Warm Season Droughts in South Korea Using North Atlantic SST

AbstractUnderstanding and predicting warm season (May–October) droughts is critically important in South Korea for agricultural productivity and water resource management. Using a 6-month standardized precipitation index ending in October (SPI6_Oct), we investigate the interannual variability of warm season droughts and the related large-scale atmospheric circulations for the most recent 20-yr period (1995–2014). Cyclonic (anticyclonic) circulations to the east of Japan (in the North Pacific) tend to induce warm season droughts (wetness) by suppressing (enhancing) moist water transport from the south of the Korean Peninsula. These circulation patterns to the east of Japan are linked to a barotropic Rossby wave–like teleconnection pattern from the North Atlantic to East Asia, which is found to be responsible for the interannual variability of SPI6_Oct. This teleconnection pattern is highly correlated with the difference in sea surface temperature (SST) between the Norwegian Sea and the Barents Sea (referred to as NA_dipole) in January–March (r = 0.68), which modulates the snow depth over the Ural Mountains in spring and the sea ice concentration over the Barents Sea during the entire warm season. Two drought prediction models, an empirical model and a hybrid machine learning model, are developed and tested for their predictive skills for South Korea. An empirical prediction model using NA_dipole as one of the predictors is found to accurately capture the interannual variability of SPI6_Oct (r2 = 53%). NA_dipole is found to improve the predictive skills of the hybrid machine learning drought prediction model, especially for longer lead times. Our results emphasize the significant role of North Atlantic SST anomalies in warm season medium-range droughts in South Korea.

Structure and dynamics of a springtime atmospheric wave train over the North Atlantic and Eurasia

The leading pattern of boreal spring 250-hPa meridional wind anomalies over the North Atlantic and mid-high latitude Eurasia displays an obvious wave train. The present study documents the structure, energy source, relation to the North Atlantic sea surface temperature (SST), and impacts on Eurasian climate of this wave train during 1948–2018. This atmospheric wave train has a barotropic vertical structure with five major centers of action lying over subtropics and mid-latitudes of the North Atlantic, northern Europe, central Eurasia, and East Asia, respectively. This spring wave train can efficiently extract available potential energy from the basic mean flow. The baroclinic energy conversion process and positive interaction between synoptic-scale eddies and the mean flow both play important roles in generating and maintaining this wave train. The North Atlantic horseshoe-like (NAH) SST anomaly contributes to the persistence of the wave train via a positive air–sea interaction. Specifically, the NAH SST anomaly induces a Rossby wave-type atmospheric response, which in turn maintains the NAH SST anomaly pattern via modulating surface heat fluxes. This spring atmospheric wave train has significant impacts on Eurasian surface air temperature (SAT) and rainfall. During the positive phase of the wave train, pronounced SAT warming appears over central Eurasia and cooling occurs over west Europe and eastern Eurasia. In addition, above-normal rainfall appears over most parts of Europe and around the Lake Baikal, accompanied by below-normal rainfall to east of the Caspian Sea and over central Asia.

Strengthened Relationships of Northwest China Wintertime Precipitation with ENSO and Midlatitude North Atlantic SST since the Mid-1990s

AbstractThe present study investigates the interdecadal changes in the relationship between El Niño–Southern Oscillation (ENSO) and midlatitude North Atlantic (MNA) sea surface temperature (SST) with northwest China (NWC) winter precipitation (WP) variability and the plausible causes. Results show that ENSO and MNA SST have weak correlations with NWC WP before the mid-1990s, whereas the connections are enhanced sharply afterward, with above (below) normal precipitation occuring when there are positive (negative) ENSO SST and negative (positive) MNA SST anomalies (SSTA). Remarkable differences are found in the atmospheric circulations. After the mid-1990s, there is a pronounced Pacific–North American–Eurasian (PNA-EU)-like pattern in the Northern Hemisphere, whereas an Arctic Oscillation–like pattern is found before the mid-1990s. The change in the relationships between NWC WP and SSTs is likely attributable to the enhanced connection between ENSO and MNA SST after the mid-1990s. It is found that ENSO and MNA SSTA can cause NWC WP variation independently through atmospheric teleconnections. In addition, significant precipitation anomalies also occur when concurrent but oppositely signed SSTs anomalies in the two regions are observed. The reinforced negative correlations between ENSO and MNA SST after the mid-1990s act in concert on NWC WP by exciting a PNA-EU-like pattern. This information would help us to better understand the physical processes of the teleconnections between NWC WP variability and the ENSO/MNA SST, in which the strength of the correlation between ENSO and MNA SST should be taken into account.

On the Emergence of the Atlantic Multidecadal SST Signal: A Key Role of the Mixed Layer Depth Variability Driven by North Atlantic Oscillation

AbstractDespite its wide-ranging potential impacts, the exact cause of the Atlantic multidecadal oscillation/variability (AMO/AMV) is far from settled. While the emergence of the AMO sea surface temperature (SST) pattern has been conventionally attributed to the ocean heat transport, a recent study showed that the atmospheric stochastic forcing is sufficient. In this study, we resolve this conundrum by partitioning the multidecadal SST tendency into a part caused by surface heat fluxes and another by ocean dynamics, using a preindustrial control simulation of a state-of-the-art coupled climate model. In the model, horizontal ocean heat advection primarily acts to warm the subpolar SST as in previous studies; however, when the vertical component is also considered, the ocean dynamics overall acts to cool the region. Alternatively, the heat flux term is primarily responsible for the subpolar North Atlantic SST warming, although the associated surface heat flux anomalies are upward as observed. Further decomposition of the heat flux term reveals that it is the mixed layer depth (MLD) deepening that makes the ocean less susceptible for cooling, thus leading to relative warming by increasing the ocean heat capacity. This role of the MLD variability in the AMO signature had not been addressed in previous studies. The MLD variability is primarily induced by the anomalous salinity transport by the Gulf Stream modulated by the multidecadal North Atlantic Oscillation, with turbulent fluxes playing a secondary role. Thus, depending on how we interpret the MLD variability, our results support the two previously suggested frameworks, yet slightly modifying the previous notions.

Identifying a human signal in the North Atlantic warming hole

North Atlantic sea surface temperatures have large climate impacts affecting the weather of the Northern Hemisphere. In addition to a substantial warming over much of the North Atlantic, caused by increasing greenhouse gases over the 21st century, climate projections show a surprising region of considerable future cooling at midlatitudes, referred to as the North Atlantic warming hole. A similar pattern of surface temperature trends has been observed in recent decades, but it remains unclear whether this pattern is of anthropogenic origin or a simple manifestation of internal climate variability. Here, analyzing state-of-the-art climate models and observations, we show that the recent North Atlantic warming hole is of anthropogenic origin. Our analysis reveals that the anthropogenic signal has only recently emerged from the internal climate variability, and can be attributed to greenhouse gas emissions. We further show that a declining northward oceanic heat flux in recent decades, which is linked to this surface temperature pattern, is also of anthropogenic origin.

Reintensification of the Anomalous Western North Pacific Anticyclone during the El Niño Modoki Decaying Summer: Relative Importance of Tropical Atlantic and Pacific SST Anomalies

AbstractThe El Niño Modoki–induced anomalous western North Pacific anticyclone (WNPAC) undergoes an interesting reintensification process in the El Niño Modoki decaying summer, the period when El Niño Modoki decays but warm sea surface temperature (SST) anomalies over the tropical North Atlantic (TNA) and cold SST anomalies over the central-eastern Pacific (CEP) dominate. In this study, the region (TNA or CEP) in which the SST anomalies exert a relatively important influence on reintensification of the WNPAC is investigated. Observational analysis demonstrates that when only anomalous CEP SST cooling occurs, the WNPAC experiences a weak reintensification. In contrast, when only anomalous TNA SST warming emerges, the WNPAC experiences a remarkable reintensification. Numerical simulation analysis demonstrates that even though the same magnitude of CEP SST cooling and TNA warming is respectively set to force the atmospheric general circulation model, the response of the WNPAC is still much stronger in the TNA warming experiment than in the CEP cooling experiment. Further analysis demonstrates that this difference is caused by the distinct location of the effective tropical forcing between the CEP SST cooling and TNA SST warming for producing a WNPAC. The CEP cooling-induced effective anomalous diabatic cooling is located in the central Pacific, by which the forced anticyclone becomes gradually weak from the central Pacific to the western North Pacific. Thus, a weak WNPAC is produced. In contrast, as the TNA SST warming–induced effective anomalous diabatic cooling is just located in the western North Pacific via a Kelvin wave–induced Ekman divergence process, the forced anticyclone is significant and powerful in the western North Pacific.