Atlantic Ocean Sea Surface Temperature Research Articles

Inter‐Basin Versus Intra‐Basin Sea Surface Temperature Forcing of the Western North Pacific Subtropical High's Westward Extensions

AbstractZonal extensions of the Western Pacific subtropical high (WPSH) strongly modulate extreme rainfall activity and tropical cyclone (TC) landfall over the Western North Pacific (WNP) region. These zonal extensions are primarily forced on seasonal timescales by inter‐basin zonal sea surface temperature (SST) gradients. However, despite the presence of large‐scale zonal SST gradients, the WPSH response to SSTs varies from year to year. In this study, we force the atmosphere‐only NCAR Community Earth System Model version 2 simulations with two real‐world SST patterns, both featuring the large‐scale zonal SST gradient characteristic of decaying El Niño‐developing La Niña summers. For each of these patterns, we performed four experimental sets that tested the relative contributions of the tropical Indian Ocean, Pacific, and Atlantic basin SSTs to simulated westward extensions over the WNP during June–August. Our results indicate that the subtle differences between the two SST anomaly patterns belie two different mechanisms forcing the WPSH's westward extensions. In one SST anomaly pattern, extratropical North Pacific SST forcing suppresses the tropical Pacific zonal SST gradient forcing, resulting in tropical Atlantic and Indian Ocean SSTs being the dominant driver. The second SST anomaly pattern drives a similar westward extension as the first pattern, but the underlying SST gradient driving the WPSH points to intra‐basin forcing mechanisms originating in the Pacific. The results of this study have implications for understanding and predicting the impact of the WPSH's zonal variability on tropical cyclones and extreme rainfall over the WNP.

The 2022-23 drought in the South American Altiplano: ENSO effects on moisture flux in the western Amazon during the pre-wet season

The 2022-23 hydrological year in the Lake Titicaca, Desaguadero River, and Lake Poopó hydrological system (TDPS) over the South American Altiplano constituted a historically dry period. This drought was particularly severe during the pre-wet season (October–December), when the TDPS and the adjacent Andean-Amazon region experienced as much as 60% reductions in rainfall. Consequently, Titicaca Lake water levels decreased by 0.05 m from December to January, which is part of the rising lake level period of normal conditions. Such conditions have not been seen since the El Niño-related drought of 1982-83. Using a set of hydroclimatic, Sea Surface Temperature (SST) and atmospheric reanalysis datasets, we find that this new historical drought was associated with enhanced southerly moisture flux anomalies, reducing the inflow of moisture-laden winds from the Amazon basin to the TDPS. Such anomalies in moisture transport were not seen since at least the 1950s. The atmospheric dynamics associated with this drought are related to La Niña SST anomalies via subtropical teleconnections associated with Rossby wave trains towards South America, further extended by subtropical Atlantic Ocean SST anomalies. This feature reduced the atmospheric moisture inflow from the Amazon and weakened the development of the Bolivian High in the upper troposphere. These results document a new atmospheric mechanism related to extreme droughts in the TDPS associated with La Niña SST anomalies during the pre-wet season. This goes beyond the traditional understanding of El Niño events, especially the strongest ones, being associated with dry conditions in the TDPS during the wet season (December–March).

Rapid high-latitude cooling in the southeastern Pacific sector driven by North Atlantic warming during 1979–2013 in CESM1

During the 1979–2013 satellite observation period, the sea surface temperature (SST) has cooled substantially in the high-latitude Southern Ocean, with the most pronounced cooling tendency centered in the southeastern Pacific domain. Previous hypotheses have commonly ascribed the recent Southern Ocean cooling to either the tropical eastern Pacific cooling or North Atlantic and tropical Indian Ocean SST warming. However, the mechanisms underpinning significant cooling in the southeastern Pacific sector remain debatable. By diagnosing three pacemaker experiments with a state-of-the-art global climate model in which SSTs in the North Atlantic, tropical central-eastern Pacific, and tropical Indian Ocean-western Pacific are individually nudged to mimic the observed trajectory, we show that the North Atlantic dominates in the cold SST response of the southeastern Pacific sector during 1979–2013. Anomalous North Atlantic warming initiates a quasi-stationary Rossby wave response to a south-to-north cross-equatorial Hadley circulation strengthening, leading to an enhanced Amundsen Sea Low. As a result, due primarily to the increased low-level marine cloud cover, the net surface shortwave radiation reduction triggers rapid SST cooling in the southeastern Pacific domain. The southeastern Pacific cold SST anomalies are further maintained via the shortwave radiation-low-cloud-SST positive feedback on decadal timescales. Our results suggest that the shortwave radiation-low-cloud SST feedback is fundamental to the observed long-term cooling of the high-latitude Southern Ocean, with profound climate consequences worldwide.

Interdecadal changes in the interannual variations in spring precipitation over the Tarim Basin

AbstractThe year‐to‐year fluctuation and total amount of spring precipitation over the Tarim Basin (TB) (SPTB) have both significantly increased after 1985. The data are then divided into two subperiods, that is, 1961–1985 (P1) and 1986–2020 (P2), and the contributions from various climate factors, including the North Atlantic and Indian Ocean sea surface temperatures (SSTs), as well as from Tibetan Plateau (TP) snow, to the interdecadal changes from P1 to P2 are examined. In P1, the SPTB‐related atmospheric circulation anomalies display a zonal wave‐like pattern dominating the middle–high North Atlantic‐Eurasian continent. In contrast, in P2, the SPTB‐related atmospheric circulation anomalies feature a meridional tripole pattern prevailing over the North Atlantic and western Europe. The observational analysis indicates that abnormally low pressure in the north and high pressure in the southwest of the TB are the key atmospheric systems that influence moisture transportation to the Tarim Basin and consequently impact the variability of the SPTB. The anomalous SSTs in the North Atlantic and Indian Ocean play a significant role in inducing changes in the SPTB by regulating the moisture transport pathway in the vicinity of the TB. In P1, the SSTs in both the North Atlantic and the tropical Indian Ocean contribute to the SPTB mainly in the periphery of the TB, while in P2, they contribute to the SPTB in most of the TB. In addition, the snow cover over TP also contributes to the SPTB changes. Distinguished from the effects of oceanic SST, the TP snow primarily influences the variability of the SPTB within the central region of the TP during P1 and around the periphery of the TP during P2.

What caused the interdecadal shift in the El Niño–Southern Oscillation (ENSO) impact on dust mass concentration over northwestern South Asia?

Abstract. Changes in large-scale circulation, especially El Niño–Southern Oscillation (ENSO), have significant impacts on dust activities over the dust source and downwind regions. However, these impacts present an interdecadal pattern, and it remains less clear which factors lead to the interdecadal variability of the ENSO impact on dust activities over northwestern South Asia, although previous studies have discussed the response of interannual dust activities over northwestern South Asia to the ENSO circle. Based on the linear regression model and MERRA-2 atmospheric aerosol reanalysis data, this study investigated the interdecadal variability of the ENSO impact on dust activities as well as the associated possible atmospheric drivers under two different warming phases over northwestern South Asia. Results indicated that the relationship between ENSO and dust column mass density (DUCMASS) experienced an obvious shift from the accelerated global warming period (1982–1996) to the warming hiatus period (2000–2014). The change in Atlantic and Indian Ocean sea surface temperature anomaly (SSTA) patterns weakened the impact of ENSO on dust activities over northwestern South Asia during 1982–1996, while the change in Pacific Decadal Oscillation (PDO) strengthened ENSO's effect when it was in phase with ENSO. Both the Atlantic and Indian Ocean SSTA patterns were modulated by the duration of ENSO events (i.e., continuing and emerging ENSO). This study provides new insights into numerical simulation involving the influence of atmospheric teleconnections on the variability of dust activities and their influence mechanisms.

Evolving Sahel Rainfall Response to Anthropogenic Aerosols Driven by Shifting Regional Oceanic and Emission Influences

Abstract Sahel summertime precipitation declined from the 1950s to 1970s and recovered from the 1970s to 2000s. Anthropogenic aerosol contributions to this evolution are typically attributed to interhemispheric gradient changes of Atlantic Ocean sea surface temperature (SST). However recent work by Hirasawa et al. indicates a more complex picture, with the response being a combination of “fast” direct atmospheric (DA) processes and “slow” ocean-mediated (OM) processes. Here, we extend this understanding using the Community Atmosphere Model 5 to determine the role of regional ocean-basin perturbations and regional aerosol emission changes in the overall aerosol-driven OM and DA responses, respectively. From the 1950s to 1970s, there was an OM Sahel wetting response due to Pacific Ocean cooling that was offset by drying due to Atlantic cooling. By contrast, from the 1970s to 2000s, Atlantic trends reversed and amplified the Pacific cooling-induced wetting. This wetting was partially offset by drying driven by Indian Ocean cooling. Thus, the OM Sahel precipitation response to aerosol crucially depends on the balance of responses to Atlantic, Pacific, and Indian Ocean SST anomalies. From the 1950s to 1970s, there is DA Sahel drying that was principally due to North American aerosol emissions, with negligible effect from European emissions. DA drying from the 1970s to 2000s was mainly due to African aerosol emissions. Thus, the shifting roles of regional OM and DA effects reveal a complex interplay of direct driving and remote teleconnections in determining the time evolution of Sahel precipitation due to aerosol forcing in the late twentieth century. Significance Statement Studies of global climate models consistently indicate that anthropogenic aerosol emissions were a significant contributor to a severe drought that occurred in the Sahel region of Africa in the late twentieth century. The drying influence of aerosol forcing is the combined result of rapid atmospheric responses directly due to the forcing and slower responses due to forced ocean temperature changes. Using a set of simulations targeted at determining the influences from different ocean basins and different emission regions for two periods in the late twentieth century, we find there is a surprising range of mechanisms through which aerosol emissions affect the Sahel. This results in a complex interplay of at times competing and at times complementary regional influences.

Stratospheric temperature features over Saudi Arabia and their relationship to Atlantic SSTs and surface temperatures in winter

Stratospheric temperature is an important climatic factor regionally and globally. This paper investigates temperature trends in the lower stratosphere at 50 hPa (T50), the mid-stratosphere at 30 hPa (T30), and the upper stratosphere at 10 hPa (T10), as well as their impacts on Atlantic Ocean sea surface temperature (SST) and Saudi Arabian surface air temperature (SAT) during the entire winter seasons of 1979-2019. The results show significant cooling for the T50 linear trend, progressive cooling for the T30 linear trend, and cooling for the T10 linear trend during the study period over Saudi Arabia. The results also indicate a significant nonlinear cooling trend for stratospheric temperature at T50 and T30, while a weak cooling at T10 is observed. Abrupt climatic changes towards warmth exist at all three levels of stratospheric temperature, which occur in 1992 for T50 and T30 and in 1983 for T10. These abrupt climate changes may be related to volcanic eruptions. Our results also indicate that a strong negative relationship exists between T50 and the SST of the tropical South Atlantic (TSA) and the Atlantic Multi-Decadal Oscillation (AMO), while T30 indicates a statistically negative relationship with the AMO. The lead-lag cross-correlation suggests that the SST of the Atlantic Ocean (tropical North Atlantic [TNA], TSA, and AMO) are linked to stratospheric temperatures at three lead winters. As a result of the teleconnection between SAT and stratospheric temperature over Saudi Arabia, the coupling of these two features occurs in winter, especially in the lower to mid-stratosphere layers.

Space–Time Variability of Summer Hydroclimate in the U.S. Prairie Pothole Region

Abstract The Prairie Pothole Region (PPR) experiences considerable space–time variability in temperature and precipitation, and this variability is expected to increase. The PPR is sensitive to this variability—it plays a large role in the water availability of the region. Thousands of wetlands in the region, sometimes containing ponds, provide habitats and breeding grounds for various species. Many wildlife management decisions are planned and executed on subseasonal-to-seasonal time scales and would benefit from knowledge of seasonal conditions at longer lead times. Therefore, it is important to understand potential driving mechanisms and teleconnections behind space–time climate variability in the PPR. We performed principal component analysis on summer precipitation of the southeastern PPR (SEPPR) to determine the leading principal components (PCs) of variability. These PCs were used to establish teleconnections to large-scale climate variables and indices. They were also used to determine potential mechanisms driving the precipitation variability. There were teleconnections to Pacific and Atlantic Ocean sea surface temperatures (SST) resembling the Pacific decadal oscillation and El Niño–Southern Oscillation, low 500-hPa heights over the western United States, and the Palmer drought severity index over the SEPPR. A large-scale low pressure region over the northwestern United States and a pattern like the Great Plains low-level jet, observed in the 500- and 850-hPa heights and winds, are a potential mechanism of the precipitation variability by increasing precipitation during wet PC1 years. These findings can inform management actions to maintain and restore wildlife habitat and the resources used for those actions in the PPR.

Holocene hydroclimate changes on the north‐eastern Tibetan Plateau inferred from geochemical records in Lake Gyaring

ABSTRACTThe north‐eastern Tibetan Plateau (TP) is an ideal region for understanding the interaction between the Asian Summer Monsoon (ASM) and the Westerlies. Here we present a high‐resolution elemental record from Lake Gyaring in the source region of the Yellow River to investigate the paleo‐hydroclimate changes over the past ~10 ka. Our results show that lithogenic elements (Ti, Rb, Mn, Ti, Zn and Ni) increased while carbonate‐related elements (Ca, Sr and Mg) decreased up the core sediments, indicating that the effective moisture on the north‐eastern TP has steadily increased throughout the Holocene. This inference is also supported by the gradual augmentation and diminution of Rb/Sr and Ca/(Ti + Fe + Al) ratios, respectively. The persistent wetting trend on the north‐eastern TP inferred from our study disagrees with the Holocene pattern of the ASM but corresponds well with that of the Westerlies. This implies that the strengthening Westerlies since the Holocene might have played a more important role than monsoon circulation in controlling the hydroclimate changes on the north‐eastern TP. However, the significant increase in the mid‐Holocene might be related to the combined effects of the strengthened ASM and Westerlies. Ultimately, Northern Hemisphere solar insolation and the substantial remnant of ice sheets might have controlled Holocene moisture availability on the north‐eastern TP through their influence on the intensity of the Westerlies and ASM as well as Atlantic Ocean sea surface temperatures.

Teleconnections between Rainfall in Equatorial Africa and Tropical Sea-Surface Temperatures: A Focus on Western Uganda

AbstractSubstantial research on the teleconnections between rainfall and sea-surface temperatures (SSTs) has been conducted across equatorial Africa as a whole, but currently no focused examination exists for western Uganda, a rainfall transition zone between eastern equatorial Africa (EEA) and central equatorial Africa (CEA). This study examines correlations between satellite-based rainfall totals in western Uganda and SSTs – and associated indices – across the tropics over 1983-2019. It is found that rainfall throughout western Uganda is teleconnected to SSTs in all tropical oceans, but much more strongly to SSTs in the Indian and Pacific Oceans than the Atlantic Ocean. Increased Indian Ocean SSTs during boreal winter, spring, and autumn and a pattern similar to a positive Indian Ocean Dipole during boreal summer are associated with increased rainfall in western Uganda. The most spatially complex teleconnections in western Uganda occur during September-December, with northwestern Uganda being similar to EEA during this period and southwestern Uganda being similar to CEA. During boreal autumn and winter, northwestern Uganda has increased rainfall associated with SST patterns resembling a positive Indian Ocean Dipole or El Niño. Southwestern Uganda does not have those teleconnections; in fact, increased rainfall there tends to be more associated with La Niña-like SST patterns. Tropical Atlantic Ocean SSTs also appear to influence rainfall in southwestern Uganda in boreal winter as well as in boreal summer. Overall, western Uganda is a heterogeneous region with respect to rainfall-SST teleconnections; therefore, southwestern Uganda and northwestern Uganda require separate analyses and forecasts, especially during boreal autumn and winter.

Eritrean central‐highland precipitation and associations with sea‐surface temperature and atmospheric circulation

AbstractThis study explores the teleconnection between the boreal summer precipitation for the central highlands of Eritrea, with respect to global sea‐surface temperature (SST) and atmospheric parameters (winds, pressure, humidity, and vertical velocity) as indirect indicators of atmospheric circulation. The central‐highland summer (Jul–Aug) precipitation (1914–2015) is significantly negatively correlated with the Atlantic Niño (ATL3) index. Similarly, with a 2‐month (May) or 1‐month lead (Jun), SSTs over the equatorial Atlantic (Gulf of Guinea) are inversely correlated with the central‐highland summer precipitation. Moreover, the regional atmospheric circulation reveals that the boreal summer precipitation series has a significant positive correlation with the westerly wind over the Gulf of Guinea at 700 hPa and a negative correlation with the Tropical Easterly Jet (TEJ) at 200 hPa. Overall, these findings depict the African Walker circulation cell, whereby a low‐level westerly wind flows from the Gulf of Guinea across central Africa, coupled with a rising branch over north‐eastern Africa, and reverses as an easterly wind at a higher level. These significant correlations between Atlantic Ocean SST as the precipitation‐driving factor enabled the development and validation of an area‐specific seasonal precipitation prediction model, as precipitation in this region is temporally and spatially highly variable.

Holocene vegetation and hydrology variations and their associations with climate changes: a multi-proxy analysis of a sediment core from an alpine basin in the middle Tianshan Mountains

We in this paper present a multi-proxy study of a 186-cm-long lacustrine sediment core (i.e., BY Core) obtained from the Youerdusi Basin in the middle Tianshan Mountains in an attempt to disentangle the causal associations among climate changes, ecological responses and hydrological variations. Palynological, assemblages and the associated biome scores indicate that the upper limit of the forest zone was closer to the Youerdusi Basin with a larger wetland extent in the Youerdusi Basin during the early part (i.e., from ~ 10.5 to ~ 5.8 cal. kyr BP) of the data-covering period (i.e., from ~ 10.5 to ~ 2.4 cal. kyr BP). The upper limit of the forest zone was farther away from the Youerdusi Basin with a smaller wetland extent in the Youerdusi Basin during the late part (i.e., from ~ 5.8 to ~ 2.4 cal. kyr BP). The changes of taiga biome score and the AP% ratio indicate a persistent cooling trend during the data-covering period which is well corroborated by various proxy data from nearby sites and the cooling trend is chronologically consistent with the trend of Northern Hemisphere temperature anomalies that was interpreted to be a delayed response to the summer solar irradiance at 50° N. The tundra biome score and the associated percentages of Cyperaceae in the Youerdusi Basin show two major stages of wetland extent variation. The wetland extent variation in the Youerdusi Basin might have been controlled by precipitation and/or by glacier-melting potential that was in turn controlled by both temperature and the extent of remained ice cover in higher elevations. The δ13Corg values obtained from the BY Core indicate a wetting trend during the data-covering period which is reasonably corroborated by various proxy data from nearby sites. We infer that the wetting trend was causally associated with the Holocene increasing trend of the North Atlantic Ocean SST (sea surface temperature) and also with the Holocene increasing trend of the westerlies’ intensity.

U.S. Tropical Cyclone Activity in the 2030s Based on Projected Changes in Tropical Sea Surface Temperature

AbstractWe use a statistical tropical cyclone (TC) model, the North Atlantic Stochastic Hurricane Model (NASHM), in combination with sea surface temperature (SST) projections from climate models, to estimate regional changes in U.S. TC activity into the 2030s. NASHM is trained on historical variations in TC characteristics with two SST indices: global–tropical mean SST and the difference between tropical North Atlantic Ocean (NA) SST and the rest of the global tropics, often referred to as “relative SST.” Testing confirms the model’s ability to reproduce historical U.S. TC activity as well as to make skillful predictions. When NASHM is driven by SST projections into the 2030s, overall NA annual TC counts increase, and the fractional increase is the greatest at the highest wind intensities. However, an eastward anomaly in mean TC tracks and an eastward shift in TC formation region result in a geographically varied signal in U.S. coastal activity. Florida’s Gulf Coast is projected to see significant increases in TC activity relative to the long-term historical mean, and these increases are fractionally greatest at the highest intensities. By contrast, the northwestern U.S. Gulf Coast and the U.S. East Coast will see little change.

Mechanisms associated with September to November (SON) rainfall over Uganda during the recent decades

This study revisits teleconnections associated with the anomalous events of September to November (SON) rainfall over Uganda during 1981-2019, owing to the recent intensification of extreme events. Empirical Orthogonal Function (EOF), Composite and Correlation analysis are employed to examine the variability of SON rainfall over the study domain and associated circulations anomalies. The first EOF mode (dominant mode) displays a positive monopole pattern and explains 67.2% of the variance. The results revealed that SON rainfall is largely influenced by a Walker circulation mode over the Indian Ocean, whereby, wet events are associated with an ascending limb of the Walker circulation on the western part of the Indian Ocean characterized by convergence at low levels and divergence at upper level. The study showed that SON rainfall is positively (negatively) correlated with Indian Ocean (Atlantic Ocean) sea surface temperatures (SST). Furthermore, Indian Ocean Dipole (IOD) events have impact on SON rainfall with strong positive correlation, whereas Southern Oscillation Index (SOI) revealed negative correlation. The results also reveal that there is a lag in ENSO and IOD episodes during wet/dry events over the region. ENSO and IOD also tend to extend the rainfall season of SON and thus study of extreme events may not be well captured by studies focusing on SON. Future studies might consider the season of October to December or December to February. These phenomena need to be closely monitored and considered when making seasonal forecasts.

Dual response of Arabian Sea cyclones and strength of Indian monsoon to Southern Atlantic Ocean

Variability and trends of the south Asian monsoon at different time scales makes the region susceptible to climate-related natural disasters such as droughts and floods. Because of its importance, different studies have examined the climatic factors responsible for the recent changes in monsoon strength. Here, using observations and climate model experiments we show that monsoon strength is driven by the variations of south Atlantic Ocean sea surface temperature (SASST). The mechanism by which SASST is modulating the monsoon could be explained through the classical Matsuno-Gill response, leading to changes in the characteristics of vertical wind shear in the Arabian Sea. The decline in the vertical wind shear to the warming of SASST is associated with anomalous lower (upper)-level easterlies (westerlies). This further leads to a strong increase in the frequency of the Arabian Sea cyclones; and also prohibits the transport of moisture to the Indian landmass, which eventually reduces the strength of monsoon. The conditions in the SASST which drove these responses are aggravated by greenhouse gas emission, revealing the prominent role played by anthropogenic warming. If, with proper mitigation, these emissions are not prevented, further increases in the SASST is expected to result in increased Arabian sea cyclones and reduced monsoon strength.

Role of the Atlantic multidecadal variability in modulating East Asian climate

We assess the effects of the North Atlantic Ocean Sea Surface Temperature (NASST) on North East Asian (NEA) surface temperature. We use a set of sensitivity experiments, performed with MetUM-GOML2, an atmospheric general circulation model coupled to a multi-level ocean mixed layer model, to mimic warming and cooling over the North Atlantic Ocean. Results show that a warming of the NASST is associated with a significant warming over NEA. Two mechanisms are pointed out to explain the NASST—North East Asia surface temperature relationship. First, the warming of the NASST is associated with a modulation of the northern hemisphere circulation, due to the propagation of a Rossby wave (i.e. the circumglobal teleconnection). The change in the atmosphere circulation is associated with advections of heat from the Pacific Ocean to NEA and with an increase in net surface shortwave radiation over NEA, both acting to increase NEA surface temperature. Second, the warming of the NASST is associated with a cooling (warming) over the eastern (western) Pacific Ocean, which modulates the circulation over the western Pacific Ocean and NEA. Additional simulations, in which Pacific Ocean sea surface temperatures are kept constant, show that the modulation of the circumglobal teleconnection is key to explaining impacts of the NASST on NEA surface temperature.

Dynamic Origin of the Interannual Variability of West China Autumn Rainfall

AbstractThe dynamic origin of the interannual variability of West China autumn rainfall (WCAR), a special weather/climate phenomenon over western-central China in September and October, was investigated via observational diagnosis and numerical simulations. Here we found that the interannual variability of WCAR is closely related to the local horizontal trough, which is passively induced by two lower-level anticyclonic (high pressure) anomalies over East Asia. The anticyclonic anomaly over the south is a Gill-type response to the central and eastern Pacific diabatic cooling, while that over the north is part of the mid- to high-latitude barotropic Rossby wave train, which could be induced by either the thermal forcing of the central and eastern Pacific Ocean sea surface temperature (SST) cooling or that of the subtropical northern Atlantic Ocean SST warming. The quasi-barotropic high pressure anomaly over East Asia acts as an “invisible mountain” that steers the low-level anomalous southwesterly into a southeasterly and hinders the water vapor going farther to the north, leading to enhanced WCAR. However, the real mountain ranges in the region (the Qinglin and Ba Mountains) have no essential impact on the formation and interannual variability of WCAR.

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.

Interdecadal Change in the Relationship of the Western North Pacific Tropical Cyclogenesis Frequency to Tropical Indian and North Atlantic Ocean SST in Early 1990s

AbstractThe present study investigates reasons of interdecadal changes in the relationship between the western North Pacific (WNP) tropical cyclone (TC) genesis and tropical Indian Ocean (TIO) and tropical North Atlantic Ocean (TNA) sea surface temperature (SST) in early‐1990s. It is found that the above interdecadal changes are related to changes in the coherence of TIO and TNA SST variations with equatorial central‐eastern Pacific Ocean (EPO) SST variations. During 1970s through mid‐1980s, TIO and TNA SSTs tend to vary in phase with EPO SST on both interannual and interdecadal time scales. The combined effects of TIO and EPO SST anomalies induce opposite environment and TC changes in southeastern and northwestern WNP, leading to a weak correlation of basin‐wide WNP TC and TIO SST. Similarly, TNA and EPO SST anomalies induce opposite environment and TC changes in northeastern and southeastern WNP and thus basin‐wide WNP TC‐TNA SST correlation is weak. During late 1990s through early 2000s, TIO and EPO SST variations have a weak coherence due to their opposite relations on interannual and interdecadal time scales. TIO SST anomalies alone induce anomalous circulation over most WNP so that basin‐wide WNP TC‐TIO SST relationship is strong. The interannual and interdecadal SST variations remain coherent in TNA and EPO but with a switch of sign in their relationship in early 1990s. TNA and EPO SST anomalies induce same environment and TC changes in northeastern and southeastern WNP, and thus, basin‐wide WNP TC‐TNA SST relationship is strong. Numerical model experiments validate the importance of coherent regional SST anomalies.

Interannual and interdecadal impact of Western North Pacific Subtropical High on tropical cyclone activity

In this study, we analyzed the impacts of Western North Pacific Subtropical High (WNPSH) on tropical cyclone (TC) activity on both interannual and interdecadal timescales. Based on a clustering analysis method, we grouped TCs in the Western North Pacific into three clusters according to their track patterns. We mainly focus on Cluster 1 (C1) TCs in this work, which is characterized by forming north of 15° N and moving northward. On interannual timescale, the number of C1 TCs is influenced by the intensity variability of the WNPSH, which is represented by the first Empirical Orthogonal Function (EOF) of 850 hPa geopotential height of the region. The WNPSH itself is modulated by the El Niño–Southern Oscillation at its peak phase in the previous winter, as well as Indian and Atlantic Ocean sea surface temperature anomalies in following seasons. The second EOF mode shows the interdecadal change of WNPSH intensity. The interdecadal variability of WNPSH intensity related to the Pacific climate regime shift could cause anomalies of the steering flow, and lead to the longitudinal shift of C1 TC track. Negative phases of interdecadal Pacific oscillation are associated with easterly anomaly of steering flow, westward shift of C1 TC track, and large TC impact on the East Asia coastal area.