Negative Sea Surface Temperature Anomalies Research Articles

Dynamic Control of the Dominant Modes of Interannual Variability of Snowfall Frequency in China

AbstractThis study investigates the first two leading modes of the interannual variability of frequency of snowfall events (FSE) over China in the winter during 1986–2018. The positive phase of the first leading mode (EOF1) is mainly characterized by positive FSE anomalies in northeastern–northwestern China and negative FSE anomalies in the three-river-source region. In contrast, the positive phase of the second leading mode (EOF2) is mainly characterized by positive FSE anomalies in central-eastern China (CEC). EOF1 is affected by the synoptic-scale wave activity over the midlatitudes of the East Asian continent, where active synoptic-scale wave activity over the midlatitudes may cause increased FSE over northeastern–northwestern China, and vice versa. In a winter of a negative phase of the North Atlantic Oscillation, an anomalous deep cold low may occur over Siberia, which may induce increased meridional air temperature gradient, increased atmospheric baroclinicity, and hence increased FSE over the midlatitudes of the East Asian continent. The EOF2 is affected by the interaction between anomalous northerly cold advection and anomalous southerly water vapor transport over CEC. The positive phase of EOF2 is associated with negative sea ice anomalies in the Barents Sea–Kara Sea region and negative sea surface temperature anomalies in the central-eastern tropical Pacific. Reduced sea ice in the Barents Sea–Kara Sea during January–February may cause increased northerly cold advection over CEC, while a La Niña–like condition during January may induce southerly water vapor transport anomalies over CEC.

Which Features of the SST Forcing Error Most Likely Disturb the Simulated Intensity of Tropical Cyclones?

Among all of the sources of tropical cyclone (TC) intensity forecast errors, the uncertainty of sea surface temperature (SST) has been shown to play a significant role. In the present study, we determine the SST forcing error that causes the largest simulation error of TC intensity during the entire simulation period by using the WRF model with time-dependent SST forcing. The SST forcing error is represented through the application of a nonlinear forcing singular vector (NFSV) structure. For the selected 12 TC cases, the NFSV-type SST forcing errors have a nearly coherent structure with positive (or negative) SST anomalies located along the track of TCs but are especially concentrated in a particular region. This particular region tends to occur during the specific period of the TCs life cycle when the TCs present relatively strong intensity, but are still intensifying just prior to the mature phase, especially within a TC state exhibiting a strong secondary circulation and very high inertial stability. The SST forcing errors located along the TC track during this time period are verified to have the strongest disturbing effect on TC intensity simulation. Physically, the strong inertial stability of TCs during this time period induces a strong response of the secondary circulation from diabatic heating errors induced by the SST forcing error. Consequently, this significantly influences the subsidence within the warm core in the eye region, which, in turn, leads to significant errors in TC intensity. This physical mechanism explains the formation of NSFV-type SST forcing errors. According to the sensitivity of the NFSV-type SST forcing errors, if one increases the density of SST observations along the TC track and assimilates them to the SST forcing field, the skill of TC intensity simulation generated by the WRF model could be greatly improved. However, this adjustment is most advantageous in improving simulation skill during the time period when TCs become strong but are still intensifying just prior to reaching full maturity. In light of this, the region along the TC track but in the time period of TC movement when the NFSV-type SST forcing errors occur may represent the sensitive area for targeting observation for SST forcing field associated with TC intensity simulation.

Synergistic effect of SST anomalies in the North Pacific and North Atlantic on summer surface air temperature over the Mongolian Plateau

The impact of sea surface temperature anomalies (SSTAs) over the North Pacific (32°–45° N, 140° E–150° W) and North Atlantic (north: 52°–68° N, 60°–20° W; south: 0°–30° N, 100°–40° W) on the summer surface air temperature (SAT) over the Mongolian Plateau (MP) is studied using NCEP/NCAR and Climatic Research Unit (CRU) reanalysis data. The results show that the circumglobal teleconnection (CGT) wave train related to the SSTAs in the North Atlantic propagates along with the westerly jet, resulting in a geopotential height anomaly at 200 hPa that favors an SAT anomalies (SATAs) on the MP. In addition, an anticyclonic (cyclonic) circulation anomaly with the characteristics of an atmospheric ultralong wave at 200 hPa over the Eurasia–Pacific region associated with the SSTAs in the North Pacific is also responsible for the positive (negative) SATAs on the MP. The results further reveal that the SSTAs in the North Pacific and the northern North Atlantic act synergistically on the SATAs on the MP. If there are strongly positive (negative) SSTAs in the North Pacific and the northern North Atlantic at the same time, the probability of a strongly positive (negative) SATAs on the MP is 67% (100%) for the research period of this paper. However, if the strongly positive (negative) SSTAs only appear in the North Pacific or the northern North Atlantic, the probability of a strongly positive (negative) SATAs on the MP is less than 34% (31%). The synergism of the SSTAs in the North Pacific and the northern North Atlantic enhances Rossby wave energy in Eurasia, with more Rossby wave energy spreading to the MP that in turn causes the circulation anomaly on the MP. Meanwhile, baroclinic conditions at 850 hPa north of the North Pacific, the northern and southern North Atlantic favor a northward shift of the westerly jet that guides the Rossby wave downstream so that the circulation anomaly that produces the SATAs on the MP is located in the mid–high latitudes.

Predictability of the Western North Pacific Subtropical High Associated with Different ENSO Phases in GloSea5

The western North Pacific subtropical high (WNPSH) dominates the summer climate over East Asia. The intensity, position, and shape of WNPSH influence the spatiotemporal distributions of precipitation, temperature, and tropical cyclone activities in this region. This paper intends to investigate the performance of the UK Met Office Global Seasonal forecast system version 5 (GloSea5) in simulation/prediction of the WNPSH based on a hindcast dataset. Analyses of the hindcast data show a systematic bias in the mean circulation over West Pacific, with negative geopotential height anomalies over the western North Pacific (WNP) and cyclonic anomalies in the 850-hPa winds and water vapor transport, indicating a weakening and eastward shift of the WNPSH. Despite the model’s bias in the climatology, it well captured the interannual variability of the monthly and seasonal-mean intensity of the WNPSH and the position of its ridge line in boreal summer from 1993 to 2015. The seasonal hindcasts indicate that there is significant prediction skill at up to three-month lead time for both the intensity and position of the WNPSH ridge line. The relationship between the WNPSH and different phases of the El Niño-Southern Oscillation (ENSO) in both the observational data and GloSea5 hindcasts was then investigated. The model captured the summer WNPSH anomalies well during most of the ENSO phases, except in the La Niña decaying and neutral summers. The intensity of the anticyclone in the WNP is weak in the decaying phase of El Niño in the GloSea5 hindcasts compared with the reanalysis data. GloSea5 is capable of representing the lagged teleconnection between El Niño events in the previous winter and the intensity of the WNPSH in the following summer. Regression analysis reveals weakened negative sea surface temperature anomalies (SSTAs) over the WNP in GloSea5, which reduced the gradient between the tropical western Pacific and the tropical Indian Ocean, resulting in a weaker easterly anomaly and stronger westerly anomaly, contributing to the weak anomalous anticyclone over the WNP and the weakened WNPSH relative to the reanalysis data.

Anticyclonic Eddies Enhance the Winter Barrier Layer and Surface Cooling in the Bay of Bengal

AbstractOceanic barrier layer impedes the vertical exchanges of heat and biogeochemical substances between the mixed layer and deep ocean, playing essential roles in modulating air‐sea heat flux and oceanic biogeochemical cycles. Recent studies revealed that mesoscale eddies can modify the vertical motions of the mixed layer. But their influence on the barrier layer has not been determined. By analyzing more than a decade of in situ, satellite, and reanalysis data, this study investigated the influence of eddies on barrier layer in the Bay of Bengal. The results show that the barrier layer in anticyclonic eddies is significantly thickened in winter (by 50% on average). This thickening consists of the deepening of isothermal layer by eddy downwelling and the shoaling of mixed layer by near‐surface salinity restratification. The thickening of the barrier layer enlarges with latitude, in consistent with the reinforcement in salinity stratification and surface cooling. The thickened barrier layer impedes the upward supply of heat into the mixed layer from the below, promoting the winter surface cooling (by 0.7°C on average) and subsurface temperature inversion (by 0.6°C on average). The feature is contrary to the intuitive expectation that anticyclonic eddies possess high sea surface temperature in eddy interiors. As a consequence, both cyclonic and anticyclonic eddies result in negative sea surface temperature anomalies, indicative of a net decrease in the local sea surface temperature. These effects of eddies may be a common feature in salinity‐stratified oceans in winter and have potential implications for the local biogeochemistry and air‐sea coupled models.

Persistence and Nonpersistence of East and Southeast Asian Rainfall Anomaly Pattern From Spring to Summer

AbstractThe present study analyzes the connection of rainfall anomalies over East and Southeast Asia between spring and summer. Spatial patterns of contrasting rainfall anomalies between East Asia and the Indochina Peninsula are identified in both spring and summer. The persistence of more rainfall over East Asia and less rainfall over the Indochina Peninsula from spring to summer is associated with the maintenance of an anomalous low‐level anticyclone over the tropical western North Pacific. Further analysis shows that this anomalous anticyclone is attributed to the combined effects of persisting positive sea surface temperature anomalies (SSTAs) in the tropical Indian Ocean and negative SSTAs in the subtropical western Pacific. In contrast, in the nonpersistent rainfall anomaly years, the anomalous anticyclone over the western North Pacific displaces southward from spring to summer. One factor accounting for such a change is the dissipation of the negative SSTAs in the subtropical western Pacific from spring to summer, which weakens the SST forcing on the western North Pacific anticyclone. The other factor is an atmospheric wave train associated with the negative SSTAs in the eastern North Atlantic Ocean. The atmospheric wave train propagates eastward across the Eurasian continent and reaches East Asia. The wave pattern contributes to the development and southward displacement of an anomalous cyclone over East Asia, which causes a southward shift in the western North Pacific anticyclone. The effect of the North Atlantic negative SSTAs on the atmospheric wave pattern is supported by numerical experiments.

Indian Ocean tripole mode and its associated atmospheric and oceanic processes

Differing from the Indian Ocean dipole (IOD) that has sea surface temperature anomalies (SSTAs) of opposing signs over the tropical southeastern and western Indian Ocean, a tripole pattern, characterized by positive (negative) SSTAs over the tropical central (southeastern and western) Indian Ocean, is observed and named the Indian Ocean tripole (IOT). This study proposes the concept of the IOT and further investigates the associated atmospheric and oceanic processes. Using empirical orthogonal function (EOF) analysis, the IOT (IOD) is represented by the third (second) leading mode of the monthly SSTAs in the tropical Indian Ocean, explaining about 8.2% (10.3%) of the total variance. The IOT peaks in boreal summer, while the IOD has its mature phase in boreal fall. The spatio-temporal differences, together with the significant separation of these two EOF patterns, illustrate that the IOT is independent of the IOD. Heat budget diagnoses indicate that the anomalous temperature over the southeastern and western Indian Ocean is mainly generated by the net heat flux during April–May and by the total ocean advection in June–August. In contrast, the anomalous temperature is mainly driven by the advection of the mean temperature by the anomalous current in April and the residual term in May–June over the central Indian Ocean, which is replaced by advection of the anomalous temperature by the mean zonal current in July.

Melting of Perennial Sea Ice in the Beaufort Sea Enhanced Its Impacts on Early-Winter Haze Pollution in North China after the Mid-1990s

AbstractIn recent years, haze pollution has become the most concerning environmental issue in China due to its tremendous negative effects. In this study, we focus on the enhanced responses of December–January haze days in North China to September–October sea ice in the Beaufort Sea during 1998–2015. Via both observation and numerical approaches, compared with an earlier period (1980–97), the sea ice concentration in the Beaufort Sea presented large variability during 1998–2015. During 1980–97, the Beaufort Sea was mainly covered by perennial ice, and the ablation and freezing of sea ice mainly occurred at the south edge of the Beaufort Sea. Thus, heavy sea ice in autumn induced negative sea surface temperature anomalies across the Gulf of Alaska in November. However, the colder sea surface in the Gulf of Alaska only induced a weak influence on the haze-associated atmospheric circulations. In contrast, during 1998–2015, a drastic change in sea ice existed near the center of the Arctic Ocean, due to the massive melting of multiyear sea ice in the western Beaufort Sea. The perennial ice cover in the western Beaufort Sea was replaced by seasonal ice. The broader sea ice cover resulted in positive sea surface temperature anomalies in the following November. Then, suitable atmospheric backgrounds were induced for haze pollution in December and January. Simultaneously, the response of the number of haze days over North China to sea ice cover increased. These findings were verified by the CESM-LE simulations and aided in deepening the understanding of the cause of haze pollution.

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.

Generalized Synchronization Between ENSO and Hydrological Variables in Colombia: A Recurrence Quantification Approach

We use Recurrence Quantification Analysis (RQA) to study features of Generalized Synchronization (GS) between El Niño-Southern Oscillation (ENSO) and monthly hydrological anomalies (HyAns) of rainfall and streamflows in Colombia. To that end, we check the sensitivity of the RQA concerning diverse HyAns estimation methods, which constitutes a fundamental procedure for any climatological analysis at inter-annual timescales. In general, the GS and its sensitivity to HyAns methods are quantified by means of time-lagged joint recurrence analysis. Then, we link the GS results with the dynamics of major physical mechanisms that modulate Colombia's hydroclimatology, including the Caribbean, the CHOCO and the Orinoco Low-Level Jets (LLJs), and the Cross-Equatorial Flow (CEF) over northwestern Amazonia (southern Colombia). Our findings show that RQA exhibits significant differences depending on the HyAns methods. GS results are similar for the HyAns methods with variable annual cycle but the time-lags seem to be sensitive. On the other hand, our results make evident that HyAns in the Pacific, Caribbean and Andean regions of Colombia exhibit strong (weak) GS with the ENSO signal during La Niña (El Niño), when hydrological anomalies are positive (negative). Results from the GS analysis allow us to identify spatial patterns of non-linear dependence between ENSO and the Colombian's climatology. The mentioned moisture transport sources constitute the interdependence mechanism and contribute to explain hydrological anomalies in Colombia during the phases of ENSO. During La Niña (El Niño), GS is strong (weak) for the Caribbean and the CHOCO LLJs. Moreover, moisture advection by the Caribbean and CHOCO LLJs exhibit synchrony with HyAns at 0 to 2 (2 to 4) month-lags over north-western Colombia. Furthermore, our results suggest a strong (weak) GS between negative (positive) Sea Surface Temperatures (SST) anomalies in the Eastern Pacific and rainfall anomalies in Colombia. In contrast, GS is strong (weak) for positive (negative) SST anomalies in the Central Pacific. Our GS results contribute to advance our understanding on the regional effects of ENSO in Colombia, whose socio-economical, environmental and ecological impacts cannot be overstated. This work provides a novel approach that reveals new insights into the impact of ENSO on northern South America.

Connections between Spring Arctic Ozone and the Summer Circulation and Sea Surface Temperatures over the Western North Pacific

AbstractUsing various observations, reanalysis datasets, and a general circulation model (CESM-WACCM4), the relationship between the Arctic total column ozone (TCO) and the tropospheric circulation and sea surface temperatures (SSTs) over the western North Pacific (30°–45°N, 130°E–170°W) was investigated. We find that anomalies in the circulation and SSTs over the western North Pacific in June are closely related to anomalies in the Arctic TCO in March; that is, when the Arctic TCO in March decreases, the anomalous tropospheric cyclone and negative SST anomalies (SSTAs) will occur over the western North Pacific in June. Further analysis indicates that the decreased Arctic TCO in March tends to result in positive Victoria mode (VM)-like SSTAs over the North Pacific in April, which persist and develop an anomalous cyclone over the eastern North Pacific in May via atmosphere–ocean coupling. This anomalous cyclone over the eastern North Pacific subsequently induces an anomalous cyclone over the western North Pacific in June via westward-propagating Rossby waves in the lower troposphere. Furthermore, the negative SSTAs over the western North Pacific are enhanced by the anomalous northerly wind related to the anomalous cyclone in June. The effects of increased Arctic TCO in March on the tropospheric circulation and SSTs are almost opposite to those of decreased Arctic TCO. These results are also supported by our numerical simulations. Moreover, 10%–20% of the anomalies in the tropospheric circulation and SSTs over the western North Pacific in June are affected by the anomalies in the Arctic TCO in March.

Assessment of PMIP3 model simulations of megadroughts over the eastern China during the last millennium

AbstractThe simulated spatiotemporal characteristics and variability of decadal megadroughts over the eastern China during the last millennium of 7 model simulations from the Paleoclimate Modelling Intercomparison Project Phase 3 (PMIP3) archive were assessed, and the corresponding mechanisms of the megadroughts were also investigated. The simulated megadrought features of frequencies, durations, magnitudes and spatial distributions were compared with reconstructed data. Comparison showed that climate models can reasonably reproduce the major features of megadroughts, however, the simulated megadroughts are not temporal synchronous with the reconstructed megadroughts, indicating that the internal variability rather than external forcing initiates the observed megadroughts. The simulated spatial distributions of wind and sea level pressure (SLP) anomalies during megadrought years showed that megadroughts over the eastern China corresponded to weakening of the East Asia Summer Monsoon (EASM) and strengthening of the East Asia Winter Monsoon (EAWM). A pattern of sea surface temperature anomalies (SSTA) with significant negative SSTA over the western central North Pacific and significant positive SSTA over eastern North Pacific and equatorial Pacific, resembling a positive PDO‐like pattern, was found to suppress precipitation in eastern China. Therefore, it could be concluded that that internal climate variability is the main trigger of the megadroughts over the eastern China, while the North Pacific decadal variability is considered the key driving factor influencing the EASM and EAWM.

Connections Between Stratospheric Ozone Concentrations Over the Arctic and Sea Surface Temperatures in the North Pacific

AbstractWe examined the relationship between sea surface temperatures (SSTs) in the North Pacific and the concentration of stratospheric ozone in the northern hemisphere in February–March from 1980 to 2017 using reanalysis and satellite datasets. Our results show that the concentration of stratospheric ozone can be modulated by the principle mode of the North Pacific SSTs—that is, there are negative and zonally asymmetrical ozone anomalies in the Arctic stratosphere, but positive ozone anomalies in the lower stratosphere at mid‐latitudes during the positive phases of the North Pacific SSTs. The North Pacific SST anomalies account for about 20% of the linear variance of ozone concentrations in the lower stratosphere over the Arctic. Negative SST anomalies in the central North Pacific tend to result in a strengthened Western‐Pacific like teleconnection, which favors the propagation of more planetary wavenumber‐1 and ‐2 waves into the stratosphere. The North Pacific SSTs‐related upwelling branch of the Brewer–Dobson circulation in the mid‐latitude stratosphere strengthens, but its downwelling branch in the Arctic stratosphere weakens. This results in decreased ozone anomalies in the lower stratosphere over the Arctic and increased ozone anomalies in the lower stratosphere at mid‐latitudes. The zonally asymmetrical distribution of ozone related to the positive phase of SSTs over the North Pacific may be related to the shifting and strengthening of the stratospheric Arctic vortex.

How much of monthly mean precipitation variability over global land is associated with SST anomalies?

The role of sea surface temperature (SST) in determining the predictability of monthly mean precipitation over the global land is assessed by analyzing the Atmospheric Model Intercomparison Project (AMIP)-like simulations forced by observed SST, which provides a benchmark for the impact of SST on the precipitation. The correlations of monthly mean precipitation anomalies between the ensemble mean of the AMIP simulations and observations are dominated by positive values with maxima around 0.3–0.4 in the tropical North Africa along 15° N and northeastern Brazil. The SST forcing for the precipitation variability is mainly associated with the El Nino-Southern Oscillation (ENSO) and in the tropical Indian Ocean. Statistically, positive and negative SST anomalies associated with an ENSO cycle have a comparable influence on precipitation variability over the land. In addition to the spatial variations, the precipitation responses to SST also vary with season and decade. Pattern correlations are larger in boreal winter than in boreal summer in the Northern Hemisphere, and relatively larger in April-June and September–November in the Southern Hemisphere. The global average of correlation is lower during 1957–1980 and 2000–2018, and higher in between. The interdecadal fluctuation of the pattern correlations is coherent with the interdecadal variation of the amplitude of ENSO.

Precessional Forced Zonal Triple-Pole Anomalies in the Tropical Pacific Annual Cycle

Abstract Based on a transient simulation of the Community Earth System Model, we identified two anomalous “zonal triple-pole type” annual cycles in the equatorial Pacific sea surface temperature (SST), which were induced by precessional evolution of the summer-minus-winter insolation difference and the autumn-minus-spring insolation difference, respectively. For example, due to the increased summer–winter insolation contrast, a zonal positive–negative–positive pattern of equatorial SST anomalies was detected after subtracting basin-scale summer SST warming. The positive SST anomalies were associated with anomalous upward air flows over the western Pacific and eastern Pacific, whereas the negative SST anomalies in the central Pacific were coupled with anomalous downward air flows, oceanic upwelling, and thermocline cooling. These central Pacific anomalies were due to multiple air–sea interactions, particularly zonal advection feedback and Bjerknes feedback. This anomalous annual cycle also included winter equatorial air–sea coupled anomalies with similar spatial patterns but opposite signs. The annual mean equatorial rainfall was significantly increased west of 135°E but decreased between 135°E and 160°W in response to the moderately intensified Walker circulation west of 160°W. The autumn–spring insolation contrast induced similar seasonal reversed anomalies during autumn and spring, but the annual means were only weakly enhanced for the Walker circulation and the rainfall anomalies had smaller magnitudes east of 160°E. These distinct responses of the annual mean climate indicated different seasonal biases in terms of the equatorial SST and associated Walker circulation anomalies due to forcing by the two seasonal insolation contrasts, and these findings had meaningful implications for paleoceanographic studies.

El Niño–East Asian monsoon teleconnection and its diversity in CMIP5 models

In this study, we assess the performance of state-of-the-art coupled models in reproducing El Nino diversity and its impacts on the East Asian Summer Monsoon (EASM), based on simulations taken from the Coupled Model Intercomparison Project phase 5 (CMIP5). All of the 17 models used in this study are identified to give reasonable eastern-Pacific (EP) El Nino-related sea surface temperature anomalies (SSTA) and anomalous Walker circulation compared with observations, whereas one-third of the models fail to reproduce the Walker circulation anomalies for central-Pacific (CP) El Nino. In these models, negative SSTA over the eastern-to-central Pacific are found to be too strong, while positive SSTA shift to the western Pacific during CP El Nino. There is thus sinking (rising) motion over the central (western) part of the Pacific basin, implying westward displaced anomalous Walker cells, relative to the observed. This results in simulated low-level cyclonic flow and positive rainfall anomalies over the far western north Pacific/South China Sea during and after the peak phase of CP El Nino, which are opposite to the observed anomalous climate state. The other two-thirds of models, with realistic CP El Nino-related anomalous Walker cell, can better reproduce the corresponding wind circulation and rainfall anomalies over East Asian (EA) region. The contrasting CP El Nino-related teleconnection among models mainly arises from the different simulated CP El Nino SSTA, which in turn determine the circulation anomalies through a Gill-type response. Further analyses reveal that, in the mean state of models with erroneous teleconnection, there exists a large cold SST bias over the eastern Pacific. The stronger Walker circulation, and possibly the stronger Bjerknes feedback, might be responsible for the overly negative SSTA associated with CP El Nino in the vicinity. Our results highlight the sensitivity of El Nino–EA climate linkage to the oceanic state in models, especially for CP El Nino events.

Evaluation of CMIP5 models in simulating the respective impacts of East Asian winter monsoon and ENSO on the western North Pacific anomalous anticyclone

AbstractThis article evaluates the linear isolated impacts of the East Asian winter monsoon (EAWM) and El Niño–Southern Oscillation (ENSO) on the evolution of the western North Pacific anticyclone (WNPAC) using 18 Coupled Model Inter‐comparison Project phase 5 (CMIP5) models. The independent effects of ENSO and EAWM (ENSOres and EAWMres) are obtained by removing the ENSO–EAWM interaction via linear regression. For the ENSO high‐skill (HS) models, which can well simulate an ENSO event, the anomalous WNPAC is confined to low latitudes and sustains from winter to late summer, which bears great resemblance to observational results. In contrast, the ENSO low‐skill (LS) models induce an insignificant and weak WNPAC from winter to the following summer. This is related to the simulation biases of ENSOres sea surface temperature anomalies (SSTAs). The ENSO LS models simulate a weak and slowly decaying ENSOres. This bias can induce unreasonable evolution of the WNPAC via producing an unrealistic intensity and location of the anomalous convection activity over the western Pacific, which is the key factor for yielding an ENSOres‐related WNPAC. The observed EAWMres‐related WNPAC appears only in winter and covers a large region. The EAWM HS models, which can well simulate EAWM features, reasonably reproduce the WNPAC in winter. However, the WNPAC is significant and persists to the following summer, which is inconsistent with observations. This bias is related to the EAWMres‐induced negative SSTA bias in the tropical central Pacific from spring to summer in the EAWM HS models. In contrast, the EAWM LS models, which cannot reasonably simulate EAWM features, are unable to reproduce the WNPAC in winter and the following seasons. Among the 18 state‐of‐the‐art CMIP5 models, the CNRM‐CM5 model yields a better performance in simulating the evolution of WNPAC compared with the ENSO HS models and EAWM HS models.

Strengthening of the Indian Ocean Dipole With Increasing Seasonal Cycle in the Mid‐Holocene

AbstractThis study presents modeling evidence that the Indian Ocean Dipole (IOD) is enhanced in its frequency and amplitude under mid‐Holocene (8‐ka) conditions. This enhancement is identified in a global climate model simulation driven by the 8‐ka insolation, in which negative sea surface temperature anomalies grow more than the present‐day simulation in the eastern Indian Ocean. The mechanism of IOD amplification is explained in terms of a positive feedback among sea surface temperature, precipitation, and wind speeds over the eastern Indian Ocean. A series of idealized experiments in which the present‐day insolation seasonal cycle over the tropics is systematically perturbed in the model demonstrates that the IOD frequency and amplitude are proportional to the seasonal cycle strength of the atmospheric circulation off Sumatra‐Java, consistent with proxy records in the mid‐Holocene. Greater surface warming over Eurasia due to increased insolation in boreal summer is not critical to the amplification of the IOD.

Variation of Anomalous Convergence Around Kalimantan Island in Lower Troposphere and Its Role in Connecting the East Asian Summer Monsoon and Australian Winter Monsoon

AbstractUsing the data from ERA‐interim, the Hadley Centre Global Sea Surface Temperature data set, and the Climate Prediction Center Merged Analysis of Precipitation, after removing the simultaneous El Niño and Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) signals, we have investigated variations of anomalous convergence around Kalimantan Island in lower troposphere in associations with the East Asian summer and Australian winter monsoons. Results indicate that when negative sea surface temperature anomalies appear in the western Pacific on both sides of the equator, the atmospheric cooling over the northwestern Pacific and from the South Pacific Convergence Zone (SPCZ) to Australia is favorable for the maintenance of anticyclonic anomalies over the areas from the western Pacific to the South China Sea and from the south of the equator to the northeast of Australia. These anomalous anticyclonic circulations can induce equatorial easterly anomalies, which subsequently facilitate the maintenance of anomalous convergence around Kalimantan Island. In turn, the anomalous convergence in Kalimantan Island is favorable for the maintenance of anomalous anticyclonic over the northwestern Pacific and SPCZ and thereby intensifies both the western Pacific subtropical high and the Australian High, inducing concurrent intensifications of the East Asian summer monsoon and Australian winter monsoon. Related to the anomalous convergence over Kalimantan Island, precipitation and temperature decrease from the western Pacific to SPCZ but increase from the Indian Ocean to western Maritime Continent and Australia, while temperature is lower than normal in Kalimantan. The above results have remarkable implications for in‐depth understanding of the mechanism behind anomalies of the East Asian summer monsoon and Australian winter monsoon.

Atmospheric Response to Oceanic Cold Eddies West of Luzon in the Northern South China Sea

Using the compositing method, two kinds of sea surface temperature (SST) anomalies associated with mesoscale ocean eddies and their effects on the atmosphere over the northern South China Sea were investigated. We focused on Luzon cold eddies (LCEs), which form during the winter monsoon and occur repeatedly to the west of Luzon Island, where a SST front exists. Using satellite and reanalysis data, 20 LCEs from 2000–2016 were classified into two types according to their impact on the atmosphere. One type consisted of cold SST anomalies within the eddy interior; subsequent turbulent heat flux and surface wind speed decreased over the cold core, presenting a monopole pattern. The second type comprised SST anomalies on either side of the eddy, which mostly propagated along the SST front. For this type of LCEs, cyclonic eddy currents acting on the SST front led to the SST anomalies. They produced a dipole, with surface wind deceleration and acceleration over negative and positive SST anomalies, respectively, on either side of the eddy’s flank. Dynamically, for both types of LCE, a vertical mixing mechanism appeared to be responsible for the wind anomalies. Moreover, anomalous vertical circulations developed over the LCEs that extended over the whole boundary layer and penetrated into the free atmosphere, leading to an anomalous convective rain rate. Quantitatively, the surface wind speed changed linearly with SST; atmospheric anomalies related to LCEs explained 5%–14% of the total daily variance.