Anomalous Zonal Wind Research Articles

Summer Russian Heat Waves Linked to Arctic Sea Ice Anomalies in 2010 and 2016

Abstract This study investigates dominant features of the atmospheric circulation evolution associated with extreme heat waves (HWs) in Russia during the summers of 2010 and 2016, respectively, and their possible association with Arctic sea ice loss. Results show that a region of Russia (20°–70°E, 45°–65°N) experienced a lasting 44-day HW event from 4 July to 16 August 2010 and a 26-day HW event from 2 to 27 August 2016. The associated atmospheric circulation anomalies are characterized by the summer Arctic cold anomaly in the mid- to low troposphere and an anticyclonic circulation anomaly over the Ural Mountains. Simulation experiments forced by summer Arctic sea ice anomalies reproduce the major characteristics of observational associations. Observations and numerical simulations indicate that summer Arctic sea ice anomaly is conducive to the formation of the summer Arctic cold anomaly, which is often accompanied by the enhanced baroclinicity in most of the Arctic troposphere and increased and decreased meridional temperature gradient in the high and midlatitudes, respectively. Such a configuration strengthens westerly winds over most of the Arctic and weakens zonal westerly over the southern Ural Mountains. This anomalous zonal wind pattern establishes the background conditions for the sustained positive geopotential height anomaly in the mid- to low troposphere that dynamically facilitates the prevalence of Russian HW events. Moreover, when compared with 2016, the weaker meridional potential vorticity gradient anomaly in the summer of 2010 prolonged the persistence of Ural blocking, which may lead to longer HW events in Russia.

MJO Seasonality in Its Scale Selection: Perspectives From Space‐Time Spectral Analysis of Moisture Budget

AbstractThe Madden–Julian Oscillation (MJO) displays an evident seasonality in its spatiotemporal scale selection. In boreal winter, the MJO is best behaved at wavenumber 2 and oscillates on a narrow time scale centering on a ∼55‐day period. In contrast, a wavenumber‐1 selection and a broad oscillation frequency centering on a ∼33‐day period are observed in boreal summer. Using the space‐time cross‐spectral analysis between convection and moisture budget, we reveal that column processes determine seasonal variations in the MJO spatial organization, while the MJO is strongly damped by the horizontal moisture advection mainly due to its meridional component (Vadv). A timescale decomposition analysis suggests that the damping effect of Vadv results primarily from high‐frequency synoptic‐scale disturbances, and the Vadv component related to the seasonal‐mean moisture generally supports the MJO growth at large wavenumbers while inhibits the growth of small wavenumbers, which is the most evident in boreal summer. The advection of seasonal‐mean moisture by anomalous zonal winds supports the MJO growth at all wavenumbers in boreal winter, but in boreal summer the growth becomes weak and even turns negative for small wavenumbers. Furthermore, different MJO timescale selections in winter versus summer are rooted in the Vadv, as both the column processes and zonal moisture advection mainly support the propagation of high‐frequency modes. Observational results related to the horizontal seasonal‐mean moisture advection are reasonably validated in a dynamical moisture model. These findings advance our understanding of the MJO seasonality and offer alternative diagnoses for evaluating the MJO simulation fidelity in contemporary climate models.

Effect of quasi‐biennial oscillation on the intensity of the South China Sea summer monsoon

AbstractThe interannual relationship between the quasi‐biennial oscillation (QBO) and the South China Sea (SCS) summer monsoon intensity (SCSSMI) is investigated using the ERA5 data set in the period of 1953–2020. The QBO–SCSSMI connection is weak in El Niño‐Southern Oscillation (ENSO)‐related summer, but significant in neutral ENSO summer. In neutral ENSO summer, there is significant positive correlation between QBO and SCSSMI, that is, the SCSSMI is enhanced in the QBO westerly phase, while it is weakened in the QBO easterly phase with larger amplitude of the SCSSMI anomaly. QBO may modulate the SCSSMI in three possible ways. The QBO‐related temperature anomalies downward to the lower stratosphere directly affect the SCS monsoon convection by modulating the upper‐tropospheric stability and tropopause height over the SCS region. Moreover, the QBO‐related anomalous zonal wind downward to the lower stratosphere may regulate cross‐tropopause vertical wind shear and then modulate the convection development. Besides, the QBO‐related zonal wind and temperature signals propagating downward into the upper troposphere can affect the horizontal divergence by influencing the vertical vorticity and the geopotential via thermal forcing. Through those three processes, QBO directly affects the deep convection activities and then modulates the SCSSMI via internal dynamic adjustment. In the southern SCS, cross‐tropopause vertical wind shear makes a main contribution to deep convections, while upper‐tropospheric static stability makes a major contribution to deep convection in the northern SCS. The conclusions are also generally verified by the CMIP6 model simulation.

Revisiting the effect of increasing horizontal resolution on the evolution of El Niño in a coupled model

The ENSO (El Niño southern oscillation) in the tropical Pacific plays a crucial role in regulating the Indian summer monsoon. Many mechanisms have been proposed to explain the positive and negative feedback associated with the ENSO through air-sea interaction in the tropical Pacific. Historical run data at varying oceanic and atmospheric resolutions from the MPI-ESM model is analyzed and compared with observations to determine the possible reasons for the much earlier onset and delayed withdrawal of ENSO in the coupled model. Increasing the resolution of the oceanic and atmospheric components of the coupled model helps in well-resolving the annual cycle of sea surface temperature in the tropical Pacific. There is a significant improvement in the early onset and late withdrawal of El Niño in the coupled model with increasing horizontal resolution of the model components. The weak subsurface propagation along the 20 °C isotherm in the equatorial Pacific improves on increasing the horizontal resolution of the coupled model. Similar to observations, there is a clear eastward extension of convection, 20° east of the dateline, in the equatorial tropical Pacific in the coupled model during ENSO years. Increasing the resolution of the oceanic and atmospheric components of the coupled model improves the simulation of convective activity over the western Pacific along with the improved magnitude of anomalous zonal wind strength during El Niño years. However, the presentation of convective activity over the equatorial Pacific deteriorated on increasing atmospheric resolution further to T255 due to the enhanced value of the interannual amplitude of intraseasonal zonal wind around the international dateline. Also, the coupled model does not capture the accurate eastward propagation of the Madden Julian Oscillation envelope over the western Pacific. While higher resolutions showed improvement in the representation of El Niño events in terms of their number and duration, challenges persisted in accurately capturing the frequency of these events and their related triggering mechanisms. Understanding the intricate interplay of factors influencing the evolution of El Niño requires further research and progress in modeling strategies.

Role of Ocean Advections during the Evolution of the Pacific Meridional Modes

Abstract The Pacific meridional modes (PMMs) are the leading ocean–atmosphere coupled modes in the subtropical northeastern (NPMM) and southeastern (SPMM) Pacific, respectively, and have been suggested to be key precursors to equatorial Pacific variability. Previous studies pointed out that both NPMM- and SPMM-related sea surface temperature (SST) anomalies are primarily driven by net surface heat flux variations during their equatorward evolution. However, whether oceanic heat advective processes would play a role during the evolution remains unclear. To address this issue, we perform an ocean mixed layer heat budget analysis based on observations and three ocean reanalysis datasets, and then reveal the effect of ocean advections on the evolution by comparing a fully coupled dynamic ocean model (DOM) to a slab ocean model (SOM). Our results suggest that for the NPMM evolution, ocean advections—primarily by anomalous meridional Ekman heat advections driven by mean and anomalous zonal wind stresses—play a damping role in the south of the NPMM. Thus, the NPMM SST anomalies appear to instead exhibit a poleward shift, although still freely propagating westward from the preceding boreal winter to the following summer. This finding challenges the traditional view that the NPMM propagates equatorward through the wind–evaporation–SST feedback. For the SPMM evolution, ocean advections play a damping role in the center of the SPMM from boreal spring to summer, as well as an intensification role in the southwest Pacific during summer. However, the effect of the intensification on the SPMM evolution is hard to determine due to the strong simulation bias of the SPMM evolution in the DOM. Significance Statement While it is known that both NPMM- and SPMM-associated SST anomalies are primarily driven by net surface heat flux variations during their evolution, whether ocean advections would play a role remains unknown. Here, we show that ocean advections play a role in the evolution of both PMMs. In particular, for the NPMM evolution, ocean advections play a damping role in the south of the NPMM center, causing a tendency for the NPMM to be displaced northward. The role of ocean advection challenges the prevailing notion that the NPMM simply evolves equatorward through the wind–evaporation–SST feedback.

Distinct MJOs Under the Two Types of La Niña

AbstractEl Niño‐Southern Oscillation (ENSO) strongly influences the interannual variations of Madden‐Julian Oscillation (MJO)—the dominant mode of tropical intraseasonal variability. Previous studies have revealed more about the impacts of the diversified El Niño on MJO, but less about those of the La Niña diversity. Using observations and global climate model experiments, this study demonstrates that MJO undergoes distinct variations and Pacific teleconnections when La Niña varies between eastern‐Pacific (EP) and central‐Pacific (CP) types. Compared to the CP La Niña, MJO under the EP La Niña is more energetic over the southern Maritime Continent–South Pacific Convergence Zone and can propagate faster and further with a broader zonal circulation scale over the Indo‐Pacific warm pool. The stronger and faster MJO under the EP La Niña is associated with more vigorous ascending motions, higher moisture content and stronger moistening in the lower‐level atmosphere ahead of the MJO deep convection. Although the meridional moisture advection exists as the fundamental mechanism of MJO eastward propagation, the distinct MJO under the two types of La Niña primarily results from the different lower‐tropospheric moistening due to the zonal moisture advection. Both the background mean and anomalous zonal winds and moisture gradients are stronger under the EP La Niña, thereby causing a remarkable invigoration effect of zonally advective moistening. Diverse extratropical circulation responses of distinct MJOs are also discussed. These results should improve our understanding of the whole ENSO diversity paradigm that influences MJO and teleconnections.

Surface ocean current variations in the North Pacific related to Arctic stratospheric ozone

Using reanalysis datasets and a coupled general circulation model, the relationship between springtime Arctic total column ozone (TCO) and surface (5 m) ocean currents in the North Pacific is investigated. We found that as March Arctic TCO decreases, a statistically significant northwestward ocean current anomaly occurs in the northern North Pacific in surface layer, but an anomalous southward ocean current appears in the central North Pacific in April, and vice versa. The decreased Arctic TCO favors an enhanced Arctic stratospheric circulation, which tends to induce the tropospheric positive Arctic Oscillation anomaly with easterly anomalies over the midlatitude eastern Asia in late March through stratosphere-troposphere dynamical coupling. The easterly anomaly over eastern Asia in late March further extends eastward and induces an easterly anomaly over the midlatitude North Pacific, which favors negative North Pacific Oscillation (–NPO)-like circulation anomaly via anomalous zonal wind shear and the interactions between synoptic scale eddies and the mean flow in early-middle April. The –NPO anomaly forces anomalous northwestward/southward surface ocean currents in the northern/central North Pacific through direct friction of wind and the Coriolis force. Our coupled numerical simulations with high- and low-ozone scenarios also support that the Arctic stratospheric ozone affects the North Pacific surface ocean currents by NPO anomalies. Moreover, the ozone-related ocean current anomalies contribute to Victoria mode-like sea surface temperature anomalies in the North Pacific by horizontal heat advection. These results imply that Arctic ozone signal could be a predictor for variations of the North Pacific surface ocean currents.

Mechanism for Southward Shift of Zonal Wind Anomalies During the Mature Phase of ENSO

AbstractThe cause of southward shift of anomalous zonal wind in the central equatorial Pacific (CEP) during ENSO mature winter was investigated through observational analyses and numerical model experiments. Based on an antisymmetric zonal momentum budget diagnosis using daily ERA-Interim data, a two-step physical mechanism is proposed. The first step involves advection of the zonal wind anomaly by the climatological mean meridional wind. The second step involves the development of an antisymmetric mode in the CEP, which promotes a positive contribution to the observed zonal wind tendency by the pressure gradient and Coriolis forces. Two positive feedbacks are responsible for the growth of the antisymmetric mode. The first involves the moisture–convection–circulation feedback, and the second involves the wind–evaporation–SST feedback. General circulation model experiments further demonstrated that the boreal winter background state is critical in generating the southward shift, and a northward shift of the zonal wind anomaly is found when the same SST anomaly is specified in boreal summer background state.

Impact of Developing ENSO on Tibetan Plateau Summer Rainfall

AbstractThe year-to-year variations of Tibetan Plateau (TP) summer rainfall have tremendous climate impacts on the adjoining and even global climate, attracting extensive research attention in recent decades to understand the underlying mechanism. In this study, we investigate an open question of how El Niño–Southern Oscillation (ENSO) influences the TP precipitation. We show that the developing ENSO has significant impacts on the summer rainfall over the southwestern TP (SWTP), which is the second EOF mode of the interannual variability of summer rainfall over the TP. The moisture budget indicates that both the suppressed vertical motion and the deficit of moisture contribute to the reduction of SWTP rainfall during El Niño’s developing summer, with the former contribution 4 times larger than the latter. Moist static energy analyses indicate that the anomalous advection of climatological moist enthalpy by anomalous zonal wind is responsible for the anomalous descending motions over the SWTP. The El Niño–related southward displacements of the South Asian high and the upper-level cyclonic anomalies over the west of TP stimulated by the suppressed Indian summer monsoon precipitation are two key processes dominating the anomalous zonal moist enthalpy advection over SWTP. Meanwhile, the India–Burma monsoon trough is strengthened during El Niño developing summer, which prevents the advection of water vapor into the SWTP, and thus contributes to the deficit of summer SWTP rainfall. Our results help to understand the complicated ENSO-related air–sea interaction responsible for the variability of TP precipitation and have implications for seasonal prediction of the TP climate.

Attenuation of Central Pacific El Niño Amplitude by North Pacific Sea Surface Temperature Anomalies

AbstractThere exists a pronounced asymmetry between the amplitudes of central Pacific (CP) and eastern Pacific (EP) El Niño sea surface temperature anomalies (SSTA). The present study examines such an asymmetry and its relationship with the North Pacific SSTA. Results indicate that the weaker CP El Niño amplitude can be attributed to the weaker anomalous zonal wind response to the east–west equatorial SSTA gradient during its growing phase compared with EP El Niño. Furthermore, the occurrence of CP El Niño is closely associated with southwesterly surface wind anomalies in the subtropical North Pacific, as well as ocean warming reminiscent of the North Pacific Gyre Oscillation (NPGO) pattern in its vicinity. Both the observations as well as the pacemaker experiments with a coupled global climate model suggest that the anomalous low-level southwesterlies, induced by the North Pacific Oscillation (NPO)-like atmospheric variability, can enhance anomalously positive SST signals and extend them southwestward to the central equatorial Pacific via the wind–evaporation–SST feedback. This will further attenuate the atmospheric response to zonal SSTA gradient, and hence weaken the amplitude of CP El Niño. Therefore, anomalous low-level southwesterlies over the subtropical North Pacific can effectively act as a conduit for tropical–subtropical air–sea interaction in that region, and can play an important role in limiting the intensity of CP El Niño.

Modulation of orbitally forced ENSO variation by Tibetan Plateau topography

The modulation of the orbitally induced variation in the El Niño/Southern Oscillation (ENSO) by Tibetan Plateau (TP) topography was investigated with four fully coupled ocean–atmosphere model experiments under the modern boundary conditions. The modeled orbitally induced ENSO change was strongly affected by the presence of the TP. Specifically, in the experiments with orbital forcing at 6 ka in comparison to the counterpart experiments with modern orbital forcing, it was shown that the orbitally forced ENSO variability was slightly weakened by 3% in the presence of the TP but increased significantly by 13% in the simulation of the absence of the TP. This difference was primarily attributed to the anomalous zonal winds over the tropical central-to-eastern Pacific in response to the increased summertime incoming insolation, which was intrinsically related to the different changes in the Asian monsoon and tropical Pacific Walker circulation. Under the absence of modulation by the TP, the anomalous westerly winds dominated the equatorial Pacific and favored the development of a stronger El Niño event and, in turn, the further intensification of La Niña due to the stronger heat discharge during the recession of the stronger El Niño. In contrast, the simulation with the presence of the TP showed the enhanced easterly wind anomalies in the boreal summer over the tropical Pacific, which further reduced (increased) the magnitude of the El Niño (La Niña) event in the subsequent winter. These results suggest that the TP is a potentially crucial boundary condition for evaluating the changes in the ENSO dynamics on orbital scales during the Cenozoic.

Interannual variability and triggers of the South China Sea summer monsoon withdrawal

The interannual variability of the South China Sea (SCS) summer monsoon (SM) withdrawal and associated factors are examined using NCEP/NCAR reanalysis, CMAP rainfall, and OLR data. During late-withdrawal years of the SCSSM, a significant cyclonic anomaly is observed over southern China and northern SCS, accompanied by pronounced anomalous low-level westerly winds over the Bay of Bengal that extend eastward to the Philippine Sea. At the upper level, significant easterly wind anomalies are seen over the SCS. Corresponding to these circulation anomalies, a dipole rainfall anomaly mode is observed, with significant positive (negative) rainfall anomalies over the SCS and Philippine Sea (equatorial Indian Ocean). The seasonal progressions of several monsoonal parameters (e.g., zonal wind, rainfall, convection activity) over the SCS also show significant differences between late- and early-withdrawal years. Observational evidence shows that tropical cyclone (TC) activities over the SCS and Philippine Sea and the active/suppressed phases of 30‒60 day oscillations are two important factors that may contribute to the interannual variability of SCSSM withdrawal. Both statistical evidence and case studies verify that the 30‒60 day oscillation impacts SCSSM withdrawal by modulating convection activity and anomalous zonal winds over the SCS. In addition to the TC activities and 30‒60 day oscillations, the interannual variability of the SCSSM withdrawal can also be affected by synoptic systems, including easterly wave and cold front.

The tropical and extratropical-origin summer meridional teleconnections over East Asia

The meridional teleconnection over East Asia (EA-MT) is one of the major features of East Asian summer monsoon (EASM), and it is also the important pathway for the impacts from both tropical and extratropical middle-high latitudes on the EASM. To reveal the temporal-spatial structure of summer EA-MT, multivariate EOF is applied to the latitude-pressure cross section of combined summer precipitation and zonal wind anomalies over East Asia, using GPCP precipitation and NCEP/NCAR reanalysis dataset during the period 1979–2016. Two leading modes of summer EA-MT are obtained and identified as the tropical and extratropical-origin EA-MT in terms of their specific characteristics and major sources, named “EA-TMT” and “EA-XTMT”, respectively. Both of them show strong interannual variabilities, and they demonstrate a quasi-barotropic structure of westerly wind anomalies that slightly tilts northward with height in the mid-high latitudes, and accordingly significant precipitation anomalies are constrained right underneath the anomalous zonal wind around the Yangtze River valley (YRV), which in turn helps to establish and maintain the summer EA-MT. The EA-TMT is triggered by anomalous convective activities in the western North Pacific and propagates northward in the mid-low troposphere, it has close relationship with the anomalies of East Asian summer westerly jet (EASWJ) intensity and the YRV precipitation, which is superior over the East Asia/Pacific and Pacific-Japan teleconnection patterns in representing the tropical-origin EA-MT. The EA-XTMT is however mainly initiated in the southeast of Kara Sea and propagating southeastward in the whole troposphere, it is dominated by the North Asian dipole mode and mainly related to the EASWJ position anomaly, favoring a “South Flood North Drought” pattern of precipitation anomalies over East China. Meanwhile, the EA-TMT is closely related to the summer SST anomalies in the tropical Pacific-Indian ocean and partly correlated with ENSO, whereas the EA-XTMT is possibly linked to the spring sea ice concentration anomalies in the western Arctic and Barents Sea. Furthermore, the joint contributions of EA-TMT and EA-XTMT are essential for the major variability of EASM, wherein the EA-TMT plays a primary role.

Impact of atmospheric model resolution on simulation of ENSO feedback processes: a coupled model study

This study examines El Nino-Southern Oscillation (ENSO)-related air-sea feedback processes in a coupled general circulation model (CGCM) to gauge model errors and pin down their sources in ENSO simulation. Three horizontal resolutions of the atmospheric component (T42, T63 and T106) of the CGCM are used to investigate how the simulated ENSO behaviors are affected by the resolution. We find that air-sea feedback processes in the three experiments mainly differ in terms of both thermodynamic and dynamic feedbacks. We also find that these processes are simulated more reasonably in the highest resolution version than in the other two lower resolution versions. The difference in the thermodynamic feedback arises from the difference in the shortwave-radiation (SW) feedback. Due to the severely (mildly) excessive cold tongue in the lower (higher) resolution version, the SW feedback is severely (mildly) underestimated. The main difference in the dynamic feedback processes lies in the thermocline feedback and the zonal-advection feedback, both of which are caused by the difference in the anomalous thermocline response to anomalous zonal wind stress. The difference in representing the anomalous thermocline response is attributed to the difference in meridional structure of zonal wind stress anomaly in the three simulations, which is linked to meridional resolution.

Physical processes controlling earlier and later onset of a typhoon season in the western North Pacific

The interannual variability of typhoon onset date in western North Pacific (WNP) during 1979–2015 is investigated. The average date in the earlier (later) onset group is early April (late July). A diagnosis of the genesis potential index shows that the key factor affecting the earlier and later onset lies in the background moisture field. According to their evolution feature, the earlier onset cases are further divided into two sub-groups. In the first sub-group, positive specific humidity anomalies were confined in the western WNP and caused primarily by circulation anomalies associated with a La Nina in the equatorial eastern Pacific. In the second sub-group, positive specific humidity anomalies were confined in the eastern WNP and caused by anomalous convection associated with the Pacific Meridional Mode (PMM) in the tropical and subtropical eastern Pacific. In the later onset composite, negative specific humidity anomalies appear in the WNP, and were accompanied by an anomalous anticyclone in situ and a V-shape negative SSTA pattern in the western Pacific. In addition, the change of background vertical wind shear (VWS) also influences the TC onset date. A decrease (increase) in the VWS magnitude and an easterly (westerly) shear anomaly occur in the earlier (later) onset composite. Physically it is argued that both the magnitude of the VWS and the sign of anomalous zonal wind shear may affect TC formation in the WNP.

Causes of ENSO Weakening during the Mid-Holocene

The causes of the change in amplitude of El Niño–Southern Oscillation (ENSO) during the mid-Holocene were investigated by diagnosing the model simulations that participated in the Paleoclimate Modelling Intercomparison Project phases 2 and 3. Consistent with paleoclimate records, 20 out of the 28 models reproduced weaker-than-preindustrial ENSO amplitude during the mid-Holocene. Two representative models were then selected to explore the underlying mechanisms of air–sea feedback processes. A mixed layer heat budget diagnosis indicated that the weakened ENSO amplitude was primarily attributed to the decrease in the Bjerknes thermocline feedback, while the meridional advective feedback also played a role. During the mid-Holocene, the thermocline response to a unit anomalous zonal wind stress forcing in the equatorial Pacific weakened in both models because of the increased ENSO meridional scale. A further investigation revealed that the greater ENSO meridional width was caused by the strengthening of the Pacific subtropical cell, which was attributed to the enhanced mean trade wind that resulted from the intensified Asian and African monsoon rainfall and associated large-scale east–west circulation in response to the mid-Holocene orbital forcing.

Analysis of the Southward Wind Shift of ENSO in CMIP5 Models

During the mature phase of El Niño–Southern Oscillation (ENSO) events there is a southward shift of anomalous zonal winds (SWS), which has been suggested to play a role in the seasonal phase locking of ENSO. Motivated by the fact that coupled climate models tend to underestimate this feature, this study examines the representation of the SWS in phase 5 of the Coupled Model Intercomparison Project (CMIP5). It is found that most models successfully reproduce the observed SWS, although the magnitude of the zonal wind stress anomaly is underestimated. Several significant differences between the models with and without the SWS are identified including biases in the magnitude and spatial distribution of precipitation and sea surface temperature (SST) anomalies during ENSO. Multiple-linear regression analysis suggests that the climatological meridional SST gradient as well as anomalous ENSO-driven convective activity over the northwest Pacific both might play a role in controlling the SWS. While the models that capture the SWS also simulate many more strong El Niño and La Niña events peaking at the correct time of year, the overall seasonal synchronization is still underestimated in these models. This is attributed to underestimated changes in warm water volume (WWV) during moderate El Niño events so that these events display relatively poor seasonal synchronization. Thus, while the SWS is an important metric, it is ultimately the magnitude and zonal extent of the wind changes that accompany this SWS that drive the changes in WWV and prime the system for termination.

Influence of subtropical dipole pattern in Southern Indian Ocean on ENSO event

Subtropical Dipole Pattern (SDP) in southern Indian Ocean is another significant dipolelike sea surface temperature anomalies (SSTA) phenomena besides tropical Indian Ocean dipole (TIOD). It is very clear in interannual and interdecadal variations. But many recent studies have mainly aimed at the SST of equatorial Indian Ocean and less at southern Indian Ocean, especially less at the relationship between SSTA in southern Indian Ocean and El NinoSouthern Oscillation (ENSO). Present works investigated primarily the correlation between the SDP and ENSO. It is found that SDP event act as a crucial linkage between positive and negative ENSO phases. The SSTAs in equatorial eastern Pacific Ocean are completely reversed before and after SDP event. Further, we investigated the possible impact of SDP on ENSO event in interannual scale by researching the anomalous characteristics of sea temperature, wind, convergence and divergence, total cloud amount, radiation and so on. The results showed that the influence of SDP on ENSO involve not only the process of positive and negative air-sea feedback, but also the interaction between atmospheric circulation over tropical and subtropical, specially the eastward propagation of anomalous zonal wind from the equatorial Indian Ocean to the Pacific Ocean. Otherwise, it also relates, to some extent, to the transition of large-scale climatic mean flow over the Indian Ocean and Pacific Ocean.

Modulation of western North Pacific tropical cyclone activity by the Atlantic Meridional Mode

This study examines the year-to-year modulation of the western North Pacific (WNP) tropical cyclones (TC) activity by the Atlantic Meridional Mode (AMM) using both observations and the Geophysical Fluid Dynamics Laboratory Forecast-oriented Low Ocean Resolution Version of CM2.5 (FLOR) global coupled model. 1. The positive (negative) AMM phase suppresses (enhances) WNP TC activity in observations. The anomalous occurrence of WNP TCs results mainly from changes in TC genesis in the southeastern part of the WNP. 2. The observed responses of WNP TC activity to the AMM are connected to the anomalous zonal vertical wind shear (ZVWS) caused by AMM-induced changes to the Walker circulation. During the positive AMM phase, the warming in the North Atlantic induces strong descending flow in the tropical eastern and central Pacific, which intensifies the Walker cell in the WNP. The intensified Walker cell is responsible for the suppressed (enhanced) TC genesis in the eastern (western) part of the WNP by strengthening (weakening) ZVWS. 3. The observed WNPTC–AMM linkage is examined by the long-term control and idealized perturbations experiment with FLOR-FA. A suite of sensitivity experiments strongly corroborate the observed WNPTC–AMM linkage and underlying physical mechanisms.

Annular modes and apparent eddy feedbacks in the Southern Hemisphere.

Lagged correlation analysis is often used to infer intraseasonal dynamical effects but is known to be affected by nonstationarity. We highlight a pronounced quasi 2 year peak in the anomalous zonal wind and eddy momentum flux convergence power spectra in the Southern Hemisphere, which is prima facie evidence for nonstationarity. We then investigate the consequences of this nonstationarity for the Southern Annular Mode and for eddy momentum flux convergence. We argue that positive lagged correlations previously attributed to the existence of an eddy feedback are more plausibly attributed to nonstationary interannual variability external to any potential feedback process in the midlatitude troposphere. The findings have implications for the diagnosis of feedbacks in both models and reanalysis data as well as for understanding the mechanisms underlying variations in the zonal wind.