Introduction

This study explores the impacts of the Indian summer monsoon (ISM) on the El Niño–Southern Oscillation (ENSO) evolution by utilizing two coupled general‐circulation model simulations. The first simulation uses the Center for Ocean–Land–Atmosphere Studies anomaly‐coupled model and the second simulation employs a new coupling strategy, interactive ensembles, that is designed to increase the signal‐to‐noise ratio of boundary forcing. The effects of monsoon variability related and unrelated to ENSO are separated through composites in terms of both sea surface temperature (SST) over the Niño‐3.4 area (170°W–120°W,5°S–5°N) and Indian summer monsoon rainfall (IMR) anomalies. It is found that ENSO‐related monsoon variability has significant impacts on warm events. In the interactive‐ensemble simulation, a weak (strong) monsoon enhances (weakens) an ongoing warm event. The monsoon impacts are manifested in the surface zonal wind‐stress anomalies in the western central equatorial Pacific. In the anomaly‐coupled simulation, the monsoon–ENSO relationship is difficult to detect. The ongoing cold events are only weakly affected by monsoon variability. Monsoon variability that is unrelated to ENSO also induces noticeable SST anomalies in the equatorial central Pacific in the following winter. In the interactive‐ensemble model, a weak (strong) monsoon induces noticeable warm (cold) SST anomalies.The long‐term change of the ISM–ENSO relationship is also analysed. Fluctuations of the IMR–Niño‐3.4 SST correlation are larger in the anomaly‐coupled model than in the interactive‐ensemble model. No apparent relation is found between the long‐term change of the correlation and that of the monsoon and ENSO anomalies or standard deviations. In the interactive‐ensemble simulation, the IMR and concurrent El Niño‐3.4 SST standard deviations show in‐phase long‐term changes. In the anomaly‐coupled simulation, the long‐term changes of the IMR and summer Niño‐3.4 SST standard deviations have an out‐of‐phase relation. Copyright © 2003 Royal Meteorological Society

The interannual variability associated with the El Nino/Southern Oscillation (ENSO) cycle is investigated using a relatively high-resolution (T42) coupled general circulation model (CGCM) of the atmosphere and ocean. Although the flux correction is restricted to annual means of heat and freshwater, the annual as well as the seasonal climate of the CGCM is in good agreement with that of the atmospheric model component forced with observed sea surface temperatures (SSTs). During a 100-year simulation of the present-day climate, the model is able to capture many features of the observed interannual SST variability in the tropical Pacific. This includes amplitude, lifetime and frequency of occurrence of El Nino events and also the phase locking of the SST anomalies to the annual cycle. Although the SST warming during the evolution of El Ninos is too confined spatially, and the warming along the Peruvian coast is much too weak, the patterns and magnitudes of key atmospheric anomalies such as westerly wind stress and precipitation, and also their eastward migration from the western to the central equatorial Pacific is in accord with observations. There is also a qualitative agreement with the results obtained from the atmospheric model forced with observed SSTs from 1979 through 1994. The large-scale dynamic response during the mature phase of ENSO (December through February) is characterized by an eastward displacement and weakening of the Walker cell in the Pacific while the Hadley cell intensifies and moves equatorward. Similar to the observations, there is a positive correlation between tropical Pacific SST and the winter circulation in the North Pacific. The deepening of the Aleutian low during the ENSO winters is well captured by the model as well as the cooling in the central North Pacific and the warming over Canada and Alaska. However, there are indications that the anomalies of both SST and atmospheric circulation are overemphasized in the North Pacific. Finally, there is evidence of a coherent downstream effect over the North Atlantic as indicated by negative correlations between the PNA index and the NAO index, for example. The weakening of the westerlies across the North Atlantic in ENSO winters which is related to a weakening and southwestward displacement of the Icelandic low, is in broad agreement with the observations, as well as the weak tendency for colder than normal winters in Europe.

Ocean salinity variability provides a new way to study the evolution of the the El Niño-Southern Oscillation (ENSO). Comparisons between the salinity variation and related processes responsible for sea surface temperature anomaly (SSTA) were extensively examined for the two strong El Niño (EN) events in 1997/1998 and 2015/2016, and a special EN event in 2014/2015. The results show that the development of EN is significantly correlated with a sea surface salinity anomaly (SSSA) in the tropical western-central Pacific. In the spring of 1997 and 2015 with strong EN events, the western-central equatorial Pacific exhibited significant negative SSSA that propagated eastward to the west of the dateline. The negative SSSA induced increased barrier layer thickness (BLT) which enhanced sea surface temperature (SST) warming in the tropical central Pacific. In contrast, although a negative SSSA occurred during April of the 2014/2015 weak EN event in the western-central equatorial Pacific, this SSSA was mainly confined to between 160° E and 180° E without significant eastward movement, resulting in a weakened BLT thickening process and a weak modulation effect on SST. We also confirm that the surface forcing associated with fresh water flux (FWF: evaporation (E) minus precipitation (P)) plays a prominent role in the surface salinity tendency in the tropical Pacific during EN events. Moreover, the negative FWF anomaly leads a strong negative SSSA by two months. Compared with the two strong ENs, the early negative FWF anomaly in the weak 2014/2015 EN did not present distinct development and eastward propagation and weakened rapidly in the summer of 2015. We demonstrate that change in salinity can modulate the ENSO, and the variation of SSSA and associated physical processes in the tropical western-central Pacific and could be used as an indicator for predicting the development of ENSO.

<sec><p indent="0mm">The impacts of anthropogenic climate change may be felt through changes in modes of natural climatic variability. The El Niño-Southern Oscillation (ENSO) is the most important year-to-year fluctuation of the climate system, varying between anomalously cold (La Niña) and warm (El Niño) conditions. Underpinning occurrences of ENSO events is the positive feedback between trade wind intensity and zonal contrasts in sea surface temperature (SST), referred to as the Bjerknes feedback. The trade winds normally pile up warm surface water in the western Pacific while upwelling colder subsurface water in the east along the equator and off the west coast of South America. The resulting east-west surface temperature contrast reinforces an east-west air pressure difference across the basin that in turn drives the trade winds. During La Niña, the system strengthens; but during El Niño, the trade winds weaken as atmospheric pressure rises in the western Pacific and falls in the eastern Pacific. The Bjerknes feedback now operates in reverse, with weakened trade winds and SST warming tendencies along the Equator reinforcing one another. These swings in temperature are accompanied by changes in the structure of the subsurface ocean, the position of atmospheric convection and associated global teleconnection patterns. </sec><sec> ENSO is spawned in the tropical Pacific, but its reach is global. It alters the general circulation of the atmosphere from one season to the next through far-field teleconnections, leading to droughts, floods, heat waves, and other extreme weather events across our planet. These natural hazards have widespread impacts on human and natural systems, including agriculture, public health, power generation and consumption, financial markets, transportation, tourism, freshwater resources, civil conflict, wildfires, fisheries, marine and terrestrial ecosystems, and biodiversity. The character of ENSO depends on the long-term average background climatic conditions on which it develops. However, because of human activities since the start of the Industrial Revolution in the mid-18th century, these background conditions have been changing at a pace that has greatly accelerated in recent decades. We have come to appreciate just how profound these changes are, thanks to five Intergovernmental Panel on Climate Change assessments since 1990. </sec><sec> In this review, we provide an update from the community since 2021, on the progress of changing ENSO and its impacts under greenhouse warming. Latest climate models suggest that a warming pattern featuring faster warming in the eastern Pacific than in the surrounding regions is possible as greenhouse warming progressively overwhelm other factors including decadal climate variability; that ENSO variability post-1960s has already increased from that pre-1960 characterized by an increase in strong El Niño, strong La Niña, and multiyear La Niña events, consistent with the projected increase pre-2100; and that the long-term change in ENSO SST variability is likely to be detected by 2030 in the eastern equatorial Pacific. Further, the changing mean state and increase in ENSO variability have intensified two-way interactions between the tropical and extratropical North Pacific, increased variability of the Western Pacific Subtropical High, increased SST variability over the north tropical Atlantic, hastened warming over the Antarctic shelf ocean, and exacerbated the economic cost of global warming. Under persistent greenhouse warming into 2300, the increased ENSO pre-2100 reverses to a subsequent reduction as the faster warming in the equatorial eastern Pacific develops into a permanent El Niño-like condition, highlighting a strong nonlinear ENSO response. </sec>

Idealized model experiments using the NCAR CESM1.0.5 under equinox conditions are designed and performed to address two fundamental questions about the effects of the sea surface temperature (SST) variation associated with the Atlantic multidecadal oscillation (AMO) on circulation and precipitation in North America and Europe: 1) Is the observed relationship between the AMO SST and the warm-season precipitation in North America a statistical coincidence? and 2) Why is the response of negative precipitation anomaly to warm SST in the AMO fairly uniform across most of North America, whereas the positive precipitation anomaly in the cold SST rather spotty? Model experiments are done with either a warm or cold SST anomaly in an aquaplanet, a planet with idealized continents, and a planet with both idealized continents and orography. Major results show that the atmospheric response to warm SST anomaly in the North Atlantic is fairly similar among the three sets of experiments. In the lower troposphere, the response has a significant negative geopotential anomaly from the SST anomaly center to the east and a positive geopotential anomaly in upstream North America. However, the response to the cold SST anomaly changes considerably among these experiments, particularly in North America. These results provide a foundation to answer the abovementioned two questions. First, they show that there is physical connection of the AMO SST and atmospheric circulation anomalies in North America. Moreover, the rather stable atmospheric response to the warm SST may explain the observed largely consistent response to the warm SST anomaly. The varying responses of the atmosphere to the cold SST indicate a strong sensitivity of the atmosphere to other forcings during the cold SST anomaly in the North Atlantic. This sensitivity could explain the varying and less stable response of the atmosphere to the cold SST during the AMO.

The subsurface ocean temperature response to El Niño–Southern Oscillation (ENSO) is examined based on 31-yr (1981–2011) simulations with the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) coupled model. The model sea surface temperature (SST) in the tropical Pacific is relaxed to observations to ensure realistic ENSO variability in the simulations. In the tropical Pacific, the subsurface temperature response to the ENSO SST is closely related to the variability of thermocline. The subsurface response is stronger and deeper in the tropical Indian Ocean than in the tropical Atlantic. The analysis at three selected locations reveals that the peak response of the subsurface temperature to ENSO lags the Niño-3.4 SST by 3, 6, and 6 months, respectively, in the southern tropical Indian Ocean, the northern tropical Atlantic, and the North Pacific, where SSTs are also known to be strongly influenced by ENSO. The ENSO-forced temperature anomalies gradually penetrate to the deeper ocean with time in the North Pacific and the tropical Atlantic, but not in the tropical Indian Ocean where the subsurface response at different depths peaks almost at the same time (i.e., at about 3–4 months following ENSO). It is demonstrated that the ENSO-induced surface wind stress plays an important role in determining the time scale and strength of the subsurface temperature response to ENSO in the North Pacific and the northern tropical Atlantic. Additionally, the ENSO-related local surface latent heat flux also contributes to the subsurface response to ENSO in these two regions.

Coupled ocean–atmosphere variability in the tropical Indian Ocean is explored with a multicentury integration of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Mark 3 climate model, which runs without flux adjustment. Despite the presence of some common deficiencies in this type of coupled model, zonal dipolelike variability is produced. During July through November, the dominant mode of variability of sea surface temperature resembles the observed zonal dipole and has out-of-phase rainfall variations across the Indian Ocean basin, which are as large as those associated with the model El Niño–Southern Oscillation (ENSO). In the positive dipole phase, cold SST anomaly and suppressed rainfall south of the equator on the Sumatra–Java coast drives an anticyclonic circulation anomaly that is consistent with the steady response (Gill model) to a heat sink displaced south of the equator. The northwest–southeast tilting Sumatra–Java coast results in cold sea surface temperature (SST) centered south of the equator, which forces anticylonic winds that are southeasterly along the coast, which thus produces local upwelling, cool SSTs, and promotes more anticylonic winds; on the equator, the easterlies raise the thermocline to the east via upwelling Kelvin waves and deepen the off-equatorial thermocline to the west via off-equatorial downwelling Rossby waves. The model dipole mode exhibits little contemporaneous relationship with the model ENSO; however, this does not imply that it is independent of ENSO. The model dipole often (but not always) develops in the year following El Niño. It is triggered by an unrealistic transmission of the model’s ENSO discharge phase through the Indonesian passages. In the model, the ENSO discharge Rossby waves arrive at the Sumatra–Java coast some 6 to 9 months after an El Niño peaks, causing the majority of model dipole events to peak in the year after an ENSO warm event. In the observed ENSO discharge, Rossby waves arrive at the Australian northwest coast. Thus the model Indian Ocean dipolelike variability is triggered by an unrealistic mechanism. The result highlights the importance of properly representing the transmission of Pacific Rossby waves and Indonesian throughflow in the complex topography of the Indonesian region in coupled climate models.

We investigate the relationship between oceanic and atmospheric anomaly fields in the tropical Pacific on the quasidecadal (QD; 10 to 18 years) scale is explored. It is suggested that the anomalous wind stress curl in the tropical South Pacific can generate the equatorial upper ocean heat content (OHC) anomalies on the QD scale via the Sverdrup transport resembling that of the recharge‐discharge oscillator model for the dynamics of El Niño‐Southern Oscillation (ENSO). The results also show that the wind stress curl anomalies in the tropical South Pacific are accompanied by ENSO‐like, QD‐scale anomaly patterns of the sea level pressure and sea surface temperature. Furthermore, the results indicate that the equatorial QD‐scale OHC anomalies can also be generated by the ENSO‐like propagations of the OHC anomalies in the tropical North and South Pacific. This study presents a systematic linkage between the ENSO‐like, QD‐scale OHC propagation in the tropical North and South Pacific, the Sverdrup transport variation in the tropical South Pacific, and the ENSO‐like, QD‐scale SST and SLP anomalies in the tropical Pacific.

While significant improvements have been made in understanding how the El Nino–Southern Oscillation (ENSO) impacts both North American and Asian climate, its relationship with the North Atlantic Oscillation (NAO) remains less clear. Observations indicate that ENSO exhibits a highly complex relationship with the NAO-associated atmospheric circulation. One critical contribution to this ambiguous ENSO/NAO relationship originates from ENSO’s diversity in its spatial structure. In general, both eastern (EP) and central Pacific (CP) El Nino events tend to be accompanied by a negative NAO-like atmospheric response. However, for two different types of La Nina the NAO response is almost opposite. Thus, the NAO responses for the CP ENSO are mostly linear, while nonlinear NAO responses dominate for the EP ENSO. These contrasting extra-tropical atmospheric responses are mainly attributed to nonlinear air-sea interactions in the tropical eastern Pacific. The local atmospheric response to the CP ENSO sea surface temperature (SST) anomalies is highly linear since the air-sea action center is located within the Pacific warm pool, characterized by relatively high climatological SSTs. In contrast, the EP ENSO SST anomalies are located in an area of relatively low climatological SSTs in the eastern equatorial Pacific. Here only sufficiently high positive SST anomalies during EP El Nino events are able to overcome the SST threshold for deep convection, while hardly any anomalous convection is associated with EP La Nina SSTs that are below this threshold. This ENSO/NAO relationship has important implications for NAO seasonal prediction and places a higher requirement on models in reproducing the full diversity of ENSO.

&amp;lt;p&amp;gt;In February 2012 a strong wind event over the Adriatic Sea lead to extreme air-sea interactions. The extreme event consisted of strong to severe cold wind (bura) event that last over three weeks accompanied by accumulation of snow on the coastline in the beginning of the event. As a result, the sea surface cooled rapidly and mixed throughtout the vertical column. The ALADIN System operational forecast used the sea surface temperature (SST) available from the global meteorological model ARPEGE. It was a practical solution for an operational application, since the same source is used also for forecast lateral boundary conditions (LBCs). Here we first compare the SST available from operational global atmospheric models ARPEGE and IFS to in situ measurements, SST analyses and ocean model output. The global models underestimated the SST cooling while the available satellite based observations do not resolve the eastern Adriatic coastline due to numerous islands. Both global models overestimated SST over the Adriatic for up to 10oC when compared to in situ measurements. Afterwards, we runset of forecasts using the SST from the OSTIA and MUR analyses or the ROMS ocean model forecast. The impact of changing SST field on forecast atmospheric fields is mostly reflected on precipitation. Model forecasts more precipitation using warmer SST. Changes in fluxes of evaporation, heat and momentum through the sea surface reflect the changes in the SST. Too warm SST yields too intense fluxes. Colder SST in the Velebit Channel causes reduced momentum flux that allows stronger bura jets above the open sea away from coastlines.&amp;lt;br&amp;gt;&amp;lt;br&amp;gt;&amp;lt;/p&amp;gt;

&lt;p&gt;This thesis investigates synoptic variability in the Ross Sea region, Antarctica and develops geochemical proxies of this variability from an ice core record in Southern Victoria Land. Particular focus is given to the influence of decadal climate oscillations on synoptic conditions and potential records of these oscillations in ice core proxy records as long-­‐term records of these oscillations are important for understanding future climate change. I present an investigation of the joint influence of the El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) on variability in the Amundsen Sea Low (ASL), a dominant climatological feature that strongly influences the weather in the Ross Sea region. It is shown that the positive phase of each oscillation is associated with significant strengthening of the ASL, while negative phases are associated with a weakening. Through regression analysis I show that a simple linear combination of indices representing these oscillations can explain more than 40% of the geopotential height variance in the AS region at a seasonal scale and over 70% of the variance at an annual scale. These results are consistent with the known mechanisms of ENSO and SAM interaction in the region and show that while SAM is dominant hemispherically, ENSO is only influential in the Pacific Sector. Finally it is demonstrated that a simple model of linear reinforcement and interference between the oscillations describes their influence on the variability in the ASL better than models incorporating more complex interactions. Atmospheric back-­‐trajectory modeling and cluster analysis are used to investigate synoptic variability at the Gawn Ice Piedmont (GIP) ice core site in the Ross Sea Region, Antarctica. I identify two dominant air-­‐mass trajectory clusters: oceanic – cyclonic and continental trajectories. My analysis shows that oceanic – cyclonic trajectories peak during April (southern hemisphere winter), while continental trajectories reach their maximum during December (summer). A causal association is demonstrated between ENSO and the frequency of oceanic – cyclonic trajectories originating from the Ross Sea region. In contrast, it is shown that the Southern Annular Mode has little influence on the frequency of cyclonic clusters. I then develop proxy records for the synoptic variability using a shallow firn core from the GIP site containing 8 years of geochemical record. Continental trajectories correlate with concentrations of nitrate (NO3), which is sourced from stratospheric air-­‐masses descending over the Antarctic interior. Oceanic – cyclonic trajectory clusters strongly correlate with deuterium excess at seasonal and inter-­‐annual scales, a proxy sensitive to changes in relative humidity and sea surface temperature (SST) in the in the Ross and Amundsen Seas. Decadal variability in the frequency of oceanic – cyclonic trajectories is discussed with respect to ENSO, SAM, and changes in SST and sea ice extent.&lt;/p&gt;

Climate variability has distinct spatial patterns with the strongest signal of sea surface temperature (SST) variance residing in the tropical Pacific. This interannual climate phenomenon, the El Niño-Southern Oscillation (ENSO), impacts weather patterns across the globe via atmospheric teleconnections. Pronounced SST variability, albeit of smaller amplitude, also exists in the other tropical basins as well as in the extratropical regions. To improve our physical understanding of internal climate variability across the global oceans, we here make the case for a conceptual model hierarchy that captures the essence of observed SST variability from subseasonal to decadal timescales. The building blocks consist of the classic stochastic climate model formulated by Klaus Hasselmann, a deterministic low-order model for ENSO variability, and the effect of the seasonal cycle on both of these models. This model hierarchy allows us to trace the impacts of seasonal processes on the statistics of observed and simulated climate variability. One of the important outcomes of ENSO’s interaction with the seasonal cycle is the generation of a frequency cascade leading to deterministic climate variability on a wide range of timescales, including the near-annual ENSO Combination Mode. Using the aforementioned building blocks, we arrive at a succinct conceptual model that delineates ENSO’s ubiquitous climate impacts and allows us to revisit ENSO’s observed statistical relationships with other coherent spatio-temporal patterns of climate variability—so called empirical modes of variability. We demonstrate the importance of correctly accounting for different seasonal phasing in the linear growth/damping rates of different climate phenomena, as well as the seasonal phasing of ENSO teleconnections and of atmospheric noise forcings. We discuss how previously some of ENSO’s relationships with other modes of variability have been misinterpreted due to non-intuitive seasonal cycle effects on both power spectra and lead/lag correlations. Furthermore, it is evident that ENSO’s impacts on climate variability outside the tropical Pacific are oftentimes larger than previously recognized and that accurately accounting for them has important implications. For instance, it has been shown that improved seasonal prediction skill can be achieved in the Indian Ocean by fully accounting for ENSO’s seasonally modulated and temporally integrated remote impacts. These results move us to refocus our attention to the tropical Pacific for understanding global patterns of climate variability and their predictability.

The NCEP Climate Forecast System version 2 (CFSv2) provides important source of information about the seasonal prediction of climate over the Indo-Pacific oceans. In this study, the authors provide a comprehensive assessment of the prediction of sea surface temperature (SST) in the tropical Indian Ocean (IO). They also investigate the impact of tropical IO SST on the summer anomalous anticyclonic circulation over the western North Pacific (WNPAC), focusing on the relative contributions of local SST and remote forcing of tropical IO SST to WNPAC variations. The CFSv2 captures the two most dominant modes of summer tropical IO SST: the IO basin warming (IOBW) mode and the IO dipole (IOD) mode, as well as their relationship with El Nino-Southern Oscillation (ENSO). However, it produces a cold SST bias in IO, which may be attributed to deeper-than-observed mixed layer and smaller-than-observed total downward heat flux in the tropical IO. It also overestimates the correlations of ENSO with IOBW and IOD, but underestimates the magnitude of IOD and summer IOBW. The CFSv2 captures the climate anomalies related to IOBW but not those related to IOD. It depicts the impact of summer IOBW on WNPAC via the equatorial Kelvin wave, which contributes to the maintenance of WNPAC in July and August. The WNPAC in June is mostly forced by local cold SST, which is better predicted by the CFSv2 compared to July and August. The mechanism for WNPAC maintenance may vary with lead time in the CFSv2.

The relative roles of internal atmospheric dynamics, land surface evaporation and sea surface temperature (SST) forcings on the coupling between the Asian monsoon (AM) and the Southern Oscillation (SO) are investigated in a series of GCM experiments. Results confirm previous studies indicating that the characteristic large-scale pattern of the SO is due primarily to SST anomaly (SSTA) forcing. The AM circulation anomalies are coupled to the SO via a characteristic upper level circulation couplet over the equatorial central Pacific. This couplet acts as a radiating node for teleconnection signals originating from the AM region to the extratropics. Generally, a weak AM is associated with warm SST over the eastern equatorial Pacific, concomitant with the negative phase of the SO, i.e., low (high) surface pressure over Tahiti (Darwin). The reverse holds for strong AM. Two wavetrains associated with the AM fluctuation have been identified: one arcing over northeastern Asia via the Aleutians to North American, and another emanating from northwestern Europe, via Siberia to northern India. Internal dynamics appear to underpin the origin of these wavetrains, which are strongly tempered by SSTA forcing and to a lesser degree by interactive land processes. Regionally, land-atmosphere interaction seems to have the strongest impact over East Asia/Indochina and the adjacent oceanic region of the South China Sea. Here, land-atmosphere interaction is responsible for the enhancement of a subseasonal scale see-saw oscillation in precipitation between land and the adjacent oceans. A local land-atmosphere feedback mechanism involving strong coupling between the hydrologic and energy cycles is identified. It is suggested that the interaction among precipitation, moisture convergence and land surface turbulent heat fluxes and radiation processes play key roles in determining the fast (subseasonal and shorter scales) response of the AM. On these time scales, the occurrences of cool/wet and hot/dry states associated with the precipitation seesaw appear to be chaotic. However, the preferred occurrence of a given state and the abrupt transition between states are dependent on the large-scale circulation and radiation forcings induced by the SO. One of the more provocative findings here is that effects of land-atmosphere interaction do not seem to alter the basic planetary scale features of the AM-SO system. As a result, the interannual variability of the coupled AM-SO is relatively small in the absence of anomalous SST forcing. Yet, the local effect of land-atmosphere interaction on AM is quite pronounced and dependent upon the large-scale forcings related to SO.

High-resolution marine data and transient simulations support orbital forcing of ENSO amplitude since the mid-Holocene