Equatorial Variability Research Articles

The Influence of Equatorial Cooling on Axially Symmetric Atmospheric Circulation Forced by Annually Averaged Heating

Abstract Axisymmetric theory of atmospheric circulation is extended for the case of concentrated equatorial cooling and annually averaged heating. The solutions are derived in a 1.5-layer shallow water model on the spherical Earth, which includes vertical mixing with diagnostic surface momentum. The axisymmetric solutions capture the sensitivity of the large-scale circulation to equatorial cooling seen in observations and in eddy-permitting models, namely, (i) weakening and widening of the meridional overturning circulation (MOC) and (ii) weakening and poleward shift of the subtropical jet. For sufficiently large equatorial cooling, a tropical anti-Hadley cell emerges that transports energy equatorward, balancing the equatorial energetic sink. The analytic solutions predict the critical cooling required for the emergence of the anti-Hadley cell and provide a simple mechanism for the response of the MOC to equatorial cooling. Specifically, equatorial cooling reduces net tropical heating, which weakens the circulation and shifts the edge of the rising branch poleward. This in turn reduces upper-level momentum which is set by surface momentum in the rising branch. The subtropical meridional temperature gradient decreases with upper-level momentum, requiring a widening of the circulation to close the energy budget. The subtropical jet therefore shifts poleward with the edge of the MOC and weakens due to the reduced upper-level angular momentum. The strong sensitivity to equatorial cooling seen in the axisymmetric system suggests that the above mechanism may have an important role in the sensitivity of the MOC to equatorial temperature anomalies on seasonal or longer time scales. Significance Statement Axisymmetric solutions for the response of the atmospheric circulation to concentrated equatorial cooling are derived, motivated by the equatorial cooling by the cold tongues of the Pacific and Atlantic. The solutions capture the response of the large-scale realistic atmosphere to equatorial cooling, namely a weakening and widening of the meridional overturning circulation and a weakening and poleward shift of the subtropical jet. In addition, for sufficiently strong cooling, a tropical anti-Hadley cell emerges. The solutions provide insight into the influence of biases in the representation of the cold tongues in climate models, the relation of interannual equatorial variability to the large-scale circulation, and changes in the large-scale atmospheric circulation on geological time scales.

The 2021 Atlantic Niño and Benguela Niño Events: external forcings and air–sea interactions

AbstractThis paper presents a comprehensive analysis of the extreme Atlantic and Benguela Niño events that occurred during the boreal spring–summer of 2021. We conducted sensitivity experiments with a regional ocean–atmosphere coupled model of the tropical Atlantic to investigate the phenology of these interannual events, unravel their triggering mechanisms, and quantify the contributions of local and remote processes. The results revealed that both the 2021 Atlantic and Benguela Niños were triggered by anomalous atmospheric fluxes at the model southern boundary (32° S), leading to a significant and persistent weakening of the South Atlantic Anticyclone. The associated poleward anomalous coastal wind off Africa reduced coastal upwelling and evaporation south of 15° S, initiating the Benguela Niño. Then, the relaxation of the equatorial trade winds forced a downwelling equatorial Kelvin wave, which warmed the eastern equatorial region, marking the onset of the Atlantic Niño. The equatorial event reached full maturity in July 2021 through ENSO-like air-sea interactions in the equatorial basin, enhanced by the atmospheric connection associated with low-level winds converging toward the distant coastal warming. While air–sea interactions in the tropical Atlantic acted as a negative feedback for the coastal warming, the ocean connection with the equatorial variability through the propagation of equatorially-forced downwelling coastal waves intensified the coastal warming, peaking end of May 2021. Overall, this research provides valuable insights into the complex dynamics of Atlantic and Benguela Niños, emphasizing the interconnectedness between these two systems. This has important implications for improving Earth system models which currently struggle to simulate these extreme events.

Spatial and seasonal variability of horizontal temperature fronts in the Mozambique Channel for both epipelagic and mesopelagic realms

IntroductionOcean fronts are moving ephemeral biological hotspots forming at the interface of cooler and warmer waters. In the open ocean, this is where marine organisms, ranging from plankton to mesopelagic fish up to megafauna, gather and where most fishing activities concentrate. Fronts are critical ecosystems so that understanding their spatio-temporal variability is essential not only for conservation goals but also to ensure sustainable fisheries. The Mozambique Channel (MC) is an ideal laboratory to study ocean front variability due to its energetic flow at sub-to-mesoscales, its high biodiversity and the currently debated conservation initiatives. Meanwhile, fronts detection relying solely on remotely-sensed Sea Surface Temperature (SST) cannot access aspects of the subsurface frontal activity that may be relevant for understanding ecosystem dynamics.MethodIn this study, we used the Belkin and O’Reilly Algorithm on remotely-sensed SST and hindcasts of a high-resolution nested ocean model to investigate the spatial and seasonal variability of temperature fronts at different depths in the MC.ResultsWe find that the seasonally varying spatial patterns of frontal activity can be interpreted as resulting from main features of the mean circulation in the MC region. In particular, horizontally, temperature fronts are intense and frequent along continental shelves, in islands’ wakes, at the edge of eddies, and in the pathways of both North-East Madagascar Current (NEMC) and South-East Madagascar Current (SEMC). In austral summer, thermal fronts in the MC are mainly associated with the Angoche upwelling and seasonal variability of the Mozambique current. In austral winter, thermal fronts in the MC are more intense when the NEMC and the Seychelles-Chagos and South Madagascar upwelling cells intensify. Vertically, the intensity of temperature fronts peaks in the vicinity of the mean thermocline.DiscussionConsidering the marked seasonality of frontal activity evidenced here and the dynamical connections of the MC circulation with equatorial variability, our study calls for addressing longer timescales of variability to investigate how ocean ecosystem/front interactions will evolve with climate change.

Marine heatwaves in the Humboldt current system: from 5-day localized warming to year-long El Ni\xf1os

During the last 4 decades punctual occurrences of extreme ocean temperatures, known as marine heatwaves (MHWs), have been regularly disrupting the coastal ecosystem of the Peru-Chile eastern boundary upwelling system. In fact, this coastal system and biodiversity hot-spot is regularly impacted by El Niño events, whose variability has been related to the longest and most intense MHWs in the world ocean. However the intensively studied El Niños tend to overshadow the MHWs of shorter duration that are significantly more common in the region. Using sea surface temperature data from 1982 to 2019 we investigate the characteristics and evolution of MHWs, distinguishing events by duration. Results show that long duration MHWs (> 100 days) preferentially affect the coastal domain north of 15° S and have decreased in both occurrence and intensity in the last four decades. On the other hand, shorter events, which represent more than 90% of all the observed MHWs, are more common south of 15° S and show an increase in their thermal impact as well as on the number of affected days, particularly those spanning 30–100 days. We also show that long duration MHWs variability in the coastal domain is well correlated with the remote equatorial variability while the onset of short events (< 10 days) generally goes along with a relaxation of the local coastal wind.

Impact of Intraseasonal Waves on Angolan Warm and Cold Events

AbstractThe intraseasonal variability of the tropical eastern boundary upwelling region in the Atlantic Ocean is investigated using multiyear mooring and satellite data. Pronounced oscillations of alongshore velocity and sea level off Angola at periods of about 90 and 120 days are observed. Similar spectral peaks are detected along the equator suggesting an equatorial forcing via equatorial and coastally trapped waves. Equatorial variability at 90 days is enhanced only in the eastern Atlantic likely forced by local zonal wind fluctuations. Variability at 120 days is generally stronger and linked to a second equatorial basin mode covering the whole equatorial basin. Besides forcing of the 120‐day variability by equatorial zonal winds, additional forcing of the resonant basin mode likely originates in the central and western tropical North Atlantic. The coastally trapped waves generated at the eastern boundary by the impinging equatorial Kelvin waves that are detected through their variations in sea level anomaly are associated with corresponding sea surface temperature anomalies delayed by about 14 days. Off Angola, those intraseasonal waves interfere with major coastal warm and cold events that occur every few years by either enhancing them as for the Benguela Niño in 1995 or damping them as for the warm event in 2001.

Observations of long period HF Doppler oscillations over the dip equatorial ionosphere in the Indian longitudinal sector

Background/ Objective: The signatures of long period High Frequency Doppler (HFD) oscillations over a dip equatorial station, Trivandrum (8.33◦N, 77◦E, dip 0.4◦N) in the Indian longitudinal sector have been investigated. Methods: High cadence vertical plasma drift measurements obtained using a Multi frequency HF Doppler Radar over Trivandrum along with ground based magnetometer observations over several latitudes/longitudes have been used to address this very aspect. Findings: It has been observed that long period (~15 min.) oscillations of high latitude origin can instantaneously reflect over the equatorial latitudes in the form of coherent HFD oscillations. The study is very important as it provided a unique opportunity not only to understand various characteristics of HFD oscillations, but also expected to have significant implications in the understanding of equatorial ionospheric variability in context of solar wind-magnetosphere-ionosphere coupling. Novelty: In a complementary point of view, the study also highlights the potential of HF Doppler measurements to capture such transient events originating from interplanetary medium with an unprecedented temporal resolution. Keywords: HF Doppler radar; equatorial ionosphere; HF Doppler oscillations

A modified seasonal cycle during MIS31 super-interglacial favors stronger interannual ENSO and monsoon variability

Abstract. It has long been recognized that the amplitude of the seasonal cycle can substantially modify climate features in distinct timescales. This study evaluates the impact of the enhanced seasonality characteristic of the Marine Isotope Stage 31 (MIS31) on the El Niño–Southern Oscillation (ENSO). Based upon coupled climate simulations driven by present-day (CTR) and MIS31 boundary conditions, we demonstrate that the CTR simulation shows a significant concentration of power in the 3–7-year band and on the multidecadal timescale between 15 and 30 years. However, the MIS31 simulation shows drastically modified temporal variability of the ENSO, with stronger power spectrum at interannual timescales but the absence of decadal periodicity. Increased meridional gradient of sea surface temperature (SST) and wind stress in the Northern Hemisphere subtropics are revealed to be the primary candidates responsible for changes in the equatorial variability. The oceanic response to the MIS31 ENSO extends to the extratropics, and fits nicely with SST anomalies delivered by paleoreconstructions. The implementation of the MIS31 conditions results in a distinct global monsoon system and its link to the ENSO in respect to current conditions. In particular, the Indian monsoon intensified but no correlation with ENSO is found in the MIS31 climate, diverging from conditions delivered by our current climate in which this monsoon is significantly correlated with the NIÑO34 index. This indicates that monsoonal precipitation for this interglacial is more closely connected to hemispherical features than to the tropical–extratropical climate interaction.

Subseasonal Coastal‐Trapped Wave Propagations in the Southeastern Pacific and Atlantic Oceans: 2. Wave Characteristics and Connection With the Equatorial Variability

AbstractThe objective of this study is to compare the characteristics of the oceanic teleconnection with the linear equatorial dynamics of two upwelling systems along the southwestern South American and African continents at subseasonal time scales (&lt;120 days). Altimetric data analysis shows that the coastal variability remains coherent with the equatorial signal until 27°S in the southeastern Pacific (SEP), while in the southeastern Atlantic (SEA) it fades out south of 12°S. To explain this striking difference, our methodology is based on the experimentation with twin regional model configurations of the SEP and SEA Oceans. The estimation of free Coastal‐Trapped Waves (CTWs) modal structures and associated contribution to coastal variability allows inferring and comparing the characteristics of each CTW mode in the two systems; namely, their forcings, amplitude, dissipation rate, and scattering. Results show that the Pacific subseasonal equatorial forcing is only 20% larger than in the Atlantic, but important differences in the relative contribution of each baroclinic mode are reported. The first baroclinic mode dominates the eastern equatorial Pacific variability, while in the eastern equatorial Atlantic, the second mode is the most energetic. This leads to a drastic increase in the dissipation and scattering of the remotely forced CTW in the SEA sector, compared to the coastal SEP. Concomitantly, south of 15°S, the subseasonal coastal wind stress forcing is substantially more energetic in the SEA and participates in breaking the link between the equatorial forcing and the coastal variability. Our results are consistent with the solutions of a simple multimode CTW model.

Subseasonal Coastal‐Trapped Wave Propagations in the Southeastern Pacific and Atlantic Oceans: 1. A New Approach to Estimate Wave Amplitude

AbstractThe Humboldt and the Benguela upwelling systems are connected to the equatorial variability through the coastal waveguide, so that a large variance of the coastal sea level and current variability can be described as an infinite sum of orthonormal free Coastal‐Trapped Wave (CTW) modes. The objective of this study is to infer the CTW mode contributions to the coastal variability in both systems at subseasonal timescales (&lt;120 days) from regional ocean circulation model simulations. We develop and validate twin regional model configurations of the southeastern Pacific and Atlantic Oceans. Cross‐shore spatial structures of the first four free CTW modes are then derived from model mean stratification and topography along the southwestern African and South American continents. We introduce and validate a new methodology to estimate the gravest CTW mode contributions to model pressure and alongshore current. Our formulation draws on the orthonormality of the CTW modal structures, and uses a simple projection of the coastal and bottom model pressure onto each CTW structure. Results give confidence in the ability of this modal decomposition methodology to disentangle CTW mode contributions from complex nonlinear coastal processes that control the coastal subseasonal variability. In both systems, it allows to successfully extract the gravest poleward propagating CTW modes with velocities close to the theoretical values and amplitudes consistent with the solutions of a simple multimode linear CTW model. Furthermore, results show that both systems exhibit relatively different CTW dynamics and forcings which are discussed in the companion paper (Illig et al., 2018).

Forcing mechanisms of intraseasonal SST variability off central Peru in 2000–2008

AbstractThe Sea Surface Temperature (SST) intraseasonal variability (40–90 days) along the coast of Peru is commonly attributed to the efficient oceanic connection with the equatorial variability. Here we investigate the respective roles of local and remote equatorial forcing on the intraseasonal SST variability off central Peru (8°S–16°S) during the 2000–2008 period, based on the experimentation with a regional ocean model. We conduct model experiments with different open lateral boundary conditions and/or surface atmospheric forcing (i.e., climatological or not). Despite evidence of clear propagations of coastal trapped waves of equatorial origin and the comparable marked seasonal cycle in intraseasonal Kelvin wave activity and coastal SST variability (i.e., peak in Austral summer), this remote equatorial forcing only accounts for ∼20% of the intraseasonal SST regime, which instead is mainly forced by the local winds and heat fluxes. A heat budget analysis further reveals that during the Austral summer, despite the weak along‐shore upwelling (downwelling) favorable wind stress anomalies, significant cool (warm) SST anomalies along the coast are to a large extent driven by Ekman‐induced advection. This is shown to be due to the shallow mixed layer that increases the efficiency by which wind stress anomalies relates to SST through advection. Diabatic processes also contribute to the SST intraseasonal regime, which tends to shorten the lag between peak SST and wind stress anomalies compared to what is predicted from an advective mixed‐layer model.

Improved Representation of Tropical Pacific Ocean–Atmosphere Dynamics in an Intermediate Complexity Climate Model

Abstract A new anomaly coupling technique is introduced into a coarse-resolution dynamic climate model [the Liège Ocean Carbon Heteronomous model (LOCH)–Vegetation Continuous Description model (VECODE)–Earth System Models of Intermediate Complexity Climate deBilt (ECBILT)–Coupled Large-Scale Ice–Ocean model (CLIO)–Antarctic and Greenland Ice Sheet Model (AGISM) ensemble (LOVECLIM)], improving the model’s representation of eastern equatorial Pacific surface temperature variability. The anomaly coupling amplifies the surface diabatic atmospheric forcing within a Gaussian-shaped patch applied in the tropical Pacific Ocean. It is implemented with an improved predictive cloud scheme based on empirical relationships between cloud cover and key state variables. Results are presented from a perturbed physics ensemble systematically varying the parameters controlling the anomaly coupling patch size, location, and amplitude. The model’s optimal parameter combination is chosen through calibration against the observed power spectrum of monthly-mean surface temperature anomalies in the Niño-3 region. The calibrated model exhibits substantial improvement in equatorial Pacific interannual surface temperature variability and robustly reproduces El Niño–Southern Oscillation (ENSO)-like variability. The authors diagnose some of the key atmospheric and oceanic feedbacks in the model important for simulating ENSO-like variability, such as the positive Bjerknes feedback and the negative heat flux feedback, and analyze the recharge–discharge of the equatorial Pacific ocean heat content. They find LOVECLIM robustly captures important ocean dynamics related to thermocline adjustment and equatorial Kelvin waves. The calibrated model demonstrates some improvement in simulating atmospheric feedbacks, but the coupling between ocean and atmosphere is relatively weak. Because of the tractability of LOVECLIM and its consequent utility in exploring long-term climate variability and large ensemble perturbed physics experiments, improved representation of tropical Pacific ocean–atmosphere dynamics in the model may more readily allow for the investigation of the role of tropical Pacific ocean–atmosphere dynamics in past climate changes.

ENSO regimes and the late 1970's climate shift: The role of synoptic weather and South Pacific ocean spiciness

South Pacific subtropical density compensated temperature and salinity (spiciness) anomalies are known to be associated with decadal equatorial variability, however, the mechanisms by which such disturbances are generated, advect and the degree to which they modulate the equatorial thermocline remains controversial. During the late 1970's a climate regime transition preceded a period of strong and sustained El Nino events. Using an ocean general circulation model forced by the constituent mechanical and thermodynamic components of the reanalysed atmosphere we show that the late 1970's transition coincided with the arrival of a large-scale, subsurface cold and fresh water anomaly in the central tropical Pacific. An ocean reanalysis for the period 1990–2007 that assimilates subsurface Argo, XBT and CTD data, reveals that disturbances occur due to the subduction of negative surface salinity anomalies from near 30° S, 100° W which are advected along the σ=25–26 kgm−3 isopycnal surfaces. These anomalies take, on average, seven years to reach the central equatorial Pacific where they may substantially perturb the thermocline before the remnants ultimately ventilate in the region of the western Pacific warm pool. Positive (warm–salty) disturbances, known to occur due to late winter diapycnal mixing and isopycnal outcropping, arise due to both subduction of subtropical mode waters and subsurface injection. On reaching the equatorial band (10° S–0° S) these disturbances tend to deepen the thermocline reducing the model's ENSO. In contrast the emergence of negative (cold–fresh) disturbances at the equator are associated with a shoaling of the thermocline and El Nino events. Process studies are used to show that the generation and advection of anomalous density compensated thermocline disturbances critically depend on stochastic forcing of the intrinsic ocean by weather. We further show that in the absence of the inter-annual component of the atmosphere forcing Central Pacific El Nino events are manifest.

Basin Resonances in the Equatorial Indian Ocean

Abstract Previous studies have investigated how second-baroclinic-mode (n = 2) Kelvin and Rossby waves in the equatorial Indian Ocean (IO) interact to form basin resonances at the semiannual (180 day) and 90-day periods. This paper examines unresolved issues about these resonances, including the reason the 90-day resonance is concentrated in the eastern ocean, the time scale for their establishment, and the impact of complex basin geometry. A hierarchy of ocean models is used: an idealized one-dimensional (1D) model, a linear continuously stratified ocean model (LCSM), and an ocean general circulation model (OGCM) forced by Quick Scatterometer (QuikSCAT) wind during 2000–08. Results indicate that the eastern-basin concentration of the 90-day resonance happens because the westward-propagating Rossby wave is slower, and thus is damped more than the eastward-propagating Kelvin wave. Results also indicate that superposition with other baroclinic modes further enhances the eastern maximum and weakens sea level variability near the western boundary. Without resonance, although there is still significant power at 90 and 180 days, solutions have no spectral peaks at these periods. The key time scale for the establishment of all resonances is the time it takes a Kelvin wave to cross the basin and a first-meridional-mode (ℓ = 1) Rossby wave to return; thus, even though the amplitude of the 90-day winds vary significantly, the 90-day resonance can be frequently excited in the real IO, as evidenced by satellite-observed and OGCM-simulated sea level. The presence of the Indian subcontinent enhances the influence of equatorial variability in the north IO, especially along the west coast of India. The Maldives Islands weaken the 180-day resonance amplitude but have little effect on the 90-day resonance, because they fall in its “node” region. Additionally, resonance at the 120-day period for the n = 1 mode is noted.

The Climatology of the Middle Atmosphere in a Vertically Extended Version of the Met Office’s Climate Model. Part II: Variability

Abstract Stratospheric variability is examined in a vertically extended version of the Met Office global climate model. Equatorial variability includes the simulation of an internally generated quasi-biennial oscillation (QBO) and semiannual oscillation (SAO). Polar variability includes an examination of the frequency of sudden stratospheric warmings (SSW) and annular mode variability. Results from two different horizontal resolutions are also compared. Changes in gravity wave filtering at the higher resolution result in a slightly longer QBO that extends deeper into the lower stratosphere. At the higher resolution there is also a reduction in the occurrence rate of sudden stratospheric warmings, in better agreement with observations. This is linked with reduced levels of resolved waves entering the high-latitude stratosphere. Covariability of the tropical and extratropical stratosphere is seen, linking the phase of the QBO with disturbed NH winters, although this linkage is sporadic, in agreement with observations. Finally, tropospheric persistence time scales and seasonal variability for the northern and southern annular modes are significantly improved at the higher resolution, consistent with findings from other studies.

Equatorial Atlantic interannual variability: Role of heat content

The dynamics of the equatorial Atlantic zonal mode are studied using observed sea surface height (SSH), sea surface temperature (SST), and heat flux and reanalysis wind stress and upper ocean temperature. Principal oscillation pattern (POP) analysis shows that the zonal mode is an oscillatory normal mode of the observed coupled system, obeying the delayed action/recharge oscillator paradigm for El Niño Southern Oscillation. Variations in equatorial averaged SSH, a proxy for upper ocean heat content, precede SST anomalies in the cold tongue by 4–5 months, about a quarter of the POP period. Positive subsurface temperature anomalies appear in the west, as a delayed response to the preceding cold event. These propagate eastward, where due to the shallow thermocline they can influence SST, leading to the next warm event. Although SST variations exhibit weak westward propagation during some zonal mode events, POP analysis indicates that to first order there is no zonal propagation in SST. Net surface heat flux anomalies generally act to damp SST anomalies. The zonal mode explains a large amount (70%) of SST variability in the east and a significant fraction (19%) of equatorial variability. Thus, the predictability potential in the equatorial Atlantic on seasonal time scales may be considerably higher than currently thought.

Low-Frequency Modulation of Intraseasonal Equatorial Kelvin Wave Activity in the Pacific from SODA: 1958–2001

Abstract Intraseasonal equatorial Kelvin wave activity (IEKW) at a low frequency in the Pacific is investigated using the Simple Ocean Data Assimilation (SODA) oceanic reanalyses. A vertical and horizontal mode decomposition of SODA variability allows estimation of the Kelvin wave amplitude according to the most energetic baroclinic modes. A wavenumber–frequency analysis is then performed on the time series to derive indices of modulation of the IEKW at various frequency bands. The results indicate that the IEKW activity undergoes a significant modulation that projects onto baroclinic modes and is not related in a straightforward manner to the low-frequency climate variability in the Pacific. Linear model experiments corroborate that part of the modulation of the IEKW is tightly linked to change in oceanic mean state rather than to the low-frequency change of atmospheric equatorial variability.

Vertical structure variability in a seasonal simulation of a medium-resolution regional model of the Eastern South Pacific

A seasonal simulation from a medium-resolution ocean general circulation mode (OGCM) is used to investigate the vertical structure variability of the Southeast Pacific (SEP). The focus is on the extra-tropical Rossby wave (ETRW) variability and associated forcing mechanism. Some aspects of the model mean state are validated from available observations, which justifies a vertical mode decomposition of the model variability. The analysis of the baroclinic mode contributions to sea level indicates that the gravest mode is dominant over most of the domain at all frequencies. Annual variability is on average twice as large as the semi-annual variability which is confined near the coast for all the modes. The first baroclinic mode contribution to the annual cycle exhibits a clear westward propagation north of the critical latitude. The higher-order modes only contribute near the coast where they are associated with vertically propagating energy. The residual variability, which is the energy at all timescales other than annual and semi-annual periods peaks offshore between ∼20°S and ∼30°S for all baroclinic modes. The third baroclinic mode also exhibits a relative maximum variability off the coast of Peru south of the critical latitude of the annual cycle (∼13°S), where the Peru–Chile Undercurrent is the most intense. Sensitivity experiments to the atmospheric and boundary forcing suggest that the residual variability results from the non-linear interaction between annual Rossby waves and the mean flow, while the annual ETRWs in the model result from the summed-contribution from both the local wind stress and remote equatorial forcing. Overall the study extends the classical analysis of sea level variability in the SEP based on linear theory, and suggests that the peculiarities of the baroclinic modes need to be taken into account for interpreting the sea level variability and understanding its connection with the equatorial variability.

Vertical propagation of extratropical Rossby waves during the 1997–1998 El Niño off the west coast of South America in a medium‐resolution OGCM simulation

A medium‐resolution ocean general circulation model (OGCM) simulation is used to investigate the subthermocline interannual variability in the eastern South Pacific. The focus is on isotherm vertical displacement variability associated with extratropical Rossby waves (ETRW) and their connection with equatorial Kelvin waves (EKW). The WKB theory is used to interpret the modeled subsurface variability. The analysis reveals vertical propagation of energy associated with the 1997–1998 El Niño near the coast at various latitudes. Consistent with theory, WKB raypaths are steeper southward and do not extend more than ∼600 km from the coast at ∼2000 m depth. A vertical mode decomposition of model variability showed that vertical propagation mostly involves the first three baroclinic modes. The vertical isotherm displacements along the raypaths, associated with the downwelling EKW of the 1997–1998 El Niño, are negative (rising isotherms) and peak (minimum isotherm) as El Niño reverses to La Niña conditions. The relationship between vertical propagation of ETRW and equatorial variability is interpreted in the light of EKW sequence. The evolution of the vertical isotherm displacements along the raypath at extratropical latitudes is controlled by the vertical structure of the EKW. As El Niño develops, high‐order baroclinic mode contributions to the EKW progressively increase, leading to the dominance of finer vertical scales of variability along the coast, triggering the vertical propagation of ETRW. This mechanism of connection between equatorial variability and subsurface variability at the extratropical latitudes illustrates how sensitive the ETRW characteristics in the eastern South Pacific are to equatorial forcing.

Variability in the subtropical-tropical cells and its effect on near-surface temperature of the equatorial Pacific: a model study

Abstract. A set of experiments utilizing different implementations of the global ORCA-LIM model with horizontal resolutions of 2°, 0.5° and 0.25° is used to investigate tropical and extra-tropical influences on equatorial Pacific SST variability at interannual to decadal time scales. The model experiments use a bulk forcing methodology building on the global forcing data set for 1958 to 2000 developed by Large and Yeager (2004) that is based on a blend of atmospheric reanalysis data and satellite products. Whereas representation of the mean structure and transports of the (sub-) tropical Pacific current fields is much improved with the enhanced horizontal resolution, there is only little difference in the simulation of the interannual variability in the equatorial regime between the 0.5° and 0.25° model versions, with both solutions capturing the observed SST variability in the Niño3-region. The question of remotely forced oceanic contributions to the equatorial variability, in particular, the role of low-frequency changes in the transports of the Subtropical Cells (STCs), is addressed by a sequence of perturbation experiments using different combinations of fluxes. The solutions show the near-surface temperature variability to be governed by wind-driven changes in the Equatorial Undercurrent. The relative contributions of equatorial and off-equatorial atmospheric forcing differ between interannual and longer, (multi-) decadal timescales: for the latter there is a significant impact of changes in the equatorward transport of subtropical thermocline water associated with the lower branches of the STCs, related to variations in the off-equatorial trade winds. A conspicuous feature of the STC variability is that the equatorward transports in the interior and along the western boundary partially compensate each other at both decadal and interannual time scales, with the strongest transport extrema occurring during El Niño episodes. The behaviour is rationalized in terms of a wobbling in the poleward extents of the tropical gyres, which is manifested also in a meridional shifting of the bifurcation latitudes of the North and South Equatorial Current systems.

Local Coupled Equatorial Variability versus Remote ENSO Forcing in an Intermediate Coupled Model of the Tropical Atlantic

Abstract The relative roles played by the remote El Niño–Southern Oscillation (ENSO) forcing and the local air–sea interactions in the tropical Atlantic are investigated using an intermediate coupled model (ICM) of the tropical Atlantic. The oceanic component of the ICM consists of a six-baroclinic mode ocean model and a simple mixed layer model that has been validated from observations. The atmospheric component is a global atmospheric general circulation model developed at the University of California, Los Angeles (UCLA). In a forced context, the ICM realistically simulates both the sea surface temperature anomaly (SSTA) variability in the equatorial band, and the relaxation of the Atlantic northeast trade winds and the intensification of the equatorial westerlies in boreal spring that usually follows an El Niño event. The results of coupled experiments with or without Pacific ENSO forcing and with or without explicit air–sea interactions in the equatorial Atlantic indicate that the background energy in the equatorial Atlantic is provided by ENSO. However, the time scale of the variability and the magnitude of some peculiar events cannot be explained solely by ENSO remote forcing. It is demonstrated that the peak of SSTA variability in the 1–3-yr band as observed in the equatorial Atlantic is due to the local air–sea interactions and is not a linear response to ENSO. Seasonal phase locking in boreal summer is also the result of the local coupling. The analysis of the intrinsic sustainable modes indicates that the Atlantic El Niño is qualitatively a noise-driven stable system. Such a system can produce coherent interdecadal variability that is not forced by the Pacific or extraequatorial variability. It is shown that when a simple slab mixed layer model is embedded into the system to simulate the northern tropical Atlantic (NTA) SST variability, the warming over NTA following El Niño events have characteristics (location and peak phase) that depend on air–sea interaction in the equatorial Atlantic. In the model, the interaction between the equatorial mode and NTA can produce a dipolelike structure of the SSTA variability that evolves at a decadal time scale. The results herein illustrate the complexity of the tropical Atlantic ocean–atmosphere system, whose predictability jointly depends on ENSO and the connections between the Atlantic modes of variability.