Interannual Variability in the Southeastern Tropical Atlantic Ocean: Benguela Ninos, Equatorial Atlantic Ninos and Interaction with ENSO

Interannual sea surface temperature (SST) variations of the tropical oceans are of particular interest due to their close relation to rainfall variability over adjacent land regions. For the eastern tropical Atlantic two recurring El Nino-like phenomena with high interannual SST anomalies have been described, one centered in the equatorial region as part of the Atlantic zonal mode and one off Angola referred to as Benguela Nino. Both events are supposed to be generated not locally but by a relaxation of the trade winds in the western equatorial Atlantic. Despite their similar assumed forcing mechanisms, until recently Atlantic and Benguela Ninos have been viewed as separate events. In this thesis the connection between Benguela and equatorial Atlantic Ninos is investigated with observational data sets as well as ocean model simulations. The study shows that they are correlated to such an extent that they should rather be viewed as one Atlantic Nino. An intriguing feature, counterintuitive in view of the remote forcing mechanism, is that SST anomalies off Angola precede those in the cold tongue region by one to three months. This behaviour is shown to be caused by differences in thermocline depths and in the seasonality of interannual SST variability in the two regions. While the subsurface-surface coupling in the cold tongue region reaches its maximum in June/July when the thermocline is shallowest, SST anomalies off Angola appear rather to be phase-locked to the season in which the Angola Benguela Front is at its southernmost position and the interannual variability in the strength of equatorial and subsequent coastal Kelvin waves is highest in February-March-April. Sensitivity experiments with artificial perturbations in the forcing configuration confirm the importance of remote forcing from the equator for SST variability off Angola. They also demonstrate the leading role of wind stress, in particular the Southeasterly Trades, in the generation of near surface temperature anomalies in the eastern Tropical Atlantic which are further shown to be linked to variations in the strength of the South Atlantic Anticyclone. The Trade Wind variations also affect the equatorial current system and thus also contribute to SST anomalies in the cold tongue region. Variability in the strength of the southern subtropical cell is shown to have a weak influence on eastern tropical Atlantic SST on decadal time scales. SST variations in the Tropical Atlantic are not only determined by processes within the Atlantic but appear to be influenced remotely from the Tropical Pacific; recent studies indicate that variability in the Tropical Atlantic might have an impact on ENSO as well. Interactions between SST variations in the Tropical Atlantic and Pacific are discussed in the last part of the thesis taking into account sensitivity experiments with an intermediate complexity model. The results suggest that warm (cold) anomalies in the eastern Tropical Pacific favors the developement of Atlantic cold (warm) events while warm events in the Atlantic appear to be linked to subsequent cold events in the Pacific. (übersetzt): Zwischenjährliche Schwankungen der Meeresoberflächentemperatur (SST) der tropischen Ozeane sind aufgrund ihres engen Zusammenhanges mit der Niederschlagsvariabilität angrenzender Landregionen von besonderer Bedeutung. Für den östlichen tropischen Atlantik sind zwei wiederkehrende, El Nino-ähnliche Phänomene beschrieben worden, eines zentriert im äquatorialen Bereich als Teil des zonales Modus und eines vor Angola, welches als Benguela Nino bezeichnet wird. Von beiden Ereignissen wird angenommen, dass sie nicht lokal, sondern durch eine Relaxation der Passatwinde im westlichen äquatorialen Atlantik erzeugt werden. Obwohl für beide ein ähnlicher Entstehungsmechanismus vermutet wird, sind Atlantik und Benguela Ninos bis vor kurzem als getrennte Ereignisse betrachtet worden. In dieser Arbeit wird die Verbindung zwischen Benguela und äquatorialen Atlantik Ninos sowohl mit Beobachtungsdaten als auch mit Ozeanmodellsimulationen untersucht. Es wird gezeigt, dass sie in einem solchen Ausmaß korreliert sind, dass sie besser gemeinsam als ein Atlantik Nino bezeichnet werden sollten. Verblüffenderweise, kontraintuitiv in Anbetracht des Fernantriebs, treten die SST Anomalien vor Angola ein bis drei Monate vor denen im Bereich der Kaltwasserzunge auf. Es wird gezeigt, dass dieses Verhalten durch Unterschiede in der Tiefe der Temperatursprungschicht sowie in der Saisonabhängigkeit der zwischenjährlichen SST Schwankungen verursacht wird. Während die Kopplung zwischen Oberfläche und darunter liegender Schicht im Bereich der Kaltwasserzunge im Juni/Juli, zur Zeit der flachsten Temperatursprungschicht, am größten ist, erscheinen SST Anomalien vor Angola eher an die Jahreszeit gebunden, in der die Angola Benguela Front ihre südlichste Position erreicht und die zwischenjährliche Variabilität der Stärke der äquatorialen und nachfolgenden Küstenkelvinwelle am höhsten ist. Dies ist zwischen Februar und April der Fall. Sensitivitätsexperimente mit künstlichen Störungen in der Antriebskonfiguration bestätigen die Wichtigkeit des Fernantriebs vom äquator für SST Schwankungen vor Angola. Zudem demonstieren sie die führende Rolle der Windschubspannung in der Erzeugung von oberflächennahen Temperaturanomalien im östlichen tropischen Atlantik. Weiterhin wird gezeigt, dass diese mit Schwankungen in der Stärke der subtropischen Antizyklone verbunden sind. Auch die äquatorialen Strömungen reagieren auf Passatwindänderungen und tragen so zu SST Anomalien im Bereich der Kaltwasserzunge bei. Es wird gezeigt, dass Variabilität in der Stärke der südlichen subtropischen Zelle die SST im östlichen tropischen Atlantik auf dekadischer Zeitskala schwach beinflußt. Schwankungen der Meeresoberflächentemperatur im tropischen Atlantik werden nicht nur von Prozessen innerhalb des Atlantiks bestimmt, sondern sind auch vom tropischen Pazifik beeinflußt. Neuere Studien legen nahe, dass umgekehrt auch Variabilität im Atlantik einen Einfluss auf ENSO hat. Wechselwirkungen zwischen SST Schwankungen im tropischen Atlantik und Pazifik werden im letzten Teil der Arbeit unter Einbeziehung von Experimenten mit einem Modell mittlerer Komplexität diskutiert. Die Ergebnisse lassen darauf schließen, dass warme (kalte) Anomalien im östlichen Pazifik die Entwicklung von kalten (warmen) Ereignissen im Atlantik begünstigen, während warme Ereignisse im Atlantik mit darauf folgenden kalten Episoden im Pazifik verbunden zu sein scheinen

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

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

A Comparative Stability Analysis of Atlantic and Pacific Nino Modes

El Niño–Southern Oscillation (ENSO) in the Pacific and the analogous Atlantic Niño mode are generated by processes involving coupled ocean–atmosphere interactions known as the Bjerknes feedback. It has been argued that the Atlantic Niño mode is more strongly damped than ENSO, which is presumed to be closer to neutrally stable. In this study the stability of ENSO and the Atlantic Niño mode is compared via an analysis of the Bjerknes stability index. This index is based on recharge oscillator theory and can be interpreted as the growth rate for coupled modes of ocean–atmosphere variability. Using observational data, an ocean reanalysis product, and output from an ocean general circulation model, the individual terms of the Bjerknes index are calculated for the first time for the Atlantic and then compared to results for the Pacific. Positive thermocline feedbacks in response to wind stress forcing favor anomaly growth in both basins, but they are twice as large in the Pacific compared to the Atlantic. Thermocline feedback is related to the fetch of the zonal winds, which is much greater in the equatorial Pacific than in the equatorial Atlantic due to larger basin size. Negative feedbacks are dominated by thermal damping of sea surface temperature anomalies in both basins. Overall, it is found that both ENSO and the Atlantic Niño mode are damped oscillators, but the Atlantic is more strongly damped than the Pacific primarily because of the weaker thermocline feedback

Assessing the Twenty-First-Century Shift in ENSO Variability in Terms of the Bjerknes Stability Index*

A decadal change in the character of ENSO was observed around year 2000 toward weaker-amplitude, higher-frequency events with an increased occurrence of central Pacific El Niños. Here these changes are assessed in terms of the Bjerknes stability index (BJ index), which is a measure of the growth rate of ENSO-related SST anomalies. The individual terms of the index are calculated from ocean reanalysis products separately for the time periods 1980–99 and 2000–10. The spread between the products is large, but they show a robust weakening of the thermocline feedback due to a reduced thermocline slope response to anomalous zonal wind stress as well as a weakened wind stress response to eastern equatorial Pacific SST anomalies. These changes are consistent with changes in the background state of the tropical Pacific: cooler mean SST in the eastern and central equatorial Pacific results in reduced convection there together with a westward shift in the ascending branch of the Walker circulation. This shift leads to a weakening in the relationship between eastern Pacific SST and longitudinally averaged equatorial zonal wind stress. Also, despite a steeper mean thermocline slope in the more recent period, the thermocline slope response to wind stress anomalies weakened due to a smaller zonal wind fetch that results from ENSO-related wind anomalies being more confined to the western basin. As a result, the total BJ index is more negative, corresponding to a more strongly damped system in the past decade compared to the 1980s and 1990s

Symmetry of the Atlantic Nino mode

El Niño-Southern Oscillation (ENSO) in the Pacific is asymmetric for warm and cold events with respect to amplitude, spatial patterns and temporal evolution. Here the symmetry of the Atlantic Niño mode, which many previous studies have argued is governed by atmosphere–ocean dynamics similar to those of ENSO, is investigated using two different ocean reanalysis products. Calculation of Bjerknes feedback terms for the Pacific reveals a pronounced asymmetry between warm and cold events, though unlike most previous studies, the largest asymmetry is found in the relationship between eastern Pacific thermocline depth and SST anomalies. For the Atlantic, cold events are effectively mirror images of warm events with Bjerknes feedbacks of similar strength. The analysis supports not only the conclusion that Atlantic Niños are more symmetric than ENSO, but the hypothesis itself that the Bjerknes feedback is operative in the Atlantic given the strength of the relationship between the key variables involved

Stand‐Alone Eastern Pacific Coastal Warming Events

A pronounced warm anomaly occurred at the Peruvian coast in early 2017. This “Coastal Niño” caused heavy rainfalls, leading to flooding in Peru and Ecuador. At the same time, neutral conditions prevailed in the equatorial Pacific. Using observational sea surface temperature data sets and an ocean reanalysis product for the time period 1900 to 2010, previous similar events are investigated. Eighteen coastal warming events without corresponding equatorial Pacific warming are identified. Further analysis shows, however, that only four of these events are not connected to the central equatorial Pacific. All other periods of strong coastal warm anomalies are directly followed or preceded by El Niño‐like conditions. The “stand‐alone” coastal warming events are characterized by comparatively low equatorial heat content. We thus hypothesize that the depleted heat content in the equatorial Pacific in the wake of the strong 2015/2016 El Niño prevented the warming to spread westward in 2017

Contribution of increased Agulhas leakage to tropical Atlantic warming

The upper tropical Atlantic Ocean has markedly warmed since the 1960s. It has been shown that this warming was not due to local heat fluxes, and that the trade winds that drive the coastal and equatorial upwelling have intensified rather than weakened. Remote forcing might thus have played an important role. Here model experiments are used to investigate the contribution from an increased inflow of warm Indian Ocean water through Agulhas leakage. A high-resolution hindcast experiment with interannually varying forcing for the time period 1948 to 2007, in which Agulhas leakage increases by about 45% from the 1960s to the early 2000s, reproduces the observed warming trend. To tease out the role of Agulhas leakage, a sensitivity experiment designed to only increase Agulhas leakage is used. Compared to a control simulation it shows a pronounced warming in the upper tropical Atlantic Ocean. A Lagrangian trajectory analysis confirms that a significant portion of Agulhas leakage water reaches the upper 300m of the tropical Atlantic Ocean within two decades, and that the tropical Atlantic warming in the sensitivity experiment is mainly due to water of Agulhas origin. Therefore, it is suggested that the increased trade winds since the 1960s favor upwelling of warmer subsurface waters, which in parts originate from the Agulhas, leading to higher SSTs in the tropic