Description: Previous studies have shown that ENSO's anomalous equatorial winds, including the observed southward shift of zonal winds that occurs around the event peak, can be reconstructed with the first two Empirical Orthogonal Functions (EOFs) of equatorial region wind stresses. Using a high-resolution ocean general circulation model, we investigate the effect of these two EOFs on changes in warm water volume (WWV), interhemispheric mass transports, and Indonesian Throughflow (ITF). Wind stress anomalies associated with the first EOF produce changes in WWV that are dynamically consistent with the conceptual recharge oscillator paradigm. The ITF is found to heavily damp these WWV changes, reducing their variance by half. Wind stress anomalies associated with the second EOF, which depicts the southward wind shift, are responsible for WWV changes that are of comparable magnitude to those driven by the first mode. The southward wind shift is also responsible for the majority of the observed interhemispheric upper ocean mass exchanges. These winds transfer mass between the Northern and the Southern Hemisphere during El Niño events. Whilst water is transferred in the opposite direction during La Niña events, the magnitude of this exchange is roughly half of that seen during El Niño events. Thus, the discharging of WWV during El Niño events is meridionally asymmetric, while the WWV recharging during a La Niña event is largely symmetric. The inclusion of the southward wind shift is also shown to allow ENSO to exchange mass with much higher latitudes than that allowed by the first EOF alone.

Description: On the basis of integrations of an eddy-permitting coupled physical-biological model of the tropical Pacific we explore changes in the simulated mean circulation as well as its intraseasonal to interannual variability driven by the biologically modulated vertical absorption profiles of solar radiation. Three sensitivity ocean hind-cast experiments, covering the period from 1948 to 2003, are performed. In the first one, simulated chlorophyll affects the attenuation of light in the water column, while in the second experiment, the chlorophyll concentration is kept constant in time by prescribing an empirically derived spatial pattern. The third experiment uses a spatially and temporally constant value for the attenuation depth. The biotically induced differential heating is generated by increased absorption of light in the surface layers, leading to a surface warming and subsurface cooling. The effect is largest in the eastern equatorial Pacific. However, the initial vertical redistribution of heat leads to considerable changes of the near-surface ocean circulation subsequently influencing the near-surface temperature structure. In general, including biophysical coupling improves the model performance in terms of temperature and ocean circulation patterns. In particular, the upwelling in the eastern equatorial Pacific is enhanced, the mixed layer becomes shallower, the warm bias in the eastern Pacific is reduced, and the zonal temperature gradient increases. This leads to stronger La Niña events and an associated increase in the variability of the Niño3 SSTA time series. Furthermore, the eddy kinetic energy (EKE) associated with mesoscale eddies in the eastern equatorial Pacific increases by almost 100% because of enhanced EKE production due to enhanced horizontal and vertical shear of the mean currents.

Description: We present a numerical eigenmode analysis of an intermediate El Nin˜o–Southern Oscillation (ENSO) model which is driven by present-day observed background conditions as well as by simulated background conditions for the Last Glacial Maximum (LGM) about 21,000 years ago. The background conditions are obtained from two LGM simulations which were performed with the National Center for Atmospheric Research climate system model (CSM1.4) and an Earth system model of intermediate complexity (ECBilt-CLIO). Our analysis clearly shows that the leading present-day unstable recharge-discharge mode changes its stability as well as its frequency during LGM conditions. Simulated LGM background conditions were favorable to support large-amplitude self-sustained interannual ENSO variations in the tropical Pacific. Our analysis indicates that off-equatorial climate conditions as well as a shoaling of the thermocline play a crucial role in amplifying the LGM ENSO mode.

Description: We use a global coupled atmosphere‐ocean sea‐ice model of intermediate complexity to demonstrate that wind‐forcing is a crucial element to sustain meridional overturning flow in the Atlantic. Neglecting wind‐stress in our multi‐century‐long simulations leads to a complete shutdown of the conveyor belt circulation. This result may have tremendous impacts for an assessment of the sensitivity of 2‐d climate models which typically do not capture wind‐driven gyres. It is argued that wind effects may be a key element in determining the fate and length of a collapsed THC state. Possible paleo implications will be discussed.

Description: Unraveling the processes responsible for Earth’s climate transition from an “El Niño–like state” during the warm early Pliocene into a modern‐like “La Niña–dominated state” currently challenges the scientific community. Recently, the Pliocene climate switch has been linked to oceanic thermocline shoaling at ∼3 million years ago along with Earth’s final transition into a bipolar icehouse world. Here we present Pliocene proxy data and climate model results, which suggest an earlier timing of the Pliocene climate switch and a different chain of forcing mechanisms. We show that the increase in North Atlantic meridional overturning circulation between 4.8 and 4.0 million years ago, initiated by the progressive closure of the Central American Seaway, triggered overall shoaling of the tropical thermocline. This preconditioned the turnaround from a warm eastern equatorial Pacific to the modern equatorial cold tongue state about 1 million years earlier than previously assumed. Since ∼3.6–3.5 million years ago, the intensification of Northern Hemisphere glaciation resulted in a strengthening of the trade winds, thereby amplifying upwelling and biogenic productivity at low latitudes.

Description: The Galápagos Islands provide a topographic barrier for the Southern Equatorial Current (SEC) and the Equatorial Undercurrent (EUC). An island wake effect can be diagnosed from the difference of an ocean general circulation model simulation which includes the Galápagos Islands and one which ignores their presence. Cold thermocline water upwells on the western side of the islands, and only during boreal winter season these cold waters can linger around the Islands at a depth of about 80 m and affect the far eastern equatorial Pacific surface waters. This effect is partly offset by the westward transport of cold surface waters by the SEC which creates a wake on the western side of the Islands. It is furthermore shown that changes in horizontal current shear, induced by the presence of the Galápagos Islands modify the generation of tropical instability waves and lead to a basin scale SST anomaly pattern.

Description: Based on a Coupled General Circulation Model (CGCM) simulation we study the influence of anthropogenic greenhouse warming on the stability of the El Niño-Southern Oscillation phenomenon (ENSO). The linear stability of such a complex model cannot be assessed directly, hence we will derive empirical low order models for ENSO from the CGCM simulation under consideration. These models capture essential features of ENSO and are sensitive also to temporal changes in ENSO statistics. An eigenvalue analysis of these reduced models reveals that as greenhouse warming progresses a transition takes place from a stable oscillatory behavior to an unstable oscillation. This transition coincides with an abrupt change in simulated ENSO activity and can be explained in terms of changing ocean dynamics.

Description: Based on the analysis of a low-order tropical atmosphere-ocean model we propose a nonlinear mechanism explaining several features of the observed El Niño-Southern Oscillation (ENSO) phenomenon: ENSO irregularity, ENSO Amplitude Modulations and decadal tropical climate variability. The mechanism suggested here is based on the idea of homoclinic/heteroclinic orbits, an inherently nonlinear concept. It turns out that this mechanism operates even in the presence of wind noise and is consistent with results from intermediate ENSO model simulations.

Description: We use a global coupled atmosphere-ocean sea-ice model of intermediate complexity to demonstrate that wind-forcing is a crucial element to sustain meridional overturning flow in the Atlantic. Neglecting wind-stress in our multi-century-long simulations leads to a complete shutdown of the conveyor belt circulation. This result may have tremendous impacts for an assessment of the sensitivity of 2-d climate models which typically do not capture wind-driven gyres. It is argued that wind effects may be a key element in determining the fate and length of a collapsed THC state. Possible paleo implications will be discussed.

Description: Global climate during the last glacial period was punctuated by abrupt warmings and occasional pulses of freshwater into the North Atlantic that disrupted deepwater production. These massive freshwater pulses known as Heinrich events arose, in part, from instabilities within the Laurentide ice sheet. Paleoevidence from the North Atlantic suggests that these events altered the production of deep water and changed downstream climate throughout the Northern Hemisphere. In the tropical western Pacific sea, surface temperatures and salinity varied together with ocean and climate changes at high latitudes. Here we present results from coupled modeling experiments that shed light on a possible dynamical link between the North Atlantic Ocean and the western tropical Pacific. This link involves a global oceanic standing wave pattern brought about by millennial-scale glacial density variations in the North Atlantic, atmospheric teleconnections triggered by meridional sea surface temperature gradients, and local air-sea interactions. Furthermore, our modeling results are compared with hydrological records from the Cariaco basin, the Indian Ocean, the Sulu Sea, and northern Australia.