Beschreibung: 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.

Beschreibung: The predictability of the coupled ocean–atmosphere climate system on interannual to decadal timescales has been studied by means of ensemble forecast experiments with a global coupled ocean–atmosphere general circulation model. Over most parts of the globe the model’s predictability can be sufficiently explained by damped persistence as expected from the stochastic climate model concept with damping times of considerably less than a year. Nevertheless, the tropical Pacific and the North Atlantic Ocean exhibit oscillatory coupled ocean–atmosphere modes, which lead to longer predictability timescales. While the tropical mode shares many similarities with the observed ENSO phenomenon, the coupled mode within the North Atlantic region exhibits a typical period of about 30 yr and relies on an interaction of the oceanic thermohaline circulation and the atmospheric North Atlantic oscillation. The model’s ENSO-like oscillation is predictable up to one-third to one-half (2–3 yr) of the oscillation period both in the ocean and the atmosphere. The North Atlantic yields considerably longer predictability timescales (of the order of a decade) only for quantities describing the model’s thermohaline circulation. For surface quantities and atmospheric variables only marginal predictability (of the order of a year) was obtained. The predictability of the coupled signal at the surface is destroyed by the large amount of internally generated (weather) noise. This is illustrated by means of a simple conceptual model for coupled ocean–atmosphere variability and predictability.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: This paper investigates the impact of the new ICE-5G paleotopography dataset for Last Glacial Maximum (LGM) conditions on a coupled model simulation of the thermal and dynamical state of the glacial atmosphere and on both land surface and sea surface conditions. The study is based upon coupled climate simulations performed with the ocean–atmosphere–sea ice model of intermediate-complexity Climate de Bilt-coupled large-scale ice–ocean (ECBilt-Clio) model. Four simulations focusing on the Last Glacial Maximum [21 000 calendar years before present (BP)] have been analyzed: a first simulation (LGM-4G) that employed the original ICE-4G ice sheet topography and albedo, and a second simulation (LGM-5G) that employed the newly constructed ice sheet topography, denoted ICE-5G, and its respective albedo. Intercomparison of the results obtained in these experiments demonstrates that the LGM-5G simulation delivers significantly enhanced cooling over Canada compared to the LGM-4G simulation whereas positive temperature anomalies are simulated over southern North America and the northern Atlantic. Moreover, introduction of the ICE-5G topography is shown to lead to a deceleration of the subtropical westerlies and to the development of an intensified ridge over North America, which has a profound effect upon the hydrological cycle. Additionally, two flat ice sheet experiments were carried out to investigate the impact of the ice sheet albedo on global climate. By comparing these experiments with the full LGM simulations, it becomes evident that the climate anomalies between LGM-5G and LGM-4G are mainly driven by changes of the earth’s topography.

Beschreibung: A coupled air–sea mode in the Northern Hemisphere with a period of about 35 years is described. The mode was derived from a multicentury integration with a coupled ocean–atmosphere general circulation model and involves interactions of the thermohaline circulation with the atmosphere in the North Atlantic and interactions between the ocean and the atmosphere in the North Pacific. The authors focus on the physics of the North Atlantic interdecadal variability. If, for instance, the North Atlantic thermohaline circulation is anomalously strong, the ocean is covered by positive sea surface temperature (SST) anomalies. The atmospheric response to these SST anomalies involves a strengthened North Atlantic Oscillation, which leads to anomalously weak evaporation and Ekman transport off Newfoundland and in the Greenland Sea, and the generation of negative sea surface salinity (SSS) anomalies. These SSS anomalies weaken the deep convection in the oceanic sinking regions and subsequently the strength of the thermohaline circulation. This leads to a reduced poleward heat transport and the formation of negative SST anomalies, which completes the phase reversal. The Atlantic and Pacific Oceans seem to be coupled via an atmospheric teleconnection pattern and the interdecadal Northern Hemispheric climate mode is interpreted as an inherently coupled air–sea mode. Furthermore, the origin of the Northern Hemispheric warming observed recently is investigated. The observed temperatures are compared to a characteristic warming pattern derived from a greenhouse warming simulation with the authors’ coupled general circulation model and also with the Northern Hemispheric temperature pattern associated with the 35-yr climate mode. It is shown that the recent Northern Hemispheric warming projects well onto the temperature pattern of the interdecadal mode under consideration.

Beschreibung: This study explores the influence of phytoplankton on the tropical Pacific heat budget. A hybrid coupled model for the tropical Pacific that is based on a primitive equation reduced-gravity multilayer ocean model, a dynamic ocean mixed layer, an atmospheric mixed layer, and a statistical atmosphere is used. The statistical atmosphere relates deviations of the sea surface temperature from its mean to wind stress anomalies and allows for the rectification of the annual cycle and the El Niño–Southern Oscillation (ENSO) phenomenon through the positive Bjerknes feedback. Furthermore, a nine-component ecosystem model is coupled to the physical variables of the ocean. The simulated chlorophyll concentrations can feed back onto the ocean heat budget by their optical properties, which modify solar light absorption in the surface layers. It is shown that both the surface layer concentration as well as the vertical profile of chlorophyll have a significant effect on the simulated mean state, the tropical annual cycle, and ENSO. This study supports a previously suggested hypothesis (Timmermann and Jin) that predicts an influence of phytoplankton concentration of the tropical Pacific climate mean state and its variability. The bioclimate feedback diagnosed here works as follows: Maxima in the subsurface chlorophyll concentrations lead to an enhanced subsurface warming due to the absorption of photosynthetically available shortwave radiation. This warming triggers a deepening of the mixed layer in the eastern equatorial Pacific and eventually a reduction of the surface ocean currents (Murtugudde et al.). The weakened south-equatorial current generates an eastern Pacific surface warming, which is strongly enhanced by the Bjerknes feedback. Because of the deepening of the mixed layer, the strength of the simulated annual cycle is also diminished. This in turn leads to an increase in ENSO variability.

Beschreibung: 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.

Beschreibung: The role of mean and stochastic freshwater forcing on the generation of millennial-scale climate variability in the North Atlantic is studied using a low-order coupled atmosphere–ocean–sea ice model. It is shown that millennial-scale oscillations can be excited stochastically, when the North Atlantic Ocean is fresh enough. This finding is used in order to interpret the aftermath of massive iceberg surges (Heinrich events) in the glacial North Atlantic, which are characterized by an excitation of Dansgaard–Oeschger events. Based on model results, it is hypothesized that Heinrich events trigger Dansgaard–Oeschger cycles and that furthermore the occurrence of Heinrich events is dependent on the accumulated climatic effect of a series of Dansgaard–Oeschger events. This scenario leads to a coupled ocean–ice sheet oscillation that shares many similarities with the Bond cycle. Further sensitivity experiments reveal that the timescale of the oscillations can be decomposed into stochastic, linear, and nonlinear deterministic components. A schematic bifurcation diagram is used to compare theoretical results with paleoclimatic data.

Beschreibung: Oceanic ecosystems altered by interdecadal climate variability may provide a feedback to the physical climate by phytoplankton affecting heat fluxes into the upper ocean and dimethylsulfide fluxes into the atmosphere