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Karl Stein
,
Niklas Schneider
,
Axel Timmermann
, and
Fei-Fei Jin

Abstract

A simple model of ENSO is developed to examine the effects of the seasonally varying background state of the equatorial Pacific on the seasonal synchronization of ENSO event peaks. The model is based on the stochastically forced recharge oscillator, extended to include periodic variations of the two main model parameters, which represent ENSO’s growth rate and angular frequency. Idealized experiments show that the seasonal cycle of the growth rate parameter sets the seasonal cycle of ENSO variance; the inclusion of the time dependence of the angular frequency parameter has a negligible effect. Event peaks occur toward the end of the season with the most unstable growth rate.

Realistic values of the parameters are estimated from a linearized upper-ocean heat budget with output from a high-resolution general circulation model hindcast. Analysis of the hindcast output suggests that the damping by the mean flow field dominates the seasonal cycle of ENSO’s growth rate and, thereby, seasonal ENSO variance. The combination of advective, Ekman pumping, and thermocline feedbacks plays a secondary role and acts to enhance the seasonal cycle of the ENSO growth rate.

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Shang-Ping Xie
,
Yuko Okumura
,
Toru Miyama
, and
Axel Timmermann

Abstract

Recent global coupled model experiments suggest that the atmospheric bridge across Central America is a key conduit for Atlantic climate change to affect the tropical Pacific. A high-resolution regional ocean–atmosphere model (ROAM) of the eastern tropical Pacific is used to investigate key processes of this conduit by examining the response to a sea surface temperature (SST) cooling over the North Atlantic. The Atlantic cooling increases sea level pressure, driving northeasterly wind anomalies across the Isthmus of Panama year-round. While the atmospheric response is most pronounced during boreal summer/fall when the tropical North Atlantic is warm and conducive to deep convection, the Pacific SST response is strongest in winter/spring when the climatological northeast trade winds prevail across the isthmus. During winter, the northeasterly cross-isthmus winds intensify in response to the Atlantic cooling, reducing the SST in the Gulf of Panama by cold and dry advection from the Atlantic and by enhancing surface turbulent heat flux and mixing. This Gulf of Panama cooling reaches the equator and is amplified by the Bjerknes feedback during boreal spring. The equatorial anomalies of SST and zonal winds dissipate quickly in early summer as the seasonal development of the cold tongue increases the stratification of the atmospheric boundary layer and shields the surface from the Atlantic influence that propagates into the Pacific as tropospheric Rossby waves. The climatological winds over the far eastern Pacific warm pool turn southwesterly in summer/fall, superimposed on which the anomalous northesterlies induce a weak SST warming there.

The ROAM results are compared with global model water-hosing runs to shed light on intermodel consistency and differences in response to the shutdown of the Atlantic meridional overturning circulation. Implications for interpreting paleoclimate changes such as Heinrich events are discussed. The results presented here also aid in understanding phenomena in the present climate such as the Central American midsummer drought and Atlantic multidecadal oscillation.

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Flávio Justino
,
Axel Timmermann
,
Ute Merkel
, and
Enio P. Souza

Abstract

A coupled global atmosphere–ocean model of intermediate complexity is used to study the influence of glacial boundary conditions on the atmospheric circulation during the Last Glacial Maximum in a systematical manner. A web of atmospheric interactions is disentangled, which involves changes in the meridional temperature gradient and an associated modulation of the atmospheric baroclinicity. This in turn drives anomalous transient eddy momentum fluxes that feed back onto the zonal mean circulation. Moreover, the modified transient activity (weakened in the North Pacific and strengthened in the North Atlantic) leads to a meridional reorganization of the atmospheric heat transport, thereby feeding back onto the meridional temperature structure. Furthermore, positive barotropic conversion and baroclinic production rates over the Laurentide ice sheets and the far eastern North Pacific have the tendency to decelerate the westerlies, thereby feeding back to the stationary wave changes triggered by orographic forcing.

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Ben Marzeion
,
Axel Timmermann
,
Ragu Murtugudde
, and
Fei-Fei Jin

Abstract

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.

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Yoshimitsu Chikamoto
,
Axel Timmermann
,
Matthew J. Widlansky
,
Shaoqing Zhang
, and
Magdalena A. Balmaseda

Abstract

Performance of a newly developed decadal climate prediction system is examined using the low-resolution Community Earth System Model (CESM). To identify key sources of predictability and determine the role of upper and deeper ocean data assimilation, we first conduct a series of perfect model experiments. These experiments reveal the importance of upper ocean temperature and salinity assimilation in reducing sea surface temperature biases. However, to reduce biases in the sea surface height, data assimilation below 300 m in the ocean is necessary, in particular for high-latitude regions. The perfect model experiments clearly emphasize the key role of combined three-dimensional ocean temperature and salinity assimilation in reproducing mean state and model trajectories. Applying this knowledge to the realistic decadal climate prediction system, we conducted an ensemble of ocean assimilation simulations with the fully coupled CESM covering the period 1960–2014. In this system, we assimilate three-dimensional ocean temperature and salinity data into the ocean component of CESM. Instead of assimilating direct observations, we assimilate temperature and salinity anomalies obtained from the ECMWF Ocean Reanalysis version 4 (ORA-S4). Anomalies are calculated relative to the sum of the ORA-S4 climatology and an estimate of the externally forced signal. As a result of applying the balanced ocean conditions to the model, our hindcasts show only very little drift and initialization shocks. This new prediction system exhibits multiyear predictive skills for decadal climate variations of the Atlantic meridional overturning circulation (AMOC) and North Pacific decadal variability.

Open access
Shayne McGregor
,
Axel Timmermann
,
Niklas Schneider
,
Malte F. Stuecker
, and
Matthew H. England

Abstract

During large El Niño events the westerly wind response to the eastern equatorial Pacific sea surface temperature anomalies (SSTAs) shifts southward during boreal winter and early spring, reaching latitudes of 5°–7°S. The resulting meridional asymmetry, along with a related seasonal weakening of wind anomalies on the equator are key elements in the termination of strong El Niño events. Using an intermediate complexity atmosphere model it is demonstrated that these features result from a weakening of the climatological wind speeds south of the equator toward the end of the calendar year. The reduced climatological wind speeds, which are associated with the seasonal intensification of the South Pacific convergence zone (SPCZ), lead to anomalous boundary layer Ekman pumping and a reduced surface momentum damping of the combined boundary layer/lower-troposphere surface wind response to El Niño. This allows the associated zonal wind anomalies to shift south of the equator. Furthermore, using a linear shallow-water ocean model it is demonstrated that this southward wind shift plays a prominent role in changing zonal mean equatorial heat content and is solely responsible for establishing the meridional asymmetry of thermocline depth in the turnaround (recharge/discharge) phase of ENSO. This result calls into question the sole role of oceanic Rossby waves in the phase synchronized termination of El Niño events and suggests that the development of a realistic climatological SPCZ in December–February/March–May (DJF/MAM) is one of the key factors in the seasonal termination of strong El Niño events.

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Ryan L. Sriver
,
Axel Timmermann
,
Michael E. Mann
,
Klaus Keller
, and
Hugues Goosse

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.

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Yuko M. Okumura
,
Clara Deser
,
Aixue Hu
,
Axel Timmermann
, and
Shang-Ping Xie

Abstract

Sudden changes of the Atlantic meridional overturning circulation (AMOC) are believed to have caused large, abrupt climate changes over many parts of the globe during the last glacial and deglacial period. This study investigates the mechanisms by which a large freshwater input to the subarctic North Atlantic and an attendant rapid weakening of the AMOC influence North Pacific climate by analyzing four different ocean–atmosphere coupled general circulation models (GCMs) under present-day or preindustrial boundary conditions. When the coupled GCMs are forced with a 1-Sv (Sv ≡ 106 m3 s−1) freshwater flux anomaly in the subarctic North Atlantic, the AMOC nearly shuts down and the North Atlantic cools significantly. The South Atlantic warms slightly, shifting the Atlantic intertropical convergence zone southward. In addition to this Atlantic ocean–atmosphere response, all of the models exhibit cooling of the North Pacific, especially along the oceanic frontal zone, consistent with paleoclimate reconstructions. The models also show deepening of the wintertime Aleutian low.

Detailed analysis of one coupled GCM identifies both oceanic and atmospheric pathways from the Atlantic to the North Pacific. The oceanic teleconnection contributes a large part of the North Pacific cooling: the freshwater input to the North Atlantic raises sea level in the Arctic Ocean and reverses the Bering Strait throughflow, transporting colder, fresher water from the Arctic Ocean into the North Pacific. When the Bering Strait is closed, the cooling is greatly reduced, while the Aleutian low response is enhanced. Tropical SST anomalies in both the Atlantic and Pacific are found to be important for the equivalent barotropic response of the Aleutian low during boreal winter. The atmospheric bridge from the tropical North Atlantic is particularly important and quite sensitive to the mean state, which is poorly simulated in many coupled GCMs. The enhanced Aleutian low, in turn, cools the North Pacific by increasing surface heat fluxes and southward Ekman transport. The closure of the Bering Strait during the last glacial period suggests that the atmospheric bridge from the tropics and air–sea interaction in the North Pacific played a crucial role in the AMOC–North Pacific teleconnection.

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Axel Timmermann
,
Tobias Friedrich
,
Oliver Elison Timm
,
Megumi O. Chikamoto
,
Ayako Abe-Ouchi
, and
Andrey Ganopolski

Abstract

The effect of obliquity and CO2 changes on Southern Hemispheric climate is studied with a series of numerical modeling experiments. Using the Earth system model of intermediate complexity Loch–VECODE–ECBilt–CLIO–Agism Model (LOVECLIM) and a coupled general circulation model [Model for Interdisciplinary Research on Climate (MIROC)], it is shown in time-slice simulations that phases of low obliquity enhance the meridional extratropical temperature gradient, increase the atmospheric baroclinicity, and intensify the lower and middle troposphere Southern Hemisphere westerlies and storm tracks. Furthermore, a transient model simulation is conducted with LOVECLIM that covers the greenhouse gas, ice sheet, and orbital forcing history of the past 408 ka. This simulation reproduces reconstructed glacial–interglacial variations in temperature and sea ice qualitatively well and shows that the meridional heat transport associated with the orbitally paced modulation of middle troposphere westerlies and storm tracks partly offsets the effects of the direct shortwave obliquity forcing over Antarctica, thereby reinforcing the high correlation between CO2 radiative forcing and Antarctic temperature. The overall timing of temperature changes in Antarctica is hence determined by a balance of shortwave obliquity forcing, atmospheric heat transport changes, and greenhouse gas forcing. A shorter 130-ka transient model experiment with constant CO2 concentrations further demonstrates that surface Southern Hemisphere westerlies are primarily modulated by the obliquity cycle rather than by the CO2 radiative forcing.

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Matthew J. Widlansky
,
Axel Timmermann
,
Shayne McGregor
,
Malte F. Stuecker
, and
Wenju Cai

Abstract

During strong El Niño events, sea level drops around some tropical western Pacific islands by up to 20–30 cm. Such events (referred to as taimasa in Samoa) expose shallow reefs, thereby causing severe damage to associated coral ecosystems and contributing to the formation of microatolls. During the termination of strong El Niño events, a southward movement of weak trade winds and the development of an anomalous anticyclone in the Philippine Sea are shown to force an interhemispheric sea level seesaw in the tropical Pacific that enhances and prolongs extreme low sea levels in the southwestern Pacific. Spectral features, in addition to wind-forced linear shallow water ocean model experiments, identify a nonlinear interaction between El Niño and the annual cycle as the main cause of these sea level anomalies.

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