Equatorial Pacific Variability in Climate Models

In this dissertation, equatorial Pacific variability in climate models is investigated. Of particular interest are the eastern equatorial Pacific sea surface temperature annual cycle and the El Niño/Southern Oscillation (ENSO). The simulation of these modes of variability in climate models is studied, identifying significant model biases and their origins. Furthermore, possibilities to alleviate model biases are addressed

Weakened SST variability in the tropical Atlantic Ocean since 2000

A prominent weakening in equatorial Atlantic sea surface temperature (SST) variability, occurring around the year 2000, is investigated by means of observations, reanalysis products and the linear recharge oscillator (ReOsc) model. Compared to the time period 1982–1999, during 2000–2017 the May–June–July SST variability in the eastern equatorial Atlantic has decreased by more than 30%. Coupled air–sea feedbacks, namely the positive Bjerknes feedback and the negative net heat flux damping are important drivers for the equatorial Atlantic interannual SST variability. We find that the Bjerknes feedback weakened after 2000 while the net heat flux damping increased. The weakening of the Bjerknes feedback does not appear to be fully explainable by changes in the mean state of the tropical Atlantic. The increased net heat flux damping is related to an enhanced response of the latent heat flux to the SST anomalies (SSTa). Strengthened trade winds as well as warmer SSTs are suggested to increase the air–sea specific humidity difference and hence, enhancing the latent heat flux response to SSTa. A combined effect of those two processes is proposed to be responsible for the weakened SST variability in the eastern equatorial Atlantic. The ReOsc model supports the link between reduced SST variability, weaker Bjerknes feedback and stronger net heat flux damping

How linear is the ENSO Teleconnection to the North Pacific? The Role of ENSO Atmospheric Feedbacks for Rainfall in California

El Niño/Southern Oscillation has global teleconnections. Precipitation on the US East Coast, and in particular Southern California, is strongly dependent on ENSO variability in the tropical Pacific: More rainfall is expected during El Niño episodes, and reduced rainfall during La Niña. While this teleconnection is highly dependent on the location, timing, and strength of the sea surface temperature (SST) signal in the tropical Pacific, the associated nonlinearities are often not well represented in current climate models. Moreover, the location and strength of convection over the equatorial Pacific has been shown to be linked to the strength of atmospheric feedbacks in the tropical Pacific, i.e. the wind-SST feedback and the heat flux-SST feedback. The strength of the local atmospheric feedbacks is here shown to not only affecting tropical Pacific ENSO dynamics, but also the teleconnection to California: A strengthening of the atmospheric feedback tends to initiate a stronger wave train to California, bringing significantly higher rainfall. In addition to feedback strength, this study compares coupled and atmosphere-only models with observations in terms of the ENSO teleconnection to California

Improving ENSO in a Climate Model: Tuning vs. Flux correction

Despite improvements in simulating El Niño/Southern Oscillation (ENSO) in the last decades, current climate model still suffering in simulating important ENSO properties of ENSO, like the amplitude, the frequency, the phase locking to the annual cycle or the asymmetry between El Niño and La Nina (e.g. Bellenger et al. 2014). In a recent study of Bayr et al. (2018) the equatorial cold sea surface temperature (SST) bias in the tropical Pacific could be highlighted as one large contributor to biased ENSO dynamics in climate models. The cold equatorial SST bias, a common problem in climate models, causes a La Niña-like mean state with a too westward position of the rising branch of the Walker Circulation (by up to 30°), resulting in an erroneous convective response during ENSO events. This in turn biases the two most important atmospheric feedbacks, the positive (amplifying) wind-SST and negative (damping) heat flux-SST feedback, with error compensation between these two. Bayr et al. (2018) also give some indications how ENSO is tunable, as the study shows that it is possible to reproduce the same spread in cold SST bias and atmospheric feedback strength as seen in a CMIP5 multi-model ensemble with a perturbed physics ensemble of the Kiel Climate Model (KCM). This was achieved by changes in the cloud and convection parameters which are usually used to tune the climate models. Here we present a more detailed and systematic analyses of the influence of the tuning parameters on ENSO simulation in the KCM which employs ECHAM5 as the atmospheric component. We show the impact of the tuning parameters on the cold SST bias, the ENSO atmospheric feedbacks and important ENSO properties and demonstrate the possibility to tune ENSO and important global climate properties at the same time

Improved silencing properties using small internally segmented interfering RNAs

RNA interference is mediated by small interfering RNAs (siRNAs) that upon incorporation into the RNA-induced silencing complex (RISC) can target complementary mRNA for degradation. Standard siRNA design usually feature a 19–27 base pair contiguous double-stranded region that is believed to be important for RISC incorporation. Here, we describe a novel siRNA design composed of an intact antisense strand complemented with two shorter 10–12 nt sense strands. This three-stranded construct, termed small internally segmented interfering RNA (sisiRNA), is highly functional demonstrating that an intact sense strand is not a prerequisite for RNA interference. Moreover, when using the sisiRNA design only the antisense strand is functional in activated RISC thereby completely eliminating unintended mRNA targeting by the sense strand. Interestingly, the sisiRNA design supports the function of chemically modified antisense strands, which are non-functional within the context of standard siRNA designs. This suggests that the sisiRNA design has a clear potential of improving the pharmacokinetic properties of siRNA in vivo

What Controls ENSO-Amplitude Diversity in Climate Models?

Climate models depict large diversity in the strength of the El Niño/Southern Oscillation (ENSO) (ENSO amplitude). Here we investigate ENSO-amplitude diversity in the Coupled Model Intercomparison Project Phase 5 (CMIP5) by means of the linear recharge oscillator model, which reduces ENSO dynamics to a two-dimensional problem in terms of eastern equatorial Pacific sea surface temperature anomalies (T) and equatorial Pacific upper ocean heat content anomalies (h). We find that a large contribution to ENSO-amplitude diversity originates from stochastic forcing. Further, significant interactions exist between the stochastic forcing and the growth rates of T and h with competing effects on ENSO amplitude. The joint consideration of stochastic forcing and growth rates explains more than 80% of the ENSO-amplitude variance within CMIP5. Our results can readily explain the lack of correlation between the Bjerknes Stability index, a measure of the growth rate of T, and ENSO amplitude in a multimodel ensemble

Seasonal ENSO phase locking in the Kiel Climate Model: The importance of the equatorial cold sea surface temperature bias

The El Niño/Southern Oscillation (ENSO) is characterized by a seasonal phase locking, with strongest eastern and central equatorial Pacific sea surface temperature (SST) anomalies during boreal winter and weakest SST anomalies during boreal spring. In this study, key feedbacks controlling seasonal ENSO phase locking in the Kiel Climate Model (KCM) are identified by employing Bjerknes index stability analysis. A large ensemble of simulations with the KCM is analyzed, where the individual runs differ in either the number of vertical atmospheric levels or coefficients used in selected atmospheric parameterizations. All integrations use the identical ocean model. The ensemble-mean features realistic seasonal ENSO phase locking. ENSO phase locking is very sensitive to changes in the mean-state realized by the modifications described above. An excessive equatorial cold tongue leads to weak phase locking by reducing the Ekman feedback and thermocline feedback in late boreal fall and early boreal winter. Seasonal ENSO phase locking also is sensitive to the shortwave feedback as part of the thermal damping in early boreal spring, which strongly depends on eastern and central equatorial Pacific SST. The results obtained from the KCM are consistent with those from models participating in the Coupled Model Intercomparison Project phase 5 (CMIP5)

Mean-State Dependence of ENSO atmospheric feedbacks and ENSO dynamics in climate models

ENSO atmospheric feedbacks are strongly underestimated in state-of-the-art climate models (Bellenger et al. 2014)⁠. Therefore we investigate in a perturbed atmospheric physics ensemble with the Kiel Climate Model (KCM) and in CMIP5 models how ENSO atmospheric feedbacks depend on the mean state of the tropical Pacific. Additionally, uncoupled simulations are conducted with the atmospheric component of the KCM to obtain further insight into the mean state dependence. It is found that the strengths of the positive zonal wind feedback µ and the negative heat flux feedback α are both strongly linearly related equatorial sea surface temperature (SST) bias, while at least in the KCM differences in model physics seem to be less important (Bayr et al. 2017)⁠. In observations, strong zonal wind and heat flux feedbacks are caused by a convective response in the western central equatorial Pacific (Niño4 region), resulting from an eastward (westward) shift of the rising branch of the Walker Circulation (WC) during El Niño (La Niña). Climate models with a La Niña-like mean state, i.e. an equatorial SST cold bias in the Niño4 region (a common problem in many state-of-the-art climate models), simulate a too westward located rising branch of the WC (by up to 30°) and only a weak convective response. Thus, the position of the WC determines the strength of both the wind and heat flux feedback, which also explains why biases in these two feedbacks partly compensate in many climate models. Furthermore, a too eastward position of the WC leads to a fundamental change in ENSO dynamics, as ocean-atmosphere coupling shifts from a predominantly wind-driven to a more solar radiation-driven mode. On the other hand, enhanced atmospheric feedbacks lead to a substantial improvement of the non-linearity of ENSO. Differences in the mean state SST are suggested to be a major source of ENSO diversity in current climate models. References: Bayr, T., M. Latif, D. Dommenget, C. Wengel, J. Harlaß, and W. Park, 2017: Mean-State Dependence of ENSO Atmospheric Feedbacks in Climate Models. Clim. Dyn., doi:10.1007/s00382-017-3799-2. Bellenger, H., E. Guilyardi, J. Leloup, M. Lengaigne, and J. Vialard, 2014: ENSO representation in climate models: From CMIP3 to CMIP5. Clim. Dyn., 42, 1999–2018, doi:10.1007/s00382-013-1783-z

The mean state dependence of ENSO atmospheric feedbacks and ENSO dynamics in climate models

ENSO atmospheric feedbacks are strongly underestimated in state-of-the-art climate models (Bellenger et al. 2014)⁠. Therefore we investigate in a perturbed atmospheric physics ensemble with the Kiel Climate Model (KCM) and in CMIP5 models how ENSO atmospheric feedbacks depend on the mean state of the tropical Pacific. Additionally, uncoupled simulations are conducted with the atmospheric component of the KCM to obtain further insight into the mean state dependence. It is found that the strengths of the positive zonal wind feedback µ and the negative heat flux feedback α are both strongly linearly related equatorial sea surface temperature (SST) bias, while at least in the KCM differences in model physics seem to be less important (Bayr et al. 2017)⁠. In observations, strong zonal wind and heat flux feedbacks are caused by a convective response in the western central equatorial Pacific (Niño4 region), resulting from an eastward (westward) shift of the rising branch of the Walker Circulation (WC) during El Niño (La Niña). Climate models with a La Niña-like mean state, i.e. an equatorial SST cold bias in the Niño4 region (a common problem in many state-of-the-art climate models), simulate a too westward located rising branch of the WC (by up to 30°) and only a weak convective response. Thus, the position of the WC determines the strength of both the wind and heat flux feedback, which also explains why biases in these two feedbacks partly compensate in many climate models. Furthermore, a too eastward position of the WC leads to a fundamental change in ENSO dynamics, as ocean-atmosphere coupling shifts from a predominantly wind-driven to a more solar radiation-driven mode. On the other hand, enhanced atmospheric feedbacks lead to a substantial improvement of the non-linearity of ENSO. Differences in the mean state SST are suggested to be a major source of ENSO diversity in current climate models. References: Bayr, T., M. Latif, D. Dommenget, C. Wengel, J. Harlaß, and W. Park, 2017: Mean-State Dependence of ENSO Atmospheric Feedbacks in Climate Models. Clim. Dyn., doi:10.1007/s00382-017-3799-2. Bellenger, H., E. Guilyardi, J. Leloup, M. Lengaigne, and J. Vialard, 2014: ENSO representation in climate models: From CMIP3 to CMIP5. Clim. Dyn., 42, 1999–2018, doi:10.1007/s00382-013-1783-z