Impact of internal climate variability on the relationship between spring northern tropical Atlantic SST anomalies and succedent winter ENSO: the role of the North Pacific Oscillation

Previous studies suggested that spring sea surface temperature anomalies (SSTAs) in the northern tropical Atlantic (NTA) have a marked influence on the succedent winter El Niño-Southern Oscillation (ENSO). In this study, we examine the spring NTA SSTA-winter ENSO connection in a 50-member large ensemble simulation conducted with the Canadian Centre for Climate Modeling and Analysis second generation Canadian Earth System Model (CanESM2) and a 100-member ensemble simulation conducted with the Max Planck Institute Earth System Model (MPI-ESM). The observed out-of-phase relation of spring NTA SSTA with winter ENSO can be captured by the multi-member ensemble means of the large ensemble simulations from both models. However, the relation shows a large diversity among different ensemble members attributing to the internal climate variability. The preceding winter North Pacific Oscillation (NPO) is suggested to be an important source of the internal climate variability that modulates the spring NTA SSTA-ENSO connection. The modulation of the winter NPO on the subsequent spring NTA SSTA-winter ENSO relation is seen in both climate modeling and observational datasets. When winter NPO and spring NTA SSTA indices have the same (opposite) sign, the linkage between the spring NTA SSTA and the following winter ENSO tends to be weak (strong). The NPO modulates the spring NTA SSTA-winter ENSO relation mainly via changing the zonal wind anomalies over the tropical western-to-central Pacific induced by the spring NTA SSTA. In addition, our analysis indicates that winter NPO may have a marked effect on the predictability of winter ENSO based on the condition of spring NTA SSTA.

Two Tropical Routes for the Remote Influence of the Northern Tropical Atlantic on the Indo−western Pacific Summer Climate

This study investigates the influence of sea surface temperature (SST) in the northern tropical Atlantic (NTA) on the Indo−western Pacific summer climate by analyzing record-high NTA SSTs summer in 2010. In that time, a decaying El Niño and developing La Niña were accompanied by widespread anomalous climate conditions in the Indo-western Pacific. These conditions are typical of summers that follow El Niño events and are often explained as being due to the influence of Indian Ocean warming induced by the El Niño. Meanwhile, the record high NTA SSTs that also resulted from the influence of the El Niño, the negative phase of the North Atlantic Oscillation as well as the interdecadal-and-longer NTA SST variability, is one of possible causes of anomalous conditions in the Indo−western Pacific. The results of sensitivity experiments using a coupled atmosphere−ocean model clearly indicate that the high NTA SSTs had a considerable influence on the summer weather in the Indo−western Pacific via two tropical routes: an eastbound route that involved a reinforcement of the atmospheric equatorial Kelvin wave and a westbound route that involved altering the Walker circulation over the Atlantic−Pacific region. The altered Walker circulation facilitated the transition to La Niña, amplifying the impact on the western North Pacific monsoon. Further evaluation reveals that both the interannual and interdecadal-and-longer variability of the NTA SST contributed to the anomalous Indo−western Pacific summer. The results highlight the interannual to multidecadal predictability of the Indo−western Pacific summer climate that originates in the NTA.

Contributions of tropical-extratropical oceans to the prediction skill of ENSO after 2000

<p>The skills of most ENSO prediction models have declined significantly since 2000. This decline may be due to a weakening of the correlation between tropical predictors and ENSO. Moreover, the effects of extratropical ocean variability on ENSO have increased during this period. To improve ENSO predictability, we investigate the influence of the tropical-extratropical Atlantic and Pacific sea surface temperature(SST) on ENSO during the periods of pre-2000 and post-2000. We find that the influence of the northern tropical Atlantic(NTA) SST on ENSO has significantly increase since 2000. Meanwhile, there is a much earlier and stronger SST responses between NTA SST and ENSO over the central-eastern Pacific during June–July–August in the post-2000 period compared with the pre-2000 period. Furthermore, the extratropical Pacific SST predictors for ENSO still retain a ~10-month lead time after 2000. We use SST signals in the extratropical Atlantic and Pacific to predict ENSO using a statistical prediction model. These results reveal a significant improvement in ENSO prediction skills. These results indicate that the Atlantic and Pacific SSTAs can make substantial contributions to ENSO prediction, and can be further used to enhance ENSO predictability after 2000.</p>

Contribution of Different Time-Scale Variations to the Tropical Cyclogenesis Environment over the Northern Tropical Atlantic and Comparison with the Western North Pacific

The present study investigates relative contributions of interannual, intraseasonal, and synoptic variations of environmental factors to tropical cyclone (TC) genesis over the northern tropical Atlantic (NTA) during July–October. Analysis shows that convection, lower-level vorticity, and midlevel specific humidity contribute to TC genesis through intraseasonal and synoptic variations with a larger contribution of the latter. The relative contribution of three components of vertical wind shear depends largely on its magnitude. The contribution of sea surface temperature (SST) to TC genesis is mainly due to the interannual component when total SST is above 27.5°C. The barotropic energy for the development of synoptic-scale disturbances comes mainly from climatological mean flows and intraseasonal wind variations. The proportion of contribution between synoptic and intraseasonal variations of convection, relative vorticity, and specific humidity is larger over the eastern NTA than over the western NTA. The barotropic energy conversion has a larger part related to climatological mean flows and intraseasonal wind variations over the eastern and western NTA, respectively. There are notable differences between the NTA and the western North Pacific (WNP). One is that the relative contribution of synoptic variations of convection, relative vorticity, and specific humidity is larger over the NTA, whereas that of intraseasonal variations is larger over the WNP. The other is that the barotropic energy conversion related to climatological mean flows and intraseasonal wind variations is comparable over the NTA, whereas that related to climatological mean flows is larger over the WNP.