The Tropical Pacific Cold Tongue Mode And Its Impacts On ENSO

The cold tongue mode(CTM) is the second EOF mode of SSTA variability over the tropical Pacific, and represents the out–of–phase relationship in SST variability between the Pacific cold tongue region and elsewhere in the tropical and subtropical Pacific. A positive CTM is characterized by the cold SST anomalies in the Pacific cold tongue region and the warm SSTA in the rest of the tropical and subtropical Pacific, with conditions reversed for a negative CTM. Its normalized principal component time series(NPC2) exhibits a strong long–term variability in SSTA and it is highly correlated with global warming, with a negative phase before the late 1930 s, oscillation during the period 1940–1980, and a positive phase after the early 1980 s. The CTM is a coupled air-sea mode, which is associated with the atmospheric fields and subsurface sea temperature.Based on the observational long-term change of oceanic temperature in the tropical Pacific, this study firstly reconfirms that the CTM is a real physical mode. The ocean dynamical processes of the CTM were established, and the relationships between these processes and global warming were discussed. Meanwhile, it is proposed the physical mechanism, which the CTM acted background regulates the thermocline feedback of ENSO. In addition, we evaluate the simulated ability of CMIP5 models in reproducing the spatial and temporal characters of the observed CTM. Finally, we study the interannual variability of the CTM in preindustrial(piControl) scenario with no forcing attributed to global warming. It is further reconfirmed that the CTM is also a real physical mode under piControl scenario. Major conclusion can be summarized as follows:(1) The tropical Pacific cold tongue mode and its dynamical processes under global warmingThe CTM is responsible for the background change of the tropical Pacific under global warming, as demonstrated here using the multi sea surface temperature and subsurface temperature datasets. It is suggested that the CTM is a real physical mode. That is, independent of the ENSO signal, the cooling trend of the eastern equatorial Pacific is robust in both datasets used. The CTM is a coupled air–sea mode, and its long–term variability is induced by ocean dynamical processes in response to global warming. To the lack of the ocean dynamical processes in the past research, this paper systematically establishs the ocean dynamical processes of the CTM under global warming. A heat budget diagnosis of ocean temperature in the eastern equatorial Pacific shows that the net heat flux plays a damping role and the four ocean advection terms(-u′ ?T ?x,-v ?T ′ ?y,-w ?T ′ ?z, and-w′ ?T ?z) contribute to the temperature change associated with the CTM. Among them, the vertical advection of the anomalous temperature by the mean upwelling(-w ?T ′ ?z) makes a dominant contribution to the long–term change in the CTM. The long–term change of the term-w ?T ′ ?z is controlled mainly by the decreasing vertical gradient of the ocean temperature anomaly( ?T ′ ?z). The other three advection terms make a minor contribution to the long–term change in the CTM.(2) The impacts of the tropical Pacific cold tongue mode on the ENSO regimes change under global warmingSince the CTM plays a background change in the tropical Pacific under global warming, we proposed the physical mechanism with the CTM regulating the thermocline feedback of ENSO. When a strong positive CTM is superposed on El Ni?o, the eastern equatorial cooling SSTA in the CTM can weaken the eastern equatorial SSTA of El Ni?o leading to a westward spreading of the El Ni?o center along the equator, which is closely associated with the frequently CP–El Ni?o. In addition, the warming SSTA of the positive CTM can modify the El Ni?o’s SSTA structure by adding warming outside the eastern equatorial Pacific, which induces warming of the western equatorial and off-equatorial SSTA and thus broadens the zonal and meridional widths of the El Ni?o. Conversely, when a strong positive CTM is superposed on La Ni?a, the additional cool SSTA in the eastern equatorial Pacific may increase the equatorial cooling SSTA of La Ni?a. However, the center of the La Ni?a is also located near the eastern equatorial Pacific. Meanwhile, the CTM’s warm SSTA outside the eastern equatorial Pacific warms the western equatorial and off-equatorial SSTA of La Ni?a and thus narrows its zonal and meridional widths. Because the different responses of the equatorial Pacific cold tongue and the rest of the tropical Pacific oceanic temperature to global warming, the CTM can affect the ENSO regimes change. Based on(1) result, the long-term change of the CTM is mainly corresponding to the-w ?T ′ ?z term, which also corresponds to the thermocline feedback in ENSO theory. Therefore, if a strong positive CTM is superposed on El Ni?o, it can weaken the thermocline feedback in the eastern equatorial Pacific and thus decrease the intensity of El Ni?o. But for La Ni?a, it can strengthen the thermocline feedback and thus increase the intensity of La Ni?a. In addition, these results are also confirmed in the Recharge Oscillator model.(3) Simulation of the tropical Pacific cold tongue mode in the CMIP5 historical scenarioBased on the 20 CMIP5 models under the historical scenario, the spatial and temporal characteristics of the CTM and ENSO are evaluated. Five models(CESM1-CAM5,CMCC-CM, FGOALS-g2, IPSL-CM5B-LR,NorESM1-M) can roughly simulate the location of the ENSO center, the ENSO horseshoe pattern, and the ENSO interannual variability(2-7 year), as well as the spatial characteristics of the CTM and its long-term change. In addition, these five models can reproduce the response of the oceanic temperature in the tropical Pacific cold tongue to global warming. That is, these five models are corresponding to the cooling vertical gradient of the ocean temperature anomaly( ?T ′ ?z) in the eastern equatorial Pacific. Accompanied with the strong climate upwelling, it induces the cooling oceanic advection term(-w ?T ′ ?z), which contributes to the long-term change of the CTM.(4) The interannual variability of the tropical Pacific cold tongue mode in the CMIP5 piControl scenarioIt is analyzed that the interannual variability of the CTM in preindustrial(piControl) scenario with no forcing attributed to global warming and associated with physical mechanism. This is further reconfirms that the CTM is also a real physical mode under piControl scenario. In this paper, it is found the close relationship between the interannual ENSO and CTM through the equatorial Pacific eastward Kelvin wave and Bjerknes feedback. Under the piControl scenario, the linking mechanism is El Ni?oâ†'CTMâ†'La Ni?aâ†'-CTMâ†'El Ni?o, where the arrows show the cause-and-effect relationships. Although the interannual signal of the CTM is covered by its long-term change, the CTM still has interannual signal in the observation. For example, the CTM is corresponding to the interannual oscillation during the period 1940–1980. The El Ni?oâ†'CTM or La Ni?aâ†'-CTM in the observation is same as that in the model, but the CTMâ†'La Ni?a or-CTMâ†'El Ni?o is not significant in the observation. Meanwhile, the oceanic temperature intensity in the eastern equatorial Pacific caused by the decay of El Ni?o is weaker than that of CMIP5 model, which is probably reason from the strong long-term change of the CTM but the weak interannual signal of the CTM in the observation.