III. T HE T ROPICAL R ESPONSE TO P ERIODIC E XTRATROPICAL T HERMAL
3.1 I NTRODUCTION
This chapter investigates the sequential mechanism of extratropics-to-tropics teleconnection.
The previous chapter shows that the temporal condition sufficiently controls the extratropics- to-tropics teleconnection, mainly accompanied with changes of lower boundaries. Therefore, a mechanism that governs the SST propagation would explain the sensitivity of extratropics- to-tropics teleconnection. We utilize the advantages of periodic time-varying forcing that allows objective identification of multiple processes based on lead-lag phase comparison.
Also, we build and adapt simplified models as fundamental tools to describe outlined atmospheric dynamics.
While most modeling studies examine the equilibrium tropical response to a time- invariant extratropical forcing, some studies have evaluated the transient features of extratropics-to-tropics teleconnection. Chiang and Bitz (2005) proposed the wind- evaporation-SST (WES) feedback as the dominant mechanism for the equatorward propagation of the high-latitude signal. This would be quite similar the role of WES feedback on the evolution of Pacific Meridional Mode (PMM) (e.g., Amaya 2019). However, suppressing the WES feedback had little impact on the tropical response to extratropical forcing in different climate models (Mahajan et al. 2011; Kang et al. 2012; Kang et al. 2014).
Their modification is made in the bulk aerodynamic formula of surface turbulent fluxes over the ocean. The parameterizations of surface turbulent heat fluxes is governed by following equations:
SHF = {4;|}U X(>*(?Y− ?) and LHF = {4;|}U X(@)(AY− A) (3.1) with ;|U the drag coefficient, {4 air density, AY the surface saturation specific humidity, A the specific humidity of the atmospheric bottom level, >* the specific heat capacity of moist
air, @) the latent heat of vaporization of water, and }X( the wind speed at 10 m reference height. The }X( is nudged or prescribed to the climatology, resulting in breaking the WES feedback. Nonetheless, if the energetic imbalance is stably imposed in the extratropics, atmosphere tends to compensate the interhemispheric energy contrast, resulting in ITCZ shift even without surface wind speed changes. We emphasize that the WES feedback would take sufficient role in the extratropics-to-tropics teleconnection for controlling magnitude.
However, it is obvious that the WES feedback is neither essential nor fundamental mechanism for the tropical responses to the extratropical changes. If it is essential, such as ocean dynamics for ENSO, the tropics would not response to the extratropical forcing.
The energetic theory has been developed over the past two dacades to understand the meridional position of ITCZ (e.g., Kang 2020), which is a main consequence of the extratropics-to-tropics teleconnection. The main idea is that an energetic imbalance in one hemisphere necessitates an anomalous cross-equatorial energy transport into the hemisphere with net energy loss. This is accomplished with baroclinic eddies spreading high-latitude energetic imbalance toward the tropics, followed by shifted ascending branch of tropical general circulation, leading to net energy transport following the upper branch to the hemisphere. Although the energetic framework is proven useful for interpreting the ITCZ response to radiative perturbations, it is diagnostic expression of cause-and-effect relationship rather than prognostic illustration of propagation process, providing little information about the teleconnection itself, repeatedly pointed in previous studies (e.g., Chapter 4 in Chiang and Friedman 2012; Emerging Challenges in Kang 2020). The energetic constraint itself hinted the possible roles of transient eddies in the extratropics and the Hadley circulation in the tropics based on their relative importance in transporting the moist static
in an experiment where energy flux perturbations are abruptly introduced, it is often difficult to disentangle the sequential order of multiple dynamical processes at work without a large number of ensemble members. Therefore, despite its implication for real world, a mechanistic theory of the teleconnection from extratropical cooling to the tropical ITCZ shift and Hadley circulation is yet to be fully understood.
In the previous chapter, we show a temporal requirement of the extratropics-to- tropics teleconnection by considering the transient facet of the energetic framework. The energetic imbalance in extratropics is almost completely resolved by local processes (i.e., local atmospheric dynamics and radiative response) when it fluctuates with high-frequency.
The compensation is hardly related to the tropical climate, hence with muffled ITCZ shift.
The ITCZ response happens when the energetic imbalance occurs in low latitudes that tropical general circulation compensates. And, with sufficient time scale, the energetic imbalance emerges in the tropics with SST responses. To understand the dependency, we set out to understand how the SST propagates into the tropics, the sequential processes of extratropics-to-tropics teleconnection. For this purpose, our experiment setup with periodical forcing is particularly helpful as that allows us to objectively differentiate the sequential order of different physical processes based on the phase lag between the extratropical forcing and the response of any variable of interest.
The chapter is organized as follows. In section 3.2, we again outline overall strategies to examine the sequential mechanism of extratropics-to-tropics teleconnection. Here, we mainly explain idealized and theoretical models that we adapt to underpin how atmospheric dynamics govern the propagation. In section 3.3, the propagation mechanism is suggested.
An energy balance model suggests that diffusive eddies are in charge of an equatorward SST propagation in the mid-latitudes. As the eddy activities weaken in the subtropics, the Hadley
circulation takes over to advect moist static energy into the deep tropics, as demonstrated by an idealized thermodynamical-advective model. Then we additionally discuss the intermediate links between governing dynamics offers conclusions in section 3.4.