Evolution of the Tropical Response to Periodic Extratropical Thermal Forcing

Abstract

This study examines the temporal evolution of the extratropically forced tropical response in an idealized aquaplanet model under equinox condition. We apply a surface thermal forcing in the northern extratropics that oscillates periodically in time. It is shown that tropical precipitation is unaltered by sufficiently high-frequency extratropical forcing. This sensitivity to the extratropical forcing periodicity arises from the critical time required for sea surface temperature (SST) adjustment. Low-frequency extratropical forcing grants sufficient time for atmospheric transient eddies to diffuse moist static energy to perturb the mid-latitude SSTs outside the forcing region, as demonstrated by a one-dimensional energy balance model with a fixed diffusivity. As the transient eddies weaken in the subtropics, a further equatorward advection is accomplished by the Hadley circulation. The essential role of Hadley cell advection in connecting the subtropical signal to the equatorial region is supported by an idealized thermodynamical-advective model. Associated with the SST changes in the tropics is a meridional shift of the Intertropical Convergence Zone. Since the time needed for SST adjustment increases with increasing mixed layer depth, the critical forcing period at which the extratropical forcing can affect the tropics scales linearly with the mixed layer depth. Our results highlight the important role of decadal-and-longer extratropical climate variability in shaping the tropical climate system. We also raise the possibility that the transient behavior of a tropical response forced by extratropical variability may be strongly dependent on cloud radiative effects.

How Does the High‐Latitude Thermal Forcing in One Hemisphere Affect the Other Hemisphere?

“…The SST response propagates into the tropics with a 1.67‐year lag, measured by the lag time at which the correlation is maximized between the SST anomaly at the forcing edge (40°N) and that at the equator. This timescale is consistent with the previously suggested timescale of the tropical response to an extratropical forcing (e.g., Figure 3b in Woelfle et al., 2015, Figure 3 in Shin et al., 2021). The response timescale in the latitude‐pressure domain indicates that the equatorward propagation preferentially occurs through the lower atmosphere (Figure S3 in Supporting Information ), hence mediated by SST response (e.g., Voigt et al., 2017).…”

“…This phenomenon is reported as the "blocking effect" by the climatological ITCZ (Kang et al, 2020). As the lower branch of the Hadley cell is responsible for the equatorward SST propagation in the tropics (Shin et al, 2021), its reversed direction at the climatological ITCZ hampers further propagation. The ITCZ blocking effect is evident in Figure S3 in Supporting Information S1, which shows a sharp jump in the response timescale at the equator below 800 hPa.…”

“…Put together, the cross‐equatorial anomalous Hadley circulation, that is, the ITCZ shift is a prerequisite of the pole‐to‐pole teleconnection. Only a sufficiently low‐frequency extratropical perturbation can instigate the ITCZ shift because of the critical time needed for the SST adjustment (Shin et al., 2021). Hence, the pole‐to‐pole teleconnection emerges preferentially for a lower frequency extratropical forcing (Figures S6a–S6h in Supporting Information ) while it is absent in the case with a 1‐year forcing period due to a quiescent ITCZ (Figures S6i and S6j in Supporting Information ).…”

“…We impose the Qflux under perpetual equinox conditions (denoted PERI) and integrate it for 330 years (i.e., 11 cycles). The periodic Qflux itself neither adds nor subtracts energy from the global system over one forcing cycle, but the global‐mean surface temperature slightly decreases by 0.4 K during the initial 30‐year adjustment period (Shin et al., 2021). This drift occurs because the surface temperature is more sensitive to an imposed cooling than to an imposed warming associated with nonlinear cloud radiative effect (CRE) (e.g., Shaw et al., 2015; Shin et al., 2017).…”

The Role of Clouds in Shaping Tropical Pacific Response Pattern to Extratropical Thermal Forcing

“…Recently, the non‐local extratropical effects in driving tropical Pacific SST trends have received increasing attention. Couplings between atmospheric adjustments, surface fluxes, and oceanic tunnels could translate anomalies in the extratropics into tropical Pacific SST changes (Amaya et al., 2019; Hwang et al., 2021; Luo et al., 2017; Shin et al., 2021). Localized energy perturbations are commonly present in the extratropics, such as those due to anthropogenic aerosol emissions and polar sea ice losses.…”