The surface temperature response to greenhouse gas forcing displays a characteristic pattern of polar-amplified warming 1-5 , particularly in the Northern Hemisphere. However, the causes of this polar amplification are still debated. Some studies highlight the importance of surface-albedo feedback 6-8 , while others find larger contributions from longwave feedbacks 4,9,10 , with changes in atmospheric and oceanic heat transport also thought to play a role [11][12][13][14][15][16] . Here, we determine the causes of polar amplification using climate model simulations in which CO 2 forcing is prescribed in distinct geographical regions, with the linear sum of climate responses to regional forcings replicating the response to global forcing. The degree of polar amplification depends strongly on the location of CO 2 forcing. In particular, polar amplification is found to be dominated by forcing in the polar regions, specifically through positive local lapse-rate feedback, with ice-albedo and Planck feedbacks playing subsidiary roles. Extra-polar forcing is further shown to be conducive to polar warming, but given that it induces a largely uniform warming pattern through enhanced poleward heat transport, it contributes little to polar amplification. Therefore, understanding polar amplification requires primarily a better insight into local forcing and feedbacks rather than extra-polar processes.Polar amplification-commonly defined as the ratio of polar warming to tropical warming 4,10 -is a robust feature of climate change seen in historical observations and climate model simulations [1][2][3][4][5] . Accurate predictions of polar warming are critical given the fundamental role that polar ice plays in the climate system, terrestrial and marine ecosystems, and human society.A key challenge is identifying the roles that local (that is, polar) and remote (that is, extra-polar) processes play in polar amplification within the inherently coupled climate system. Indeed, different conclusions have been reached as to which feedbacks most contribute to polar amplification and whether poleward heat transport plays a significant role 4,5,[7][8][9][10][11][12][13][14]17,18 . These differences may, in part, be due to different analysis methods. For instance: using simulations with prescribed changes in sea-ice and sea-surface temperatures (SSTs), Screen et al. 7 argue that sea-ice loss is the main contributor to Arctic

Here we show that the characteristics of the Indian Ocean Dipole (IOD), such as its power spectrum and phase relationship with the El Niño–Southern Oscillation (ENSO), can be succinctly explained by ENSO combination mode (C‐mode) wind and heat flux forcing together with a seasonal modulation of the air/sea coupled Indian Ocean (IO) Bjerknes feedback. This model explains the observed high‐frequency near‐annual IOD variability in terms of deterministic ENSO/annual cycle interactions. ENSO‐independent IOD events can be understood as a seasonally modulated ocean response to white noise atmospheric forcing. Under this new physical null hypothesis framework, IOD predictability is determined by both ENSO predictability and the ENSO signal‐to‐noise ratio. We further emphasize that lead/lag correlations between different climate variables are easily misinterpreted when not accounting properly for the seasonal modulation of the underlying climate phenomena.

Abstract. A high O 3 episode with the large increases in surface ozone by 21-42 ppbv and the nocturnal surface O 3 levels exceeding 70 ppbv was observed in the region between Xiamen and Quanzhou over the southeastern coast of China during 12-14 June 2014, before the Typhoon Hagibis landing. Variations in the surface O 3 , NO 2 , CO and meteorology during the Typhoon Hagibis event clearly suggest a substantial impact of the peripheral downdrafts in the large-scale typhoon circulation on such an O 3 episode excluding the contributions of photochemical production and the horizontal transport. The influence of vertical O 3 transport from the upper troposphere and lower stratosphere (UTLS) region on high surface O 3 levels is further confirmed by a negative correlation between surface O 3 and CO concentrations as well as dry surface air observed during the O 3 episode. This study provides observational evidence of typhoon-driven intrusion of O 3 from the UTLS region to surface air, revealing a significant effect of such a process of stratosphere-troposphere exchange (STE) of O 3 on tropospheric O 3 and ambient air quality.