Recent autumn sea ice loss in the eastern Arctic enhanced by summer Asian-Pacific Oscillation

Domain: earth_science
Published: 2024-03-30
Editorial outcome: major_revision

Abstract
Recent rapid Arctic sea ice loss was documented as combined results from anthropogenic forcing and climate system internal variability. However, the role of internal variability is not well understood. Here, we propose that the Asian-Pacific Oscillation (APO), an intrinsic atmospheric mode featuring out-of-phase variations in upper-tropospheric temperatures between Asia and the North Pacific, is one driver for autumn sea ice variability in the eastern Arctic. The positive summer APO favors warming of the mid-latitude North Atlantic sea surface temperatures. This warming persists to autumn and in turn triggers strong anticyclonic anomalies over the Barents-Kara-Laptev Seas and weak lower-tropospheric cyclonic anomalies over the East Siberian Sea, enhancing moisture transport into the eastern Arctic. Such changes consequently increase lower-tropospheric humidity, downwelling longwave radiation, and surface air temperature in the eastern Arctic, thereby melting sea ice. Hence, a recent tendency of the summer APO towards the positive phase accelerates autumn sea ice loss in the eastern Arctic.

Introduction
In the context of global warming, the Arctic has warmed faster than global average by a factor of 2 or more since the mid-20th century1,2. Under such a background, the Arctic sea ice has melted at an accelerating pace particularly in autumn during recent decades3. A number of studies have documented impacts of Arctic sea ice loss on local and remote climate4–15. For instance, autumn sea ice loss in the Barents-Kara Seas tends to increase the frequency of cold winters and snow storms in Eurasia16–20 via influencing the Eurasian blocking21,22, the East Asian winter monsoon circulations23,24, the land process25 and so on. The decline of autumn sea ice in the Kara-Laptev Seas even affects the subsequent summer precipitation over East Asia through the seasonal persistence of snow depth and soil moisture26. The sea ice reduction in the Chukchi-East Siberian Sea is inclined to cool temperatures in central North America27 and that around Greenland is closely linked to the variability of surface temperatures in northern Europe and eastern North America28,29. Nevertheless, some studies have reservations about the effects of Arctic sea ice loss 30–32. For example, Refs. 33, 34 reported a weak response of the Eurasian winter climate to the reduction of Arctic sea ice.

Although no consensus is reached for the climate effects of sea ice loss in the Arctic, the physical drivers for Arctic sea ice loss have sparked increasing interest. It is indicated that the secular trend of Arctic sea ice loss is attributed to anthropogenic forcing1,35. The internal variability within the climate system also plays important roles36–42. For example, Ref. 37 revealed that anomalous anticyclonic circulation over Greenland and the Arctic Ocean in summertime may contribute as much as 60% to the September Arctic sea ice loss since 1979. Some atmospheric modes of internal variability, such as the Arctic Dipole43, the Arctic Oscillation/North Atlantic Oscillation44–46, and the Pacific North American pattern47, have also been highlighted as physical drivers for the Arctic sea ice variability.

The Asian-Pacific Oscillation (APO)48, featuring a seesaw change in the upper-tropospheric temperatures between Asia and the North Pacific, is an atmospheric mode of internal variability over the extra-tropical Asian-Pacific sector and most dominant in the summertime49. When the troposphere is warming over the Asian continent, the troposphere is cooling over the North Pacific, and vice versa. In essence, the APO pattern reflects the variability of zonal thermal contrast between Asia and the North Pacific. Due to its hemispheric nature, summer APO exerts prominent effects on large-scale atmospheric circulations and climate in and beyond the Asian-North Pacific sector48–54. An emerging question is whether it also plays a role in the variability of Arctic sea ice. In this study, we are motivated to examine their linkage based on the reanalysis data including the HadISST1 sea ice concentration (SIC), the ERSSTv5 sea surface temperature (SST), and the NCEP/NCAR atmospheric reanalysis as well as the Community Earth System Model Large Ensemble (CESM-LE) simulation data (see “Data and Methods” section for details), and demonstrate that summer APO may act as a driver of autumn sea ice variability in the eastern Arctic.

Results
We first examine the co-variability of summer (June-July-August, JJA) upper-tropospheric (500-200 hPa) eddy temperature (UTT, defined as the departure of temperature from its zonal mean) over the Asian-Pacific sector (25°−65°N, 75°E-120°W) and autumn (September-October-November, SON) SIC in the eastern Arctic (70°−83°N, 30°E-180°) by the maximum covariance analysis (MCA). The spatial patterns associated with the first leading MCA mode, accounting for 62.9% of the squared covariance between the two fields analyzed, characterize that positive UTT anomalies over Asia and negative UTT anomalies over the North Pacific (Fig. 1a) during summer are coupled with autumn sea ice decline in the eastern Arctic (Fig. 1b). The correlation coefficients of their corresponding temporal expansion series, exhibiting consistent interannual and interdecadal variations over the record (Fig. 1c), are significantly correlated at r=0.62 (p<0.01). A similar result can be generally obtained if we redo the analysis over the satellite record period (1980–2019) directly using the satellite data of SIC (Supplementary Fig. 1).

Of note, the UTT spatial distribution in the leading MCA mode (Fig. 1a) highly resembles its first leading empirical orthogonal function (EOF1) pattern, featured by a zonal seesaw UTT oscillation over the Asian-Pacific sector (Supplementary Fig. 2a). The EOF1 mode, explaining 31% of the total variance, denotes the dominant pattern of the APO variability48. Here, we define the time series of the leading MCA of UTT (red line in Fig. 1c) as the APO index for observational analysis. This index is highly correlated with the principal component (PC1) series of the EOF1 pattern (Supplementary Fig. 2b, r=0.98, p<0.01). Such a high correlation suggests that the index we define can measure the variability of summer APO. Similar to the previous studies53–55 which reported decadal variations, the summer APO over the course of 1950–2019 exhibits decadal variability with the positive phase from the 1950s to the mid-1960s as well as after the mid-2000s and the negative phase in between (Fig. 1c and Supplementary Fig. 2b). Similarly, the temporal expansion series of the leading MCA for autumn SIC (blue line in Fig. 1c) is employed as the SIC index (SICI). This index is also strongly correlated with the area-weighted mean of autumn SIC over the eastern Arctic (r=−0.98, p<0.01), indicating that it can represent autumn sea ice variability in the eastern Arctic. A positive SICI value signifies a lower-than-normal SIC in the eastern Arctic, and vice versa. The area-weighted mean of autumn SIC over the eastern Arctic and the summer APO are correlated at r=−0.59 (p<0.01).

The close connection of summer APO with the autumn sea ice variability in the eastern Arctic is also supported by the regression of autumn SIC anomalies (relative to the climatology of 1950–2019) against the summer APO index. As shown in Fig. 1d, a positive phase of summer APO (i.e., warmer UTT over Asia accompanied by cooler UTT over the North Pacific) well corresponds to a uniform reduction of sea ice in the eastern Arctic during the following autumn.

The squared wavelet coherence analysis for summer APO and autumn SIC indices (Supplementary Fig. 3) further illustrates that they show a predominant in-phase coherency in 5–8 year, 8–12 year, and 16–24 year bands. Therefore, we hypothesize that summer APO may act as a precursor driving eastern Arctic sea ice variability in autumn on interannual-to-decadal timescales. When summer APO is in the positive phase, the eastern Arctic sea ice tends to decline in the subsequent autumn. Conversely, if summer APO lies in the negative phase, the eastern Arctic is inclined to experience considerably higher-than-normal sea ice during autumn.

To address the physical link, we investigate the summer APO-related dynamic and thermodynamic processes. Figure 2a, b show 200 hPa and 850 hPa horizontal wind anomalies in autumn regressed against the summer APO index, respectively. Associated with the positive APO phase, an equivalent barotropic structure characterized by strong anticyclonic circulation anomalies dominating the Barents-Kara-Laptev Seas is observed. In contrast, anomalous anticyclonic and cyclonic circulations reside in the upper and lower troposphere of the East Siberian Sea, respectively. The anticyclonic circulation anomaly over the Barents-Kara-Laptev Seas and the cyclonic circulation anomaly over the East Siberian Sea in the lower troposphere both favor the advection of warm and moisture air flows into the eastern Arctic, leading to significant increases of humidity in situ (Fig. 2c). As an important greenhouse gas, the increase in the lower-tropospheric moisture can further enhance downwelling longwave radiation (DLR, Fig. 2d) and warm up surface air temperature (SAT, Fig. 2e)56–59, which are key to the eastern Arctic sea ice variability. Correlations indicate that the low-level humidity correlates well with the DLR (r=0.95, p<0.01) and SAT (r=0.91, p<0.01) over the eastern Arctic, and the eastern Arctic-averaged low-level humidity, DLR and SAT are all highly correlated to the SICI (r=0.66, 0.70, 0.74, respectively, p<0.01), manifesting as increased low-level humidity, DLR and SAT conducive to autumn sea ice reduction in the eastern Arctic (Supplementary Fig. 4). The correlations of summer APO with the autumn low-level humidity, DLR, and SAT averaged over the eastern Arctic are 0.52, 0.58, and 0.56 (p<0.01), respectively. This suggests that the thermodynamic conditions associated with the positive summer APO are conducive to a decline in eastern Arctic sea ice in autumn.

Certainly, due to short memory of the atmosphere, the APO signal cannot persist from summer to autumn by itself. How does summer APO exert impacts on the following autumn thermodynamic processes over the eastern Arctic? We hypothesize that the North Atlantic and the North Pacific SSTs may play roles.

According to the static equilibrium relationship, warming (cooling) of air column tends to induce an increase (a decrease) in the upper-level pressure and a decrease (an increase) in the lower-level pressure via the expansion (contraction) of the air column. Thus, during the positive APO phase, the warming in Asia yields an anticyclonic circulation anomaly in the upper troposphere (Fig. 3a) and a cyclonic circulation anomaly in the lower troposphere (Fig. 3b). The situation is reversed over the North Pacific due to cooling, showing an anomalous cyclonic circulation in the upper troposphere and an anomalous anticyclonic circulation in the lower troposphere. Furthermore, the anomalous pattern over the Asian-North Pacific sector is teleconnected with an anomalous cyclonic circulation between 40° and 60°N and an anomalous anticyclonic circulation south of it in the upper troposphere of the North Atlantic (Fig. 3a). The westerly anomalies in between indicates an intensification of the westerly jet stream over the North Atlantic, which can induce anomalous descending motion on the right-hand side of the exit of jet stream through a secondary circulation due to the effect of geostrophic deviation60. To compensate the downward outflow, a mass divergence is introduced, yielding an anticyclonic circulation anomaly in the lower troposphere of the mid-latitude North Atlantic (Fig. 3b).

We hypothesize the anomalous patterns in the upper troposphere of the Asian-North Pacific sector and the North Atlantic are linked via a zonal wave train propagating from the Asian-North Pacific region to the North Atlantic via North America, with positive stream function anomalies over East Asia and eastern North America and negative anomalies over the western coast of North America and the North Atlantic north of 40°N (Fig. 3c). Note that the wave activity over eastern North America is relatively weak. Such a downstream propagation of wave train was previously described in Ref. 52, but the wave activity seems weaker in the North Atlantic compared to Fig. 3c, which may be due to that it used different domains of Asia (10°–40°N, 30°–140°E) and the North Pacific (10°–40°N, 180°–90°W) to measure the APO during May–August. Ref. 61 argued that the Asian land heating induces a westward propagation of tropospheric temperatures to the Atlantic, leading to increased tropospheric temperatures over the North America–Atlantic region and an anomalous high in the troposphere of the Atlantic. It suggests a consistently varying feature of tropospheric temperatures between Asia and the North America-Atlantic sector, which differs from the APO-related tropospheric temperatures that show positive values over the extratropics of Eurasia and negative values over the extratropics of the North Pacific, North America, and the North Atlantic48. Thus, the above two processes may link to different phenomena although both affect atmospheric circulations over the North Atlantic. However, the relative importance of the upstream and downstream wave propagations on the North Atlantic climate remains an open issue, which needs more studies in the future.

The low-level anticyclonic anomalies over the subtropical North Pacific and North Atlantic hint at enhanced subtropical highs, since these regions are climatologically controlled by high pressure system. Underlying the intensification of subtropical highs, warm SST anomalies can be induced51,61–64 (Fig. 4a). Because of long memory of the SSTs, these warming anomalies can persist from summer to autumn (Fig. 4b). We define the NP_sst and NA_sst indices, respectively, as the area-averaged SSTs over the North Pacific (30°–50°N, 145°E–150°W) and the North Atlantic (30°–50°N, 75°–25°W). The lead-lag correlations show that the correlations of summer APO leading the NA_sst (NP_sst) are higher than that of summer APO lagging the NA_sst (NP_sst) (Fig. 4c, d). This finding suggests that the main physical process of air-sea interaction in the two regions is the APO forcing on the ocean rather than its response to the oceanic forcing. This is consistent with the result from the modeling studies61,62, which showed that when a NA_sst-like or NP_sst-like anomaly is imposed in a general circulation model, a local circulation response is modeled and no APO-like pressure anomalies are generated in the Asia-Pacific sector.

Previous studies documented the role of the North Atlantic and the Pacific SSTs in the Arctic sea ice change46,65–67. Thus, the APO resultant SST warming in autumn is expected to affect the eastern Arctic sea ice through influencing atmospheric circulations. By examining the autumn NA_sst-related wave activity flux (Fig. 5a), we notice a well-organized atmospheric wave train propagating from the North Atlantic northeastward to the Barents-Kara-Laptev Seas and then shifting southeastward to central Eurasia in the upper troposphere. Accordingly, the Barents-Kara-Laptev Seas are occupied by an anomalous anticyclonic circulation. The strong anticyclonic circulation anomaly extends from the upper troposphere to the lower troposphere, displaying a barotropic structure (Fig. 5). In addition, a relatively weak cyclonic circulation anomaly appears over the lower troposphere of the East Siberian Sea (Fig. 5b). Generally, the NA_sst-related pattern shown in Fig. 5b highly resembles that associated with the positive phase of summer APO (Fig. 2b), favoring the intrusion of warm and moisture air flows into the eastern Arctic. As a consequence, the low-level moisture, DLR and SAT increase in the eastern Arctic, leading to a decrease of sea ice (Supplementary Fig. 5). The simultaneous correlations of the NA_sst with the low-level moisture, DLR, and SAT averaged over the eastern Arctic and the SICI in autumn are 0.49, 0.50, 0.45, and 0.47 (p<0.01), respectively.

In comparison, autumn NP_sst is related to a Pacific-North America (PNA)-like wave train (Supplementary Fig. 6a), which emanates from the mid-latitude North Pacific towards North America and shows less conspicuous effects on atmospheric circulations over the eastern Arctic. Instead, anomalous cyclonic circulation over the East Siberian Sea which accompanies the anticyclonic circulation anomaly over the North Pacific in the lower troposphere (Supplementary Fig. 6b) may favor the transportation of warm and moisture air flows from lower latitudes to the eastern Arctic. The simultaneous correlations of the NP_sst with the eastern Arctic-averaged low-level moisture, DLR, and SAT in autumn are 0.39, 0.39, and 0.33, respectively, all of which are lower than the counterparts (r=0.49, 0.50, 0.45, respectively, p<0.01.) with autumn NA_sst.

Considering the collinearity (r=0.65, p<0.01) between autumn NP_sst and NA_sst time series which implies that the SSTs over the two domains are dependent on each other, partial correlations are performed to identify unique and independent influence. When the effect of the NA_sst signal is controlled, the partial correlations between autumn NP_sst and the eastern Arctic low-level moisture (r=0.11), DLR (r=0.09), and SAT (r=0.05) become insignificant. However, in the absence of the NP_sst signal, the relationships of autumn NA_sst with the eastern Arctic low-level moisture (r=0.33), DLR (r=0.35), and SAT (r=0.33) remain substantial. This result suggests a primary role of the North Atlantic SSTs in linking summer APO to autumn thermodynamic processes and hence the sea ice variability in the eastern Arctic.

Methods
Reanalysis data
The monthly SIC with a resolution of 1.0°×1.0° is obtained from the Met Office Hadley Centre Sea Ice and SST dataset (HadISST1)75. The SIC was estimated from satellite retrievals after 1978, prior to which it was reconstructed from different sources of data. The monthly SST dataset used is the Extended Reconstructed SST version 5 (ERSSTv5) at 2.0°×2.0° from the National Oceanic and Atmospheric Administration (NOAA)76. The atmospheric reanalysis data at 2.5°×2.5° are provided by the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR)77.

Community earth system model large ensemble (CESM-LE) simulations
Large “initial condition” ensemble experiments have been developed and widely used to understand the impact of internal variability in recent years. To validate the observational analysis, we use the CESM-LE simulations78, in which 40 members performed by the Community Earth System Model (CESM) with atmosphere-ocean-land-sea ice fully coupled at 0.9°×1.25° (latitude×longitude). Each member is subject to the same historical forcing from 1920 to 2005, but with a small random noise perturbation to their initial air temperature fields. In this study, three members that reproduce a significant negative correlation (p<0.1 with the Monte Carlo test79) between summer APO and autumn SIC averaged over the eastern Arctic for the period 1950–2005, which is consistent with the observation, are selected as an ensemble named BMME, given that this relationship is not captured by each of the 40 members. For comparison, four members that show a significant positive correlation (p<0.1 with the Monte Carlo test79) between summer APO and autumn SIC over the eastern Arctic for the period 1950–2005, which is contrary to the observation, are selected as an ensemble named WMME. The ensemble mean is firstly calculated by individual members and then averaged with equal weight.

In addition to the time series of the leading MCA of UTT (red line in Fig. 1c), another convenient method to construct the APO index is referring to the locations of the teleconnection centers48, because the calculation by this method is independent on the length of the study time. According to Supplementary Fig. 2a, if we define the arithmetic difference of the UTT between the Asian region (30°–55°N, 80°–135°E) and the North Pacific region (30°-55°N, 170°E-125°W) as the APO index, this index is highly correlated with both the PC1 series of the UTT pattern (r=0.95, p<0.01) and the time series of the leading MCA of UTT (r=0.95, p<0.01) over the Asian-Pacific sector. To facilitate analysis, this definition is adopted to measure the APO variability in the simulations.

Statistical analyses
The MCA method80, performed by singular value decomposition of the covariance matrix between two fields, is adopted to identify dominant modes of co-variability between summer UTT over the Asian-Pacific region and autumn SIC in the eastern Arctic. The leading pair of singular vectors and the associated expansion series obtained by projecting the singular vectors onto the original fields indicate the spatial pattern and temporal variation of the two fields that are optimally coupled. The squared singular value explains the contribution of the pair of singular vectors to the total covariance between them.

The horizontal wave activity flux (W) is applied to detect the energy propagation of quasi-stationary Rossby waves, which is calculated in term of the equation81:

W= 1/(2|U|) * [U(ψ'x^2 - ψ'ψ'xx) + V(ψ'xψ'y - ψ'ψ'xy), U(ψ'xψ'y - ψ'ψ'xy) + V(ψ'y^2 - ψ'ψ'yy)] (1)

in which ψ' is the perturbation stream function, |U| is the horizontal wind speed, U(V) is the zonal (meridional) component of the basic flow for the whole period, and p is the pressure divided by 1000 hPa. The anomalous perturbation stream function associated with the given index is calculated by the regression.

Regression, correlation, EOF, and wavelet transform coherence (WTC) analyses are also employed. The statistical significance of the results for regression, correlation, and WTC analyses is determined by the Monte Carlo test79 with 1000 simulations. In the Monte Carlo test, the null hypothesis is that two variables X and Y are not related (i.e., the correlation coefficient r=0), and the alternative hypothesis assumes a significant relationship between the two variables. For a given significance level α=0.1, for example, we first randomly sample X 1000 times, and then calculate r values of each sample with Y. The obtained 1000 simulated r values are ranked from the smallest to the largest, and the 5th (95th) percentile value is used as the critical value of r (r0.05, r0.95). If the actual r is smaller than r0.05 or greater than r0.95, the null hypothesis is rejected and we consider it significant at α=0.1. The time period used for analysis in this study is from 1950 to 2019. Before analysis, all the data are linearly detrended.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of study of Autumn sea ice loss in the eastern Arctic mediated by summer Asian-Pacific\nOscillation by Zhou et al\nThe authors provide a clear analysis of a new (?) Asian-Pacific Oscillation driver of Eastern sea ice\nvariability on annual to decadal timescales. The arguments are well structured in a series of clear\nfigures that 1) identify teleconnections between APO and SIC 2) propose mechanistic linkages\nbetween APO, SST and SIC 3) contextualise the results against a CMIP class model (CESM). I was not\naware of the role of APO in controlling SIC and as such find the paper interesting. I would consider the\npaper for publication but there are several major issues that I would like the authors to consider and\naddress beforehand (see below). Overall I am worried that the importance of the findings are\nsomewhat exaggerated in that they do not give sufficient credit to similar results in the literature\n(even if APO was not mentioned explicitly there), they don’t provide an assessment of the relative\nimportance of this APO-SIC teleconnection against other well known drivers of sea ice variability, they\nconfuse variability on long term changes and inter-annual variability in their analysis and use a\ndataset that is not observation per se.\nGeneral Comments:\n- The authors should redo their analysis on the satellite record period directly on the satellite data of\nsea ice concentration / extent. Or at least characterise the realism of the analysis product they use. It\nis not clear as well which version of HadISST they use. Quote also\nhttps://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2013JD020316 and product link.\n- The physical processes leading to correlation between summer APO and Autumn SIC are hinting at a\npathway via SST but the authors do not demonstrate that causality directly nor do they reference the\nliterature on SST to SIC teleconnections that is very rich in both Pacific and Atlantic sectors.\n- I find the literature review very partial with a lot of references ignored that have looked at very\nsimilar SST to SIC teleconnections, for example a few below but there are many more. How does this\nwork relate to this literature and what is new (apart from the APO definition and use).\nBonan et al (2020) https://doi.org/10.1029/2019GL085666\nLiu et al (2021) https://www.nature.com/articles/s41467-021-21830-z\nKapsch et al (2013) https://www.nature.com/articles/nclimate1884\n- Can you expand as to why the SST driven SIC variability is showing lower correlation scores than the\nAPO to SIC.\n- One more proof reading required to iron out some typos and small grammatical errors.\n- The procedure of selection of CESM ensemble members needs to be clarified.\n- Regression is done at multiannual time scales (i.e. there is no detrending) and yet mechanisms are\ndiscussed at inter-annual scale. Are you claiming that APO drives inter annual variability or variability\non\nvariability on longer time scales? Clarify\n- Your extrapolations to a negative feedback on sea ice decline by the end of the century seem a little\nweak. In particular against the context of rapid Arctic amplification and disintegration of the sea ice. In\ngeneral something that is missing from your analysis is the relative importance of your findings to\nother more well known drivers of sea ice variability (i.e. AO) and decline (CO2 emissions). See for\nexamples papers by Dirk Notz and Julienne Stroeve.\n- It is well know that regression between indices (AO, etc…) and SIC are not stationary and change\ndrastically depending on the duration of the time period studied. The authors should assess their\nresults over the satellite period only and use satellite products only. I would also like to see more\nanalysis of the time scale over which their teleconnections are effective. You could for example\nproduce a spectral analysis and look at drivers of the time series for different amounts of smoothing.\nWould the authors care to expand to teleconnections between other seasons and if not explain why\nsummer to autumn is the most clear result.\nSpecific Comments:\n- Fig 2f is quoted before Fig2 a-e\n- Fig 2a and 2b hard to see the land contours\n- Fig 2 needs clarifying as it is not clear in the title that we are looking at regressions\n- L140 define the static equilibrium relationship", "reviewer_3": "Review of the paper\n'Autumn sea ice loss in the eastern Arctic mediated by summer\nAsian-Pacific Oscillation'\nby B. Zhou, Z. Song, Z. Yin, X. Xu, B. Sun, P. Hsu, and H. Chen\nsubmitted to 'Nature Communication'\nThe aim of the manuscript is to analyse the role of a specific atmospheric variability pattern in\nsummer, namely the Asian-Pacific Oscillation (APO), as a potential driver of autumn sea ice variability\nin the eastern Arctic. The authors proposed a chain of dynamical processes linking variations in the\nAPO to autumn sea ice loss in the eastern Arctic, whereby the recent trend of the summer APO\ntowards its positive phase may have contributed to accelerated autumn sea ice loss in the eastern\nArctic in autumn.\nThis study continues a series of papers by the lead author dealing with the APO and its relationship\nwith climate variability over the North Pacific or North Atlantic, respectively.\nThis study contributes to the large body of literature investigating the connections between the Arctic\nand the mid-latitudes. While the majority of papers investigate how the dramatic Arctic changes in\nautumn and winter, in particular sea ice retreat and increases in near-surface temperature, may\ntrigger tropospheric and stratospheric pathways from the Arctic to the mid-latitudes, this study\ncontributes to the less studied link between changes in the mid-latitude circulation and the Arctic\nclimate system, in particular Arctic sea ice retreat in autumn (which in turn may trigger the above-\nmentioned Arctic-mid-latitude linkages in autumn and winter). This study is therefore timely and\nrelevant.\nThe manuscript is clear and mostly easy to follow, but in my opinion several aspects of the proposed\nchain of dynamical processes linking variations in the APO to autumn sea ice loss in the eastern Arctic\nlack evidence, in particular the link from the anomalous circulation over the Asian-North Pacific sector\nto the anomalous circulation in the mid-latitudes of the North Atlantic. Because this is a key link in\nconnecting SUMMER APO to AUTUMN sea-ice loss, in my view it is necessary to provide more evidence\nin this regard.\nIn addition, the description of the methods needs to be improved to ensure that the reader can\nunderstand all the analyses and hence the subsequent conclusions. This explicitly includes the analysis\nof the modelling results, in particular the choice of the members for the sub-ensemble.\nAt this stage, the submitted manuscript needs careful and major revision.\nMajor comments:\n(1) My main concern is the missing evidence for the link from the anomalous circulation over the\nAsian-North Pacific sector to the anomalous circulation in the mid-latitudes of the North Atlantic. This\nis critical, because this is a key link in connecting SUMMER APO to AUTUMN sea-ice loss. The authors\nargue that \" the anomalous pattern over the Asian-North Pacific sector is teleconnected with an\nanomalous anticyclonic circulation in the mid-latitudes of the North Atlantic through a zonal wave\ntrain\" (L145-147) with reference to a previous paper by the lead author (Zhou and Wang, JGR, 2015).\nIn my opinion, Fig. 3 alone does not show the real extent of the suggested wave train. It is not clear,\nwhich centers of action are really connected via wave activity fluxes, nor the start and end of the wave\ntrain. In the referenced earlier study (Fig. 9a in Zhou and Wang, JGR, 2015), wave activity fluxes for\nthe suggested wave train are shown, but in my view, the wave train ends over eastern North America\ndue to only very weak wave activity fluxes originating from the stream function anomaly over eastern\nNorth America towards the mid-latitude North Atlantic. I would like to ask the authors to provide\nstronger evidence for the suggested link from the anomalous circulation over the Asian-North Pacific\nsector to the anomalous circulation in the mid-latitudes of the North Atlantic.\nMoreover, in L160, the authors give the reference Zhao et al., JC (2012). These authors argue that\n\"...the westward propagation of the positive anomalies results in an increase of tropospheric\ntemperature over the North America–Atlantic region. As seen later from section 4d, this westward\npropagation may be forced by Asian surface heating anomalies. Accordingly, significant positive\nanomalies appear in the troposphere over North America and the Atlantic, .... Thus the tropospheric\ntemperature over North America and the Atlantic is high (low) when Asian tropospheric temperature is\nhigh (low), suggesting a consistently varying feature of tropospheric temperature between Asia and\nthe North America–Atlantic sector.\" (Zhao et al., JC (2012), p. 6600). Here, a different mechanisms is\nsuggested. Could the authors should analyse and discuss these different mechanisms.\n(2) Parts of the \"Data and Methods\" section have to be improved by providing necessary information\nto enable the reader to better understand the analyses performed in the study.\nWith regard to the simulations: more details have to be given, how those ensemble members have\nbeen selected, which simulate an summer APO- autumn SIC relation, how many ensemble members\nfrom the 40 members simulate the APO-SIC relation, and how the ensemble patterns shown in\nsupplementary Figs. 6 and 7 have been calculated. What are potential reasons that not all ensemble\nmembers reproduce the observed summer APO- autumn SIC relation?\nWith regard to statistical analyses: Please provide information, how the perturbations\n(needed for the calculation of the wave activity fluxes) have been calculated, since the definition of the basic flow has\nan impact on the calculated wave fluxes.\n(3) The authors have used seasonally (3 months) averaged data for their analyses. How robust are\ntheir results when using data with higher temporal resolution (montly means or means over pentads)?\nMinor comments\n(1) General commnt for figures 2, 3, 5, S4, S7: I strongly recommend to show the same area in all\nthese figures for better comparability, and in particular to extend the area shown in fig. 5 to the west\n(to allow a better identification of the begin and end of wave trains).\n(2) Introduction L32-67: For some statements, newer references should be added, I here give one\nexample: Arctic amplification: Arctic warming nearly four times faster than the globe (Rantanen et al.,\n2022). Please check newer publications also for other statements.\nRantanen, M., Karpechko, A. Y., Lipponen, A., Nordling, K., Hyvärinen, O., Ruosteenoja, K., ... &\nLaaksonen, A. (2022). The Arctic has warmed nearly four times faster than the globe since 1979.\nCommunications Earth & Environment, 3(1), 168.\n(3) L39: What means etc.?\n(4) L34-L44: Please give also refernces for the uncertainties and knowledge gaps with regard to the\nimpacts of Arctic changes on the local and remote weather and climate.\n(5) L65: \"the reanalysis and simulation data\" please, be more specific.\n(6) L70 and following: I strongly recommend to reformulate the frist sentences of the results section\nto allow the reader an easier entry into the results section.\n(7) L74-L76: Please give lat-lon information for the Asian-Pacific region in the text.\n(8) L74-L76: How sensitive are the results of MCA (or EOF) on the chosen area for the Asian-Pacific\nregion? Does the dipole pattern of the APO appear as first mode for a larger region as well?\n(9) L98: How are SIC anomalies calculated?\n(10) L488/Fig.1: Please indicate the correlation coefficient between the time series in Fig. 1c. Same\nfor Fig. S5c.\n(11) Fig. S1: The shown timeseries for the APO (PC1 of EOF1 mode over Asian-Pacific sector) deviates\nfrom that shown in Zhou et al., 2015 over the period 1965-2000. In Zhou et al. (2015), Fig. 3, I do\nsee interannual variability of the index with no preference of a negative phase, whereas the timeseries\nshown in Fig. S1 does show a prolonged negative phase from ~1965 to ~2000. Please, could you\ncomment on these differences?"}, "author_response": "Response to the Reviewer #1\nThe authors provide a clear analysis of a new (?) Asian-Pacific Oscillation driver of\nEastern sea ice variability on annual to decadal timescales. The arguments are well\nstructured in a series of clear figures that 1) identify teleconnections between APO\nand SIC 2) propose mechanistic linkages between APO, SST and SIC 3) contextualise\nthe results against a CMIP class model (CESM). I was not aware of the role of APO\nin controlling SIC and as such find the paper interesting. I would consider the\npaper for publication but there are several major issues that I would like the authors to\nconsider and address beforehand (see below). Overall I am worried that the\nimportance of the findings are somewhat exaggerated in that they do not give\nsufficient credit to similar results in the literature (even if APO was not mentioned\nexplicitly there), they don’t provide an assessment of the relative importance of this\nAPO-SIC teleconnection against other well known drivers of sea ice variability, they\nconfuse variability on long term changes and inter-annual variability in their analysis\nand use a dataset that is not observation per se.\nReply: Positive comments and helpful suggestion from the reviewer are highly\nappreciated. We have carefully considered all the comments and made substantial\nrevisions. The revisions in the manuscript are highlighted by red color.\nGeneral Comments:\n- The authors should redo their analysis on the satellite record period directly on the\nsatellite data of sea ice concentration /extent. Or at least characterise the realism of the\nanalysis product they use. It is not clear as well which version of HadISST they use.\nQuote also https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2013JD020316\nand product link.\nReply: The sea ice concentration (SIC) we use is from the Met Office Hadley Centre\nSea Ice and SST dataset, HadISST1. The SIC was estimated from satellite retrievals\naals\na\nafter 1978, prior to which it was reconstructed from different sources of data. This has\nbeen clarified in the subsection “Reanalysis data” of the revised manuscript (Please\nsee Lines 280-2823.\nAccording to the comment, we have also redone the analysis over the satellite record\nperiod (1980-2019), and a similar result can be generally obtained (Please see\nLines88-90 and Supplementary Fig. 1). We also redo the analysis using the SIC data\nof the HadISST v2.2 (Titchner and Rayner, 2014) as mentioned above. The results\nshown in Fig. R1 below resemble that of Fig. 1 a-c in the manuscript.\nFig. R1:a–b Spatial patterns of the first leading MCA mode for a summer UTT over\nthe Asian-Pacific sector and b autumn SIC in the eastern Arctic based on the\nHadISST.2.2 dataset over the period 1950–2019. Areas significant above the 90%\\nconfidence level are dotted. c Time series of the UTT (red line) and SIC (blue line)\npatterns shown in a and b,respectively.\n- The physical processes leading to correlation between summer APO and Autumn\nSIC are hinting at a pathway via SST but the authors do not demonstrate that causality\ndirectly nor do they reference the literature on SST to SIC teleconnections that is very\nrich in both Pacific and Atlantic sectors.\nReply: According to the comment, we have added some references on the SST to SIC\nteleconnections to provide more evidence explaining the causality between summer\nAPO and autumn SIC in the revised manuscript (Please see L179-180).\n- I find the literature review very partial with a lot of references ignored that have\nlooked at very similar SST to SIC teleconnections, for example a few below but there\nare many more. How does this work relate to this literature and what is new (apart\nfrom the APO definition and use).\nBonan et al (2020) https://doi.org/10.1029/2019GL085666\nLiu et al (2021) https://www.nature.com/articles/s41467-021-21830-z\nKapsch et al (2013) https://www.nature.com/articles/nclimate1884\nReply: We have added these and more references in the revised manuscript. Our work,\nbased on these literatures, advances the understanding of Arctic sea ice loss by taking\na step forward to summer signal.\n- Can you expand as to why the SST driven SIC variability is showing lower\ncorrelation scores than the APO to SIC.\nReply: Our proposed candidate mechanism underlying the linkage between summer\nAPO and autumn sea ice in the eastern Arctic focuses on the pathway via the North\nAtlantic SSTs. Given the complexity of climate system, other processes may also act\nin their linkage, which may explain that the SST driven SIC variability is showing\nlower correlation scores than the APO to SIC. The discussion for this issue has been\nadded in the revised manuscript (Please see L272-278).\n- One more proof reading required to iron out some typos and small grammatical\nerrors.\nReply: A careful check has been carried out to fix typo and grammatical errors.\n- The procedure of selection of CESM ensemble members needs to be clarified.\nReply: It has been clarified in the subsection “Community Earth System Model Large\nEnsemble (CESM-LE) Simulations”. (Please see L296-303)\n- Regression is done at multiannual time scales (i.e. there is no detrending) and yet\nmechanisms are discussed at inter-annual scale. Are you claiming that APO drives\ninter annual variability or variability on longer time scales? Clarify\nReply: Note that all the data are detrended before analysis in our study (Please see\nL334-335 for this statement). Thus, there is no influence of the trends. The summer\nAPO drives sea ice variability on interannual-to-decadal time scales. This has been\nclearly stated in the revised manuscript (Please see L114-119).\n- Your extrapolations to a negative feedback on sea ice decline by the end of the\ncentury seem a little weak. In particular against the context of rapid Arctic\namplification and disintegration of the sea ice. In general something that is missing\nfrom your analysis is the relative importance of your findings to other more well\nknown drivers of sea ice variability (i.e. AO) and decline (CO2 emissions). See for\nexamples papers by Dirk Notz and Julienne Stroeve.\nReply: I agree with the reviewer that the external forcing (CO2 emissions) is the main\ndriver for the projected Arctic sea ice loss. In the revised manuscript, we have\nrewritten this part to make it more balance, which is as: “Under future warming\nscenarios, the autumn Arctic sea ice loss is projected to continue due to the effect of\ngreenhouse gases67, with more rapid changes for higher warming levels1,68. This\ndeclining rate may be accelerated or counteracted by future changes in climate system\ninternal variability. The CMIP5 and CMIP6 project a weakening in summer APO\ntoward the end of this century in a warmer world69,70. Given the results presented in\nthis study, such a tendency is speculated to increase autumn sea ice in the eastern\nArctic, which may, to some degree, counteract the sea ice loss resulting from the\nanthropogenic warming and thus is a source of uncertainty in projections of Arctic sea\nice, although its relative importance is unclear”. (Please see L262-270)\n- It is well know that regression between indices (AO, etc…) and SIC are not\nstationary and change drastically depending on the duration of the time period studied.\nThe authors should assess their results over the satellite period only and use satellite\nproducts only. I would also like to see more analysis of the time scale over which\ntheir teleconnections are effective. You could for example produce a spectral analysis\nand look at drivers of the time series for different amounts of smoothing. Would the\nauthors care to expand to teleconnections between other seasons and if not explain\nwhy summer to autumn is the most clear result.\nReply: According to the comment, we have redone the analysis over the satellite\nrecord period (1980-2019). A similar result can be generally obtained. This content\nhas been added in the revised manuscript (Please see Lines88-90 and Supplementary\nFig. 1). We also perform the squared wavelet coherence analysis (Please see\nSupplementary Fig. 3) in the revision to investigate the time scale over which the\nAPO-SIC teleconnections are effective. It is indicated that they show a predominant\nin-phase coherency in 5-8 year, 8-12year, and 16-24 year bands (Please see\nL114-117). The reason why we care about summer APO is due to that the APO is\nmost dominant in the summertime, which has been clarified in the revised manuscript\n(Please see L61).\nSpecific Comments:\n- Fig 2f is quoted before Fig2 a-e\nReply: In the revised manuscript, we have moved it to Fig. 1 (as Fig. 1d).\n- Fig 2a and 2b hard to see the land contours\nReply: The land contours have been thickened.\n- Fig 2 needs clarifying as it is not clear in the title that we are looking at regressions\nReply: Clarified in the figure caption.\n- L140 define the static equilibrium relationship\nReply: It has been defined in the revised manuscript. (Please see L152-154)\nResponse to the Reviewer #2\nGeneral comments\nThe authors consider the question of whether the June-July-August (JJA)\nAsian-Pacific Oscillation (APO) exerts an influence on the\nSeptember-October-November (SON) sea-ice in the eastern Arctic. They find a\nsignificant correlation between two associated indices of 0.62, and argue that this\nreflects a causal forcing from the APO. The pathway is suggested to be mediated by\nthe North Atlantic SSTs: the APO forces anomalies in North Atlantic SSTs in JJA,\nwhich persist to SON, triggering circulation anomalies that are conducive to ice loss.\nDecadal variability or future trends in the APO may therefore exacerbate or offset ice\nloss due to global warming. If true, this would be a noteworthy result, since model\nbiases in the APO may affect their projections of sea ice loss.\nI enjoyed reading this paper. It was generally easy to read and follow, and follows a\nclear and logical order. The figures showing wave activity fluxes are particularly neat\nand convincing. However, I have a few major comments which need to be addressed,\nin particular about the role of the North Atlantic. I am therefore recommending major\nrevisions. I hope the authors will be able to address my comments without too much\ntrouble.\nReply: Positive comments and helpful suggestion from the reviewer are highly\nappreciated. We have carefully considered all the comments and made substantial\nrevisions. The revisions in the manuscript are highlighted by red color.\nMajor comments\n1. The North Atlantic as a mediator\nThe authors argue that the correlations between the APO and Eastern Arctic sea-ice\ncan be explained by a pathway via the North Atlantic. While physical motivation for\nthis is given by the figures provided, some basic statistical testing is missing.\nThe basic method for testing if something is a mediator is described e.g. on Wikipedia.\nThe authors do not seem to state what the correlation is between their NA_SST index\nand Eastern Arctic sea-ice index, nor what happens if you do a multiple regression\nwith the APO and NA_SST as predictors and eastern Arctic sea-ice as the dependent\nvariable. Can the authors please explicitly state the NA_SST-Ice correlation and carry\nout this test for mediation and discuss the results?\nThe correlation between APO and sea-ice is 0.62, explaining ~38% of the ice variance.\nWhile NA_SSTs and sea-ice will surely also be correlated, it can’t be ruled out that\nthe NA_SST related ice-variance is disjoint from the 38% APO-related variance. If\nthere were actually a different pathway from APO to ice (e.g. via the stratosphere?)\nthen this could easily be the case. The above test for mediation should help clarify\nthis.\nReply: Thanks for useful suggestion. According to the comment, we have explicitly\nshown the correlation between the NA_SST and SIC indices in the revised manuscript\n(Please see L194-196). The regression of SIC with the NA_sst index is provided in\nSupplementary Fig. 5.\nGiven high linear correlations between APO and NA_SST, the multivariate linear\nregression may give rise to covariance problems. Instead, we use the CESM-LE\nensemble simulation results to verify the proposed physical process. We select the\nmembers that can reproduce a significant negative correlation between summer APO\nand autumn SIC averaged in the eastern Arctic, which is consistent with the\nobservation, as an ensemble named BMME. We also select the members that show a\nsignificant positive correlation between summer APO and autumn SIC averaged in\nthe eastern Arctic, which is contrary to the observation, as an ensemble named\nWMME. Overall, the BMME simulation results are broadly comparable to the\nobservation, showing that associated with the positive APO pattern is the warming of\nsummer SSTs in the mid-latitudes of the North Atlantic and the SST warming can\npersist to the subsequent autumn (Supplementary Fig. 8). The North Atlantic SST\nwarming in autumn further excites a wave train and triggers barotropical anticyclonic\nanomalies over the Barents-Kara-Laptev Seas (Supplementary Fig. 9), beneficial for\nthe increase in the low-level moisture (r=0.34, p<0.05) and SAT (r=0.27, p<0.05) and\nhence reducing sea ice in the eastern Arctic. In comparison, for the WMME\nsimulation, the persistent SST warming in the North Atlantic cannot be simulated\n(Supplementary Fig. 10). This discrepancy in the results between the BMME\nsimulation and the WMME simulation suggests the importance of the North Atlantic\nSSTs in the lagged impact of summer APO on autumn SIC in the eastern Arctic\n(Please see L233-255). In addition, based on the partial correlation, when controlling\nthe NA_sst signal, the APO-SICI correlation decreases from 0.62 to 0.46, which may\nalso suggest the role of the NA_SST.\nIt is worth noting that our proposed candidate mechanism underlying the linkage\nbetween summer APO and autumn sea ice in the eastern Arctic only focuses on the\npathway via the North Atlantic SSTs. Given that the North Atlantic SST driven SIC\nvariability shows lower correlation scores than the APO to SIC, this reminds us that\nother processes may also act in their linkage, which deserves further in-depth\ninvestigation in the future in order to get a broad picture. This has been added in the\nrevised manuscript (Please see L272-278)\n2. Causality between APO and North Atlantic\nThe obvious question which arises when reading this paper is whether it’s actually\njust the North Atlantic SSTs doing everything. The authors include an argument for\nwhy the APO is driving the NA_SSTs and not vice versa, by considering the lag-lead\ncorrelations in Fig 4c. However, I do not find this convincing. The difference\ndepending on whether APO leads or not looks by eye to be 0.6 (APO leads) and 0.4 to\n0.45 (APO lags). Because the total APO-ice correlation to be explained is only 0.62,\nthis means that a hypothesis of the North Atlantic independently forcing some\nproportion of both the APO and ice variance might explain almost the correlations\nseen, once you allow for some noise. In addition, in the CESM_LE models, the\nequivalent lead-lag plot shows an even less pronounced difference between APO\nleading and lagging, making the authors argument less convincing still. Finally, there\nis considerable persistence in the SSTs, meaning that even though the forcing is on\nthe face of it smaller, the fact that it lasts for longer also needs to be factored in.\nI think this point is made more acute by the fact that the APO seems to undergo some\ndecadal variability. By eye this variability looks consistent with that seen in the North\nAtlantic in the same period. This makes it more tempting to wonder if it’s the North\nAtlantic doing the heavy lifting.\nCan the authors do some further tests for whether the NA_SSTs explain what’s going\non? Again, you can use mediation tests (see previous comment) to examine to what\nextent e.g. JJA or MJJ NA_SSTs account for the APO-Ice correlations.\nReply: This issue is related to the point 1. We have carefully considered it in the\nrevision and believe that we have addressed it to the extent we can. Below are some\nexplanations:\nFirst, according to the previous studies, the correlation of X leading Y higher than that\nof X lagging Y suggests the influence of X on Y. Thus, the lead-lag correlations\nbetween summer APO and NA_sst may suggest the impact of summer APO on the\nNorth Atlantic SSTs. In addition, in line with the previous studies, “warm SST–ridge”\nrelationship also implies the influence of atmosphere on ocean. From Fig. 3b and Fig.\n4a in the manuscript, we can observe such a “warm SST–ridge” phenomenon. Thus,\nthe summer APO signal may be preserved in the ocean, and then affects autumn\natmospheric circulations through the feedback of the ocean. However, there is\npossibility that summer SST itself gives rise to the changes in autumn atmospheric\ncirculations.\nSecond, the previous modeling studies indicated that the sensitive experiments with\nthe summer SST changed cannot force the APO phenomenon, suggesting the air-sea\ncoupled relationship is the APO forcing on the ocean.\nThird, we add more analyses based on the CESM-LE simulations. We select the\nmembers that can reproduce a significant negative correlation between summer APO\nand autumn SIC averaged in the eastern Arctic, which is consistent with the\nobservation, as an ensemble named BMME. In general, the BMME simulation results\nare broadly comparable to the observation, showing that associated with the positive\nAPO pattern is the warming of summer SSTs in the mid-latitudes of the North\nAtlantic and the SST warming can persist to the subsequent autumn (Supplementary\nFig. 8). For comparison, we also select the members that show a significant positive\ncorrelation between summer APO and autumn SIC averaged in the eastern Arctic,\nwhich is contrary to the observation, as an ensemble named WMME. It is interesting\nto find that the persistent SST warming in the North Atlantic disappears in the\nWMME simulation (Supplementary Fig. 10). This discrepancy in the results between\nthe BMME simulation and the WMME simulation suggests the importance of the\nNorth Atlantic SSTs in the lagged impact of summer APO on autumn SIC in the\neastern Arctic.\nCertainly, the North Atlantic SSTs cannot do everything. Although our proposed\ncandidate mechanism underlying the linkage between summer APO and autumn sea\nice in the eastern Arctic focuses on the pathway via the North Atlantic SSTs, other\nprocesses may also act in their linkage given that the North Atlantic SST driven SIC\nvariability shows lower correlation scores than the APO to SIC, which needs further\nin-depth investigation in the future.\nAll of these have been presented in the revised manuscript.\n3. Statistical significance tests\nThe authors use a basic t-test to assess significance of correlations throughout.\nHowever, this is not appropriate when considering SSTs or sea-ice timeseries. The\nt-test assumes that samples are independent, but there is considerable interannual\nautocorrelation in SSTs and ice, meaning this assumption is not valid. In particular,\nthe p-values reported are almost certainly much lower than they should be.\nThe authors should use a more appropriate method for assessing significance of\ncorrelations in cases involving SSTs and ice. The most transparent way is probably\nvia Monte Carlo sampling using an explicit null hypothesis. For example, if\ncorrelating some atmospheric variable X and sea-ice Y, you can model X as\nindependent Gaussian and Y as an AR-1 process. By fitting these to the data you can\ngenerate 1000 random draws and assess what correlations are expected by chance.\nOther methods are also possible.\nReply: According to the suggestion, we have changed the t-test to the Monte Carlo\ntest with 1000 simulations for the significance test throughout the manuscript.\n4. Decadal APO variability\nThere are implicit suggestions of decadal variability in the APO in the manuscript (e.g.\nL81 and L236). Can this point be made explicit? That is, what does prior literature\nsuggest about decadal APO variability? Does the decadal variability seen in Figure 1c\nrequire an explanation (i.e. a source of decadal timescale external forcing) or is it\nconsistent with random internal variability of the APO? These questions have obvious\npotential implications for what might happen in the future.\nReply: According to the comment, we have explicitly stated decadal variability in\nsummer APO in the revised manuscript (Please see L99-103). To our knowledge, the\ndecadal variability of APO is internal variability.\nMinor comments\nL1 (Title): I think the use of the word ‘mediated’ is not ideal, since the content of the\npaper will go on to discuss a pathway under which the APO’s influence on sea-ice is\nmediated via the North Atlantic. This can cause confusion. Can the authors please\nrephrase or choose another word? E.g. “enhanced” or “amplified” could work.\nReply: It has been changed to “enhanced” in the title.\nL35: The extent to which the Arctic has “exerted profound impacts” on remote\nclimate is actively debated, and there are many papers which argue it has only had a\nsmall impact. The authors should recognize the lack of consensus here. Please\nrephrase to clarify this, and cite some relevant papers arguing for a more minimal or\nunclear influence, e.g.\nhttps://doi.org/10.1038%2Fs41467-022-28283-y\nhttps://doi.org/10.1038/s41558-020-00954-y\nhttps://doi.org/10.1038/s41558-019-0662-y\nor any number of other references therein.\nReply: We have rewording this part and the lack of consensus has been clarified.\n(Please see L35-36 and L44-47)\nL43: “closely links to”  “is closely linked to”\nReply: Changed. (Please see L43)\nL45: Again, the “pronounced climate impacts” are debatable. Please rephrase to\nreflect the lack of consensus in the literature.\nReply: We have rephrased it as “Although no consensus is reached for the climate\neffects of sea ice loss in the Arctic, the physical drivers for Arctic sea ice loss have\nsparked increasing interest”. (Please see L48-49)\nL49: This doesn’t read very easily: “that summertime atmospheric circulation towards\na stronger anticyclonic circulation”. Do you mean recent trends in summertime\ncirculation? Or something else? Maybe it’s easier and clearer to just write “Ref.31\nindicated that anomalous anticyclonic circulation over Greenland and the Arctic\nOcean in summertime may contribute…”?\nReply: Changed as suggested. (Please see L52-53)\nL60: “essential”  essence\nReply: Changed. (Please see L63)\nL80: I’d write “correlation coefficients” instead of “temporal expansion coefficients”,\nto avoid any potential confusion.\nReply: Changed. (Please see L86)\nL102-103: “may act as a precursor to drive the eastern Arctic”  “may act as a\nprecursor driving eastern Arctic …”\nReply: Changed. (Please see L117-118)\nL130-131: It’s too early in the paper to assert that everything shown is causally\nresponding to the APO (“as a response to the aforementioned … APO …”). At this\npoint there are still outstanding questions about causality which the authors have not\ncommented on. Please rephrase to something more cautious, e.g. “This suggests that\nthe thermodynamic conditions associated with the positive summer APO are\nconducive to a decline in eastern Arctic sea ice in the autumn”, or similar.\nReply: Rephrase as suggested. (Please see L146-147)\nL199: “concerned”  “concerning”\nReply: Changed. (Please see L221)\nL201: “makes an advance to this understanding”  “advances this understanding”\nReply: Changed. (Please see L223)\nL202: I suggest removing the “known as”. It’s not that the APO is known as a mode\nof variability, it’s that it is a mode of variability.\nReply: Deleted.\nL205: North Atlantic SSTs should be plural?\nReply: Changed. (Please see L227)\nL206: Again, SST should be plural (warmer SSTs). Please check other instances in\nthe paper for this.\nReply: We have checked it throughout the manuscript and changed “SST” to “SSTs”.\nL216: Should be an “a” between reproduce and significant.\nReply: Added. (Please see L235)\nL223: “produces”  “shows”\nReply: Changed. (Please see L242)\nL244: The use of the word ‘contradicts’ seems a bit odd to me. I think ‘counteracts’\nwould be better.\nReply: It has been changed to “counteract”. (Please see L268)\nL245: “in the projection of Arctic sea ice”  “in projections of Arctic sea ice”.\nReply: Changed. (Please see L270)\nL265-266: What exactly is the selection criteria? Is it that only members showing\nstatistically significant correlations are chosen? If so, with respect to what p-value and\nwhat test (see major comment!) Or correlations close in magnitude to observations? If\nso, what exact threshold? Also, how many members are actually selected in this way?\nThe answer to this would help give an idea of how robust the APO-ice link really is in\nCESM (and by extension other models).\nReply: These issues have been clarified in the subsection “Community Earth System\nModel Large Ensemble (CESM-LE) Simulations”. (Please see L296-304)\nComment on Figures: I found it very difficult to see the landmasses in Figures 2a and\n2b. Is it possible for the authors to make the land more visible? Also, in Figures 4c\nand 4d, can the authors add information (in caption or as text within the subplots)\nabout when it’s the APO leading and lagging?\nReply: The land contours have been thickened in Figures 2a and 2b. The information\nabout when it’s the APO leading and lagging in Figures 4c and 4d has been added in\nthe caption.\nResponse to the Reviewer #3\nThe aim of the manuscript is to analyse the role of a specific atmospheric variability\npattern in summer, namely the Asian-Pacific Oscillation (APO), as a potential driver\nof autumn sea ice variability in the eastern Arctic. The authors proposed a chain of\ndynamical processes linking variations in the APO to autumn sea ice loss in the\neastern Arctic, whereby the recent trend of the summer APO towards its positive\nphase may have contributed to accelerated autumn sea ice loss in the eastern Arctic in\nautumn.\nThis study continues a series of papers by the lead author dealing with the APO and\nits relationship with climate variability over the North Pacific or North Atlantic,\nrespectively.\nThis study contributes to the large body of literature investigating the connections\nbetween the Arctic and the mid-latitudes. While the majority of papers investigate\nhow the dramatic Arctic changes in autumn and winter, in particular sea ice retreat\nand increases in near-surface temperature, may trigger tropospheric and stratospheric\npathways from the Arctic to the mid-latitudes, this study contributes to the less\nstudied link between changes in the mid-latitude circulation and the Arctic climate\nsystem, in particular Arctic sea ice retreat in autumn (which in turn may trigger the\nabove-mentioned Arctic-mid-latitude linkages in autumn and winter). This study is\ntherefore timely and relevant.\nThe manuscript is clear and mostly easy to follow, but in my opinion several aspects\nof the proposed chain of dynamical processes linking variations in the APO to autumn\nsea ice loss in the eastern Arctic lack evidence, in particular the link from the\nanomalous circulation over the Asian-North Pacific sector to the anomalous\ncirculation in the mid-latitudes of the North Atlantic. Because this is a key link in\nconnecting SUMMER APO to AUTUMN sea-ice loss, in my view it is necessary to\nprovide more evidence in this regard.\nIn addition, the description of the methods needs to be improved to ensure that the\nreader can understand all the analyses and hence the subsequent conclusions. This\nexplicitly includes the analysis of the modelling results, in particular the choice of the\nmembers for the sub-ensemble.\nAt this stage, the submitted manuscript needs careful and major revision.\nReply: Positive comments and helpful suggestion from the reviewer are highly\nappreciated. We have carefully considered all the comments and made substantial\nrevisions. The revisions in the manuscript are highlighted by red color.\nMajor comments:\n(1) My main concern is the missing evidence for the link from the anomalous\ncirculation over the Asian-North Pacific sector to the anomalous circulation in the\nmid-latitudes of the North Atlantic. This is critical, because this is a key link in\nconnecting SUMMER APO to AUTUMN sea-ice loss. In my opinion, Fig. 3 alone\ndoes not show the real extent of the suggested wave train. It is not clear, which centers\nof action are really connected via wave activity fluxes, nor the start and end of the\nwave train. In the referenced earlier study (Fig. 9a in Zhou and Wang, JGR, 2015),\nwave activity fluxes for the suggested wave train are shown, but in my view, the wave\ntrain ends over eastern North America due to only very weak wave activity fluxes\noriginating from the stream function anomaly over eastern North America towards the\nmid-latitude North Atlantic. I would like to ask the authors to provide stronger\nevidence for the suggested link from the anomalous circulation over the Asian-North\nPacific sector to the anomalous circulation in the mid-latitudes of the North Atlantic.\nMoreover, in L160, the authors give the reference Zhao et al., JC (2012). These\nauthors argue that \"...the westward propagation of the positive anomalies results in an\nincrease of tropospheric temperature over the North America–Atlantic region. As seen\nlater from section 4d, this westward propagation may be forced by Asian surface\nheating anomalies. Accordingly, significant positive anomalies appear in the\ntroposphere over North America and the Atlantic, .... Thus the tropospheric\ntemperature over North America and the Atlantic is high (low) when Asian tropospheric\ntemperature is high (low), suggesting a consistently varying feature of tropospheric\ntemperature between Asia and the North America–Atlantic sector.\" (Zhao et al., JC\n(2012), p. 6600). Here, a different mechanisms is suggested. Could the authors should\nanalyse and discuss these different mechanisms.\nReply: Thanks for the suggestion. According to the comment, we have added summer\nhorizontal wave activity flux and stream function associated with the summer APO\n(Fig. 3c) as well as relevant description (Please see L166-170) in the revised\nmanuscript. Seen from Fig.3, a branch of wave train propagates from the Asian-North\nPacific sector to North America and then to the North Atlantic. We also add the\nmechanism proposed by Zhao et al. (2012) for discussion in the revised manuscript\n(Please see L162-166).\n(2) Parts of the \"Data and Methods\" section have to be improved by providing\nnecessary information to enable the reader to better understand the analyses\nperformed in the study.\nWith regard to the simulations: more details have to be given, how those ensemble\nmembers have been selected, which simulate an summer APO- autumn SIC relation,\nhow many ensemble members from the 40 members simulate the APO-SIC relation,\nand how the ensemble patterns shown in supplementary Figs. 6 and 7 have been\ncalculated. What are potential reasons that not all ensemble members reproduce the\nobserved summer APO- autumn SIC relation?\nWith regard to statistical analyses: Please provide information, how the perturbations\n(needed for the calculation of the wave activity fluxes) have been calculated, since the\ndefinition of the basic flow has an impact on the calculated wave fluxes.\nReply: According to the suggestion, we have added more details in the subsection\n“Community Earth System Model Large Ensemble (CESM-LE) Simulations” to\naddress the issues concerned by the reviewer (Please see L296-304). The description\nfor the calculation of the perturbations has also been added in the revised manuscript\n(Please see L328-330).\n(3) The authors have used seasonally (3 months) averaged data for their analyses.\nHow robust are their results when using data with higher temporal resolution (montly\nmeans or means over pentads)?\nReply: Thanks for the comment. This study focuses on the seasonally averaged data\nin order to filter some high frequency signal. However, as a response to the comment,\nwe have calculated the results using the data month by month, although it is beyond\nthe scope of this study. In general, similar results can be obtained. Below (Figs. R2-4)\nshows the couple of August UTT with the SIC in September, October, and November\nas an example.\nFig. R2 a–b Spatial patterns of the first leading MCA mode for a August UTT over\nthe Asian-Pacific sector and b September SIC in the eastern Arctic over the period\n1950–2019. Areas significant above the 90% confidence level are dotted. c Time\nseries of the UTT (red line) and SIC (blue line) patterns shown in a and b,\nrespectively.\nFig. R3 Same as Fig. R2, but for August UTT and October SIC\nFig. R4 Same as Fig. R2, but for August UTT and November SIC\nMinor comments:\n(1) General commnt for figures 2, 3, 5, S4, S7: I strongly recommend to show the\nsame area in all these figures for better comparability, and in particular to extend the\narea shown in fig. 5 to the west (to allow a better identification of the begin and end\nof wave trains).\nReply: According to the comment, we have adjusted the areas. We extend the\nlongitudes in fig. 5 and S7 (now as S9) to -180°~180°, same as that (0°~360°) in S4\n(now as S6). For better shows, Fig.5 and S9 is displayed form the Western\nHemisphere to the Eastern Hemisphere, while S6 is displayed from the Eastern\nHemisphere to the Western Hemisphere. However, to keep figure clear, Figures 2 and\n3 are unchanged.\n(2) Introduction L32-67: For some statements, newer references should be added, I\nhere give one example: Arctic amplification: Arctic warming nearly four times faster\nthan the globe (Rantanen et al., 2022). Please check newer publications also for other\nstatements.\nRantanen, M., Karpechko, A. Y., Lipponen, A., Nordling, K., Hyvärinen, O.,\nRuosteenoja, K., ... & Laaksonen, A. (2022). The Arctic has warmed nearly four times\nfaster than the globe since 1979. Communications Earth & Environment, 3(1), 168.\nReply: We have added the newer publications in this part.\n(3) L39: What means etc.?\nReply: We change it to “and so on”. (Please see L39)\n(4) L34-L44: Please give also refernces for the uncertainties and knowledge gaps with\nregard to the impacts of Arctic changes on the local and remote weather and climate.\nReply: We have clarified this issue in the revised manuscript. (Please see L44-49)\n(5) L65: \"the reanalysis and simulation data\" please, be more specific.\nReply: We have specified it in the revised manuscript. (Please see L68-71)\n(6) L70 and following: I strongly recommend to reformulate the frist sentences of the\nresults section to allow the reader an easier entry into the results section.\nReply: Reformulated. (Please see L77-81)\n(7) L74-L76: Please give lat-lon information for the Asian-Pacific region in the text.\nReply: We have added the lat-lon information in the revised manuscript. (Please see\nL79-80)\n(8) L74-L76: How sensitive are the results of MCA (or EOF) on the chosen area for\nthe Asian-Pacific region? Does the dipole pattern of the APO appear as first mode for\na larger region as well?\nReply: We perform the MCA and EOF analyses for a larger region over the\nAsian-Pacific region (15°-75°N, 60°E-105°W). Similar results can be obtained\n(please see Figs. R5-6 below).\nFig. R5 a–b Spatial patterns of the first leading MCA mode for a summer UTT over\nthe Asian-Pacific sector and b autumn SIC in the eastern Arctic over the period\n1950–2019. Areas significant above the 90% confidence level are dotted. c Time\nseries of the UTT (red line) and SIC (blue line) patterns shown in a and b,respectively.\nFig. R6 a EOF1 mode of summer UTT (×10-2°C) over the Asian-Pacific sector for the\nperiod 1950–2019. b Normalized time series (PC1; bar) of the EOF1 mode from 1950\nto 2019.\n(9) L98: How are SIC anomalies calculated?\nReply: The SIC anomalies are calculated relative to the climatology of 1950-2019.\n(Please see L110-111)\n(10) L488/Fig.1: Please indicate the correlation coefficient between the time series in\nFig. 1c. Same for Fig. S5c.\nReply: Added in Fig. 1c and Fig. S5c (now as Fig. S7c).\n(11) Fig. S1: The shown timeseries for the APO (PC1 of EOF1 mode over\nAsian-Pacific sector) deviates from that shown in Zhou et al., 2015 over the period\n1965-2000. In Zhou et al. (2015), Fig. 3, I do see interannual variability of the index\nwith no preference of a negative phase, whereas the timeseries shown in Fig. S1 does\nshow a prolonged negative phase from ~1965 to ~2000. Please, could you comment\non these differences?\nReply: This is due to the difference in the length of time period and months used in\nthis study from that used in Zhou et al. (2015). Note that the data in both studies are\ndetrended. In Zhou et al. (2015), the time period is 1958-2002. After detrending, it\nmainly reflects interannual variability. In this study, the time period is much longer\n(1950-2019). After detrending, it reflects both interannual and decadal variability. In\nfact, the time period in Zhou et al. (2015) lies in the negative phase of this study. If\nwe extract the time period 1965-2000 from the figure of this study, it also only present\ninterannual variability.\nREVIEWER COMMENTS\nReviewer #2 (Remarks to the Author):\nI thank the authors for addressing my comments, as well as those of the other two reviewers. I think\nthe paper now only requires what I’d consider “minor revisions” in order for me to recommend for\npublication.\nMinor comments:\nL108: The authors have previously stated that the ice and atmosphere MCA timeseries enjoy a\ncorrelation of 0.62 with each other. For completeness, it would be good if the authors can also add\n(after L108) what the correlation is between your APO and SIC indices. Because your indices correlate\nstrongly with the MCA timeseries, I expect it’s probably close to 0.62 as well, but I think it would be\nbetter to just state the correlation anyway here to avoid any confusion.\nL178: The authors refer to earlier modelling studies here, and you also referred to these in your\nresponse to my major comments. However, the content of these modelling studies was not really\nmade clear in both cases. After looking at Ref. 60 myself, I would strongly urge the authors to\nexplicitly spell out the key result from Ref. 60 here in the manuscript: when Zhao et al. impose the NA\nSST pattern in a GCM it generates a local response that differs from the APO-like response diagnosed\nusing regression analysis, and furthermore does not trigger an actual APO anomaly in\nAsia/Pacific; on the other hand, when APO anomalies are imposed, an NA\nSST/circulation response is generated which looks similar to the diagnosed one. To me, this is a very strong bit of evidence for the dominant role of the APO, as opposed to just North Atlantic SSTs by themselves. I still think the lead-lag analysis is not\nconvincing by itself, but when combined with Ref 60 and your new BMME/WMME\nensemble it becomes much more convincing.\nFor example, you could do it like this: (L175) “This finding suggests that the main\nphysical process of air-sea interaction in the two regions is the APO forcing on the\nocean rather than its response to the oceanic forcing. This is consistent with results\nfrom the modeling studies 60,61, which show that when an APO anomaly is imposed\nin a general circulation model, a circulation and SST response is generated in the\nNorth Atlantic which resembles that diagnosed by regression against the APO index\n(Figures 3 and 4). On the other hand, when a NA_sst-like anomaly is imposed, a\ndifferent local circulation response is observed and no APO-like pressure anomalies\nare generated in the Asia-Pacific sector.”\nThe authors should feel free to do it some other way or reword my suggestion, but\nplease do include a description of the results of the modelling studies one way or\nanother.\nL252: It seems a bit strong to say that persistent warming “cannot be simulated”,\ngiven that Figure S10 shows that the signal is overall a warming one. I suggest\nrephrasing to rather say that the persistent warming is substantially reduced and no\nlonger statistically significant (p<0.05).\nFigure comments: I appreciate the changes made to the continents in the revised manuscript.\nHowever, the colour of continents is currently very inconsistent: sometimes they are grey, sometimes\nthey are white (e.g. Figure 1a vs Figure 1b, and Figure 1b vs Figure 2c). For 850hpa variables regions above this level (Greenland, mountains…) are also shown in grey with white continents. I feel it would\ngreatly improve legibility of your figures if you made this consistent across all figures. My\nopinion is that the figures where continents are clearly grey are much easier to look at\nthan ones where continents are white, like Figure 2 where there are colours, dots and\narrows covering everything and making it hard to see the continents underneath. I\nrecommend making the continents grey in all figures, and then highlighting regions\nabove 850hPa with a different shade of grey (or pure white?).\nIt is possible that doing such changes could make figures like Figure 2 even harder to\nread, because of the need for a good contrast between continents and black arrows. I\ntherefore won’t insist on these changes, and leave it to the author’s discretion to make\nthe final decision. It is primarily Figures 1a and 2 (and corresponding Supp. Figures) that are difficult to look at: it would be good if the authors explore possible ways of\nimproving legibility one way or another.\nComment on statistics: “The Monte Carlo test” is completely ambiguous and doesn’t clarify in any way\nwhat statistical test has been done. Monte Carlo just means you wrote down an explicit null hypothesis\nmodel and sampled from it many times. The key thing is that you need to explicitly state what the null\nhypothesis was. In this case, you need to explicitly state how you are choosing to model SSTs, ice and\natmospheric variables. Please write down in the Methods what the null hypothesis you used is exactly\nfor your Monte Carlo test. If you went with my suggestion, you are modelling SSTs and ice using an\nAR1 process (you should clarify that this means ‘autoregressive process with a lag of 1 year’ in your\ncontext) and atmospheric variables as white noise.\nReviewer #3 (Remarks to the Author):\nI very much appreciated the efforts made by the authors to improve the manuscript, and the very\ncareful and detailed responses to all the reviewer's comments. The manuscript has clearly improved,\nbut still several aspects of the proposed chain of dynamical processes linking variations in the APO to\nautumn sea ice loss in the eastern Arctic lack convincing explanations and evidence, in particular the\nlink from the anomalous circulation over the Asian-North Pacific sector to the anomalous\ncirculation in the mid-latitudes of the North Atlantic.\nBecause this is a key link in connecting SUMMER APO to AUTUMN sea-ice loss, in my view it is still\nnecessary to provide better explanations and in-depth discussions of uncertainties in this regard.\nTherefore, I still see the need for major revision.\nMajor comments:\n(1) Still, my main concern is the missing evidence for the link from the anomalous circulation over the\nAsian-North Pacific sector to the anomalous circulation in the mid-latitudes of the North Atlantic. This\nis critical, because this is a key link in connecting SUMMER APO to AUTUMN sea-ice loss.\nAs far as I understood from the literature there have been two mechanisms suggested:\n(a) \" the anomalous pattern over the Asian-North Pacific sector is teleconnected with an anomalous\nanticyclonic circulation in the mid-latitudes of the North Atlantic through a zonal wave train\npropagating from the Asian North Pacific region to the North Atlantic via North America (Fig. 3c).\"\n(L160-163). I appreciated the inclusion of Fig. 3c, but I am struggling to see anomalous anticyclonic\ncirculation in the mid-latitudes of the North Atlantic. I recognize cyclonic anomalies over the North\nAtlantic between 40-60N, and anticyclonic anomalies south of it. In addition, the\nanticyclonic center of the wave train over eastern North America is only weakly\nconnected to the anomalies over the North Atlantic. Please, clarify this and be more\nprecise in the description of the locations of the centers of the wave train.\nA similar plot was shown in a previous paper by the lead author (Zhou and Wang, JGR, 2015, Fig. 9a).\nAs far as I understand, this plot should be directly comparable to Fig. 3c. The wave trains visible in\nthese two plots do show strong similarities over the North Pacific and over North America, but in Fig.\n9a from Zhou and Wang (2015), the wave train does not extend into the North Atlantic. I would like to\nask the authors again (see also major comment 1 from my first review) to discuss these different\nresults, and give arguments, why you consider the weak extension of the wave train\ninto the North Atlantic visible in Fig. 3c as a strong evidence for the link from the\nanomalous circulation over the Asian-North Pacific sector to the anomalous\ncirculation in the mid-latitudes of the North Atlantic.\n(b) In that respect, I also found arguments for another process leading to increased\ntemperatures over the North Atlantic in the literature which I mentioned in my first\nreview. Zhao et al., JC (2012) argue that \"...the westward propagation of the positive\nanomalies results in an increase of tropospheric temperature over the North America-Atlantic region. As seen later from section 4d, this westward propagation may be forced by Asian surface heating anomalies. Accordingly, significant positive anomalies appear in the troposphere over North America and the Atlantic, .... Thus the tropospheric temperature over North America and the Atlantic is high (low) when Asian tropospheric temperature is high (low), suggesting a consistently varying feature of tropospheric temperature between Asia and the North America-Atlantic sector.\" (Zhao et al., JC (2012), p. 6600).\nAlthough the authors included now a statement in response to my comment from the first review, they\ndo not discuss the importance of each of these two different mechanisms (a) and (b) nor they\nprovided any analysis to show the westward propagation of the Eurasian tropospheric temperature\nanomalies.\nOverall, I still do not see enough evidence for the link from the anomalous circulation\nover the Asian-North Pacific sector to the anomalous circulation in the mid-latitudes\nof the North Atlantic, which is really key in the proposed chain of processes to\nexplain the connection from SUMMER APO to AUTUMN sea-ice loss.\nResponse to the Reviewer #2\nGeneral comments\nI thank the authors for addressing my comments, as well as those of the other two\nreviewers. I think the paper now only requires what I’d consider “minor revisions” in\norder for me to recommend for publication.\nReply: We highly appreciate positive comments and helpful suggestion. We have\nrevised the manuscript according to these comments. The revisions in the manuscript\nare highlighted by red color.\nMinor comments\nL108: The authors have previously stated that the ice and atmosphere MCA timeseries\nenjoy a correlation of 0.62 with each other. For completeness, it would be good if the\nauthors can also add (after L108) what the correlation is between your APO and SIC\nindices. Because your indices correlate strongly with the MCA timeseries, I expect\nit’s probably close to 0.62 as well, but I think it would be better to just state the\ncorrelation anyway here to avoid any confusion.\nReply: As suggested, we have added their correlation coefficient in the revised\nmanuscript (Please see L108-109).\nL178: The authors refer to earlier modelling studies here, and you also referred to\nthese in your response to my major comments. However, the content of these\nmodelling studies was not really made clear in both cases. After looking at Ref. 60\nmyself, I would strongly urge the authors to explicitly spell out the key result from\nRef. 60 here in the manuscript: when Zhao et al. impose the NA SST pattern in a\nGCM it generates a local response that differs from the APO-like response diagnosed\nusing regression analysis, and furthermore does not trigger an actual APO anomaly in\nAsia/Pacific; on the other hand, when APO anomalies are imposed, an NA\nSST/circulation response is generated which looks similar to the diagnosed one. To\nme, this is a very strong bit of evidence for the dominant role of the APO, as opposed\nto just North Atlantic SSTs by themselves. I still think the lead-lag analysis is not\nconvincing by itself, but when combined with Ref 60 and your new BMME/WMME\nensemble it becomes much more convincing.\nFor example, you could do it like this: (L175) “This finding suggests that the main\nphysical process of air-sea interaction in the two regions is the APO forcing on the\nocean rather than its response to the oceanic forcing. This is consistent with results\nfrom the modeling studies 60,61, which show that when an APO anomaly is imposed\nin a general circulation model, a circulation and SST response is generated in the\nNorth Atlantic which resembles that diagnosed by regression against the APO index\n(Figures 3 and 4). On the other hand, when a NA_sst-like anomaly is imposed, a\ndifferent local circulation response is observed and no APO-like pressure anomalies\nare generated in the Asia-Pacific sector.”\nThe authors should feel free to do it some other way or reword my suggestion, but\nplease do include a description of the results of the modelling studies one way or\nanother.\nReply: Many thanks for the comment. According to the suggestion, we have added\nthe description of the results of the modeling studies in the revised manuscript (Please\nsee L204-206).\nL252: It seems a bit strong to say that persistent warming “cannot be simulated”,\ngiven that Figure S10 shows that the signal is overall a warming one. I suggest\nrephrasing to rather say that the persistent warming is substantially reduced and no\nlonger statistically significant (p<0.05).\nReply: We have rephrased it as suggested (Please see L280-281).\nFigure comments: I appreciate the changes made to the continents in the revised\nmanuscript. However, the colour of continents is currently very inconsistent:\nsometimes they are grey, sometimes they are white (e.g. Figure 1a vs Figure 1b, and\nFigure 1b vs Figure 2c). For 850hpa variables regions above this level (Greenland,\nmountains…) are also shown in grey with white continents. I feel it would greatly\nimprove legibility of your figures if you made this consistent across all figures. My\nopinion is that the figures where continents are clearly grey are much easier to look at\nthan ones where continents are white, like Figure 2 where there are colours, dots and\narrows covering everything and making it hard to see the continents underneath. I\nrecommend making the continents grey in all figures, and then highlighting regions\nabove 850hPa with a different shade of grey (or pure white?).\nIt is possible that doing such changes could make figures like Figure 2 even harder to\nread, because of the need for a good contrast between continents and black arrows. I\ntherefore won’t insist on these changes, and leave it to the author’s discretion to make\nthe final decision. It is primarily Figures 1a and 2 (and corresponding Supp. Figures)\nthat are difficult to look at: it would be good if the authors explore possible ways of\nimproving legibility one way or another.\nReply: Thanks for the comment. We have re-plotted figures to improve their legibility.\nIn the revised manuscript, after several tries, we set the colour of continents white for\nconsistency except that regions above 850hPa are highlighted in grey. We also\nthicken continents and set the colour of arrows purple to make continents underneath\nclearly visible.\nComment on statistics: “The Monte Carlo test” is completely ambiguous and doesn’t\nclarify in any way what statistical test has been done. Monte Carlo just means you\nwrote down an explicit null hypothesis model and sampled from it many times. The\nkey thing is that you need to explicitly state what the null hypothesis was. In this case,\nyou need to explicitly state how you are choosing to model SSTs, ice and\natmospheric variables. Please write down in the Methods what the null hypothesis you used is\nexactly for your Monte Carlo test. If you went with my suggestion, you are modelling\nSSTs and ice using an AR1 process (you should clarify that this means\n‘autoregressive process with a lag of 1 year’ in your context) and atmospheric\nvariables as white noise.\nReply: According to the suggestion, we have added details about the null hypothesis\nand the calculation process of the Monte Carlo test in the revised manuscript (Please\nsee L375-383).\nResponse to the Reviewer #3\nI very much appreciated the efforts made by the authors to improve the manuscript,\nand the very careful and detailed responses to all the reviewer's comments. The\nmanuscript has clearly improved, but still several aspects of the proposed chain of\ndynamical processes linking variations in the APO to autumn sea ice loss in the\neastern Arctic lack convincing explanations and evidence, in particular the link from\nthe anomalous circulation over the Asian-North Pacific sector to the anomalous\ncirculation in the mid-latitudes of the North Atlantic.\nBecause this is a key link in connecting SUMMER APO to AUTUMN sea-ice loss,\nin my view it is still necessary to provide better explanations and in-depth discussions of\nuncertainties in this regard. Therefore, I still see the need for major revision.\nReply: We highly appreciate positive comments by the reviewer. We have carefully\nconsidered the comments in the revised manuscript, and the revisions are highlighted\nby red color. We believe we have addressed these comments to the extend we can.\nMajor comments:\n(1) Still, my main concern is the missing evidence for the link from the anomalous\ncirculation over the Asian-North Pacific sector to the anomalous circulation in the\nmid-latitudes of the North Atlantic. This is critical, because this is a key link in\nconnecting SUMMER APO to AUTUMN sea-ice loss.\nAs far as I understood from the literature there have been two mechanisms suggested:\n(a) \" the anomalous pattern over the Asian-North Pacific sector is teleconnected with\nan anomalous anticyclonic circulation in the mid-latitudes of the North Atlantic\nthrough a zonal wave train propagating from the Asian North Pacific region to the\nNorth Atlantic via North America (Fig. 3c).\" (L160-163). I appreciated the inclusion\nof Fig. 3c, but I am struggling to see anomalous anticyclonic circulation in the\nmid-latitudes of the North Atlantic. I recognize cyclonic anomalies over the North\nAtlantic between 40-60N, and anticyclonic anomalies south of it. In addition, the\nanticyclonic center of the wave train over eastern North America is only weakly\nconnected to the anomalies over the North Atlantic. Please, clarify this and be more\nprecise in the description of the locations of the centers of the wave train.\nA similar plot was shown in a previous paper by the lead author (Zhou and Wang,\nJGR, 2015, Fig. 9a). As far as I understand, this plot should be directly comparable to\nFig. 3c. The wave trains visible in these two plots do show strong similarities over the\nNorth Pacific and over North America, but in Fig. 9a from Zhou and Wang (2015),\nthe wave train does not extend into the North Atlantic. I would like to ask the authors\nto discuss these different results, and give arguments, why you consider the weak\nextension of the wave train into the North Atlantic visible in Fig. 3c as a strong\nevidence for the link from the anomalous circulation over the Asian-North Pacific\nsector to the anomalous circulation in the mid-latitudes of the North Atlantic.\n(b) In that respect, I also found arguments for another process leading to increased\ntemperatures over the North Atlantic in the literature which I mentioned in my first\nreview. Zhao et al., JC (2012) argue that \"...the westward propagation of the positive\nanomalies results in an increase of tropospheric temperature over the North\nAmerica-Atlantic region. As seen later from section 4d, this westward propagation\nmay be forced by Asian surface heating anomalies. Accordingly, significant positive\nanomalies appear in the troposphere over North America and the Atlantic, .... Thus the\ntropospheric temperature over North America and the Atlantic is high (low) when\nAsian tropospheric temperature is high (low), suggesting a consistently varying\nfeature of tropospheric temperature between Asia and the North America-Atlantic\nsector.\" (Zhao et al., JC (2012), p. 6600).\nAlthough the authors included now a statement in response to my comment from the\nfirst review, they do not discuss the importance of each of these two different\nmechanisms (a) and (b) nor they provided any analysis to show the westward\npropagation of the Eurasian tropospheric temperature anomalies.\nOverall, I still do not see enough evidence for the link from the anomalous circulation\nover the Asian-North Pacific sector to the anomalous circulation in the mid-latitudes\nof the North Atlantic, which is really key in the proposed chain of processes to\nexplain the connection from SUMMER APO to AUTUMN sea-ice loss.\nReply: We have rewritten this part to address the concern of the reviewer (Please see\nL162-191).\n(a) We reword the sentences to describe the locations more precisely. “the anomalous\npattern over the Asian-North Pacific sector is teleconnected with an anomalous\ncyclonic circulation between 40°-60°N and an anomalous anticyclonic circulation\nsouth of it in the upper troposphere of the North Atlantic (Fig. 3a). The westerly\nanomalies in between indicates an intensification of the westerly jet stream over the\nNorth Atlantic, which can induce anomalous descending motion on the right-hand\nside of the exit of jet stream through a secondary circulation due to the effect of\ngeostrophic deviation60. To compensate the downward outflow, a mass divergence is\nintroduced, yielding an anticyclonic circulation anomaly in the lower troposphere of\nthe mid-latitude North Atlantic (Fig. 3b)” (Please see L162-169).\nWe also polish the tone about the wave train and clarify what the reviewer mentioned.\nThat is, “We hypothesize the anomalous patterns in the upper troposphere of the\nAsian-North Pacific sector and the North Atlantic are linked via a zonal wave train\npropagating from the Asian-North Pacific region to the North Atlantic via North\nAmerica, with positive stream function anomalies over East Asia and northeastern\nNorth America and negative anomalies over the western coast of North America and\nthe North Atlantic north of 40°N (Fig. 3c). Note that the wave activity over eastern\nNorth America is relatively weak” (Please see L170-176). Although the wave activity\nis relatively weak over eastern North America, the wave propagation from eastern\nNorth America toward the North Atlantic is observed if we narrow the region of Fig.\n3c in the manuscript to that shown in Fig. R1 below.\nFig. R1 Horizontal wave activity flux (vectors, m2 s-2) and stream function (shading,\n105 m2 s-1) at 300 hpa in association with the normalized summer APO index from\n1950-2019. Areas significant above the 90% confidence level are dotted.\nThe comparison of downstream propagation of wave train in this study with that in\nZhou and Wang (JGR, 2015) is added in the revised manuscript. That is, “Such a\ndownstream propagation of wave train was previously described in Ref.52, but the\nwave activity seems weaker in the North Atlantic compared to Fig. 3c, which may be\ndue to that it used different domains of Asia (10°-40°N, 30°-140°E) and the North\nPacific (10°-40°N, 180°-90°W) to measure the APO during May-August” (Please see\nL176-180).\n(b) We add the discussion of the physical mechanism with that of Zhao et al. (JC,\n2012). Ref.61 argued that the Asian land heating induces a westward propagation of\ntropospheric temperatures to the Atlantic, leading to increased tropospheric\ntemperatures over the North America–Atlantic region and an anomalous high in the\ntroposphere of the Atlantic. It suggests a consistently varying feature of tropospheric\ntemperatures between Asia and the North America-Atlantic sector, which differs from\nthe APO-related tropospheric temperatures that show positive values over the\nextratropics of Eurasia and negative values over the extratropics of the North Pacific,\nNorth America, and the North Atlantic48. Thus, the above two processes may link to\ndifferent phenomena although both affect atmospheric circulations over the North\nAtlantic. However, the relative importance of the upstream and downstream wave\npropagations on the North Atlantic climate remains an open issue, which needs more\nstudies in the future (Please see L179-191).\n(2) I have some questions/comments to the results from the model ensemble.\nDo all ensemble members show an APO?\nWhat are potential reasons that not all ensemble members reproduce the observed\nsummer APO-autumn SIC relations (I have asked this already in my first review!)\nIn my understanding, the main difference between the BMME and WMME ensemble\nis the difference in the lead-lag correlation between APO and North Atlantic SST\n(persistent North Atlantic warming in the BMME ensemble). Please, comment on\npossible reasons for those differences. Is it due to differences in the atmospheric wave\ntrain in summer (proposed as reason for increased North Atlantic SST) or does the\nbackground ocean circulation play a role or are there other reasons? How do you\ninterpret the low sizes of the BMME and WMME ensembles (only about 10% of the\nensemble size of the full CESM Large Ensemble)?\nReply: As seen from Fig. R2 below, all ensemble members can generally reproduce\nthe APO structure.\nFig. R2 Regressions of summer UTT (°C) over the Asian-Pacific sector with the\nnormalized APO index for each member over the period 1950–2005. Areas significant\nabove the 90% confidence level are dotted.\nThe purpose of using large ensemble simulations is to validate the physical process\nthat the North Atlantic SSTs play a role in linking summer APO and autumn SIC in\nthe eastern Arctic. Given that not all the members can reproduce the summer\nAPO-autumn SIC relationship, we select the BMME and WMME ensembles\nmembers rather than all members to outline the differences in the simulated physical\nprocess. If using all members as an ensemble, the signals may be smoothed. We have\nclarified this in the revised manuscript (please see L341-342).\nThe discussion regarding possible reasons for the differences between the BMME and\nWMME in simulating the relationship of the APO with the North Atlantic SST is also\nadded (please see L284-296). One candidate reason may be due to different seasonal\nevolution of the oceanic temperatures associated with the APO. Previous studies\nshowed that the oceanic temperature anomalies extending to deep mixed layer are\nessential to the seasonal persistence of SST anomalies via advection and mixing\nprocesses68-70. In the BMME simulation (Supplementary Fig. 11a), the APO-related\noceanic warming in the North Atlantic extends from the sea surface to a depth of 150\nmeters in summer. This deep sub-surface warming lasts from summer to autumn,\nconducive to the maintenance of SST warming. In contrast, for the WMME\nsimulation (Supplementary Fig. 11b), a shallow oceanic warming which only extends\nto a 50-meter depth is modeled in summer, failing to yield SST warming in autumn.\nCertainly, this issue needs further in-depth investigation in the future.\nREVIEWERS' COMMENTS\nReviewer #2 (Remarks to the Author):\nI thank the authors for their efforts, especially with regards to streamlining the figures. I am now\nrecommending publication with no further revisions.\nReviewer #3 (Remarks to the Author):\nI very much appreciated the efforts made by the authors to improve the manuscript, and the very\ncareful and detailed responses to my comments from the last review round. In particular I value the\nimproved description and explanation of the link from the anomalous circulation over the Asian-North\nPacific sector to the anomalous circulation in the mid-latitudes of the North Atlantic.\nIn my opinion the manuscript is now ready for publication.\nResponse to the Reviewer #2\nI thank the authors for their efforts, especially with regards to streamlining the figures.\nI am now recommending publication with no further revisions.\nReply: Positive comments are highly appreciated.\nResponse to the Reviewer #3\nI very much appreciated the efforts made by the authors to improve the manuscript,\nand the very careful and detailed responses to my comments from the last review\nround. In particular I value the improved description and explanation of the link from\nthe anomalous circulation over the Asian-North Pacific sector to the anomalous\ncirculation in the mid-latitudes of the North Atlantic.\nIn my opinion the manuscript is now ready for publication.\nReply: Positive comments are highly appreciated."}]

Decision letter