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Article Seasonality and Dynamics of Atmospheric Teleconnection from the Tropical Indian and the Western Pacific to West

Hyun-Ju Lee and Emilia-Kyung Jin *

Division of Glacial Environment Research, Korea Polar Research Institute, 26 Songdomirae-ro, Yeonsu-gu, Incheon 21900, Korea; [email protected] * Correspondence: [email protected]

Abstract: The global impact of the tropical and the Western Pacific (IOWP) is expected to increase in the future because this area has been continuously warming due to global warming; however, the impact of the IOWP forcing on West Antarctica has not been clearly revealed. Recently, ice loss in West Antarctica has been accelerated due to the basal melting of ice shelves. This study examines the characteristics and formation mechanisms of the teleconnection between the IOWP and West Antarctica for each using the theory. To explicitly understand the role of the background flow in the teleconnection process, we conduct linear baroclinic model (LBM) simulations in which the background flow is initialized differently depending on the season. During JJA/SON, the barotropic Rossby wave generated by the IOWP forcing propagates into the through the climatological northerly wind and arrives in West Antarctica; meanwhile, during DJF/MAM, the wave can hardly penetrate the tropical . This indicates that during   the Austral winter and spring, the IOWP forcing and IOWP-region variabilities such as the (IOD) and Indian Ocean Basin (IOB) modes should paid more attention to in order to Citation: Lee, H.-J.; Jin, E.-K. Seasonality and Dynamics of investigate the ice change in West Antarctica. Atmospheric Teleconnection from the Tropical Indian Ocean and the Keywords: tropical-Antarctic teleconnection; barotropic Rossby wave theory; Indian Ocean and Western Pacific to West Antarctica. Western Pacific forcing; climate of West Antarctica Atmosphere 2021, 12, 849. https:// doi.org/10.3390/atmos12070849

Academic Editor: Markus Donat 1. Introduction West Antarctica is experiencing severe ice loss due to global warming. The is Received: 1 June 2021 losing mass faster than in early 2020 due to an increase of ice discharge [1,2]. The enhanced Accepted: 28 June 2021 ice flow from the glaciers of West Antarctica is mainly attributed to the basal melting of Published: 30 June 2021 ice shelves caused by the inflow of the Circumpolar Deep Water (CDW) from the nearby ocean. Since the ice shelf supports ice flows on the ground, a thinning of the ice shelves Publisher’s Note: MDPI stays neutral by basal melting produces huge amounts of meltwater from glaciers and consequently with regard to jurisdictional claims in causes level rise [3]. These changes in Antarctic ice are closely related to atmospheric published maps and institutional affil- disturbance. The zonal wind at high latitudes derives Ekman transport offshore and iations. the upwelling of the CDW along the coast, generating flow toward the underside of ice shelves, and leads to the basal melting of ice shelves (e.g., the Thwaite and Pine Island glaciers) [4–6]. The consequent retreat of the grounding line modifies ice-sheet instability, which induces massive ice discharge into the ocean. The meridional wind is also important Copyright: © 2021 by the authors. for the poleward advection of warm moist air, which is responsible for sea ice melting, the Licensee MDPI, Basel, Switzerland. accumulation of snowfall, and ice movement due to wind shear [7,8]. This article is an open access article These seemingly regional circulation changes are mainly associated with global-scale distributed under the terms and atmospheric variability [9]. Many studies have found that high-latitude atmospheric conditions of the Creative Commons variability such as the Southern Annular Mode (SAM), the Pacific- (PSA) Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ pattern, and the Zonal Wavenumber 3 (ZW3) pattern are responsible for changes in sea 4.0/). ice, ice discharge, and the accumulation of snowfall over West Antarctica [10–14]. Other

Atmosphere 2021, 12, 849. https://doi.org/10.3390/atmos12070849 https://www.mdpi.com/journal/atmosphere Atmosphere 2021, 12, 849 2 of 12

studies have suggested that tropical forcing can be a trigger for the high-latitude circulation anomaly [15,16]. For example, the Eastern Pacific variability such as the El Niño-Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO) modulates the anomalous SAM and ZW3 pattern by forming teleconnection [17–19] and in particular causes the Amundsen Sea Low (ASL) anomaly [20]. Furthermore, related to the recent dramatic ice discharge in West Antarctica, Nicolas et al. [21] and Holland et al. [22] found that the Eastern Pacific variability can accelerate the effect of global warming on ice loss in West Antarctica. Additionally, the Atlantic Multidecadal Oscillation (AMO) contributes a dipole-like sea ice concentration anomaly in West Antarctica during the austral winter by deepening the surface pressure in the Amundsen Sea as a part of globe-crossing teleconnection and has been revealed to account for the recent sea ice trend, which cannot be explained by the ENSO [23]. Since tropical forcing has a large influence in the high latitudes, there have been many attempts to examine the connection between the tropics and ice changes in West Antarctica. However, the impact of the tropical Indian Ocean and the Western Pacific (IOWP) has not yet been fully understood because it is located far upstream. By conducting a coupled climate model simulation and regression analysis, Purich and England [24] and Wang et al. [25] documented that, unlike the teleconnection from the Pacific Ocean that may be reinforcing the ASL deepening, the teleconnection from the Indian Ocean tends to contribute to the formation of the ZW3 pattern. Several studies reported that during the austral winter, the influence of the IOWP can extend to West Antarctica [26,27], whereas Nunci and Yuan [28] argued that the wave train induced by the Indian Ocean Dipole (IOD) mode does not reach the Amundsen Sea and beyond during the austral spring. In the IOWP, the has shown progressive warming due to global warming, and this region is thus expected to be increasingly influential for global circulation as a potential source of convection in the future. Therefore, it is essential to understand the dynamics of teleconnection from the IOWP to West Antarctica. Since the teleconnection shows different features depending on the season, each season should be examined separately. Based on the Rossby wave theory, the formation of teleconnection from tropical forcing can be explained by two consecutive processes: the generation of the Rossby wave source (RWS) and the propagation of the barotropic Rossby wave. Diabatic forcing induces a divergent wind anomaly in the upper atmosphere and leads to the generation of the RWS by interacting with the climatological absolute vorticity [29–31]. The RWS is a dynamic source of propagating barotropic Rossby waves and acts as a catalyst in energy conversion from potential energy to rotational kinetic energy [32,33]. To adapt to the growth of the RWS, the barotropic Rossby wave consecutively arises and spreads, releasing kinetic energy. Generally, when the barotropic Rossby waves propagate, they are directed toward the pole at which the potential vorticity is large and the teleconnection is well-constructed in the winter hemisphere. There are characteristics of the Rossby wave propagation for identifying the preferred propagation region. For the stationary Rossby wave, (1) the wave is trapped inside the jet stream and moves eastward up to the exit of the jet, (2) the wave is reflected at a reflecting latitude where the meridional wavenumber of the wave is near zero, and (3) the wave dissipates its energy at a critical latitude where the background zonal wind is near zero and the meridional wavenumber is infinite [34–36]. Thus, the wave propagation depends on the background wind flow. In this study, we theoretically explore the characteristics and dynamics of the IOWP- West Antarctica teleconnection using a primitive model simulation and Rossby wave theory. Its main goal is to determine the seasonal differences in the teleconnection for the four .

2. Materials and Methods 2.1. Data Used Monthly sea surface temperature (SST) data for the period 1870–2019 are obtained from the Met Office Hadley Center SST dataset (HadISST) version 1.1. For atmospheric Atmosphere 2021, 12, 849 3 of 12

variables, daily ERA5 reanalysis data for 250 hPa over the period 1979–2019 are obtained from the European Centre for Medium-range Forecasts (ECMWF). The anomaly for each variable is calculated by subtracting the mean value of each month or day to remove the annual cycle. December–February (DJF), March–May (MAM), June–August (JJA), and September–November (SON) are considered as the Austral summer, autumn, winter, and spring, respectively.

2.2. Model and Experimental Design To investigate the circulation response generated by tropical forcing, a numerical experiment is performed using an atmospheric linear baroclinic model (LBM) that is available from: http://ccsr.aori.u-tokyo.ac.jp/~lbm/sub/lbm.html, accessed on 10 June 2021. The model has a horizontal resolution of T42 with 20 sigma levels. The horizontal diffusion has an e-holding decay time of 6 h and the vertical diffusion has a damping timescale of 1000 days. The timescale of linear drag is set to 0.5 days for the lowest three levels, 1 day for the topmost two levels, and 20 days for the middle levels to mimic Rayleigh friction and Newtonian damping. Each climatological seasonal-mean field of the three- dimensional wind, temperature, geopotential height, specific humidity, and mean sea level pressure is initialized as a background field in the model. To imitate the diabatic heating, circle-shaped heating rate with the 20◦ longitudinal and latitudinal scales is added to the temperature tendency equation at longitudes ranging from 40◦ E to 180◦ E at intervals of 20◦ along the equator. The vertical heating profile has a peak of 2.25 K day−1 around the sigma level of 0.454. The wide IOWP forcing region is arbitrarily partitioned into the several independent forcing , but it helps to better understand the explicit teleconnection process. To concentrate the extratropical response and suppress the continuous tropical response, the forcing is turned on until day 15 (i.e., no forcing after day 15), so the modeled intensity of the response is slightly smaller than the observed intensity; however, this difference does not change our conclusion. The simulated circulation anomaly is plotted as an average over 20–25 days as the steady response. The simulation performed here is designed to examine the steady response to the short-lived tropical forcing, but the result will be nearly similar to the response to the steady (or long-lived) forcing if the spatial features of the basic state and the forcing are maintained. Because the response to the steady forcing is structurally identical to the sum of the response to a sequence of pulses of the forcing [37]. Thus, the model has been used in teleconnection analysis on the inter-annual timescale as well.

2.3. Teleconnectivity Teleconnectivity is calculated with the daily 250-hPa zonal-eddy stream function anomaly on a 2.5 × 2.5◦ grid. To reveal the strength of teleconnection, one-point correlation analysis is conducted for every grid point, then the strongest negative correlation in their respective correlation maps is plotted at the location of each grid point in the same way as in Wallace and Gutzler [38], i.e.,  Ti = Rij minimum for all j , (1)

where i and j are the grid points and Rij is the correlation between i and j. The absolute value of the teleconnectivity is taken and is multiplied by 100 for the sake of convenience. In the teleconnectivity map, the arrow connects the grid points for which the strongest negative correlation is exhibited in the respective one-point correlation maps.

2.4. Stationary Total Wavenumber From the modified dispersion relationship [39,40], the stationary total wavenumber is calculated as follows: q k − q l 2 y x Ks = , (2) uMk + vMl − ω Atmosphere 2021, 12, 849 4 of 12

where ω is the angular frequency; k and l are the zonal and meridional wavenumbers, respectively; and uM and vM are the basic zonal and meridional flow, respectively, on a Mercator projection. The case of ω = 0 is determined to describe the stationary Rossby wave. qx and qy represent the zonal and meridional gradients of the absolute vorticity, respectively, and are expressed as follows:

1  ∂2v ∂2u ∂u  q = − cos ϕ + sin ϕ (3) x a2 cos ϕ ∂λ2 ∂λ∂ϕ ∂λ

and ∂2v ∂v q = β + + tan ϕ , (4) y M ∂λ∂ϕ ∂λ 2 2 where βM = ∂ f /∂y − ∂ uM/∂y , where f is the planetary vorticity. Here, k is normally given as an integer and l should be calculated by a cubic polynomial equation written as follows:

3 2  2  h 2 i f (l) = vMl + k(uM − c)l + k vM + qx l + k (uM − c) –qy k, (5)

which is driven by the dispersion relationship. The parameter l has at least one and at most three solutions. Positive and negative values of l are associated with waves propagating northward and southward, respectively. If there is a local maximum Ks ranging from K1 to K2, a wave with a zonal wavenumber between K1 and K2 is trapped in the localized Ks region. In general, the stationary total wavenumber is plotted to identify the preferred region for the barotropic Rossby wave propagation since the wave is refracted toward the region with higher values of Ks [35].

3. Results 3.1. Basic Flow and Teleconnectivity in Different Seasons Before investigating tropical-extratropical teleconnection, the basic flow in different seasons should be examined. In Figure1, the climatological zonal wind shows strong values in westerly regions near 30◦ S and 50◦ S—that is, the locations of the subtropical and polar jets, respectively—regardless of the season. The subtropical jet is strong in JJA and SON with a maximum near 180◦ E (Figure1c,d) and disappears in DJF (Figure1a). The polar jet has a similar intensity in all seasons but the longitudinal location of its center differs depending on the season; in DJF, the polar jet has two centers around 30–60◦ E and 100◦ W (Figure1a), while the latter center does not exist in MAM/JJA/SON (Figure1b–d). Thus, the polar jet in MAM/JJA/SON becomes zonally localized. An important feature of the austral winter is the presence of a double jet which is identified as the westerly regions with values greater than 35 m/s centered near and 50◦ S, 40◦ E (Figure1c). In the tropics, there is a region where the zonal wind is near to or less than zero, which extends from the tropical West to the date line and South America in DJF/MAM (Figure1a,b). According to the classic Rossby wave theory, the barotropic Rossby wave cannot penetrate this easterly region, so the wave only crosses the equator through the westerly region over the Central and Eastern Pacific; this tropical westerly region is referred to as “westerly ducts” [41]. During JJA/SON, since the easterly region appears over the tropics (Figure1c,d), the cross-equatorial wave propagation cannot be easily explained [ 36]. However, in the modified Rossby wave theory considering an effect of the meridional wind, the background meridional flow makes it possible for the Rossby wave to pass through the easterly region [39,40]. Particularly, the dominant northerly over the eastern Indian Ocean and the Western Pacific in JJA/SON allows inter-hemispheric propagation, and these “northerly ducts” can be a waveguide for the southward-propagating Rossby wave (Figure1g,h) [27,42]. Atmosphere 2021, 12, x FOR PEER REVIEW 5 of 12 Atmosphere 2021, 12, 849 5 of 12

FigureFigure 1 1.. TheThe climatological climatological 250 250-hPa-hPa zonal zonal (left panels; intervals of 5 m s−–11) )and and meridional meridional (middle (middle panels;panels; intervals intervals of of 2 2 m m s −1)1 )winds winds and and the the climatological climatological mean mean sea sea level level pressure pressure (MSLP) (MSLP) (right (right –1 panels)panels) for for four four seasons. seasons. The The red red lines lines in in the left and middle panels denote 0 m s−1 ofof zonal zonal wind, wind, while the thick black lines in the left panels denote 25 and 35 m s−1 of zonal wind. The MSLP is while the thick black lines in the left panels denote 25 and 35 m s−1 of zonal wind. The MSLP is represented by contours at 978, 982, and 985 hPa. (a,e,i) DJF; (b,f,j) MAM; (c,g,k) JJA; (d,h,i) SON. represented by contours at 978, 982, and 985 hPa. (a,e,i) DJF; (b,f,j) MAM; (c,g,k) JJA; (d,h,l) SON.

ThThee s seasonaleasonal mean mean sea level pressure (MSLP)(MSLP) forfor thethe fourfour seasonsseasons is is shown shown in inFigure Figure1 1to to demonstrate demonstrate the the annualannual cyclecycle ofof thethe ASL,ASL, whichwhich is closely associated with the the climate climate ofof West West Antarctica Antarctica and and the the surrounding surrounding ocean. ocean. The The cyclonic cyclonic circulation circulation over over the the Bel Belling-lings- hausenshausen and and Amundsen Amundsen seas shows shows a a seasonal seasonal difference difference in in terms terms of of its its location location and and strength;strength; the the longitudinal longitudinal position position is confinedconfined toto thethe AmundsenAmundsen seaseain in MAM/JJA/SON MAM/JJA/SON ((FigureFigure 11jj–l)–l) and the the deepest deepest low low pressure pressure appears appears in in SON SON (Figure (Figure 1l).1l). By By developing developing an ASLan ASL index, index, Hosking Hosking et al. et al.[43] [43 calculated] calculated the the relative relative ASL ASL pressure pressure with with regard regard to to the the backgroundbackground pressure pressure by by subtracting subtracting the the area area-averaged-averaged MSLP MSLP over over the the Bellingshausen Bellingshausen and and AmundsenAmundsen seas seas (60 (60–70–70°◦ S,S, 170 170–290–290°◦ EE)) to to isolate isolate the the regional regional ASL ASL variability variability and and found found thatthat the the strength strength of of the the relative relative ASL ASL pressure pressure is is greatest greatest in in JJA JJA ( (notnot shown) shown).. This This seasonal seasonal ALSALS climatology is generated byby thethe interactioninteraction between between the the topography topography of of West West Antarctica Antarc- ticaand and the the westerly westerly wind wind jet [ 44jet]; [44]; therefore, therefore, it is speculatedit is speculated that thethat annual the annual zonal-wind zonal-wind cycle cycleplays plays an important an important role in role the in seasonal the seasonal modification modification of the ASL.of the If ASL. atmospheric If atmospheric disturbance dis- turbanceoccurs by occurs teleconnection, by teleconnection, the ASL becomesthe ASL becomes deeper or deeper weaker or over weaker time. over time. ToTo identify identify teleconnect teleconnectionion structures structures in in the the mid mid-to-high-to-high latitudes latitudes of of the the Southern Southern Hemisphere,Hemisphere, teleconnectivity teleconnectivity maps maps are are presented presented in in Figure Figure 2 2.. In DJF, the the regions regions ofof strongeststrongest teleconnection areare found found at at latitudes latitudes lower lower than than 60◦ 60S and° S and are alignedare aligned in the in zonal the zonaldirection direction (Figure (2Figurea). In MAM,2a). In the MAM, strongest the strongest teleconnection teleconnection occurs farther occurs south farther (Figure south2b), (andFigure in JJA/SON,2b), and in a JJA/SON, new region a new of maximum region of teleconnectionmaximum teleconnection emerges near emerges the north near of the the northAmundsen of the SeaAmundsen and the AntarcticSea and the Peninsula Antarctic (Figure Peninsula2c,d). This(Figure indicates 2c,d). thatThis teleconnection indicates that teleconnectionpatterns tend topatt occurerns moretend to in occur the higher more latitudesin the higher over latitudes the Western over Hemisphere the Western during Hem- ispherethe austral during winter the thanaustral the winter austral than summer. the austral Most summer. of the strong Most teleconnectivity of the strong teleconnec- exists near tivityjet streams. exists near In JJA jet when streams. the doubleIn JJA when jet occurs, the double the region jet occurs, with thethe strongestregion with teleconnection the strong- estis locatedteleconnection between is Australia located between and southern Australia South and America southern along South the America subtropical along jet andthe ◦ subtropicalit is located jet between and it is the located South between Atlantic the Ocean South and Atlantic the south Ocean Indian and Oceanthe south near Indian 45 S Oceanalong near the polar 45° S jet.along It is the suggested polar jet. that It is thesuggested jet acts that as a the waveguide jet acts as in a thewaveguide teleconnection in the teleconnectionprocess. A similar process. structure A similar is observed structure in is standard observed deviation in standard maps deviation of the eddymapsstream of the eddyfunction stream anomaly function (Figure anomaly3). In ( DJF,Figure the 3). region In DJF, of largestthe region standard of largest deviation standard is confined deviation to

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is confined to the lower latitudes (Figure 3a), but in JJA, it gradually migrates to the high is confined to thethe lower lower latitudes latitudes ( (FigureFigure 3a),3a), but in JJA, it gradually migrates to the high latitudes and latitudes and almost reaches West Antarctica (Figure 3b,c). This corresponds well to the latitudes and almostalmost reaches reaches West West Antarctica Antarctica (Figure (Figure 3b3b,c).,c). This This corresponds corresponds well well to to the the annual cycle ofannual the ASL, cycle which of the inASL, the which austral in winter the austral is located winter farther is located southwest farther and southwest has the and lowest has annual cycle of the ASL, which in the austral winter is located farther southwest and has depth,the lowest while depth, in the while austral in summerthe austral is locatedsummer farther is locate tod the farther northeast to the and northeast has the and highest has the lowest depth, while in the austral summer is located farther to the northeast and has depththe highest [43,45 ,depth46]. In [4 MAM,3,45,46 a] relatively. In MAM, small a relatively standard small deviation standard is observed deviation over is Australia,observed the highest depth [43,45,46]. In MAM, a relatively small standard deviation is observed andover this Australia, area of low and standard this area deviation of low standardmoves southeastward deviation moves beyond southeastward New Zealand beyond during over Australia, and this area of low standard deviation moves southeastward beyond theNew next Zealand two seasons during (Figurethe next3 c,d).two seasons The existence (Figure of 3c the,d). area The ofexistence low standard of the area deviation of low New Zealand during the next two seasons (Figure 3c,d). The existence of the area of low indirectlystandard deviation represents indirectly that waves represents detour aroundthat waves this detour area. This around behavior this area. is also This observed behav- standard deviation indirectly represents that waves detour around this area. This behav- inior the is also teleconnectivity observed in map,the teleconnectivity which shows that map, strong which teleconnection shows that strong appears teleconnection at 30–40◦ S ior is also observed in the teleconnectivity map, which shows that strong teleconnection andappears 50–60 at◦ 30S,– but40° notS and at 40–5050–60◦° SS, near but Newnot at Zealand 40–50° S (Figure near New2c); theZealand former (Figure two latitude 2c); the appears at 30–40° S and 50–60° S, but not at 40–50° S near New Zealand (Figure 2c); the rangesformer aretwo associated latitude ranges with theare associated double jets. with Thus, the it double can be jets. inferred Thus, that it can the be Rossby inferred wave that former two latitude ranges are associated with the double jets. Thus, it can be inferred that propagationthe Rossby wave along propagation the polar jet along is very the likely polar to jet affect is very the likely atmospheric to affect disturbance the atmospheric over the Rossby wave propagation along the polar jet is very likely to affect the atmospheric Westdisturbance Antarctica over during West Antar the australctica during winter. the austral winter. disturbance over West Antarctica during the austral winter.

FigureFigure 2.2. TheThe teleconnectivityteleconnectivity ofof thethe dailydaily 250-hPa250-hPa eddyeddy streamstream functionfunction anomalyanomaly forfor thethe fourfour seasons.seasons. TheThe valuesvalues areare Figure 2. The teleconnectivity of the daily 250-hPa eddy stream function anomaly for the four seasons. The values are multipliedmultiplied byby −−100100 and values smaller thanthan 4040 areare notnot expressed.expressed. ContoursContours greatergreater thanthan40 40 m m s −s−11 areare denoteddenoted withwith thethe multiplied by −100 and values smaller than 40 are not expressed. Contours greater than 40 m s−1 are denoted with the interval of 5 m s−−11. Arrows indicate locations with maximum teleconnectivity. (a) DJF; (b) MAM; (c) JJA; (d) SON. interval of 5 m s−1interval. Arrows of indicate 5 m s . locations Arrows indicate with maximum locations teleconnectivity. with maximum (a teleconnectivity.) DJF; (b) MAM;(a ()c DJF;) JJA; ( b(d)) MAM; SON. (c) JJA; (d) SON.

Figure 3. The standard deviation of the daily 250-hPa eddy stream function anomaly for the four Figure 3. TheFigure standard 3. The deviation standard of deviation the daily of 250-hPa the daily eddy 250 stream-hPa eddy function stream anomaly function for anomaly the four for seasons. the four The contour seasons. The contour interval is 2.0 × 106 m2 s−1 and the regions with values greater than 10 × 106 m2 interval is 2.0seasons.× 106 mThe2 s −contour1 and the interval regions is with2.0 × values106 m2 greaters−1 and thanthe regions 10 × 10 with6 m2 valuess−1 are greater shaded. than (a) DJF;10 × 10 (b)6 MAM;m2 (c) JJA; s−1 are shaded.(a) DJF; (b) MAM; (c) JJA; (d) SON. (d) SON. s−1 are shaded.(a) DJF; (b) MAM; (c) JJA; (d) SON. 3.2. Simulated Circulation in the LBM 3.2. Simulated Circulation3.2. Simulated in the Circulation LBM in the LBM To understand the formation process of the teleconnection developed by the tropical To understand Tothe understand formation process the formation of the teleconnection process of the teleconnectiondeveloped by the developed tropical by the tropical forcing over the IOWP, model simulations are performed. External diabatic heating is forcing over theforcing IOWP, over model the IOWP, simulations model are simulations performed. are External performed. diabatic External heating diabatic is heating is forced at varying longitudes at 20◦° intervals from 40 to 100◦° E for the Indian Ocean forcing forced at varyingforced longitudes at varying at 20 longitudes° intervals at from 20 40intervals to 100° from E for 40the to Indian 100 E Ocean for the forcing Indian Ocean forcing and from 120 to 180◦° E for the Western Pacific forcing. For the sake of convenience, the and from 120 toand 180 from° E for 120 the to Western 180 E for Pacific the Western forcing. Pacific For the forcing. sake of For convenience, the sake of the convenience, the simulation results are presented with the IOWP forcing divided into the Indian Ocean simulation resultssimulation are presented results with are presented the IOWP with forcing the IOWPdivided forcing into the divided Indian into Ocean the Indian Ocean forcing and the Western Pacific forcing, because the wave emitted from the former forcing forcing and theforcing Western and Pacific the Western forcing,Pacific because forcing, the wave because emitted the from wave the emitted former from forcing the former forcing penetrates the tropics through the northerly duct over the eastern Indian Ocean, while the penetrates the tropicspenetrates through the tropics the northerly through duct the northerlyover the eastern duct over Indian the easternOcean, while Indian the Ocean, while the wave from the latter forcing passes through the northerly duct over the Western Pacific, wave from the wavelatter fromforcing the passes latter forcingthrough passes the northerly through duct the northerlyover the Western duct over Pacific, the Western Pacific,

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whichwhich hashas been documented inin previous studiesstudies [[2727,42],42].. The simulations are re-performedre-performed withwith differentdifferent backgroundsbackgrounds for eacheach season with fixedfixed forcing toto isolateisolate thethe effecteffect ofof thethe seasonal cyclecycle in in the the background background state. state. Figure Figure4 presents 4 presents the the circulation circulation response response to diabatic to dia- forcingbatic forcing at 80◦ atE 80° and E 160and◦ E160° as representativeE as representative experiments experiments for the for Indianthe Indian Ocean Ocean and and the Westernthe Western Pacific Pacific forcing, forcing, respectively. respectively. Forthe For response the response to the to 80 the◦ E 80° forcing, E forcing, the simulation the simu- resultlation showsresult shows a seasonal a seasonal difference difference in the wavein the propagationwave propagation pattern. pattern. There isThere a weak is a waveweak trainwave for train the for SON/DJF/MAM the SON/DJF/MAM background background states states (Figure (Figure4a,b,d), 4a,b while,d), while a strong a strong wave trainwave appearstrain appears for the for JJA the background JJA background state (Figure state (Figure4c). In JJA,4c). theIn JJA, wave the starting wave starting near East reachesAfrica reaches the Ross the Sea Ross after Sea propagating after propagating southeastward southeastward and returns and returns to the to lower the lower latitudes lat- nearitudes South near AmericaSouth America forming forming an arc-like an arc route.-like route. It develops It develops an anticyclonic an anticyclonic circulation circula- centeredtion centered in the in Rossthe andSea and a cyclonic a cyclonic circulation circulation in the in the ocean ocean north north of theof the Amundsen Amund- Sea.sen Sea. In response In response to the to 160the◦ 160°E forcing, E forcing, similar similar circulation circulation anomalies anomalies are observedare observed in the in Easternthe Hemisphere in all in seasons, all seasons, although although the anomaly the anomaly is strongest is strongest in JJA in (Figure JJA (4Figuree–h). The4e–h) wave. The arises wavenear arises the near Indian the OceanIndianand Ocean moves and to moves the Ross to the Sea, Ross generating Sea, generating a cyclonic a circulationcyclonic circulation at the date at line the atdate 55◦ lineS, and at 55 subsequently° S, and subsequently propagates propagates until the Weddell until the Sea Wed- with adell weak Sea anticyclonic with a weak circulation anticyclonic there circulation in JJA. This there result in JJA. suggests This result that the suggests teleconnection that the fromteleconnection the tropical from forcing the tropical arrives inforcing West Antarcticaarrives in West in the Anta australrctica winter, in the whereas austral winter, it does notwhereas in other it does seasons. not in During other seasons. winter, theDuring teleconnection winter, the isteleconnection likely to contribute is likely to to heavy con- snowfalltribute to inheavy West snowfall Antarctica, in West where Antarctica the climatological, where the snowfallclimatological is highest snowfall from is Mayhighest to Septemberfrom May to (not September shown). An(not analogous shown). An result analogous is obtained result in previous is obtained studies in previous which revealed studies thatwhich ENSO revealed teleconnection that ENSO toteleconnection the Amundsen to the Sea Amundsen is not strong Sea in is thenot australstrong in autumn the austral and summerautumn and [19,46 summer]. [19,46].

Figure 4.4. TheThe simulated simulated zonal-eddy zonal-eddy stream stream function function anomalies anomalies at σ at= 0.229σ = 0.229 averaged averaged over 20–25over 20 days–25 ofdays simulation of simulation integration integration in response in response to the to tropical the tropical forcing forcing at 80◦ atE (left80° E panels) (left panels) and 160 and◦ E 160 (right° E panels).(right panels). The initiated The initiated forcing forcing is represented is represented as blue as dot.blue Eachdot. Each experiment experiment is initialized is initialized with with the climatologicalthe climatological background background states states of (ofa, e(a), DJF,e) DJF, (b ,(fb), MAM,f) MAM, (c ,(gc),g JJA,) JJA, and and ( d(d,h,h)) SON. SON. The The contourcontour 6 2 −1 intervalinterval isis 2.02.0 × 10106 mm 2ss−. 1.

Figure5 shows standard deviation maps of the circulation responses to the various forcings located between 40–100◦ E and 120–180◦ E. During JJA, the forcing over the Indian

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Atmosphere 2021, 12, 849 8 of 12 Figure 5 shows standard deviation maps of the circulation responses to the various forcings located between 40–100° E and 120–180° E. During JJA, the forcing over the In- dianOcean Ocean induces induces strong strong atmospheric atmospheric variation variation over over the Rossthe Ross and and Bellingshausen Bellingshausen seas, seas, and andthe the Western Western Pacific Pacific forcing forcing leads leads to variation to variation over over the Rossthe Ross and and Weddell Weddell seas seas (Figure (Figure5c,g). 5cAlthough,g). Although in the in figure, the figure, a subtropical a subtropical response response seems seems to initiate to initiate in the westin th ore west east sidesor east of sidesthe Indian of the OceanIndian in Ocean response in response to the Indian to the Ocean Indian forcing Ocean or forcing the Western or the Pacific Western forcing, Pacific the forcing,individual the subtropicalindividual subtropical response to response each forcing to each arises forcing near the arises location near ofthe the location forcing. of Since the forcing.the divergent Since the wind divergent anomaly wind emitted anomaly from emitted the forcing from produces the forcing the produces RWS, the the longitude RWS, theof thelongitude RWS is of nearly the RWS in the is nearly vicinity in ofthe the vicinity tropical of forcingthe tropical even forcing though even the RWSthough can the be RWSamplified can be by amplified the background by the background flow [29,31 ].flow Therefore, [29,31]. theTherefore, effect of the the effect season of the tends season to be tendsmore to critical be more for critical wave propagation for wave propagation than for RWS tha generation.n for RWS generation.

FigureFigure 5. 5. StandardStandard deviation deviation maps maps of the of thesimulation simulation results. results. The s Theimulated simulated zonal- zonal-eddyeddy stream stream func- tionfunction anomalies anomalies are the are same the same as in as Figure in Figure 4 except4 except for for the the location location of of the the tropical tropical forcing forcing.. The The left left panelspanels are (a– ford) arethe forsimulations the simulations forced forcedat various at various longitudes longitudes ranging ranging from 40 from to 100 40° to E 100and◦ Ethe and right the panelsright panelsare for ( ethe–h )simulations are for the simulationsforced at var forcedious longitudes at various longitudesranging from ranging 120 to from 180° 120E at to 20° 180 inter-◦ E at 6 2 −1 vals.20◦ intervals.The contour The interval contour is interval2.0 × 10 is m 2.0 s×. 106 m2 s−1.

3.3.3.3. Stationary Stationary Total Total Wavenumber Wavenumber ToTo clarify clarify the the wave wave propagation, propagation, the the stationary stationary total total wavenumber wavenumber (Ks (Ks) )is is plotted plotted in in FigureFigure 66 forfor the the negative meridional wavenumber (l < 0) 0) that that is is associated associated with with the the wave wave headingheading toward toward the the south. south. By By examining examining KsKs, ,we we can can diagnose diagnose where where the the wave wave propagates propagates becaubecausese the the wave wave refracts refracts toward toward the the region region with with higher higher KsKs valuesvalues than than the the wavenumber wavenumber ofof the the wave. wave. The The cubic cubic polynomial polynomial equation equation solution solution of of Eq Equationuation (5 (5)) for for l lisis solved solved by by priority,priority, so so the the Ks Ks value value is is calculated calculated where where there there is isa solution a solution for for l; wavel; wave propagation propagation is nisot nottheoretically theoretically achieved achieved except except for where for where the solution the solution exists. exists. In the Infigure, the figure,the Ks thevalue Ks existsvalue in exists the climatological in the climatological northerly northerly region regionin the subtropics in the subtropics (see Figure (see Figure1a–h).1 Ta–h).he wave The canwave propagate can propagate into the into Southern the Southern Hemisphere Hemisphere through through the northerly the northerly near thenear IOWP the during IOWP JJAduring and JJAnear and the near western the western Pacific Pacificduringduring SON, while SON, whilethe wave the wavecan hardly can hardly penetrate penetrate the entirethe entire tropics tropics except except for the for Eastern the Eastern Pacific Pacific during during DJF/MAM DJF/MAM due to duethe existence to the existence of the southerlyof the southerly in this region in this. regionThis is (Figurethe reason6a,b). why This the is simulation the reason result whythe shows simulation a weak resultwave shows a weak wave train response when the climatological DJF/MAM background state is initialized (Figures4 and5).

Atmosphere 2021, 12, x FOR PEER REVIEW 9 of 12

train response when the climatological DJF/MAM background state is initialized (Figures 4 and 5). Meanwhile, it is suggested that the wave propagation in the mid-latitudes is more affected by the climatological zonal wind because the localized stationary total wave- number is placed in the jet stream. During JJA and SON, the subtropical jet wave guide is placed along the strong westerly region near 25° S where the localized Ks appears (Figure 6c,d). The wave guide traps waves with shorter wavelengths in the jet and makes them propagate eastward to the exit of the jet. The trapped waves are well represented in the response to the Indian Ocean forcing, and have less impact on West Antarctica (Figure 4c,d). During JJA, to the south of the subtropical jet around 40° S, there is a region where the stationary total wavenumber is smaller than 2; this region is referred to as the reflect- ing latitude (Figure 6c). At this latitude, the meridional direction of wave propagation changes. For example, when the wave propagating southward reaches the reflecting lati- tude, the wave is reflected and moves towards the equator. In the same manner, if the wave propagates from high latitudes and is close to the reflecting latitude, the wave turns poleward; this forces wave trapping in the high latitudes [26,36]. In general, waves with longer wavelength propagate farther poleward than those with shorter wavelengths [34]. Thus, waves with longer wavelengths, which reach the high latitudes as they move south- ward from the Indian Ocean, are blocked by the reflecting latitude and cannot return to Atmosphere 2021, 12, 849 the tropics. Consequentially, the waves zonally propagate and arrive in West Antarctica9 of 12 (Figure 4c,g).

FigureFigure 6. The(a) DJF; 250- (hPab) MAM; stationary (c) JJA; total (d )wavenumber SON. The 250-hPa (Ks) (units stationary of a−1, total where wavenumber a is the radius (Ks) of (units the −1 )of a for, where a zonal a iswavenumber the radius of of the 3 and earth) for negative for a zonal meridional wavenumber wavenumbers. of 3 and forWhite negative areas merid-repre- sentional areas wavenumbers. where there White is no solution areas represent for l. Red areas contours where represent there is nozero solution climatological for l. Red zonal contours wind andrepresent black contours zero climatological denote climatological zonal wind zonal and blackwind contoursof 25 and denote35 m s− climatological1. zonal wind of 25 and 35 m s−1. 4. Summary and Discussion ThisMeanwhile, study investigates it is suggested how that the theteleconnection wave propagation between in the the IOWP mid-latitudes and West is moreAntarc- af- ticafected differs by the seasonally climatological and what zonal mechanisms wind because affect the this localized difference. stationary We used total the wavenumber modified Rossbyis placed wave in the theory jet stream. in which During the effect JJA andof the SON, background the subtropical meridional jet wave wind guide is considered is placed along the strong westerly region near 25◦ S where the localized Ks appears (Figure6c,d). to examine the practical wave propagation in the teleconnection process. The simulation The wave guide traps waves with shorter wavelengths in the jet and makes them propagate results showed that, during JJA/SON, the wave response to the tropical IOWP forcing eastward to the exit of the jet. The trapped waves are well represented in the response to the Indian Ocean forcing, and have less impact on West Antarctica (Figure4c,d). During JJA, to the south of the subtropical jet around 40◦ S, there is a region where the stationary total wavenumber is smaller than 2; this region is referred to as the reflecting latitude (Figure6c). At this latitude, the meridional direction of wave propagation changes. For example, when the wave propagating southward reaches the reflecting latitude, the wave is reflected and moves towards the equator. In the same manner, if the wave propagates from high latitudes and is close to the reflecting latitude, the wave turns poleward; this forces wave trapping in the high latitudes [26,36]. In general, waves with longer wavelength propagate farther poleward than those with shorter wavelengths [34]. Thus, waves with longer wavelengths, which reach the high latitudes as they move southward from the Indian Ocean, are blocked by the reflecting latitude and cannot return to the tropics. Consequentially, the waves zonally propagate and arrive in West Antarctica (Figure4c,g).

4. Summary and Discussion This study investigates how the teleconnection between the IOWP and West Antarctica differs seasonally and what mechanisms affect this difference. We used the modified Rossby wave theory in which the effect of the background meridional wind is considered to examine the practical wave propagation in the teleconnection process. The simulation results showed that, during JJA/SON, the wave response to the tropical IOWP forcing propagates into the Southern Hemisphere through the climatological northerly wind and moves southeastward along the jet waveguide. Waves with long wavelengths reach an area near to West Antarctica by trapping in the high latitudes due to the reflecting latitude. Atmosphere 2021, 12, 849 10 of 12

Meanwhile, during DJF/MAM, since the climatological southerly wind is located in the IOWP region, the wave is more likely to propagate toward the and not reach West Antarctica even if it moves southward. This result demonstrates the importance of the background flow to the teleconnection between the IOWP and West Antarctica and the significant effect of the IOWP on West Antarctica during the austral winter and spring. The RWS is important in the teleconnection process since it provides information about where the wave practically starts. It is known that the location and the amplitude of the RWS depend on the season. Generally, the RWS is induced near the subtropical jet where the climatological absolute vorticity gradient is strong, and it acts as a medium of energy conversion from divergent flow to rotational flow, indicating that it is indeed a remote source of the barotropic Rossby wave [32,33]. Here, The RWS generated by the 80◦ E or 160◦ E forcing is mainly located near the subtropical jet close to the forcing, regardless of the season, and the amplitude is stronger in JJA and SON (not shown). It may induce the enhanced Rossby wave response in those seasons, but it does not mean that the stronger RWS helps the wave reach West Antarctica. Rather, the effect of seasonal modulation of the wave propagation by the basic state is more important to the teleconnection toward the high latitudes. In an additional simulation in which the forcing near the Eastern Pacific is initiated for JJA, the cyclonic circulation over the Bellingshausen and Amundsen seas is roughly twice as strong as that of the IOWP forcing (not shown). It indicates that the strength of the response of the IOWP forcing is less than that of the Eastern Pacific forcing. Nevertheless, our result suggests that the tropical IOWP forcing during the austral winter and spring should be more investigated in order to better understand the ice change in West Antarctica. Contrary to the ENSO, which shows a peak in the austral summer, the Indian Ocean Basin (IOB) mode following an El Nino event has a peak in the early austral autumn and persists through winter, and the IOD known as an intrinsic mode peaks in the austral winter. The teleconnection generated by these IOWP-related variabilities modulates the ASL, which can influence the ice mass balance and total melting rate during winter, when snowfall peaks in West Antarctica. Recently, accelerated SST warming over the Indo-Pacific warm pool has been observed with the steepest trend slope than other , so the global influence of the warming trend has been revealed as external convective forcing strengthens under global warming [47–50]. Thus, the impact of the IOWP forcing on the climate of West Antarctica is expected to grow in the future. In this study, we analyze wave propagation operating in the climatological state. However, if there is a change in the background state, wave propagation can be expected to exhibit different behavior. The latest Coupled Model Inter-comparison Project phase 6 (CMIP6) model simulations reported poleward movement of the subtropical jet in re- sponse to CO2 forcing (although its poleward shift is smaller than in the Coupled Model Inter-comparison Project phase 5 (CMIP5)), and the poleward jet shift is unanimously accepted [51,52]. Not only the tropical SST trend, but also the long-term climate variabil- ity, modifies the strength and location of the jet [15,53]. The changes in the jet and the strengthening of the forcing over the IOWP region have potential implications for the teleconnection between the IOWP and West Antarctica; however, the impact and detailed processes need to be further investigated.

Author Contributions: Conceptualization and formal analysis, H.-J.L. and E.-K.J.; data curation, investigation, and methodology, H.-J.L.; writing—original draft preparation, H.-J.L.; writing—review and editing, E.-K.J. Both authors have read and agreed to the published version of the manuscript. Funding: This work was sponsored by a research grant from the Korean Ministry of Oceans and Fisheries (KIMST20190361; PM21020). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Atmosphere 2021, 12, 849 11 of 12

Data Availability Statement: All data are available to readers upon request. Acknowledgments: The authors are grateful to the editor and two anonymous reviewers for their helpful comments on the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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