Evolving Perspectives on Abrupt Seasonal Changes of the...

ADVANCES IN ATMOSPHERIC SCIENCES, VOL. 34, OCTOBER 2017, 1185–1194

• Review •

Evolving Perspectives on Abrupt Seasonal Changes of the General Circulation

Jianhua LU∗1 and Tapio SCHNEIDER2

1School of Atmospheric Science, Sun Yat-sen University, Zhuhai, Guangdong 519082, China2Department of Environmental Science and Engineering, California Institute of Technology,

Pasadena, California, CA91125, USA

(Received 29 March 2017; revised 21 June 2017; accepted 28 June 2017)

ABSTRACT

Professor Duzheng YE (Tu-cheng YEH) was decades ahead of his time in proposing a model experiment to investigatewhether abrupt seasonal changes of the general circulation can arise through circulation feedbacks alone, unrelated to un-derlying inhomogeneities at the lower boundary. Here, we introduce Professor YEH’s ideas during the 1950s and 1960s onthe general circulation and summarize the results and suggestions of Yeh et al. (1959) on abrupt seasonal changes. We thenreview recent advances in understanding abrupt seasonal changes arising from model experiments like those proposed byYeh et al. (1959). The model experiments show that circulation feedbacks can indeed give rise to abrupt seasonal transitions.In these transitions, large-scale eddies that originate in midlatitudes and interact with the zonal mean flow and meridionaloverturning circulations in the tropics play central roles.

Key words: abrupt change, general circulation, Hadley cell, large-scale eddies

Citation: Lu, J. H., and T. Schneider, 2017: Evolving perspectives on abrupt seasonal changes of the general circulation.Adv. Atmos. Sci., 34(10), 1185–1194, doi: 10.1007/s00376-017-7068-4.

1. Historical backgroundRossby (1951) wrote in a short note published in Tellus

on cooperative research projects that “there is [. . . ] everyreason to expect, during the next few years, an extremelyvigorous development of Chinese meteorology and, as a re-sult, many significant realistic contributions from that partof world”. In addition to the establishment of the People’sRepublic of China, one reason Rossby must have had inmind when he wrote this was the return to China in 1950of his three proteges: Tu-cheng YEH (Duzheng YE) and Yi-ping SHIEH (Yibing XIE) from the University of Chicago,and Chen-Chao KOO (Zhengchao GU) from the Universityof Stockholm. Both T.-C. YEH and C.-C. KOO went tothe Institute of Geophysics and Meteorology of the ChineseAcademy of Sciences. There, they were not only responsi-ble for leading the meteorological research program, but theywere also key figures in the establishment of a modern opera-tional weather forecast system. Later in the 1950s, Rossbywas able to see for himself the contributions his Chineseproteges had made, such as the first thorough analysis of

∗ Corresponding author: Jianhua LUEmail: [email protected]

the general circulation over eastern Asia, which was invitedto be published in Tellus (Staff Members of the Section ofSynoptic and Dynamic Meteorology, Institute of Geophysicsand Meteorology, Academia Sinica, Peking

a, 1957, 1958;

StaffMembers of the Section of Synoptic and Dynamic Mete-orology, Institute of Geophysics and Meteorology, AcademiaSinica, 1958). However, the scope of meteorological researchin China was not limited to topics directly related to Chinaor East Asia, but extended to fundamental problems of thefield. Blumen and Washington (1973) surveyed the researchactivities and developments in atmospheric dynamics and nu-merical weather predictions in China during 1949–66 andfound “on the basis of accumulated evidence” that “the fieldof meteorology had become a well established and contin-ually growing scientific activity.” Blumen and Washington(1973) focused on cumulus and turbulent boundary layer dy-namics and the dynamics of meso-, synoptic-, and planetary-scale motions, but they acknowledged that “a more exhaus-tive overview of the contributions made by Chinese meteo-rologists to the theory of the general circulation appears war-ranted.”

Indeed, a unique contribution by Chinese meteorologiststo the theory of the general circulation had been the mono-graph by Professor YEH and his colleague Pao-chen CHU,

a The 1957 paper lists the Staff Members of the Section of Synoptic and Dynamic Meteorology of the Chinese Academy of Sciences who contributedto the study: Cheng-yu CHANG, Jih-ping CHAO, Lung-shun CHEN, Pao-chen CHU, Shih-yen DAO, Pei-lan KUEI, Chang JU, Yu-hsi KAO, Chen-chaoKOO, Kwang-nan LIU, Szu-wei LO, Jun-fang PAN, Hung-shun WU, Chien-chu YANG, and Tu-cheng YEH. Dao, Koo, and Yeh are identified as maincontributors.

© Institute of Atmospheric Physics/Chinese Academy of Sciences, and Science Press and Springer-Verlag Berlin Heidelberg 2017

1186 ABRUPT SEASONAL CHANGES OF THE GENERAL CIRCULATION VOLUME 34

entitled “Some Fundamental Problems of the General Circu-lation of the Atmosphere” (Yeh and Chu, 1958). The chap-ter headings (Fig. 1) show the breadth of the contents and“give an idea of the broad sweep and ground-breaking natureof his ideas at this time” (Hoskins, 2014). The monograph

was not only a survey of Chinese contributions, largely ledby T.-C. YEH and C.-C. KOO, to the theory of the generalcirculation—it was also a survey of the contemporary theo-retical advances in the world, organized by the two authors’vision of the nature of the general circulation. Many insights

Fig. 1. Chapter headings of “Some fundamental problems of the general circulation of the atmosphere” (Yeh and Chu, 1958).

OCTOBER 2017 LU AND SCHNEIDER 1187

Fig. 1. (continued)

in Yeh and Chu (1958), such as the mechanisms responsiblefor the formation and maintenance of the mean meridionalcirculation and the westerly jet streams, the joint effects of to-pography and thermal forcing on the location of mean ridgesand troughs (stationary waves), laid foundations for later ad-vances in the theory of the general circulation (e.g., Held,1983; Held et al., 2002; Kaspi and Schneider, 2011; Birneret al., 2013). Although the monograph was published onlytwo years after the first successful numerical simulation of thegeneral circulation by Phillips (1956), Yeh and Chu (1958) at-tempted as much as possible at the time to give an internallyconsistent picture of the general circulation. They assigneda central role in the maintenance of jets and the mean circu-lation to large-scale eddies and their transport of energy, an-gular momentum, and mass—insights that had grown out ofProfessor YEH’s PhD thesis on energy dispersion in Rossbywaves (Yeh, 1949).

A thorough summary of the Yeh and Chu (1958) mono-graph is beyond the scope of this paper; readers are referredto Lu (2016) for more details. Lu (2016) particularly em-

phasized the scientific style of Yeh and Chu (1958), whostrove for conciseness in mathematics, lucidity in physics,and overall internal consistency in their quest to identify gen-eral laws from regional and special phenomena. Here, it suf-fices to mention that Yeh and Chu (1958) stressed, citingthe then-unpublished results by Yeh, Dao, and Li, that theobserved abrupt seasonal change of the general circulationover Asia may be an important and possibly broader phe-nomenon in need of a general explanation. The present paperfocuses on this abrupt change of the general circulation, re-viewing Professor Tu-cheng YEH’s contributions along withrecent advances. Professor YEH and his colleagues weredecades ahead of their time in contemplating the universal-ity of abrupt seasonal changes and in proposing model ex-periments to elucidate their mechanisms. Such model ex-periments have recently uncovered mechanisms involved inabrupt seasonal changes—mechanisms that are independentof inhomogeneities at the lower boundary, such as those aris-ing from land–sea contrast or topography but that are region-ally modulated by such inhomogeneities.


We first review the main results of Yeh et al. (1959), re-ferred to as Yeh-Dao-Li (1959) hereafter, in section 2. In sec-tion 3, we review results from statistically zonally symmet-ric numerical simulations that exhibit rapid seasonal transi-tions (Bordoni and Schneider, 2008; Schneider and Bordoni,2008). Section 4 discusses the implications and open ques-tions, followed by conclusions in section 5.

2. Yeh-Dao-Li (1959) on abrupt changes of thegeneral circulation during June and Octo-ber

In a footnote of the Tellus paper led by T.-C. YEH, S.-Y. DAO, and C.-C. KOO on the general circulation overEast Asia, the authors mentioned that “according to our re-cent study (to be published) the sudden northward shift ofwesterlies in June is not limited to Asia but it is a world-wide phenomenon, being earliest over Asia and latest overN. America. In the period from middle September to mid-dle October, there is a world-wide sudden onset of westerliessouthward” (Staff Members of the Section of Synoptic andDynamic Meteorology, Institute of Geophysics and Meteo-rology, Academia Sinica, Peking, 1957). Subsequently, Yehand Chu (1958, chapter 1) documented abrupt changes of thegeneral circulation during June and October over the MiddleEast, Tibetan Plateau, the coast of East Asia, central Pacific,and North America, followed by a publication by Yeh, Dao,and Li in Acta Meteorologica Sinica in Chinese. The au-thors also prepared an English version of the paper for a vol-ume celebrating Rossby’s 60th birthday, which was eventu-ally published in a volume commemorating Rossby becausehe suddenly died on 19 August 1957. The paper is entitled“The abrupt change of circulation over northern hemisphereduring June and October” and is authored by T.-C. YEH, S.-Y. DAO, and M.-T. LI; we refer to it as Yeh-Dao-Li (1959).We base our introduction to Professor YEH’s contribution tothe description of abrupt changes of the general circulationon this version of their publication.

Although abrupt changes of the circulation had been ob-served over the South Asian monsoon region and the MiddleEast, it was Yeh-Dao-Li (1959) who first stated clearly, withthe limited observational data available then, that the seasonalvariation of zonal winds and associated circulation patterns isabrupt not only over the monsoon region, but that it may bea broader phenomenon at least in the Northern Hemisphere,and possibly also over the Southern Hemisphere. The mainobservations were summarized in the abstract of Yeh-Dao-Li(1959):

“In this paper we have shown that there is an abruptchange of the upper-air circulation over the Northern Hemi-sphere in June and October. In June this change is character-ized by a sudden northward shift of the westerlies and east-erlies. Associated with this[,] a marked change in the upperflow pattern takes place[,] followed by the establishment ofthe typical summer circulation. In October the abrupt changeis characterized by a sudden southward shift of the westerlies

and easterlies. This is also accompanied by a marked changein the upper flow pattern, after which the typical upper wintercirculation is established.

The onset of the summer circulation is associated withthe outbreak of the SW monsoon in India and Mai-yu inChina and Japan and a rapid northward displacement of theintertropical convergence zone (ICZ). The onset of the win-ter circulation is followed by the retreat of the SW monsoonand the ICZ. The synoptic sequence of these development isdescribed.”

As an example, Fig. 2, adapted from Yeh-Dao-Li (1959,Figs. 2 and 8), depicts the abrupt northward shift of the west-erlies and easterlies at the end of May and the southward shiftof the circulation during October in the South Asian monsoonregion. Note in particular the northward shift of upper-leveleasterlies at the monsoon onset (end of May) and their down-ward extension to the surface on the southern flank of theTibetan Plateau, as well as the transition from easterlies towesterlies in the lower troposphere.

By considering abrupt seasonal changes of the circulationas a broad phenomenon, Yeh-Dao-Li (1959) went beyond theregional dynamics of monsoons, unlike previous work goingback to Halley (1686), by not focusing exclusively on the un-derlying regional inhomogeneities at the lower boundary asthe main factors responsible for the abrupt seasonal changes.They stated explicitly:

“Concerning the cause of the abrupt changes, we shallonly propose the following reasons as a conjecture: Fromwinter to summer the inclination of the sun over the North-ern Hemisphere gradually increases. With this increase thetemperature contrast between the equator and pole graduallydecreases. When it has decreased to a certain value, a certaintype of ‘instability’ in the atmosphere appears and the abruptchange of the upper-air circulation takes place. From sum-mer to winter the ‘reversed’ sequence of events would occur.Since the temperature and velocity fields are not independentof each other, a sudden change of the temperature would alsofollow such a change of the circulation.”

“To answer conclusively the above conjecture,” Yeh-Dao-Li proposed “the following model experiment: In a rotatinghalf sphere or two coaxial cylinders [. . . ] we heat differentlythe inner and outer part, then gradually decrease or increasethe heating difference and observe whether we get the abrupttransition of the circulation as observed in the atmosphere.”

Their conjecture about the mechanisms of the abrupt tran-sitions and the proposed experiment did not assign centralroles to topography and/or land–sea contrasts, but rather toa conjectured internal instability of the general circulation,which can be triggered by gradual variations of the differen-tial heating between the equator and poles. Consistent withthe view of Yeh and Chu (1958), who put large-scale eddiesin a central position in the maintenance of the general circu-lation, it is likely that Yeh-Dao-Li also thought eddies wouldplay a role in the conjectured instability of the general circu-lation.

It is remarkable that they took such a broad view of mon-soon transitions as an instability, given they were working at


Fig. 2. Pentad-mean cross-sections of the zonal wind (m s−1) along 90◦E (a) from May to June and (b) during October,1956. E and W indict easterlies and westerlies, respectively. [Adapted from Figs. 2 and 8 of Yeh et al. (1959)].

a time when data were necessarily regional and systematicmodel experiments were in their infancy. Regime transitionsof the general circulation were found around the time of Yeh-Dao-Li (1959) in the now classical laboratory experiments inrotating annuli by D. Fultz, R. Hide, and R. Pfeffer (see thereview by Read et al., 2015). For instance, transitions be-tween different circulation regimes, such as regular waves orirregular geostrophic turbulence, occur as varying parametersthat relate to the thermal contrast between the inner and outerannulus or the rotation rate. However, the regime transitionsobserved in these laboratory experiments were different fromthe abrupt seasonal transitions of the general circulation seenin the atmosphere.

The advent of numerical general circulation models(GCMs) put the experiments proposed by Yeh-Dao-Li (1959)within easy reach. Indeed, numerical simulations success-

fully reproduce abrupt seasonal transitions of the general cir-culation (e.g., Zeng et al., 1988; Prell and Kutzbach, 1992;Wu et al., 1997; Chao and Chen, 2001). However, the setup ofthese simulations included lower-boundary inhomogeneitiessuch as land–sea contrasts and topography, which preventedthe modeling studies from identifying instabilities or othermechanisms that contribute to abrupt transitions but may beindependent of lower-boundary inhomogeneities. Thus, thesemodeling studies were still not the numerical version of themodel experiment proposed by Yeh-Dao-Li (1959) to testtheir conjecture about the mechanism of the observed abruptseasonal transitions.

Only half a century after the publication of Yeh-Dao-Li (1959), GCM experiments corresponding to the labora-tory experiment proposed by Yeh-Dao-Li (1959) were con-ducted (Bordoni and Schneider, 2008; Schneider and Bor-


doni, 2008), without the authors of these studies being awareof Yeh-Dao-Li’s proposal decades earlier.

3. Abrupt transition of the general circulationin a zonally symmetric setting

In the model experiment proposed by Yeh-Dao-Li (1959),the boundary conditions were to be axisymmetric and onlythe thermal contrast between the equator and the poles was tobe varied gradually. Building on steady-state simulations byWalker and Schneider (2006), Schneider and Bordoni carriedout just such numerical experiments with idealized GCMs,first in a dry GCM without a hydrologic cycle (Schneider andBordoni, 2008), and then in a moist GCM with a hydrologicalcycle (Bordoni and Schneider, 2008). In the simulations witha hydrological cycle, the lower boundary is a homogeneousslab ocean whose thickness is varied from very thin (a swampplanet with low thermal inertia) to very thick (a thick oceanplanet with high thermal inertia).

Abrupt seasonal transitions of the general circulationfrom an equinox to a solstice regime indeed occur in the sim-ulations when the lower boundary has sufficiently low ther-mal inertia (Fig. 3a). They occur irrespective of the presenceof a hydrological cycle and latent heat release (Schneider andBordoni, 2008). That is, the onset or retreat of intense off-equatorial precipitation during the circulation transitions inthe simulations is a byproduct of the circulation changes, nota driver of it. The transitions resemble the circulation tran-sitions observed in Earth’s atmosphere in the South Asianmonsoon region at the onset (May/June) and retreat (Septem-ber/October) of the summer monsoon, with abrupt changesin both precipitation (Fig. 3b) and zonal winds (Fig. 4). Inparticular, the simulated circulation changes at the transitionsbetween the equinox and solstice regimes exhibit the samerearrangement of the zonal wind structure as that describedby Yeh-Dao-Li (cf. Figs. 2 and Figs. 5c, d).

The key to understanding the dynamics of the circula-tion changes is a regime transition of the tropical merid-ional overturning circulation. During the equinox regime,

Fig. 3. (a) Zonal- and pentad-mean precipitation (color contours; units: mm d−1; the intervalis 2 mm d−1 with maximum identified by cross) and sea-level air temperature (grey contours;the contour interval is 2 ◦C, with the solid line being 24 ◦C isoline) in aquaplanet simulationwith a thin slab ocean [Reprinted from (Bordoni and Schneider, 2008)]. (b) Precipitation (colorscale) and surface winds (vectors; The longest vector at 18◦S in September indicts a wind speedof 9.1 m s−1 and vector components to the left and right indict westward and eastward windcomponents, respectively) averaged zonally over the South Asian monsoon sector (65◦–95◦E),in which the ITCZ (precipitation maxima) is marked by red lines[Reprinted from (Schneider etal., 2014)].


0 90 180 270 360

20

10

0

-10

Fig. 4. Seasonal cycle of the zonal- and pentad-mean near-surface zonal wind at 15◦N from observations in the Asian mon-soon sector (green) and from aquaplanet simulations with a thinslab ocean (yellow) and with a thick slab ocean (red) [Reprintedfrom (Bordoni and Schneider, 2008)].

the Hadley cells are close to symmetric about the equator,and the strength of the poleward flow in the upper branchesis primarily controlled by the eddy angular momentum fluxdivergence in the upper troposphere (Fig. 5a). During thesolstice regime, there is a strong cross-equatorial Hadley cellwith an ascending branch in the summer hemisphere, whilea second, much weaker Hadley cell is confined to the win-ter hemisphere (Fig. 5b). The strong cross-equatorial Hadleycell is much closer to the angular momentum–conservingregime described by Schneider (1977), Held and Hou (1980),Lindzen and Hou (1988), and Plumb and Hou (1992). In thisregime, the poleward flow in the upper branches is more di-rectly thermally controlled (by differential diabatic heatingand eddy energy flux divergences) than mechanically con-trolled (by eddy angular momentum flux divergence).

The abrupt transition of the circulation in the simulationsis one between these two regimes, as can be ascertained bycomputing a local Rossby number: Ro = −ζ/ f , where ζ isthe mean relative vorticity and f is the planetary vorticity inthe upper branch of the overturning circulation. A larger Ro(close to 1) means the upper branch is closer to the angu-lar momentum–conserving limit, and the strength of the cir-culation responds directly to the thermal forcing; a smallerRo (close to 0) indicates an eddy-mediated regime in whichthe circulation strength is influenced by the eddy angular mo-mentum fluxes and does not directly respond to the thermalforcing (Schneider, 2006; Walker and Schneider, 2006). Bor-doni and Schneider (2008) found that Ro changes from lessthan 0.4 to larger than 0.7 during the transition between theequinox and solstice regimes in their aquaplanet simulationwith a thin slab ocean. A similar transition in Ro occurs forthe zonal-mean Hadley circulation (Walker and Schneider,2006) and for the meridional overturning circulation in theSouth Asian monsoon sector in Earth’s atmosphere (Bordoniand Schneider, 2008).

These simulation results and observations suggest thatthe monsoonal transitions in Earth’s atmosphere may indeedarise through some kind of “instability in the atmosphere”,as Yeh-Dao-Li (1959) surmised. The question is, what is themechanism responsible for this “instability”? Several mecha-

nisms appear to play a role. One is that a Hadley cell can am-plify rapidly once it is in the angular momentum–conservingregime. The rapid amplification in this regime is made pos-sible by the nonlinearity of the angular momentum balance,provided the thermal inertia of the atmosphere and surface issufficiently low to allow rapid adjustments (Lindzen and Hou,1988, Plumb and Hou, 1992). Another involves an eddy-mean flow feedback: as the ascending branch of a Hadleycell moves into the summer hemisphere, it moves along withthe lower-level temperature (or, more precisely, moist staticenergy) maximum (Prive and Plumb, 2007a, b). Thermal orgradient wind balance implies that upper-level easterlies de-velop within the cross-equatorial Hadley cell, above the re-gion where temperatures increase going meridionally towardsthe lower-level temperature maximum under the ascendingbranch. This region of upper-level easterlies extends fromthe ascending branch deep into the winter hemisphere (Figs.5c and d)—similar to the zonal wind changes Yeh-Dao-Li de-scribed (Fig. 2). The upper-level easterlies shield the interiorof the cross-equatorial Hadley cell from extratropical eddieswith westerly phase speeds, which cannot propagate on theeasterlies. This allows the cross-equatorial Hadley cell to becloser to the angular momentum–conserving regime, whereit can amplify rapidly with strengthening differential heating(Schneider and Bordoni, 2008).

It is clear that these mechanisms act in the simulations aswell as in observations. It is also clear that, in the simula-tions, they suffice to trigger rapid circulation transitions be-tween the equinox and solstice regimes, with attendant rapidchanges in zonal winds and precipitation that have tradition-ally been taken as defining monsoon transitions. A low ther-mal inertia of the surface and atmosphere is necessary for thecirculation changes to occur rapidly; otherwise, the adjust-ments in temperature and zonal winds would be thermallydamped. If these are indeed the essential mechanisms formonsoon transitions, they suggest the role of the subtropicalland surfaces in the South Asian monsoon is primarily to of-fer a surface of low thermal inertia. Land–sea contrasts arenot necessary for such monsoon transitions. The upshot isthat the heating contrast between the Indian Ocean and thesubtropical continental mass to the north is certainly help-ful for triggering monsoon transitions, but it is not essen-tial for monsoon transitions. Monsoon transitions can arisemore universally through internal rearrangements of the cir-culation, confirming the conjecture in Yeh-Dao-Li’s proposal.Nonetheless, it is without question that land–sea contrast andtopographic features such as the Tibetan Plateau, which fea-tured prominently in Professor Yeh’s work, also play impor-tant roles (e.g., Boos and Kuang, 2010, Wu et al., 2012).

4. Discussion and open questionsSimulation studies such as those we discuss in this paper

have over the past decade substantially broadened our viewof what controls monsoons. The description of their dynam-ics has evolved from being rooted in regional peculiarities


Fig. 5. Zonal- and temporal-mean circulation at two 20-day periods (a, c) before (Julian Day 101–120) and (b, d) after(Julian Day 145–165) monsoon onset in the aquaplanet simulation of Bordoni and Schneider (2008) with a thin slabocean: (a, b) streamfunction of meridional overturning circulation (black contours; contour interval: 50× 109 kg s−1;solid contours for anticlockwise rotation and dashed contours for clockwise rotation), and transient eddy angular mo-mentum flux divergence [color contours; contour interval: 0.6×10−5 m s−2 in (a) and 1.2×10−5 m s−2 in (b), with redtones for positive and blue tones for negative values]; (c, d) zonal wind (black contours; contour interval: 6 m s−1) andthe same eddy angular momentum flux divergence (color contours) as in (a, b) [Reprinted from (Bordoni and Schneider,2008)].

to being understood from broader and more universal prin-ciples. Several new mechanisms controlling their dynamicshave been identified, and new ones continue to be added, suchas stationary-eddy effects, especially in the East Asian mon-soon (e.g., Park et al., 2012; Shaw, 2014). What remainsmissing, more than half a century after Yeh-Dao-Li suggestedto put the study of seasonal transitions on a systematic foot-ing, is a comprehensive theoretical framework that describesquantitatively how the different mechanisms interact.

Such a framework would have to account for the effectsof transient eddy energy and angular momentum fluxes, theirinteraction with the mean meridional circulation and zonalwinds, and their modulation through stationary eddies. Itwould have to be able to provide answers to questions suchas: (1) How does the strength and structure of a Hadley celldepend on external factors such as the solar declination an-gle? (2) What controls when and how rapidly the circu-lation undergoes transitions between the equinox and sol-stice regimes? (3) How and to what extent do surface in-homogeneities such as land–sea contrasts modulate the in-ternal rearrangement of the atmospheric circulation duringthe seasonal transitions? A comprehensive theoretical frame-work should also provide an answer to the question origi-nally posed by Yeh-Dao-Li: Can the seasonal transitions beunderstood as an instability of the circulation as forcing pa-rameters move through a critical region? To quantitativelyanswer these questions, at a minimum we need a closed the-ory of the Hadley circulation that couples the dynamics of

tropical overturning circulation to extratropical wave dynam-ics (Schneider et al., 2010).

5. Conclusions

Professor YEH and his colleagues were decades aheadof their time in hypothesizing that the abrupt seasonal tran-sitions of the general circulation may be a circulation phe-nomenon that can occur independently of regional surfaceinhomogeneities, and that this can be probed in a model ex-periment that abstracts from surface inhomogeneities. Yehand Chu (1958) had earlier expressed the view that the meanmeridional circulation is largely controlled by eddies, so wemay surmise that they would have been pleased with themore recent discovery of how important a role eddies ap-pear to play in seasonal transitions of the general circula-tion, through their transports of angular momentum and en-ergy (e.g., Walker and Schneider, 2006; Bordoni and Schnei-der, 2008; Schneider and Bordoni, 2008) as well as moisture(Boos and Kuang, 2010).

The question remains open as to how the many differ-ent processes contributing to seasonal circulation transitionsare quantitatively linked. We are still missing a theoreticalframework that consolidates our understanding of the gen-eral circulation into a set of mathematical equations that areamenable to analytical insight and that would allow us to an-swer, among other questions, whether the seasonal transition


of the circulation can be understood as “a certain type ofinstability,” as conjectured by Yeh-Dao-Li. Unlike in Pro-fessor YEH’s time, progress towards reaching this goal isnot impeded by difficulties to experiment with the generalcirculation systematically. Numerical models today give usthe means to do so. There seems to be little that obstructsprogress towards completion of the program began by pio-neers such as ROSSBY, CHARNEY, and YEH: developing acomprehensive theory of the general circulation grounded inthe universal laws of physics.

Acknowledgements. J. H. LU is indebted to Professor YEH’smentoring over more than two decades. Both authors acknowledgethe Organization Committee of Professor YEH’s Centenary Sympo-sium for the invitation to the symposium. J. H. LU is grateful for thesupport from the LASG during his visit to the lab. J. H. LU is sup-ported by the National Nature Science Foundation of China (GrantNo. 4157060636) and the Hundred Talents Program of Sun Yat-senUniversity.

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