15 JANUARY 2021 H O S K I N S A N D Y A N G 805 The Detailed Dynamics of the Hadley Cell. Part II: December–February a,b a,c B. J. HOSKINS AND G.-Y. YANG a Department of Meteorology, University of Reading, Reading, United Kingdom b Grantham Institute for Climate Change, Imperial College, London, United Kingdom c Climate Directorate, National Centre for Atmospheric Science, University of Reading, Reading, United Kingdom (Manuscript received 7 July 2020, in final form 15 October 2020) ABSTRACT: This paper complements an earlier paper on the June–August Hadley cell by giving a detailed analysis of the December–February Hadley cell as seen in a 30-yr climatology of ERA-Interim data. The focus is on the dynamics of the upper branch of the Hadley cell. There are significant differences between the Hadley cells in the two solsticial seasons. These are particularly associated with the ITCZs staying north of the equator and with mean westerlies in the equatorial regions of the east Pacific and Atlantic in December–February. The latter enables westward-moving mixed Rossby–gravity waves to be slow moving in those regions and therefore respond strongly to upstream off-equatorial active convection. However, the main result is that in both seasons it is the regions and times of active convection that predominantly lead to upper-tropospheric outflows and structures that average to give the mean flow toward the winter pole, and the steady and transient fluxes of momentum and vorticity that balance the Coriolis terms. The response to active convection in preferred regions is shown by means of regressions on the data from the climatology and by synoptic examples from one season. Eddies with tropical origin are seen to be important in their own right and also in their interaction with higher-latitude systems. There is support for the relevance of a new conceptual model of the Hadley cell based on the sporadic nature of active tropical convection in time and space. KEYWORDS: Convection; Hadley circulation; Potential vorticity; Waves, atmospheric; Vortices 1. Introduction decades been seen as a limiter on the latitudinal extent of the Hadley cell. In their axisymmetric simulations using a dy- The concept of the Hadley cell (HC) in the zonally averaged namical core, Davis and Birner (2019) found that the Hadley view of the atmosphere is at the core of the traditional view of cell only extended through the depth of the atmosphere to the the general circulation of the atmosphere as discussed, for lower boundary if they included significant viscosity or pre- example, in Lorenz (1967). As reviewed by Held (2018), there scribed eddy driving. Schneider (2006), stressed the impor- have been contrasting views about the relative importance of tance for the Hadley cells of the extratropical eddies, and in tropical and midlatitude phenomena and processes for the subsequent years the emphasis has often been on their strong, existence, strength, and extent of Earth’s Hadley cells and the and even controlling, influence on the HC. strength of the upper-tropospheric subtropical jets on their However, from the tropical perspective, the improvement of poleward flank. From the midlatitude perspective, Jeffreys data availability permitted Starr et al. (1970) to calculate mo- (1926) raised the importance for the maintenance of the sur- mentum transport for the whole Northern Hemisphere and for face westerlies in midlatitudes of angular momentum transfers the four seasons. In winter the upper-tropospheric poleward from the subtropics by midlatitude eddies. The early datasets momentum flux by transient eddies occurred started from the for computation of momentum fluxes (e.g., Starr 1948) were deep tropics, but there were indications of a reversal of the only for the extratropical region. Eady (1950) saw that main- direction of transport by both standing and transient eddies in tenance of thermal wind balance would require that the mid- the lowest latitudes. Rosen and Salstein (1980) also saw these latitude eddy-induced poleward momentum transfer in the features in their improved datasets, and they were discussed by upper troposphere must be accompanied by a Hadley cell, Dima and Wallace (2003). Dima et al. (2005) associated the though this cell is weak, as was shown for the life cycle of a reversed tropical momentum transports with Rossby waves baroclinic wave by Simmons and Hoskins (1978). As was forced in the tropics, and in particular the horizontal tilts of quoted by Held (2018), baroclinic instability has also for many waves generated as a response to the regions of tropical heat- ing. However, Hartley and Black (1995) found that in their Denotes content that is immediately available upon publica- climate model the momentum fluxes on the equator that are tion as open access. equatorward in the Northern Hemisphere winter were asso- ciated with convective outflow rather than Rossby wave propagation and tilted troughs. Also, Zurita-Gotor (2019) Supplemental information related to this paper is available at found that the near equatorial momentum transfers were the Journals Online website: https://doi.org/10.1175/JCLI-D-20- dominated by the correlation of the divergent meridional wind 0504.s1. and the rotational zonal wind. A zonally symmetric and temporally uniform atmosphere Corresponding author: Gui-Ying Yang, [email protected] and angular momentum conservation in the tropical upper DOI: 10.1175/JCLI-D-20-0504.1 Ó 2020 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 10/04/21 03:27 AM UTC 806 JOURNAL OF CLIMATE VOLUME 34 troposphere formed the basis of the simple model of the longitudinal variation of the convection and meridional motion equinoctial HC given by Held and Hou (1980) in their seminal was seen to be fundamental in the dynamics of the HC. paper, which followed the study of Schneider (1977). In this In this paper the dynamics of the December–February model the HCs exist to reduce the midtropospheric tempera- (DJF) HC are analyzed in a manner similar to that used in ture gradient, and therefore to reduce the upper-tropospheric HC1 for the other solsticial season. The aims of the study are winds implied by thermal wind balance to those which are at- the following: tainable by angular momentum (AM) conservation from zero d to investigate the similarities and differences between the zonal wind at the equator. The latitudinal extent of the HC is two seasons, determined by the latitudinal range over which the reduction d to determine to what extent this further study reinforces the of the latitudinal temperature gradient is needed, along with picture given in HC1 that longitudinal and temporal varia- energy conservation. In this model, the subtropical jet (STJ) is tions are of order one importance for the HC, and at the latitude of the edge of the HC and its magnitude is that d to explore the interaction of the tropics with higher latitudes given by angular momentum conservation. The role of eddies is that is initiated by flaring in regional tropical convection. restricted to removing the discontinuity between this STJ wind and the zero wind in latitudes outside the HC and to reducing Section 2 gives a summary of the equations on which the the speed of the STJ. A factor of about 3 would be required to analysis is based and of the reanalysis dataset that is employed. give the observed values. The Held and Hou model for an Further details of both are given in HC1. An analysis from a equinoctial HC was extended to include the solsticial seasons conventional time and zonally averaged perspective is pre- by Lindzen and Hou (1988). sented in section 3.Theninsection 4 the time-mean 3D A fundamental issue associated with the dynamics of the behavior is analyzed. Section 5 produces a view of the upper branch of the HC was raised with one of us by John temporal behavior in terms of spectra and of regressions on Sawyer in the 1970s (J. Sawyer, personal communication). In a OLR in a number of tropical regions. In section 6 the be- solsticial season, as the air in the upper branch of the HC moves havior through one season is looked at in detail in order to from the tropics of the summer hemisphere to near 308 in the illustrate the synoptic behavior and to investigate the rele- winter hemisphere the change in its potential vorticity (PV) vance for it of the regressions in the previous section. A can be expected to be small. Therefore, this air will carry PV of discussion of the results for the HC in DJF is given in the opposite sign to that of the air in the rest of this winter section 7 and conclusions from this study and that in HC1 for hemisphere. There is then the possibility of instabilities or a JJA are drawn. return to the summer hemisphere much as described in the inertial motion discussed by Paldor and Killworth (1988). Rodwell and Hoskins (1995) found both such behaviors in their 2. Data and diagnostics study of the low-level inflow to the Indian summer monsoon. The basis for the analysis is ECMWF interim reanalysis However, in this contribution to the lower branch of the HC, (ERA-Interim) horizontal winds (u, y), and vorticity fields for surface processes were usually found to be able to produce the the 30-yr period from 1981 to 2010. The fields are available 6- PV modification that enabled a steady monsoon inflow from hourly with horizontal resolution of about 0.78 and at 37 the southern Indian Ocean. pressure levels from 1000 to 1 hPa.