## 1. Introduction

[2] By affecting winds, clouds, humidity, precipitation and energy transport in the tropics, the Hadley cell plays a key role in the climate system. It is therefore important to understand the mechanisms underpinning its variability. Previous work [*Oort and Yienger*, 1996; *Quan et al.*, 2004] has shown significant co-variability between Hadley cell mass flux and the El Niño/Southern Oscillation (ENSO). Both these papers used composite analysis to show that the warm phase of ENSO is associated with a stronger and more equatorially symmetric Hadley circulation. On the other hand, recent work using simplified atmosphere models has shown that the propagation of midlatitude eddies into the tropics can also strongly affect Hadley cell strength [*Becker et al.*, 1997; *Kim and Lee*, 2001; *Walker and Schneider*, 2006], suggesting that much of the variability in the Hadley cell could be a response to changes in eddy forcing from the midlatitudes. Here, we explore this hypothesis through an observational assessment of the role played by eddy momentum transport in the interannual variability of Hadley cell strength.

[3] Momentum transport is connected to the Hadley cell mass flux through the upper-tropospheric zonal momentum budget, which to leading order is a balance between zonal acceleration by meridional advection of absolute vorticity, and deceleration by eddy stresses:

where overbars represent a zonal climatological mean and primes a deviation from the zonal mean, = −(*a* cos ϕ)^{−1}∂(cos ϕ )/∂ϕ is the relative vorticity, *Ro* = −/*f* is a meridionally varying Rossby number [*Walker and Schneider*, 2006], and

is the horizontal eddy momentum flux divergence; other notation is standard.

[4] If *S* is small, then angular momentum is conserved, *Ro* ≈ 1 and the system is close to the nearly-inviscid axisymmetric regime in which the mass flux is determined purely by the diabatic forcing [*Held and Hou*, 1980]. If eddy momentum transport is very strong, on the other hand, then the subtropical jet is weak, *Ro* is small and the Hadley cell mass flux (proportional to the mass-weighted vertical integral of the meridional velocity *v*) is determined solely by the eddy stress, *v* ≈ *S*/*f*. The observed value of *Ro* (Figure 1a) peaks at about 0.5 for the northern hemisphere winter cell, suggesting that this cell is in an intermediate regime between the eddy-driven and axisymmetric limits. The summer cell, with *Ro* ≈ 0.2, is closer to the eddy-driven limit. There are thus grounds for expecting an important influence of eddy stress on the variability of both cells.

[5] Interannual variability in Hadley cell strength turns out to be relatively small compared to the mean (section 3.1), so it is reasonable to linearize the momentum budget (1). With some rearrangement, we obtain

where *δ* indicates an interannual fluctuation around the climatological mean. Thus, increased mass flux can be balanced by increased *Ro*, or by increased eddy stress. Below, we will study the interannual variability of the momentum budget by regressing indices of Hadley cell strength onto the terms in (3). This analysis will show that changes in eddy stress do indeed play a crucial role in the momentum adjustment.