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P2.18 Recent Trend of Hadley and Walker Circulation Shown in Water Vapor Transport Potential

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P2.18 Recent Trend of Hadley and Walker Circulation Shown in Water Vapor Transport Potential P2.18 Recent trend of Hadley and Walker circulation shown in water vapor transport potential Seong-Chan Park and *Byung-Ju Sohn School of Earth and Environmental Sciences Seoul National University, Seoul, Korea 1. Introduction Although these previous studies have In recent years, there are many studies provided useful information of interannual focusing on satellite observations of long- variations of Hadley and Walker circulations, term changes in tropical cloud and radiation some questions should be explored: (1) How [Chen et al., 2002; Wielicki et al., 2002; Allan are the recent trend of Hadley and Walker and Slingo 2002; Wang et al., 2002]. The circulation associated with the hydrological analysis of satellite-based outgoing longwave cycle from satellite observations and the radiation (OLR) at the top of the atmosphere global reanalysis data? (2) Are these trends (TOA) over the period 1985-2000 suggested robust among satellite and reanalysis data? that there has been an increase in the To address these questions, we examine the tropical mean OLR at TOA [Wielicki et al., interannual variation of Hadley and Walker 2002]. In addition, Chen et al. [2002] argued circulation by using satellite measurements that changes in radiation budget are due to together with various reanalysis data in an increase in the strength of the Hadley association with hydrological cycle. circulation. Attentions were given to the long-term 2. Methodology and datasets variability of the Hadley and Walker The methodology used here for deriving circulations using the various reanalysis data water vapor transports over the global [Oort and Yienger, 1996; Trenberth et al., oceans from satellite-retrieved precipitation 2000; Wang, 2002]. Mitas and Clement (P) and evaporation (E) is described in detail [2005] suggested that the NCEP and ERA40 in Sohn et al. [2004] and Park et al. [2007]. appear to be in agreement with the Briefly, for a given atmospheric column, the strengthening of the Hadley circulation in water balance is achieved as follows: recent decades, and Tanaka et al. [2004] ∂W also indicated the strengthened trend of + ∇•Q = E − P , (1) Hadley circulation by using the maximum ∂t velocity potential of the upper troposphere where W is the total amount of precipitable obtained from NCEP reanalysis data. water, E and P are evaporation and On the other hand, it is suggested that precipitation, respectively. Q is the horizontal there has already been a weakening of the water vapor transport vector defined by Walker circulation in the past century, and 1 ps that the observed changes are consistent Q = ∫ q V dp , (2) g p0 with those expected as a response to increases in anthropogenic greenhouse where g is the acceleration of gravity, q is gases [Vecchi et al., 2006]. specific humidity, V is the horizontal wind vector, ps is the pressure at the surface, and p0 is the pressure at the top of the atmosphere. *Corresponding author's address In the case of satellite measurements, Prof. Byung-Ju Sohn the Special Sensor Microwave Imager School of Earth and Environmental Sciences (SSM/I) onboard the Defense Meteorological Seoul National University, NS80 Satellite Program (DMSP) satellite, by Seoul, 151-747, Korea separating the water vapor transport vector E-mail: [email protected] (Q) into rotational (QR) and divergent (QD) Phone:+82-2-880-7783, Fax:+82-2-872-8156 components [Chen, 1985; Sohn et al., 2004] and introducing the water vapor transport the Hadley circulation is an axisymmetric part potential function (Φ) we effectively remove of the circulation. The monsoon circulation is the rotational component (QR), i.e.: considered as a part of the seasonal change ∂ of the deviation field. Thus, the seasonal W 2 (3) ∇ •QD = E − P − = − ∇ Φ mean is subtracted from the deviation field to ∂t define the interannual variation of the monsoon circulation, i.e., Φ *' (t,x, y) . Finally, = − ∇Φ (4) Q D the Walker circulation is defined by the Equations (3) and (4) are solved for Φ by annual mean of the zonal deviation field, i.e., * using a spectral method on a global domain Φ (x, y) . with inputs of evaporation, precipitation, and The SSM/I-derived precipitation and total total precipitable water. precipitable water [Wentz and Spencer 1998] In the reanalyses, however, water vapor for the May-September period of the years transports are directly calculated from q and from 1988 to 2000 were obtained (from V data for more realistic depiction of water http://www.ssmi.com). The evaporation data vapor transport rather than indirectly were obtained from the products based upon calculated from the E-P field. It is because q an algorithm developed by Chou [1993]. and V fields are produced by assimilating the The used datasets for specific humidity model outputs to various observations. In (q), and horizontal wind fields (V) were doing so, vertically integrated water vapor obtained from following three reanalysis transport vectors (Q) are obtained by products: applying Eq. (2) to reanalysis data, and then • NCEP/NCAR reanalysis (NCEP, 1979- a similar methodology used for the SSM/I 2006 [Kalnay et al., 1996]) calculation is applied for calculating the • ECMWF 40-year reanalysis (ERA40, potential function, i.e.: 1979-2001 [Simmons and Gibson, 2000]) • Japanese 25-year reanalysis (JRA25, 2 ∇ •Q = − ∇ Φ (5) 1979-2004 [Onogi et al., 2005]) The same spherical harmonics approach 3. Interannual variability of the Hadley and is employed to avoid the boundary value Walker circulation problems for solving potential functions. The associated divergent component of water 3.1 Derived water vapor transport and vapor transport (Q ) in the reanalsyes is then D zonal mean of potential function obtained by taking the gradient of potential Using the definition of Hadley and Walker function as in Eq. (4). circulation index as described in Eq. (6), we In this study, we used the simple first examine the characteristics of seasonal separation of the tropical circulations change of the Hadley circulation. Figure 1 proposed by Tanaka et al., [2004] to define presents the spatial pattern of derived water the Hadley and Walker circulations. The vapor transport (left panels) and the zonal water vapor transport potential function (Φ) is mean of water vapor transport potential separated into the following linear function (Φ) (right panels) from ERA40 for the combinations of spatial patterns: climate (1979-2001). During the DJF period Φ(t,x,y)=[Φ(t,y)] + Φ* (t,x,y) (Figure 1a), the divergent water vapor is (6) transported from the positive peak of Φ over =[Φ(t,y)] + Φ* (x,y) + Φ*'(t,x,y) the subtropical northwestern Pacific to the minimum of it over the western Pacific warm where x, y, and t are longitude, latitude, and pool region. The zonal mean of Φ is positive time. [()] and ()* represent the zonal mean in the Northern Hemisphere (NH) with a peak and the deviation from the zonal mean, at 25°N. The gradient of Φ over the respectively. Also denoted are () and ()' subtropics in the NH yields the divergent the annual mean and the deviation from the component of water vapor transport so that annual mean, respectively. From Eq. (6), the the water vapor is transported toward both Hadley circulation is defined by zonal mean north and south of 25°N where moisture field of (Φ), i.e., [Φ (t, y)] by assuming that divergence is formed. On the other hand, the local saddle point of Φ is located near 5°N so is measured by the difference of area- that moisture convergence is formed by averaged water vapor transport potential moisture transports from the NH. The function shown in Figure 1c between the pronounced meridional convergent flow over tropical western Pacific (10°S–10°N, 80°E– the equator represents the lower branch of 160°E) and the tropical eastern Pacific the Hadley circulation in the boreal winter. (10°S–10°N, 160°W–80°W). Based on the In the JJA mean climatology (Figure 1b), definition of Hadley and Walker circulation strong divergent water vapor transport from indices, the interannual variation of the the SH to the NH is found over the Asian intensity of the Hadley and Walker circulation monsoon region. The zonal mean of Φ is from satellite and reanalyses is given in the negative in the NH and positive in the SH. following section. The meridional water vapor transport is thus obviously stronger than that in the boreal winter. The zonal mean of Φ is positive in the SH with a local maximum at about 35°S where the moisture divergence is established. On the other hand, the local saddle point of (Φ) is located near the ITCZ where moisture convergence is formed by moisture transports from the SH. The strong water vapor is thus transported from the SH to the NH, indicating the overturning of the Hadley cell over the equator during the boreal summer. Since the intensity of water vapor transport is determined by the gradient of its potential function, the Hadley circulation index is defined here by using the difference between positive and negative peaks of the zonal mean water vapor transport potential function. A stronger Hadley circulation can be expected when the difference in potential function at those two points is larger. On the other hand, for separating the Walker circulation from the divergent circulation, we construct the deviation field from the zonal mean water vapor transport potential function representing the Hadley circulation, and then take its annual mean as described in Eq. (6). Figure 1c shows the Figure 1: Long-term mean water vapor annual mean of the zonal deviation field of transport [QD] (arrow) and potential function water vapor transport potential function and [Φ] (contour) in (a) DJF, (b) JJA, and (c) associated moisture transports from ERA40.
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