The Role of Deformation and Potential Vorticity in Southern Hemisphere Blocking Onsets
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THE ROLE OF DEFORMATION AND POTENTIAL VORTICITY IN SOUTHERN HEMISPHERE BLOCKING ONSETS Li Dong and Stephen J. Colucci Department of Earth and Atmospheric Sciences Cornell University Ithaca, New York 1. INTRODUCTION extensively investigated as their Northern Hemisphere (NH) counterparts, we have Theoretical studies (e.g. Shutts, 1983) applied this model to SH blocking cases, and observational investigations (e.g. although our diagnostic model is equally Colucci, 2001) have shown that locally applicable to the Northern Hemisphere. The diffluent planetary-scale flow precedes the aim of this paper is to contribute to our onset of blocking. According to Shutts understanding of block formation over the (1983), diffluent or deformed flow can SH from the perspective of the interaction interact with the vorticity field so as to between deformation and PV. initiate and maintain the blocking structure (anticyclonic circulation poleward of a 2. DIAGNOSTIC MODEL cyclonic circulation). Mak (1991) has shown how the deformation field can interact with A diagnostic equation in which the the eddy wind field to energetically maintain deformation term appears as a part of the a blocking system. This energy exchange was forcing to the local change of the u- shown by Marques and Rao (2000) to component of the geostrophic wind can be account for the distinction between years written as with low and high blocking frequency over the Southern Hemisphere (SH). From a ä 1 ä ä u ç 2 f 2 g = different perspective, Trenberth (1986) and 3 p 0 ä p ä p 4 ä t Marques and Rao (1999) have shown how ä q ä u ä q ä v ä q V ô ç g ï g ï k ô D \ ç q eddies in the deformed flow associated with g p ä y ä x ä y ä x ä x g p a block can locally decelerate the westerly (1) flow, thereby maintaining the block. In addition, it is known that a local D is the geostrophic deformation weakening of geostrophic westerlies or g increasing of geostrophic easterlies often pseudovector ( , ), where D1 D2 occurs prior to blocking onset. It would seem ä u ä v D 1 g ï g is the geostrophic stretching from the above considerations that 1 ä x ä y ä v ä u deformation should appear as part of the deformation and D 1 g g is the forcing of the geostrophic u-component wind 2 ä x ä y field during block development. However, to geostrophic shearing deformation. the authors' knowledge, such a diagnostic Due to some cancellation among the relationship has not been presented in the second through fourth terms on the RHS of literature. We therefore have established a equation (1), alternatively, it may be written diagnostic quasigeostrophic (QG) tendency as model for the u-component of the ä u geostrophic wind, in which forcing functions 2 2 ä 1 ä g ç p f 0 = are expressed in terms of deformation and 3 ä p ä p 4 ä t ä q ä u ä q ä v ä q potential vorticity (PV). Furthermore, V ô ç g g considering that SH blocks have not been as g p ä y 3 ä y ä x ä y ä y 4 (2) such that the bracketed second and third thin arrows indicate the local coordinates. The terms on the RHS of equation (2) represent regions of meridional change of QGPV are denoted by the dashed circles. CPVA is the abbreviation of the net effect of deformation interacting with Cyclonic Potential Vorticity Advcetion, and APVA QGPV in forcing the geostrophic wind Anticyclonic Potential Vorticity Advection. tendency. Following the way that Holton (1992) interpreted the y component of Fig. 2 schematically shows how the Q interaction of PV and deformation can vector as the forcing of the meridional locally weaken the westerlies. Equatorward temperature gradient by horizontal shear increasing zonal winds (cyclonic shear in the deformation and stretching deformation SH) coinciding with eastward increasing PV respectively, we interpret the bracketed two would result in a pattern of equatorward terms in the RHS of Eq (2) as the forcing of increasing CPVA, which could force the geostrophic zonal wind tendency by poleward increasing height rises. horizontal shear deformation and stretching Accordingly, the local weakening of deformation, respectively. In the present westerlies would occur as displayed in Fig. 2 study, we define the first term on the RHS of (a). Similarly, anticyclonic shear embedded Eq (2) as the advection effect, and the in an eastward decreasing PV field would remaining two terms together as the also favor local weakening of westerlies (not interaction effect. shown). In Fig. 2 (b) the meridional gradient The decreasing of the geostrophic of PV (i.e. equatorward increasing PV) westerlies due to the advection effect is would be weakened by a purely diffluent schematically illustrated in Fig. 1. The background flow, which stretches the PV advection of equatorward increasing cyclonic dipoles along the dilatation axis y and ä q PV, V ôç 0 , would favor a compacts them along the contraction axis x. 3 g p ä y 4 Thus, the originally existing westerlies pattern of cyclonic PV advection (CPVA) would be weakened by the dilution of the equatorward of the anticyclonic PV meridional PV gradient. advection (APVA). This in turn would result in a geostrophic height tendency pattern with (a) Shearing contribution (b) Stretching contribution height falls situated equatorward of height rises. Thus locally decreasing westerlies would likely result. FIG.2. Schematic illustration of formation of decreasing geostrophic westerlies associated with (a) QGPV-shearing interactions and (b) QGPV- stretching interactions in the SH. Every notation is as FIG.1. Schematic illustration of formation of in FIG.1, except the dashed ellipses in (b) represent decreasing geostrophic westerlies associated with the PV fields after being stretched along the advection of meridional QGPV (denoted as q) dilatation axis, which is denoted as y axis (pointing to gradient in the SH. Positive (negative) signs inside the equator). The x axis is the contraction axis the ellipses indicate the sense of QGPV. The heavy (pointing to east). arrows show the background geostrophic wind and q 3. DATA 0 block onset region where y . This Fifty years of gridded SH 500-mb combination favors a local increase in heights from the NCEP daily average westerlies, but is overwhelmed by the reanalysis (Kalnay et al. 1996) were screened westerly-decreasing contribution from the by the Watson and Colucci (2002) algorithm advection effect. Compared to the analyzed for block detection. Thirty of these blocking weakening of westerlies over the block-onset cases were selected and comprehensively region on 20 July, the total calculated wind examined with the QG wind tendency tendencies are qualitatively similar. equation (2). The forcing effects and other The December 1965 blocking case is quantities were calculated from the NCEP- dominated by the interaction effect. Over the NCAR reanalysis of six-hourly SH western half of the block-onset region on 14 temperatures and geopotential heights at 10 December 1965 (three days prior to the u vertical levels ( P = 1000, 925, 850, 700, block-onset), cyclonic shear, g 0 , y 600, 500, 400, 300, 200, 100 ) on a 2.5º by coincides with eastward increasing PV, 2.5º grid. The geostrophic u-component wind resulting a weakening of the geostrophic tendencies are resolved into contributions westerlies. Meanwhile, a strong diffluent from the advection and interaction effects v flow g 0 is overwhelming over the respectively. y equatorward half of the block onset region. 4. THE BLOCKING AND An equatorward increasing PV, embedded in NONBLOCKING CASES this diffluent flow, can weaken the westerlies. In most blocking cases diagnosed by While two types of forcing patterns the QG zonal wind tendency equation, one or have been identified as favoring the the other but not both forcing effects weakening of westerlies, one may wonder if appeared to primarily contribute to the wind these forcing signals are unique to the tendency fields prior to the block-onset. Two initiation of blocking. Two nonblocking of these cases, July 1999 and December events were thus also examined. Compared 1965, are discussed in detail due to an to the blocking cases, the calculated and apparent dominance of one effect over the analyzed wind tendencies are both positive, other in each case. opposing block onset, in the two The July 1999 blocking event features nonblocking cases. the advection forcing effect. Following the geostrophic wind vectors from southwestern 5. SCALE INTERACTIONS IN BLOCK corner to the eastern side of the block-onset ONSETS region, the meridional PV gradient changes q q 0 0 Scale partitioning was applied to the signs from y to y . There two forcing effects of Eq (2) and it appears appears to be an advection of a dipolar that the weakening of westerlies during pattern with low (cyclonic) PV equatorward block onsets is due primarily to the of high (anticyclonic) PV, which would advection of equatorward increasing cyclonic locally force a weakening of westerlies from synoptic-scale PV by the planetary-scale previous arguments and as calculated and wind, or eastward increasing synoptic-scale analyzed. As expected from the previous PV embedded in a cyclonically sheared discussion, the calculated wind tendencies synoptic-scale flow and equatorward attributable to the interaction effect offset the increasing synoptic-scale PV coincident with advection effect. In fact, the flow is v a synoptic-scale diffluent flow. In other geostrophically confluent g 0 , y words, synoptic-to-planetary-scale especially over the eastern portion of the interactions between PV and deformation appear to often oppose block onsets. Briefly System (GrADS) from the Center for Ocean- speaking, when geostrophic deformation is Land_Atmosphere Studies.