1NOVEMBER 2004 CHIANG AND VIMONT 4143

Analogous Paci®c and Atlantic Meridional Modes of Tropical Atmosphere±Ocean Variability*

JOHN C. H. CHIANG 1Department of Geography, and Center for Atmospheric Sciences, University of , Berkeley, Berkeley, California

DANIEL J. VIMONT Department of Atmospheric and Oceanic Sciences, University of WisconsinÐMadison, Madison, Wisconsin

(Manuscript received 4 December 2003, in ®nal form 28 May 2004)

ABSTRACT From observational analysis a Paci®c mode of variability in the intertropical convergence zone (ITCZ)/cold tongue region is identi®ed that possesses characteristics and interpretation similar to the dominant ``meridional'' mode of interannual±decadal variability in the tropical Atlantic. The Paci®c and Atlantic meridional modes are characterized by an anomalous (SST) gradient across the mean latitude of the ITCZ coupled to an anomalous displacement of the ITCZ toward the warmer hemisphere. Both are forced by trade wind variations in their respective northern subtropical oceans. The Paci®c meridional mode exists independently of ENSO, although ENSO nonlinearity projects strongly on it during the peak anomaly of boreal spring. It is suggested that the Paci®c and Atlantic modes are analogous, governed by physics intrinsic to the ITCZ/ cold tongue complex.

1. Introduction mode (hereafter the meridional mode, also known as the gradient or interhemispheric mode) arises. Nobre and The dominant statistical mode of tropical Atlantic in- Shukla (1996) argued from observational analysis that terannual±decadal atmosphere±ocean variability is an trade wind variations in the north tropical Atlantic pre- anomalous meridional SST gradient across the mean intertropical convergence zone (ITCZ) latitude and a cede the tropical basinwide anomalies that in turn pro- cross-gradient atmospheric boundary layer ¯ow toward duced SST anomalies there (and hence an anomalous the anomalously warmer hemisphere (Nobre and Shukla SST gradient across the mean ITCZ latitude), implying 1996; Chang et al. 1997). Hastenrath and Greischar that the mode is externally forced. Two sources of ex- (1993) proposed that this boundary layer ¯ow is driven ternal forcing have been identi®ed: ENSO and the North by the anomalous meridional SST gradient through its Atlantic Oscillation (NAO; e.g., Curtis and Hastenrath hydrostatic effect on sea level pressure (Lindzen and 1995; Nobre and Shukla 1996; Xie and Tanimoto 1998; Nigam 1987). The cross-gradient ¯ow implies a shift of Giannini et al. 2000). They directly perturb the boreal the ITCZ and associated convection toward the anom- winter north tropical Atlantic (NTA) trade wind alously warmer hemisphere (e.g., Hastenrath and Heller strength, changing the underlying SST through latent 1977). This basic pictureÐlinking anomalous SST, heat ¯uxes. The resulting NTA SST perturbation peaks winds, and convectionÐhas been supported in subse- in boreal spring, creating the near-equatorial meridional quent observational and modeling studies (e.g., Ruiz- SST gradient. Barradas et al. 1999; Chang et al. 2000; Chiang et al. Chang et al. (1997) proposed an alternative expla- 2002). nation for the origins of the meridional mode, pointing Two interpretations had been proposed for how this to a [sometimes known as wind±evap- oration±SST (WES) feedback (e.g., Xie 1999)] between the SST gradient and the cross-gradient ¯ow. The cross- * National Oceanic and Atmospheric Administration Contribution Number 1010. gradient ¯ow reduced the strength of the in the anomalously warmer hemisphere and increased the trades in the cooler hemisphere, and this reinforced the Corresponding author address: John Chiang, Department of Ge- SST gradient through wind speed impact on anomalous ography, University of California, 547 McCone Hall, Berkeley, Berkeley, CA 94720-4740. evaporation. Idealized modeling studies (e.g., Chang et E-mail: [email protected] al. 1997; Xie 1999) have shown that this feedback can

᭧ 2004 American Meteorological Society

Unauthenticated | Downloaded 09/30/21 06:48 PM UTC 4144 JOURNAL OF VOLUME 17 qualitatively reproduce the observed behavior, though march lagging by a quarter cycle the seasonal cycle of it was found that under realistic coupling strength the the ITCZ over land (Mitchell and Wallace 1992). An model behavior was not self-sustaining, and that exter- idealized tropical coupled general circulation model nal forcing was required to sustain the variability. Sub- study (Xie and Saito 2001) shows that in the absence sequent modeling and observational studies show this of thermocline±SST feedback central to the ENSO phys- feedback is relatively weak and limited to the deep Trop- ics, interannual variations in the north±south oceanic ics (Chang et al. 2000; Chiang et al. 2002; Czaja et al. ITCZ position are present, suggesting that this vari- 2002), further underlining the need for external forcing. ability is intrinsic to the ITCZ/cold tongue climate. In- A more recent idealized model study by Kushnir et al. deed, a mode in the Paci®c with spatial pattern remi- (2002) incorporating the deep Tropics±limited WES niscent of the Atlantic meridional mode was identi®ed feedback suggests, however, that the feedback is essen- in a recent coupled model study with realistic con®g- tial to the decadal nature of meridional mode variability. uration (Yukimoto et al. 2000). Finally, recent detailed Our current knowledge of the meridional mode physics observational and general circulation model (GCM) thus suggests that it is externally forced through trade analyses by Vimont and collaborators (Vimont et al. wind variations, with limited WES feedback in the deep 2001, 2003b) shows that a leading mode of wintertime Tropics. atmospheric variability in the North Paci®c [the North An outstanding issue with the meridional mode is the Paci®c Oscillation (NPO); Rogers 1981] forces varia- issue of (out of phase) coherence between the SST tions of the north tropical Paci®c (NTP) SST through anomalies on either side of the anomalous SST gradient. its control of the subtropical trades there, a mechanism Houghton and Tourre 1992) showed that the dipolelike similar to the external forcing of the tropical Atlantic nature of the anomalies derived from a straight empirical meridional mode by the NAO. orthogonal function (EOF) analysis of tropical Atlantic These similarities motivate us to search for analogous SST anomalies do not survive under varimax rotation; tropical meridional modes in the two basins. We will instead, the two SST lobes appear to vary independently show that once the ENSO in¯uence is linearly removed of each other, a result supported by subsequent obser- from the SST and wind data we analyze, the dominant vational analysis (Mehta and Delworth 1995; En®eld et mode of atmosphere±ocean variability extracted sepa- al. 1999), and also the modeling study by Dommenget rately from the tropical Paci®c and tropical Atlantic ba- and Latif (2000). On the other hand, Xie and Tanimoto sin resembles each other in spatial and temporal char- (1998) argue using observational and modeling evi- acteristics (section 2). The Atlantic pattern is the me- dence for anticoherence between the two lobes at de- ridional mode pattern, whose interpretation is fairly well cadal time scales and part of a coherent pan-Atlantic established from previous studies. A nonlinear com- decadal oscillation characterized by zonal bands of SST ponent of ENSO does, however, project strongly on the and wind anomalies with alternate polarities from the derived Paci®c mode. We show that by additionally ®l- tropical South Atlantic to Greenland. The ``dipole'' is- tering out the strong ENSO years from our analysis, the sue is intimately tied to the physical interpretation of derived Paci®c mode stays essentially the same, indi- the mode, since independent behavior between the cating that this mode does not depend on ENSO for its northern and southern SST lobes suggests an externally existence (section 3). We then show that the Paci®c and forced mechanism with little to no positive feedback in Atlantic modes share the same physical interpretation, the Tropics, whereas dipole behavior between the north- and on that basis we argue that the Paci®c mode is the ern and southern lobes implies a stronger role for pos- analogue to the Atlantic meridional mode pattern (sec- itive feedback. At present, the prevailing evidence sug- tion 4). We reinforce our interpretation by showing the gests that the SST lobes act essentially independently, similarity between the two basins using a different in agreement with the prevailing physical interpretation; (composite) analysis (section 5). A summary and dis- although we emphasize that this issue is not yet fully cussion of the potential implications of our results is resolved in large part because of inadequate length of given in section 6. data, but also in part because simple models hint at dipolelike anomalous SST structures even with WES 2. Data, method, and results feedback limited to the deep Tropics (Kushnir et al. 2002). We use monthly mean SST and 10-m winds from the A similar tropical Paci®c mode of variability (here- National Centers for Environmental Prediction±Nation- after the meridional mode, following Servain et al. al Center for Atmospheric Research (NCEP±NCAR) re- 1999) has heretofore not been identi®ed, in large part analysis (Kalnay et al. 1996, hereafter simply the re- because the El NinÄo±Southern Oscillation (ENSO) dom- analysis) spanning January 1948±December 2001. For inates the variability there (Wallace et al. 1998). There each ®eld, we spatially averaged over six adjoining (two are, however, compelling reasons for its existence. Each latitude by three longitude) grid points, computed de- basin possesses similar mean states: namely, a cold trended monthly mean anomalies, and applied a 3-month tongue over the equatorial oceans (weighted to the east) running mean. The spatial averaging does not unduly and an ITCZ at its northern edge that follows an annual impact the result, as the extracted dominant mode pos-

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FIG. 1. Spatial properties of the leading MCA mode 1 in the (left) Paci®c, (right) Atlantic. (a), (b) Regression maps of the MCA leading mode SST normalized expansion coef®cients on SST and 10-m wind vectors. Wind vectors are plotted where the geometric sum of their correlation coef®cients exceeds 0.27 (the 95% con®dence level). (c), (d) Same as (a), (b) but for precipitation (mm day Ϫ1). In general, shaded regions in all panels exceed the 95% con®dence level. sesses comparatively large space scales; similarly for the Atlantic basin, we show results from the original the temporal smoothing. Since ENSO is not the vari- method. ability of interest, we subtract the linear least squares The spatial structure of the leading MCA mode is ®t to a commonly used ENSO index [cold tongue index depicted by regression maps of SST and winds onto the (CTI), SST anomalies averaged over 6ЊS±6ЊN and 180Њ± normalized SST expansion coef®cients for the Paci®c 90ЊW] from all ®elds prior to analysis. (Fig. 1a) and Atlantic (Fig. 1b). The Atlantic analysis We apply maximum covariance analysis (MCA; also identi®es the meridional mode as the leading one. The known as singular value decomposition, Bretherton et Paci®c pattern strongly resembles the Atlantic pattern, al. 1992) to the cross-covariance matrix between SST with anomalously warm SST in the NTP region pro- and both components of 10-m winds to extract the lead- ducing an anomalous meridional SST gradient. South- ing mode of coupled variability. Two domains that pos- erly atmospheric ¯ow occurs over the strongest anom- sess qualitatively similar mean ITCZ/cold tongue cli- alous SST gradient in both patterns. The divergence of mates are separately and independently analyzed: 32ЊN± the anomalous surface winds (not shown) gives a di- 21ЊS and 175ЊE±95ЊW for the Paci®c, and 32ЊN±21ЊS pole with increased convergence to the north of the and 74ЊW to the West African coastline for the Atlantic. mean ITCZ position (around 5ЊN for the Atlantic, 8ЊN The meridional offset in both domains is in keeping for the Paci®c) and increased divergence to the south, with the northward bias of the mean ITCZ position. The indicating an anomalous northward displacement of the squared covariance fractions for the leading modes are ITCZ. A simultaneous regression on a satellite precip- each 53% for the Paci®c and the Atlantic, explaining itation dataset 1979±2001 (Xie and Arkin 1997) (Figs. the majority of the total squared covariance. The nor- 1c,d) shows anomalous rainfall generally consistent malized root-mean-square covariance, which measures with the anomalous northward displacement of the the strength of the relation between two ®elds (in this ITCZ, although the Paci®c precipitation pattern resem- case SST and 10-m winds in the Paci®c or Atlantic) is bles more a strengthening of the climatological mean 0.16 for the Paci®c and 0.17 for the Atlantic, indicating rainfallÐmore precipitation in the mean ITCZ lati- that the SST and wind ®elds are closely coupled. We tudes north of the cold tongue, and less precipitation found that the leading MCA mode calculated in this over the cold tongue region itself. We will address this method (in which the CTI has been removed via linear issue in section 3. regression) is identical to the second MCA mode of the The temporal characteristics of the leading modes are original data (in which the CTI was not removed via also similar. The month-by-month variance of the SST linear regression). For consistency with the analysis in expansion coef®cient (Fig. 2, top row) peaks in boreal

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obtain is insensitive to reasonable variations in domain size (e.g., from 120ЊEto75ЊW in the Paci®c, and from 32ЊSto32ЊN in either basin). We repeated the analysis for subsets of the data periods, in particular separating the pre- and postsatellite data periods of 1948±79, and 1979±2001, as well as excluding the ®rst 10 yr of data (1948±57) when a different set of observing times was used in the upper-air data going into the reanalysis (Kis- tler et al. 2001). All three subsets produced comparable results. The same analysis using an independent tropical Paci®c wind stress product 1961±2001 (Legler and O'Brien 1988) and a reduced space optimal analysis SST product (Kaplan et al. 1998) gives essentially the same leading Paci®c pattern. Similarly, using an Atlan- tic wind stress and SST product (Servain 1991) 1964± 88 gives the same leading Atlantic pattern. Insofar as these datasets are independent of the reanalysis, they suggest that our results are robust. We also repeated the MCA after removing ENSO-related variability from the ®elds by linearly regressing out the leading complex empirical orthogonal function of the global SST anom- aly ®eld (instead of the CTI) that has been shown to represent ENSO and its teleconnected response (En®eld and Mestas-Nunez 1999). Our results remain essentially the same. FIG. 2. Temporal properties of the leading MCA mode: (left) Paci®c and (right) Atlantic. (top row) Variance computed by the month for the SST (light shading) and wind (dark shading) expansion coef®- cients. (second row) Lagged correlation between the SST and wind 3. ENSO nonlinearity and the Paci®c meridional expansion coef®cients. The simultaneous correlation is the black bar, mode and dark (light) shaded bars are for winds leading (lagging) SST. For reference, correlations above 0.27 are signi®cant at the 95% con®- Since ENSO so dominates in the Paci®c variability, dence level assuming a decorrelation time scale of 1 yr (54 inde- the question arises as to whether linear removal of CTI pendent samples). (third and fourth rows) SST expansion coef®cients from our analysis ®elds removes ENSO suf®ciently so for the Paci®c and Atlantic MCA mode 1. that the dominant pattern arising from what remains is in fact independent of ENSO. It turns out that ENSO appears to project nonlinearly on the Paci®c meridional spring, following the late winter/early spring peak var- mode. Figure 3 shows a scatterplot of an interannual iance in the winds. A lag correlation between the ex- ENSO indexÐCTI averaged over November±Decem- pansion coef®cients (Fig. 2, second row) shows the wind ber±January (NDJ) and normalized (hereafter ctiNDJ)Ð leading SST by a month in both basins, implying that against March±April±May (MAM)-averaged values of the winds drive SST variability. The symmetry around the Paci®c meridional mode SST expansion coef®cients zero lag implies, however, that a signi®cant coupled (also normalized) following the NDJ months of the in- component is also captured. This symmetry is not an terannual ENSO index. We choose MAM for the Paci®c artifact of the initial temporal smoothing on the analyses meridional mode index as that is the season when the ®elds; the symmetry in the lagged correlations seen in behavior is most pronounced. Visual examination of the Fig. 2 results when the MCA is repeated without ®rst scatterplot suggests that extreme (positive and negative) applying a 3-month running mean to the data. A no- ENSO events tend to favor low values of the Paci®c ticeable difference is the long persistence in the Paci®c MAM meridional mode index. lag correlation relative to the Atlantic, which we will In order to see if the extreme ENSO years affected address in the next section. Both SST expansion coef- our MCA result, we repeated the analysis but using only ®cients vary across many time scales (Fig. 2, bottom those years with low or no ENSO activity. We divided two rows) including interannual and decadal variations. the 53 yr of the analysis into three bins based on In the present manuscript we do not argue for any pre- ctiNDJ. We use the NDJ period rather than the MAM ferred time scale of the variability. We note that the period as NDJ is the season when ENSO peaks; how- correlation between the Paci®c and Atlantic SST ex- ever, the results remain the same whether we use NDJ pansion coef®cients is weak (r ϭ 0.18), indicating that or MAM, with 9 yr in each of the extreme ``high'' and the modes vary independently in each basin (recall, the ``low'' index years (these years are indicated by light analysis is conducted separately for each basin). shading in Fig. 5), and 35 ``neutral'' years. We de®ned How robust are these results? The leading mode we the year to be from the previous September through

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ing change, however, is the marked decrease in the long- range (greater than 5 months) persistence in the lag correlation between the SST and wind expansion co- ef®cients (cf. Fig. 4f to Fig. 2, second row, left). Thus, with the extreme ENSO years removed, the temporal properties of this Paci®c MCA mode 1 fall in line with the Atlantic meridional mode. The persistence seen in the original Paci®c MCA mode 1 arises from nonlinear ENSO behavior. To dem- onstrate this, we repeated the MCA analysis using only the 18 extreme ENSO years that were omitted in the neutral MCA analysis. The results for mode 1 (Fig. 4b) show similar SST and wind spatial pattern as before, but with a stronger equatorial emphasis (in particular, a stronger zonal SSTA gradient and anomalous equatorial easterlies). The regression of the expansion coef®cients from this MCA mode on the available precipitation data (Fig. 4dÐnote that there are only 8 yr in this regression) resembles the same from the MCA analysis using all years (Fig. 1c). This indicates that the extreme ENSO FIG. 3. MAM-averaged values of the Paci®c MCA 1 SST expansion years basically determine the ``all years'' regression for coef®cients vs CTI averaged over NDJ (ctiNDJ). The reverse U shape precipitation. The wind and SST expansion coef®cients of the scatterplot suggests that extreme ENSO events (both positive (Fig. 4g) possess more uniform variance across all and negative) favor low values of the MAM Paci®c meridional mode. months. The most pronounced difference between the temporal structure of the ``neutral'' and ``strong'' ENSO the following August in order to include the crucial years is seen in the strong lagged correlation between boreal winter (for ENSO) and spring (for the meridi- the wind and SST time series out to several months onal mode) in the same year. The result of the (Fig. 4h). This explains the strong persistent lagged cor- MCA analysis with only neutral years (Figs. 4a,e,f) relation in the original MCA time series. The combined essentially reproduces the same mode 1 as the previous SST expansion coef®cients from the neutral ENSO MCA analysis using all 53 years: the SST expansion years analysis (second curve in Fig. 5, nonshaded re- coef®cients from this analysis (middle curve in Fig. 5, gions) and the extreme ENSO years (second curve in the coef®cients are in the nonshaded region of the Fig. 5, shaded regions) are nearly identical to the SST graph) are correlated at r ϭ 0.99 with the same time expansion coef®cient from the entire record (Fig. 5, ®rst period from the MCA analysis with all data included curve), indicating that the original MCA was not unduly (top curve in Fig. 5). affected by the inclusion of the extreme ENSO years There are slight but noticeable differences between and ENSO nonlinearity. the original Paci®c MCA 1 pattern and the one derived What does it mean to say that nonlinear ENSO pro- using only the neutral years: the spatial pattern in the jects on the meridional mode? To clarify, we follow an neutral case (Fig. 4a) has slightly greater amplitude in analysis similar to Hoerling et al. (1997) to show how the northern tropical Paci®c features relative to the equa- strong El NinÄo and La NinÄa events spatially differ from torial features compared to the original (Fig. 1a), and each other. SST anomaly (SSTA) composites, weighted the peaks in the variance by the month (Fig. 4e) for the by CTI, for each of the high (El NinÄo) and low (La SST (in boreal spring) and wind expansion (in boreal NinÄa) ENSO cases are computed using the same extreme winter) coef®cients are more sharply de®ned than the ENSO years identi®ed earlier: original (Fig. 2, top left). The regression map of pre- cipitation (note that this is computed using only 12 years ͸ SSTЈ all࿞࿞months࿞࿞in࿞࿞composite of data, so interpretation of precipitation is speculative) ͗SSTЈ͘ ϭ . shows a substantially different precipitation response ͸ CTI all࿞࿞months࿞࿞in࿞࿞composite than in Fig. 1Ðit shows primarily a reduction in the southern half of the mean ITCZ latitude of ϳ8ЊN, and Here we use the ``full'' anomaly data, that is, without only a suggestion of increased precipitation to the north CTI removed. The denominator acts to normalize the of 10ЊN. This pattern implies a northward shift in the sum relative to the CTI. We also computed the same position of maximum ITCZ rainfall (making the inter- composites but for the 10-m zonal and meridional winds pretation more consistent with the ITCZ response for using the same technique (by replacing the SSTЈ in the the Atlantic meridional mode), although in this case it above equation with the appropriate wind component). is achieved largely by reducing rainfall in the southern The weighted composites for ENSO warm and cold half of the climatological Paci®c ITCZ. The most strik- events are plotted in Figs. 6a and 6b, respectively (note

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FIG. 4. Results of the MCA on the tropical Paci®c domain, using only (left column) ENSO ``neutral'' years and (right column) only extreme ENSO years to highlight the in¯uence of ENSO nonlinearity on the meridional mode. (top row) SST and wind ®elds regressed onto the SST expansion coef®cient for (a) neutral and (b) extreme ENSO years. The contour interval is 0.1ЊC (std dev)Ϫ1. Wind vectors are plotted where the geometric sum of their correlation coef®cients exceeds the 95% con®dence level. The reference wind vector is 0.5 m s Ϫ1. Shading denotes SST regions where the correlation coef®cients exceed the 95% con®dence level. (middle row) Precipitation regressed onto the SST expansion coef®cient for (c) neutral and (d) extreme ENSO years. Contour interval: (c) 0.2 mm day Ϫ1 (std dev)Ϫ1, (d) 0.4 mm dayϪ1 (std dev)Ϫ1. (a)±(d) Solid contours denote positive anomalies, dashed contours denote negative anomalies, and the zero contour has been omitted. (bottom row) (e) and (g) Variance computed by the month for the SST (light gray) and wind (dark gray) expansion coef®cients, similar to Fig. 2, top row. (f) and (h) Lagged correlation between the SST and wind expansion coef®cients, similar to Fig. 2, second row. Dark (light) gray indicates that the wind expansion coef®cient leads (lags) the SST expansion coef®cient. Lagged correlations greater than 0.34 in (f) [greater than 0.46 in (h)] exceed the 95% con®dence level.

that scaling by the CTI reverses the polarity of the neg- plitude in the northern than negative ENSO ative ENSO events in Fig. 6b). Both positive- and neg- events (La NinÄas). Thus, both warm and cold ENSO ative-weighted ENSO composites show the familiar events will tend to project onto the negative meridional equatorial SSTA change associated with ENSO. The mode index, as seen in Fig. 3. part of the ENSO response linear with CTI is estimated So, while nonlinear ENSO projects strongly onto the by the sum of the weighted means (Fig. 6c); this ap- Paci®c meridional mode, the existence of this mode proximates the SST pattern extracted by the linear re- does not depend on ENSO. Once the nonlinear ENSO moval of the CTI. What is not removed by the CTIÐ in¯uence is largely removed, the resulting Paci®c me- that is, the nonlinear partÐis the difference between the ridional mode shares similar characteristics to the At- weighted means (Fig. 6d). The resemblance of the SSTA lantic meridional mode in both spatial and temporal and surface wind ®elds here to the Paci®c meridional characteristics. These similarities suggest, therefore, mode pattern is evident. The results here indicate that that the meridional modes have similar physical inter- positive ENSO events (El NinÄos) tend to have a stronger pretation in each basinÐan interpretation we explore amplitude in the equatorial Paci®c and a weaker am- in the next section.

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FIG. 5. Mode 1 SST expansion coef®cients from the Paci®c MCA of the entire record (top curve) for the ENSO neutral years only (middle curve, within the nonshaded regions) and for the extreme ENSO years only (middle curve, within the shaded regions), plotted together with the CTI (bottom curve). The months corresponding to the extreme ENSO years are denoted by the shading. Note that the expansion coef®cient for the entire record (top curve) is nearly identical to the expansion coef®cients for the neutral and extreme ENSO years combined (middle curve).

FIG. 6. A demonstration of the how ``nonlinear ENSO'' projects on the meridional mode. (a) The CTI-weighted mean of the nine high ctiNDJ years, and (b) the same for the nine low ctiNDJ years. Note that in (a) and (b) the mean SST and winds are divided by the sum of the ctiNDJ, which is negative for the ``low'' years, so that the polarity of (b) is opposite what would be expected from a negative ENSO event. (c) An estimate of ``linear'' ENSO, the sum of (a) and (b). (d) The nonlinear component of ENSO estimated as the difference between (a) and (b). (a)±(d) SST is contoured [contour interval (a) and (b) 0.4 K, in (c) 0.8 K, and (d) 0.2 K] and vectors denote 10-m winds. Solid contours denote positive SST anomalies, dashed contours denote negative SST anomalies, and the zero contour has been omitted.

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FIG. 7. Deviations of the Mar±May climatological SST and winds from the annual mean, showing their resemblance to the meridional mode patterns. To facilitate comparison with Fig. 1, the sign of every ®eld has been reversed, and the color scale offset so that white represents values of SST between Ϫ1.5 and Ϫ0.5 K. Note also that the SST color scale relative to the magnitude of the wind vector is the same as in Fig. 1. Hence, the magnitude of the wind vector can be directly compared to the strength of the SST gradient as indicated by the colors. The zero contour has been added as a solid line. Winds are plotted where the amplitude of the vector anomaly exceeds 1msϪ1.

4. Interpretation for a description) AGCM at standard T42 resolution, using different SST boundary conditions for each en- Following a previous lead (Mitchell and Wallace semble. The initial conditions in each ensemble were 1992), we examine the spatial characteristics of the sea- taken as different Januaries from a long simulation of sonal cycle in both the Paci®c and Atlantic basins. Fig- CCM3.10 forced by climatological SSTs. In the ®rst two ure 7 shows the deviation of the boreal spring (MAM) ensembles, we forced CCM3.10 with the positive and seasonal mean SST and surface winds from the annual negative polarity of the Paci®c meridional mode SST mean. Recall that the peak variance in the meridional map (shown in Fig. 1a), respectively. Similarly, the last mode SST expansion coef®cients occurs in this season. two ensembles were forced by the meridional mode SST The resemblance in the deep Tropics to the correspond- map in the Atlantic (Fig. 1b). We averaged the precip- ing meridional mode spatial patterns (cf. Fig. 1, top row) itation and lowest model-level wind output for each set is striking. This observation has been previously noted of eight ensemble runs, subtracted the mean of the ``neg- for the Atlantic (Mitchell and Wallace 1992). A similar ative polarity'' runs from the mean of the ``positive pattern results if we apply MCA to the annual cycle in tropical SST and winds (not shown). Note that the qual- polarity'' runs, and divided the result by 2. The result itative differences between the Paci®c and Atlantic me- (Fig. 8) shows the SST-forced response in the surface ridional mode spatial patterns are reproduced in the winds and precipitation. It clearly shows a deep tropical MAM deviation too: the more zonal equatorial ¯ow and surface wind response similar to that derived from the equatorially con®ned negative SST lobe in the Paci®c MCA analysis (Fig. 1), as well as a meridionally dis- as compared to the Atlantic. The point made here is that placed ITCZ. On the other hand, the northern subtrop- the meridional mode pattern emerges in different phys- ical wind anomalies present in the MCA mode 1 Paci®c ical situations and constitutes an independent extraction. and Atlantic patterns are conspicuously absent from the While it suggests that the physics of the meridional model anomaly ®eld. This suggests that the underlying mode is related to the physics of the seasonal cycle, we SST do not force the subtropical wind anomalies. do not necessarily interpret the former as a modulation We now interpret the anomalous trade winds to the of the latter. north of the mean ITCZ coincident with the northern How are the SST and surface wind patterns of the lobe of the meridional mode SST anomaly. Their ab- MCA mode 1 related? Previous modeling (Chang et al. sence in the AGCM simulations indicates that they are 2000; Chiang et al. 2001; Sutton et al. 2000) and ob- not forced by the underlying SST anomaly. This sug- servational (Chiang et al. 2002) work have suggested gests that they are either spurious [this is doubtful, given that the SST-forced surface wind response is limited to that the Atlantic (Paci®c) meridional mode expansions the deep Tropics, suggesting the same for the Paci®c coef®cients are signi®cantly correlated to the observed MCA mode 1 pattern. In order to determine the SST- NTA (NTP) surface wind anomalies] or they are forcing forced surface , we devised a rather than responding to the underlying SST anomalies. simple atmospheric general circulation model (AGCM) This latter interpretation is supported through lead±lag experiment. We ran four separate ensemble simulations analysis of zonal mean SST, wind, and heat ¯ux anom- (eight members in each ensemble) using the National alies, similar to the analysis done by Czaja et al. (2002) Center for Atmospheric Research Community Climate for the Atlantic. We created an annual index of winter- model, version 3.10 (CCM3.10; see Kiehl et al. 1998, time atmospheric circulation anomalies associated with

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FIG. 8. Results of the CCM3 model experiments with imposed SST anomalies from the leading MCA mode, showing the SST-forced component of the surface wind and precipitation associated with the meridional mode. Shown is one-half of the difference between the mean of an ensemble of eight simulations with the polarity of SST anomalies in Fig. 1, and the same but with SST anomalies of the opposite polarity. Vectors denote 995-mb winds (reference vector is 2 m s Ϫ1) and gray shading denotes precipitation anomalies in mm day Ϫ1. Winds are plotted only where signi®cant at the 95% con®dence level (based on a multivariate t test for both components of the wind). In general, shaded regions (precipitation) exceed the 95% con®dence level (based on a two-tailed t test). the MCA modes by averaging the December±January± dient anomalous surface ¯ow, indicating a positive feed- February (DJF) MCA wind expansion coef®cients, after back between the tropical meridional wind response and which we lag regressed this index against anomalies of the SST anomalies through boreal spring and into sum- SST, winds, and surface net heat ¯ux arranged by the mer. This feedback is, however, not readily apparent in month and zonally averaged across the domain of the the Paci®c, though we note that Vimont et al. (2003a) MCA analyses (Fig. 9). Extreme ENSO years (as de- ®nd a positive feedback between similar subtropical ®ned in the previous section) are not included in the SST anomalies and surface winds in coupled simula- Paci®c analysis. Recall that December±March are the tions of the Commonwealth Scienti®c and Industrial Re- months of maximum variance in the MCA wind ex- search Organisation (CSIRO) general circulation model. pansion coef®cients, while March±May are the months Where do the northern trade wind anomalies origi- of maximum variance of the SST expansion coef®cients. nate? To explore this question, we regressed the DJF In both basins, decreased northern subtropical trade wind expansion coef®cients on concurrent DJF sea level winds in DJF coincide with a decrease in surface ¯uxes pressure (SLP) anomalies (Fig. 10). The patterns strong- (primarily decreased latent heat ¯ux) out of the ocean ly resemble the NPO (Rogers 1981) and NAO (Hurrell and an increase in the SST there. This result suggests et al. 2002) pattern for the Paci®c and Atlantic cases, that the trade wind anomalies are forcing the northern respectively, suggesting that the northern trade wind subtropical SST anomalies. anomalies originate in large part from wintertime at- Figure 9 shows other interesting characteristics of the mospheric variability over their respective northern ba- meridional mode evolution in both basins. In the At- sins. This interpretation for variability of the northern lantic, the decay in the subtropical SST anomaly (around wintertime subtropical trades and their impact on the 20ЊN) past boreal spring is coincident with increased underlying SST through resulting latent heat ¯ux anom- ¯uxes out of the ocean. The increased ¯ux out of the alies has already been established for both the Paci®c ocean is not as clear in the Paci®c, though there are (e.g., Vimont et al. 2003a,b) and the Atlantic (e.g., Czaja very weak upward net heat ¯ux anomalies north of about et al. 2002; Xie and Tanimoto 1998), so we will not go 20ЊN (this may help explain the enhanced persistence into depth here. We stress that our analysis here does of SST anomalies in the Paci®c well into boreal summer not preclude other sources of forcing on the meridional and fall). In the Atlantic, the meridional near-equatorial mode; indeed, ENSO is known to force the Atlantic ¯ow in this analysis is coincident with the establishment meridional mode, but is ®ltered out of our analysis. of northern tropical SST anomalies and the meridional The interpretation of a purely unidirectional in¯u- SST gradient in the deep Tropics, consistent with our enceÐforcing from the northern extratropics, and re- interpretation of this ¯ow as an SST-forced response. sponse in the TropicsÐis likely too simplistic, given Likewise for the Paci®c anomalies, in boreal spring the possible feedback from the Tropics to the northern there is a clear tendency for a meridional wind to blow extratropical circulation. Our results showing the north- across the maximum SST gradient. In agreement with ern midlatitude atmospheric variability preceding the earlier work by Chang et al. (1997), there is a clear meridional mode event do suggest, however, that the indication in the deep Tropics of increased heat ¯uxes tropical meridional mode response can be initiated by into the ocean associated with the Atlantic cross-gra- wintertime northern midlatitude events, and that the pat-

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terns of the wintertime forcing resemble intrinsic modes of the midlatitude variability. The issue of feedback can- not be resolved purely from analysis of observational data, and GCM studies will be required; indeed, a feed- back mechanism has been proposed from a GCM study for the Atlantic case (Okumura et al. 2001); and recent work by Cassou et al. (2004) using the Action de Re- cherche Petite Echelle Grande Echelle (ARPEGE) at- mospheric general circulation model also suggests forc- ing on the NAO by SST anomalies in the north tropical Atlantic. On the other hand, we note that our ensemble GCM simulations, while responding strongly in the Tropics to the meridional mode SST anomalies, has SLP anomalies that do not project on the NPO (NAO) pattern in the Paci®c (Atlantic) (not shown). A detailed analysis of our model extratropical response to meridional mode SST anomalies is beyond the scope of our study; we simply note that there is possibility for Atlantic (Paci®c) tropical feedback on the NAO (NPO), but this knowl- edge requires more comprehensive analysis, and taking into account the sensitivity of this connection across different atmospheric models. The Southern Hemisphere does not appear to play much of a role in the mode 1 results of our MCA anal- ysis. The SST and surface wind amplitude in the south- ern lobe is generally much smaller in both the mode 1 spatial patterns (Fig. 1) and also in the lag regression of zonally averaged SST and winds onto the MCA 1 DJF wind expansion coef®cients (Fig. 9). Furthermore, the SLP regressed onto the DJF MCA 1 wind expansion coef®cients does not project signi®cantly on the south- ern Tropics (Fig. 10). The MCA 1 SST pattern in the Southern Hemisphere also appears to be different be- tween the two basins (Fig. 1), although we noted a sim- ilar difference with the MAM deviation of SST from the annual mean (Fig. 7). While it does not preclude signi®cant involvement by the Southern Hemisphere in our mode 1, establishing such involvement requires fur- ther investigation that is beyond the scope of our study. For our purposes, it is suf®cient to note that both the FIG. 9. Lead±lag regression of monthly mean, zonally averaged Atlantic and Paci®c MCA mode 1 have primarily North- anomalies onto the normalized DJF wind expansion coef®cient of the ern Hemisphere origins and with impact on the deep (top) Paci®c and (bottom) Atlantic MCA leading mode (see Fig. 1) Tropics. from Jul prior to the wind DJF through the Jan following. This follows a technique used by Czaja et al. (2002). Data are zonally averaged from 165Њ to 115ЊW in the Paci®c, and from 60ЊWto0Њ in the Atlantic. 5. Composite analysis A 3-month running mean is applied to the data, and the cold tongue index is linearly removed prior to analysis; also, extreme ENSO years As a ®nal step, we composite NCEP reanalyses (as de®ned in section 3) are not used in the Paci®c analysis. Colors monthly anomaly SST, surface wind, and surface pres- indicate SST anomalies, vectors are 10-m winds, and contours denote sure ®elds based on the high and low years of the DJF- net surface heat ¯ux (contour interval is 2 W mϪ2). The surface wind vectors are plotted where the magnitude of either component of the averaged wind expansion coef®cients of the meridional correlation vector exceeds 0.27 (the 95% con®dence level). Solid modes. For the Paci®c, we begin by excluding strong contours denote downward heat ¯ux anomalies (into the ocean), ENSO years (based on the same criteria as in section dashed contours denote upward heat ¯ux anomalies (out of the ocean), 3) from the analysis. Composites are taken around the and the zero contour has been omitted. In general, SST amplitudes greater than about 0.075ЊC (std dev)Ϫ1 and heat ¯ux amplitudes great- nine largest positive and negative values of the DJF- er than about 3 W mϪ2 (std dev)Ϫ1 exceed the 95% con®dence level. averaged wind expansion coef®cient (taken from the 35 ENSO neutral years). In the Atlantic, composites are taken around the nine largest positive and negative val- ues of the DJF-averaged wind expansion coef®cient

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FIG. 10. Simultaneous regression maps of DJF sea level pressure onto the standardized DJF wind expansion coef®cients for (a) the Paci®c MCA mode 1 and (b) the Atlantic MCA mode 1, suggesting that the origins of the north tropical Paci®c and Atlantic trade wind forcing originates from the NPO and NAO, respectively. Contour interval is 0.4 mb (std dev) Ϫ1. Light shading denotes regions where the correlation coef®cient exceeds the 95% con®dence level.

(taken from the full 53 years available). For each basin, tures and physical mechanisms in the Paci®c (Fig. 11) we compute means of the high and low years for each and Atlantic (Fig. 12) basins is striking. 3-month interval from the previous September±Novem- The composites for DJF, MAM, and JJA remain es- ber through June±August, take their difference, and di- sentially the same if we use the MAM SST expansion vide by 2. A two-sided t test shows signi®cance of the coef®cients instead of the DJF wind coef®cients to cre- difference in the means. The important thing to note ate the composites. The Paci®c composite for the pre- here is that we use the ``full'' anomaly data without ceding September±October±November (SON) period linear removal of the CTI in this analysis, as we want using the MAM SST expansion coef®cients, however, to show that the Paci®c meridional mode pattern comes does change in this fashion: while the SON SST com- out even in the absence of this ®lter. posite pattern resembles the Paci®c SON pattern in Fig. The composites (Fig. 11 for the Paci®c, Fig. 12 for 12, the peak SST amplitudes in the pattern increases by the Atlantic) show the evolution of the Paci®c and At- around 0.2 K in the central Paci®c warm patch, and lantic meridional modes for SST and surface wind these SST differences survive the signi®cance test. In anomalies (Figs. 11 and 12, left) and sea level pressure other words, there is a suggestion of a precursor event anomalies (Figs. 11 and 12, right). All that we had said in the SON equatorial Paci®c SST prior to the peak before is apparent in these composites: the northern mid- MAM period. We think this difference comes about be- latitude circulation anomalies in boreal winter led to cause the SST expansion coef®cients preferentially altered trade winds in the northern tropical region, and picks years with stronger ENSO events (while our com- anomalous SST therein that maximizes in boreal spring. posite excludes strong ENSO years, it does not remove The SST-driven meridional circulation is coincident all years with ENSO events, obviously), more so that with the maximum meridional gradient in SST anom- the wind expansion coef®cients and the ``precursor'' alies; and the northern tropical SST anomalies begin to SST in the central Paci®c are part of the nonlinear ENSO decay by boreal summer. The composites also hint at signature [recall that the spatial pattern of MCA mode similarities not mentioned before, in particular the equa- 1 in Fig. 4b, which was constructed using only extreme torial southerlies in the western portion of the Paci®c ENSO years, shows a sizable SST amplitude in the cen- and Atlantic basins in June±August (JJA). Though there tral equatorial Paci®c that is absent in the MCA mode are also notable differences in the evolution of the me- 1 using neutral ENSO years (Fig. 4a)]. Given this result, ridional modes in the two basinsÐin particular, the we leave open the possibility of a tropical precursor in southeastern tropical ocean response differs between the our interpretation of the Paci®c and Atlantic meridional basinsÐthe overall similarity between the spatial struc- modes. However, establishing the existence of precur-

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FIG. 11. Composite analysis of the Paci®c meridional mode based on positive and negative values of the DJF wind expansion coef®cients of MCA mode 1. Only neutral ENSO years (as described in section 3) are used. The composites are in 3-month averages from Sep±Nov through the following Jun±Aug (thus, the ®rst row indicates variability that occurs 6 months before the peak of the MAM SST expansion coef®cient, and so forth). The maps are generated by subtracting the mean of the ``negative'' years from the mean of the ``positive'' years, and dividing by 2. (left column) SST (shaded) and surface wind (reference vector 1 m s Ϫ1) differences. Regions where the SST is statistically signi®cant at the 95% level are enclosed by the black contour line. For graphical purposes only, wind contours are plotted only where the F value exceeds the 80% level based on a multivariate t test. (right column) Sea level pressure (contour interval 0.4 mb) differences. Signi®cant regions (based on 95% con®dence of a two-sided t test) are lightly shaded. Solid contours denote positive anomalies, dashed contours denote negative anomalies, and the zero contour has been omitted.

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FIG. 12. Same as in Fig. 11 but for the Atlantic meridional mode. Unlike the Paci®c composite, the positive and negative composite years here are taken from the full 53 yr of the available data.

Unauthenticated | Downloaded 09/30/21 06:48 PM UTC 4156 JOURNAL OF CLIMATE VOLUME 17 sors requires further elucidation that is beyond the scope SST variability there through modulation of latent heat of this paper (we note also another hint of an Atlantic ¯ux, thus giving rise to the near-equatorial meridional meridional mode precursor in the Fig. 12 SON SST SST gradient in boreal spring; (iv) it did not appear, anomaly map in the western midlatitude Atlantic, al- based on a ensemble GCM experiment, that the tropical though the area represented is very small). Our purpose response fed back onto the trade wind forcing. We ad- is to point out the similarities between the Paci®c and ditionally showed evidence that the anomalous trades the Atlantic, and to this end we believe the composites were associated with NAO and NPO wintertime mid- reinforce this comparison. latitude atmospheric variability in the Atlantic and Pa- ci®c, respectively, though this had been previously es- tablished. 6. Summary and discussion What are the potential implications of this study? The tropical Atlantic has a well-documented ``merid- First, we note a pleasing symmetry that the NAO and ional'' mode of interannual±decadal atmosphere±ocean NPO are leading modes of wintertime atmospheric var- variability, distinct from the ``zonal'' ENSO-like mode. iability (Hsu and Wallace 1985) that can drive the me- Several factors motivated us to look for an analogous ridional modes in their respective basins. This leads us mode of variability in the tropical Paci®c, which has to postulate that the meridional mode is an effective heretofore not been discovered: the fact that the two conduit for extratropical atmospheric in¯uence on the basins share similar ITCZ/cold tongue mean state cli- Tropics. We think this is the consequence of two par- mates and possess qualitatively similar midlatitude win- ticular features of the atmosphere±ocean climate: the tertime variability (NAO and NPO); also note the sug- ability of the wintertime midlatitude variability to leave gestion by idealized model studies that this variability an anomalous ``footprint'' on surface ocean temperature should exist as part of the intrinsic variability of the (e.g., Vimont et al. 2001), and of the tropical atmosphere ITCZ/cold tongue climate. To proceed, we employed to be sensitive to relatively small (meridional) SST gra- MCA analysis to ®nd coupled modes of variability be- dients (e.g., Chiang et al. 2002). If this is indeed correct, tween SST and surface wind in climatically equivalent it means that the midlatitude atmospheric in¯uence has regions of both tropical basins. The MCA had been to be given serious consideration in tropical marine cli- employed successfully in previous studies (e.g., Chang mate variability. In particular, should the meridional et al. 1997) to extract the Atlantic meridional mode. mode event forced by the midlatitudes be picked up by Because we were not interested in ENSO (though ENSO the Bjerknes feedback in that basin, it could imply sig- dominates tropical Paci®c variability), we removed ni®cant midlatitude control of ENSO and Atlantic NinÄo ENSO from all ®elds prior to the analysis through linear events. Indeed, such scenarios have already been pro- regression onto CTI. posed and explored (Servain et al. 1999; Vimont et al. The dominant mode in the Atlantic was indeed the 2003b) with potentially important implications for trop- meridional mode, and the Paci®c mode 1 resembled the ical climate prediction. Atlantic meridional mode in both spatial and temporal There is no intrinsic time scale associated with the characteristics. A notable exception, however, was the meridional mode as described here, beyond what is im- substantially larger temporal persistence in the Paci®c plied by the ocean surface mixed-layer thermodynamics mode 1 expansion coef®cients relative to the Atlantic. and the adjustment time of the atmosphere. The merid- We realized, however, that the ENSO nonlinearity (the ional mode physics should be equally applicable to part of ENSO evolution not taken out by linear regres- mean state climate changes and to climate variability. sion on the CTI) was projecting on the Paci®c merid- In particular, if something causes (say) the northern At- ional mode during extreme ENSO events. By repeating lantic trade wind intensity to change, the tropical At- the MCA without the extreme ENSO years, we showed lantic climate and in particular the mean position of the that the original Paci®c MCA mode 1 pattern derived Atlantic ITCZ should change accordingly. This scenario was in fact robust, and that furthermore without the has been proposed to explain the apparent southward extreme ENSO years, the temporal persistence of the shift in the mean position of the tropical Atlantic ITCZ Paci®c MCA mode reduced to the same levels as that during the Last Glacial Maximum (LGM) 21 000 yr ago for the Atlantic meridional mode. (Chiang et al. 2003). The mechanism for the northern We tentatively labeled the Paci®c MCA 1 the Paci®c trade wind increase during LGM is through perturbation meridional mode and proceeded to show that it had the of the North Atlantic stationary wave circulation same physical interpretation as the Atlantic meridional brought about by the presence of the Laurentide ice mode. In particular, we showed that (i) the deviation of sheet over North America. boreal spring climate from the annual mean climate in It could still be argued that the Paci®c mode we obtain both basins had spatial patterns that resembled the me- is a result of statistical artifactÐperhaps there is still ridional mode in the respective basins; (ii) that the near- nonlinearity in ENSO that has not been accounted for. equatorial surface ¯ow across the meridional SST gra- While in principle we acknowledge that possibility, dient was coupled to the SST gradient; (iii) the anom- there is one compelling and general argument we can alous trades in the northern tropical region drove the make against it: that the striking similarity of the Paci®c

Unauthenticated | Downloaded 09/30/21 06:48 PM UTC 1NOVEMBER 2004 CHIANG AND VIMONT 4157 mode to the well-established Atlantic meridional mode northeast Brazil rainfall anomalies. J. Geophys. Res., 98D, 5093± in both spatial and temporal characteristics, and in their 5102. Hoerling, M. P., A. Kumar, and M. Zhong, 1997: El NinÄo, La NinÄa, interpretation, makes it unlikely that our result arises and the nonlinearity of their . J. Climate, 10, from statistical artifact. Our interpretationÐthat they 1769±1786. arise from analogous processes given similar mean Houghton, R. W., and Y. M. Tourre, 1992: Characteristic low-fre- ITCZ/cold tongue climatesÐis simpler and more gen- quency sea surface temperature ¯uctuations in the tropical At- lantic. J. Climate, 5, 765±771. eral. Hsu, H. H., and J. M. Wallace, 1985: Vertical structure of wintertime patterns. J. Atmos. Sci., 42, 1693±1710. Acknowledgments. Special thanks to J. M. Wallace Hurrell, J. W., Y. Kushnir, G. Ottersen, and M. Visbeck, 2002: An for his advice and encouragement. We also thank M. overview of the North Atlantic Oscillation. The North Atlantic Biasutti, A. Czaja, C. Deser, S. Hastenrath, Y. Kushnir, Oscillation: Climatic Signi®cance and Environmental Impact, Geophys. Monogr., No. 134, Amer. Geophys. Union, 1±35. and S.-P. Xie for useful discussions and comments. This Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Re- research is funded by the NOAA Postdoctoral Program analysis Project. Bull. Amer. Meteor. Soc., 77, 437±471. in Climate and Global Change, administered by the Uni- Kaplan, A., M. A. Cane, Y. Kushnir, A. C. Clement, M. B. Blumen- versity Corporation for Atmospheric Research, and by thal, and B. Rajagopalan, 1998: Analyses of global sea surface the Joint Institute for the Study of the Atmosphere and temperature 1856±1991. J. Geophys. Res., 103C, 18 567±18 589. Kiehl, J. T., J. J. Hack, G. B. Bonan, B. A. Boville, D. L. Williamson, Ocean (JISAO) under NOAA Cooperative Agreement and P. J. Rasch, 1998: The National Center for Atmospheric NA17RJ1232. The authors contributed equally to this Research Community Climate Model: CCM3. J. Climate, 11, work. 1131±1149. Kistler, R., and Coauthors, 2001: The NCEP±NCAR 50-year reanal- ysis: Monthly means CD-ROM and documentation. Bull. Amer. REFERENCES Meteor. Soc., 82, 247±267. Kushnir, Y., R. Seager, J. Miller, and J. C. H. Chiang, 2002: A simple Bretherton, C. S., C. Smith, and J. M. Wallace, 1992: An intercom- coupled model of tropical Atlantic decadal climate variability. parison of methods for ®nding coupled patterns in climate data. Geophys. Res. Lett., 29, 2133, doi:10.1029/2002GL015874. J. Climate, 5, 541±560. Legler, D. M., and J. J. O'Brien, 1988: Tropical Paci®c wind stress Cassou, C., L. Terray, J. W. Hurrell, and C. Deser, 2004: North At- analysis for TOGA, IOC time series of ocean measurements. lantic winter climate regimes: Spatial asymmetry, stationarity IOC Tech. Series 33, Vol. 4, UNESCO, 11±17. with time, and oceanic forcing. J. Climate, 17, 1055±1068. Lindzen, R. S., and S. Nigam, 1987: On the role of sea-surface tem- Chang, P., L. Ji, and H. Li, 1997: A decadal climate variation in the perature gradients in forcing low-level winds and convergence tropical from thermodynamic air±sea interac- in the Tropics. J. Atmos. Sci., 44, 2440±2458. tions. Nature, 385, 516±518. Mehta, V. M., and T. Delworth, 1995: Decadal variability of the ÐÐ, R. Saravanan, L. Ji, and G. C. Hegerl, 2000: The effect of local tropical Atlantic Ocean surface temperature in shipboard mea- sea surface temperatures on the atmospheric circulation over the surements and in a global ocean±atmosphere model. J. Climate, tropical Atlantic sector. J. Climate, 13, 2195±2216. 8, 172±190. Chiang, J. C. H., S. E. Zebiak, and M. A. Cane, 2001: Relative roles Mitchell, T. P., and J. M. Wallace, 1992: The annual cycle in equatorial of elevated heating and surface temperature gradients in driving convection and sea surface temperature. J. Climate, 5, 1140± anomalous surface winds over tropical oceans. J. Atmos. Sci., 1156. 58, 1371±1394. Nobre, P., and J. Shukla, 1996: Variations of sea surface temperature, ÐÐ, Y. Kushnir, and A. Giannini, 2002: Deconstructing Atlantic wind stress, and rainfall over the tropical Atlantic and South ITCZ variability: In¯uence of the local cross-equatorial SST America. J. Climate, 9, 2464±2479. gradient, and remote forcing from the eastern equatorial Paci®c. Okumura, Y., S.-P.Xie, A. Numaguti, and Y. Tanimoto, 2001: Tropical J. Geophys. Res., 107, 4004, doi:10.1029/2000JD000307. Atlantic air±sea interaction and its in¯uence on the NAO. Geo- ÐÐ, M. Biasutti, and D. S. Battisti, 2003: Sensitivity of the Atlantic phys. Res. Lett., 28, 1507±1510. Intertropical Convergence Zone to Last Glacial Maximum Rogers, J. C., 1981: The North Paci®c Oscillation. J. Climatol., 1, boundary conditions. Paleoceanography, 18, 1094, doi:10.1029/ 39±57. 2003PA000916. Ruiz-Barradas, A., J. A. Carton, and S. Nigam, 1999: Structure of Curtis, S., and S. Hastenrath, 1995: Forcing of anomalous sea-surface interannual-to-decadal climate variability in the tropical Atlantic temperature evolution in the tropical Atlantic during Paci®c sector. J. Climate, 12, 1±43. warm events. J. Geophys. Res., 100C, 15 835±15 847. Servain, J., 1991: Simple climatic indices for the tropical Atlantic Ocean Czaja, A., P. van der Vaart, and J. Marshall, 2002: A diagnostic study and some applications. J. Geophys. Res., 96, 15 137±15 146. of the role of remote forcing in tropical Atlantic variability. J. ÐÐ, I. Wainer, J. P. McCreary, and A. Dessier, 1999: Relationship Climate, 15, 3280±3290. between the equatorial and meridional modes of climatic vari- Dommenget, D., and M. Latif, 2000: Interannual to decadal variability ability in the tropical Atlantic. Geophys. Res. Lett., 26, 485±488. in the tropical Atlantic. J. Climate, 13, 777±792. Sutton, R. T., S. P. Jewson, and D. P. Rowell, 2000: The elements of En®eld, D. B., and A. M. Mestas-Nunez, 1999: Multiscale variabil- climate variability in the tropical Atlantic region. J. Climate, 13, ities in global sea surface temperatures and their relationships 3261±3284. with tropospheric climate patterns. J. Climate, 12, 2719±2733. Vimont, D. J., D. S. Battisti, and A. C. Hirst, 2001: Footprinting: A ÐÐ, ÐÐ, D. A. Mayer, and L. Cid-Serrano, 1999: How ubiquitous seasonal connection between the Tropics and mid-latitudes. Geo- is the dipole relationship in tropical Atlantic sea surface tem- phys. Res. Lett., 28, 3923±3926. peratures? J. Geophys. Res., 104C, 7841±7848. ÐÐ, ÐÐ, and ÐÐ, 2003a: The seasonal footprinting mechanism Giannini, A., Y. Kushnir, and M. A. Cane, 2000: Interannual vari- in the CSIRO general circulation models. J. Climate, 16, 2653± ability of Caribbean rainfall, ENSO, and the Atlantic Ocean. J. 2667. Climate, 13, 297±311. ÐÐ, J. M. Wallace, and D. S. Battisti, 2003b: The seasonal foot- Hastenrath, S., and L. Heller, 1977: Dynamics of climatic hazards in printing mechanism in the Paci®c: Implications for ENSO. J. northeast Brazil. Quart. J. Roy. Meteor. Soc., 103, 77±92. Climate, 16, 2668±2675. ÐÐ, and L. Greischar, 1993: Circulation mechanisms related to Wallace, J. M., E. M. Rasmusson, T. P. Mitchell, V. E. Kousky, E.

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S. Sarachik, and H. von Storch, 1998: The structure and evo- ÐÐ, and Y. Tanimoto, 1998: A pan-Atlantic decadal climate oscil- lution of ENSO-related climate variability in the tropical Paci®c: lation. Geophys. Res. Lett., 25, 2185±2188. Lessons from TOGA. J. Geophys. Res., 103C, 14 241±14 259. ÐÐ, and K. Saito, 2001: Formation and variability of a northerly Xie, P. P., and P. A. Arkin, 1997: Global precipitation: A 17-year ITCZ in a hybrid coupled AGCM: Continental forcing and oce- monthly analysis based on gauge observations, satellite esti- anic±atmospheric feedback. J. Climate, 14, 1262±1276. mates, and numerical model outputs. Bull. Amer. Meteor. Soc., Yukimoto, S., M. Endoh, Y. Kitamura, A. Kitoh, T. Motoi, and A. 78, 2539±2558. Noda, 2000: ENSO-like interdecadal variability in the Paci®c Xie, S.-P., 1999: A dynamic ocean±atmosphere model of the tropical Ocean as simulated in a coupled general circulation model. J. Atalantic decadal variability. J. Climate, 12, 64±70. Geophys. Res., 105C, 13 945±13 963.

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