Revised and Resubmitted to Journal of Climate November 2005 Abstract




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Some of the major progress made in Pacific ENSO and its decadal changes during the initial phase of CLIVAR can be briefly summarized as follows:
  1. Further progress has been made in the fundamental understanding of ENSO physics, particularly in the area concerning the effect of atmospheric internal variability (both in the tropics and extratropics) on the development and maintaince of ENSO. This has led to a better understanding of how atmospheric internal variability can limit ENSO predictability.
  2. Although many issues remain unresolved, the early years of CLIVAR have witnessed a number of advances in documenting decadal changes in ENSO and in understanding the underlying processes of these changes. A wide range of theoretical frameworks have been proposed to understand the dynamics of the low frequency variability of ENSO, including stochastic mechanism, mean state changes, anthropogenic forcing and nonlinear dynamics.
  3. The general concept of the STCs has emerged as a useful framework for understanding the potential role of the ocean in decadal changes of ENSO, because the maintenance of the equatorial thermocline is closely linked to the STC dynamics. Decadal changes in the STCs have been demonstrated by both observational and modeling studies.

With the initial progress, there are still fundamental gaps in our understanding of ENSO dynamics and decadal changes in ENSO. Some of the key remaining issues and major uncertainties in this area of research are:


  1. There is still a lack of complete understanding of the factors that limit the predictability of ENSO. The intrinsic limit of ENSO predictability remains uncertain,

  2. Quantifying the cause of low frequency variability of ENSO has proven to be very difficult, because of insufficient observations and imperfect climate models. As a result, large uncertainties exist in assessing low-frequency changes in ENSO induced by anthropogenic climate change or by natural climate variability. There is little current understanding of the predictability associated with the decadal variation of ENSO.

  3. Our current understanding of the role of the ocean in decadal changes of ENSO is incomplete. Some of key elements concerning the STC variation and its connection to the atmosphere are still missing.



3. Tropical Atlantic Variability (TAV)


The dominant climate fluctuations in the tropical Atlantic sector seem to include two distinctive patterns associated with the year-to-year variation in the annual migration of the Atlantic marine ITCZ complex. These two patterns manifest themselves most clearly during the two seasons when the Atlantic ITCZ moves furthest south during the boreal spring (March-April) and furthest north during the boreal summer (June-August):


During boreal spring the warmest SSTs appear in the deep tropics and maximum seasonal precipitation moves to the western equatorial Atlantic and the adjacent land region of tropical South America. During this season the anomaly of rainfall from its seasonal cycle is characterized by a dipolar pattern across the thermal equator, as shown by the leading EOF of the March-April rainfall anomaly (Fig. 7).


Correlated with this precipitation anomaly is an anomalous meridional SST gradient and cross-equatorial winds. These correlations reflect a dynamically consistent situation where a stronger than normal northward SST gradient drives northward cross-equatorial winds with weaker than normal trades in the north and stronger than normal trades in the south. Precipitation and SST are below normal on the southern flank of the climatological ITCZ position and above normal to the north. This circulation pattern implies a weakening and northward shift of the ITCZ towards the warmer hemisphere. Because the coupled variability in this pattern exhibits a north-south contrast, it is often refered to as the “meridional mode” of variability (Servain et al. 1999). In passing, we note a similar meridional mode has been identified in the tropical Pacific which also has peak variability during the boreal spring (Chiang and Vimont, 2004).


The SST anomaly associated with the meridional mode also forces precipitation anomalies on neighboring continents. Indeed, Wallace et al (1998) assert that the link between rainfall over northeast Brazil and the meridional mode is the most robust signal outside of tropical Pacific Ocean. For more discussion of the impact of the meridional mode, readers are refered to Marshall et al. (2001) and Xie and Carton (2004).


During boreal summer a cold tongue of SST develops in the equatorial eastern Atlantic Ocean, while the ITCZ moves to its northernmost position, extending over the adjacent land region of West Africa. The leading EOF of precipitation during this season shows an anomaly pattern that is maximum along the northern coast of the Gulf of Guinea. The corresponding anomalous pattern of SST is maximum in the eastern basin with a convergent pattern of equatorial trade winds (Fig. 7).


The coupled pattern described above bears a certain resemblance to the Pacific ENSO, it is often referred to as the “Atlantic Niño” mode or the “equatorial mode” (Merle 1980; Hisard 1980; Zebiak 1993; Carton and Huang 1994; Ruiz-Barradas 2000). Here, we simply refer it to as the “zonal mode’’ of the tropical Atlantic. However, it is important to note that the relationship of the zonal mode to the annual cycle of SST differs markedly from ENSO. The differences, discussed below, hint that the processes involved may be quite different, despite of the similarity in their appearance. It is also worth pointing out the distinction between the rainfall pattern associated with this coupled mode and the rainfall variability over the semi-arid Sahel between 10°N and 20°N. The former has an interannual timescale and owes its existence largely to equatorial SST variability, while the latter varies on interdecadal timescales and represents an internal mode of the African summer monsoon variability (Giannini et al 2003).


South of the equator along the western coast of Africa warm and cold episodes occur that are confined to the coastal zone of Angola and Namibia (Shannon et al. 1986; Gammelsrød et al. 1998, Florenchie, et al., 2004). These episodes, referred to as Benguela Niños/Niñas, occur following some, but not all extrema of the equatorial zonal mode (for example, in year 1984) and have an effect on regional rainfall variability (Rouault et al. 2003) and fish distribution and abundance (Boyer et al. 2001).

a. Coupled air-sea feedbacks in TAV


1) THERMODYNAMIC FEEDBACKS


The meridional mode involves off-equatorial SST changes. These SST changes are intimately linked to changes in surface heat fluxes, particularly latent heat flux, as shown by a large number of observational and modeling studies (e.g. Carton et al. 1996). This linkage gives rise to the Wind-Evaporation-SST (WES) feedback mechanism (Xie and Philander 1994) which is inherently a tropical mechanism. The feedback works as follows: Imagine that a small change in hemispheric SST gradient is introduced. The atmosphere will respond with a change in meridional pressure gradient through hydrostatic adjustment of the atmospheric boundary layer (Lindzen and Nigam 1987; Hastenrath and Greischar 1993) as well as mid-tropospheric diabatic heating, which together drive a cross-equatorial boundary layer flow, changing the meridional position of maximum surface wind convergence and therefore the ITCZ. The anomalous cross-equatorial flow is deflected by the Coriolis force in the Southern and Northern hemispheres in such a way that it increases the wind speed over the hemisphere where a negative SST anomaly exists, cooling it further through surface evaporation, and it decreases the wind speed over the hemisphere where a positive SST anomaly exists, warming it further. The net effect is a positive feedback on the original SST anomaly (Chang et al. 1997). The weakness of the feedback (a few W/m2) implies that this mechanism must have long timescales. In the tropical Atlantic, the WES feedback mechanism is a defining characteristic of the meridional mode.


There is growing observational evidence supporting WES feedback in the development phase of the meridional mode (Ruiz-Barradas et al. 2000; Chiang et al. 2002; Czaja et al. 2002; Kushnir et al. 2002; Frankignoul et al. 2004). Atmospheric GCMs have also been used to test the WES hypothesis by examining the response of the GCMs to SST forcing (Chang et al. 2000; Sutton et al. 2000; Okumura et al. 2001; Terray and Cassou 2002). Nearly all the GCMs show a cross-equatorial circulation pattern in response to a hemispheric SST gradient in the deep tropics, as in the observations. However, the surface heat flux response is more perplexing. While some models show a positive thermodynamic feedback in the deep tropics (e.g., Chang et al. 2000; Saravanan and Chang 2004), others show a negative feedback (e.g., Sutton 2000). Wang and Carton (2003) and Frankignoul et al. (2004) compared different atmospheric and coupled GCM simulations of TAV and discuss the difficulties faced by current GCMs in reproducing the WES feedback. Besides WES, there seem to be additional feedback mechanisms that act between SST and clouds (Wang and Enfield 2001; Tanimoto and Xie 2002). The lack of reliable surface heat flux measurements has prevented a rigid test of these hypotheses.


2) DYNAMIC FEEDBACKS


In contrast to the thermodynamic meridional mode, the underlying feedback in the zonal mode, like the Pacific ENSO mode, is thought to be the dynamical Bjerknes mechanism (Bjerknes 1969). In contrast to ENSO, however, compelling evidence is less forthcoming. This is partly because the Atlantic signal is much weaker and less coherent than Pacific ENSO. The first well-documented SST-wind relationship was probably provided by Merle (1980) for the 1963 warm event, one of strongest warm events on the record. The connection between changes in the trade winds and SST was confirmed by Servain et al. (1982) in a statistical analysis. The warm event that occurred in 1984 also received considerable attention because it coincided with the SEQUAL/FOCAL program which provided an extensive array of subsurface ocean measurements. The studies that focused on this event (discussed later) revealed some similarity in SST, winds, convection, and upper ocean circulation anomalies to those during an El Niño. But again the short observational record prevented a robust analysis of the relationship between SST and the atmospheric response.


A comprehensive investigation was carried out by Zebiak (1993) who contrasted the statistical relationship between the surface wind stress and SST for Pacific ENSO to that for the Atlantic zonal mode using historical observational data sets. The study showed that although there is a common overall SST-wind relationship between the two oceans in the sense that for either ocean, the appearance of coherent warm (cold) SST anomalies coincides with westerly (easterly) changes in zonal wind to the west, there are important differences (Fig. 8). These differences include 1) the correlation between the zonal wind and equatorial SST anomaly is considerably lower in the tropical Atlantic than in the Pacific (0.4 vs. 0.7), while the correlation between the meridional wind in the vicinity of the ITCZ and equatorial SST anomaly remains comparable in both oceans (0.5 vs 0.6); 2) the spatial correlation structure between the zonal wind and SST anomaly is narrower in the tropical Atlantic than in the tropical Pacific; 3) the coherence between equatorial and eastern coastal SST is much less in the Atlantic and the zonal wind variability is displaced proportionally farther to the west in the basin than in the Pacific; 4) while in the Pacific, the ENSO related SST anomalies in the eastern and central basin tend to vary out of phase with those in the western basin, the SST anomalies associated with the Atlantic zonal mode tend to vary together at all longitudes with nearly equal amplitude (although an eastward shift of thermocline water is evident); 5) in the Pacific, the zonal wind anomaly appears to migrate eastward during an ENSO event, whereas in the Atlantic, there is no migratory component to the zonal wind variability and the strongest variability is situated in the western portion of the basin as opposed to the interior basin in the Pacific ENSO. The more recent study by Ruiz-Barradas et al. (2000) employed a joint ocean-atmosphere analysis based on NCEP and SODA reanalysis data sets to further validate the statistical relationship revealed by Zebiak (1993).


Recent atmospheric GCM studies (Chang et al. 2000; Sutton et al. 2000; Okumura and Xie 2004; Frankignoul et al. 2004) show that during boreal summer the atmosphere does respond to equatorial Atlantic SST anomalies in a manner that is consistent with the Bjerknes feedback, even though the details are different in different models. These studies further confirmed that in spite of the relatively weak SST perturbations in the equatorial Atlantic, the atmosphere is capable of generating a wind response that can potentially trigger a positive feedback along the equatorial waveguide following the Bjerknes mechanism. However, whether such a positive feedback can sustain itself depends on whether the subsurface ocean response is strong enough and whether this subsurface response can induce a sizable change in SST. This oceanic aspect of the Bjerknes feedback in the tropical Atlantic is less well-established.


b. External sources of atmospheric influences


Because the coupled feedbacks involved in TAV are much weaker than those in ENSO, external influences are likely to play an important role. At least two external sources of influences on the meridonal mode have been proposed: Pacific ENSO and the NAO. Both of these phenomena peak during the boreal winter. One common mechanism through which they exert their influences on TAV is by altering the strength of the northeasterly trade winds in the northern Tropical Atlantic, which in turn causes changes in the surface latent and sensible heat fluxes. The altered heat flux then forces the ocean mixed layer to produce a maximum SST response in the boreal spring.


1) REMOTE INFLUENCE OF ENSO


The existence of a statistical relation in boreal spring between ENSO and the tropical Atlantic has been noted for some time now (e.g., Covey and Hastenrath 1978; Aceituno 1988; Enfield and Mayer 1997; Giannini et al. 2000; Mestas-Nunes and Enfield 2001; Alexander and Scott 2002). The most significant influences of El Niño in the tropical Atlantic sector include: 1) a zonal seesaw in sea level pressure between the eastern equatorial Pacific and Atlantic Oceans during the onset and peak phase of ENSO, with a high sea level pressure anomaly in the northern tropical Atlantic, 2) a weakening in the meridional sea level pressure gradient between the North Atlantic subtropical high and the ITCZ accompanied by weaker-than-average northeasterly trades, 3) a warming of SST during boreal spring following the mature phase of ENSO, and 4) a northward shift of the ITCZ and a decrease of rainy season precipitation in northeastern Brazil. Some of these features are shown in Fig. 9a. However, the detailed dynamical processes responsible for setting up this ENSO remote influence are still not entirely clear. Among the proposed mechanisms are a tropical mechanism via anomalous Walker circulation (Klein et al. 1999; Giannini et al. 2000; Saravanan and Chang 2000; Chiang and Sobel 2002; Huang et al. 2002) and an extratropical mechanism via a stationary Rossby Wave train teleconnection pattern (e.g., Wallace and Gutzler 1981; Horel and Wallace 1981; Hoskins and Karoly 1981; Giannini et al. 2000).


A more recent study by Giannini et al. (2004) suggests that ENSO does not passively exert its influence on the tropical Atlantic, but rather interferes actively with coupled air-sea feedbacks in the region. When, in seasons prior to the mature phase of ENSO, the air-sea feedback in the tropical Atlantic happens to be evolving consistently with the expected development of the ENSO teleconnection, ENSO and TAV act in concert to force large climate anomalies in the region. When it happens to be evolving in opposition to the canonical development of ENSO, then the net outcome is less obvious and less predictable. Giannini et al. call this the “preconditioning” role of TAV in the development of the ENSO teleconnection. These constructive and destructive interferences between ENSO and TAV have been shown by Barriero et al. (2004) to have an important consequence on the seasonal predictability of boreal spring climate condition in the tropical Atlantic sector. For example, during the years when the existing Atlantic conditions reinforce the remote influence of ENSO (constructive interference), the boreal spring climate anomaly can be forecasted skillfully up to two seasons in advance.


In terms of ENSO's influence on the zonal mode, reports from existing literature are inconsistent. On the one hand, some observational analysis (e.g. Zebiak 1993, Enfield and Mayer 1997) finds no statistical evidence for linkage between the SST variability of Pacific ENSO and the Gulf of Guinea, suggesting that the coupled variability in the equatorial Atlantic is largely independent of ENSO. On the other hand, the modeling study by Latif and Barnett (1995) suggests that Pacific ENSO does influence the eastern equatorial Atlantic through an adjustment of the entire tropical Walker cell. There is some evidence that certain events in the equatorial Atlantic are indeed linked to strong ENSO events. For example, Delecluse et al. (1994) and Carton and Huang (1994) showed that the strong 1984 warming in the eastern equatorial Atlantic resulted from the zonal wind anomaly related to the severe 1982-83 ENSO.


One reason that the zonal mode appears to be less affected by the remote influences than the meridional mode may be related to the different seasons to which these two phenomena are phase-locked. The fact that the equatorial mode appears primarily in the boreal summer makes it difficult for ENSO and the NAO to have a direct influence on it, because both of these phenomena peak in the boreal winter. This, however, raises the question whether the zonal mode is related to the variability in the Southern Hemisphere. Only a few recent studies (Venegas et al. 1997; Wu and Liu 2002; Sterl and Hazeleger 2003; Barriero et al. 2004) have begun to investigate this issue. Some of the initial findings suggest that the southern summer atmospheric variability (and to a less extent the winter variability) can play a pre-conditioning role in the onset of the anomalies in the deep tropics during the following austral fall.


2) REMOTE INFLUENCE OF THE NAO


The notion that the NAO can have an impact on TAV can be traced back to at least Namias (1972) who pointed out that increased cyclonic activity in the Newfoundland area is related to abundant rainfall in the Nordeste. Studies that followed show that the NAO is correlated with a tripole pattern of SST anomalies over the North Atlantic in boreal winter/spring. This pattern of SST arises primarily from the oceanic response to month-to-month atmospheric fluctuations associated with the NAO (e.g., Seager et al. 2000). The southernmost lobe of the SST tripole reaches down into the subtropics/tropics of the north Atlantic, potentially affecting the coupled variability in the deep tropics. There appears to be little dispute that the NAO is one major source of external influence on TAV and the former exerts its influence on the latter through modulating the intensity of the semi-permanent subtropical high pressure system centered around 40ºN over the North Atlantic, which in turn affects the northeasterly trades and the latent heat flux at the ocean surface (Fig. 9b) (Xie and Tanimoto 1998; Chang et al. 2001, Czaja et al. 2002, and Kushnir et al. 2002).


There is, however, disagreement on the extent to which the NAO can affect TAV:

Is the deep tropical variability simply a mirror image of the changes in the strength of the subtropical high? Or does the NAO merely play a role in exciting TAV whose evolution is also influenced by the coupled feedback in the deep tropics? Czaja et al. (2002) argue that the SST variability within the latitudinal band of 10ºN-20ºN can be largely explained as the passive response of a slab mixed layer ocean to changes in wind-induced latent heat flux associated with fluctuations in the subtropical high, without the need to invoke a significant role for local air-sea feedback. Others (e.g., Xie and Tanimoto 1998; Chang et al. 2001 and Kushnir et al. 2002) reason that the local feedback is essential in maintaining an organized deep tropical response that consists of a strong cross-equatorial wind and a cross-equatorial SST gradient during the boreal spring.


Some of the disagreements between these two schools of thought are probably due to the different emphasis in each of these studies. Czaja et al. (2002) place their emphasis on NAO's influence on SST variability between 10ºN and 20ºN in the NTA, while others focus their attention on the cross-equatorial SST gradient variability in the deep tropics. In fact, Czaja et al. did hint at the existence of the WES feedback in their observational analysis (see Fig. 9a), but concluded that the feedback is too far south to have a significant impact on the evolution of the NTA SST variability. The other differences are more fundamental, and lie very much at the heart of controversial debate on whether TAV can have a remote influence on the NAO, or the latter only involves dynamic processes local to the mid- to high-latitude regions. The interested readers are referred to recent reviews by Kushnir et al. (2004) and Xie and Carton (2004).


c. Role of the ocean in TAV

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