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VOLUME 14 JOURNAL OF CLIMATE 1MARCH 2001

LETTERS

On the Relationship between Tropical Convection and Sea Surface

ADRIAN M. TOMPKINS* Max-Planck-Institut fuÈr Meteorologie, Hamburg, Germany

9 August 2000 and 12 September 2000

ABSTRACT Tropical observations show convective activity increasing sharply above sea surface (SSTs) of around 26ЊC and then decreasing as the SST exceeds 30ЊC, with maximum observed SSTs of around 32ЊC. Although some aspects of this relationship are reasonably well understood, as of yet no theory has explained the decrease in convective activity above 30ЊC. Here it is suggested that this aspect of the relationship could result from a organizational positive feedback, sometimes termed ``self aggregation,'' whereby the occurrence of convection makes future convection more likely to occur in the same location. Using -resolving simulations, it is shown that the feedback between convection and the vapor ®eld is a good candidate for this role.

Deep convection in the Tropics is intimately tied to dynamics (Newell 1979; Ramanathan and Collins the general dynamical circulation. Thus to understand 1991; Fu et al. 1992; Wallace 1992; Clement et al. 1996; the observed relationship between convection and sea and Liu 1996). All current theories in- surface temperature (SST) in the Tropics is also to gain volve convection. For example, Ramanathan and Collins a general understanding of tropical dynamics. The ob- (1991) proposed that cirrus associated with con- servations show that above a ``threshold'' SST of 26ЊC, vection regulate SST by the reduction of the surface convective activity increases sharply (Graham and Bar- incident (SW) . Wallace (1992) nett 1987; Waliser and Graham 1993; Zhang 1993). By states that surface ¯uxes and the large-scale ¯ow will sorting the observations into classes of large-scale dy- control SSTs even in the absence of SW cloud forcing. namical activity (i.e., convergence or divergence re- However, convection is still invoked to ``vent'' the high gimes), Lau et al. (1997) and Bony et al. (1997) revealed SST area, and prevent the (BL) and that this increase is due to the horizontal SST gradient, surface temperatures from warming in unison. The role and in fact convection is largely independent of the of convection via both enhanced surface ¯uxes and SW absolute value of the local SST. This has been con®rmed forcing has recently been emphasized (Sud et al. 1999). using cloud-resolving models (CRMs; Lau et al. 1994; Other theories involve convection in ocean Tompkins and Craig 1999). Thus we can view convec- dynamics (Anderson et al. 1996) or as part of a coupled tive activity and the general tropical circulation as syn- large-scale circulation that forces the upwelling of cool onymous; an increase in convective activity must be (Clement et al. 1996; Sun and Liu 1996). concurrent with an increase in mean atmospheric ascent. This leads us to the third aspect of the convection± The observations also show that SST is limited to SST relationship, namely the reduction in convective about 32ЊC, and the fact that temperatures in the ex- activity above SSTs of around 30ЊC. As documented by tensive ``warm pool'' region of the western Paci®c are Waliser (1996), these warmest SSTs often occur within within one or two degrees of this maximum has led to the Paci®c warm pool region or the Indian Ocean, with the suggestion of a number of SST thermostat mecha- timescales of weeks to a few months, and are termed nisms (i.e., negative feedbacks), involving cloud radi- SST ``hot spots'' or ``warm anomalies.'' These regions ative forcing, surface latent ¯uxes, and atmospheric and of warmest SST are often free from convection. Thus the argument is that a lack of local convection (for what- ever reason) allows the SST to increase due to increased * Current af®liation: ECMWF, Reading, United Kingdom. solar insulation and reduced surface latent ¯uxes. The SST does not run away, however, since it is even- tually controlled by the thermostat mechanisms involv- Corresponding author address: Dr. A. M. Tompkins, ECMWF, Shin®eld Park, Reading RG2 9AX, United Kingdom. ing convection. E-mail: [email protected] However, as it stands this argument leads to an ap-

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Unauthenticated | Downloaded 09/29/21 06:41 PM UTC 634 JOURNAL OF CLIMATE VOLUME 14 parent paradox. If convection is generally increasing (1998) and references therein, while the experimental with SSTs above 26ЊC, and SSTs are ef®ciently limited setup is discussed in more detail in Tompkins (2001). by convective , how can developing SST hot A uniform 2 K dayϪ1 cooling provides the forcing for spots remain free from convection for periods of weeks convection. An underlying SST gradient is imposed to months, allowing the SST anomaly to amplify? It is along the 1024-km axis, taking the form of one sine sometimes claimed (e.g., Waliser 1996; Lau and Sui wave with SSTs ranging from 299.5 to 300.5 K, giving 1997) that ``remotely forced'' atmospheric descent pro- SST gradients comparable to those observed along the vides the mechanism for convective suppression. This equatorial Paci®c. The SST gradient is imposed for a leads to a picture where a negative feedback thermostat period of 5 days, in order to establish convection over prevents runaway SSTs, with large-scale ¯ow variability the warmest SSTs. Over the cooler SSTs, convection shutting off convection, allowing hot spot development. should become suppressed by mean subsidence. Thus a But there is a problem with this view. To illustrate large-scale overturning circulation is established, mod- this, we use the example of an incipient SST hot spot. eled for the ®rst time in a 3D framework with resolved For current thermostat theories to , the BL above deep convection. this warm patch must be reasonably closely related to Over the coolest SSTs, where deep convection is sup- the surface thermodynamically, although the water va- pressed, the increase in shortwave incoming ¯ux, and por difference between the surface and is the reduction of surface ¯ux, would result likely to increase due to low mean in the sup- in a surface ¯ux imbalance. Within the warm pool and pressed region (Zhang et al. 1995). Thus under current Indian Ocean regions, this imbalance can cause SSTs theories, one would expect positive SST anomalies to to increase (e.g., Lau and Sui 1997; Woolnough et al. be accompanied by higher values of boundary layer 2000b), perhaps eventually creating a SST warm anom- equivalent potential energy (␪e). The reason why this aly. We simulate the warm anomaly development in a high ␪e does not lead to deep convection, removing the very idealized manner, by simply reversing the SST SST perturbation on the timescale of a day or less, is gradient at day 5 of the experiment, to create a surrogate that remote forcing prevents this. This seems quite rea- hot spot. Although extremely idealized, this experiment sonable until we realize that we are deceiving ourselves allows us to examine the atmospheric dynamical re- with a subtle terminology change. What remote forcing sponse to SST anomalies, in an framework where both actually refers to is ``remote convection,'' since we have large- and convective-scale 3D circulations are explic- seen that convection and large-scale ¯ow are commen- itly represented. surate. Thus, the assertion that hot spots are the result Figure 1 shows the surface , averaged of remote forcing implicitly contains the assumption that across the 64-km axis, revealing the location of con- some, as yet undetermined, process ensures that regions vection during the experiment. After an initial period over remote cooler SSTs remain more favorable to deep of random convection, the large-scale circulation is es- convection over a period of time long enough to allow tablished during the ®rst 5 days, with ascending hot spot development. Either some unspeci®ed process over the warmest SSTs as expected. After the SST re- prevents the ␪e of BL air over the warmest SSTs from versal on day 5, the convection dies out quickly over exceeding that of remote cooler SST regions, or if this the cool SSTs. However, convection does not sponta- is not the case, the BL ␪e air over the warmest SSTs is neously ¯are up over the new SST hot spot, but instead somehow prevented from forming deep convection. propagates slowly toward it. Computing resources limit Convection over land, for example associated with the the experiment to ten days, but judging from the prop- Asian monsoon, can not be invoked as an alternative agation speeds, at least two weeks would be required remote forcing candidate, since there is no obvious rea- for the convection to reach the highest SSTs. son that convection over higher SSTs will be suppressed Closer examination of the thermodynamic ®elds re- in preference to lower SST areas. veals a possible reason for thisÐthe interaction with To shed light on this problem, an idealized experiment . Figure 2 shows how, even by day 10, the is conducted, using a 3D model with a 2-km horizontal region centered at x ϭ 768 km, that was free from resolution to resolve the dynamics of convection, and convection during the ®rst 5 days of the experiment, is that represents many ice and warm microphysical very dry. It is well known that convection locally moist- processes that occur in clouds. The model uses a domain ens its environment by detraining water vapor and cloud of 1024 km by 64 km in the horizontal, and 20 km deep, condensate. On the other hand, the subsidence with horizontal periodic boundary conditions. The hor- is spread out quickly over the deformation radius by izontal domain is a compromise enforced by compu- waves. Even if an ensemble of cumulus clouds tational limitations. The long 1024-km axis allows rea- has no net effect on atmospheric water vapor, the regions sonably large-scale circulations to be represented in a local to convection are moistened while remote clear- CRM framework, while the limited third dimension en- sky areas are dried (see discussion by Randall and Huff- sures that the organizational scales are not arti®cially man 1980). This is seen directly in observations (Udel- affected by 2D geometry (Tompkins 2000). Details of hofen and Hartmann 1995; Liao and Rind 1997). the model are documented in Tompkins and Craig Dry air acts to suppress convection. Firstly, dry air

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FIG. 2. Vertical section taken on day 10 of the normalized water vapor perturbation, qЈ/␴(q), averaged across the short 64-km axis, where qЈ is the water vapor perturbation about the horizontal mean, and ␴(q) represents the standard deviation of q. The contours show where the total cloud mass mixing ratio (ice ϩ cloud) averaged across the short axis equals 10Ϫ7 kg kgϪ1.

FIG. 1. HovmoÈller rainfall plot for the CRM experiment. Surface rainfall is summed across the short 64-km axis. The light and dark surface winds (Fig. 3b). The latent heat ¯ux will not shading represent a rain rate of 0.1 mm hrϪ and1mmhrϪ1, respec- lead to a local runaway since the tively. For days 0±5 a sine wave SST is imposed with the maximum SST at x ϭ 256 km. This is reversed on day 5. moisture is advected laterally into neighboring con- vecting regions rather than being vented vertically by local convection. As the BL air is advected, surface reduces in-cloud when entrained into up- ¯uxes further increase the water vapor content, and a drafts. Additionally, dry boundary layers (either result- shallow convective layer develops, capped by the dry ing from convective downdrafts on a local scale, or due air above. This is visible in Fig. 2, and is more obvious to dry air far from convection subsiding into the BL) in Fig. 4, which reveals how the dryness of the free are associated with low ␪e values that are less likely to troposphere clearly delineates the ``'' rain-free form deep convective events. Observations show ex- areas from the deep convecting regions, despite the fact amples of intrusions of extratropical dry air suppressing that the BL ␪e is often comparable. convection (Numaguti et al. 1995; Yoneyama and Fu- Thus, a quasi-stable circulation exists, but although jitani 1995), and lower-tropospheric has been convection is suppressed by dryness over the highest shown to be an important precursor for tropical deep SST regions, it will propagate preferentially toward convection (Sherwood 1999). Thus it is only a small them at the rate at which it can moisten the atmosphere. step to suggest the existence of a positive organizational This is because a state with the convection centered over feedback, sometimes referred to as ``self-aggregation,'' the warmest SSTs is also one of lowest potential energy where convection locally moistens its atmosphere, mak- (i.e., the difference between mean atmospheric and sur- ing it favorable for future convection. face temperature is a minimum). On reaching the SST Adding a positive organizational feedback between hot spot, both solar and surface latent ¯uxes act in uni- convection and water vapor allows us to attempt an son, via the established thermostat mechanisms, and the explanation for the tropical observations that is illus- SST anomaly will disappear on a much faster timescale trated schematically in Fig. 3. When we now invoke than the complementary warming phase with which it atmospheric variability that suppresses convection, the was established (Fig. 3c). This difference in warming local atmosphere above the BL is also dried (Fig. 3a), and cooling timescales is documented in observations since it is a long distance from deep convection. Even (Sud et al. 1999), and it is the reason that the mean if the increasing SST creates a hot spot, convection will convective activity reduces with surface temperature not break out due to the inhibition of this dry atmo- with SST Ͼ 30ЊC, since the atmosphere spends more sphere, and SSTs can increase further. Eventually the time in the drought warming phase. The two-week hot SST will be arrested by increasing surface ¯uxes of spot recovery timescale in the model experiment is latent heat, offsetting the solar radiation anomaly, al- slightly shorter than observed, which is to be expected, though this ``brake'' is made less ef®cient by lower since the horizontal scale of the SST perturbation is

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FIG. 4. Surface rainfall, water vapor (q␷ ) at 1520 m, ␪e at 50 m, and SST at day 10 of the experiment. The lines are the average and the dotted lines the maximum value across the 64-km y axis. FIG. 3. Schematic of interaction between deep convection and SST in the Tropics. The cloud shows the location of convection, the large red arrows show the mean large-scale circulation, and the striped bar found in 3D CRM simulations (Tompkins 2001), and shows schematically the SST evolution with red colors representing in a 2D CRM experiment with interactive radiation and the warmest SSTs and blue the coolest SSTs. The small arrows near a swamp ocean Grabowski (2000) observed that con- the surface represent surface latent heat ¯uxes, with the thickness vection did not occur over the highest SSTs, but con- indicating the relative magnitude of the mean ¯uxes. The total time- scale of the SST progression is several weeks. tinuously propagated toward them, while new warm anomalies developed away from the convective region, exactly as suggested here. Moreover, Woolnough et al. limited to a few hundred kilometers, smaller than the (2000a) have shown that the feedback between convec- typical observed value that exceeds 1000 km. tion and water vapor appears to be important in regu- The idea that the feedback between water vapor and lating the strength and propagation speed of the Mad- convection can cause self-aggregation has been previ- den±Julian oscillation, indicating that the feedback op- ously discussed by Esbensen (1978), Johnson (1978), erates on larger spatial and longer temporal scales than Nicholls and Lemone (1980), and Randall and Huffman can be currently simulated in a 3D CRM framework. (1980), mostly in the context of shallow cumulus de- In summary, it appears that surface latent ¯uxes limit velopment. Here we are extending this idea to suggest SSTs to 32ЊC, but that this regime is unstable. Even- that the self-aggregation feedback between deep con- tually convection must propagate into the high SST ar- vection and water vapor could play a fundamental role eas, and shortwave forcing, possibly in conjunction with in the relationship between convection, large-scale trop- ocean dynamical effects, provide a lower SST maxi- ical dynamics, and the ocean surface. In observations, mum. The closing remarks of Wallace (1992) should be Brown and Zhang (1997) and Lucas and Zipser (2000) emphasized. None of the thermostat theories based on documented substantial differences in low- to midtro- convection will prevent maximum tropical SSTs from pospheric moisture between ``rainy'' and ``drought'' pe- increasing in a hypothetical future climate. The crucial riods during the Tropical Ocean and Global Atmosphere point that we have attempted to add to the debate is that Coupled Ocean±Atmosphere Response Experiment current theories on their own do not explain the reduc- (TOGA±COARE) but the causal relationship was dif- tion in convective activity with SSTs greater than 30ЊC. ®cult to determine. However, strong evidence of a two- With the addition of an organizational positive feedback, way positive feedback relationship has recently been an explanation of the full convection±SST relationship

Unauthenticated | Downloaded 09/29/21 06:41 PM UTC 1MARCH 2001 LETTERS 637 and the existence of convection-free SST hot spots can Lucas, C., and E. J. Zipser, 2000: Environmental variability during be attempted. The CRM experiment suggested that the TOGA COARE. J. Atmos. Sci., 57, 2333±2350. Newell, R. E., 1979: Climate and the ocean. Amer. Sci., 67, 405± feedback between convection and the water vapor ®eld 416. is a strong candidate for this role, but other positive Nicholls, S., and M. A. Lemone, 1980: The fair weather boundary feedback mechanisms exist that could enhance this. For layer in GATE: The relationship of subcloud ¯uxes and structure example, convectively generated cold pools can trigger to the distribution and enhancement of cumulus clouds. J. Atmos. Sci., 37, 2051±2067. new convective cells (e.g., Simpson 1980), causing con- Nilsson, J., and K. A. Emanuel, 1999: Equilibrium of a vective clustering. 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