Discussion

In Section 4, it has been shown that, in general, the frequency of the simulated TCs is substantially and significantly reduced when the concentration of atmospheric CO2 is increased. In order to understand this result, we discuss here how the global warming affects those characteristics of the tropical atmosphere which are of relevance for the development of the TCs. In particular, we consider the two major basic mechanisms, dynamical and thermodynamical, that can oppose the development, and hence the occurrence, of these phenomena: the vertical wind shear and the stability of the atmosphere.

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Fig. 10 Probability density function (PDF) of total precipitation (upper row), convective precipitation (middle row) and large scale precipitation (bottom row) associated with the simulated TCs in the different regions for the PREIND case (dashed line), 2C02 case (solid line) and 4C02 experiment (dotted line). On the x-axis is the value of precipitation in mm/day. On the y-axis is the (density of) frequency of events corresponding to a certain amount of rainfall

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Fig. 10 Probability density function (PDF) of total precipitation (upper row), convective precipitation (middle row) and large scale precipitation (bottom row) associated with the simulated TCs in the different regions for the PREIND case (dashed line), 2C02 case (solid line) and 4C02 experiment (dotted line). On the x-axis is the value of precipitation in mm/day. On the y-axis is the (density of) frequency of events corresponding to a certain amount of rainfall

o vo

4C02-PREIND WNP 4C02-PREIND ATL

Fig. 11 Left panels: composite of TC precipitation for the PREIND experiment over the WNP region (shaded pattern) along with the difference 2CO2-PREIND (upper panel) and 4CO2-PREIND (lower panel) shown by the contour patterns. The composites represent the mean rainfall rate averaged over the TC life time and over the number of TCs for the considered regions. The means have been computed for a domain centered on the core of the cyclones and extending 5° each side. Right panels: as for the left panels but for the ATL region. The PREIND rainfall composite (shaded patterns) have a shaded contour interval of 5 mm/day. The difference between the 2CO2 and PREIND composite and the 4CO2 and PREIND composite (contour patterns) have a contour interval of 1 mm/day. The contour lines show only the values that are statistically significant at a 95% level. The significance test has been performed using the bootstrap method

Fig. 11 Left panels: composite of TC precipitation for the PREIND experiment over the WNP region (shaded pattern) along with the difference 2CO2-PREIND (upper panel) and 4CO2-PREIND (lower panel) shown by the contour patterns. The composites represent the mean rainfall rate averaged over the TC life time and over the number of TCs for the considered regions. The means have been computed for a domain centered on the core of the cyclones and extending 5° each side. Right panels: as for the left panels but for the ATL region. The PREIND rainfall composite (shaded patterns) have a shaded contour interval of 5 mm/day. The difference between the 2CO2 and PREIND composite and the 4CO2 and PREIND composite (contour patterns) have a contour interval of 1 mm/day. The contour lines show only the values that are statistically significant at a 95% level. The significance test has been performed using the bootstrap method

It is well known that the vertical wind shear is one of the dynamical parameters that controls the formation of TCs. Specifically, strong large scale vertical wind shear represent unfavorable environmental conditions to the development of TCs (Gray 1968, Emanuel 2003). Therefore, a change in the climatological wind shear induced by greenhouse warming over a certain region might affect the TC frequency there.

Figure 12 shows the vertical wind shear for the PREIND and 4CO2 experiments and the difference 4CO2-PREIND (for the sake of brevity, we omit the 2CO2 experiment, whose results are fully consistent with the 4CO2 case, though with smaller amplitudes). Here, the wind shear is defined as the absolute value of the vector wind difference at 300 hPa and 850 hPa (i.e., winds shear = sqrt{(u_{300}-u_{850})A2+(v_{300}-v_{850})A2}). Both in the PREIND and 4CO2 case, the

Tropics are characterized by a minimum of vertical wind shear, which is particularly weak in the summer hemisphere. The difference 4CO2-PREIND (bottom panels) indicates a general reduction of the wind shear over most of the Tropics. This result is in agreement with previous studies, which have shown the weakening of the tropical circulation with the increasing of the atmospheric CO2 (e.g. Knutson and Manabe 1995, Vecchi and Soden 2007a).

A notable exception is the reinforcement of the vertical wind shear in the 4CO2 experiment visible both in winter and in summer over the north tropical Atlantic. The increase of the tropical Atlantic wind shear in a warmer climate is consistent with the findings of Vecchi and Soden 2007b, and might be one of the possible causes of the TCs reduction found over this area. Interestingly, the warming patterns in the tropical Pacific SSTs found in the 4CO2 case (Fig. 7) resemble the SST anomalies occurring during El Nino events. It is known that ENSO affects the TC activity over the north tropical Atlantic and one hypothesized mechanism is wind shear (V300-VB50) PREIND DJFMA wind sneor (V300-V850) PREIND JASO

wind shear (V300-VB50) PREIND DJFMA wind sneor (V300-V850) PREIND JASO

wind a hear (V300 VS5D) 4C02 DJFMA wind aheor (V300 V85Q) 4C02 JJASO

Fig. 12 Seasonal mean of the vertical wind shear defined as the difference between the wind at 300 hPa and at 850 hPa (winds shear = sqrt{(u_{300}-u_{850})A2+(v_{300}-v_{850})A2}). On the left panels are the results for the Southern Hemisphere extended summer (DJFMA), whereas on the right panels the values for the Northern Hemisphere extended summer (JJASO). The upper panels show the fields for the PREIND experiment. The middle panels the results from the 4CO2 experiments. For these plots, the contour interval is 5 m/s. The lower panels show the difference between the 4CO2 and the PREIND case. For these plots the contour interval is 1 m/s and negative values are shaded

Fig. 12 Seasonal mean of the vertical wind shear defined as the difference between the wind at 300 hPa and at 850 hPa (winds shear = sqrt{(u_{300}-u_{850})A2+(v_{300}-v_{850})A2}). On the left panels are the results for the Southern Hemisphere extended summer (DJFMA), whereas on the right panels the values for the Northern Hemisphere extended summer (JJASO). The upper panels show the fields for the PREIND experiment. The middle panels the results from the 4CO2 experiments. For these plots, the contour interval is 5 m/s. The lower panels show the difference between the 4CO2 and the PREIND case. For these plots the contour interval is 1 m/s and negative values are shaded the modulation of the vertical wind shear strength (Goldenberg and Shapiro 1996). Therefore, the stronger response of the tropical eastern Pacific SSTs to the global warming might induce a reduction of the TC activity in the ATL region in a way (and through mechanisms) similar to the influence exerted by El Nino (see also Aiyyer and Thorncroft 2006, Latif et al. 2007).

The strengthening of the vertical wind shear, however, does not explain the reduction of the TCs over the WNP. Fig. 12, in fact, shows that the wind shear reinforces over this region only during the northern winter, whereas it remains substantially unalterated in boreal summer, i.e. the TC season for this area. Therefore, there must be some other explanation for the reduced TC activity in this area.

Another important parameter that may regulate the development of TCs is the vertical stability of the atmospheric column (e.g., Gray 1979, DeMaria 2001). If the atmosphere becomes more stable, the occurrence of phenomena based on the development of organized convective systems, such as TCs, becomes more unlikely. In Section 4.1, it has been shown that the increase of atmospheric CO2 is accompanied by a reduction of the convective precipitation in the Tropics. The latter, in turn, might be the sign of an increase of the vertical stability in this region. In order to investigate possible changes in the stability of the tropical troposphere, we have assessed how the Convective Available Potential Energy (CAPE) and the Convective Inhibition (CIN) (Stevens, 2005) might be affected by the greenhouse warming.

The annual mean values of CAPE and CIN have been computed for the three experiments. The results (not shown) indicate that, in general, CAPE tends to increase with the increasing of CO2 over most of the Tropics. Exceptions are found in the eastern equatorial Indian Ocean, subtropical eastern Pacific and central Atlantic, where a slight reduction of CAPE is recorded. Table 5 shows the mean value of CAPE computed over the Tropics and over the tropical oceans only. When the atmospheric CO2 concentration is doubled, on average CAPE has increased by about 20% over the Tropics and 17% over the tropical oceans, with respect to the PREIND case. The further doubling of CO2 (4CO2) leads to only a slight increase of CAPE (3% with respect to 2CO2 and 24% with respect to PREIND) in the tropical belt. However, even more interesting, over the tropical oceans, the 4CO2 CAPE increases only by about 15% compared to the PREIND value and decreases by about 2% compared to the 2CO2 case. Therefore, the increment of tropical CAPE that appears to accompany the doubling of atmospheric CO2 seems to saturate, especially over the oceans, when the CO2 concentration is further augmented.

Table 5 shows also the mean value of CIN. Similarly to CAPE, also CIN tends to increase with the CO2 concentration, and at an even greater rate. However, different from CAPE, CIN does not appear to saturate when the CO2 concentration is quadrupled. Therefore, in the model, the increased atmospheric CO2 appears to cause an increase of CAPE, i.e. an augmented conditional instability, but also an even more pronounced increment of CIN, i.e. a higher energy barrier preventing the convection from occurring spontaneously.

Figure 13 shows the pdf of the level of free convection (LFC) for the PREIND, 2CO2 and 4CO2 cases over the WNP and ATL areas. The results suggest that the LFC tends to be higher when the atmospheric CO2 concentration increases. Con-

Table 5 Spatial average of mean convective available potential energy (CAPE) and mean convective inhibition (CIN). The mean CAPE and CIN are obtained by averaging over the 30-year periods of the PREIND, 2CO2 and 4CO2 experiments. The spatial average are computed over the whole tropical belt and over the tropical oceans only. Values in parenthesis are the percent increment with respect to the PREIND case

Table 5 Spatial average of mean convective available potential energy (CAPE) and mean convective inhibition (CIN). The mean CAPE and CIN are obtained by averaging over the 30-year periods of the PREIND, 2CO2 and 4CO2 experiments. The spatial average are computed over the whole tropical belt and over the tropical oceans only. Values in parenthesis are the percent increment with respect to the PREIND case

PREIND

2CO2

4CO2

CAPE Tropical mean (J/Kg)

109.09

131.39 (+20%)

135.24 (+24%)

CAPE Tropical oceans only (J/Kg)

132.41

155.21 (+17%)

152.40 (+15%)

CIN Tropical mean (J/Kg)

13.06

16.04 (+23%)

18.73 (+43%)

CIN Tropical oceans only (J/Kg)

8.16

9.85 (+21%)

11.46 (+40%)

sistent with the larger CIN, the effects of the higher LFCs is to reduce the chance for convective instabilities to develop.

The generally larger potential energy barrier (CIN) and the shift of the LFC to higher levels, making less likely the development of convective systems, might be responsible for both the general diminishing of convective precipitation and, at least in part, for the reduced occurrence of TCs, especially in the WNP region. For the ATL region, on the other hand, the decreased number of TCs appears to be probably due to both the increased vertical wind shear and the reduced instability of the atmosphere.

Importantly, the reduction of the convective activity suggested by the results of this work is fully consistent with the findings of other studies, where the effects of atmospheric CO2 concentration on the tropical convection have been investigated (e.g., Knutson and Manabe 1995, Sugi and Yoshimura 2004, Held and Soden 2006, Vecchi and Soden 2007a).

The warming of the tropical troposphere is accompanied by an increase of water vapor, especially in the lower layers (not shown), which, in general, leads to an increase of the potential energy available for convection. In fact, as we have seen in Table 5, the tropical CAPE increases in the 2CO2 and 4CO2 experiments compared with the PREIND case. Therefore, when convection occurs it has more potential energy available and the events might be more intense. In other words, the increase of CIN makes the triggering of convective episodes more difficult, but the larger CAPE makes the convective episodes stronger. This might explain the increased intensity of TC precipitation, found in Section 4.2 (Fig. 10 and Fig. 11).

In order to further substantiate our findings, we have assessed how other (empirical) indices related to the TC activity are changed as a consequence of the greenhouse warming. Specifically, parameters like the mid-tropospheric relative humidity over the oceans, the maximum potential index (MPI, Bister and Emanuel 2002) and the genesis potential (GP) index (Emanuel and Nolan, 2004) have been found to be related with the TC activity (e.g., Camargo et al. 2004). In Fig. 14 we show the differences between the 30-year mean values of these parameters from the 4CO2 and the PREIND case.

The tropical mean of the 4CO2-PREIND difference of the 700-hPa relative humidity (RH700) exhibits a very small increase, consistent with Held and Soden 2007. However, locally some considerable change is visible (Fig. 14, upper panels).

Level of Free Convection monthly mean jjaso ATL

O.Ol

O.Ol

500 550 600 650 700 750 800

500 550 600 650 700 750 800

Level of Free Convection monthly mean jjaso WNP

Fig. 13 Probability density function (PDF) of the level of free convection (LFC) for the PREIND case (dashed line), the 2CO2 case (solid line) and the 4CO2 experiment (dotted curve) over the ATL region (panel a) and the WNP region (panel b) during northern summer (JJASO). On the x-axis is the value of vertical levels in Millibar (mb), and on the y-axis is the (density of) frequency of occurrence at which free convection can be triggered at that level

Fig. 13 Probability density function (PDF) of the level of free convection (LFC) for the PREIND case (dashed line), the 2CO2 case (solid line) and the 4CO2 experiment (dotted curve) over the ATL region (panel a) and the WNP region (panel b) during northern summer (JJASO). On the x-axis is the value of vertical levels in Millibar (mb), and on the y-axis is the (density of) frequency of occurrence at which free convection can be triggered at that level

A substantial increase, for example, is found in the equatorial band of the Pacific ocean, whereas reductions are found the tropical Indian Ocean, subtropical Pacific and Atlantic oceans. In northern summer (left panel), the RH700 appears to decrease in the North Atlantic, whereas a very slight increased is found in the WNP region.

When averaged over the Tropics, the MPI difference (panels c and d) shows a reduction of this index. Consistent with Vecchi and Soden (2007b), the patterns of MPI change are similar to the patterns of SST change (Fig. 7). MPI increases (decreases) over the region where the SST warming is more (less) intense. Thus, substantial increase of MPI is found in the equatorial Pacific and western Indian Ocean, while the index decreases over the southern Pacific, eastern Indian Ocean, WNP and tropical Atlantic, especially during northern summer (left panel).

The 4CO2-PREIND difference of the GP index (lower panels) reveals an increase of this parameter in the tropical Pacific, north of the equator, during northern summer. In most of the WNP sector, the difference is not statistically significant, though there is a portion of the region where the GP exhibits a significant increment. The increase is more pronounced in the central-eastern North Pacific. Noteworthy, from the results shown in Section 4 (e.g., Fig. 8), this is also the region where the TC activity does not appear to be reduced by the CO2 increase.

Overall, the results obtained from the parameters shown in Fig. 14 are consistent with the findings we obtained with the TC tracking methods described in Section 2.4. The agreement appears to be particularly evident in the ATL region, where all of the empirical parameters suggest a reduction of the TC activity, consistent with the results shown in Section 4. In the WNP area, on the other hand, the agreement is less obvious. While the MPI shows a slight but visible decrease, the GP index exhibits some increment, especially in the eastern part.

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