Tropical Sea Surface Temperature, Vertical Wind Shear, And Hurricane Development - GRL

[hatrack]

<1> The anomalously strong hurricane activity in the Atlantic sector during the recent years led to a controversy about the impact of global warming on hurricane activity in the Atlantic sector. Here we show that the temperature difference between the tropical North Atlantic and the tropical Indian and Pacific Oceans (Indo-Pacific) is a key parameter in controlling the vertical wind shear over the Atlantic, an important quantity for hurricane activity. The stronger warming of the tropical North Atlantic relative to that of the Indo-Pacific during the most recent years drove reduced vertical wind shear over the Atlantic and is thus responsible for the strong hurricane activity observed. In 2006, however, the temperature difference between the tropical North Atlantic and the tropical Indian and Pacific Oceans is much reduced, which explains the relatively weak hurricane season.



EDIT

<6> The simulated vertical wind shear, defined by the zonal wind difference between the upper (200 hPa) and lower (850 hPa) atmosphere, is averaged over the tropical North Atlantic (10–14°N and 20–70°W) as in the work by Goldenberg et al. <2003> and shown in Figure 2 for the hurricane season June–November (JJASON). There is a rather good agreement among the five realisations, indicating a strong role of the boundary forcing for the vertical wind shear in the tropical Atlantic sector. The ensemble mean, a measure of the SST-forced signal, compares well with two observational estimates of the vertical wind shear from the ECMWF and NCEP reanalysis projects, although the observed changes are somewhat stronger. Furthermore, the variations of the vertical wind shear correspond nicely to those of the ACE Index (Figure 1a): Reduced wind shear goes along with enhanced tropical storm activity and vice versa. This indicates the important roles of the vertical wind shear in controlling tropical storm activity in the Atlantic sector during the last 130 years and of the global SSTs in driving the wind shear. The strongest correlation based on the raw summer/fall (JJASON) values amounting to −0.7 is found between the ECMWF vertical wind shear and the ACE Index, the NCEP vertical wind shear exhibits a correlation with the ACE Index of −0.5, and the model simulation of −0.6. The correlations were computed for the corresponding overlapping periods. It should be noted, however, that there remains a large fraction of the variance in the ACE Index that is not explained by the vertical wind shear.




<9> The inverted SST difference, tropical North Atlantic minus Indo-Pacific, is plotted in Figure 4 together with the inverted ensemble mean wind shear from the T106 model simulation (Figure 2) and the ACE Index (Figure 1a). The timeseries are shown only from 1940 onwards, which is the period of most reliable observations. All three indices show a remarkable correspondence at decadal and longer timescales. Apparently, the warming trends of the three tropical oceans cancel with respect to their effects on the vertical wind shear over the tropical North Atlantic, so that the tropical cyclone activity remained rather stable and mostly within the range of the natural multidecadal variability. The correlation between the SST difference and the ACE Index amounts to 0.7 for the period 1940 to present and to 0.6 for the full overlapping period 1870 to present. The most recent period is characterized by an increased tropical North Atlantic/Indo-Pacific SST difference indicating that the tropical North Atlantic warmed more rapidly than the Indo-Pacific. This led to reduced vertical wind shear and thus to enhanced tropical storm activity. In contrast, summer and fall of 2006 are characterized by El Niño conditions in the Indo-Pacific, leading to a rather small temperature difference between the tropical North Atlantic and the tropical Indian and Pacific Oceans, and this explains the weak tropical storm activity.

<10> Thus, the future evolution of Atlantic tropical storm activity will critically depend on the warming of the tropical North Atlantic relative to that in the Indo-Pacific region. Changes in the MOC and their effect on tropical Atlantic SST have to be considered in this context. Likewise changes in ENSO statistics in the tropical Pacific may become important, as they affect the SSTs in all three tropical oceans. Other parameters than SST, however, such as the vertical stability of the atmosphere or changes in oceanic mixed layer depth also need to be considered in future projections of hurricane activity over the Atlantic.

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[Igel]

1. Thanks.

Missed pieces of what it said, but I get the gist, the conclusion, and what researchers (and therefore non-researchers) should be looking for.

[phantom power]

2. That isn't good news. It means we were right to suspect 2005 is the new normal.