Slow down of the THC and increasing hurricane activity Two articles in Nature and the role of multidecadal variability in the Atlantic sector Emanuel, 2005: Increasing hurricane destructiveness Bryden et al., 2005: Slow down of the THC by 30% What we see is mostly multidecadal variability and the response to global warming is tricky (Latif et al., 2006; Latif, Keenlyside, Bader, 2006)

Multidecadal variability in the North Atlantic region

„The day after tomorrow“

„Alarming“ MOC change at 25°N Slowing (30%) of the Atlantic meridional overturning circulation at 25°N, Bryden et al. (2005), Nature*). The Atlantic heat conveyor slows, Quadfasel (2005), Nature. The Atlantic conveyor may have slowed, but don´t panic, Kerr (2005), Science. *)northward heat transport ca. 1.3-1.4 PW (1957, 1981, 1992) northward heat transport ca. 1.1 PW (1998, 2004) Is the MOC weakening already underway?

Assesment of density changes in the „overflows“ using ocean model experiments Δρobs ~ 1Sv models change in MOC (Sv) observed change in overflow density (kg/m3)

Observed SST trend 1980-2004, global mean subtracted MOC fingerprint: ΔSST

The SST dipole index, a measure for the MOC strength The multidecadal variability is much larger than the linear trend. The variations in the dipole index correspond to about ±1.5-3 Sv (MPI-model).

Ocean model simulations support the MOC strenghtening full forcing add. realizations heat flux only anomaly relative to control run (Sv) ORCA-LIM, ~0.5°  (45°N) time (years)

The dipole index and the NAO (low-pass filterted) The dipole index (MOC) follows the NAO with a time lag of about 10 years SST dipole NAO

The new IPCC (4AR) simulations „best estimate“ -25% until 2100 (-43Sv) „best estimate“

Conclusions I The SST of the Atlantic may serve as a fingerprint to detect MOC changes. If so, a significant weakening of the MOC cannot be detected against the strong multidecadal variability. The multidecadal MOC variability can be understood as the response to the NAO. The latest IPCC models simulate only a moderate weakening of the MOC, with a best estimate of about 25% reduction until 2100. An anthropogenic weakening of the MOC is unlikely to leave the range of the internal variability during the next few decades.

Hurricane Katrina

The ACE Index, a measure of kinetic energy (~v2) 2005

Trends in tropical sea surface temperature Trop. NA Indo-Pacific

The role of the vertical wind shear (ECHAM5-T106)

ACE Index, vertical wind shear, and SST difference tropical North Atlantic/Indo-Pacific r(ACE/SSTdiff.)=0.7 1940-2005

Conclusions II Tropical Atlantic SST exhibits a strong trend over the last century. Hurricane activity in the Atlantic sector does not exhibit a sustained long-term trend. This is due to the competing effects of the tropical oceans on the vertical wind shear over the Atlantic. The SST difference between the tropical North Atlantic and the Indo-Pacific correlates well with the hurricane activity. Thus, the Atlantic needs to warm more than the Indo-Pacific for the hurricane activity to increase.