Mojib Latif

Tropical sea surface temperature, vertical wind shear, and hurricane development

[1]xa0The 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.

Generation of hyper climate modes

[1]xa0It is shown that some important aspects of the space-time structure of multidecadal sea surface temperature (SST) variability can be explained by local air-sea interactions. A concept for “Global Hyper Climate Modes” is formulated: surface heat flux variability associated with regional atmospheric variability patterns is integrated by the large heat capacity of the extra-tropical oceans, leading to a continuous increase of SST variance towards longer timescales. Atmospheric teleconnections spread the extra-tropical signal to the tropical regions. Once SST anomalies have developed in the Tropics, global atmospheric teleconnections spread the signal around the world creating a global hyper climate mode. A simple model suggests that hyper climate modes can vary on timescales longer than 1,000 years. Ocean dynamics may amplify theses modes and influence the regional expression of the variability, but are not at the heart of the mechanism which produces the hyper modes.

North Atlantic Oscillation Response to Anomalous Indian Ocean SST in a Coupled GCM

The dominant pattern of atmospheric variability in the North Atlantic sector is the North Atlantic Oscillation (NAO). Since the 1970s the NAO has been well characterized by a trend toward its positive phase. Recent atmospheric general circulation model studies have linked this trend to a progressive warming of the Indian Ocean. Unfortunately, a clear mechanism responsible for the change of the NAO could not be given. This study provides further details of the NAO response to Indian Ocean sea surface temperature (SST) anomalies. This is done by conducting experiments with a coupled ocean–atmosphere general circulation model (OAGCM). The authors develop a hypothesis of how the Indian Ocean impacts the NAO.

Reply to a comment of Behera et al. on "A cautionary note on the interpretation of EOFs"

EOF and rotated EOF analyses are widely used tools in climate research. In recent years there have been several cases in which the EOFor rotated EOF analyses were used to identify physical modes. These are the tropical Atlantic and the tropical Indian Ocean SST dipole modes and the di erent modes of the Northern Hemisphere winter surface air pressure variability. Here we would like to discus the problems in interpreting these statistically derived modes as physical modes. By constructing an arti cial example we shall show that the patterns derived from EOFor rotated EOF analysis can lead to misinterpretations. This study should be seen as a cautionary note to highlight the pitfalls which may occur when using the EOF or rotated EOF techniques.

Southern Ocean Decadal Variability and Predictability

The Southern Ocean featured some remarkable changes during the recent decades. For example, large parts of the Southern Ocean, despite rapidly rising atmospheric greenhouse gas concentrations, depicted a surface cooling since the 1970s, whereas most of the planet has warmed considerably. In contrast, climate models generally simulate Southern Ocean surface warming when driven with observed historical radiative forcing. The mechanisms behind the surface cooling and other prominent changes in the Southern Ocean sector climate during the recent decades, such as expanding sea ice extent, abyssal warming, and CO2 uptake, are still under debate. Observational coverage is sparse, and records are short but rapidly growing, making the Southern Ocean climate system one of the least explored. It is thus difficult to separate current trends from underlying decadal to centennial scale variability. Here, we present the state of the discussion about some of the most perplexing decadal climate trends in the Southern Ocean during the recent decades along with possible mechanisms and contrast these with an internal mode of Southern Ocean variability present in state-of-the art climate models.

Internal Southern Ocean Centennial Variability: Dynamics, Impacts and Implications for Global Warming. Climate Change: Multidecadal and Beyond

It is well established that centennial climate variability can be externally forced by, e.g., quasi-oscillatory fluctuations of the solar constant or slowly varying atmospheric aerosol concentrations in association with changes of volcanic activity. Climate models recently suggested that substantial centennial variability can be also produced internally, and different competing mechanisms have been proposed. This paper deals with the internal centennial variability originating in the Southern Ocean Sector simulated by the Kiel Climate Model (KCM). In that model, the Southern Ocean centennial variability (SOCV) is linked to Weddell Sea deep convection activity and drives regional as well as global climate variations, as witnessed, for example, by coherent changes in Antarctic sea ice extent and globally averaged surface air temperature (SAT). Furthermore, the SOCV is associated with changes in deep Southern Ocean temperature in the KCM. Interestingly, a warming of the abyssal Southern Ocean has been observed during the recent decades, suggesting a contribution from SOCV. Another important impact of the SOCV in the model concerns the Atlantic Meridional Overturning Circulation (AMOC). The AMOC strengthens and deepens after the cessation of Weddell Sea deep convection and Antarctic Bottom Water (AABW) formation with a time delay of several decades to a century. Internal North Atlantic sea level variations can be as large as ± 15cm/century in the model with a strong contribution from the SOCV. Such regional sea level variations are of the same order of magnitude as the observed globally averaged 20th century sea level rise amounting to about 15-20cm. Finally, the KCM simulation suggests that the SOCV may have contributed to the current hiatus in global warming through an enhanced deep ocean heat uptake.