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Katja Lohmann
and
Mojib Latif

Abstract

This study investigates the influence of El Niño on the upper-ocean circulation in the tropical Atlantic Ocean (via changes in the Atlantic trade winds) by analyzing observed sea surface temperature (SST) together with an ocean general circulation model integration forced by the NCEP–NCAR reanalysis. During periods with anomalously warm (cold) eastern equatorial Pacific SST, the southern Atlantic tropical cell is strengthened (weakened). The difference of the cell strength between El Niño and La Niña years is about 20% of the mean cell strength. However, the variability of the cell is not dominated by the remote forcing from the eastern equatorial Pacific but seems to be caused by intrinsic tropical Atlantic variability. A strengthening (weakening) for periods with anomalously warm (cold) eastern equatorial Pacific SST is also found for the zonal surface and subsurface currents. TOPEX/Poseidon altimetry data are used to validate the results based on the OGCM integration.

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Katja Lohmann
and
Mojib Latif

Abstract

The decadal-scale variability in the tropical Pacific has been analyzed herein by means of observations and numerical model simulations. The two leading modes of the sea surface temperature (SST) variability in the central western Pacific are a decadal mode with a period of about 10 yr and the ENSO mode with a dominant period of about 4 yr. The SST anomaly pattern of the decadal mode is ENSO like. The decadal mode, however, explains most variance in the western equatorial Pacific and off the equator. A simulation with an ocean general circulation model (OGCM) forced by reanalysis data is used to explore the origin of the decadal mode. It is found that the variability of the shallow subtropical–tropical overturning cells is an important factor in driving the decadal mode. This is supported by results from a multicentury integration with a coupled ocean–atmosphere general circulation model (CGCM) that realistically simulates tropical Pacific decadal variability. Finally, the sensitivity of the shallow subtropical–tropical overturning cells to greenhouse warming is discussed by analyzing the results of a scenario integration with the same CGCM.

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Jon Robson
,
Rowan Sutton
,
Katja Lohmann
,
Doug Smith
, and
Matthew D. Palmer

Abstract

In the mid-1990s, the subpolar gyre of the North Atlantic underwent a remarkable rapid warming, with sea surface temperatures increasing by around 1°C in just 2 yr. This rapid warming followed a prolonged positive phase of the North Atlantic Oscillation (NAO) but also coincided with an unusually negative NAO index in the winter of 1995/96. By comparing ocean analyses and carefully designed model experiments, it is shown that this rapid warming can be understood as a delayed response to the prolonged positive phase of the NAO and not simply an instantaneous response to the negative NAO index of 1995/96. Furthermore, it is inferred that the warming was partly caused by a surge and subsequent decline in the meridional overturning circulation and northward heat transport of the Atlantic Ocean. These results provide persuasive evidence of significant oceanic memory on multiannual time scales and are therefore encouraging for the prospects of developing skillful predictions.

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Rohit Ghosh
,
Dian Putrasahan
,
Elisa Manzini
,
Katja Lohmann
,
Paul Keil
,
Ralf Hand
,
Jürgen Bader
,
Daniela Matei
, and
Johann H. Jungclaus

Abstract

The North Atlantic subpolar gyre (SPG) plays a crucial role in determining the regional ocean surface temperature (SST), which has profound implications on surrounding continental and coastal climate. Here, we analyze the Max-Planck-Institute Grand Ensemble global warming experiments and show that the SPG can evolve in two distinct phases under continuous global warming. In the first phase, as the global mean surface temperature approaches 2 K warming, the eastern SPG intensifies in combination with a weakening Atlantic meridional overturning circulation (AMOC), accompanied by a cooling of subpolar North Atlantic SST, known as the warming hole. The associated oceanic fingerprint matches with the observations over the last 15 years, where an intensification and cooling of the eastern SPG is related to salinity reduction at the eastern side of the SPG. However, for further warming beyond 2 K, in spite of a continuous decline in the AMOC, a northward shift of the mean zonal wind extends the subtropical gyre northward with an associated disruption of the eastern SPG intensification, resulting in the cessation of the warming hole. Therefore, a shift from the initially dominating oceanic drivers to the atmospheric driver results into a two-phase evolution of the North Atlantic Ocean SPG circulation and the associated SST under continuous global warming.

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Alexander J. Baker
,
Malcolm J. Roberts
,
Pier Luigi Vidale
,
Kevin I. Hodges
,
Jon Seddon
,
Benoît Vannière
,
Rein J. Haarsma
,
Reinhard Schiemann
,
Dimitris Kapetanakis
,
Etienne Tourigny
,
Katja Lohmann
,
Christopher D. Roberts
, and
Laurent Terray

Abstract

Tropical cyclones undergo extratropical transition (ET) in every ocean basin. Projected changes in ET frequency under climate change are uncertain and differ between basins, so multimodel studies are required to establish confidence. We used a feature-tracking algorithm to identify tropical cyclones and performed cyclone phase-space analysis to identify ET in an ensemble of atmosphere-only and fully coupled global model simulations, run at various resolutions under historical (1950–2014) and future (2015–50) forcing. Historical simulations were evaluated against five reanalyses for 1979–2018. Considering ET globally, ensemble-mean biases in track and genesis densities are reduced in the North Atlantic and western North Pacific when horizontal resolution is increased from ∼100 to ∼25 km. At high resolution, multi-reanalysis-mean climatological ET frequencies across most ocean basins as well as basins’ seasonal cycles are reproduced better than in low-resolution models. Skill in simulating historical ET interannual variability in the North Atlantic and western North Pacific is ∼0.3, which is lower than for all tropical cyclones. Models project an increase in ET frequency in the North Atlantic and a decrease in the western North Pacific. We explain these opposing responses by secular change in ET seasonality and an increase in lower-tropospheric, pre-ET warm-core strength, both of which are largely unique to the North Atlantic. Multimodel consensus about climate change responses is clearer for frequency metrics than for intensity metrics. These results help clarify the role of model resolution in simulating ET and help quantify uncertainty surrounding ET in a warming climate.

Open access