Modulation of cross-isothermal velocities with ENSO in the tropical Pacific cold tongue

Published in Journal of Physical Oceanography, 2021

The tropical Pacific Ocean cold tongue (CT) plays a major role in the global climate system. The strength of the CT sets the zonal temperature gradient in the Pacific that couples with the atmospheric Walker circulation. This coupling is an essential component of the El Niño–Southern Oscillation (ENSO). The CT is supplied with cold water by the Equatorial Undercurrent that follows the thermocline as it shoals toward the east, adiabatically transporting cold water toward the surface. As the thermocline shoals, its water is transformed through diabatic processes, producing water mass transformation (WMT) that allows water to cross mean isotherms. Here, we examine WMT in the cold-tongue region from a global high-resolution ocean simulation with saved budget terms that close its heat budget exactly. Using the terms of the heat budget, we quantify each individual component of WMT (vertical mixing, horizontal mixing, eddy fluxes, and solar penetration) and find that vertical mixing is the single most important contribution in the thermocline and solar heating dominates close to the surface. Horizontal diffusion is much smaller. During El Niño events, vertical mixing, and hence cross-isothermal flow as a whole, are much reduced, whereas, during La Niña periods, strong vertical mixing leads to strong WMT, thereby cooling the surface. This analysis demonstrates the enhancement of diabatic processes during cold events, which in turn enhances cooling of the CT from below the surface.

Recommended citation: Deppenmeier, A., Bryan, F.O., Kessler, W.S., Thompson, L. Modulation of Cross-Isothermal Velocities with ENSO in the Tropical Pacific Cold Tongue. JPO (2021).

The effect of vertical ocean mixing on the tropical Atlantic in a coupled global climate model

Published in Climate Dynamics, 2020

In this study we tested the effect of enhanced vertical ocean mixing on the tropical Atlantic warm sea surface temperature (SST) bias, and biases related to the SST bias. We find that increased vertical mixing efficiency reduces the SST bias in the cold tongue region, where the thermocline is shallow and warm biases are strong. Associated biases such as marine precipitation are reduced. We then continued the model simulation into the 21st century and find that the climate change signal depends on the ocean vertical mixing parameterization.

Recommended citation: Deppenmeier, A., Haarsma, R.J., LeSager, P. et al. The effect of vertical ocean mixing on the tropical Atlantic in a coupled global climate model. Clim Dyn (2020).

The Southeastern Tropical Atlantic SST bias investigated with a coupled atmosphere-ocean single column model at a PIRATA mooring site

Published in Journal of Climate, 2020

In this study we investigate the sea surface temperature (SST) evolution in the southeastern tropical Atlantic with a single column model (SCM). We demonstrate that the SCM develops a warm bias similar to that of the three dimensional host model EC-Earth, and that this bias can largely be reduced by enhancing the vertical mixing efficieny within the physically possible range. Reducing the atmospheric biases (and therefore improve surface fluxes into the ocean) reduces the SST bias by 40%, while enhancing the vertical mixing efficiency can reduce the bias by 70%.

Recommended citation: Deppenmeier, A., R.J. Haarsma, C. van Heerwaarden, and W. Hazeleger, 0: The Southeastern Tropical Atlantic SST bias investigated with a coupled atmosphere-ocean single column model at a PIRATA mooring site. J. Climate, 0,

The Bjerknes Feedback in the Tropical Atlantic

Published in Climate Dynamics, 2016

In this first study we investigate the Bjerknes Feedback (BF) in the tropical Atlantic in an ocean reanalysis and in the CMIP5 model ensemble. We find the BF active in ORA-S4, and quantify it in the climate models against the reanalysis. While the effects of sea surface temperature (SST) anomalies on zonal wind stress and zonal wind stress on the thermocline depth are reproduced well in the models, the response of SST to the thermocline depth anomaly is lacking. Most models suffer from sever biases in the ocean structure, which results in too deep thermoclines. This severly dampens the ability of thermocline anomalies to influence SST.

Recommended citation: Deppenmeier, Anna-Lena, Reindert J. Haarsma, and Wilco Hazeleger. "The Bjerknes feedback in the tropical Atlantic in CMIP5 models." (2016):Climate Dynamics . 47.7-8 2691-2707.