Diapycnal heat flux and mixed layer heat budget within the Atlantic Cold Tongue

Rebecca Hummels,Peter Brandt,Marcus Dengler,Michael Schlundt

doi:10.1007/s00382-014-2339-6

Rebecca Hummels, Peter Brandt + Show 2 more

Open Access

https://doi.org/10.1007/s00382-014-2339-6

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Abstract

Sea surface temperatures (SSTs) in the eastern tropical Atlantic are crucial for climate variability within the tropical belt. Despite this importance, state-of-the-art climate models show a large SST warm bias in this region. Knowledge about the seasonal mixed layer (ML) heat budget is a prerequisite for understanding SST mean state and its variability. Within this study all contributions to the seasonal ML heat budget are estimated at four locations within the Atlantic cold tongue (ACT) that are representative for the western (0°N, 23°W), central (0°N, 10°W) and eastern (0°N, 0°E) equatorial as well as the southern (10°S, 10°W) ACT. To estimate the contribution of the diapycnal heat flux due to turbulence an extensive data set of microstructure observations collected during ten research cruises between 2005 and 2012 is analyzed. The results for the equatorial ACT indicate that with the inclusion of the diapycnal heat flux the seasonal ML heat budget is balanced. Within the equatorial region, the diapycnal heat flux is essential for the development of the ACT. It dominates over all other cooling terms in the central and eastern equatorial ACT, while it is of similar size as the zonal advection in the western equatorial ACT. In contrast, the SST evolution in the southern ACT region can be explained entirely by air-sea heat fluxes.

Highlights

The tropical Atlantic ocean plays a key role for climate variability in sensitive regions of the surrounding continents: Previous studies suggest that inter-annual variability of sea surface temperature (SST) in the equatorial Atlantic impacts rainfall variability over northeast Brazil and the coastal regions surrounding the Gulf of Guinea (Chang et al 2006; Kushnir et al 2006; Brandt et al 2011b)
A unique multi-cruise microstructure data set, observations from the PIRATA buoy network as well as climatological data sets are used to investigate the individual contributions to the seasonal mixed layer (ML) heat budget at four different locations within the Atlantic cold tongue (ACT) region with a special focus on the role of the diapycnal heat flux
The ML budgets within different phases of the seasonal cycle associated with the absence, development and mature phase of the ACT are described

Summary

Introduction

The tropical Atlantic ocean plays a key role for climate variability in sensitive regions of the surrounding continents: Previous studies suggest that inter-annual variability of sea surface temperature (SST) in the equatorial Atlantic impacts rainfall variability over northeast Brazil and the coastal regions surrounding the Gulf of Guinea (Chang et al 2006; Kushnir et al 2006; Brandt et al 2011b). Sea surface temperature variability in the tropical Atlantic is dominated by an annual signal formed by the seasonal appearance of the ACT (Fig. 1). The net surface heat fluxes within this region are dominated by a semi-annual signal due to the meridional migration of the Inter Tropical Convergence Zone (ITCZ). The discrepancy between the semi-annual atmospheric forcing and the annual cycle of SSTs suggests ocean dynamics to be of fundamental importance within this region. In order to reduce SST during ACT development, oceanic processes need to redistribute large amounts of heat to explain the observed seasonal decrease of SST. To further understand the seasonal cycle of SSTs within this region, an analysis of all individual contributions to the mixed layer (ML) heat budget is inevitable

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