On the variability of turbulent mixing within the upper layers of the Atlantic Cold Tongue region

Sea surface temperature (SST) variability within the equatorial Atlantic is of climatic relevance for the surrounding continents. A striking feature of this SST variability is the annual appearance of the Atlantic cold tongue (ACT). The respective contributions to the ML heat budget forming the seasonal cycle of SSTs within the ACT could up to date not be clarified. Especially the role of the diapycnal heat flux due to turbulence in cooling SSTs is still controversially discussed. The main focus of this study is to infer regional and seasonal variability of upper ocean turbulent mixing and the inferred diapycnal heat flux within the ACT region using a multi cruise data set of microstructure observations. The assessed varibility of the diapycnal heat flux is then integrated into the ML heat budget within different regions and seasons of the ACT. In addition, the variability in mixing intensity is related to the variability in large scale background conditions, which were additionally observed during the cruises. The observations indicate fundamental differences in background conditions in terms of shear and stratification below the mixed layer (ML) for the western and eastern equatorial as well as the southern ACT region. This leads to the occurrence of critical Froude numbers (Fr), which points towards elevated mixing intensity, most frequently in the western equatorial ACT. The distribution of critical Fr below the ML reflects the regional and seasonal variability of mixing intensity. Turbulent dissipation rates (ε) at the equator (2°N-2°S) are strongly increased in the upper thermocline compared to off-equatorial locations. In addition, ε is elevated in the western equatorial ACT compared to the east from May to November, whereas boreal summer appears as the season of highest mixing intensities throughout the equatorial ACT region, coinciding with ACT development. Diapycnal heat fluxes at the base of the ML in the western equatorial ACT region inferred from ǫ and stratification range from a maximum of 90 W/m² in boreal summer to 40 W/m² in November. In the eastern equatorial ACT region maximum values of about 25 W/m² were estimated during boreal summer. Outside the equatorial region, inferred diapycnal heat fluxes are comparably low rarely exceeding 10 W/m². Critical to the enhanced diapycnal heat flux in the western equatorial ACT region during boreal summer and autumn is elevated meridional velocity shear in the upper thermocline. It is thus suggested that TIWs are crucial contributors to mixing within this region during this time period. Integrating the obtained heat flux estimates in the ML heat budget accentuates the diapycnal heat flux as the largest ML cooling term during boreal summer and early autumn in the entire equatorial ACT region and crucial for decreasing SSTs for ACT development. Within the southern ACT region SST cooling is dominated by atmospheric forcing. Additionally, it is shown that most of the existing parametrization schemes for the equatorial thermocline, which are supposed to estimate the general magnitude of mixing related parameters without cost-intensive observations, tend to overestimate turbulent mixing intensity and the inferred diapycnal heat fluxes within the equatorial ACT region

3. Wochenbericht M145

Mindelo-Recife 26.02.-04.03.201

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

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

4. Wochenbericht M145

Mindelo-Recife 05.03.-11.03.201

Deep Intraseasonal Variability in the Central Equatorial Atlantic

Besides the zonal flow that dominates the seasonal and long-term variability in the equatorial Atlantic, energetic intraseasonal meridional velocity fluctuations are observed in large parts of the water column. We use 15 years of partly full-depth velocity data from an equatorial mooring at 23°W to investigate intraseasonal variability and specifically the downward propagation of intraseasonal energy from the near-surface into the deep ocean. Between 20 and 50 m, intraseasonal variability at 23°W peaks at periods between 30 and 40 days. It is associated with westward-propagating tropical instability waves, which undergo an annual intensification in August. At deeper levels down to about 2000 m considerable intraseasonal energy is still observed. A frequency–vertical mode decomposition reveals that meridional velocity fluctuations are more energetic than the zonal ones for periods < 50 days. The energy peak at 30–40 days and at vertical modes 2–5 excludes equatorial Rossby waves and suggests Yanai waves to be associated with the observed intraseasonal energy. Yanai waves that are considered to be generated by tropical instability waves propagate their energy from the near-surface west of 23°W downward and eastward to eventually reach the mooring location. The distribution of intraseasonal energy at the mooring position depends largely on the dominant frequency and the time, depth, and longitude of excitation, while the dominant vertical mode of the Yanai waves plays only a minor role. Observations also show the presence of weaker intraseasonal variability at 23°W below 2000 m that cannot be associated with tropical instability waves

An elevated turbulent mixing event caused by a near-inertial wave in the mixed layer

Between 2005 and 2016, an extensive shipboard and autonomous microstructure measurement program was carried out in the proximity of PIRATA sites in the central and northeastern tropical Atlantic. The data reveal regional variability of upper ocean mixing processes from diurnal to seasonal time scales. Here, we discuss an elevated turbulent mixing event below the mixed layer caused by surface near-inertial waves (NIWs) and address the impact of these mixing events on the mixed layer heat balance at the PIRATA site at 11.5°N, 23°W. Altogether, microstructure data at this site was collected during 8 different cruises. During one incident, sampling was conducted during the presence of an elevated NIW. Velocities associated with the NIW were above 0.6ms-1 in the mixed layer and decreased to near zero below the stratification maximum at 30m depth. Mixing during the presence of the NIW was strongly elevated and dissipation rates of turbulent kinetic energy exceeded 1x10-5m2s-3 in the stratified region below the mixed layer in some profiles. Associated cooling of the sea surface temperature was also elevated. Diapycnal heat flux was above 140Wm-2 10m below the mixed layer and more than 300Wm-2 in the region 5m below the mixed layer. Near-inertial wind stress magnitude (NIWSM) during the period war particularly high. Wind energy flux to NIWs from a slab ocean model is used to estimate the frequency of the occurrence of the elevated NIW ocean velocity