Using NIW Observations to Assess Mixed Layer Parameterizations: A Case Study in the Tropical Atlantic
Marta MrozowskaMarkus JochumSwantje BastinRebecca HummelsА. В. КолдуновMarcus DenglerTim FischerRoman NutermanRagnhild Hansen
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Abstract Tropical sea surface temperature (SST) biases can cause atmospheric biases on global scales, hence SST needs to be represented well in climate models. A major source of uncertainties is the representation of turbulent mixing in the oceanic boundary layer, or mixed layer (ML). In the present study we focus on near‐inertial wave (NIW) induced mixing. The performance of two mixing schemes, Turbulent Kinetic Energy and K‐profile parameterization (KPP), is assessed at two sites (11.5°N, 23°W and 15°N, 38°W) in the tropical Atlantic. At 11.5°N, turbulence observations (eddy diffusivities, shear and stratification) are available for comparison. We find that the schemes differ in their representation of NIWs, but both under‐represent the observed enhanced diffusivities below the observed ML. However, we find that the models do mix below the ML at 15°N when a storm passes nearby. The near‐inertial oscillations remain below the ML for the following 10 days. Near‐inertial kinetic energy (NIKE) biases in the models are not directly correlated with the wind speed, the MLD biases, or the stratification at the ML base. Instead, NIKE biases are sensitive to the vertical mixing scheme parameterization. NIKE biases are lowest when the KPP scheme is used.Keywords:
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Abstract Variations in tropical Atlantic SST are an important factor in seasonal forecasts in the region and beyond. An analysis is given of the capabilities of the latest generation of coupled GCM seasonal forecast systems to predict tropical Atlantic SST anomalies. Skill above that of persistence is demonstrated in both the northern tropical and equatorial Atlantic, but not farther south. The inability of the coupled models to correctly represent the mean seasonal cycle is a major problem in attempts to forecast equatorial SST anomalies in the boreal summer. Even when forced with observed SST, atmosphere models have significant failings in this area. The quality of ocean initial conditions for coupled model forecasts is also a cause for concern, and the adequacy of the near-equatorial ocean observing system is in doubt. A multimodel approach improves forecast skill only modestly, and large errors remain in the southern tropical Atlantic. There is still much scope for improving forecasts of tropical Atlantic SST.
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Seasonal mean values of tropical Sea Surface Temperature (SST) and Atlantic/European Mean Sea Level Pressure (MSLP) from a 301-year coupled ocean/atmosphere model run are analysed statistically. Relations between the two fields are identified on both interannual and interdecadal timescales. It is shown that tropical SST variability affects Atlantic/European MSLP in winter. In particular, there appears to be a statistically significant relation, between the leading modes of variability, the El Niño/Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO). During cold ENSO (La Niña) years the NAO tends to be in its positive phase, while the opposite is the case during warm ENSO (El Niño) years, although to a lesser extent. Similar analyses that are presented for gridded observational data, confirm this result, although here tropical Atlantic SST appears to be stronger related to the NAO than tropical Pacific SST. The linear predictability of a model simulated NAO index is estimated by making statistical predictions that are based on model simulated tropical SST. It is shown that the predictive skill is rather insensitive to the length of the training period. On the other hand, the skill score estimate can vary significantly as a result of interdecadal variability in the climate system. These results are important to bear in mind when making statistical seasonal forecasts that are based on observed SST.
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This observational study focuses on remote forcing of the dominant pattern of north tropical Atlantic sea surface temperature (SST) anomalies by ENSO and NAO. Based on a spring SST index of the north tropical Atlantic (NTA) SST (5°–25°N), it is shown that almost all NTA–SST extreme events from 1950 to the present can be related to either ENSO or NAO. Since the SST NTA events lag NAO and ENSO events, NTA variability is interpreted as being largely a response to remote NAO or ENSO forcing. The local response of the tropical Atlantic to these external sources—whether it be ENSO or the NAO—is observed to be rather similar: changes in surface winds induce changes in latent heating that, in turn, generate SST anomalies. Once generated, the latter are damped through local air–sea interaction, at a rate estimated to be 10 W m−2 K−1. Experiments with simple models, but driven by observations, strongly suggests that variability on interannual to interdecadal timescales—both time series and spectral signatures—can be largely explained as a result of direct atmospheric forcing, without the need to invoke a significant role for local unstable air–sea interactions or ocean circulation.
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The tropical Atlantic region, unlike the tropical Pacific, is not dominated by any single mode of climate variability such as the El Niño–Southern Oscillation (ENSO). Rather, this region is subject to multiple competing influences of comparable importance. The nature and potential predictability of these various influences has been investigated by analysis of an ensemble of atmospheric GCM integrations forced with observed SST for the period December 1948–November 1993. The dominant modes of internal atmospheric and SST-forced variability are determined. Internal variability in the tropical Atlantic region is dominated by the equatorward extension of extratropical patterns, especially the North Atlantic oscillation. Three different SST-forced signals are identified. These are (a) a remote response to ENSO, (b) a response to the so-called Atlantic Dipole SST pattern, and (c) a response to equatorial Atlantic SST anomalies. The spatial structure and seasonality of these different elements of climate variability are diagnosed and feedbacks onto the ocean are assessed. The evidence presented supports the possibility of ENSO-like variability in the equatorial Atlantic, but does not support the suggestion that the Atlantic Dipole is a coupled ocean–atmosphere mode of variability. An important feature of this study is that the results include quantitative estimates of the comparative importance, in different regions and different seasons, of the various influences on tropical Atlantic climate variability. These estimates are used to assess the potential predictability of key climatic indices.
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Abstract The direct response of the tropical mixed layer to near-inertial waves (NIWs) has only rarely been observed. Here, we present upper-ocean turbulence data that provide evidence for a strongly elevated vertical diffusive heat flux across the base of the mixed layer in the presence of a NIW, thereby cooling the mixed layer at a rate of 244 W m −2 over the 20 h of continuous measurements. We investigate the seasonal cycle of strong NIW events and find that despite their local intermittent nature, they occur preferentially during boreal summer, presumably associated with the passage of atmospheric African Easterly Waves. We illustrate the impact of these rare but intense NIW induced mixing events on the mixed layer heat balance, highlight their contribution to the seasonal evolution of sea surface temperature, and discuss their potential impact on biological productivity in the tropical North Atlantic.
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Abstract Tropical Atlantic Variability (TAV) is simulated in a coupled GCM. The TAV seems to be consistent with a dipole mode that involves both surface and subsurface oceanic dynamics. The poor correlation of the tropical North and South Atlantic SST is suggested to be distorted by the presence of a symmetric tropical Atlantic mode. Copyright © 2000 Royal Meteorological Society.
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Based on Levitus 94’ climatological temperature and salinity data, the temporal and spatial features of the mixed layer in the South China Sea (SCS) are derived, and the regular pattern of the mixed layer depth and temperature are analysed. Data analysis indicates that the monsoon has an obvious impact on the temporal and spatial features of the mixed layer in the SCS through the adjustment of the current field. The complicated effect of the wind includes not only influence on the mixed layer depth directly through sea surface Ekman process, but also divergence or convergence forced by the large scale circulation, which would favorite or suspend development of the mixed layer depth. The relationship between the mixed layer depth and temperature over the SCS is revealed. The formation of the maximum mixed layer depth in summer exhibits a process that the 28℃ isotherm line overlaps the bottom of the mixed layer, whereas the formation of the maximum mixed layer depth in winter is a process that the 28℃ warm water completely disappears and the isotherm lines intersect to the bottom of the mixed layer, and the seasonal thermocline is ventilated in the northern SCS.
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