operate in the model. Due to the small zonal extent of the equatorial Atlantic, the observed Bjerknes feedback acts quasi-instantaneously during the dynamically active periods of boreal summer and early boreal winter. Then, all elements of the observed Bjerknes feedback operate simultaneously. The model cannot reproduce this, although it hints at a better performance when using bias reduction.
We quantify seasonal prediction skill of tropical winter rainfall in 14 climate forecast systems. High levels of seasonal prediction skill exist for year-to-year rainfall variability in all tropical ocean basins. The tropical East Pacific is the most skilful region, with very high correlation scores, and the tropical West Pacific is also highly skilful. Predictions of tropical Atlantic and Indian Ocean rainfall show lower but statistically significant scores.We compare prediction skill (measured against observed variability) with model predictability (using single forecasts as surrogate observations). Model predictability matches prediction skill in some regions but it is generally greater, especially over the Indian Ocean. We also find significant inter-basin connections in both observed and predicted rainfall. Teleconnections between basins due to El Niño-Southern Oscillation (ENSO) appear to be reproduced in multi-model predictions and are responsible for much of the prediction skill. They also explain the
Due to strong mean state‐biases most coupled models are unable to simulate equatorial Atlantic variability. Here, we use the Kiel Climate Model to assess the impact of bias reduction on the seasonal prediction of equatorial Atlantic sea surface temperature (SST). We compare a standard experiment (STD) with an experiment that employs surface heat flux correction to reduce the SST bias (FLX) and, in addition, apply a correction for initial errors in SST. Initial conditions for both experiments are generated in partially coupled mode, and seasonal hindcasts are initialized at the beginning of February, May, August and November for 1981–2012. Surface heat flux correction generally improves hindcast skill. Hindcasts initialized in February have the least skill, even though the model bias is not particularly strong at that time of year. In contrast, hindcasts initialized in May achieve the highest skill. We argue this is because of the emergence of a closed Bjerknes feedback loop in boreal summer in FLX that is a feature of observations but is missing in STD.
Near-inertial oscillations are ubiquitous in the ocean and are believed to play an important role in the global climate system. Studies on wind power input to near-inertial motions (WPI) have so far focused primarily on estimating the time-meanWPI, with little attention being paid to its temporal variability. In this study, a combination of atmospheric reanalysis products, a high-resolution ocean model and linear regression models are used to investigate for the first time the relationship between interannual variability of WPI in the North Atlantic and the North Atlantic Oscillation (NAO), motivated by the idea that the NAO serves as a good indicator for storminess over the North Atlantic and that storms account for the majority of WPI. It is found that WPI at low and high latitudes of the North Atlantic is significantly correlated to the NAO, owing to its influence on the configuration of the storm track. Positive (negative) NAO conditions are associated with increased WPI in the subpolar (subtropical) ocean. Basin-wide WPI is found to be significantly enhanced under negative NAO conditions, but is not significantly different from the climatological average under positive NAO conditions. This indicates a weak inverse relationship between basin-wide WPI and the NAO, contradicting intuitive expectations. The asymmetric impact of the NAO on basin-wide WPI results from greater sensitivity of WPI to near-inertial wind forcing at lower latitudes due to the variation of the Coriolis parameter with latitude
The Bjerknes feedback is the dominant positive feedback in the equatorial ocean basins. To examine the seasonality, symmetry, and stationarity of the Pacific and Atlantic Bjerknes feedbacks we decompose them into three feedback elements that relate thermocline depth, sea surface temperature, and western basin wind stress variability to each other. We partition feedback elements into composites associated with positive or negative anomalies. Using robust regression, we diagnose the strength of each composite. For the recent period 1993–2012, composites of the Pacific Bjerknes feedback elements agree well with previous work. Positive composites are generally stronger than negative composites, and all feedback elements are weakest in late boreal spring. In the Atlantic, differences between positive and negative composites are less consistent across feedback elements. Specifically, wind variability seems to play a less important role in shaping atmosphere‐ocean coupling in the Atlantic when compared to the Pacific. However, a clear seasonality emerges: Feedback elements are generally strong in boreal summer and, for the negative composites, again in boreal winter. The Atlantic Bjerknes feedback is dominated by subsurface‐surface coupling. Applying our analysis to overlapping 25‐year periods for 1958–2009 shows that the strengths of feedback elements in both ocean basins vary on decadal time scales. While the overall asymmetry of the Pacific Bjerknes feedback is robust, the strength and symmetry of Atlantic feedback elements vary considerably between decades. Our results indicate that the Atlantic Bjerknes feedback is nonstationary on decadal time scales.
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