Seasonal variation of atmospheric coupling with oceanic mesoscale eddies in the North Pacific Subtropical Countercurrent
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Abstract: This study investigated the seasonal variation in the atmospheric response to oceanic mesoscale eddies in the North Pacific Subtropical Countercurrent (STCC) and its mechanism, based on satellite altimetric and reanalysis datasets. Although mesoscale eddy in the study area is more active in summer, the sea surface temperature (SST) anomaly associated with mesoscale eddies is more intense and dipolar in winter, which is largely due to the larger background SST gradient. Similarly, the impact of the oceanic eddy on sea surface wind speed and heat flux is strongest in winter, whereas its effect on precipitation rate is more significant in summer. The study revealed that the SST gradient in STCC could impact the atmosphere layer by up to 800 hPa (900 hPa) in boreal winter (summer) through the dominant vertical mixing mechanism. Moreover, the intensity of the SST gradient causes such seasonal variation in mesoscale air-sea coupling in the study region. In brief, a stronger (weaker) background SST gradient field in wintertime (summertime) leads to a larger (smaller) eddy-induced SST anomaly, thus differently impacting atmosphere instability and transitional kinetic energy flux over oceanic eddies, leading to seasonal variation in mesoscale air-sea coupling intensity.
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Figure 1. Climatological monthly EKE (black dots and curve) spatially averaged in STCC area (15°–28°N, 123°–170°E). Gray shading indicates standard deviation of EKE.
Figure 2. Seasonal variation of eddy SST anomaly (color shading) and sea surface height (SSH) anomaly (contours; unit: cm; interval: 3 cm) structures. Upper (bottom) panels are for cyclonic (anticyclonic) eddies. The left panels are for July and the right are for January. The dotted areas mean the t-test is passed at a 95% confidence level.
Figure 3. Meridional component of eddy rotational velocity in the east-west section across composite eddy center in STCC.
Figure 4. Seasonal variation of composite SST anomaly (contours, unit: °C) and WSA (color shading) structures associated with mesoscale eddies in July for summer and January for winter.
Figure 5. Composite latent heat flux anomaly (color shading) on the sea surface and SST anomaly (contours, unit: °C) of cyclonic eddies (a, b) and anticyclonic eddies (c, d).
Figure 6. Composite sensible heat flux anomaly (color shading) on the sea surface and SST anomaly (contours, unit: °C) of cyclonic eddies (a, b) and anticyclonic eddies (c, d).
Figure 7. Composite rain rate anomaly (color shading) and SST anomaly (contours, unit: °C) of cyclonic eddies (a, b) and anticyclonic eddies (c, d).
Figure 8. Correlations between SST anomaly and atmospheric variable anomaly for cyclonic (blue) and anticyclonic (red) eddies, respectively. Blue (red) shading is the standard deviation for cyclonic (anticyclonic) eddy. Variables from left to right panels: sea surface wind speed anomaly, rain rate anomaly, latent heat flux anomaly, and sensible heat flux anomaly. The slope (s1, s2) of straight lines and correlation coefficients (r1, r2) between SST anomaly and atmospheric parameters overlying the mesoscale eddies are listed at the bottom of each panel. (s1, r1) are for cyclonic eddies, and (s2, r2) are for anticyclonic eddies. The anomalous variables are obtained from the pre-filtered parameters within the scope of eddies.
Figure 9. Climatological seasonal background SST (isolines, unit: °C) and SST gradient (color shading) (a, c), wind directional steadiness (color shading, unit: 1) and wind speed (isolines, m/s) (b, d).
Figure 10. Temporal (upper panel) and spatial (lower panel) correlation coefficients between wind divergence item and downwind SST gradient (a, c) and SST Laplacian (b, d) item. A 10-m wind field is used. The monthly average magnitude of the mesoscale temperature gradient (MTG) is represented as red circles and curve in c, and gray shading in c and d is the standard deviation of spatial correlation coefficient.
Figure 11. Spatial correlation coefficient between downwind SST gradient and wind divergence at vertical levels from 1 000 hPa to 750 hPa in July and January. The red and blue shadings are the standard deviation of the correlation coefficient for July and January, respectively.
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