Climatic Effects of Spring Mesoscale Oceanic Eddies in the North Pacific: A Regional Modeling Study

A high-resolution atmospheric model of the Weather Research and Forecast (WRF) is used to investigate the climatic effects of mesoscale oceanic eddies (OEs) in the North Pacific (NPac) in spring and the respective effects of OEs in the northern NPac associated with the Kuroshio Extension (KE) and of OEs in the southern NPac related to the subtropical countercurrent. Results show that mesoscale OEs in the NPac can strengthen the upper-level ridge (trough) in the central (eastern) subtropical NPac, together with markedly weakened (strengthened) westerly winds to its south. The mesoscale OEs in the whole NPac act to weaken the upper-level storm track and strengthen lower-level storm activities in the NPac. However, atmospheric responses to the northern and southern NPac OEs are more prominent. The northern NPac OEs can induce tropospheric barotropic responses with a tripole geopotential height (GPH) anomaly pattern to the north of 30° N, while the OEs in both the northern and southern NPac can enhance the upper-level ridge (trough) in the central (eastern) subtropical NPac. Additionally, the northern NPac OEs can shrink the lower-level subtropical high and weaken the easterly trade winds at the low latitudes, while the southern NPac OEs result in a southward shift of the lower-level subtropical high and an eastward shift of the upper-level westerly jet stream. The southern and northern NPac OEs have similar effects on the storm track, leading to an enhanced lower-level storm track over the KE via moistening the atmospheric boundary layer; and they can also exert significant remote influences on lower- and upper-level storm activities over the Northeast Pacific off the west coast of North America. When the intensities of OEs are doubled in the model, the spatial distribution of atmospheric responses is robust, with a larger and more significant magnitude. Additionally, although OEs are part of the mesoscale oceanic processes, the springtime OEs play an opposite role in mesoscale sea-surface temperature anomalies. These findings point to the potential of improving the forecasts of extratropical springtime storm systems and the projections of their responses to future climate change, by improving the representation of ocean eddy-atmosphere interaction in forecast and climate models.

Local and Remote Forcing Effects of Oceanic Eddies in the Subtropical Front Zone on the Mid-latitude Atmosphere in Winter

Multiple oceanic eddies coexist in the North Pacific subtropical front zone (STFZ) in winter, which can be classified into the isolated single eddies (ISO), the combined double isotropic eddies (DBL) and pairs of anisotropic eddies (PAIR). The forcings of these eddies on the mid-latitude atmosphere are investigated using Climate Forecast System Reanalysis (CFSR) data from year 1979 to 2009, which are divided into the remote and local effects in this research. In the stronger STFZ years,there are more ISO and DBL cyclonic eddies to the north but more ISO and DBL anticyclonic eddies to the south of the STFZ, meanwhile more PAIR eddies with cold to the north and warm to the south concentrated around the main axis of the STFZ. These eddy distributions enhance the strength of STFZ, intensify the propagation of upwards baroclinic waves in the lower atmosphere, and finally enhance the zonal wind at upper atmosphere, which is defined as the remote effects of the eddies. However, distinct from this basin-scale remote forcings, three types of oceanic eddies also have different local forcings on the maritime atmospheric boundary layer (MABL) over these eddies. The local effects of the ISO and DBL eddies on MABL entirely depend on the numbers and polarity of the eddy center, while the MABL response to the PAIR eddies appears at the boundary of the two eddies. Furthermore, the local effects of the three types of eddies can be traced to the middle atmosphere accompanied by local precipitation differences.

Atmospheric Responses to Mesoscale Oceanic Eddies in the Winter and Summer North Pacific Subtropical Countercurrent Region

In the winter and summer North Pacific Subtropical Countercurrent region, the atmospheric responses to 20,000+ mesoscale oceanic eddies (MOEs) are examined using satellite and reanalysis data from 1999 to 2013. The composite results indicate that surface wind speed, cloud, and precipitation anomalies are positively correlated with sea surface temperature anomalies in both seasons. The surface wind speed anomalies and convective precipitation anomalies show dipolar structures centering on MOEs in winter and on unipolar structures in summer. In both seasons, the vertical mixing mechanism plays an obvious role in the atmospheric responses to MOEs. In addition, the distributions of sea level pressure anomalies in winter reflects the effects of the pressure adjustment mechanism. Due to the seasonal variations in the atmospheric background state and the MOEs, the sensitivities of surface wind speeds, clouds, and precipitation responses to MOEs in summer are over 30% higher than those in winter.

Regional Dependence of Atmospheric Responses to Oceanic Eddies in the North Pacific Ocean

Based on sea surface height anomaly (SSHA) from satellite altimeter and microwave radiometer datasets, this study investigates atmospheric responses to oceanic eddies in four subdomains of the North Pacific Ocean with strongest eddy activity: Kuroshio Extension (KE), Subtropical Front (SF), California Coastal Current (CC) and Aleutian Islands (AI). Analyses show that anticyclonic eddies cause sea surface temperature, surface wind speed and precipitation rate to increase in all four subdomains, and vice versa. Through a further examination of the regional dependence of atmospheric responses to oceanic eddies, it is found that the strongest and the weakest surface wind speed responses (in winter and summer) are observed in the KE and AI region, respectively. For precipitation rate, seasonal variation of the atmospheric responses to oceanic eddies is strongest in winter and weakest in summer in the KE, CC and AI regions, but stronger in summer in the SF area. The reasons for such regional dependence and seasonality are the differences in the strength of SST anomalies, the vertical kinetic energy flux and atmospheric instability in the four subdomains.