Sea Surface Cooling Induced by Extratropical Cyclones in the Subtropical North Pacific: Mechanism and Interannual Variability
Sea Surface Cooling Induced by Extratropical Cyclones in the Subtropical North Pacific: Mechanism and Interannual Variability
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DOI：
10.1029/2018jc014632
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发表时间：
2019-03-01
影响因子：
3.6
通讯作者：
Iwasaka, Naoto
中科院分区：
文献类型：
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作者：
Kobashi, Fumiaki;Doi, Haruki;Iwasaka, Naoto
研究了热带气旋对海洋的影响，重点是北太平洋副热带锋（STF），它是一个位于北纬25-30度的高海表温度（SST）梯度区。STF在春季变得最明显，并且容易受到气旋频繁通过的影响。卫星观测显示，SST的显着下降，同时与气旋通过沿着STF。SST下降约20天，然后逐渐恢复约50天。降温幅度约为0.4摄氏度。海洋再分析资料显示，随着混合层的加深，类似的表面冷却。冷却似乎达到了季节性温跃层，并持续了一个多星期。在STF上空的气旋是一个次天气气旋，伴随着一个更大的天气高压在低压的西北部。地面冷却的主要原因是由于低层云带导致的短波辐射减少以及低、高压系统之间偏北风增强导致的潜热和海洋卷吸的增加。STF的强度在年际时间尺度上变化，以响应西太平洋模式，改变上覆大气的斜压性和气旋的活动。春季强气旋活动减弱了季节性海温增暖。这表明，气旋不仅影响混合层的变化，也影响季节性温跃层的变化。简单的语言摘要热带气旋是一种影响日常天气和气候的低压系统。众所周知，强烈的热带气旋，如台风和飓风，会在其下方和后方留下寒冷的海面尾迹。然而，人们对热带气旋知之甚少。在这项研究中，我们研究了热带气旋如何改变海洋，重点是北太平洋副热带锋（STF），这是一个位于北纬25-30度的高海面温度梯度区。STF在春季变得最明显，并且容易受到气旋频繁通过的影响。我们发现，温带气旋导致海面冷却沿着STF，主要是由于云屏蔽传入的太阳辐射和增强的蒸发冷却和海洋混合。冷却并不局限于表面附近，但似乎达到了约100米的地下深度，并保持在这个深度比在表面更长。STF的年际变化改变了锋面气旋的发生和发展，影响春季的季节变暖趋势。大量的气旋过境减弱了季节变暖。
The influence of extratropical cyclones on the ocean is examined with a focus on the North Pacific subtropical front (STF), which is a zone of high sea surface temperature (SST) gradients located approximately 25-30 degrees N. The STF becomes most pronounced and susceptible to frequent passages of cyclones in spring. Satellite observations reveal a significant decrease in SST concurrent with cyclone passage along the STF. The SST decreases for approximately 2days and then gradually recovers over approximately 5days. The magnitude of the cooling is approximately 0.4 degrees C. The ocean reanalysis data show a similar surface cooling with a deepening of the mixed layer. The cooling seems to reach the seasonal thermocline and persist there for more than a week. A cyclone over the STF is a subsynoptic cyclone accompanied by a larger synoptic high to the northwest of the low. The surface cooling is mainly attributable to the decrease in shortwave radiation due to the cloud band of the low and the increases in the latent heat and the ocean entrainment, which are associated with intensified northerly winds between the low- and high-pressure systems. The STF varies in strength on interannual timescales in response to the western Pacific pattern, changing the baroclinicity of the overlying atmosphere and the activity of the cyclones. The intense cyclone activity weakens the seasonal SST warming in spring. It is suggested that the cyclones affect the variability of not only the mixed layer but also the seasonal thermocline.Plain Language Summary An extratropical cyclone is a low-pressure system that affects daily weather and climate. It is well known that intense tropical cyclones such as typhoons and hurricanes leave a cold sea surface wake beneath and behind them. However, little is known about extratropical cyclones. In this study, we examine how extratropical cyclones change the ocean, with a focus on the North Pacific subtropical front (STF), which is a zone of high sea surface temperature gradients located approximately 25-30 degrees N. The STF becomes most pronounced and susceptible to frequent passages of cyclones in spring. We found that extratropical cyclones cause sea surface cooling along the STF, mainly due to the cloud shielding of incoming solar radiation and the enhanced evaporative cooling and ocean mixing. The cooling is not limited to near the surface but seems to reach subsurface depths of approximately 100m and remains longer at this depth than at the surface. The year-to-year variability of the STF alters the occurrence and growth of cyclones over the front, affecting the seasonal warming trend in spring. A large number of cyclone passages weakens the seasonal warming.