4:15 PM - 4:30 PM
[ACG38-16] Influence of marginal seas' hotspot on extratropical cyclones: explosive and binary cyclogenesis in idealized experiments
Keywords:extratropical cyclone, binary cyclone, marginal seas' hotspot
Over the marginal seas of the western North Pacific, the active region of extratropical cyclones is divided into two (the Sea of Japan and the southern coast of the Japanese main islands) and some cyclones often develop explosively. In addition, binary extratropical cyclones, called twin extratropical cyclones, are also often observed. A few works have studied cases of binary lows associated with (i) upper-level double jets (Ogura et al. 2006; Hitsuma 2006), (ii) the formation of binary lows in single jets (Yamamoto 2012, 2018), and (iii) transition from double jets to single jets (Yokoyama & Yamamoto 2019). At the present stage, one does not fully understand why binary cyclone structures often form around Japan. In winter, high sea surface heat fluxes are concentrated into the marginal seas of the western North Pacific. Recently, such a localized, strong surface heat flux at midlatitudes is called a "hotspot" in the climate system (e.g., Nakamura et al. 2015). The atmospheric response to the hotspots in the marginal seas has been studied in idealized atmospheric general circulation models from the viewpoint of geophysical fluid dynamics, to discuss the temperature decrease around the east coast of the continent and cyclonic activity over the North Pacific Basin (e.g., Kaspi & Schneider 2011).
The purposes of this study are to reconsider the atmospheric response to localized heating over marginal seas of the western North Pacific, and to clarify the impact of the sea hotspot on binary cyclones near Japan. In idealized experiments used, the setup of Held & Suarez (1994) was applied to the global Weather Research and Forecasting model (e.g., Dias Pinto & Mitchell, 2014). The present work assumed that the North Pacific marginal seas' hotspot heats the lower atmosphere. The steady and localized heating was given by the triangular region. Several sensitivity experiments were conducted by changing the location and thickness of this heating region.
In the area of enhanced westerly, the baroclinicity and short-period disturbance activity at low levels are enhanced by the hotspot. In addition, the frequency of explosive cyclones increases around the hotspot. Over the marginal seas of the western North Pacific, a pressure trough extends in a north-south direction and two maxima of cyclone activity (intensity of short-period disturbances) forms. These two maxima form even when the heating parameters are altered, and thus the hotspot is an important factor in forming the bifurcation of the cyclone activity area around Japan (Japan Sea path and south coast path). In the time-latitude cross-section of sea level pressure over the hotspot, a binary cyclone appears under the condition that a deep pressure trough has developed. A binary cyclone system appears in the early stage of the cyclone development, and the contact cyclone pair merges during the fully developed stage. This is consistent with several case studies of explosive, binary cyclones (Yamamoto 2018; Yokoyama & Yamamoto 2019).
A simple framework for investigating the atmospheric response to an oceanic hotspot in the western North Pacific shows that the heating of the lower atmosphere by the hotspot leads to the formation of a north-south bifurcation of the active cyclone area and increases the frequency of explosive cyclones. The formation of binary lows in the absence of the Japanese Islands suggests the importance of enhanced pressure troughs associated with heating over the marginal seas' hotspot.
The purposes of this study are to reconsider the atmospheric response to localized heating over marginal seas of the western North Pacific, and to clarify the impact of the sea hotspot on binary cyclones near Japan. In idealized experiments used, the setup of Held & Suarez (1994) was applied to the global Weather Research and Forecasting model (e.g., Dias Pinto & Mitchell, 2014). The present work assumed that the North Pacific marginal seas' hotspot heats the lower atmosphere. The steady and localized heating was given by the triangular region. Several sensitivity experiments were conducted by changing the location and thickness of this heating region.
In the area of enhanced westerly, the baroclinicity and short-period disturbance activity at low levels are enhanced by the hotspot. In addition, the frequency of explosive cyclones increases around the hotspot. Over the marginal seas of the western North Pacific, a pressure trough extends in a north-south direction and two maxima of cyclone activity (intensity of short-period disturbances) forms. These two maxima form even when the heating parameters are altered, and thus the hotspot is an important factor in forming the bifurcation of the cyclone activity area around Japan (Japan Sea path and south coast path). In the time-latitude cross-section of sea level pressure over the hotspot, a binary cyclone appears under the condition that a deep pressure trough has developed. A binary cyclone system appears in the early stage of the cyclone development, and the contact cyclone pair merges during the fully developed stage. This is consistent with several case studies of explosive, binary cyclones (Yamamoto 2018; Yokoyama & Yamamoto 2019).
A simple framework for investigating the atmospheric response to an oceanic hotspot in the western North Pacific shows that the heating of the lower atmosphere by the hotspot leads to the formation of a north-south bifurcation of the active cyclone area and increases the frequency of explosive cyclones. The formation of binary lows in the absence of the Japanese Islands suggests the importance of enhanced pressure troughs associated with heating over the marginal seas' hotspot.