Uncovering impacts of the midlatitude ocean is a part of the “Climatic hotspot project, as profiled here by Professor Hisashi Nakamura from the Research Center for Advanced Science and Technology, at the University of Tokyo in Japan
By way of background, it has long been believed that the extratropical ocean is passive to atmospheric variability and, therefore, anomalies in extratropical sea-surface temperature (SST) provide no predictability in climatic condition. In fact, the current operational seasonal prediction for the extratropics finds its basis on remote influence from tropical ocean-atmosphere variability which is typified by El Niño/La Niña. This conventional notion was established on the basis of previous findings that extratropical SST anomalies form as a response to BOTH wind and temperature fluctuations, which have been derived from SST data based on in-situ observations and output data from atmospheric general circulation models with low spatial resolution.
However, recent satellite data and high-resolution numerical modelling have started offering a different view. Utilising such high-resolution data, our project “Hotspots in the Climate System” or “Hotspot Project”, aims to present convincing evidence that the mid-latitude ocean, especially, an intense warm current along the extreme western portion of each ocean basin, such as the Gulf Stream or Kuroshio, can exert thermodynamic forces on the atmosphere through heat and moisture release.
The eastward extension of the warm current merges with a cold current, forming an oceanic frontal zone with a pronounced SST gradient. Variability of the frontal zone as a response of the ocean gyre to anomalous wind stress can, therefore, yield strong SST anomalies and thermodynamic forcing on the atmosphere by modifying heat and moisture release. We regard these warm western boundary currents (WBCs) and associated frontal zones as climatic ‘hotspots’ that must be significant in shaping the tropospheric circulation and variability.
Well-designed structure for the challenge and capacity building
Our hotspot project challenges the aforementioned established notion by exploring the climatic impacts of the WBCs and SST fronts through the effective combination of numerical, analytical and observational studies. Under five-year funding from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), the project was initiated in July 2012 as the collective endeavour of 18 universities and four research institutes, where over 100 researchers and graduate students across Japan are involved.
Led by Professor Hisashi Nakamura, the project consists of nine main programmes and a dozen of smaller supplementary programmes, with umbrella projects under which crosscutting research activity is promoted for challenging four important topics: Oceanic jets and SST fronts, Pacific decadal climate variability, and air-sea interactions under the East Asian summer/winter monsoons. The project encourages close collaborations between physical oceanographers and atmospheric/climate scientists, as well as international collaboration, therefore, offering an excellent environment for capacity building. In fact, the number of PhD students in the project increased by 150% during the five years.
Unique field observations
One of the main programmes is specially designed for field observations. Experts in the programme led observation campaigns just east of Japan, including the one in early July 2012. It featured a unique strategy where three research vessels aligned meridionally at fixed intervals crossed the Kuroshio Extension and associated SST front repeatedly from the south and north over five days, while conducting GPS sonde and expendable bathythermograph observations for the atmosphere and upper ocean, respectively and synchronously every two hours.
Unlike many previous attempts by single-vessel observations, our campaign was the first to unambiguously reveal a detailed vertical structure of the SST front and cross-frontal contrasts in the atmosphere, including the vertical structure of the boundary layer and low-level clouds. These findings are valuable as a benchmark for numerical models.
Outcomes and achievements
The outcome of the hotspot project was truly substantial and represented as 400 peer-reviewed papers in international journals after a five-year funding period, and new papers are still coming out. Some of them are archived in a special section of the Journal of Oceanography (2015) and a special collection of the American Meteorological Society “Climate Implications of Frontal Scale Air–Sea Interaction”.
Jointly with the Japan Meteorological Agency, the project has produced a new product of global atmospheric reanalysis with high-resolution SST (JRA55-CHS). We presented solid evidence that midlatitude oceanic frontal zones favour the recurrent development of cyclones and anticyclones to form regions called “storm tracks”. Moisture supply from the warm WBCs favours explosive development of cyclones. Wintertime cyclone tracks are found sensitive to the Kuroshio meanders, modulating snowfall probability in Tokyo.
Recurrent cyclone development shapes low-level cloud distribution, which influences the Earth’s radiation budget, and organises large-scale rain bands along the frontal zones. Through storm track formation, the midlatitude oceanic frontal zones influence basin-scale or even hemispheric-scale westerly jet streams and their variability. For example, persistent SST anomalies in the Kuroshio/Oyashio Extensions act to force basin-scale variability in upper-level westerlies over the Pacific and semi-permanent surface low-pressure system (the Aleutian Low), whose climatic influence extends into North America.
SST frontal zones in the southern oceans are found to be essential for the dominant hemispheric-scale mode of westerly variability, through which extensive climatic trends were induced over the late 20th Century by the formation of the stratospheric Antarctic ozone hole. In summer, particularly high SSTs along the Gulf Stream or Kuroshio are found to organise deep convective clouds, especially under warm, moist air flows from the tropics, which sometimes give rise to torrential rainfall in coastal regions.
Towards the next stage
Our findings mentioned above urge the international community of climate science to promote high-resolution modelling of the ocean and atmosphere to resolve narrow midlatitude hotspots and their climatic impacts. During the 20th Century, the hotspots underwent locally enhanced warming, and their climatic role is, therefore, expected to increase under future warming.
In fact, recent ocean warming around Japan appears to yield enhanced surface evaporation and, thereby, increases the likelihood of torrential precipitation over Japan. The next stage of the project is under preparation to expand its scope into clarifying the role of the climatic hotspots in a future projection of the occurrence of extreme weather and climatic conditions, under on-going global warming. We will also explore similar hotspots in the Arctic, as we have found that declining sea-ice cover in the Barents/Kara Seas acts to strengthen the Siberian High and, thereby, induce severe winters over midlatitude Eurasia.
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Research Center for Advanced Science and
Technology, the University of Tokyo
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