Characteristics of 2018/2019 winter monsoon in Japan Masashi Sumitomo Tokyo Climate Center Japan Meteorological Agency I’m Masashi Sumitomo and an assistant forecaster at Tokyo Climate Center of JMA. Today I’d like to talk about the characteristics of 2018/19 winter monsoon in Japan.

Outline Overview of winter climate over Japan Warm winter factors Cold spells factors This is the outline of my presentation. Firstly, I will talk about overview of winter climate over Japan. After that, I will talk about two extreme events. One is that it was warm winter mainly in Okinawa and Amami. The other is that cold spells sometimes covered northern Japan.

Overview of Temperature Anomalies in this Winter Time series of 5-day running mean temperature anomaly for subdivisions Dec Jan Feb Northern Japan +0.4 ℃ Cold spells Eastern Japan +1.1 ℃ Western Japan +1.3 ℃ Okinawa Amami +1.8 ℃ In this winter, most areas of Japan experienced very warm. This figure shows time series of 5-day running mean temperature anomaly for subdivisions. From this figure, it was warm winter except in northern Japan. Especially in Okinawa and Amami, the seasonal mean temperature anomaly was +1.8 ℃ . It was the warmest winter since 1946/47. However, cold spells sometimes covered northern Japan. On early February, observed 850-hPa temperature at Sapporo was -24.4 ˚C. It was the lowest since 1957. The warmest winter since 1946/47 Warm winter In Okinawa / Amami, It was the warmest winter since 1946/47. Cold spells (Northern Japan) On early Feb, 850hPa temperature at Sapporo of −24.4 ℃ was the lowest since 1957.

Overview of Snowfall Ratios in this Winter For observation stations For subdivisions Sea of Japan side Northern Japan Pacific Ocean side 68% 61% The least since 1961/62 Sea of Japan 27% 26% Pacific Ocean 15% Eastern Japan 7% In this winter, snowfall was extremely below normal all over Japan. Left figure shows snowfall ratios for all observation stations. Right figure shows snowfall ratios for subdivisions. From these figures, snowfall was below normal mainly in western Japan. Western Japan Snowfall was extremely below normal all over Japan.

Outline Overview of winter climate over Japan Warm winter factors Cold spells factors Next, I will talk about factors of the warm winter.

5-month running mean of SST deviation Warm winter factors U 200 anomaly in winter 5-month running mean of SST deviation For NINO.3 2018 2019 Apr Jul Oct Jan Apr Jul SST : Sea Surface Temperature NINO.3 : 5°N~5°S,150°W~90°W Observed values There were two primary factors contributing the warm winter around Japan. One factor is the meandering of subtropical jet stream. Left figure shows zonal wind anomaly at 200-hPa in this winter. From this figure, the jet stream meandered northward around Japan. The other factor is the persistence of El Niño conditions. Right figure shows 5-month running mean of SST deviation for NINO.3. NINO.3 is one of El Niño monitoring areas. In this figure, red line and points show observed values and yellow boxes show predictions. From this figure, SST of NINO.3 have been more than 0.5 ℃ above reference from last autumn It is thought that El Niño conditions have persisted from last autumn. Japanese climate is often affected by El Niño. I will talk about these phenomenon in detail from next slides. Predictions by JMA/MRI-CGCM2 U 200 : Zonal wind at 200-hPa Subtropical jet stream meandered northward around Japan. El Niño conditions have persisted from autumn 2018.

Warm winter factors (meandering of jet stream) Ψ 300 anomaly and WAF in winter Max Z 500 anomaly at 40°N~80°N H Dec Blocking high L H L L Jan H L Feb H L Firstly, I will talk about meandering of subtropical jet stream. Left figure shows stream function anomaly and wave activity flux at 300-hPa. From this figure, you can see wave trains along the subtropical jet stream. The wave trains formed anticyclonic circulation anomaly over the East China Sea. Therefore, warm air often covered Japan. Right figure shows blocking high index. This index is maximum value of geopotential height anomaly at 500-hPa from 40°N to 80°N. From this figure, blocking high formed over the North Atlantic Ocean mainly in January. It is thought that the source of wave trains may be the blocking high. H Z 500 : Geopotential height at 500-hPa Ψ 300 : Stream function at 300-hPa WAF : Wave Activity Flux (Takaya and Nakamura 2001) Anticyclonic circulation anomaly formed over the East China Sea. Blocking high formed over the North Atlantic Ocean.

Warm winter factors (influence of El Niño) Percentage of mean temperature rank in El Niño events (winter 1958 - 2012) Due to statistical result of area averaged temperature in past. High Normal Low Significant difference by binomial test Secondly, I will talk about the influences of El Niño. Japanese climate is often affected by El Niño. This figure shows the percentage of mean temperature rank when El Niño events occur in winter. From this figure, when El Niño events occur in winter, temperatures tend to be normal or higher except northern Japan. Furthermore, in eastern Japan, there is significant difference from normal. When El Niño events occur in winter, temperatures tend to be normal or higher except northern Japan.

Warm winter factors (influence of El Niño) Ψ 850 : Stream function at 850-hPa Ψ 850 anomaly in winter 2018/19 Matsuno-Gill pattern H L H L Ψ 850 anomaly composite of El Niño (winter 1958/59 – 2012/13) Consistent I will talk about the response of El Niño to circulation field. Upper figure shows stream function anomaly at 850-hPa in this winter. Lower figure shows El Niño composite of the same element. From upper figures, anticyclonic circulation anomaly formed at the lower level around the Philippines and south of Japan. It is consistent with the El Niño composite. The response is called Matsuno-Gill pattern. It is associated with suppressed convection over the Maritime Continent. Along the edge of the anticyclone, warm and moist air often flowed into Okinawa and Amami. Anticyclonic circulation anomaly formed around the Philippines. Warm and moist air often flowed into Okinawa and Amami.

I focused on this Cold spells Outline Overview of winter climate over Japan Warm winter factors Cold spells factors I focused on this Cold spells Next, I will talk about factors of the cold spells. I focused on the cold spells of early February. Dec Jan Feb Northern Japan +0.4 ℃

Polar vortex shifted southward. Cold spells factors Z 500 anomaly (5th - 9th Feb) T 850 anomaly (5th – 9th Feb) Polar vortex shifted southward. Cold spell covered. C L I will talk about the example affected by cold spells in northern Japan. Left figure shows 5-day mean of geopotential height anomaly at 500-hPa from 5th February to 9th February. Right figure shows temperature anomaly at 850-hPa in the same period. From these figures, polar vortex shifted southward around northern Japan. Corresponding to the polar vortex, cold spells covered northern Japan. On 8th February, observed 850-hPa temperature at Sapporo was −24.4 ℃. It was the lowest since 1957. I will talk about one of the causes for the cold spells from next slides. Z 500 : Geopotential height at 500-hPa T 850 : Temperature at 850-hPa On 8th Feb, observed 850-hPa temperature at Sapporo was −24.4 ℃.

SSW : Stratospheric Sudden Warming Cold spells factors Z 30 : Geopotential height at 30-hPa T 30 : Temperature at 30-hPa T 30 over the North Pole Z 30 in January 2019 Major warming Weaker polar vortex over the North Pole Minor warming SSW ending SSW : Stratospheric Sudden Warming One of the causes is thought to be stratospheric sudden warming, SSW. SSW had persisted from mid December to late January. Left figure shows temperature at 30-hPa over the North Pole. From this figure, on 31st December, major warming occurred. The temperature suddenly rose by about 40 ℃. It took a month to return to the normal, and this warming had finished on 31st January. Right figure shows geopotential height at 30-hPa in January. From this figure, polar vortex shifted southward. And, it was stronger than normal over the Kamchatka Peninsula. Therefore, the Aleutian Low located in the northwest of its normal position. Cold spells sometimes covered northern Japan. Minor warming Temperature in the stratosphere rises more than 25 ℃ in one week. Major warming In addition to minor condition, zonal wind at less than 10-hPa changes east. Sep Nov Jan Mar Stronger polar vortex over the Far East SSW conditions had persisted from mid December to late January. Polar vortex sometimes shifted southward of the Far East. Cold spells sometimes covered northern Japan.

How does the SSW affect tropospheric circulation ? NAM index NAM : Northern Annular Mode NAM Pattern at 500-hPa hPa Propagate to troposphere Negative NAM / AO index → Weaker polar vortex The influence of SSW gradually propagated to the troposphere. Upper figures show NAM index and NAM pattern. Lower figure shows AO index. NAM means Northern Annular Mode, and AO means Arctic Oscillation. Both indices shows strength of polar vortex over the North Pole. If these indices are negative, it means that polar vortex is weaker than normal over the North Pole. From these figures, negative NAM index propagated from the stratosphere to the troposphere. The propagation reached the troposphere from mid January to early February. During this period, cold spells sometimes covered northern Japan. Dec Jan Feb NAM pattern : EOF-1 of monthly average geopotential height in Nov-Apr, 1985-2010 AO index AO : Arctic Oscillation

1. Meandering of subtropical jet 2. Suppressed convection Summary Siberian High 1. Bloking High 3. Polar Vortex Aleutian Low Anticyclonic anomaly at upper level 2. Anticyclonic anomaly at low level 1. Meandering of subtropical jet SST higher SST lower 2. El Niño 2. Suppressed convection Warm air often covered Japan. Wave trains Warm and moist air often flowed into Okinawa /Amami. El Niño Cold spells sometimes covered northern Japan. Stratospheric Sudden Warming (SSW) Let me summarize. This figure shows the characteristics of this winter. Firstly, warm air often covered Japan. Wave trains propagated from the North Atlantic Ocean to East Asia. As a result, subtropical jet stream meandered, and anticyclonic anomaly formed at the upper level over the East China Sea. So, warm air often covered Japan. Secondly, warm and moist air often flowed into Okinawa and Amami. El Niño conditions have persisted from last autumn, and convection was suppressed over the Maritime Continent. As a result, anticyclonic anomaly formed at the lower level around the Philippines and the south of Japan. Along its edge, warm and moist air often flowed into Okinawa and Amami. Thirdly, cold spells sometimes covered northern Japan. Stratospheric sudden warming had persisted from mid December to late January. As a result, polar vortex was stronger than normal over the Kamchatka Peninsula. In northern Japan, we observed record low temperatures in early February.

Thank you for your attention.

reference

Definition of El Niño and La Niña in JMA 5-month running mean of SST deviation for NINO.3 El Niño ⋯ more than +𝟎.𝟓℃ for more than 6 months. La Niña ⋯ less than −𝟎.𝟓℃ for more than 6 months. Time series of SST deviations for NINO.3 In JMA, El Niño and La Niña is defined by 5-month running mean of SST deviation for NINO.3. NINO.3 is one of El Niño monitoring areas. El Niño is defined that the SST deviation is more than +0.5 ℃ for more than 6 months. La Niña is defined that the SST deviation is less than −0.5 ℃ for more than 6 months. Lower figure shows the SST deviation for the last 10 years. From this figure, La Niña conditions occurred in last winter.

Past El Niño and La Niña El Niño 1950 s La Niña 1960 s SST 1970 s This figure shows the periods of El Niño and La Niña conditions. 2000 s 2010 s

L H H SLP anomaly in winter T 850anomaly in winter Northwest of normal position L H Weaker to stretch to southeast Ψ 850 anomaly in winter SLP : Sea Level Pressure, T 850 : Temperature at 850-hPa Ψ 850 : Stream function at 850-hPa Siberian high was weak in eastern / western Japan. Cold air flowed out little. T 850 was higher than normal. Aleutian Low located northwest of normal position. Cold spells sometimes covered Northern Japan. It is thought to be associated with SSW. Left figure shows sea level pressure in winter. Right figure shows temperature at 850-hPa in winter. Lower figure shows stream function at 850-hPa in winter. From these figures, Siberian High was weak in eastern and western Japan. Lower high at the East China Sea prevented cold spells from flowing out. As a result, temperature at 850-hPa was higher than normal around the East China Sea. However, Aleutian Low was located northwest than normal. Therefore, cold spells sometimes covered Northern Japan. It is thought to be associated with SSW. H

Difference from last year (2017-2018 winter) Ψ 300 anomaly and WAF in winter 2018 - 2019 winter 2017 - 2018 winter H L L H H L L L L H H H H H L L H L L L H H H Ψ 300 : Stream function at 300-hPa, WAF : Wave Activity Flux (Takaya and Nakamura 2001)

Warm winter factors SST anomaly in winter Cold Warm EQ SST : Sea Surface Temperature Cold Warm EQ OLR anomaly in winter OLR : Outgoing Longwave Radiation Active EQ Inactive El Niño conditions have persisted from autumn 2018.

Cold spells factors L C SLP anomaly (5th – 9th Feb) T 850 anomaly (5th – 9th Feb) L C Aleutian Low was stronger. Cold air covered. I will talk about the example affected by cold spells in northern Japan. Left figure shows 5-day average anomaly of sea level pressure from 5th February to 9th February. Right figure shows temperature at 850-hPa in the same period. From these figures, the Aleutian Low was stronger at northwest of its normal position. This was caused by the south bottom of the polar vortex. As a result, cold air covered mainly in northern Japan. On 8th February, observed 850-hPa temperature at Sapporo was −24.4 ℃. It was the lowest since 1957. SLP : Sea Level Pressure T 850 : Temperature at 850-hPa Polar low shifted southward. Aleutian Low was stronger at northwest of its normal position. On 8th Feb, observed 850-hPa temperature at Sapporo was −24.4 ℃.