Interannual variability of the atmosphere/snow interaction
over the Eurasian continent during the spring season
and its relation to the monsoon period
Masatake E. Hori
(University of Tsukuba, Graduate School for Environmental Sciences)
Graduate School for Enviromental Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba city,Ibaraki pref., Japan. e-mail: mhori@kankyo.envr.tsukuba.ac.jp

Abstract
Interannual variability of the spring snow cover over the Eurasian continent and its relevance towards
the following monsoon season has been investigated using the NOAA/ NESDIS northern hemisphereweekly snow-cover chart and in-situ measurement of snow- depth derived from the Historical Soviet
Daily Snow Depth Version 2 (HSDSD) dataset. Interannual variability of snow cover in the western
Russian region has a strong correlation with the January NAO (North Atlantic Oscillation) and a signalof a strong positive/negative NAO remains as a emanating early/late snow melt in the region which
persists until April. While NAO itself has little or no auto-correlation for period longer than 1 month,a consistent lag correlation was seen, where an EU-like wave-guide pattern following the NAO dipole
matures during March, which in turn allows an anomalous heat advection from the Atlantic to alter
the land surface condition. In the following April, a barotropic atmospheric anomaly responsible forthe adiabatic heating in the region can be seen. While such large-scale atmospheric teleconnection
can be seen accompanying the snow anomaly, its effect can no longer be seen past May, which could
suggest that its inﬂuence towards the monsoon onset or its maturity can be marginal.
Keywords; Snow cover, snow melt, North Atlantic Oscillation

1. Introduction

Eurasian continent which lasts up to April. Also, Robock et al. (2004)

Interannual variability of snow extent over the Eurasian continent during

the snow extent and soil moisture of the region which propagates

the winter season has long been considered to have a possible impact

eastward and lasts until April. Hori and Yasunari (2003) also showed thattowards the climatic conditions of the subsequent seasons. The so

the inﬂuence of NAO towards snow extent during Jan. is limited to thecalled "Blanford hypothesis" suggests an inverse relationship between

European region, and has a lag effect emanating northeastwards until

the winter and spring snow accumulation over the Himalayas and theApril. While some studies suggests that the effect NAO may remain as

subsequent monsoon rainfall over India. This relation was later supported

a climatic memory in the snow cover until summer (Ogi 2004), otherby Hahn and Shukla (1976) where they used satellite derived snow

studies showed that early/late snow cover does not last through May, andarea data to show a statistically signiﬁcant negative correlation against

its ability to alter the state of the monsoon may be relatively small (Ueda

the Indian monsoon rainfall. Extending this result to a continental scale,

et al. 2003; Shinoda 2001). Also, Hori and Yasunari(2003) showed that

Bamzai and Shukla (1999) showed that the inverse correlation between

the NAO inﬂuence towards the snow extent can be seen regardless of

the IMR exists only over the western European region during late winter,the state of the monsoon, which suggests that the given correlation mayand over the western Russian plains during early spring.

be accidental, with no physical background. The objective of this study

On the other hand, snow extent in this region is under strong inﬂuence

is to build upon Hori and Yasunari(2003) to describe the atmosphericof the atmospheric variability such as the North Atlantic Oscillation

variabilities accompnying the snow melt over the Eurasian continent(NAO). Based on an EOF analysis, Clark et al. (1999) showed threeduring early spring.

showed that strength of winter NAO may have an influence towards

distinct atmospheric patterns where each has a different impact towards
the snow extent, one of which is the NAO. Saito et al. (2003) showedthat the dominant mode of atmospheric variaibility (the Arctic Oscillation,or the NAO) has strong correlation with the snow extent over the2. Data and Methods
Two types of snow data are used throughout this study. The Historical7060

Std. Deviation

Std. Deviationa)

b)
0.5

50

0.4

0.3

400.2

30
7060

Difference

Differencec)

d)
1.0

0.3

50

0.1

-0.1

40

-0.3

-1.03003060900306090Fig1: a) Interannual standard deviation of snow extent on Jan. for the 1972-2002 period. b) same as a) but for Mar. c)
Climatological snow extent difference for the ﬁrst half and second half of the month of Jan. d) same as c) but for Mar.

Soviet Daily Snow Depth (Version 2.0) is used for in-situ measurement

the Iceland low weeakens while the strength of the SH intensiﬁes, whichof snow amount. NOAA/NESDIS Northern Hemisphere Snow Covercreates a strong pressure gradient in the lower troposphere whichChart is used for continent-wide snow content estimation. The original

maintains the strong southerly in 20-60E.

dataset is comprised of gridded digital data of 2x2 degrees. Snow extent

On the other hand during January, westerly wind is predominant

climatology was created using a 30-year weekly averages of NOAA/

over the climatological snow extent region, suggesting the interannual

NESDIS snow cover dataset.

variability of NAO playing a strong role in determining the existance ofAlso, daily data from the NCEP/NCAR Reanalysis dataset is used fromsnow in the region.

1972-2002. to create the monthly and weekly atmospheric datasets.

To investigate the interannual variability of snow melt during these two
seasons, we create two indieces of snow extent. We deﬁne the the ﬁrst

3. Results

index as the snow extent averaged over 45-55N, 0-30E at week 1-3

The climatological feature of the springtime snowmelt over the Eurasian

extent over 45-55N, 20-60E during weeks 8-12 as the Western-Russiancontinent is assesed using a 30 year averaged snow extent data. Figure

index(WR index).

1. shows the interannual standard deviation of snow extent during Jan.

Figure 3. shows the timeseries of the two indices, from 1972-2002. Theand Mar. Also shown is the variability in snow extent within each month,

Eu index has a strong correlation with the January NAO index, which isby taking the difference of the climatological snow extent between theconsistent with previous studies (Hori 2003, Robock 2003). The Marchﬁrst half of the month against the latter half. In January, the tnterannual

index shows strong interannual variability along with a notable decadalvariability of snow extent is large over the European region while invariability where the index shifts around 1986/87. Correlation withMarch, the region with maximum variability shifts north-eastward toMarch NAO index is better for the latter period, which is consistent

as the Eurasian-Index (Eu-index), and the second index as the snow

the western Russian plains. However, snow extent difference within thethe existence of snow is interannual by nature rather than by seasonalchange of snow extent.

a) SLP, windb) Snow extent

Aprﬁrst and second half of the month is small in January, which suggests that1023

melt season is presented. Figure 2. shows a longitude-time section of SLP,

MarNext, the seasonal change of atmospheric variables during the snow

1026

850hPa v-wind, 850hPa wind vectors and snow extent averaged over theduring late February through April, accompanied by a rapid snow melt

1020

Feb

region of 45-55N. The onset of southerly is clearly shown over 20-60E

where they showed that the snow variability during early March throughApril is governed by the climatological southerly onsetting in early Marchin the region of 30-60E.
Such southerly wind is in conjunction with the climatological westward
extension of the Siberian High (SH). During early March, the intensity ofJanin the region. This is consistent with the ﬁndings of Ueda et al. (2003)

030

60
5m/s

900

102

530

60
0.2

0.4

0.6

0.8

1.0Fig2: a) Climatological SLP (contours) and 850hPa v wind (shades)
and wind vectors averaged over 45-55N. b) climatological snow
extent over the same latitude zone.

with the findings of Saito et al. (2004) where the covariability of thesnow cover between the winter season and the spring season changed in
the mid-'80s.
To investigate the atmospheric variabilities corresponding to each indices,a)80

and low years. The composite differnence for SLP, snow extent and

40

850hPa wind is given in ﬁgure 4.

30b)

80

Eurasian continent, no signiﬁcant difference in snow extent anomaly can50

be seen.

40

During March, the atmoshpheric patterns differs between the two30c)

80

a negative anomaly can be seen over the western Russian plains. The

70

negative SLP anomalies signiﬁes a late IL retreat, and the overall pressure

60

gradient in the region becomes strong in the WR index region. Strong

50

snow extent anomaly corresponding to less snow can be seen. The SLP
anomaly over the Atlantic ocean basin shows no NAO like features,(1987-2002)

60

While strong westerly anomalies are seen over the inland region of thesouthwesterly anomaly can be seen over the region, with an negative

Week: 12-14

70

maritime advection in conjunction with the NAO westerly is strongest.

anomaly can be seen over the climatologocal center of the SH, while(1972-1986)50

and 1987-2002, and normalized for each period to derive the high yearsdefined periods. In the first period of 1972-1986, a weak positive SLP

Week: 12-1460

indices is given. The WR-index is devided into two periods of 1972-1986,extent anomaly is confined to the European region where the warm(1972-2002)

70

a composite analysis between the high years and low years of each

In ﬁgure 4 (a), the dipole structure of the NAO is clearly seen. The snow

Week: 1-3403005 m/s

which suggests weak correlation with the NAO.

906030-1.0

-0.3

-0.1

0.1

0.3

1.0

The second period of 1987-2002 shows a negtive SLP anomaly in central

Fig4: Composite difference of SLP (contour), snow extent

Eurasia corresponding to the weakened westward extention of theanomalies (shades) and 850hPa wind vector for a) high/low years

SH. A positive SLP anomaly is over the european region, accompaniedof Eu-index during weeks 1-3. Period is from 1972-2002, b) sameby a westerly anomaly extending from the Atlantic ocean towards

as a ) but for WR-index for weeks 12-14, peiod is 1972-1986, c)

the western Russian plains. A weak dipole anomaly is present in thesame as b) but for period of 1986-2002.

Atlcntic which suggests moderate correlation with the NAO. Strong
northwesterly anomaly is present over the snow extent anomaly over
20-50E, and southwesterly anomaly is present over the 50-80E region.

decadal whift in the mid 1980s, where IL/SH pressure gradien t becam

It is clear from the figure that the January snow extent is mostlye weaker, and warm advection accompanied by the anomalous westerly

governed by the NAO variabliity. March variability has shown a distinct

by the March NAO has become dominant. It should be noted that thearea of the snow anomaly mostly overlaps between the two periods, andnotable change exists only the controling atmospheric anomalies.

EU Index

To show the change in snow extent persistence betwwen the twoa) 2

periods, composite difference of snow extent for the subsequent seasons1

is given in figure 5. In the 1972-1986 period, snow extent anomaly

0

propagates northwards with sustained intensity, which suggests strong-1

correlation with the subsequent seasonal change of the snow extent. The-2snow extent anomaly persists through April, until mid May, but diminishes

WR Indexb)

2

after June (ﬁgure now shown).

1

In the 1986-2002 period, snow anomalies extends more eastwards.

0However, the anomaly becomes weaker in the subsequent weeks, and-1

the anomaly does not persist beyond April.

-2

1970

1975

1980

1985

1990

1995

2000

Fig3: a) Timeseries of snow extent averaged over 0-20E, 45-55N
for Jan. (solid line), NAO index for Jan. (dashed line). b) snow
extent averaged over 30-60E, 45-55N for Mar.

This suggests that the atmospheric control over the snow extent has
become weaker in the recent years, where a snow anomaly, once formed
do not persist through the spring season.

1972-1986

1987-2002

wk:
12-14

wk:
15-17

wk:
18-19

-1.0

-0.3

-0.1

0.1

0.3

1.0

Fig5: a)-c) Composite difference of snow extent for high/low years of WR-index during for the period of
1972-1986, b) same as a ) but for period of 1987-2002.

4. Discussions

References

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