The link between the Victoria mode in the preceding boreal winter and spring precipitation over the southeastern USA and Gulf of Mexico

PU Xiu-Shu， CHEN Qun-Ling， DING Rui-Qingnd GUO Yi-Peng，Plteu Atmosphere nd Environment Key Lortory of Sihun Provine， College of Atmospheri Sienes， Chengdu University of Informtion Tehnology， Chengdu， Chin；Stte Key Lortory of Numeril Modeling for Atmospheri Sienes nd Geophysil Fluid Dynmis （LASG），Institute of Atmospheri Physis， Chinese Ademy of Sienes， Beijing， Chin；College of Erth Siene， University of Chinese Ademy of Sienes， Beijing， Chin

The link between the Victoria mode in the preceding boreal winter and spring precipitation over the southeastern USA and Gulf of Mexico

PU Xiu-Shua，b， CHEN Quan-Lianga， DING Rui-Qianga，band GUO Yi-Pengb，caPlateau Atmosphere and Environment Key Laboratory of Sichuan Province， College of Atmospheric Sciences， Chengdu University of Information Technology， Chengdu， China；bState Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics （LASG），Institute of Atmospheric Physics， Chinese Academy of Sciences， Beijing， China；cCollege of Earth Science， University of Chinese Academy of Sciences， Beijing， China

The sea surface temperature anomalies （SSTAs） associated with the Victoria mode （VM） can persist into the following season and then infuence climate variability in the tropical Pacifc. This paper demonstrates the connection between the preceding boreal winter VM and precipitation in the following spring over the southeastern United States （SE USA） and the Gulf of Mexico （GM）. The results indicate that a positive （negative） preceding winter VM is usually followed by increased（reduced） precipitation over the SE USA and GM during the following spring. The corresponding mechanism is similar， but slightly diferent to， the seasonal footprinting mechanism. For positive VM cases， the preceding-winter VM-related SSTAs appear to persist into the following spring via airsea interactions， which then induce low-level convergence and vigorous ascending motion， leading to an adjustment of the zonal and meridional circulation. This adjustment can then infuence the local Hadley cell by weakening the downward branch. These anomalous patterns of vertical airfow enhance spring precipitation over the SE USA and GM under suitable moisture conditions. Hence，this work demonstrates that the preceding-winter VM has the potential to regulate precipitation over the SE USA and GM in the following spring.

ARTICLE HISTORY

Revised 8 March 2016

Accepted 28 March 2016

Victoria mode； spring precipitation； southeastern USA； Gulf of Mexico； air-sea interaction

北太平洋海温第二主导模态是一个呈现东北-西南“+-”偶极子型变化的海温模态，被定义为维多利亚模态（VM）。本文利用1979-2014年的逐月资料，通过偏相关分析及回归分析等气象统计方法，探究了北半球前冬VM与春季美国东南部及墨西哥湾（SE USA及GM，（24-34°N，95-80°W））降水之间的关系，结果表明两者之间存在显著的正相关关系。当前冬为正（负）VM事件，则在随之而来的春季， SE USA及GM区域往往会出现降水增加（减少）。SE USA及GM区域的环流系统对前冬正（负）VM的响应十分显著，具体表现为比湿偏高（偏低），输入此区域的水汽输送增多（减少），并且，此区域对流层基本上被异常上升（下沉）运动控制，有利于（不利于）降水发生。因此，本文的研究可能为春季美国东南部及墨西哥湾降水的季节预测提供新的预测因子。

1. Introduction

The Victoria mode （VM） is the second empirical orthogonal function mode （EOF2） of sea surface temperature anomalies （SSTAs） in the North Pacifc north of 20°N（Bond et al. 2003； Ding et al.， “The Victoria Mode，” 2015），and is distinct from the Pacifc Decadal Oscillation （Mantua et al. 1997； Zhang， Wallace， and Battisti 1997）， which is the leading mode of North Pacifc climate variability （fgure not shown）. The VM exhibits a tripole structure that is characterized by a band of positive SSTAs extending from the west coast of North America to the central tropical Pacifc， a band of negative SSTAs extending from the central North Pacifc to the northwestern tropical Pacifc， and another band of positive SSTAs in the Pacifc north of 35°N （Bond et al. 2003； Ding et al.， “The Victoria Mode”， 2015） Figure 1（a）. The VM index （VMI） is the corresponding time coefcient of the EOF2 of the monthly SSTA feld over the North Pacifc （20-61°N， 100°E-100°W） （Bond et al. 2003； Ding et al.， “The Victoria Mode”， “The Impact of South Pacifc”，2015）. Previous studies indicate that the VM is driven by the North Pacifc Oscillation （NPO； Walker and Bliss 1932；Rogers 1981； Ding et al.， “The Victoria Mode”， “The Impact of South Pacifc”， 2015）.

Figure 1.Correlation maps of the preceding winter VMI-DJF showing the three-month averaged SSTAs （shaded） and 850 hPa wind anomalies （vectors） for （a） DJF and （b） MAM.

Ding et al.， “The Victoria Mode”， （2015） suggested that the VM， as an ocean bridge through which extratropical atmospheric variability in the North Pacifc afects tropical variability， is more closely linked than the NPO to the development of ENSO. The VM can trigger the onset of ENSO via surface air-sea coupling and the evolution of subsurface ocean temperature anomalies along the equator. Meanwhile， the spring VM has been linked to variability in Pacifc ITCZ precipitation during the following summer（Ding et al.， “The Impact of South Pacifc”， 2015）. In positive VM cases， SSTAs in the subtropics associated with the spring VM persist until summer and develop towards the equator， inducing low-level convergence that leads to enhanced precipitation over the central-eastern Pacifc ITCZ region.

Many studies have focused on the correlation between the VM and the tropical Pacifc climate system （Ding et al.，“The Victoria Mode”， “The Impact of South Pacifc”， 2015）. Wang et al. （2010） found that spring precipitation over the southeastern United States （SE USA） is afected by SST patterns in the Pacifc. However， they did not address the efect of the preceding winter （December-January-February: DJF） VM on spring （March-April-May: MAM） precipitation. In this study， we explore a possible connection between the preceding winter VM and following spring precipitation over the SE USA and the Gulf of Mexico （GM）with the ENSO signal removed.

2. Data description

The following datasets were used in this study:（1） Precipitation data were obtained from the GPCP（Hufman et al. 1997）， and the CMAP data-set （Xie and Arkin 1997） was used to validate the results for precipitation. The GPCP and CMAP datasets contain monthly precipitation data at a horizontal resolution of 2.5° × 2.5°.（2） Atmospheric variables are from the NCEP-NCAR reanalysis （Kalnay et al. 1996）， which has a horizontal resolution of 2.5° × 2.5°. （3） SST data are from the HadISST data-set，gridded at a resolution of 1° × 1° （Rayner et al. 2006）.（4） The ENSO index （Ninõ3） is from the NOAA CPC website. In addition， we calculated the vertical integration of the anomalous moisture fux felds between the sea level and 300 hPa （Behera， Krishnan， and Yamagata 1999； Nnamchi and Li 2011）.

We analyzed the period from 1979 to 2014， for which satellite records are available. The signifcance of the correlation between two autocorrelated time series was assessed using the efective number of degrees of freedom.

3. Results

3.1. Connections between the preceding winter VM and spring precipitation

To investigate the link between the preceding winter VM and following spring precipitation over the USA， we show in Figure 2（a） the correlation coefcients between the DJF-averaged VMI （denoted as the VMI-DJF） and the MAM-averaged precipitation anomalies over the USA based on the GPCP data-set. Large areas with signifcant positive values occur over the SE USA and GM， which includes Louisiana， Mississippi， Alabama， Georgia， and Florida. This feature implies that a positive （negative） preceding winter VM is likely to be followed by increased （reduced）spring precipitation over the SE USA and GM （hereafter referred to as ‘the Box，' i.e. the region enclosed by （24-34°N，95-80°W））. We represent spring precipitation using the area-averaged precipitation index （PI）， which is defned as the standardized area-averaged spring precipitation for the Box region （Figure 2（c））. The preceding winter VM has a marked positive correlation with precipitation anomalies over the above region at a confdence level greater than 99%， with a correlation coefcient of 0.51. This result proves the reliability of the relationship between the preceding winter VM and spring precipitation.

Figure 2.（a） Correlation map of the VMI-DJF with the spring （MAM） precipitation anomalies based on the GPCP data-set. Positive （blue）and negative （red） precipitation anomalies， signifcant at the 0.2 level， are shaded. The crosses indicate the 90% confdence level. The green box is the positive correlation box （24-34°N， 95-80°W）， which indicates the location of the Box region. （c） Time series of VMI-DJF（red line） and MAM-averaged PI （blue line） for the Box region between 1979 and 2014. Both the VMI-DJF and PI have been detrended and standardized. （e） Regressions of the boreal winter （DJF） SSTA （°C） feld on the PI for the period 1979-2014. Shaded areas represent signifcance above the 0.1 level. （b， d， f） As in （a， c， e） but based on the CMAP data-set.

To further confrm the connection between the preceding winter VM and spring precipitation over the SE USA and GM， we regressed the preceding winter North Pacifc SSTAs onto the PI， shown in Figure 2（c）. The regression of the SSTAs （Figure 2（e）） shows a well-defned dipole structure over the North Pacifc poleward of 20°N， which closely resembles the VM-related SSTA pattern in Figure 1（a）. It appears that the VM SST pattern bears a resemblance to the optimal initial SST condition that is likely to lead to spring precipitation anomalies over the SE USA and GM. A positive （negative） VM event in the preceding winter tends to be followed by more （less） precipitation during the following spring over this region. This result agreeswith the conclusions from Figure 2（a） and （c）， and further supports the existence of a close relationship between the preceding winter VM and following spring precipitation over the SE USA and GM.

Similar correlation maps and regression patterns were also obtained when using the CMAP data-set （Figure 2（b），（d）， and （f））. These consistent results demonstrate that the spring precipitation anomalies over the SE USA and GM are closely related to the VM from the previous winter. Thus，the preceding winter VM is one possible factor that afects spring precipitation over the SE USA and GM.

Figure 3.Correlation maps of the VMI-DJF with anomalies of MAM-averaged （a） 700 hPa specifc humidity， （b） moisture transport magnitude vertically integrated from the 1，000 to 400 hPa pressure levels （shading and vectors）， （c） 500 hPa vertical pressure velocity， and （d） 200 hPa divergence.

3.2. Spring atmospheric circulation anomalies associated with the preceding winter VM

The distribution of precipitation is closely associated with the combined efect of the water vapor conditions and vertical motion. To explain the above-mentioned link between the preceding winter VM and spring precipitation over the SE USA and GM， we present the following spring meteorological variable anomalies that are correlated with the preceding winter VM （Figure 3（a-d））.

Figure 3（a） displays the correlation between the VMIDJF and spring specifc humidity at 700 hPa. During positive VM cases， positive anomalies are centered over the SE USA and GM， indicating that the air is wetter over this region. Furthermore， Figure 3（b） indicates that the vertically integrated moisture fux feld characterizes much of the specifc humidity anomaly pattern. The vector of the moisture fux shows that the moisture transport induced by the preceding winter's VM is centered over the GM. The majority of the region is dominated by strong southerly winds， which promote the moisture transport from the GM.

When the preceding winter VM is positive， anomalous upward motion prevails over the SE USA and GM （Figure 3（c））. The correlation between the VMI-DJF and spring divergence at 200 hPa is displayed in Figure 3（d）. The confguration of the divergence feld in the upper troposphere matches these upward motion anomalies well. The vertical motion associated with the positive preceding winter VM is consistent with the vapor conditions that favor the spring precipitation pattern in Figure 2（a） and （b）.

To summarize， in the case of a positive preceding winter VM， the anomalous horizontal divergence at 200 hPa favors ascending motion of wetter air over the SE USA and GM. The combination of favorable vapor conditions， vertical motion， and the divergence feld generates increased precipitation in this region.

3.3. Possible physical mechanisms

As mentioned above， the preceding winter VM has the potential to infuence the following spring precipitation over the SE USA and GM. However， the question remains as to exactly how the winter VM afects the SE USA and GM during the following spring. To provide a physicalexplanation for the observed relationship， we examined the VM-related SST and atmospheric circulation anomalies.

Figure 4.（a） Correlation map of the VMI-DJF with the MAM-averaged 850 hPa divergence anomalies. Correlation signifcant at the 0.1 level is shaded. （b） Correlation map of the VMI-DJF with anomalies of the meridional mean spring （MAM） zonal wind and omega components for 10°S-10°N. The green lines indicate the longitudinal band of the Box region. （c） Correlation map of the VMI-DJF with anomalies of the zonal mean spring（MAM） meridional wind and omega components for 95-80°W. The green lines indicate the latitudinal band of the Box region. In（b， c）， the omega value with the vector is multiplied by 10. Shading represents signifcance above the 0.1 level.

Figure 1（a） and （b） present the correlation between the VMI-DJF and SST and the 850-hPa wind anomalies in the winter and following spring. During winter， the positive VM is accompanied by a dipole-like SSTA pattern in the North Pacifc north of 20°N， and a subtropical （0°-20°N）band of positive SSTAs extending from the northeastern Pacifc to the tropical central Pacifc （Figure 1（a））. The related wind anomalies resemble those associated with the NPO （Walker and Bliss 1932； Rogers 1981）. This result is consistent with those reported by Vimont， Wallace， and Battisti （2003）， Vimont， Battisti， and Hirst （2003）， Alexander et al. （2010）， and Ding et al.， “The Victoria Mode，” （2015）. These signifcant SST and wind anomalies in the North Pacifc north of 20°N decrease quickly in the following spring （Figure 1（b））. In contrast， SSTAs in the subtropical central-eastern North Pacifc （10-20°N） can persist from the preceding winter and into spring （Figure 1（b）） via surface air-sea interactions associated with the VM （Ding， Li， and Tseng 2015； Ding et al.， “The Impact of South Pacifc”， 2015）. Specifcally， anomalous southwesterlies associated with the VM during the preceding winter reduce the upward latent heat fux （fgure not shown） and subsequently warm the ocean from the northeastern Pacifc to the equatorial central Pacifc.

In response to the warming induced by the above processes in the central-eastern tropical Pacifc， strong anomalous southwesterlies in the central-western tropical Pacifc strengthen （Figure 1（b））， leading to convergence at 850 hPa with the center located in the central-eastern North Pacifc（Figure 4（a））. These convergence zones in the lower tropospheric layers cause vigorous ascending motion centered near the dateline （10°S-10°N， 170°E-170°W）. Meanwhile，signifcant descending motion occurs over the tropical eastern Pacifc of the west coast of Colombia and Ecuador（（10°S-10°N， 95-80°W）； Figure 4（b））. This anomalous east-west oriented circulation resembles the Walker circulation across the tropical Pacifc， resulting in enhancement of the latter. In addition， the increased convection and precipitation caused by low-level convergence in the central-eastern North Pacifc may intensify the release of the latent heat of condensation into the atmosphere， which favors the ascending motion， convective precipitation， and so on （Ding et al.， “The Impact of South Pacifc”， 2015）.

Adjustment of the Walker circulation also infuences the meridional circulation over the region 95-80°W， which is the longitudinal band of the Box region. Following the increased Walker circulation， the sinking airfow over the tropical eastern Pacifc of the west coast of Colombia and Ecuador is strengthened （Figure 4（c））. Subsequently，this enhanced downward motion is superposed onto the upward branch of the local Hadley cell in the tropical eastern Pacifc， weakening the latter and thereby leading to anomalous ascending motion and precipitation over the SE USA and GM （Figure 4（c））.

Note that signifcant anomalous southerlies are seen over the Box region （Figure 4（c））， which is consistent with anomalous rising airfow there， indicating that the abundant supply of water vapor over the SE USA and GM is closely linked to the ascending motion over this region，and together they encourage the generation of local precipitation. In general， our interpretation is that a largescale convergence over the central-eastern North Pacifc induced by the VM plays an important role in causing anomalous ascending motion and increased precipitation over the SE USA and GM.

4. Conclusions and discussion

This paper focuses on the relationship between the preceding winter VM and precipitation over the SE USA and GM during the following spring. Our analysis demonstrates that the VM may have a marked efect on the interannual variation in spring precipitation over this region. A positive preceding winter VM is related to an intensifed Walker circulation across the tropical eastern Pacifc and a suppressed local Hadley cell within the longitude of the Box region. The related anomalous upward motion over the Box region，which contains large amounts of water vapor， dominates the SE USA and GM. The confguration of the atmospheric circulation and the water vapor conditions is consistent with the positive precipitation anomalies over the SE USA and GM. In brief， the underlying physical processes associated with the infuence of the preceding winter VM on spring precipitation over the SE USA and GM are similar， but slightly diferent to， the seasonal footprinting mechanism（SFM）. The SFM was proposed by Vimont， Battisti， and Hirst（2001）， Vimont， Wallace， and Battisti （2003）， Vimont， Battisti，and Hirst （2003） to explain the efects of the NPO-like variability during a particular winter on ENSO during the following winter. The preceding winter VM SST pattern displayed in Figure 1a closely resembles the SST footprint reported by Vimont， Battisti， and Hirst （2001）， Vimont， Wallace， and Battisti （2003）， Vimont， Battisti， and Hirst （2003）. However，here we emphasize the linkage between the VM and precipitation over the SE USA and GM. Specifcally， the preceding winter VM signal can persist into the following spring，inducing anomalous southwesterlies that have a potential efect on the circulation in the central-eastern tropical Pacifc through air-sea interaction. Thus， the Walker circulation and local Hadley cell act as an atmospheric bridge，which allows the North Pacifc VM to infuence precipitation over the SE USA and GM during the following spring. Our analysis suggests that the preceding winter VM provides an additional source of predictability for downscaled seasonal predictions of the following spring precipitation over the SE USA and GM. Nevertheless， the problem of how to construct a prediction model for the following spring precipitation based on the preceding winter VM remains；additional study is required in this area. Moreover， given that the VM is closely correlated with ENSO （Ding et al.，“The Victoria Mode，” 2015）， and ENSO could signifcantly infuence precipitation over the USA （Ting and Wang 1997；Gutzler， Kann， and Thornbrugh 2002； Wang et al. 2010，2012； Ciancarelli et al. 2014）， the question naturally arises as to whether the efect of the VM and ENSO on the precipitation over the SE USA and GM are independent. Further research into this issue is also necessary.

Acknowledgements

The authors thank Dr Sen Zhao for calculating the vertical integration of the anomalous moisture fux felds.

Disclosure statement

No potential confict of interest was reported by the authors.

Funding

This work was jointly supported by the China Special Fund for Meteorological Research in the Public Interest ［grant number GYHY201506013］； the National Basic Research Program of China［973 Program， grant number 2012CB955200］； the National Natural Science Foundation of China for Excellent Young Scholars［grant number 41522502］； the National Natural Science Foundation of China ［grant number 41475037］， and the Strategic Priority Research Program of the Chinese Academy of Sciences［grant number XDA11010303］.

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维多利亚模态； 春季降水；美国东南部； 墨西哥湾；海气相互作用

6 February 2016

CONTACT DING Rui-Qiang drq@mail.iap.ac.cn

© 2016 The Author（s）. Published by Informa UK Limited， trading as Taylor & Francis Group.

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