# Contrast of Evolution Characteristics of Boreal Summer and Winter Intraseasonal Oscillations over Tropical Indian Ocean

• Corresponding author: Tim LI, timli@hawaii.edu
• Funds:

Supported by the National Key Research and Development Program of China (2018YFC1505804 and 2015CB453200), National Natural Science Foundation of China (41630423 and 41875069), US Natural Science Foundation (AGS-16-43297), and US National Oceanic and Atmospheric Administration (NA18OAR4310298). There are School of Ocean and Earth Science and Technology (SOEST) contribution number 10740, International Pacific Research Center (IPRC) contribution number 1340, and Earth System Modeling Center (ESMC) number 271

• doi: 10.1007/s13351-019-9015-z
• A most striking summer–winter difference of evolution of the intraseasonal oscillation (ISO) over the equatorial Indian Ocean is a quasi-stationary oscillation in boreal summer but eastward propagation in boreal winter. This feature is consistent with the observational fact that maximum ISO variance appears only in the eastern Indian Ocean in boreal summer while it appears across the entire basin in boreal winter. The cause of the distinctive propagation and initiation characteristics is investigated through the diagnosis of observational and reanalysis data for the period of 1982–2012. It is found that when the ISO convection appears over eastern Indian Ocean, a positive (negative) moisture tendency appears to the east of the convection in boreal winter (summer). It is the moisture tendency difference that is responsible for different propagation behavior in the summer and winter. A further diagnosis of the moisture budget indicates that the major difference lies in anomalous moisture advection by the mean flow. In addition, air–sea interaction also plays a role. While boreal winter ISO starts over western Indian Ocean, boreal summer ISO is initiated over central–eastern equatorial Indian Ocean, due to boundary layer moistening. The moisture increase is caused primarily by the horizontal advection of mean specific humidity by anomalous easterlies induced by preceding suppressed-phase ISO over eastern Indian Ocean. Besides, a delayed SST feedback also plays a role. The overall difference of ISO evolution between the summer and winter is regulated by the seasonal mean state including the mean SST and water vapor content.
• Fig. 1.  The leading EOF patterns of time−longitude section of 20–90-day filtered OLR anomalies from 1982 to 2012 for boreal summer (top) and winter (bottom) seasons. For boreal summer (winter) the anomalous OLR is averaged over 10°S–10°N (15°S–5°N). The green arrow indicates the direction of propagation.

Fig. 2.  The standard deviation (shaded) of 20–90-day band-pass filtered OLR field and the propagation vector (arrow) calculated based on the OLR anomalies at the near grids. The blue box is a reference region for subsequent regression analyses.

Fig. 3.  Evolutions of regressed OLR (W m–2) and 850-hPa wind (m s–1) anomaly patterns from pentads –2 to +2 during boreal summer (left pa-nels) and winter (right panels). Pentad 0 represents the time when ISO convection is located over the central equatorial Indian Ocean (blue box in Fig. 2). The green boxes in (b, g) represent the key regions for ISO initiation in boreal summer and winter respectively. The anomalous wind that exceeds the 99% confidence level is shown, and the red dotted area indicates that the OLR field passes the 99% confidence test.

Fig. 4.  Zonal–vertical distributions of anomalous specific humidity (contour; 10–3 kg kg–1) and specific humidity tendency (shaded; 10–3 kg kg–1 day–1) fields averaged over 5°–10°S in pentad 0 during (a) boreal summer and (b) boreal winter. The dotted area indicates that the anomalous specific humidity field passes the 99% confidence test. Only specific humidity tendency that exceeds 99% confidence level is shown. Red triangles represent the location of ISO convective center.

Fig. 5.  (a) Vertically integrated (1000–500 hPa) intraseasonal moisture budget terms averaged over 5°–10°S, 120°–140°E in pentad 0 (black box in Fig. 4). From left to right, observed specific humidity tendency, horizontal moisture advection, vertical moisture advection, apparent moisture source, and sum of the last three terms. (b) Individual components of the anomalous horizontal moisture advection, with term 1–9 representing, respectively, ${\left({{\rm{ - }}\bar { {V}} \cdot \nabla {\bar q}} \right)^\prime }$, ${\left({{\rm{ - }}\bar { {V}} \cdot \nabla {q'}} \right)^\prime }$, ${\left({{\rm{ - }}\bar { {V}} \cdot \nabla {{q}^{\rm{*}}}} \right)^\prime }$, ${\left({{\rm{ - }}{ {V}}' \cdot \nabla {\bar q}} \right)^\prime }$, ${\left({{\rm{ - }}{ {V}}' \cdot \nabla {q'}} \right)^\prime }$, ${\left({{\rm{ - }}{ {V}}' \cdot \nabla {{q}^{\rm{*}}}} \right)^\prime }$, ${\left({{\rm{ - }}{{ {V}}^*} \cdot \nabla {\bar q}} \right)^\prime }$, $\left({{\rm{ - }}{{ {V}}^*} \cdot \nabla {q'}} \right)'$, and ${\left({{\rm{ - }}{{ {V}}^*} \cdot \nabla {{q}^*}} \right)^\prime }$. (c) 850-hPa intraseasonal zonal wind (m s–1) averaged over 10°–5°S, 120°–140°E in pentad 0. (d) Individual components of the anomalous vertical moisture advection, with term 1–9 representing, respectively, ${\left({{\rm{ - }}\bar \omega \frac{{\partial {\bar q}}}{{\partial p}}} \right)^\prime }$, ${\left({{\rm{ - }}\bar \omega \frac{{\partial {q'}}}{{\partial p}}} \right)^\prime }$, ${\left({{\rm{ - }}\bar \omega \frac{{\partial {{q}^{\rm{*}}}}}{{\partial p}}} \right)^\prime }$, ${\left({{\rm{ - }}\omega '\frac{{\partial {\bar q}}}{{\partial p}}} \right)^\prime }$, ${\left({{\rm{ - }}\omega '\frac{{\partial {q'}}}{{\partial p}}} \right)^\prime }$, ${\left({{\rm{ - }}\omega '\frac{{\partial {{q}^{\rm{*}}}}}{{\partial p}}} \right)^\prime }$, ${\left({{\rm{ - }}{\omega ^*}\frac{{\partial {\bar q}}}{{\partial p}}} \right)^\prime }$, ${\left({{\rm{ - }}{\omega ^*}\frac{{\partial {q'}}}{{\partial p}}} \right)^\prime }$, and ${\left({{\rm{ - }}{\omega ^*}\frac{{\partial {{q}^{\rm{*}}}}}{{\partial p}}} \right)^\prime }$.

Fig. 6.  (Top) Vertically integrated (1000–500 hPa) LFBS wind (m s–1) and intraseasonal specific humidity (shaded; 10–3 kg kg–1) fields during the active phase of ISO over EIO in (a) boreal summer and (b) winter. The dotted area indicates that the anomalous specific humidity field passes the 99% confidence test. (Bottom) Meridional–vertical distributions of LBFS vertical p-velocity (Pa s–1) averaged over 120°–140°E in (c) boreal summer and (d) winter.

Fig. 7.  Meridional–vertical structures of northward propagating BSISO averaged over 70°–90°E in pentad 0: (a) vertical velocity (Pa s–1), (b) vorticity (10–6 s–1), (c) specific humidity (10–3 kg kg–1), and (d) meridional–vertical profile of the north–south component of the summer mean flow (m s–1) averaged between 70° and 90°E. The dotted area indicates that anomalous vertical velocity, vorticity, and specific humidity fields pass the 99% confidence test.

Fig. 8.  Anomalous OLR (red for positive and green for negative, interval: 10 W m–2), SST (shaded; °C), and surface wind stress (arrow; 102 N m–2 s) fields regressed onto the intraseasonal OLR anomaly over EIO (blue box in Fig. 2). The dotted area indicates that the anomalous SST field passes the 99% confidence test. Only OLR and surface wind stress anomalies that exceed 99% confidence level are shown.

Fig. 9.  Meridional–vertical cross-sections of 20–90-day-filtered vertical velocity field (Pa s–1) averaged over 70°–90°E from days –14 to –4 in boreal summer. Day 0 represents the time when the ISO convective center appears over the equatorial EIO. The dotted area indicates that the anomalous vertical velocity field passes the 99% confidence test.

Fig. 10.  Evolutions of (a) regressed intraseasonal OLR anomaly and (b)–(d) vertical profiles of intraseasonal specific humidity, vertical velocity, and moist static energy (MSE) field averaged over 10°S–10°N, 70°–90°E (the green box in Fig. 3b). The dotted area indicates that the anomalous specific humidity, vertical velocity, and MSE fields pass the 99% confidence test.

Fig. 11.  (a) Vertically integrated (1000–700 hPa) intraseasonal moisture budget terms averaged over 10°S–10°N, 70°–90°E and from days –11 to –8. (b) Individual components of the anomalous horizontal moisture advection, with term 1–9 representing $\left({-\bar { V} \cdot \nabla {{\bar q}}} \right)^\prime$, ${\left({ - \bar { V}\cdot \nabla {{q'}}} \right)^\prime }$, ${\left({-\bar { V}\cdot \nabla {{{q}}^{\rm{*}}}} \right)^\prime }$, ${\left({-{ V}' \cdot \nabla {{\bar q}}} \right)^\prime }$, ${\left({-{ V}' \cdot \nabla {{q'}}} \right)^\prime }$, ${\left({-{ V}' \cdot \nabla {{{q}}^{*}}} \right)^\prime }$, ${\left({-{{ V}^*} \cdot \nabla {{\bar q}}} \right)^\prime }$, $\left({-{{ V}^*} \cdot \nabla {{q'}}} \right)$, and ${\left({-{{ V}^*} \cdot \nabla {{{q}}^*}} \right)^\prime }$, respectively.

Fig. 12.  (a) Vertically integrated (1000–700 hPa) intraseasonal wind anomaly (m s–1) and LFBS specific humidity (shaded; 10–3 kg kg–1) fields averaged from days –11 to –8. The green box denotes the initiation region for BSISO. (b) As in (a), but for the intraseasonal OLR anomaly (shaded; W m–2). The red dotted area indicates that the anomalous OLR passes the 99% confidence test. Only wind anomalies that exceed 99% confidence level are shown.

Fig. 13.  Evolutions of (a) regressed intraseasonal OLR anomaly and (b)–(d) vertical profiles of intraseasonal specific humidity, vertical velocity, and advection of the mean moisture by the intraseasonal flow field averaged over 15°S–5°N, 50°–70°E (the green box in Fig. 3g). The dotted area indicates that specific humidity, vertical velocity, and advection fields pass the 99% confidence test.

Fig. 14.  Climatological mean SST (shaded; °C and vertically integrated (1000–700 hPa) specific humidity (contour; 10–3 kg kg–1) fields in boreal (a) summer and (b) winter.

Fig. 15.  Evolutions of regressed SST (shaded; °C), surface wind stress (arrow; 102 N m–2 s), and OLR (red for positive and green for negative, interval: 10 W m–2) anomaly fields from pentads –1 to +3 in boreal summer (left panel) and winter (right panel). Pentad 0 represents the time when suppressed ISO center is located over the equatorial EIO (70°–90°E). The black boxes show the same ISO initiation regions as in Fig. 3. The white dotted area indicates that the anomalous SST passes the 99% confidence test. Only OLR and surface wind stress anomalies that exceed 99% confidence level are shown.

Fig. 16.  Lead–lag correlations of intraseasonal OLR and SST anomalies averaged over the initiation regions in boreal summer (red) and winter (blue). A positive value in the horizontal axis denotes days by which positive SST anomaly leads negative OLR anomaly (i.e., convection). Blue (red) dashed line denotes that the lagged correlation exceeds 95% confidence level during boreal winter (summer).

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###### 通讯作者: 陈斌, bchen63@163.com
• 1.

沈阳化工大学材料科学与工程学院 沈阳 110142

## Contrast of Evolution Characteristics of Boreal Summer and Winter Intraseasonal Oscillations over Tropical Indian Ocean

###### Corresponding author: Tim LI, timli@hawaii.edu;
• 1. Key Laboratory of Meteorological Disaster, Ministry of Education (KLME)/Joint International Research Laboratory of Climate and Environmental Change (ILCEC)/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD), Nanjing University of Information Science & Technology, Nanjing 210044, China
• 2. International Pacific Research Center and Department of Atmospheric Sciences, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA
Funds: Supported by the National Key Research and Development Program of China (2018YFC1505804 and 2015CB453200), National Natural Science Foundation of China (41630423 and 41875069), US Natural Science Foundation (AGS-16-43297), and US National Oceanic and Atmospheric Administration (NA18OAR4310298). There are School of Ocean and Earth Science and Technology (SOEST) contribution number 10740, International Pacific Research Center (IPRC) contribution number 1340, and Earth System Modeling Center (ESMC) number 271

Abstract: A most striking summer–winter difference of evolution of the intraseasonal oscillation (ISO) over the equatorial Indian Ocean is a quasi-stationary oscillation in boreal summer but eastward propagation in boreal winter. This feature is consistent with the observational fact that maximum ISO variance appears only in the eastern Indian Ocean in boreal summer while it appears across the entire basin in boreal winter. The cause of the distinctive propagation and initiation characteristics is investigated through the diagnosis of observational and reanalysis data for the period of 1982–2012. It is found that when the ISO convection appears over eastern Indian Ocean, a positive (negative) moisture tendency appears to the east of the convection in boreal winter (summer). It is the moisture tendency difference that is responsible for different propagation behavior in the summer and winter. A further diagnosis of the moisture budget indicates that the major difference lies in anomalous moisture advection by the mean flow. In addition, air–sea interaction also plays a role. While boreal winter ISO starts over western Indian Ocean, boreal summer ISO is initiated over central–eastern equatorial Indian Ocean, due to boundary layer moistening. The moisture increase is caused primarily by the horizontal advection of mean specific humidity by anomalous easterlies induced by preceding suppressed-phase ISO over eastern Indian Ocean. Besides, a delayed SST feedback also plays a role. The overall difference of ISO evolution between the summer and winter is regulated by the seasonal mean state including the mean SST and water vapor content.

Reference (53)

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