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    Monthly climatology of (a) sea level pressure (hPa; contours and color shading) and surface winds (m s−1; colored vectors), (b) skin temperature and SST (°C), and (c) SST gradient magnitude [°C (100 km)−1] in July in the Northern Hemisphere and in January in the Southern Hemisphere. For clarity, the vectors in (a) are plotted at every eight grid points (6.0° × 6.0°). The measures of the color bar in (a) indicate sea level pressure (top numbers) and surface wind speed (bottom numbers). SST is overlaid in (c) by thin and thick contours with intervals of 2° and 10°C, respectively.

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    Zonal mean of coverage ratio at which the cool air is definable over the sea in July in the Northern Hemisphere and in January in the Southern Hemisphere for isentropic surfaces of 270–300 K with an interval of 2 K. The coverage ratio is computed based on the 6-hourly data and the zonal mean is conducted only for the sea grids. The interval of the thick black contours is 6 K. Gray shading indicates the latitudes without any sea grid points owing to the Antarctic continent.

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    Monthly climatology in July in the Northern Hemisphere and in January in the Southern Hemisphere. (a) Height of the isentropic surface of 296 K (m; color shading and contours at an interval of 200 m). (b) NHC (K hPa; color shading and contours at an interval of 500 K hPa) and its flux (K hPa m s−1; fixed-length colored vectors). The measures of the color bar in (b) indicate NHC (top numbers) and its flux (bottom numbers). For clarity, the flux vectors are plotted at every eight grid points (6.0° × 6.0°). (c) Genesis/loss rate of NHC (K hPa day−1). SST is overlaid in (c) by thin and thick contours with intervals of 2° and 10°C, respectively. In these maps and others to follow, we do not plot the data at the grid points where the height of the threshold isentropic surface in the monthly climatology is lower than 10 m.

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    (a) Zonal integration of the NHC per unit length in the latitudinal direction over land (red), ocean (blue), and both (gray) in July in the Northern Hemisphere and in January in the Southern Hemisphere. (b) As in (a), but for January in the Northern Hemisphere and July in the Southern Hemisphere. (c) Percentage of the NHC integrated zonally only over ocean to that integrated zonally over both land and ocean in July in the Northern Hemisphere and in January in the Southern Hemisphere (red) and in January in the Northern Hemisphere and in July in the Southern Hemisphere (blue). The percentage of ocean coverage as a function of latitude is also shown (gray).

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    Climatological monthly variations of the total NHC in the Northern Hemisphere (NH; black) and the Southern Hemisphere (SH; gray).

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    Zonal integrations of (a) meridional NHC flux and (b) genesis/loss rate of NHC per unit length in the latitudinal direction in July in the Northern Hemisphere (NH; black) and in January in the Southern Hemisphere (SH; gray). In (a), the sign is reversed for the data of the Southern Hemisphere and thus negative values indicate equatorward flux for both hemispheres. In (b), positive and negative values indicate genesis and loss of NHC, respectively.

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    Monthly climatology of (a) surface temperature advection (K day−1; color shading) and (b) local deepening rate (hPa day−1) in July in the Northern Hemisphere and in January in the Southern Hemisphere. The surface temperature advection in (a) is calculated from 10-m horizontal wind and skin temperature or SST T (Klein et al. 1995). In (b), following the method of Kuwano-Yoshida (2014), the averaging is for the positive local deepening rate with the other values set to zero, and the values less than 2.0 hPa day−1 or between 20°N and 20°S are not plotted. SST is overlaid in both figures by thin and thick contours with intervals of 2° and 10°C, respectively.

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    Monthly climatology of (a) sensible heat flux (W m−2; color shading) and (b) latent heat flux (W m−2; color shading) in July in the Northern Hemisphere and in January in the Southern Hemisphere. Positive and negative values indicate upward and downward heat fluxes, respectively. These climatologies are constructed for the grid points at which the cool air is defined and thus upward sensible and latent heat fluxes at low latitudes are enhanced compared with the ordinary climatology in which the 6-hourly data fields are equally averaged. SST is overlaid in both panels by thin and thick contours with intervals of 2° and 10°C, respectively. Incidentally, the downward sensible heat flux in some land areas in (a) reflects the nighttime cooling of the land (e.g., in the east of Australian continent).

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    Binned scatterplots between the sensible heat flux and the genesis/loss rate of NHC (black) and between the latent heat flux and the genesis/loss rate of NHC (gray) for six geographical regions in July in the Northern Hemisphere and in January in the Southern Hemisphere. (top) Regions mainly covering (a) the subarctic North Pacific (40°–60°N, 140°E–110°W), (b) the subarctic North Atlantic (40°–60°N, 75°–10°W), and (c) the Southern Ocean (40°–60°S, all longitudes). (bottom) Regions mainly covering (d) the subtropical North Pacific (15°–40°N, 140°E–110°W), (e) the subtropical North Atlantic (15°–40°N, 75°–10°W), and (f) the subtropical oceans in the Southern Hemisphere (15°–40°S, all longitudes). The comparisons are based on monthly means for each year. The points in each panel are the means within each bin and the error bars are the plus or minus one standard deviation within each bin. The data at land grid points and the data within bins with relative frequency less than 0.5% are excluded. Each panel includes the correlation coefficients R.

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    Monthly climatology of (a) cloud water vertically integrated from the ground to the threshold potential temperature surface and (b) EIS in July in the Northern Hemisphere and in January in the Southern Hemisphere. Cloud water mixing ratio in (a) is the sum of cloud liquid and ice water mixing ratios. SST is overlaid in both panels by thin and thick contours with intervals of 2° and 10°C, respectively.

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    As in Fig. 9, but for the binned scatterplots between the EIS and the genesis/loss rate of NHC. Correlation coefficients are not shown.

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    Time–latitude diagrams of genesis/loss rate of NHC (K hPa day−1) along the longitudes (a) 147.75°E, (b) 180.00°, (c) 125.25°W, and (d) 49.50°W. Gray bars along the right ordinate indicate land grid points. A running average over five grid points in the zonal direction is applied.

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    Climatological monthly variations of the genesis/loss rate of NHC (K hPa day−1) for the selected latitudinal bands along the longitudes 147.75°E and 125.25°W. The subarctic North Pacific (black line with filled circles), the land in Siberia (gray line with filled circles), the upwelling region off the California coast (gray line with filled diamonds), and the NHC loss region in the double structure in the Pacific sector of the Southern Ocean (black line with filled diamonds).

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Low-Level Cool Air over the Midlatitude Oceans in Summer

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  • 1 Graduate School of Science and Technology, Hirosaki University, Hirosaki, Japan
  • 2 Graduate School of Science, Tohoku University, Sendai, Japan
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Abstract

The climatology of low-level cool air over the midlatitude oceans in summer is presented based on an isentropic analysis. This study focuses on isentropic surfaces of 296 K to analyze an adiabatic invariant referred to as the negative heat content representing the coldness of the air layer below the threshold isentropic surface. This approach allows a systematic analysis and a quantitative comparison of the cool air distribution and a diagnosis of diabatic heating of the air mass. The cool air covers most of the subarctic oceans and extends equatorward over the coastal upwelling regions in the east of the ocean basins. In these regions, the genesis of the cool air is diagnosed. The loss of the cool air occurs over land and the subtropical oceans, particularly on the offshore side of the coastal upwelling regions. In the Pacific sector and the Indian Ocean sector of the Southern Ocean, another large loss of the cool air occurs along the oceanic frontal zone including the Agulhas Return Current. Over the zonally extended region where the cool air is generated in the Southern Hemisphere and the coastal upwelling regions, it is suggested that diabatic cooling associated with low-level clouds overcome heating by turbulent surface heat fluxes. The genesis of the cool air over the subarctic oceans in the Northern Hemisphere in the warm season switches into the loss of the cold air in the cool season on a basin scale. Meanwhile, over the oceans in the Southern Hemisphere, there is no basin-scale seasonal switch.

© 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: T. Shimada, shimadat@hirosaki-u.ac.jp

Abstract

The climatology of low-level cool air over the midlatitude oceans in summer is presented based on an isentropic analysis. This study focuses on isentropic surfaces of 296 K to analyze an adiabatic invariant referred to as the negative heat content representing the coldness of the air layer below the threshold isentropic surface. This approach allows a systematic analysis and a quantitative comparison of the cool air distribution and a diagnosis of diabatic heating of the air mass. The cool air covers most of the subarctic oceans and extends equatorward over the coastal upwelling regions in the east of the ocean basins. In these regions, the genesis of the cool air is diagnosed. The loss of the cool air occurs over land and the subtropical oceans, particularly on the offshore side of the coastal upwelling regions. In the Pacific sector and the Indian Ocean sector of the Southern Ocean, another large loss of the cool air occurs along the oceanic frontal zone including the Agulhas Return Current. Over the zonally extended region where the cool air is generated in the Southern Hemisphere and the coastal upwelling regions, it is suggested that diabatic cooling associated with low-level clouds overcome heating by turbulent surface heat fluxes. The genesis of the cool air over the subarctic oceans in the Northern Hemisphere in the warm season switches into the loss of the cold air in the cool season on a basin scale. Meanwhile, over the oceans in the Southern Hemisphere, there is no basin-scale seasonal switch.

© 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: T. Shimada, shimadat@hirosaki-u.ac.jp

1. Introduction

Low-level air with low temperature (which we hereinafter refer to as cool air in summer and cold air in winter) is an important factor involved in climate. In summer, relatively cool air develops over the oceans at midlatitudes because of differential heating between land and ocean. In the east of the ocean basins, equatorward winds along the eastern flanks of subtropical highs are intensified by baroclinicity enhanced by the cool ocean and warm land (Fig. 1a; e.g., Burk and Thompson 1996; Miyasaka and Nakamura 2005, 2010). The intensified equatorward winds induce coastal upwelling in the east of the ocean basins (Fig. 1b; e.g., Chavez and Messié 2009). The cool air over the ocean then enhances stratification in the lower atmosphere, as does the subsidence associated with subtropical highs. The enhanced stratification promotes the formation of low-level clouds over the subarctic oceans (oceanic regions with a subarctic gyre) and cool regions such as the coastal upwelling regions (e.g., Klein and Hartmann 1993). In turn, the low-level clouds contribute to the formation of cool air by reduction of the insolation and by longwave radiative cooling at the cloud top (e.g., Wood 2012). Furthermore, the cool air over the subarctic oceans creates a large meridional gradient of air temperature across the boundary with the subtropical oceans (oceanic regions with a subtropical gyre) at latitudes of around 40° (Figs. 1b,c). Thus, the cool air over the ocean is an important aspect for understanding air–sea interaction. Many studies have indicated the important roles of the cool air over the ocean in basin-scale climate and the impact of the cool air intrusion onto the land on regional climate (e.g., Ninomiya and Mizuno 1985; Bakun 1990; Norris et al. 1998).

Fig. 1.
Fig. 1.

Monthly climatology of (a) sea level pressure (hPa; contours and color shading) and surface winds (m s−1; colored vectors), (b) skin temperature and SST (°C), and (c) SST gradient magnitude [°C (100 km)−1] in July in the Northern Hemisphere and in January in the Southern Hemisphere. For clarity, the vectors in (a) are plotted at every eight grid points (6.0° × 6.0°). The measures of the color bar in (a) indicate sea level pressure (top numbers) and surface wind speed (bottom numbers). SST is overlaid in (c) by thin and thick contours with intervals of 2° and 10°C, respectively.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0188.1

However, few studies have used an analysis method for the air mass to directly analyze the summertime cool air over the oceans; this is in stark contrast to the many studies on wintertime cold air mainly in terms of cold air outbreaks or cold surges (e.g., Shoji et al. 2014 and references therein). Although the cool air over the subarctic oceans and the coastal upwelling regions are confirmed by low-level temperature distributions, such temperature distributions represent the only one aspect of the cool air over the ocean. We propose the following issues to be addressed. First, no comparative investigation has been conducted for the distributions of the cool air over the oceans in summer, and thus a systematic quantification of the cool air is desired. Second, we need to identify where the cool air is generated and lost, and explore the similarities and differences in the distribution and amount of diabatic cooling and heating over the midlatitude oceans. Then, the seasonal persistence of the summertime features remains an open question. It is necessary to identify when the ocean and land switch roles between cooling and heating of the atmosphere. The reason why these issues have not been addressed in previous studies is because effective methods have not been applied to the analysis of the summertime cool air. Analysis methods that can quantify features of the air mass and are not region specific must serve to obtain a synthesis of the cool air in summer and to shed new light on the understanding of the phenomena associated with the cool air.

Isentropic coordinates have proved effective for airmass analysis. Iwasaki et al. (2014) proposed an isentropic method for diagnosing the geographical distributions of air masses in the lower atmosphere and of diabatic heating of the air masses. Iwasaki et al. (2014) and the subsequent studies have applied this method to the analysis of the cold air in winter and have demonstrated the effectiveness of this method in the following points (Shoji et al. 2014; Yamazaki et al. 2015; Kanno et al. 2015a,b, 2016; Papritz and Pfahl 2016; Abdillah et al. 2017; Kanno et al. 2017). First, based on the dynamical movement of the air mass, the method can evaluate the amount and flux of the cold air and show their geographical distributions. This advantage enables us to trace the evolution processes of the cold air and to identify the outflow routes of the cold air. The effective indices for defining the cold air outbreak are derived from the flux of cold air. Second, the method can quantitatively diagnose diabatic heating or cooling of the air mass, especially for climatological fields, and show the geographical distributions, which would otherwise be difficult to estimate. These two advantages allow us to examine the dynamical and thermodynamical characteristics of the air mass and to systematically compare them between regions.

This study applies the isentropic analysis proposed by Iwasaki et al. (2014) to reanalysis data to derive the climatology of the low-level cool air over the midlatitude oceans in summer. Specific questions we would like to address are the following: 1) How is the cool air distributed over the midlatitudes in summer? What are the effects of land–ocean distribution on the distribution of cool air? 2) Where is the cool air generated and lost in summer? What are the similarities and differences in the distributions of diabatic heating and cooling of the cool air between the ocean basins? 3) How long do the summertime features persist? When do the roles of ocean and land switch between heating and cooling of the atmosphere? Answering these questions provides new insights into the cool air over the oceans, beyond the conventional view that the cool air is distributed over the cool oceans, and leads to a global and seasonal synthesis of air mass in the lower atmosphere.

Data and methods are described in section 2. Sections 3 and 4 present the climatology of the cool air distributions and of the diagnosed diabatic heating and cooling, respectively. Section 5 shows seasonal persistence of the summertime features. Section 6 contains the summary and conclusions.

2. Data and methods

We used 6-hourly reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim) on a 0.75° × 0.75° grid and 60 model levels (Dee et al. 2011). Typically, over the sea surface, nine model levels are taken below the 925-hPa level, and thus the model level data are suitable for analyzing the thin layer of summertime cool air. We used skin temperature for sea surface temperature (SST) and hereinafter refer to skin temperature on the sea as SST. We mainly analyzed the monthly climatology averaged over 11 years between 2003 and 2013. The reason for the analyzed period after 2003 is that more satellite observations have been used for data assimilation and that satellite observations have been used to analyze SST fields for the reanalysis.

As typical summer months, we chose July in the Northern Hemisphere and January in the Southern Hemisphere. In these months, the cool air retreats farthest poleward. In some of the following figures, we combined data fields and plots in July in the Northern Hemisphere and in January in the Southern Hemisphere to produce one figure for summer climatology. The following are key climatological features in these months for understanding the results: the equatorward winds intensified along the eastern boundary of the basins or the eastern flank of the subtropical highs (Fig. 1a), the upwelled water with low temperature extending in the east of the subtropical oceans (Fig. 1b), and SST frontal zones along the boundaries between the subarctic and subtropical oceans and along the subtropical western boundary currents and their extensions (Figs. 1b,c).

We adopted the isentropic analysis method proposed by Iwasaki et al. (2014). This study proposed two adiabatic invariants defined below a threshold potential temperature and derived their conservation equations from the vertical integration between the surface and the isentropic surface of . Here we use one of the adiabatic invariants, negative heat content (NHC), which is defined as
e1
The NHC represents the degree of coldness weighted by the potential temperature difference with respect to . The conservation equation of NHC is derived as
e2
where is horizontal wind vector and . The term on the right-hand side of Eq. (2) is the genesis/loss rate of NHC, which represents diabatic heating or cooling within the layer below the isentropic surface of . Iwasaki et al. (2014) refer to as NHC flux. For the steady state or climatological mean, Eq. (2) gives the following equation:
e3
where the square brackets here indicate a time mean. Thus, the genesis/loss rate of NHC for the steady state or climatological mean, represented by the right-hand side of Eq. (3), can be diagnosed from the left-hand side of Eq. (3). This diagnosis is effective for climatological fields, because time mean velocity weighted with NHC is significantly different from unweighted time mean velocity especially in a baroclinic zone. In this study, we assume that Eq. (3) is satisfied for the monthly climatology, which is derived from averaging the 6-hourly data. We have confirmed that for monthly climatology the time mean of is one order of magnitude smaller than the time means of the other terms in Eq. (2). The NHC has the advantages that it reflects the air temperature profiles below the isentropic surface of and that features associated with the diabatic heating or cooling especially from the sea and land surfaces can be distinctly identified. This makes NHC particularly appropriate for the analyses of the summertime cool air, compared with the other adiabatic invariant, cold airmass amount, which is defined as and is used in the analyses of wintertime cold air (Shoji et al. 2014; Kanno et al. 2015a,b, 2016; Yamazaki et al. 2015).

We chose a threshold potential temperature of 296 K for the present analysis. The cool air is consequently defined as the air below this isentropic surface and the NHC is computed for the layer between the surface and this isentropic surface. We made this choice for two reasons, considering the coverage ratio of the cool air over the ocean (Fig. 2). 1) The isentropic surfaces of 296 K exist over the oceans at midlatitudes (especially the subarctic oceans and the subarctic frontal zones) with high coverage ratio, which allows us to construct a monthly climatology, even in the summer months when the cool air retreats poleward. In the Northern Hemisphere, the coverage ratio is more than 87% (66%) to the north of 40°N (35°N); in the Southern Hemisphere, the coverage ratio does not fall below 99% at 35°S. With a threshold potential temperature less than 294 K, it is unable to show the cool air distribution around the subarctic frontal zones. 2) The isentropic surfaces of 296 K in the Northern and Southern Hemispheres mostly reach the sea surface at low latitudes and do not connect with each other over the tropics, which allows us to distinguish the distribution of the cool air in the two hemispheres. The coverage ratio decreases to less than 5% at 15°N in the Northern Hemisphere and at 6°S in the Southern Hemisphere. With a threshold potential temperature greater than 298 K, the cool air appears over the equatorial regions. Although regions for analysis and clarity of the regional distribution depend on the threshold potential temperature, we have confirmed that the essentials of the present results on the summertime cool air are largely insensitive to the choice of threshold potential temperature around 296 K. For the seasonal analysis of the air mass, this study consistently uses the threshold potential temperature of 296 K.

Fig. 2.
Fig. 2.

Zonal mean of coverage ratio at which the cool air is definable over the sea in July in the Northern Hemisphere and in January in the Southern Hemisphere for isentropic surfaces of 270–300 K with an interval of 2 K. The coverage ratio is computed based on the 6-hourly data and the zonal mean is conducted only for the sea grids. The interval of the thick black contours is 6 K. Gray shading indicates the latitudes without any sea grid points owing to the Antarctic continent.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0188.1

3. Distribution of cool air

We examine the height of the threshold isentropic surface and the NHC at midlatitudes in July in the Northern Hemisphere and in January in the Southern Hemisphere (Figs. 3a,b). The height of the threshold isentropic surface has a maximum in the polar regions (about 3300 m at the North Pole and about 6200 m at the South Pole) and decreases equatorward in both hemispheres (Fig. 3a). The resulting NHC generally decreases equatorward from the polar regions (Fig. 3b). However, at midlatitudes, NHC larger than 500 K hPa covers the oceans preferentially and only parts of the land around the Arctic Ocean and the Southern Ocean. Thus, the distribution of the NHC at midlatitudes in summer strongly depends on the land–ocean distribution.

Fig. 3.
Fig. 3.

Monthly climatology in July in the Northern Hemisphere and in January in the Southern Hemisphere. (a) Height of the isentropic surface of 296 K (m; color shading and contours at an interval of 200 m). (b) NHC (K hPa; color shading and contours at an interval of 500 K hPa) and its flux (K hPa m s−1; fixed-length colored vectors). The measures of the color bar in (b) indicate NHC (top numbers) and its flux (bottom numbers). For clarity, the flux vectors are plotted at every eight grid points (6.0° × 6.0°). (c) Genesis/loss rate of NHC (K hPa day−1). SST is overlaid in (c) by thin and thick contours with intervals of 2° and 10°C, respectively. In these maps and others to follow, we do not plot the data at the grid points where the height of the threshold isentropic surface in the monthly climatology is lower than 10 m.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0188.1

We examine the predominant distribution of the NHC over the oceans from the zonal integrations of the NHC over ocean and land separately and their sum (Fig. 4), and from the seasonal variations in the total NHC integrated over each whole hemisphere (Fig. 5). The most notable feature of the integrated NHC in summer is the large difference between the hemispheres (Figs. 4a and 5): the total NHC in July in the Northern Hemisphere is 29% of that in January in the Southern Hemisphere. This ratio is a seasonal minimum (Fig. 5). The ratios in winter months (November–March for the Northern Hemisphere and May–September for the Southern Hemisphere) are more than 107% (Figs. 4b and 5; Fig. 3 of Kanno et al. 2015a). The other notable feature in summer is a major effect of the land–ocean distribution on the distribution of the cool air at mid-to-low latitudes (Figs. 4a,c). In summer, the percentage of the zonally integrated NHC over the ocean to the zonally integrated NHC over the ocean and land at a given latitude deviates from the percentage of ocean coverage south of 60°N in the Northern Hemisphere and north of 40°S in the Southern Hemisphere (Fig. 4c). This means that, at these latitudes in summer, the NHC is preferentially distributed over the oceans. This is in sharp contrast to the fairly zonal distribution of the cold air in winter (Figs. 4b,c).

Fig. 4.
Fig. 4.

(a) Zonal integration of the NHC per unit length in the latitudinal direction over land (red), ocean (blue), and both (gray) in July in the Northern Hemisphere and in January in the Southern Hemisphere. (b) As in (a), but for January in the Northern Hemisphere and July in the Southern Hemisphere. (c) Percentage of the NHC integrated zonally only over ocean to that integrated zonally over both land and ocean in July in the Northern Hemisphere and in January in the Southern Hemisphere (red) and in January in the Northern Hemisphere and in July in the Southern Hemisphere (blue). The percentage of ocean coverage as a function of latitude is also shown (gray).

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0188.1

Fig. 5.
Fig. 5.

Climatological monthly variations of the total NHC in the Northern Hemisphere (NH; black) and the Southern Hemisphere (SH; gray).

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0188.1

We then compare the characteristics of the NHC distribution in different basins (Fig. 3b). There are two common characteristics. First, large NHC is distributed mainly over the subarctic oceans: the subarctic North Pacific, the subarctic North Atlantic, and the Southern Ocean. Eastward NHC flux is dominant over these subarctic oceans, corresponding to the westerlies. Second, the NHC extends equatorward with anticyclonic curvature in the east of the North Pacific, the North Atlantic, the South Pacific, the South Atlantic, and the Indian Ocean, with large NHC flux. The large NHC flux in the east of each basin contributes largely to the equatorward transport of NHC and the equatorward extensions of the NHC account for the NHC over the ocean at mid-to-low latitudes. Although these extensions roughly exist over the coastal upwelling regions of the basins, the central axes of the equatorward extensions of the NHC are displaced 5°–10° longitude to the west from the eastern boundary region with lower SST. We have confirmed that the equatorward extensions of the NHC for lower threshold potential temperatures are located closer to the eastern boundary, which enhances the baroclinicity across the eastern boundaries between ocean and land.

The NHC differs between the basins in the following aspects (Fig. 3b). First, the NHC over the basins in the Southern Hemisphere is larger than that over the basins in the Northern Hemisphere at the same latitude, as also seen in Figs. 4a and 5. The NHC differs little between the basins in the Southern Hemisphere. Second, the NHC over the upwelling regions extends farther equatorward in the Southern Hemisphere than in the Northern Hemisphere, except in the Indian Ocean. The equatorward extension of the NHC in the Indian Ocean is restricted probably by the limited region of coastal upwelling owing to the shorter western coast of the Australian continent and owing to warm water of the Leeuwin Current (Fig. 1b; e.g., Domingues et al. 2007). Moreover, in the North Pacific, the maximum NHC is located at the exit region of the Bering Strait in the Bering Sea. This suggests that the cool air over the Bering Sea is almost separated from that over the Arctic Ocean because of the genesis of cool air over the Bering Sea and because of the loss of NHC over the land on both sides of the Bering Strait, as will be shown in section 4. Thus, from the results above, we could show the continuity of the cool air distribution, compare the cool air distributions between basins, and clarify the impact of the land–ocean distribution on the cool air distribution, beyond the conventional view that the cool air is distributed over the cool oceans.

4. Diagnosed diabatic heating and cooling

a. Distribution

We show the genesis/loss rate of NHC diagnosed for the monthly climatology (Fig. 3c). In the Northern Hemisphere, the NHC is generated over the Arctic Ocean, the subarctic North Pacific, and the subarctic North Atlantic. Distinct regions with high genesis rate of NHC exist in the western part of the subarctic oceans, including the marginal seas: the Okhotsk Sea and the Bering Sea for the subarctic North Pacific; and the Davis Strait, Baffin Bay, and Hudson Bay for the subarctic North Atlantic. The east–west difference is particularly distinct over the subarctic North Atlantic because the subarctic ocean gyre is deflected to the west and the Gulf Stream extends to the northeast. The NHC is also generated over the upwelling regions in the east of the basins. The region of the NHC genesis is seen over the broad upwelling region off the California coast in the North Pacific, but is confined close to the coast near the Canary Islands in the North Atlantic.

In the Northern Hemisphere, the NHC is lost over the land areas at latitudes higher than 45°N surrounding the oceans (the Arctic Ocean, the subarctic North Pacific, and the subarctic North Atlantic) where NHC is generated. This fact indicates that the cool air flowing from the Arctic region and the subarctic oceans into the land is heated and then lost over the land. Only over Greenland, NHC is weakly generated even in summer. Over the ocean, the regions of NHC loss lie to the south of the genesis regions and over the Kuroshio Extension and the Gulf Stream. In the subarctic North Atlantic, NHC loss occurs to the west of the British Isles, probably owing to the warm water of the Gulf Stream. In particular, large loss of NHC occurs on the offshore side of the upwelling regions in the east of the basins. The cool air generated over the subarctic oceans and the upwelling regions flows southwestward and disappears in the trade wind zone. About 34% of the NHC generated over the subarctic ocean is lost over the subtropical ocean and the trade wind zone in the North Pacific, while, in the North Atlantic, the corresponding percentage is about 25%.

The genesis and loss of NHC in the Southern Hemisphere share the following characteristics with the Northern Hemisphere. The NHC is generated over the polar region (or the Antarctic continent), the surrounding subarctic region (or the Southern Ocean), and the upwelling regions in the east of the basins except the Indian Ocean. Losses of NHC are large over the subtropical oceans to the north of 30°S, the subtropical western boundary currents, and especially the region on the offshore side of the upwelling regions. Meanwhile, a distinctive feature is found in the Southern Ocean. Over the Pacific sector and the Indian Ocean sector of the Southern Ocean, a roughly zonal region of NHC loss extends from south of the African continent (35°S, 15°E) to south of the South American continent (55°S, 75°W) via to south of the Australian continent (55°S, 150°E). This region of the NHC loss corresponds to the Agulhas Return Current and its extension to the west of about 70°E and, to the east of about 70°E, to the subantarctic zone and the polar frontal zone (e.g., Orsi et al. 1995; Belkin and Gordon 1996). Between this roughly zonal region and the subtropical oceans of the NHC loss, there exists a weak but well-organized zone of the NHC genesis. Thus, the genesis/loss rate of NHC over the Pacific sector and the Indian Ocean sector of the Southern Ocean shows a double structure with two pairs of roughly zonal regions with genesis and loss of NHC. This double structure is not seen over the Atlantic Ocean, where the single boundary is formed between the regions of genesis and loss.

We confirm the differences in the distribution of the genesis/loss rate of NHC from the zonal integrations of the meridional NHC flux and of the genesis/loss rate of NHC (Fig. 6). The NHC flux into the midlatitudes across a latitude of 70° from the polar regions and the diagnosed NHC genesis in the polar regions are largely the same in both hemispheres. In the Northern Hemisphere, the equatorward NHC flux reaches its maximum at 70°N. This latitude corresponds roughly to the boundary between the Arctic Ocean and the surrounding land areas. To the south of 70°N, the equatorward NHC flux decreases and NHC loss occurs over the land areas. The NHC genesis over the subarctic oceans is evident from its maximum at around 45°N (Fig. 6b). In the Southern Hemisphere, the equatorward NHC flux reaches its maximum at 56°S. Equatorward of this latitude, the equatorward NHC flux decreases monotonically and NHC loss occurs with a peak over the subtropical oceans at around 30°N. Thus, the supplies of NHC from the polar regions to the midlatitudes are comparable in both hemispheres and the differences in the distribution of NHC are made at the midlatitudes.

Fig. 6.
Fig. 6.

Zonal integrations of (a) meridional NHC flux and (b) genesis/loss rate of NHC per unit length in the latitudinal direction in July in the Northern Hemisphere (NH; black) and in January in the Southern Hemisphere (SH; gray). In (a), the sign is reversed for the data of the Southern Hemisphere and thus negative values indicate equatorward flux for both hemispheres. In (b), positive and negative values indicate genesis and loss of NHC, respectively.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0188.1

Finally, we focus on higher NHC loss over the subtropical western boundary currents in the Southern Hemisphere than in the Northern Hemisphere (Fig. 3c). For example, the NHC loss over the Gulf Stream is lower than that over the other subtropical western boundary currents in the Southern Hemisphere, despite the strongest SST fronts of the Gulf Stream. The following are suggested as the cause. Over the Gulf Stream and the Kuroshio and its extension, warm air advection associated with the subtropical high is dominant in summer (Fig. 7a) and equatorward developments of the cool air are weak owing to the summertime storm tracks located to the north of these currents (Fig. 7b). We here derived storm tracks based on the local deepening rate of surface pressure proposed by Kuwano-Yoshida (2014). In contrast, storm tracks in summer are closer to the subtropical western boundary currents and more active in the Southern Hemisphere than in the Northern Hemisphere (Fig. 7b). Thus, over the subtropical western boundary currents in the Southern Hemisphere, equatorward developments of the cool air across the oceanic fronts occur frequently in association with cyclone passages, and the heating of the cool air on the warmer flank of the oceanic fronts is accounted for by the NHC loss.

Fig. 7.
Fig. 7.

Monthly climatology of (a) surface temperature advection (K day−1; color shading) and (b) local deepening rate (hPa day−1) in July in the Northern Hemisphere and in January in the Southern Hemisphere. The surface temperature advection in (a) is calculated from 10-m horizontal wind and skin temperature or SST T (Klein et al. 1995). In (b), following the method of Kuwano-Yoshida (2014), the averaging is for the positive local deepening rate with the other values set to zero, and the values less than 2.0 hPa day−1 or between 20°N and 20°S are not plotted. SST is overlaid in both figures by thin and thick contours with intervals of 2° and 10°C, respectively.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0188.1

b. Influence of turbulent surface heat flux

We examine the contribution of turbulent surface heat fluxes to the genesis/loss rate of NHC (Figs. 8 and 9). The sensible heat flux is directly responsible for both cooling and heating of the lower atmosphere (Fig. 8a). In the Northern Hemisphere, the sensible heat flux is downward in the regions of the NHC genesis (the subarctic North Pacific and the subarctic North Atlantic except the east of the basins). In the Southern Hemisphere, the downward sensible heat flux is rather distributed sporadically in the Southern Ocean. However, the sporadic distribution of the downward sensible heat flux is quite consistent with that of the genesis rate of NHC. The regions of the NHC loss globally correspond to those of upward sensible heat flux. These collocations indicate that the sensible heat flux is important for the genesis and loss of the NHC. This is confirmed from the fact that the genesis/loss rate of NHC is linearly related to the sensible heat flux in the basins (Fig. 9). On the other hand, the latent heat flux can contribute to the NHC loss if the latent heat is released in the layer of the cool air. The latent heat flux also linearly contributes to the NHC loss. However, the slopes for the latent heat flux are smaller than those for the sensible heat flux. This is significant in a range of high latent heat flux in the subarctic regions (Figs. 9a–c) and in a whole range in the subtropical regions (Figs. 9d–f) where the height of the threshold potential temperature surface is relatively low. This is probably because the latent heat flux is partly released at levels higher than the height of the threshold potential temperature surface.

Fig. 8.
Fig. 8.

Monthly climatology of (a) sensible heat flux (W m−2; color shading) and (b) latent heat flux (W m−2; color shading) in July in the Northern Hemisphere and in January in the Southern Hemisphere. Positive and negative values indicate upward and downward heat fluxes, respectively. These climatologies are constructed for the grid points at which the cool air is defined and thus upward sensible and latent heat fluxes at low latitudes are enhanced compared with the ordinary climatology in which the 6-hourly data fields are equally averaged. SST is overlaid in both panels by thin and thick contours with intervals of 2° and 10°C, respectively. Incidentally, the downward sensible heat flux in some land areas in (a) reflects the nighttime cooling of the land (e.g., in the east of Australian continent).

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0188.1

Fig. 9.
Fig. 9.

Binned scatterplots between the sensible heat flux and the genesis/loss rate of NHC (black) and between the latent heat flux and the genesis/loss rate of NHC (gray) for six geographical regions in July in the Northern Hemisphere and in January in the Southern Hemisphere. (top) Regions mainly covering (a) the subarctic North Pacific (40°–60°N, 140°E–110°W), (b) the subarctic North Atlantic (40°–60°N, 75°–10°W), and (c) the Southern Ocean (40°–60°S, all longitudes). (bottom) Regions mainly covering (d) the subtropical North Pacific (15°–40°N, 140°E–110°W), (e) the subtropical North Atlantic (15°–40°N, 75°–10°W), and (f) the subtropical oceans in the Southern Hemisphere (15°–40°S, all longitudes). The comparisons are based on monthly means for each year. The points in each panel are the means within each bin and the error bars are the plus or minus one standard deviation within each bin. The data at land grid points and the data within bins with relative frequency less than 0.5% are excluded. Each panel includes the correlation coefficients R.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0188.1

In the upwelling regions in both hemispheres where the NHC genesis is diagnosed, the sensible heat flux is mostly upward, as well as the latent heat flux, and is downward only in the immediate vicinity of the eastern boundary of the basins (Fig. 8). Thus, linear relationships between the genesis/loss rate of NHC and sensible and latent heat fluxes as shown in Fig. 9 are not seen in the upwelling regions. The equatorward wind is predominant over the upwelling regions in summer and the temperature of the air from the subarctic oceans is lower than the SST in the upwelling regions. The resulting upward sensible heat flux is supposed to partly contribute to the NHC loss. Given that the NHC genesis is diagnosed in the upwelling regions, other factors inducing the NHC genesis are suggested.

c. Influence of low-level clouds

Diabatic cooling processes associated with low-level clouds, such as radiative cooling and evaporation of cloud water, are candidates for the NHC genesis. The distribution of the vertically integrated cloud water mixing ratio within the layer of the cool air (Fig. 10a) is consistent with the low-level cloud cover in the previous studies (e.g., Klein and Hartmann 1993; Wood 2012; Naud et al. 2014) and is generally similar to that of the NHC genesis (Fig. 3c). Low-level clouds fully cover the subarctic North Pacific, the subarctic North Atlantic, the Southern Ocean, and the upwelling regions in the east of the basins. Over the upwelling regions, the maximum cloud water mixing ratio is located away from the coastlines roughly 5°–10° longitude to the west because of the strong winds and boundary layer deepening offshore from the coast (Wood and Bretherton 2004). Thus, these consistent distributions suggest the importance of low-level clouds for the NHC genesis.

Fig. 10.
Fig. 10.

Monthly climatology of (a) cloud water vertically integrated from the ground to the threshold potential temperature surface and (b) EIS in July in the Northern Hemisphere and in January in the Southern Hemisphere. Cloud water mixing ratio in (a) is the sum of cloud liquid and ice water mixing ratios. SST is overlaid in both panels by thin and thick contours with intervals of 2° and 10°C, respectively.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0188.1

We examine the relation of the low-level clouds to the genesis/loss rate of NHC. As a measure of low-level clouds, we adopt the estimated inversion strength (EIS), which represents the inversion strength of the planetary boundary layer (Wood and Bretherton 2006). The EIS is commonly applicable to midlatitude regions. The spatial pattern of the EIS (Fig. 10b) is more consistent with that of the genesis/loss rate of NHC (Fig. 3c) than the vertically integrated cloud water is (Fig. 10a). Over the midlatitude oceans (30°–60°N and 30°–60°S, all longitudes), the genesis/loss rate of NHC is uncorrelated with the vertically integrated cloud water while it is correlated with the EIS with a correlation coefficient of 0.43. The EIS is large in the west of the subarctic North Pacific and the subarctic North Atlantic and in the upwelling regions with a maximum immediately adjacent to the coast. In the Pacific sector and the Indian Ocean sector of the Southern Ocean, the large EIS coincides with the NHC genesis within the double structure of the genesis/loss rate of NHC.

Figure 11 shows relationships between EIS and the genesis/loss rate of NHC. In the subarctic regions, the genesis rate of NHC increases with EIS (Figs. 11a–c). This indicates that low-level clouds contribute to the NHC genesis. In the range of the EIS less than 4–6 K, the NHC genesis/loss rate is negative, suggesting that heating by sensible and/or latent heat flux overcomes the cooling. On the other hand, in the subtropical regions with the small EIS (Figs. 11d–f), the genesis/loss rate of the NHC is almost constant with the EIS. However, the slight increases of the genesis/loss rate of NHC for the large EIS indicate the cooling over the upwelling regions with large EIS or large amount of low-level clouds. Thus, in the subarctic North Pacific, the subarctic North Atlantic, and the Southern Ocean, the diabatic cooling associated with low-level clouds, as well as downward surface heat flux, contributes to the cool air formation. In the upwelling regions, diabatic cooling associated with low-level clouds overcomes the sensible and latent heating from the sea surface. In the weak but well-organized zone of the NHC genesis in the Pacific sector and the Indian Ocean sector of the Southern Ocean, diabatic cooling associated with low-level clouds cancels out or slightly overcomes the sensible and latent heating from the sea surface, and consequently forms the double structure of the genesis/loss rate of NHC.

Fig. 11.
Fig. 11.

As in Fig. 9, but for the binned scatterplots between the EIS and the genesis/loss rate of NHC. Correlation coefficients are not shown.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0188.1

5. Seasonal persistence

Here we examine how long the summertime features shown so far persist from seasonal variations in the genesis/loss rate of NHC. Figure 12 shows time–latitude diagrams of the genesis/loss rate of NHC along the representative meridians of 147.75°E, 180.00°, 125.25°W, and 49.50°W. In Fig. 13, we extract the data at the four selected latitudinal bands from Figs. 12a,c. Both hemispheres share the following two characteristics. First, the polar regions (the Arctic Ocean and the Antarctic continent) contribute to the genesis of NHC throughout the year. Second, over land at midlatitudes, NHC is lost in the warm season and generated in the cool season. This seasonal variation is confirmed over Siberia (60°–71°N in Figs. 12a and 13 and 65°–69°N in Fig. 12b), over the southern Australian continent and the island of Tasmania (28°–38°S and 41°–43°N in Fig. 12a), and over the North American continent (55°–69°N in Fig. 12c). Only Greenland contributes to the NHC genesis throughout the year (63°–81°N in Fig. 12d).

Fig. 12.
Fig. 12.

Time–latitude diagrams of genesis/loss rate of NHC (K hPa day−1) along the longitudes (a) 147.75°E, (b) 180.00°, (c) 125.25°W, and (d) 49.50°W. Gray bars along the right ordinate indicate land grid points. A running average over five grid points in the zonal direction is applied.

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0188.1

Fig. 13.
Fig. 13.

Climatological monthly variations of the genesis/loss rate of NHC (K hPa day−1) for the selected latitudinal bands along the longitudes 147.75°E and 125.25°W. The subarctic North Pacific (black line with filled circles), the land in Siberia (gray line with filled circles), the upwelling region off the California coast (gray line with filled diamonds), and the NHC loss region in the double structure in the Pacific sector of the Southern Ocean (black line with filled diamonds).

Citation: Journal of Climate 31, 5; 10.1175/JCLI-D-17-0188.1

Over the subarctic North Pacific (including the Okhotsk Sea and the Bering Sea; 37°–59°N in Figs. 12a and 13 and 36°–65°N in Fig. 12b) and the subarctic North Atlantic (40°–60°N in Fig. 12d), NHC is generated in the warm season and lost in the cool season. This switch occurs on almost a basin scale. The switches from loss to genesis occur in March–May and from genesis to loss in September–November with regional differences. Above all, NHC genesis over the Labrador Current (40°–50°N in Fig. 12d) persists for more than nine months and NHC loss in the cool season is smaller than that over the sea areas to the north and south. In stark contrast to these seasonal switches, NHC is generated throughout the year over the upwelling region off the California coast, with a slight northern shift in the warm season (27°–46°N in Figs. 12c and 13). In addition, over the Kuroshio Extension (30°–40°N in Fig. 12a) and the Gulf Stream (32°–42°N in Fig. 12d), weak loss of the NHC occurs even in the warm season. Over these subtropical western boundary currents, NHC is not generated in summer and is lost throughout the year.

The basins in the Southern Hemisphere have the following features common to those in the Northern Hemisphere: throughout the year, NHC is generated over the upwelling regions in the east of the basins (except in the Indian Ocean) and is lost over the subtropical western boundary currents (the Agulhas Current, the East Australian Current, and the Brazil Current). A distinct feature over the oceans in the Southern Hemisphere is that the general structure persists throughout the year, with seasonal variations in the latitudinal location of the structure and in magnitude of the genesis/loss rate of NHC. In the Pacific sector and the Indian Ocean sector of the Southern Ocean, the double structure of the genesis/loss rate of NHC is seen throughout the year (Figs. 12a–c and 13). In the South Atlantic, a single boundary between the genesis and loss rate of NHC clearly exists throughout the year (50°S in Fig. 12d). Thus, unlike the Northern Hemisphere, the genesis and loss of NHC do not switch on a basin scale in the Southern Hemisphere. In fact, the wintertime cold air is generated over the Southern Ocean in a circumpolar fashion and lost on the equatorial side (Kanno et al. 2015a). The reason why the Southern Ocean does not contribute to the loss of the cold air may be insignificant outbreaks of the cold air originating from the high- to midlatitude land onto the relatively warm ocean in the Southern Hemisphere.

6. Summary and conclusions

We have presented a climatology of low-level cool air at midlatitudes in summer by applying isentropic analysis to reanalysis data. The main results are summarized as follows:

  1. In summer, the cool air is preferentially distributed over the subarctic oceans, with the amount decreasing equatorward from the polar regions and extending equatorward over the upwelling regions in the east of the basins. The ratio of the integrated NHC over the Northern Hemisphere in July to that over the Southern Hemisphere in January is a seasonal minimum (29%), possibly reflecting the large loss of NHC over land in the Northern Hemisphere, diabatic cooling associated with a large amount of low-level clouds over the Southern Ocean, and equatorward transport of the cool air owing to active storm tracks in the Southern Hemisphere.
  2. The NHC is generated over the polar regions, the subarctic oceans, and the upwelling regions in the east of the basins, and is lost over the subtropical oceans including western boundary currents and over land except Greenland. In particular, large loss of NHC occurs on the offshore side of the upwelling regions where the equatorward winds associated with the subtropical highs flow into the trade wind zone. In the Pacific sector and the Indian Ocean sector of the Southern Ocean, a double structure with two pairs of zonally extended regions with genesis and loss of NHC is formed. The genesis/loss rate of NHC is linearly related to the sensible and latent heat fluxes in the basins. Over the subarctic regions and the upwelling regions, the diabatic cooling associated with low-level clouds is indicated by the relation between the genesis/loss rate of NHC and the EIS.
  3. The subarctic North Pacific and the subarctic North Atlantic switch seasonally between genesis and loss of the NHC on a basin scale; the cool air is generated over the ocean in the warm season and the cold air is lost over the ocean in the cool season. In contrast, the oceans in the Southern Hemisphere do not show such a basin-scale seasonal variation in the genesis/loss rate of NHC. Throughout the year, we can see the double structure of the genesis/loss rate of NHC in the Pacific sector and the Indian Ocean sector of the Southern Ocean and the single structure in the Atlantic sector. The following characteristics are common to both hemispheres. Over the land at midlatitudes, the NHC is lost in the warm season and generated in the cool season. The polar regions and the upwelling regions in the east of the basins (except the Indian Ocean) contribute to the genesis of NHC throughout the year. Over the subtropical western boundary currents, the NHC is lost throughout the year.

Further studies of the following aspects are needed. First, examining the temporal evolution of the cool air would be a challenge. A key issue may be intermittent equatorward flow of the cool air in the storm tracks (e.g., Papritz and Spengler 2015; Papritz and Pfahl 2016). Issues to be solved include processes involved in the genesis and loss of the cool air and impacts of differences in storm-track activity between both hemispheres on the cool air distribution (Uno and Iwasaki 2006). Second, examining the regional climate from the viewpoint of the low-level cool air would merit further research. Isentropic analysis may provide a new view of the intrusion of the cool air generated over the ocean onto the land through low-altitude areas. Interannual variations in the amount of the cool air would also be an important subject. These points are being explored in our ongoing work.

Acknowledgments

The authors thank three anonymous reviewers for their careful reading and constructive suggestions. We downloaded the ERA-Interim data from the website of the European Centre for Medium-Range Weather Forecasts (http://apps.ecmwf.int/datasets/data/interim-full-daily/levtype=sfc/). This study was supported by Grants-in-Aid for Scientific Research (JSPS KAKENHI Grants 26400461, 17K00657, and 15H02129) and by Social Implementation Program on Climate Change Adaptation Technology (SI-CAT) of the Japanese Ministry of Education, Culture, Sports, Science and Technology. YK is supported by the Japan Society for the Promotion of Science through a Grant-in-Aid for Research Fellows (16J01722).

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