Response of Mode Water and Subtropical Countercurrent to Greenhouse Gas and Aerosol Forcing in the North Pacific

WANG Liyi1), LIU Qinyu1), *, XU Lixiao1), and XIE Shang-Ping2), 1)



Response of Mode Water and Subtropical Countercurrent to Greenhouse Gas and Aerosol Forcing in the North Pacific

WANG Liyi, LIU Qinyu, XU Lixiao, and XIE Shang-Ping

1),,266100,2),,,92093-0230,

The response of the North Pacific Subtropical Mode Water and Subtropical Countercurrent (STCC) to changes in greenhouse gas (GHG) and aerosol is investigated based on the 20th-century historical and single-forcing simulations with the Geophysical Fluid Dynamics Laboratory Climate Model version 3 (GFDL CM3). The aerosol effect causes sea surface temperature (SST) to decrease in the mid-latitude North Pacific, especially in the Kuroshio Extension region, during the past five decades (1950–2005), and this cooling effect exceeds the warming effect by the GHG increase. The STCC response to the GHG and aerosol forcing are opposite. In the GHG (aerosol) forcing run, the STCC decelerates (accelerates) due to the decreased (increased) mode waters in the North Pacific, resulting from a weaker (stronger) front in the mixed layer depth and decreased (increased) subduction in the mode water formation region. The aerosol effect on the SST, mode waters and STCC more than offsets the GHG effect. The response of SST in a zonal band around 40˚N and the STCC to the combined forcing in the historical simulation is similar to the response to the aerosol forcing.

North Pacific; Subtropical Countercurrent; mode water; greenhouse gas; aerosol

1 Introduction

Increasing concentrations of the greenhouse gas (GHG) in the atmosphere are considered to be the major cause of global warming (Meehl, 2007). On the other hand, the influence of atmospheric aerosol on climate is complex, including both direct radiative and indirect effects on cloud properties (Penner, 2001). Anthropogenic aerosol effect partly offsets the GHG warming effect, and the inclusion of the former in a climate model can improve the model performance (Donner, 2011).

The Subtropical Countercurrent (STCC) is a narrow eastward jet in the central North Pacific (20˚N–30˚N), flowing against the broad westward Sverdrup flow (Yoshida and Kidokoro, 1967). It is accompanied by a subsurface temperature and density front called the subtropical front (STF), in the thermal wind relationship with the STCC (Uda and Hasunuma, 1969; White, 1978; Kobashi, 2006). The STCC affects the atmosphere through its surface thermal effects, which can induce anomalous cyclonic wind curl and precipitation (Kobashi, 2008).

The North Pacific mode waters, in particular, the Sub-tropical Mode Water (STMW; Suga, 1989) and the Central Mode Water (CMW; Nakamura, 1996; Suga, 1997), play a key role in the formation and maintenance of the STCC and STF as illustrated by theoretical (Kubokawa, 1997, 1999), model (Takeuchi, 1984; Kubokawa and Inui, 1999; Yamanaka, 2008), and observational (Aoki, 2002; Kobashi, 2006) studies. Using an ocean general circulation model (GCM), Kubokawa and Inui (1999) first illustrated the mechanisms for the STCC formation: mode waters of low potential vorticity (PV) on different isopycnals are subducted at different locations along the mixed layer depth (MLD) front, advected southward by the subtropical gyre, and eventually stacked up vertically to form a thick low-PV pool, which pushs the upper pycnocline upward. The slope of the upper pycnocline causes aneastward current near the surface. The hydrographic analysis of Kobashi(2006) shows that the STCC is indeed anchored by mode waters beneath to the north. Thus mode waters are not only passive water masses but also exhibit an important dynamical effect on ocean circulation.

Based on the analysis of the output from the GFDL CM2.1 experiment under a COdoubling, the surface water in the formation region and the core layer of STMW become warmer and fresher (Lee, 2009). Based on a set of the Intergovernmental Panel on Climate Change (IPCC) Forth Assessment Report (AR4) models, Luo(2009) showed that in a warmer climate, mode waters are produced on lighter isopycnal surfaces and significantly reduced in volume. Using a 300-year control simulation from the GFDL coupled model CM2.1, Xie(2011) showed that on decadal time scales, the dominant mode of sea surface height (SSH) variability in the central subtropical gyre (170˚E–130˚W, 15˚N–35˚N) is characterized by the strengthening or weakening of the STCC as a result of variations in mode-water ventilation. This STCC mode decays, owing to weakened mode waters in both the mean state and variability (Xu, 2012b), as COconcentration increases in the 21st century.

Recent multi-model analysis confirmed that the weakened mode waters and STCC are robust among the 17 Coupled Model Intercomparison Project Phase 5 (CMIP5) models (Xu, 2012a). Except for a recent study of Suzuki and Ishii (2011) showing that STMW was warming and freshening during the period 1981–2007, no other studies have reported significant changes in the mode waters. Why are the mode waters and the STCC not reduced in the face of global mean temperature increasing? One plausible hypothesis is that the aerosol effect may offset the warming effect of GHG, but the aerosol effect on the mode waters and STCC has not been investigated in previous studies yet.

Observational analysis reveals that SST in the mid-latitude North Pacific has decreased over the past five decades (Fig.1). But in climate models, the response of SST to increased COfeatures a warming trend in the North Pacific (See Fig.1; Xie, 2010). Does the aerosol effect exceed the COeffect on the SST in the North Pacific? CMIP5 reported that global mean atmospheric sulfate aerosol (SO) concentration increases slowly since 1850, and the increase accelerates in the latter half of the 20th century (Fig.2). Sharp increases in SOconcentration have been observed over Asia (70˚E–150˚E, 0˚–60˚N) since 1950s. Aerosol optical depth (AOD) over East Asia may have important impact on cloud and shortwave radiation overthe western North Pacific, which, in turn, could induce SST response (Bao, 2009). The SST change in the North Pacific may further alter the properties of mode waters and the STCC.

The present study, using the output from historical all-forcing and single-forcing simulations and pre-industrial control experiment of GFDL CM3, investigates the aerosol cooling and GHG warming effects on the SST, mode waters and the STCC in the North Pacific. The major conclusion drawn from the study is that the aerosol effect significantly exceeds the GHG effect over the past five decades.

The rest of the paper is organized as follows. Section 2 briefly describes the model and simulations. Section 3 investigates the SST changes due to the aerosol and GHG effects in the North Pacific. Section 4 studies the responses of the STCC, mode waters and related subduction processes to different forcing. Section 5 is a summary.

Fig.2 Atmospheric sulfate aerosol (SO4) concentration based on the CMIP5 recommended data.

2 Model and Simulations

This study uses the output from the National Oceanic and Atmospheric Administration (NOAA) GFDL CM3, one of the primary models from GFDL contributed to the IPCC Fifth Assessment Report (AR5). The GFDL CM3 is formulated with the same ocean and sea ice components as the earlier CM2.1, and includes extensive development of the atmosphere and land model components (Griffies, 2011). Especially, aerosol-cloud interactions are included in GFDL CM3, as documented by Donner(2011). The atmospheric component AM3 employs a cubed-sphere implementation of a finite-volume dynamical core with horizontal resolution of approximately 200km. The ocean component MOM4 has a horizontal resolution of 1.0˚×1.0˚ and 50 layers, 22 of which are in the upper 220m. In the meridional direction the horizontal resolution increases toward the equator, and the grid scale becomes 1/3˚ between 30˚S and 30˚N.

A number of CM3 integrations were performed following the CMIP5 protocol (Taylor, 2012), which includes the pre-industrial control, a 5-member historical ensemble, and 4 future scenarios (Representative Concentration Pathway 2.6, 4.5, 6.0 and 8.5). Detection and attribution simulations were conducted to examine the model’s response to a subset of historical single forcing (GHG forcing, natural forcing, aerosol forcing, and anthropogenic forcing), each consisting of 3 runs. Historical and single-forcing simulations employ evolution of forcing agents during the period 1860–2005, and each ensemble member is initialized 50 years or 100 years apart from the pre-industrial control experiment, which runs for 800 years with time-invariant radiative forcing agents fixed at the 1860 value.

The output of GHG forcing, aerosol forcing and historical all-forcing simulations is used in the present study. First, the output is processed to obtain the 1950–2005 mean and ensemble mean of each simulation from 3 members. The combined ensemble and time means allow for a robust signal from natural variability present in an individual ensemble member. As a reference, the 100–155 years averages are used from the pre-industrial control experiment (the first year of the control run is called 001 year).

3 SST Response to GHG and Aerosol Forcing

First the the climatology mean SST in the historical and single-forcing simulations and in pre-industrial control experiment (hereafter referred to as ‘the control run’) are compared (Fig.3). The SST response to the GHG forcing is opposite to that to the aerosol forcing. The SST difference between all forcing and the control run is close to zero in much of the North Pacific except in the mid-latitude regions (Fig.3c). The result shows that the SST response in the all forcing experiment is due to the combined effect of GHG and aerosol. But, in the zonal band of the mid-latitude North Pacific (40˚N, 140˚E–150˚W), the SST rises by about 0.5℃–1.0℃ in the GHG forcing run (Fig.3a), and drops by 1.0℃–1.5℃ in the aerosol forcing run (Fig.3b) and by 0.5℃–1.0℃ in the all-forcing run (Fig.3c). Thus, in this zonal band, the SST response to all forcing is similar to the response to the aerosol forcing.Consequently, the aerosol effect is important in the observed SST dropping in the mid-latitude North Pacific, especially in the Kuroshio Extension region, during the past 56 years.

4 STCC and Mode Waters Responses

Previous studies showed that the STCC is anchored by mode waters to the north (Kubokawa, 1999; Kobashi, 2006; Yamanaka, 2008; Xie, 2011, Xu, 2012a, b), and the changes in mode waters can be further traced upstream to those in the MLD and the subduction rate in the Kuroshio-Oyashio Extension (KOE) region (Xie, 2011). In order to determine the physical mechanisms of the STCC change, the responses of the MLD, subduction, mode waters and STCC to different forcing are compared first, and then the relationship between the STCC and mode waters is investigated.

4.1 The MLD and Subduction

The MLD in CM3 reaches its seasonal maximum in the North Pacific in March. Generally, the March mixed layer is shallow in the southern subtropical gyre, but deepens northward and reaches its maximum in the mid-latitude regions. The water separating the deep mixed layer regions from the rest of the North Pacific is in a narrow transition zone called the MLD front (Xie, 2000), which is a key to the formation of STMW and CMW (Kubokawa, 1999). The MLD front varies among four runs and is strongest in the aerosol forcing run with the largest subduction rate (Fig.4).

In the control run, there are two sub-regions where the MLD exceeds 400m. On the south flank of the deepest MLD region are two subduction zones, where the MLD front intersects with the isopycnal surfaces of 25.5σand 26.0σand STMW and CMW are formed, respectively (Fig.4d). The GHG run, the aerosol runs and historical all-forcing run are compared with the control run respectively. In the GHG run (Fig.4a), the isopycnal outcrop lines move northwards, the MLD front becomes weaker, and the lateral induction decreases. In the aerosol run (Fig.4b), the situation is opposite because the mixed layer over the central North Pacific deepens by 80m and the ocean surface cools (Fig.5b). In the historical all-forcing run (Fig.4c), the overall spatial structure of MLD is still similar but the local MLD maximum associated with CMW formation deepens compared to the control run. It indicates that the aerosol effect on MLD is greater than the GHG effect in the historical run.

The vertical section of potential temperature along the maximum change of MLD (175˚W in Fig.5a; 180˚E in Figs.5b and 5c) further clarifies the cause of the MLD change. In the GHG run, the warming is greater near the surface and decreases with depth. The resultant intensification of the upper ocean stratification inhibits winter convection, and shoals the mixed layer and isothermal layers (Fig.5d). In contrast, the potential temperature in the aerosol run decreases from the surface to about 400m and the decreasing decays rapidly with depth. The reduced stratification leads to a MLD increase (Fig.5e). In the historical all-forcing run, the potential temperature decrease is similar to, but weaker than that in the aerosol run, and the MLD deepens less accordingly (Fig.5f). Here the aerosol effect is again opposite to and surpasses the GHG effect on the MLD, and other forcing seems not to effect the MLD in Fig.5.

Fig.4 Mixed layer depth (black contour line and contour interval (CI)=50m), outcrop line (blue dotted contour line (CI)=0.5kgm-3) and lateral induction (color in myear-1) in (a) the GHG run, (b) the aerosol run , (c) the historical run and (d) the control run during March.

The lateral induction as a function of outcrop density is further analyzed by integrating the local positive lateral induction at a ±0.05kgminterval about each isopycnal layer over the area (140˚E–160˚W, 20˚N–40˚N), where the STMW and CMW form. Fig.6 compares the integral of the lateral induction. All runs show at least two distinct peaks of the lateral induction, which correspond to the two different outcrop densities for the STMW and CMW: 25.7σand 26.1σin the control run; 25.4σand 26.0σin the GHG run; 25.9σand 26.5σin the aerosol run; and 25.8σand 26.3σin the historical run. In the historical and control runs the core densities of the STMW and CMW are slightly higher than the observations, especially for the STMW with an observed typical core density of 25.4σ(Qiu and Huang, 1995; Suga, 1997). As for the response to the GHG (aerosol) effect, less and lighter (more and denser) water appears in the cores of the STMW and the CMW in the GHG (aerosol) run.

4.2 Mode Waters

The volume of the low-PV layer (less than 1.5×10ms) is calculated for each density class from 25.0 to 27.0kgmbetween the layers at ±0.05kgmabout an isopycnal layer in May. In order to exclude the formation area of the eastern subtropical mode water, the calculations are conducted in two regions (140˚E–170˚W, 20˚N–40˚N and 170˚W–140˚W, 30˚N–40˚N).

As shown in Fig.7, corresponding to the STMW and the CMW, the low-PV water in different runs seems to occur in two distinct potential density layers. Specifically, the major peaks of the STMW and the CMW volumes appear at 25.8σand 26.3σin the control run, 25.5σand 26.0σin the GHG run, 26.0σand 26.5σin the aerosol run, and 25.9σand 26.4σin the historical all-forcing run. The North Pacific mode waters (STMW and CMW) form on lighter isopycnal surfaces and decrease in volume in the GHG run, which is consistent with the results of Luo(2009). The opposite is true for a cooler climate in the aerosol run. As a result that the aerosol effect surpasses the GHG effect, the North Pacific mode waters form on denser isopycnal surfaces and increase in volume in the historical all-forcing run.

4.3 STCC Response

Fig.8 shows the May climatological means of SSH and zonal current velocity at the 25-m level for four different runs. These runs all simulate on a basin scale the anti-cyclonic subtropical gyre circulation with an eastward jet (STCC) embedded in the central gyre where the Sverdrup zonal flow is sluggish. This STCC originates in the western Pacific around 20˚N, intensifies and reaches its maximum velocity near 175˚E, and then weakens gradually northeast of Hawaii. Compared to the control run (Fig.8d), the STCC in the GHG run weakens by 2cms(Fig.8a), but strengthens from 8cmsto 10cmsin the aerosol run (Fig.8b). In the historical run (Fig.8c), the STCC is stronger than in GHG run and similar to that in the aerosol run, because the aerosol effect exceeds the GHG effect.

4.4 Relationship Between Mode Waters and STCC

Fig.9 shows the eastward current speed, potential density, and PV for four different runs in a meridional section along 175˚E in May. The STCC is confined above 200m depth between 19˚N–26˚N. Because of the thermal wind balance it can be seen that the northward shoaling of the upper pycnocline is accompanied by a thick layer of low-PV water underneath in the north in all runs. In the control run (Fig.9d), the 26.2σisopycnal layer begins to shoal northward from 20˚N to 24˚N and the 25.8σisopycnal layer shows an even steeper northward shoaling, pushed by the low-PV water in between the 25.8–26.2σlayers. For the other runs (Figs.9a, 9b and 9c), the vertical structure of STCC and its relationship with mode waters do not change, indicating that the STCC is tied to mode waters. In the control run, the low-PV water core (less than 0.5×10ms) resides in between the 25.8–26.1σlayers with the STCC exceeding 8cms. Compared to the control run, the low-PV water core weakens and moves to lighter isopycnal layers (25.6–25.9σ) with a decelerated STCC in the GHG run. By contrast, the low-PV water core intensifies and moves to denser isopycnal layers (26.0–26.3σ), and the STCC is accelerated with its maximum zonal velocity over 10cmsin the aerosol run. In historical all-forcing run, the magnitudes of the low-PV water core and its residing isopycnal layers, 25.8–26.2σ, fall in between the results of the GHG and aerosol runs, the maximum speed of the STCC still exceeds 10cms, but the area with the speed exceeding 10cmsis smaller than in the aerosol run.

In order to clarify the relationship between mode waters and the STCC, the May PV distributions are calculated on the isopycnals from 25.0 to 27.0kgmat an interval of 0.1kgm. PV is selected on 25.9σin the GHG run (Fig.10a), 26.3σin the aerosol run (Fig.10b), 26.2σin the historical run (Fig.10c), and 26.1σin the control run (Fig.10d) to represent the mode waters core. Fig.10 shows that a weakened mode water causes the STCC to decelerate in the GHG run, whereas an enhanced mode water cause it to accelerate in the aerosol run as mentioned above.

Table 1 shows the regional averages of the eastward current speed in the STCC core (140˚E–160˚W, 20˚N–25˚N),the positive lateral induction in subduction region (140˚E–160˚W, 25˚N–40˚N), and the volume of the low-PV water core withσ<26.5 north of the STCC (140˚E–160˚W, 20˚N–30˚N) from different runs. In the aerosol run the lateral induction is the largest, the low-PV water and the STCC are the strongest. However, the results in the GHG run are just the opposite. In the historical run, the lateral induction rate, the volume of the low-PV water and the STCC speed are somewhere between the aerosol and GHG runs, but still larger than those in the control run, because the aerosol cooling effect is larger than the GHG warming effect.

Fig.8 May climatological means of zonal current speed (color in cms-1) and SSH (CI=10cm) over the North Pacificin (a) the GHG run, (b) the aerosol run, (c) the historical run, and (d) the control run, respectively.

Fig.9 May climatological means of eastward current speed (color contours at 2cms-1), potential density (black contours at 0.2kgm-3 intervals), and PV (gray shading in 10-10m-1s-1) for (a) the GHG run, (b) the aerosol run, (c) the historical run, and (d) the control run, respectively, in a meridional section along 175˚E.

Fig.10 May climatological means of eastward current speed (>8cms-1 contours in cms-1) and PV (color in 10-10m-1s-1) on (a) 25.9σθin the GHG run, (b) 26.3σθin the aerosol run, (c) 26.2σθin the historical run, and (d) 26.1σθin the control run in the North Pacific in May.

Table 1 The area-averaged values from different runs

5 Summary

The responses of the SST, mode waters and STCC to the GHG and aerosol effects are examined in the 20th century single-forcing and historical all-forcing simulations using GFDL CM3. The SST in the mid-latitude North Pacific increases in response to the GHG forcing, but decreases in response to the aerosol forcing. The model results show that the aerosol cooling effect is larger than the GHG warming effect, and the SST in the mid-latitude North Pacific experiences a net decreases in response to all-forcing in history. Thus, the aerosol effect appears to be a more important physical mechanism for the observed SST decreases in the North Pacific, especially in the Kuroshio Extending region, during the past 56 years (1950–2005). The GHG run corresponds to a more stratified upper ocean and a shoaled MLD while the aerosol run corresponds to a weakened ocean stratification and a deepened MLD. The maximum change in MLD appears in the KOE region where the mean MLD is the largest, and the MLD front and the subduction rate also experience changes (mainly by lateral induction). As a result of the decreased subduction rate and mixed layer density, less mode waters are formed on lighter isopycnals in the GHG run. By contrast, more mode waters are formed on denser isopycnals in the aerosol run, due to the increased subduction rate and mixed layer density. The southward advection of the weakened (strengthened) mode waters allows the upper pycnocline to rise less (more) and decelerates (accelerate) the STCC in response to the GHG (aerosol) forcing. In general, the effect of the aerosol forcing is larger than the effect of the GHG forcing, and the response of the STCC to all-forcing resembles that to the aerosol forcing in trend and spatial pattern, albeit weaker in magnitude.

One caveat from this modeling study is that the modeled mode waters and the STCC are too strong comparing with the observations, a bias common to CMIP5 models (Xu, 2012a). The future scenarios of atmospheric composition call for an eventual decrease in aerosols but a continued increase in GHG. As a result, the GHG warming effect will dominate and cause a reduction in mode-water ventilation and weakening of the STCC in the 21st century (Xu, 2012a, 2013). Further model improvements are necessary to better understand the response to anthropogenic forcing, and to obtain a more reliable projection of regional and global climate change.

Acknowledgements

This work is supported by the National Basic Research Program of China (2012CB955602), National Key Program for Developing Basic Science (2010CB428904), and Natural Science Foundation of China (41176006 and 40921004).

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(Edited by Xie Jun)

10.1007/s11802-013-2193-x

ISSN 1672-5182, 2013 12 (2): 222-229

. Tel: 0086-532-66782556 E-mail: liuqy@ouc.edu.cn

(October 29, 2012; revised January 15, 2013; accepted February 26, 2013)

© Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2013