SATELLITE-BASED RESEARCH ON OCEANIC PLANETARY WAVES: RECENT ADVANCES AND OPEN QUESTIONS

Paolo Cipollini

National Oceanography Centre, Southampton, European Way, SO14 3ZH Southampton, United Kingdom

ABSTRACT

This paper discusses several open questions relevant to the dynamics of oceanic planetary waves, to their effects on phytoplankton and to their role in climate change. These questions have been prompted by the last two decades of satellite observations of planetary waves – initially with altimeters but now also with SST and ocean colour – which have also stimulated significant advances in our theoretical understanding of these large-scale westward-propagating waves.

1. INTRODUCTION

Oceanic planetary waves, also known as Rossby waves, play a significant role in ocean circulation and climate [1][2]. Satellite techniques, and altimetry in particular, have allowed a quantum leap in our knowledge of these long-wavelength, westward-travelling waves. The progress since the late 1980s is reviewed in [3] and [4]. Although the main impetus to this advance has to be credited to altimetry, in the last 10 years there have also been observations of planetary waves in satellite-derived sea surface temperature (SST) [5] and ocean colour [6][7].

Despite the many advances in the observations and in the relevant theory, many questions still remain open on the dynamics of oceanic planetary waves, their effects on phytoplankton and their role in climate change. In this paper I briefly discuss some of these open questions.

First I introduce several issues related to the dynamics of the features, such as whether they can be considered waves or non-linear eddies on the basis of the observations and the best available theory; to the realm of ocean dynamics belong also recent studies on the relevance and detectability of different modes of propagation, and on the presence of waveguides of enhanced westward-propagating energy. After that, I discuss the potential role of waves in the ENSO cycle in the Pacific ocean, as well as in the modulation of the Meridional Overturning Circulation (MOC) in the Atlantic, and the new hypothesis of planetary wave

speed up due to anthropogenic global warming. Finally, I deal with the question on what causes the signature of planetary waves in ocean colour, and in particular whether planetary waves can modulate primary productivity in the oceans. I also suggest a possible approach to future research on this intriguing topic.

2. PLANETARY WAVES IN A CHANGING CLIMATE

As there is consensus that planetary waves are important for ocean circulation, we now need to establish what role they take in the current climate change scenario. There are essentially two ways in which planetary waves come into play: through their purely dynamical aspects (a case in point is their role in the ENSO cycle, see below) and through their effects on biology, which might (or might not) imply an effect on the carbon cycle. In the following I discuss both issues.

3. DYNAMICAL ASPECTS OF PLANETARY WAVES

3.1. Westward-propagating features: waves or eddies?

Many studies on large-scale westward propagation based on satellite altimetry in the 1990s interpreted the observed features in terms of planetary waves. Most of those studies were based on data from the TOPEX/ Poseidon (T/P) mission and its successor, Jason-1. The advent of optimally-interpolated maps of sea surface height anomaly (SSHA) from the merging of a 10-day orbital repeat mission (first T/P, then Jason-1) and a 35-day orbital repeat mission (ERS-1, ERS-2 and now Envisat) [8] has made possible for the same type of analysis to be carried out with significantly increased spatial resolution [9], therefore allowing the characterization of westward-propagating features at scales of the order of 100-500 Km. In these higher resolution data it becomes apparent that a large part of the variability at mid-latitudes takes the form of westward-propagating, eddy-like features, mildly non-linear [9] and at spatial scales close to the lower limit for planetary waves [10].

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Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)

As I will discuss in section 4, the distinction between eddies and planetary waves has important bearings on the effects of these features on the biology; however, the SSHA data show that both phenomena are present at mid-latitude, superimposed on each other, as illustrated in the example in figure 1. Some useful insight on the characteristics of the westward propagation can be obtained by looking at the spectral domain. Reference [11] analysed zonal wavenumber/ frequency spectra of SSHA in the South Pacific and found that the most recent extension of the planetary wave theory [12] fits the observations satisfactorily, except at the shortest wavelengths (wavelengths ≤ 500 km). At these wavelengths waves are expected to

become significantly dispersive from the theory, but the features seen in SSHA data remain substantially non-dispersive, which again could be a suggestion of non-linear eddies. In a Radon Transform-based analysis [10] a non-negligible part of the energy was found to be indicative of those ‘short and fast’ planetary waves or eddies. One intriguing feature of planetary wave spectra is that they are broad-banded and only rarely show clear peaks at the annual or semiannual period; this suggests that a combination of mechanisms at different frequencies may be implied both in the generation process and in the intermodal exchange of energy (see below), and calls for further observational and modelling studies on this topic.

Fig.1 – a) longitude/time plot of unfiltered (raw) SSHA data from merged T/P and ERS at 34°N in the Pacific ocean, showing westward propagation superimposed on the annual steric cycle; b) same as a) after filtering with a westward-only filter similar to the one used in [6]; c) same as in b) after the application of a low-pass filter in both spacce and

time, to select the larger-scale, longer-period features. Figure courtesy of Doug McNeall at NOCS.

3.2. Modes and waveguides

Another remarkable dynamical issue is the relative importance and observability of the different modes of propagation. A few studies have suggested that it is possible to observe distinct baroclinic modes, for instance [13]. Reference [11] found that both the 1st and 2nd baroclinic modes are observable in the South Pacific and contribute significantly to the westward propagating energy, while higher modes are negligible.

The barotropic mode is generally difficult to observe in satellite data because it propagates very quickly, however [14] has shown a significant presence of ‘fast’ westward-propagating barotropic energy in some oceanic basins, by using T/P data regridded on a 3-day sub-cycle. Recently [15] found evidence of a cyclic energy exchange between the barotropic mode and the mesoscale signals in the Argentinian Basin. Similar wave-eddy interactions might also involve the baroclinic modes and need further investigation: for instance, coupling between the baroclinic and barotropic modes has been suggested as a possible explanation for the amplification of planetary waves when crossing major topographic features [16].

Analysis of westward propagating SSHA signals in the various oceanic basins has revealed the presence of distinct ‘waveguides’ of enhanced energy. In the North Atlantic such a waveguide is around 33°-34°N [17], while [10] and [18] found waveguides in the South Pacific and South Indian, respectively. A possible theoretical explanation for the presence of such waveguides has been suggested by [19] who adopted a ray tracing approach. Their results do yield zonal waveguides of enhanced propagation, but sometimes at different latitudes from those observed in the data.

3.3. Planetary waves, climate signals and climate change

It has been suggested that planetary waves can play a role in the ENSO dynamics, both as a recharger of the western warm pool in the Cane-Zebiak delayed oscillator model, and as a delayer of El Niño effects. Reference [20] present evidence for the existence of an extra-tropical Rossby wave in the North Pacific,

generated by the 1982-83 El Nin ̃o event and suggest that this wave may have induced a shifting of the Kuroshio Current in the north-west Pacific after a delay of 11 years. The generation of North Pacific planetary waves by coastally-trapped waves propagating from the equator towards the poles as a result of El Niño, has been confirmed by analysis of in situ data. However, [21] found that boundary-generated waves normally do not propagate far, and that the westward variability in the interior of the north Pacific basin is dominated by wind-driven planetary waves.

Very recently it has been suggested [22] that some of the variability observed in the Atlantic MOC could be due to planetary waves. Reference [22] used OCCAM, an eddy-permitting numerical model, and a simple theoretical model to show that Rossby waves can generate short term MOC fluctuations of several Sverdrups.

Planetary waves are also expected to speed up in a warming ocean, as stratification increases as an effect of climate change. A new study [23] has found, from theoretical considerations and model simulations, that the waves at low latitudes should be already showing a detectable increase in their westward speed w.r.t. the 1900 value. A look at the ratio of speeds over 2003-2005 to those over 1993-1995, computed with altimeter SSHA data (figure 2), does not show any regions of systematic increase; however the areas of increase or decrease in figure 2 do show a hint of geographical correlation, which warrants further investigations.

4. PLANETARY WAVES AND BIOLOGY

4.1. The ‘rototiller effect’

The presence of an indisputable signature of planetary waves in ocean colour fields [6][7] has come as a surprise and has sparked a number of studies on its causes and on whether planetary waves can affect the carbon cycle. This issue is reviewed in detail in [24]. Here I only discuss a possible way to establish whether the waves have a significant effect on primary productivity and therefore on the carbon cycle.

Fig. 2 – ratio of 2003-2005 westward propagating speeds of planetary waves to the 1993-1995 value. The speeds have been computed from longitude/time plots of SSHA data from all available altimeters.

The way that planetary waves could affect the carbon cycle is via the vertical advection of nutrients (upwelling), illustrated in Fig. 3. Nutrient-rich water is advected upwards by the lifting of the nutricline on the leading side of the density wave (which corresponds to the trailing side of the wave in SSH) and stimulates phytoplankton growth (new production). Nutrient-depleted water is dragged down on the other side, so that the response to the linear process of upwelling is non-linear (rectification). This mechanism notably differs from eddy-induced pumping of nutrients in that eddies tend to retain water in their core and only advect water vertically when they intensify or weaken, whereas planetary waves would upwell nutrients (and downwell nutrient-depleted water) all along their propagation path – hence the name ‘rototiller effect’ [25]. Such a mechanism would be of particular interest to biologists, as planetary waves could be one of the ‘missing factors’ for the supply of nutrients to the upper layer to match observed new production, a question that is still open over large oligotrophic areas of the oceans.

Figure 3. Scheme of the upwelling mechanism for a first mode-baroclinic planetary wave field, with the

order of magnitude of the scales of the various features (‘thermocline’indicates the midlatitude permanent

thermocline). Figure from [26]

In a combined satellite and process modelling study [26] showed that, while over large part of the ocean the observed signal in ocean colour is mainly due to horizontal advection of chlorophyll gradients, the vertical mechanism could not be ruled out. Reference [27], using the same approach plus some statistical assumptions, attempted a decomposition of the observed signal in the North Atlantic and found that north of 28°N the contributions of horizontal advection and upwelling are comparable (figure 4).

A study currently being carried out within the ‘Biowaves’ project at NOCS aims at linking a state-of-the-art physical model (NEMO) with a NPZDDOM biogeochemical model, to simulate the effects on primary productivity of planetary waves and other westward propagating features. The strategy is to use the full-spectrum, unfiltered physical fields from a NEMO run to drive a control run of the biogeochemical model, and diagnose productivity; subsequently, the same biogeochemical model will be run again, driven by filtered physical fields in which the westward propagation has been suppressed. A comparison of the production from the latter run with that from the control run should elucidate the net effect that westward propagating signals have on the oceanic carbon cycle, thus confirming or disproving the ‘rototiller effect’ hypothesis.

4.2. The ‘hay rake’: a non-phytoplankton mechanism?

A possible non-phytoplankton mechanism, the ‘hay-rake’-style accumulation of particles at surface, due to convergence/divergence by planetary waves, has been suggested by [28]. The signal detected by the ocean colour sensor would detect the signature of these particles, rather than a change in chlorophyll. However this mechanism is still object of debate, as it has been noted [29] that particles may not converge in a planetary wave when wave phase speed is taken into account. The authors [30] have replied that if features are non-linear (and therefore more akin to an eddy) then the particles may converge again – in essence, this brings the argument back to the wave vs eddy argument dealt with in sections 3.1 and 4.1.

5. CONCLUSIONS

In this paper I have briefly reviewed the state of the art of the research on planetary waves. Some key questions still remain open on their dynamics, for instance on the relative importance of waves and eddies (and their interaction) at mid-latitude, on the role of waves in the ENSO cycle and in the modulation of the MOC and on the speed-up of the waves in a warming ocean. Moreover we are still searching for conclusive evidence that planetary waves can

significantly affect primary production and the global carbon cycle. A thorough analysis of multi-parameter satellite data, such as those gathered by the suite of ocean-observing instruments on board Envisat, and the synergies of surface observations from space, 3D observations of the interior of the ocean by the ARGO system and simulations by coupled physical and biogeochemical models are the key to solve many of the questions I have discussed in the present paper.

Fig 4 – contribution of different mechanisms to the planetary wave signature in ocean colour in the North

Atlantic. Figure taken from [27]

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